Doc. no. | N4485
Date: | 2015-05-23
Project: | Programming Language C++
Reply to: | Marshall Clow <lwgchair@gmail.com>

# C++ Standard Library Defect Report List (Revision R93)

Revised 2015-05-23 at 15:05:40 UTC

Reference ISO/IEC IS 14882:2014(E)

Also see:

- Table of Contents for all library issues.
- Index by Section for all library issues.
- Index by Status for all library issues.
- Library Active Issues List
- Library Closed Issues List

This document contains only library issues which have been closed by the Library Working Group (LWG) after being found to be defects in the standard. That is, issues which have a status of DR, TC1, C++11, C++14, or Resolved. See the Library Closed Issues List for issues closed as non-defects. See the Library Active Issues List for active issues and more information. The introductory material in that document also applies to this document.

## Revision History

- R93: 2014-05-22 2015 post-Lenexa mailing
  - Summary:
    - 256 open issues, down by 36.
    - 1770 closed issues, up by 48.
    - 2026 issues total, up by 12.
  - Details:
    - Added the following 2 Ready issues: 2492, 2494.
    - Added the following 10 New issues: 2493, 2495, 2496, 2497, 2498, 2499, 2500, 2501, 2502, 2503.
    - Changed the following 2 issues to Ready (from Review): 2111, 2380.
    - Changed the following 20 issues to Ready (from New): 2244, 2250, 2259, 2336, 2353, 2367, 2384, 2385, 2435, 2462, 2466, 2473, 2476, 2477, 2483, 2484, 2485, 2486, 2487, 2489.
    - Changed the following 12 issues to Ready (from Open): 1169, 2072, 2101, 2119, 2127, 2133, 2156, 2181, 2218, 2219, 2447, 2469.
    - Changed the following issue to Tentatively Ready (from Open): 2224.
    - Changed the following issue to Review (from New): 2296.
    - Changed the following issue to Review (from Open): 2328.
    - Changed the following 11 issues to Open (from New): 2262, 2289, 2338, 2348, 2349, 2370, 2398, 2402, 2422, 2450, 2456.
    - Changed the following 8 issues to Open (from SG1): 2245, 2265, 2276, 2309, 2363, 2379, 2426, 2441.
    - Changed the following issue to LEWG (from New): 2372.
    - Changed the following issue to EWG (from New): 2432.
    - Changed the following issue to Deferred (from Open): 2202.
    - Changed the following 14 issues to WP (from Ready): 2160, 2168, 2364, 2403, 2406, 2411, 2425, 2427, 2428, 2433, 2434, 2438, 2439, 2440.
    - Changed the following 18 issues to WP (from Tentatively Ready): 2059, 2076, 2239, 2369, 2378, 2410, 2415, 2418, 2437, 2448, 2454, 2455, 2458, 2459, 2463, 2467, 2470, 2482.
    - Changed the following 3 issues to WP (from New): 2420, 2464, 2488.
    - Changed the following issue to WP (from Open): 2063.
    - Changed the following 2 issues to WP (from SG1): 2407, 2442.
    - Changed the following issue to Resolved (from Review): 2228.
    - Changed the following 3 issues to Resolved (from Open): 1526, 2274, 2397.
    - Changed the following 5 issues to NAD (from New): 2079, 2251, 2351, 2373, 2386.
    - Changed the following issue to NAD (from Open): 2388.
- R92: 2015-04-09 pre-Lenexa mailing
  - Summary:
    - 292 open issues, up by 33.
    - 1722 closed issues, up by 0.
    - 2014 issues total, up by 33.
  - Details:
    - Added the following 5 Tentatively Ready issues: 2459, 2463, 2467, 2470, 2482.
    - Added the following 27 New issues: 2460, 2461, 2462, 2464, 2465, 2466, 2468, 2471, 2472, 2473, 2474, 2475, 2476, 2477, 2478, 2479, 2480, 2481, 2483, 2484, 2485, 2486, 2487, 2488, 2489, 2490, 2491.
    - Added the following Open issue: 2469.
    - Changed the following issue to Tentatively Ready (from Review): 2378.
    - Changed the following 11 issues to Tentatively Ready (from New): 2076, 2239, 2369, 2410, 2415, 2418, 2437, 2448, 2454, 2455, 2458.
    - Changed the following issue to Tentatively Ready (from Open): 2059.
    - Changed the following issue to Tentatively NAD (from New): 2337.
    - Changed the following issue to Tentatively NAD (from Open): 760.
    - Changed the following 5 issues to Open (from New): 2312, 2388, 2393, 2444, 2447.
    - Changed the following 4 issues to LEWG (from New): 2391, 2417, 2436, 2451.
    - Changed the following issue to EWG (from Open): 2089.
    - Changed the following issue to Core (from New): 2452.
    - Changed the following 13 issues to SG1 (from New): 2236, 2245, 2265, 2276, 2309, 2334, 2363, 2379, 2407, 2412, 2426, 2442, 2445.
    - Changed the following issue to SG1 (from Open): 2441.
- R91: 2014-11-23 post-Urbana mailing
  - Summary:
    - 259 open issues, up by 32.
    - 1722 closed issues, down by 20.
    - 1981 issues total, up by 12.
  - Details:
    - Added the following 12 New issues: 2447, 2448, 2449, 2450, 2451, 2452, 2453, 2454, 2455, 2456, 2457, 2458.
    - Changed the following 2 issues to Ready (from Review): 2160, 2364.
    - Changed the following 11 issues to Ready (from New): 2403, 2406, 2411, 2425, 2427, 2428, 2433, 2434, 2438, 2439, 2440.
    - Changed the following issue to Ready (from Open): 2168.
    - Changed the following issue to Review (from New): 2424.
    - Changed the following 5 issues to Open (from New): 2307, 2310, 2383, 2414, 2441.
    - Changed the following 2 issues to Open (from NAD Future): 760, 1173.
    - Changed the following 4 issues to LEWG (from New): 2419, 2430, 2443, 2446.
    - Changed the following 48 issues to LEWG (from NAD Future): 255, 423, 484, 523, 532, 708, 839, 851, 877, 933, 935, 936, 961, 1025, 1031, 1041, 1052, 1053, 1112, 1120, 1121, 1150, 1154, 1184, 1188, 1201, 1203, 1217, 1235, 1238, 1242, 1282, 1289, 1317, 1320, 1396, 1406, 1422, 1459, 1484, 1488, 1493, 1499, 1521, 2040, 2055, 2226, 2232.
    - Changed the following 2 issues to Pending NAD (from Tentatively NAD): 2302, 2382.
    - Changed the following 11 issues to WP (from Ready): 2016, 2170, 2340, 2354, 2377, 2396, 2399, 2400, 2401, 2404, 2408.
    - Changed the following 12 issues to WP (from Tentatively Ready): 2106, 2129, 2212, 2217, 2230, 2233, 2266, 2325, 2361, 2365, 2376, 2387.
    - Changed the following issue to Resolved (from Ready): 2319.
    - Changed the following issue to Resolved (from Review): 2118.
    - Changed the following issue to Resolved (from New): 2416.
    - Changed the following issue to Resolved (from Open): 2108.
    - Changed the following issue to NAD (from New): 2429.
- R90: 2014-10-13 pre-Urbana mailing
  - Summary:
    - 227 open issues, up by 31.
    - 1742 closed issues, up by 0.
    - 1969 issues total, up by 31.
  - Details:
    - Added the following 31 New issues: 2416, 2417, 2418, 2419, 2420, 2421, 2422, 2423, 2424, 2425, 2426, 2427, 2428, 2429, 2430, 2431, 2432, 2433, 2434, 2435, 2436, 2437, 2438, 2439, 2440, 2441, 2442, 2443, 2444, 2445, 2446.
    - No issues changed.
- R89: 2014-07-08 post-Rapperswil mailing
  - Summary:
    - 196 open issues, up by 14.
    - 1742 closed issues, up by 12.
    - 1938 issues total, up by 26.
  - Details:
    - Added the following 6 Ready issues: 2396, 2399, 2400, 2401, 2404, 2408.
    - Added the following 15 New issues: 2391, 2392, 2393, 2394, 2398, 2402, 2403, 2406, 2407, 2410, 2411, 2412, 2413, 2414, 2415.
    - Added the following Open issue: 2397.
    - Added the following 3 WP issues: 2390, 2395, 2409.
    - Added the following NAD issue: 2405.
    - Changed the following issue to Ready (from New): 2377.
    - Changed the following 2 issues to Ready (from Deferred): 2253, 2255.
    - Changed the following 2 issues to Tentatively Ready (from New): 2325, 2387.
    - Changed the following issue to Tentatively NAD (from New): 2382.
    - Changed the following 3 issues to Review (from New): 2364, 2378, 2380.
    - Changed the following 2 issues to Review (from Open): 2118, 2160.
    - Changed the following 3 issues to Open (from New): 2168, 2238, 2273.
    - Changed the following 3 issues to Open (from Deferred): 2254, 2264, 2277.
    - Changed the following 3 issues to WP (from New): 2371, 2374, 2389.
    - Changed the following 4 issues to Resolved (from Deferred): 2282, 2283, 2287, 2333.
    - Changed the following issue to NAD (from Deferred): 2305.
- R88: 2014-05-24 pre-Rapperswil mailing
  - Summary:
    - 182 open issues, up by 29.
    - 1730 closed issues, up by 0.
    - 1912 issues total, up by 29.
  - Details:
    - Added the following 3 Tentatively Ready issues: 2361, 2365, 2376.
    - Added the following 26 New issues: 2362, 2363, 2364, 2366, 2367, 2368, 2369, 2370, 2371, 2372, 2373, 2374, 2375, 2377, 2378, 2379, 2380, 2381, 2382, 2383, 2384, 2385, 2386, 2387, 2388, 2389.
    - Changed the following 5 issues to Tentatively Ready (from Open): 2106, 2129, 2212, 2230, 2233.

## Defect Reports

### 1. C library linkage editing oversight

Section: 17.6.2.3 [using.linkage] Status: TC1 Submitter: Beman Dawes Opened: 1997-11-16 Last modified: 2015-04-08

View all other issues in [using.linkage].

View all issues with TC1 status.

Discussion:

The change specified in the proposed resolution below did not make it into the Standard. This change was accepted in principle at the London meeting, and the exact wording below was accepted at the Morristown meeting.

Proposed resolution:

Change 17.6.2.3 [using.linkage] paragraph 2 from:

> It is unspecified whether a name from the Standard C library declared with external linkage has either extern "C" or extern "C++" linkage.

to:

> Whether a name from the Standard C library declared with external linkage has extern "C" or extern "C++" linkage is implementation defined. It is recommended that an implementation use extern "C++" linkage for this purpose.

### 3. Atexit registration during atexit() call is not described

Section: 18.5 [support.start.term] Status: TC1 Submitter: Steve Clamage Opened: 1997-12-12 Last modified: 2015-04-08

View all other issues in [support.start.term].

View all issues with TC1 status.

Discussion:

We appear not to have covered all the possibilities of exit processing with respect to atexit registration.
Example 1: (C and C++)

        #include <stdlib.h>
        void f1() { }
        void f2() { atexit(f1); }

        int main()
        {
            atexit(f2); // the only use of f2
            return 0; // for C compatibility
        }

At program exit, f2 gets called due to its registration in main. Running f2 causes f1 to be newly registered during the exit processing. Is this a valid program? If so, what are its semantics?

Interestingly, neither the C standard, nor the C++ draft standard nor the forthcoming C9X Committee Draft says directly whether you can register a function with atexit during exit processing.

All 3 standards say that functions are run in reverse order of their registration. Since f1 is registered last, it ought to be run first, but by the time it is registered, it is too late to be first.

If the program is valid, the standards are self-contradictory about its semantics.

Example 2: (C++ only)

        void F() { static T t; } // type T has a destructor

        int main()
        {
            atexit(F); // the only use of F
        }

Function F registered with atexit has a local static variable t, and F is called for the first time during exit processing. A local static object is initialized the first time control flow passes through its definition, and all static objects are destroyed during exit processing. Is the code valid? If so, what are its semantics?

Section 18.3 "Start and termination" says that if a function F is registered with atexit before a static object t is initialized, F will not be called until after t's destructor completes.

In example 2, function F is registered with atexit before its local static object O could possibly be initialized. On that basis, it must not be called by exit processing until after O's destructor completes. But the destructor cannot be run until after F is called, since otherwise the object could not be constructed in the first place.

If the program is valid, the standard is self-contradictory about its semantics.

I plan to submit Example 1 as a public comment on the C9X CD, with a recommendation that the results be undefined. (Alternative: make it unspecified. I don't think it is worthwhile to specify the case where f1 itself registers additional functions, each of which registers still more functions.)

I think we should resolve the situation in the whatever way the C committee decides.

For Example 2, I recommend we declare the results undefined.

[See reflector message lib-6500 for further discussion.]

Proposed resolution:

Change section 18.3/8 from:

> First, objects with static storage duration are destroyed and functions registered by calling atexit are called. Objects with static storage duration are destroyed in the reverse order of the completion of their constructor. (Automatic objects are not destroyed as a result of calling exit().) Functions registered with atexit are called in the reverse order of their registration. A function registered with atexit before an object obj1 of static storage duration is initialized will not be called until obj1's destruction has completed. A function registered with atexit after an object obj2 of static storage duration is initialized will be called before obj2's destruction starts.

to:

> First, objects with static storage duration are destroyed and functions registered by calling atexit are called. Non-local objects with static storage duration are destroyed in the reverse order of the completion of their constructor. (Automatic objects are not destroyed as a result of calling exit().) Functions registered with atexit are called in the reverse order of their registration, except that a function is called after any previously registered functions that had already been called at the time it was registered. A function registered with atexit before a non-local object obj1 of static storage duration is initialized will not be called until obj1's destruction has completed. A function registered with atexit after a non-local object obj2 of static storage duration is initialized will be called before obj2's destruction starts. A local static object obj3 is destroyed at the same time it would be if a function calling the obj3 destructor were registered with atexit at the completion of the obj3 constructor.

Rationale:

See 99-0039/N1215, October 22, 1999, by Stephen D. Clamage for the analysis supporting to the proposed resolution.

### 5. String::compare specification questionable

Section: 21.4.6.8 [string::swap] Status: TC1 Submitter: Jack Reeves Opened: 1997-12-11 Last modified: 2015-04-08

View all other issues in [string::swap].

View all issues with TC1 status.

Duplicate of: 87

Discussion:

At the very end of the basic_string class definition is the signature: int compare(size_type pos1, size_type n1, const charT* s, size_type n2 = npos) const; In the following text this is defined as: returns basic_string<charT,traits,Allocator>(*this,pos1,n1).compare( basic_string<charT,traits,Allocator>(s,n2);

Since the constructor basic_string(const charT* s, size_type n, const Allocator& a = Allocator()) clearly requires that s != NULL and n < npos and further states that it throws length_error if n == npos, it appears the compare() signature above should always throw length error if invoked like so: str.compare(1, str.size()-1, s); where 's' is some null terminated character array.

This appears to be a typo since the obvious intent is to allow either the call above or something like: str.compare(1, str.size()-1, s, strlen(s)-1);

This would imply that what was really intended was two signatures int compare(size_type pos1, size_type n1, const charT* s) const int compare(size_type pos1, size_type n1, const charT* s, size_type n2) const; each defined in terms of the corresponding constructor.

Proposed resolution:

Replace the compare signature in 21.4 [basic.string] (at the very end of the basic_string synopsis) which reads:

> `int compare(size_type pos1, size_type n1,
> const charT* s, size_type n2 = npos) const;`

with:

> `int compare(size_type pos1, size_type n1,
> const charT* s) const;
> int compare(size_type pos1, size_type n1,
> const charT* s, size_type n2) const;`

Replace the portion of 21.4.6.8 [string::swap] paragraphs 5 and 6 which read:

> `int compare(size_type pos, size_type n1,
> charT * s, size_type n2 = npos) const;
> ` Returns:`
> basic_string<charT,traits,Allocator>(*this, pos, n1).compare(
> basic_string<charT,traits,Allocator>( s, n2))`

with:

> `int compare(size_type pos, size_type n1,
> const charT * s) const;
> ` Returns:`
> basic_string<charT,traits,Allocator>(*this, pos, n1).compare(
> basic_string<charT,traits,Allocator>( s ))
> int compare(size_type pos, size_type n1,
> const charT * s, size_type n2) const;
> ` Returns:`
> basic_string<charT,traits,Allocator>(*this, pos, n1).compare(
> basic_string<charT,traits,Allocator>( s, n2))`

Editors please note that in addition to splitting the signature, the third argument becomes const, matching the existing synopsis.

Rationale:

While the LWG dislikes adding signatures, this is a clear defect in the Standard which must be fixed. The same problem was also identified in issues 7 (item 5) and 87.

### 7. String clause minor problems

Section: 21 [strings] Status: TC1 Submitter: Matt Austern Opened: 1997-12-15 Last modified: 2015-04-08

View all other issues in [strings].

View all issues with TC1 status.

Discussion:

(1) In 21.4.6.4 [string::insert], the description of template <class InputIterator> insert(iterator, InputIterator, InputIterator) makes no sense. It refers to a member function that doesn't exist. It also talks about the return value of a void function.

(2) Several versions of basic_string::replace don't appear in the class synopsis.

(3) basic_string::push_back appears in the synopsis, but is never described elsewhere. In the synopsis its argument is const charT, which doesn't makes much sense; it should probably be charT, or possible const charT&.

(4) basic_string::pop_back is missing.

(5) int compare(size_type pos, size_type n1, charT* s, size_type n2 = npos) make no sense. First, it's const charT* in the synopsis and charT* in the description. Second, given what it says in RETURNS, leaving out the final argument will always result in an exception getting thrown. This is paragraphs 5 and 6 of 21.4.6.8 [string::swap]

(6) In table 37, in section 21.2.1 [char.traits.require], there's a note for X::move(s, p, n). It says "Copies correctly even where p is in [s, s+n)". This is correct as far as it goes, but it doesn't go far enough; it should also guarantee that the copy is correct even where s in in [p, p+n). These are two orthogonal guarantees, and neither one follows from the other. Both guarantees are necessary if X::move is supposed to have the same sort of semantics as memmove (which was clearly the intent), and both guarantees are necessary if X::move is actually supposed to be useful.

Proposed resolution:

ITEM 1: In 21.3.5.4 [lib.string::insert], change paragraph 16 to
EFFECTS: Equivalent to insert(p - begin(), basic_string(first, last)).
ITEM 2: Not a defect; the Standard is clear.. There are ten versions of replace() in the synopsis, and ten versions in 21.3.5.6 [lib.string::replace].
ITEM 3: Change the declaration of push_back in the string synopsis (21.3, [lib.basic.string]) from:

void push_back(const charT)
to
void push_back(charT)
Add the following text immediately after 21.3.5.2 [lib.string::append], paragraph 10.
void basic_string::push_back(charT c);
EFFECTS: Equivalent to append(static_cast<size_type>(1), c);
ITEM 4: Not a defect. The omission appears to have been deliberate.
ITEM 5: Duplicate; see issue 5 (and 87).
ITEM 6: In table 37, Replace:
"Copies correctly even where p is in [s, s+n)."
with:
"Copies correctly even where the ranges [p, p+n) and [s, s+n) overlap."

### 8. Locale::global lacks guarantee

Section: 22.3.1.5 [locale.statics] Status: TC1 Submitter: Matt Austern Opened: 1997-12-24 Last modified: 2015-04-08

View all issues with TC1 status.

Discussion:

It appears there's an important guarantee missing from clause 22. We're told that invoking locale::global(L) sets the C locale if L has a name. However, we're not told whether or not invoking setlocale(s) sets the global C++ locale.

The intent, I think, is that it should not, but I can't find any such words anywhere.

Proposed resolution:

Add a sentence at the end of 22.3.1.5 [locale.statics], paragraph 2:

> No library function other than `locale::global()` shall affect the value returned by `locale()`.

### 9. Operator new(0) calls should not yield the same pointer

Section: 18.6.1 [new.delete] Status: TC1 Submitter: Steve Clamage Opened: 1998-01-04 Last modified: 2015-04-08

View all other issues in [new.delete].

View all issues with TC1 status.

Discussion:

Scott Meyers, in a comp.std.c++ posting: I just noticed that section 3.7.3.1 of CD2 seems to allow for the possibility that all calls to operator new(0) yield the same pointer, an implementation technique specifically prohibited by ARM 5.3.3.Was this prohibition really lifted? Does the FDIS agree with CD2 in the regard? [Issues list maintainer's note: the IS is the same.]

Proposed resolution:

Change the last paragraph of 3.7.3 from:

> Any allocation and/or deallocation functions defined in a C++ program shall conform to the semantics specified in 3.7.3.1 and 3.7.3.2.

to:

> Any allocation and/or deallocation functions defined in a C++ program, including the default versions in the library, shall conform to the semantics specified in 3.7.3.1 and 3.7.3.2.

Change 3.7.3.1/2, next-to-last sentence, from :

> If the size of the space requested is zero, the value returned shall not be a null pointer value (4.10).

to:

> Even if the size of the space requested is zero, the request can fail. If the request succeeds, the value returned shall be a non-null pointer value (4.10) p0 different from any previously returned value p1, unless that value p1 was since passed to an operator delete.

5.3.4/7 currently reads:

> When the value of the expression in a direct-new-declarator is zero, the allocation function is called to allocate an array with no elements. The pointer returned by the new-expression is non-null. [Note: If the library allocation function is called, the pointer returned is distinct from the pointer to any other object.]

Retain the first sentence, and delete the remainder.

18.5.1 currently has no text. Add the following:

> Except where otherwise specified, the provisions of 3.7.3 apply to the library versions of operator new and operator delete.

To 18.5.1.3, add the following text:

> The provisions of 3.7.3 do not apply to these reserved placement forms of operator new and operator delete.

Rationale:

See 99-0040/N1216, October 22, 1999, by Stephen D. Clamage for the analysis supporting to the proposed resolution.

### 11. Bitset minor problems

Section: 20.6 [template.bitset] Status: TC1 Submitter: Matt Austern Opened: 1998-01-22 Last modified: 2015-04-08

View other active issues in [template.bitset].

View all other issues in [template.bitset].

View all issues with TC1 status.

Discussion:

(1) bitset<>::operator[] is mentioned in the class synopsis (23.3.5), but it is not documented in 23.3.5.2.

(2) The class synopsis only gives a single signature for bitset<>::operator[], reference operator[](size_t pos). This doesn't make much sense. It ought to be overloaded on const. reference operator[](size_t pos); bool operator[](size_t pos) const.

(3) Bitset's stream input function (23.3.5.3) ought to skip all whitespace before trying to extract 0s and 1s. The standard doesn't explicitly say that, though. This should go in the Effects clause.

Proposed resolution:

ITEMS 1 AND 2:
In the bitset synopsis (20.6 [template.bitset]), replace the member function
`reference operator[](size_t pos);
`
with the two member functions
`bool operator[](size_t pos) const;
reference operator[](size_t pos);
`
Add the following text at the end of 20.6.2 [bitset.members], immediately after paragraph 45:

> `bool operator[](size_t pos) const;`
> Requires: pos is valid
> Throws: nothing
> Returns: `test(pos)`
> `bitset<N>::reference operator[](size_t pos);`
> Requires: pos is valid
> Throws: nothing
> Returns: An object of type `bitset<N>::reference` such that `(*this)[pos] == this->test(pos)`, and such that `(*this)[pos] = val` is equivalent to `this->set(pos, val);`

Rationale:

The LWG believes Item 3 is not a defect. "Formatted input" implies the desired semantics. See 27.7.2.2 [istream.formatted].

### 13. Eos refuses to die

Section: 27.7.2.2.3 [istream::extractors] Status: TC1 Submitter: William M. Miller Opened: 1998-03-03 Last modified: 2015-04-08

View all other issues in [istream::extractors].

View all issues with TC1 status.

Discussion:

In 27.6.1.2.3, there is a reference to "eos", which is the only one in the whole draft (at least using Acrobat search), so it's undefined.

Proposed resolution:

In 27.7.2.2.3 [istream::extractors], replace "eos" with "charT()"

### 14. Locale::combine should be const

Section: 22.3.1.3 [locale.members] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2015-04-08

View all other issues in [locale.members].

View all issues with TC1 status.

Discussion:

locale::combine is the only member function of locale (other than constructors and destructor) that is not const. There is no reason for it not to be const, and good reasons why it should have been const. Furthermore, leaving it non-const conflicts with 22.1.1 paragraph 6: "An instance of a locale is immutable."

History: this member function originally was a constructor. it happened that the interface it specified had no corresponding language syntax, so it was changed to a member function. As constructors are never const, there was no "const" in the interface which was transformed into member "combine". It should have been added at that time, but the omission was not noticed.

Proposed resolution:

In 22.3.1 [locale] and also in 22.3.1.3 [locale.members], add "const" to the declaration of member combine:

>     template <class Facet> locale combine(const locale& other) const;

### 15. Locale::name requirement inconsistent

Section: 22.3.1.3 [locale.members] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2015-04-08

View all other issues in [locale.members].

View all issues with TC1 status.

Discussion:

locale::name() is described as returning a string that can be passed to a locale constructor, but there is no matching constructor.

Proposed resolution:

In 22.3.1.3 [locale.members], paragraph 5, replace "`locale(name())`" with "`locale(name().c_str())`".

### 16. Bad ctype_byname<char> decl

Section: 22.4.1.4 [locale.codecvt] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2015-04-08

View all other issues in [locale.codecvt].

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Discussion:

The new virtual members ctype_byname<char>::do_widen and do_narrow did not get edited in properly. Instead, the member do_widen appears four times, with wrong argument lists.

Proposed resolution:

The correct declarations for the overloaded members `do_narrow` and `do_widen` should be copied from 22.4.1.3 [facet.ctype.special].

### 17. Bad bool parsing

Section: 22.4.2.1.2 [facet.num.get.virtuals] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2015-04-08

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Discussion:

This section describes the process of parsing a text boolean value from the input stream. It does not say it recognizes either of the sequences "true" or "false" and returns the corresponding bool value; instead, it says it recognizes only one of those sequences, and chooses which according to the received value of a reference argument intended for returning the result, and reports an error if the other sequence is found. (!) Furthermore, it claims to get the names from the ctype<> facet rather than the numpunct<> facet, and it examines the "boolalpha" flag wrongly; it doesn't define the value "loc"; and finally, it computes wrongly whether to use numeric or "alpha" parsing.
I believe the correct algorithm is "as if":

      // in, err, val, and str are arguments.
      err = 0;
      const numpunct<charT>& np = use_facet<numpunct<charT> >(str.getloc());
      const string_type t = np.truename(), f = np.falsename();
      bool tm = true, fm = true;
      size_t pos = 0;
      while (tm && pos < t.size() || fm && pos < f.size()) {
        if (in == end) { err = str.eofbit; }
        bool matched = false;
        if (tm && pos < t.size()) {
          if (!err && t[pos] == *in) matched = true;
          else tm = false;
        }
        if (fm && pos < f.size()) {
          if (!err && f[pos] == *in) matched = true;
          else fm = false;
        }
        if (matched) { ++in; ++pos; }
        if (pos > t.size()) tm = false;
        if (pos > f.size()) fm = false;
      }
      if (tm == fm || pos == 0) { err |= str.failbit; }
      else                      { val = tm; }
      return in;

Notice this works reasonably when the candidate strings are both empty, or equal, or when one is a substring of the other. The proposed text below captures the logic of the code above.

Proposed resolution:

In 22.4.2.1.2 [facet.num.get.virtuals], in the first line of paragraph 14, change "&&" to "&".

Then, replace paragraphs 15 and 16 as follows:

> Otherwise target sequences are determined "as if" by calling the members `falsename()` and `truename()` of the facet obtained by `use_facet<numpunct<charT> >(str.getloc())`. Successive characters in the range `[in,end)` (see [lib.sequence.reqmts]) are obtained and matched against corresponding positions in the target sequences only as necessary to identify a unique match. The input iterator `in` is compared to `end` only when necessary to obtain a character. If and only if a target sequence is uniquely matched, `val` is set to the corresponding value.

> The `in` iterator is always left pointing one position beyond the last character successfully matched. If `val` is set, then err is set to `str.goodbit`; or to `str.eofbit` if, when seeking another character to match, it is found that `(in==end)`. If `val` is not set, then err is set to `str.failbit`; or to `(str.failbit|str.eofbit)`if the reason for the failure was that `(in==end)`. [Example: for targets `true`:"a" and `false`:"abb", the input sequence "a" yields `val==true` and `err==str.eofbit`; the input sequence "abc" yields `err=str.failbit`, with `in` ending at the 'c' element. For targets `true`:"1" and `false`:"0", the input sequence "1" yields `val==true` and `err=str.goodbit`. For empty targets (""), any input sequence yields `err==str.failbit`. --end example]

### 18. Get(...bool&) omitted

Section: 22.4.2.1.1 [facet.num.get.members] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2015-04-08

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Discussion:

In the list of num_get<> non-virtual members on page 22-23, the member that parses bool values was omitted from the list of definitions of non-virtual members, though it is listed in the class definition and the corresponding virtual is listed everywhere appropriate.

Proposed resolution:

Add at the beginning of 22.4.2.1.1 [facet.num.get.members] another get member for bool&, copied from the entry in 22.4.2.1 [locale.num.get].

### 19. "Noconv" definition too vague

Section: 22.4.1.4 [locale.codecvt] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2015-04-08

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Duplicate of: 10

Discussion:

In the definitions of codecvt<>::do_out and do_in, they are specified to return noconv if "no conversion is needed". This definition is too vague, and does not say normatively what is done with the buffers.

Proposed resolution:

Change the entry for noconv in the table under paragraph 4 in section 22.4.1.4.2 [locale.codecvt.virtuals] to read:

> `noconv`: `internT` and `externT` are the same type, and input sequence is identical to converted sequence.

Change the Note in paragraph 2 to normative text as follows:

> If returns `noconv`, `internT` and `externT` are the same type and the converted sequence is identical to the input sequence `[from,from_next)`. `to_next` is set equal to `to`, the value of `state` is unchanged, and there are no changes to the values in `[to, to_limit)`.

### 20. Thousands_sep returns wrong type

Section: 22.4.3.1.2 [facet.numpunct.virtuals] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2015-04-08

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Discussion:

The synopsis for numpunct<>::do_thousands_sep, and the definition of numpunct<>::thousands_sep which calls it, specify that it returns a value of type char_type. Here it is erroneously described as returning a "string_type".

Proposed resolution:

In 22.4.3.1.2 [facet.numpunct.virtuals], above paragraph 2, change "string_type" to "char_type".

### 21. Codecvt_byname<> instantiations

Section: 22.3.1.1.1 [locale.category] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2015-04-08

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Discussion:

In the second table in the section, captioned "Required instantiations", the instantiations for codecvt_byname<> have been omitted. These are necessary to allow users to construct a locale by name from facets.

Proposed resolution:

Add in 22.3.1.1.1 [locale.category] to the table captioned "Required instantiations", in the category "ctype" the lines

>     codecvt_byname<char,char,mbstate_t>,
>     codecvt_byname<wchar_t,char,mbstate_t>

### 22. Member open vs. flags

Section: 27.9.1.9 [ifstream.members] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2015-04-08

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Discussion:

The description of basic_istream<>::open leaves unanswered questions about how it responds to or changes flags in the error status for the stream. A strict reading indicates that it ignores the bits and does not change them, which confuses users who do not expect eofbit and failbit to remain set after a successful open. There are three reasonable resolutions: 1) status quo 2) fail if fail(), ignore eofbit 3) clear failbit and eofbit on call to open().

Proposed resolution:

In 27.9.1.9 [ifstream.members] paragraph 3, and in 27.9.1.13 [ofstream.members] paragraph 3, under open() effects, add a footnote:

> A successful open does not change the error state.

Rationale:

This may seem surprising to some users, but it's just an instance of a general rule: error flags are never cleared by the implementation. The only way error flags are are ever cleared is if the user explicitly clears them by hand.

The LWG believed that preserving this general rule was important enough so that an exception shouldn't be made just for this one case. The resolution of this issue clarifies what the LWG believes to have been the original intent.

### 23. Num_get overflow result

Section: 22.4.2.1.2 [facet.num.get.virtuals] Status: CD1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2015-04-08

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Discussion:

The current description of numeric input does not account for the possibility of overflow. This is an implicit result of changing the description to rely on the definition of scanf() (which fails to report overflow), and conflicts with the documented behavior of traditional and current implementations.

Users expect, when reading a character sequence that results in a value unrepresentable in the specified type, to have an error reported. The standard as written does not permit this.

Further comments from Dietmar:

I don't feel comfortable with the proposed resolution to issue 23: It kind of simplifies the issue to much. Here is what is going on:

Currently, the behavior of numeric overflow is rather counter intuitive and hard to trace, so I will describe it briefly:

- According to 22.4.2.1.2 [facet.num.get.virtuals] paragraph 11 `failbit` is set if `scanf()` would return an input error; otherwise a value is converted to the rules of `scanf`.
- `scanf()` is defined in terms of `fscanf()`.
- `fscanf()` returns an input failure if during conversion no character matching the conversion specification could be extracted before reaching EOF. This is the only reason for `fscanf()` to fail due to an input error and clearly does not apply to the case of overflow.
- Thus, the conversion is performed according to the rules of `fscanf()` which basically says that `strtod`, `strtol()`, etc. are to be used for the conversion.
- The `strtod()`, `strtol()`, etc. functions consume as many matching characters as there are and on overflow continue to consume matching characters but also return a value identical to the maximum (or minimum for signed types if there was a leading minus) value of the corresponding type and set `errno` to `ERANGE`.
- Thus, according to the current wording in the standard, overflows can be detected! All what is to be done is to check `errno` after reading an element and, of course, clearing `errno` before trying a conversion. With the current wording, it can be detected whether the overflow was due to a positive or negative number for signed types.

Further discussion from Redmond:

The basic problem is that we've defined our behavior, including our error-reporting behavior, in terms of C90. However, C90's method of reporting overflow in scanf is not technically an "input error". The `strto_*` functions are more precise.

There was general consensus that `failbit` should be set upon overflow. We considered three options based on this:

1. Set failbit upon conversion error (including overflow), and don't store any value.
2. Set failbit upon conversion error, and also set `errno` to indicated the precise nature of the error.
3. Set failbit upon conversion error. If the error was due to overflow, store +-numeric_limits<T>::max() as an overflow indication.

Straw poll: (1) 5; (2) 0; (3) 8.

Discussed at Lillehammer. General outline of what we want the solution to look like: we want to say that overflow is an error, and provide a way to distinguish overflow from other kinds of errors. Choose candidate field the same way scanf does, but don't describe the rest of the process in terms of format. If a finite input field is too large (positive or negative) to be represented as a finite value, then set failbit and assign the nearest representable value. Bill will provide wording.

Discussed at Toronto: N2327 is in alignment with the direction we wanted to go with in Lillehammer. Bill to work on.

Proposed resolution:

Change 22.4.2.1.2 [facet.num.get.virtuals], end of p3:

> Stage 3: The result of stage 2 processing can be one of The sequence of `char`s accumulated in stage 2 (the field) is converted to a numeric value by the rules of one of the functions declared in the header `<cstdlib>`:

> - A sequence of `char`s has been accumulated in stage 2 that is converted (according to the rules of `scanf`) to a value of the type of val. This value is stored in val and `ios_base::goodbit` is stored in err. For a signed integer value, the function `strtoll`.
> - The sequence of `char`s accumulated in stage 2 would have caused `scanf` to report an input failure. `ios_base::failbit` is assigned to err. For an unsigned integer value, the function `strtoull`.
> - For a floating-point value, the function `strtold`.

> The numeric value to be stored can be one of:

> - zero, if the conversion function fails to convert the entire field. `ios_base::failbit` is assigned to err.
> - the most positive representable value, if the field represents a value too large positive to be represented in val. `ios_base::failbit` is assigned to err.
> - the most negative representable value (zero for unsigned integer), if the field represents a value too large negative to be represented in val. `ios_base::failbit` is assigned to err.
> - the converted value, otherwise.

> The resultant numeric value is stored in val.

Change 22.4.2.1.2 [facet.num.get.virtuals], p6-p7:

>     iter_type do_get(iter_type in, iter_type end, ios_base& str,
>                      ios_base::iostate& err, bool& val) const;

> > -6- Effects: If `(str.flags()&ios_base::boolalpha)==0` then input proceeds as it would for a `long` except that if a value is being stored into val, the value is determined according to the following: If the value to be stored is 0 then `false` is stored. If the value is 1 then `true` is stored. Otherwise `err|=ios_base::failbit` is performed and no value `true` is stored. and `ios_base::failbit` is assigned to err.

> > -7- Otherwise target sequences are determined "as if" by calling the members `falsename()` and `truename()` of the facet obtained by `use_facet<numpunct<charT> >(str.getloc())`. Successive characters in the range `[in,end)` (see 23.1.1) are obtained and matched against corresponding positions in the target sequences only as necessary to identify a unique match. The input iterator in is compared to end only when necessary to obtain a character. If and only if a target sequence is uniquely matched, val is set to the corresponding value. Otherwise `false` is stored and `ios_base::failbit` is assigned to err.

### 24. "do_convert" doesn't exist

Section: 22.4.1.4 [locale.codecvt] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2015-04-08

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Duplicate of: 72

Discussion:

The description of codecvt<>::do_out and do_in mentions a symbol "do_convert" which is not defined in the standard. This is a leftover from an edit, and should be "do_in and do_out".

Proposed resolution:

In 22.4.1.4 [locale.codecvt], paragraph 3, change "do_convert" to "do_in or do_out". Also, in 22.4.1.4.2 [locale.codecvt.virtuals], change "do_convert()" to "do_in or do_out".

### 25. String operator<< uses width() value wrong

Section: 21.4.8.9 [string.io] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2015-04-08

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Duplicate of: 67

Discussion:

In the description of operator<< applied to strings, the standard says that uses the smaller of os.width() and str.size(), to pad "as described in stage 3" elsewhere; but this is inconsistent, as this allows no possibility of space for padding.

Proposed resolution:

Change 21.4.8.9 [string.io] paragraph 4 from:
"... where `n` is the smaller of `os.width()` and `str.size()`; ..."
to:
"... where `n` is the larger of `os.width()` and `str.size()`; ..."

### 26. Bad sentry example

Section: 27.7.2.1.3 [istream::sentry] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2015-04-08

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Discussion:

In paragraph 6, the code in the example:

      template <class charT, class traits = char_traits<charT> >
      basic_istream<charT,traits>::sentry(
               basic_istream<charT,traits>& is, bool noskipws = false) {
          ...
          int_type c;
          typedef ctype<charT> ctype_type;
          const ctype_type& ctype = use_facet<ctype_type>(is.getloc());
          while ((c = is.rdbuf()->snextc()) != traits::eof()) {
            if (ctype.is(ctype.space,c)==0) {
              is.rdbuf()->sputbackc (c);
              break;
            }
          }
          ...
       }

fails to demonstrate correct use of the facilities described. In particular, it fails to use traits operators, and specifies incorrect semantics. (E.g. it specifies skipping over the first character in the sequence without examining it.)

Proposed resolution:

Remove the example above from 27.7.2.1.3 [istream::sentry] paragraph 6.

Rationale:

The originally proposed replacement code for the example was not correct. The LWG tried in Kona and again in Tokyo to correct it without success. In Tokyo, an implementor reported that actual working code ran over one page in length and was quite complicated. The LWG decided that it would be counter-productive to include such a lengthy example, which might well still contain errors.

### 27. String::erase(range) yields wrong iterator

Section: 21.4.6.5 [string::erase] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2015-04-08

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Discussion:

The string::erase(iterator first, iterator last) is specified to return an element one place beyond the next element after the last one erased. E.g. for the string "abcde", erasing the range ['b'..'d') would yield an iterator for element 'e', while 'd' has not been erased.

Proposed resolution:

In 21.4.6.5 [string::erase], paragraph 10, change:

> Returns: an iterator which points to the element immediately following _last_ prior to the element being erased.

to read

> Returns: an iterator which points to the element pointed to by _last_ prior to the other elements being erased.

### 28. Ctype<char>is ambiguous

Section: 22.4.1.3.2 [facet.ctype.char.members] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2015-04-08

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Duplicate of: 236

Discussion:

The description of the vector form of ctype<char>::is can be interpreted to mean something very different from what was intended. Paragraph 4 says

> Effects: The second form, for all *p in the range [low, high), assigns vec[p-low] to table()[(unsigned char)*p].

This is intended to copy the value indexed from table()[] into the place identified in vec[].

Proposed resolution:

Change 22.4.1.3.2 [facet.ctype.char.members], paragraph 4, to read

> Effects: The second form, for all *p in the range [low, high), assigns into vec[p-low] the value table()[(unsigned char)*p].

### 29. Ios_base::init doesn't exist

Section: 27.4.2 [narrow.stream.objects] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2015-04-08

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Discussion:

Sections 27.4.2 [narrow.stream.objects] and 27.4.3 [wide.stream.objects] mention a function ios_base::init, which is not defined. Probably they mean basic_ios<>::init, defined in 27.5.5.2 [basic.ios.cons], paragraph 3.

Proposed resolution:

[R12: modified to include paragraph 5.]

In 27.4.2 [narrow.stream.objects] paragraph 2 and 5, change

> ios_base::init

to

> basic_ios<char>::init

Also, make a similar change in 27.4.3 [wide.stream.objects] except it should read

> basic_ios<wchar_t>::init

### 30. Wrong header for LC_*

Section: 22.3.1.1.1 [locale.category] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2015-04-08

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Discussion:

Paragraph 2 implies that the C macros LC_CTYPE etc. are defined in <cctype>, where they are in fact defined elsewhere to appear in <clocale>.

Proposed resolution:

In 22.3.1.1.1 [locale.category], paragraph 2, change "<cctype>" to read "<clocale>".

### 31. Immutable locale values

Section: 22.3.1 [locale] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2015-04-08

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Duplicate of: 378

Discussion:

Paragraph 6, says "An instance of `locale` is immutable; once a facet reference is obtained from it, ...". This has caused some confusion, because locale variables are manifestly assignable.

Proposed resolution:

In 22.3.1 [locale] replace paragraph 6

> An instance of `locale` is immutable; once a facet reference is obtained from it, that reference remains usable as long as the locale value itself exists.

with

> Once a facet reference is obtained from a locale object by calling use_facet<>, that reference remains usable, and the results from member functions of it may be cached and re-used, as long as some locale object refers to that facet.

### 32. Pbackfail description inconsistent

Section: 27.6.3.4.4 [streambuf.virt.pback] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2015-04-08

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Discussion:

The description of the required state before calling virtual member basic_streambuf<>::pbackfail requirements is inconsistent with the conditions described in 27.5.2.2.4 [lib.streambuf.pub.pback] where member sputbackc calls it. Specifically, the latter says it calls pbackfail if:

traits::eq(c,gptr()[-1]) is false

where pbackfail claims to require:

traits::eq(*gptr(),traits::to_char_type(c)) returns false

It appears that the pbackfail description is wrong.

Proposed resolution:

In 27.6.3.4.4 [streambuf.virt.pback], paragraph 1, change:

> "`traits::eq(*gptr(),traits::to_char_type( c))`"

to

> "`traits::eq(traits::to_char_type(c),gptr()[-1])`"

Rationale:

Note deliberate reordering of arguments for clarity in addition to the correction of the argument value.

### 33. Codecvt<> mentions from_type

Section: 22.4.1.4 [locale.codecvt] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2015-04-08

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Duplicate of: 43

Discussion:

In the table defining the results from do_out and do_in, the specification for the result error says

> encountered a from_type character it could not convert

but from_type is not defined. This clearly is intended to be an externT for do_in, or an internT for do_out.

Proposed resolution:

In 22.4.1.4.2 [locale.codecvt.virtuals] paragraph 4, replace the definition in the table for the case of _error_ with

> encountered a character in `[from,from_end)` that it could not convert.

### 34. True/falsename() not in ctype<>

Section: 22.4.2.2.2 [facet.num.put.virtuals] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2015-04-08

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Discussion:

In paragraph 19, Effects:, members truename() and falsename are used from facet ctype<charT>, but it has no such members. Note that this is also a problem in 22.2.2.1.2, addressed in (4).

Proposed resolution:

In 22.4.2.2.2 [facet.num.put.virtuals], paragraph 19, in the Effects: clause for member put(...., bool), replace the initialization of the string_type value s as follows:

>     const numpunct& np = use_facet<numpunct<charT> >(loc);
>     string_type s = val ? np.truename() : np.falsename();

### 35. No manipulator unitbuf in synopsis

Section: 27.5 [iostreams.base] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2015-04-08

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Discussion:

In 27.5.6.1 [fmtflags.manip], we have a definition for a manipulator named "unitbuf". Unlike other manipulators, it's not listed in synopsis. Similarly for "nounitbuf".

Proposed resolution:

Add to the synopsis for <ios> in 27.5 [iostreams.base], after the entry for "nouppercase", the prototypes:

>     ios_base& unitbuf(ios_base& str);
>     ios_base& nounitbuf(ios_base& str);

### 36. Iword & pword storage lifetime omitted

Section: 27.5.3.5 [ios.base.storage] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2015-04-08

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Discussion:

In the definitions for ios_base::iword and pword, the lifetime of the storage is specified badly, so that an implementation which only keeps the last value stored appears to conform. In particular, it says:

The reference returned may become invalid after another call to the object's iword member with a different index ...

This is not idle speculation; at least one implementation was done this way.

Proposed resolution:

Add in 27.5.3.5 [ios.base.storage], in both paragraph 2 and also in paragraph 4, replace the sentence:

> The reference returned may become invalid after another call to the object's iword [pword] member with a different index, after a call to its copyfmt member, or when the object is destroyed.

with:

> The reference returned is invalid after any other operations on the object. However, the value of the storage referred to is retained, so that until the next call to copyfmt, calling iword [pword] with the same index yields another reference to the same value.

substituting "iword" or "pword" as appropriate.

### 37. Leftover "global" reference

Section: 22.3.1 [locale] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2015-04-08

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Discussion:

In the overview of locale semantics, paragraph 4, is the sentence

> If Facet is not present in a locale (or, failing that, in the global locale), it throws the standard exception bad_cast.

This is not supported by the definition of use_facet<>, and represents semantics from an old draft.

Proposed resolution:

In 22.3.1 [locale], paragraph 4, delete the parenthesized expression

> (or, failing that, in the global locale)

### 38. Facet definition incomplete

Section: 22.3.2 [locale.global.templates] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2015-04-08

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Discussion:

It has been noticed by Esa Pulkkinen that the definition of "facet" is incomplete. In particular, a class derived from another facet, but which does not define a member id, cannot safely serve as the argument F to use_facet<F>(loc), because there is no guarantee that a reference to the facet instance stored in loc is safely convertible to F.

Proposed resolution:

In the definition of std::use_facet<>(), replace the text in paragraph 1 which reads:

> Get a reference to a facet of a locale.

with:

> Requires: `Facet` is a facet class whose definition contains the public static member `id` as defined in 22.3.1.1.2 [locale.facet].

[ Kona: strike as overspecification the text "(not inherits)" from the original resolution, which read "... whose definition contains (not inherits) the public static member `id`..." ]

### 39. istreambuf_iterator<>::operator++(int) definition garbled

Section: 24.6.3.4 [istreambuf.iterator::op++] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2015-04-08

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Discussion:

Following the definition of istreambuf_iterator<>::operator++(int) in paragraph 3, the standard contains three lines of garbage text left over from a previous edit.

>     istreambuf_iterator<charT,traits> tmp = *this;
>     sbuf_->sbumpc();
>     return(tmp);

Proposed resolution:

In 24.6.3.4 [istreambuf.iterator::op++], delete the three lines of code at the end of paragraph 3.

### 40. Meaningless normative paragraph in examples

Section: 22.4.8 [facets.examples] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2015-04-08

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Discussion:

Paragraph 3 of the locale examples is a description of part of an implementation technique that has lost its referent, and doesn't mean anything.

Proposed resolution:

Delete 22.4.8 [facets.examples] paragraph 3 which begins "This initialization/identification system depends...", or (at the editor's option) replace it with a place-holder to keep the paragraph numbering the same.

### 41. Ios_base needs clear(), exceptions()

Section: 27.5.3 [ios.base] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2015-04-08

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Duplicate of: 157

Discussion:

The description of ios_base::iword() and pword() in 27.5.3.4 [ios.members.static], say that if they fail, they "set badbit, which may throw an exception". However, ios_base offers no interface to set or to test badbit; those interfaces are defined in basic_ios<>.

Proposed resolution:

Change the description in 27.5.3.5 [ios.base.storage] in paragraph 2, and also in paragraph 4, as follows. Replace

> If the function fails it sets badbit, which may throw an exception.

with

> If the function fails, and `*this` is a base sub-object of a `basic_ios<>` object or sub-object, the effect is equivalent to calling `basic_ios<>::setstate(badbit)` on the derived object (which may throw `failure`).

[Kona: LWG reviewed wording; setstate(failbit) changed to setstate(badbit).]

### 42. String ctors specify wrong default allocator

Section: 21.4 [basic.string] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2015-04-08

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Discussion:

The basic_string<> copy constructor:

    basic_string(const basic_string& str, size_type pos = 0,
                 size_type n = npos, const Allocator& a = Allocator());

specifies an Allocator argument default value that is counter-intuitive. The natural choice for a the allocator to copy from is str.get_allocator(). Though this cannot be expressed in default-argument notation, overloading suffices.

Alternatively, the other containers in Clause 23 (deque, list, vector) do not have this form of constructor, so it is inconsistent, and an evident source of confusion, for basic_string<> to have it, so it might better be removed.

Proposed resolution:

In 21.4 [basic.string], replace the declaration of the copy constructor as follows:

>     basic_string(const basic_string& str);
>     basic_string(const basic_string& str, size_type pos, size_type n = npos,
>                  const Allocator& a = Allocator());

In 21.4.1 [string.require], replace the copy constructor declaration as above. Add to paragraph 5, Effects:

> In the first form, the Allocator value used is copied from `str.get_allocator()`.

Rationale:

The LWG believes the constructor is actually broken, rather than just an unfortunate design choice.

The LWG considered two other possible resolutions:

A. In 21.4 [basic.string], replace the declaration of the copy constructor as follows:

>     basic_string(const basic_string& str, size_type pos = 0,
>                  size_type n = npos);
>     basic_string(const basic_string& str, size_type pos,
>                  size_type n, const Allocator& a);

In 21.4.1 [string.require], replace the copy constructor declaration as above. Add to paragraph 5, Effects:

> When no `Allocator` argument is provided, the string is constructed using the value `str.get_allocator()`.

B. In 21.4 [basic.string], and also in 21.4.1 [string.require], replace the declaration of the copy constructor as follows:

>     basic_string(const basic_string& str, size_type pos = 0,
>                  size_type n = npos);

The proposed resolution reflects the original intent of the LWG. It was also noted by Pete Becker that this fix "will cause a small amount of existing code to now work correctly."

[ Kona: issue editing snafu fixed - the proposed resolution now correctly reflects the LWG consensus. ]

### 44. Iostreams use operator== on int_type values

Section: 27 [input.output] Status: CD1 Submitter: Nathan Myers Opened: 1998-08-06 Last modified: 2015-04-08

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Discussion:

Many of the specifications for iostreams specify that character values or their int_type equivalents are compared using operators == or !=, though in other places traits::eq() or traits::eq_int_type is specified to be used throughout. This is an inconsistency; we should change uses of == and != to use the traits members instead.

Proposed resolution:

[Pre-Kona: Dietmar supplied wording]

List of changes to clause 27:

1. In lib.basic.ios.members paragraph 13 (postcondition clause for 'fill(cT)') change

   >          fillch == fill()

   to

   >          traits::eq(fillch, fill())

2. In lib.istream.unformatted paragraph 7 (effects clause for 'get(cT,streamsize,cT)'), third bullet, change

   >          c == delim for the next available input character c

   to

   >          traits::eq(c, delim) for the next available input character c

3. In lib.istream.unformatted paragraph 12 (effects clause for 'get(basic_streambuf<cT,Tr>&,cT)'), third bullet, change

   >          c == delim for the next available input character c

   to

   >          traits::eq(c, delim) for the next available input character c

4. In lib.istream.unformatted paragraph 17 (effects clause for 'getline(cT,streamsize,cT)'), second bullet, change

   >          c == delim for the next available input character c

   to

   >          traits::eq(c, delim) for the next available input character c

5. In lib.istream.unformatted paragraph 24 (effects clause for 'ignore(int,int_type)'), second bullet, change

   >          c == delim for the next available input character c

   to

   >          traits::eq_int_type(c, delim) for the next available input
   >          character c

6. In lib.istream.unformatted paragraph 25 (notes clause for 'ignore(int,int_type)'), second bullet, change

   >          The last condition will never occur if delim == traits::eof()

   to

   >          The last condition will never occur if
   >          traits::eq_int_type(delim, traits::eof()).

7. In lib.istream.sentry paragraph 6 (example implementation for the sentry constructor) change

   >          while ((c = is.rdbuf()->snextc()) != traits::eof()) {

   to

   >          while (!traits::eq_int_type(c = is.rdbuf()->snextc(), traits::eof())) {

List of changes to Chapter 21:

1. In lib.string::find paragraph 1 (effects clause for find()), second bullet, change

   >          at(xpos+I) == str.at(I) for all elements ...

   to

   >          traits::eq(at(xpos+I), str.at(I)) for all elements ...

2. In lib.string::rfind paragraph 1 (effects clause for rfind()), second bullet, change

   >          at(xpos+I) == str.at(I) for all elements ...

   to

   >          traits::eq(at(xpos+I), str.at(I)) for all elements ...

3. In lib.string::find.first.of paragraph 1 (effects clause for find_first_of()), second bullet, change

   >          at(xpos+I) == str.at(I) for all elements ...

   to

   >          traits::eq(at(xpos+I), str.at(I)) for all elements ...

4. In lib.string::find.last.of paragraph 1 (effects clause for find_last_of()), second bullet, change

   >          at(xpos+I) == str.at(I) for all elements ...

   to

   >          traits::eq(at(xpos+I), str.at(I)) for all elements ...

5. In lib.string::find.first.not.of paragraph 1 (effects clause for find_first_not_of()), second bullet, change

   >          at(xpos+I) == str.at(I) for all elements ...

   to

   >          traits::eq(at(xpos+I), str.at(I)) for all elements ...

6. In lib.string::find.last.not.of paragraph 1 (effects clause for find_last_not_of()), second bullet, change

   >          at(xpos+I) == str.at(I) for all elements ...

   to

   >          traits::eq(at(xpos+I), str.at(I)) for all elements ...

7. In lib.string.ios paragraph 5 (effects clause for getline()), second bullet, change

   >          c == delim for the next available input character c

   to

   >          traits::eq(c, delim) for the next available input character c

Notes:

- Fixing this issue highlights another sloppyness in lib.istream.unformatted paragraph 24: this clause mentions a "character" which is then compared to an 'int_type' (see item 5. in the list below). It is not clear whether this requires explicit words and if so what these words are supposed to be. A similar issue exists, BTW, for operator*() of istreambuf_iterator which returns the result of sgetc() as a character type (see lib.istreambuf.iterator::op* paragraph 1), and for operator++() of istreambuf_iterator which passes the result of sbumpc() to a constructor taking a char_type (see lib.istreambuf.iterator::operator++ paragraph 3). Similarily, the assignment operator ostreambuf_iterator passes a char_type to a function taking an int_type (see lib.ostreambuf.iter.ops paragraph 1).
- It is inconsistent to use comparisons using the traits functions in Chapter 27 while not using them in Chapter 21, especially as some of the inconsistent uses actually involve streams (eg. getline() on streams). To avoid leaving this issue open still longer due to this inconsistency (it is open since 1998), a list of changes to Chapter 21 is below.
- In Chapter 24 there are several places with statements like "the end of stream is reached (streambuf_type::sgetc() returns traits::eof())" (lib.istreambuf.iterator paragraph 1, lib.ostreambuf.iter.ops paragraph 5). It is unclear whether these should be clarified to use traits::eq_int_type() for detecting traits::eof().

### 46. Minor Annex D errors

Section: D.7 [depr.str.strstreams] Status: TC1 Submitter: Brendan Kehoe Opened: 1998-06-01 Last modified: 2015-04-08

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Discussion:

See lib-6522 and edit-814.

Proposed resolution:

Change D.7.1 [depr.strstreambuf] (since streambuf is a typedef of basic_streambuf<char>) from:

             virtual streambuf<char>* setbuf(char* s, streamsize n);

to:

             virtual streambuf* setbuf(char* s, streamsize n);

In D.7.4 [depr.strstream] insert the semicolon now missing after int_type:

         namespace std {
           class strstream
             : public basic_iostream<char> {
           public:
             // Types
             typedef char                                char_type;
             typedef typename char_traits<char>::int_type int_type
             typedef typename char_traits<char>::pos_type pos_type;

### 47. Imbue() and getloc() Returns clauses swapped

Section: 27.5.3.3 [ios.base.locales] Status: TC1 Submitter: Matt Austern Opened: 1998-06-21 Last modified: 2015-04-08

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Discussion:

Section 27.4.2.3 specifies how imbue() and getloc() work. That section has two RETURNS clauses, and they make no sense as stated. They make perfect sense, though, if you swap them. Am I correct in thinking that paragraphs 2 and 4 just got mixed up by accident?

Proposed resolution:

In 27.5.3.3 [ios.base.locales] swap paragraphs 2 and 4.

### 48. Use of non-existent exception constructor

Section: 27.5.3.1.1 [ios::failure] Status: TC1 Submitter: Matt Austern Opened: 1998-06-21 Last modified: 2015-04-08

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Discussion:

27.4.2.1.1, paragraph 2, says that class failure initializes the base class, exception, with exception(msg). Class exception (see 18.6.1) has no such constructor.

Proposed resolution:

Replace 27.5.3.1.1 [ios::failure], paragraph 2, with

> EFFECTS: Constructs an object of class `failure`.

### 49. Underspecification of ios_base::sync_with_stdio

Section: 27.5.3.4 [ios.members.static] Status: CD1 Submitter: Matt Austern Opened: 1998-06-21 Last modified: 2015-04-08

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Discussion:

Two problems

(1) 27.4.2.4 doesn't say what ios_base::sync_with_stdio(f) returns. Does it return f, or does it return the previous synchronization state? My guess is the latter, but the standard doesn't say so.

(2) 27.4.2.4 doesn't say what it means for streams to be synchronized with stdio. Again, of course, I can make some guesses. (And I'm unhappy about the performance implications of those guesses, but that's another matter.)

Proposed resolution:

Change the following sentence in 27.5.3.4 [ios.members.static] returns clause from:

> `true` if the standard iostream objects (27.3) are synchronized and otherwise returns `false`.

to:

> `true` if the previous state of the standard iostream objects (27.3) was synchronized and otherwise returns `false`.

Add the following immediately after 27.5.3.4 [ios.members.static], paragraph 2:

> When a standard iostream object str is synchronized with a standard stdio stream f, the effect of inserting a character c by

>       fputc(f, c);

> is the same as the effect of

>       str.rdbuf()->sputc(c);

> for any sequence of characters; the effect of extracting a character c by

>       c = fgetc(f);

> is the same as the effect of:

>       c = str.rdbuf()->sbumpc(c);

> for any sequences of characters; and the effect of pushing back a character c by

>       ungetc(c, f);

> is the same as the effect of

>       str.rdbuf()->sputbackc(c);

> for any sequence of characters. [Footnote: This implies that operations on a standard iostream object can be mixed arbitrarily with operations on the corresponding stdio stream. In practical terms, synchronization usually means that a standard iostream object and a standard stdio object share a buffer. --End Footnote]

[pre-Copenhagen: PJP and Matt contributed the definition of "synchronization"]

[post-Copenhagen: proposed resolution was revised slightly: text was added in the non-normative footnote to say that operations on the two streams can be mixed arbitrarily.]

### 50. Copy constructor and assignment operator of ios_base

Section: 27.5.3 [ios.base] Status: TC1 Submitter: Matt Austern Opened: 1998-06-21 Last modified: 2015-04-08

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Discussion:

As written, ios_base has a copy constructor and an assignment operator. (Nothing in the standard says it doesn't have one, and all classes have copy constructors and assignment operators unless you take specific steps to avoid them.) However, nothing in 27.4.2 says what the copy constructor and assignment operator do.

My guess is that this was an oversight, that ios_base is, like basic_ios, not supposed to have a copy constructor or an assignment operator.

Jerry Schwarz comments: Yes, its an oversight, but in the opposite sense to what you're suggesting. At one point there was a definite intention that you could copy ios_base. It's an easy way to save the entire state of a stream for future use. As you note, to carry out that intention would have required a explicit description of the semantics (e.g. what happens to the iarray and parray stuff).

Proposed resolution:

In 27.5.3 [ios.base], class ios_base, specify the copy constructor and operator= members as being private.

Rationale:

The LWG believes the difficulty of specifying correct semantics outweighs any benefit of allowing ios_base objects to be copyable.

### 51. Requirement to not invalidate iterators missing

Section: 23.2 [container.requirements] Status: TC1 Submitter: David Vandevoorde Opened: 1998-06-23 Last modified: 2015-04-08

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Discussion:

The std::sort algorithm can in general only sort a given sequence by moving around values. The list<>::sort() member on the other hand could move around values or just update internal pointers. Either method can leave iterators into the list<> dereferencable, but they would point to different things.

Does the FDIS mandate anywhere which method should be used for list<>::sort()?

Matt Austern comments:

I think you've found an omission in the standard.

The library working group discussed this point, and there was supposed to be a general requirement saying that list, set, map, multiset, and multimap may not invalidate iterators, or change the values that iterators point to, except when an operation does it explicitly. So, for example, insert() doesn't invalidate any iterators and erase() and remove() only invalidate iterators pointing to the elements that are being erased.

I looked for that general requirement in the FDIS, and, while I found a limited form of it for the sorted associative containers, I didn't find it for list. It looks like it just got omitted.

The intention, though, is that list<>::sort does not invalidate any iterators and does not change the values that any iterator points to. There would be no reason to have the member function otherwise.

Proposed resolution:

Add a new paragraph at the end of 23.1:

> Unless otherwise specified (either explicitly or by defining a function in terms of other functions), invoking a container member function or passing a container as an argument to a library function shall not invalidate iterators to, or change the values of, objects within that container.

Rationale:

This was US issue CD2-23-011; it was accepted in London but the change was not made due to an editing oversight. The wording in the proposed resolution below is somewhat updated from CD2-23-011, particularly the addition of the phrase "or change the values of"

### 52. Small I/O problems

Section: 27.5.4.2 [fpos.operations] Status: TC1 Submitter: Matt Austern Opened: 1998-06-23 Last modified: 2015-04-08

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Discussion:

First, 27.5.5.2 [basic.ios.cons], table 89. This is pretty obvious: it should be titled "basic_ios<>() effects", not "ios_base() effects".

[The second item is a duplicate; see issue 6 for resolution.]

Second, 27.5.4.2 [fpos.operations] table 88 . There are a couple different things wrong with it, some of which I've already discussed with Jerry, but the most obvious mechanical sort of error is that it uses expressions like P(i) and p(i), without ever defining what sort of thing "i" is.

(The other problem is that it requires support for streampos arithmetic. This is impossible on some systems, i.e. ones where file position is a complicated structure rather than just a number. Jerry tells me that the intention was to require syntactic support for streampos arithmetic, but that it wasn't actually supposed to do anything meaningful except on platforms, like Unix, where genuine arithmetic is possible.)

Proposed resolution:

Change 27.5.5.2 [basic.ios.cons] table 89 title from "ios_base() effects" to "basic_ios<>() effects".

### 53. Basic_ios destructor unspecified

Section: 27.5.5.2 [basic.ios.cons] Status: TC1 Submitter: Matt Austern Opened: 1998-06-23 Last modified: 2015-04-08

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Discussion:

There's nothing in 27.4.4 saying what basic_ios's destructor does. The important question is whether basic_ios::~basic_ios() destroys rdbuf().

Proposed resolution:

Add after 27.5.5.2 [basic.ios.cons] paragraph 2:

> `virtual ~basic_ios();`

> Notes: The destructor does not destroy `rdbuf()`.

Rationale:

The LWG reviewed the additional question of whether or not `rdbuf(0)` may set `badbit`. The answer is clearly yes; it may be set via `clear()`. See 27.5.5.3 [basic.ios.members], paragraph 6. This issue was reviewed at length by the LWG, which removed from the original proposed resolution a footnote which incorrectly said "`rdbuf(0)` does not set `badbit`".

### 54. Basic_streambuf's destructor

Section: 27.6.3.1 [streambuf.cons] Status: TC1 Submitter: Matt Austern Opened: 1998-06-25 Last modified: 2015-04-08

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Discussion:

The class synopsis for basic_streambuf shows a (virtual) destructor, but the standard doesn't say what that destructor does. My assumption is that it does nothing, but the standard should say so explicitly.

Proposed resolution:

Add after 27.6.3.1 [streambuf.cons] paragraph 2:

> `virtual ~basic_streambuf();`

> Effects: None.

### 55. Invalid stream position is undefined

Section: 27 [input.output] Status: TC1 Submitter: Matt Austern Opened: 1998-06-26 Last modified: 2015-04-08

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Discussion:

Several member functions in clause 27 are defined in certain circumstances to return an "invalid stream position", a term that is defined nowhere in the standard. Two places (27.5.2.4.2, paragraph 4, and 27.8.1.4, paragraph 15) contain a cross-reference to a definition in _lib.iostreams.definitions_, a nonexistent section.

I suspect that the invalid stream position is just supposed to be pos_type(-1). Probably best to say explicitly in (for example) 27.5.2.4.2 that the return value is pos_type(-1), rather than to use the term "invalid stream position", define that term somewhere, and then put in a cross-reference.

The phrase "invalid stream position" appears ten times in the C++ Standard. In seven places it refers to a return value, and it should be changed. In three places it refers to an argument, and it should not be changed. Here are the three places where "invalid stream position" should not be changed:

> 27.8.2.4 [stringbuf.virtuals], paragraph 14
> 27.9.1.5 [filebuf.virtuals], paragraph 14
> D.7.1.3 [depr.strstreambuf.virtuals], paragraph 17

Proposed resolution:

In 27.6.3.4.2 [streambuf.virt.buffer], paragraph 4, change "Returns an object of class pos_type that stores an invalid stream position (_lib.iostreams.definitions_)" to "Returns `pos_type(off_type(-1))`".

In 27.6.3.4.2 [streambuf.virt.buffer], paragraph 6, change "Returns an object of class pos_type that stores an invalid stream position" to "Returns `pos_type(off_type(-1))`".

In 27.8.2.4 [stringbuf.virtuals], paragraph 13, change "the object stores an invalid stream position" to "the return value is `pos_type(off_type(-1))`".

In 27.9.1.5 [filebuf.virtuals], paragraph 13, change "returns an invalid stream position (27.4.3)" to "returns `pos_type(off_type(-1))`"

In 27.9.1.5 [filebuf.virtuals], paragraph 15, change "Otherwise returns an invalid stream position (_lib.iostreams.definitions_)" to "Otherwise returns `pos_type(off_type(-1))`"

In D.7.1.3 [depr.strstreambuf.virtuals], paragraph 15, change "the object stores an invalid stream position" to "the return value is `pos_type(off_type(-1))`"

In D.7.1.3 [depr.strstreambuf.virtuals], paragraph 18, change "the object stores an invalid stream position" to "the return value is `pos_type(off_type(-1))`"

### 56. Showmanyc's return type

Section: 27.6.3 [streambuf] Status: TC1 Submitter: Matt Austern Opened: 1998-06-29 Last modified: 2015-04-08

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Discussion:

The class summary for basic_streambuf<>, in 27.5.2, says that showmanyc has return type int. However, 27.5.2.4.3 says that its return type is streamsize.

Proposed resolution:

Change `showmanyc`'s return type in the 27.6.3 [streambuf] class summary to `streamsize`.

### 57. Mistake in char_traits

Section: 21.2.3.4 [char.traits.specializations.wchar.t] Status: TC1 Submitter: Matt Austern Opened: 1998-07-01 Last modified: 2015-04-08

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Discussion:

21.1.3.2, paragraph 3, says "The types streampos and wstreampos may be different if the implementation supports no shift encoding in narrow-oriented iostreams but supports one or more shift encodings in wide-oriented streams".

That's wrong: the two are the same type. The <iosfwd> summary in 27.2 says that streampos and wstreampos are, respectively, synonyms for fpos<char_traits<char>::state_type> and fpos<char_traits<wchar_t>::state_type>, and, flipping back to clause 21, we see in 21.1.3.1 and 21.1.3.2 that char_traits<char>::state_type and char_traits<wchar_t>::state_type must both be mbstate_t.

Proposed resolution:

Remove the sentence in 21.2.3.4 [char.traits.specializations.wchar.t] paragraph 3 which begins "The types streampos and wstreampos may be different..." .

### 59. Ambiguity in specification of gbump

Section: 27.6.3.3.2 [streambuf.get.area] Status: TC1 Submitter: Matt Austern Opened: 1998-07-28 Last modified: 2015-04-08

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Discussion:

27.5.2.3.1 says that basic_streambuf::gbump() "Advances the next pointer for the input sequence by n."

The straightforward interpretation is that it is just gptr() += n. An alternative interpretation, though, is that it behaves as if it calls sbumpc n times. (The issue, of course, is whether it might ever call underflow.) There is a similar ambiguity in the case of pbump.

(The "classic" AT&T implementation used the former interpretation.)

Proposed resolution:

Change 27.6.3.3.2 [streambuf.get.area] paragraph 4 gbump effects from:

> Effects: Advances the next pointer for the input sequence by n.

to:

> Effects: Adds `n` to the next pointer for the input sequence.

Make the same change to 27.6.3.3.3 [streambuf.put.area] paragraph 4 pbump effects.

### 60. What is a formatted input function?

Section: 27.7.2.2.1 [istream.formatted.reqmts] Status: TC1 Submitter: Matt Austern Opened: 1998-08-03 Last modified: 2015-04-08

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Duplicate of: 162, 163, 166

Discussion:

Paragraph 1 of 27.6.1.2.1 contains general requirements for all formatted input functions. Some of the functions defined in section 27.6.1.2 explicitly say that those requirements apply ("Behaves like a formatted input member (as described in 27.6.1.2.1)"), but others don't. The question: is 27.6.1.2.1 supposed to apply to everything in 27.6.1.2, or only to those member functions that explicitly say "behaves like a formatted input member"? Or to put it differently: are we to assume that everything that appears in a section called "Formatted input functions" really is a formatted input function? I assume that 27.6.1.2.1 is intended to apply to the arithmetic extractors (27.6.1.2.2), but I assume that it is not intended to apply to extractors like

        basic_istream& operator>>(basic_istream& (*pf)(basic_istream&));

and

        basic_istream& operator>>(basic_streammbuf*);

There is a similar ambiguity for unformatted input, formatted output, and unformatted output.

Comments from Judy Ward: It seems like the problem is that the basic_istream and basic_ostream operator <<()'s that are used for the manipulators and streambuf* are in the wrong section and should have their own separate section or be modified to make it clear that the "Common requirements" listed in section 27.6.1.2.1 (for basic_istream) and section 27.6.2.5.1 (for basic_ostream) do not apply to them.

Additional comments from Dietmar Kühl: It appears to be somewhat nonsensical to consider the functions defined in 27.7.2.2.3 [istream::extractors] paragraphs 1 to 5 to be "Formatted input function" but since these functions are defined in a section labeled "Formatted input functions" it is unclear to me whether these operators are considered formatted input functions which have to conform to the "common requirements" from 27.7.2.2.1 [istream.formatted.reqmts]: If this is the case, all manipulators, not just `ws`, would skip whitespace unless `noskipws` is set (... but setting `noskipws` using the manipulator syntax would also skip whitespace :-)

It is not clear which functions are to be considered unformatted input functions. As written, it seems that all functions in 27.7.2.3 [istream.unformatted] are unformatted input functions. However, it does not really make much sense to construct a sentry object for `gcount()`, `sync()`, ... Also it is unclear what happens to the `gcount()` if eg. `gcount()`, `putback()`, `unget()`, or `sync()` is called: These functions don't extract characters, some of them even "unextract" a character. Should this still be reflected in `gcount()`? Of course, it could be read as if after a call to `gcount()` `gcount()` return `0` (the last unformatted input function, `gcount()`, didn't extract any character) and after a call to `putback()` `gcount()` returns `-1` (the last unformatted input function `putback()` did "extract" back into the stream). Correspondingly for `unget()`. Is this what is intended? If so, this should be clarified. Otherwise, a corresponding clarification should be used.

Proposed resolution:

In 27.6.1.2.2 [lib.istream.formatted.arithmetic], paragraph 1. Change the beginning of the second sentence from "The conversion occurs" to "These extractors behave as formatted input functions (as described in 27.6.1.2.1). After a sentry object is constructed, the conversion occurs"

In 27.6.1.2.3, [lib.istream::extractors], before paragraph 1. Add an effects clause. "Effects: None. This extractor does not behave as a formatted input function (as described in 27.6.1.2.1).

In 27.6.1.2.3, [lib.istream::extractors], paragraph 2. Change the effects clause to "Effects: Calls pf(*this). This extractor does not behave as a formatted input function (as described in 27.6.1.2.1).

In 27.6.1.2.3, [lib.istream::extractors], paragraph 4. Change the effects clause to "Effects: Calls pf(*this). This extractor does not behave as a formatted input function (as described in 27.6.1.2.1).

In 27.6.1.2.3, [lib.istream::extractors], paragraph 12. Change the first two sentences from "If sb is null, calls setstate(failbit), which may throw ios_base::failure (27.4.4.3). Extracts characters from *this..." to "Behaves as a formatted input function (as described in 27.6.1.2.1). If sb is null, calls setstate(failbit), which may throw ios_base::failure (27.4.4.3). After a sentry object is constructed, extracts characters from *this...".

In 27.6.1.3, [lib.istream.unformatted], before paragraph 2. Add an effects clause. "Effects: none. This member function does not behave as an unformatted input function (as described in 27.6.1.3, paragraph 1)."

In 27.6.1.3, [lib.istream.unformatted], paragraph 3. Change the beginning of the first sentence of the effects clause from "Extracts a character" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, extracts a character"

In 27.6.1.3, [lib.istream.unformatted], paragraph 5. Change the beginning of the first sentence of the effects clause from "Extracts a character" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, extracts a character"

In 27.6.1.3, [lib.istream.unformatted], paragraph 7. Change the beginning of the first sentence of the effects clause from "Extracts characters" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, extracts characters"

[No change needed in paragraph 10, because it refers to paragraph 7.]

In 27.6.1.3, [lib.istream.unformatted], paragraph 12. Change the beginning of the first sentence of the effects clause from "Extracts characters" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, extracts characters"

[No change needed in paragraph 15.]

In 27.6.1.3, [lib.istream.unformatted], paragraph 17. Change the beginning of the first sentence of the effects clause from "Extracts characters" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, extracts characters"

[No change needed in paragraph 23.]

In 27.6.1.3, [lib.istream.unformatted], paragraph 24. Change the beginning of the first sentence of the effects clause from "Extracts characters" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, extracts characters"

In 27.6.1.3, [lib.istream.unformatted], before paragraph 27. Add an Effects clause: "Effects: Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, reads but does not extract the current input character."

In 27.6.1.3, [lib.istream.unformatted], paragraph 28. Change the first sentence of the Effects clause from "If !good() calls" to Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, if !good() calls"

In 27.6.1.3, [lib.istream.unformatted], paragraph 30. Change the first sentence of the Effects clause from "If !good() calls" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, if !good() calls"

In 27.6.1.3, [lib.istream.unformatted], paragraph 32. Change the first sentence of the Effects clause from "If !good() calls..." to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, if !good() calls..." Add a new sentence to the end of the Effects clause: "[Note: this function extracts no characters, so the value returned by the next call to gcount() is 0.]"

In 27.6.1.3, [lib.istream.unformatted], paragraph 34. Change the first sentence of the Effects clause from "If !good() calls" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1). After constructing a sentry object, if !good() calls". Add a new sentence to the end of the Effects clause: "[Note: this function extracts no characters, so the value returned by the next call to gcount() is 0.]"

In 27.6.1.3, [lib.istream.unformatted], paragraph 36. Change the first sentence of the Effects clause from "If !rdbuf() is" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to gcount(). After constructing a sentry object, if rdbuf() is"

In 27.6.1.3, [lib.istream.unformatted], before paragraph 37. Add an Effects clause: "Effects: Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to gcount()." Change the first sentence of paragraph 37 from "if fail()" to "after constructing a sentry object, if fail()".

In 27.6.1.3, [lib.istream.unformatted], paragraph 38. Change the first sentence of the Effects clause from "If fail()" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to gcount(). After constructing a sentry object, if fail()

In 27.6.1.3, [lib.istream.unformatted], paragraph 40. Change the first sentence of the Effects clause from "If fail()" to "Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to gcount(). After constructing a sentry object, if fail()

In 27.6.2.5.2 [lib.ostream.inserters.arithmetic], paragraph 1. Change the beginning of the third sentence from "The formatting conversion" to "These extractors behave as formatted output functions (as described in 27.6.2.5.1). After the sentry object is constructed, the conversion occurs".

In 27.6.2.5.3 [lib.ostream.inserters], before paragraph 1. Add an effects clause: "Effects: None. Does not behave as a formatted output function (as described in 27.6.2.5.1).".

In 27.6.2.5.3 [lib.ostream.inserters], paragraph 2. Change the effects clause to "Effects: calls pf(*this). This extractor does not behave as a formatted output function (as described in 27.6.2.5.1).".

In 27.6.2.5.3 [lib.ostream.inserters], paragraph 4. Change the effects clause to "Effects: calls pf(*this). This extractor does not behave as a formatted output function (as described in 27.6.2.5.1).".

In 27.6.2.5.3 [lib.ostream.inserters], paragraph 6. Change the first sentence from "If sb" to "Behaves as a formatted output function (as described in 27.6.2.5.1). After the sentry object is constructed, if sb".

In 27.6.2.6 [lib.ostream.unformatted], paragraph 2. Change the first sentence from "Inserts the character" to "Behaves as an unformatted output function (as described in 27.6.2.6, paragraph 1). After constructing a sentry object, inserts the character".

In 27.6.2.6 [lib.ostream.unformatted], paragraph 5. Change the first sentence from "Obtains characters" to "Behaves as an unformatted output function (as described in 27.6.2.6, paragraph 1). After constructing a sentry object, obtains characters".

In 27.6.2.6 [lib.ostream.unformatted], paragraph 7. Add a new sentence at the end of the paragraph: "Does not behave as an unformatted output function (as described in 27.6.2.6, paragraph 1)."

Rationale:

See J16/99-0043==WG21/N1219, Proposed Resolution to Library Issue 60, by Judy Ward and Matt Austern. This proposed resolution is section VI of that paper.

### 61. Ambiguity in iostreams exception policy

Section: 27.7.2.3 [istream.unformatted] Status: TC1 Submitter: Matt Austern Opened: 1998-08-06 Last modified: 2015-04-08

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Discussion:

The introduction to the section on unformatted input (27.6.1.3) says that every unformatted input function catches all exceptions that were thrown during input, sets badbit, and then conditionally rethrows the exception. That seems clear enough. Several of the specific functions, however, such as get() and read(), are documented in some circumstances as setting eofbit and/or failbit. (The standard notes, correctly, that setting eofbit or failbit can sometimes result in an exception being thrown.) The question: if one of these functions throws an exception triggered by setting failbit, is this an exception "thrown during input" and hence covered by 27.6.1.3, or does 27.6.1.3 only refer to a limited class of exceptions? Just to make this concrete, suppose you have the following snippet.

      char buffer[N];
      istream is;
      ...
      is.exceptions(istream::failbit); // Throw on failbit but not on badbit.
      is.read(buffer, N);

Now suppose we reach EOF before we've read N characters. What iostate bits can we expect to be set, and what exception (if any) will be thrown?

Proposed resolution:

In 27.6.1.3, paragraph 1, after the sentence that begins "If an exception is thrown...", add the following parenthetical comment: "(Exceptions thrown from `basic_ios<>::clear()` are not caught or rethrown.)"

Rationale:

The LWG looked to two alternative wordings, and choose the proposed resolution as better standardese.

### 62. `Sync`'s return value

Section: 27.7.2.3 [istream.unformatted] Status: TC1 Submitter: Matt Austern Opened: 1998-08-06 Last modified: 2015-04-08

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Discussion:

The Effects clause for sync() (27.6.1.3, paragraph 36) says that it "calls rdbuf()->pubsync() and, if that function returns -1 ... returns traits::eof()."

That looks suspicious, because traits::eof() is of type traits::int_type while the return type of sync() is int.

Proposed resolution:

In 27.7.2.3 [istream.unformatted], paragraph 36, change "returns `traits::eof()`" to "returns `-1`".

### 63. Exception-handling policy for unformatted output

Section: 27.7.3.7 [ostream.unformatted] Status: TC1 Submitter: Matt Austern Opened: 1998-08-11 Last modified: 2015-04-08

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Discussion:

Clause 27 details an exception-handling policy for formatted input, unformatted input, and formatted output. It says nothing for unformatted output (27.6.2.6). 27.6.2.6 should either include the same kind of exception-handling policy as in the other three places, or else it should have a footnote saying that the omission is deliberate.

Proposed resolution:

In 27.6.2.6, paragraph 1, replace the last sentence ("In any case, the unformatted output function ends by destroying the sentry object, then returning the value specified for the formatted output function.") with the following text:

> If an exception is thrown during output, then `ios::badbit` is turned on [Footnote: without causing an `ios::failure` to be thrown.] in `*this`'s error state. If `(exceptions() & badbit) != 0` then the exception is rethrown. In any case, the unformatted output function ends by destroying the sentry object, then, if no exception was thrown, returning the value specified for the formatted output function.

Rationale:

This exception-handling policy is consistent with that of formatted input, unformatted input, and formatted output.

### 64. Exception handling in `basic_istream::operator>>(basic_streambuf*)`

Section: 27.7.2.2.3 [istream::extractors] Status: TC1 Submitter: Matt Austern Opened: 1998-08-11 Last modified: 2015-04-08

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Discussion:

27.6.1.2.3, paragraph 13, is ambiguous. It can be interpreted two different ways, depending on whether the second sentence is read as an elaboration of the first.

Proposed resolution:

Replace 27.7.2.2.3 [istream::extractors], paragraph 13, which begins "If the function inserts no characters ..." with:

> If the function inserts no characters, it calls `setstate(failbit)`, which may throw `ios_base::failure` (27.4.4.3). If it inserted no characters because it caught an exception thrown while extracting characters from `sb` and `failbit` is on in `exceptions()` (27.4.4.3), then the caught exception is rethrown.

### 66. Strstreambuf::setbuf

Section: D.7.1.3 [depr.strstreambuf.virtuals] Status: TC1 Submitter: Matt Austern Opened: 1998-08-18 Last modified: 2015-04-08

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Discussion:

D.7.1.3, paragraph 19, says that strstreambuf::setbuf "Performs an operation that is defined separately for each class derived from strstreambuf". This is obviously an incorrect cut-and-paste from basic_streambuf. There are no classes derived from strstreambuf.

Proposed resolution:

D.7.1.3 [depr.strstreambuf.virtuals], paragraph 19, replace the setbuf effects clause which currently says "Performs an operation that is defined separately for each class derived from strstreambuf" with:

> Effects: implementation defined, except that `setbuf(0,0)` has no effect.

### 68. Extractors for char* should store null at end

Section: 27.7.2.2.3 [istream::extractors] Status: TC1 Submitter: Angelika Langer Opened: 1998-07-14 Last modified: 2015-04-08

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Discussion:

Extractors for char* (27.6.1.2.3) do not store a null character after the extracted character sequence whereas the unformatted functions like get() do. Why is this?

Comment from Jerry Schwarz: There is apparently an editing glitch. You'll notice that the last item of the list of what stops extraction doesn't make any sense. It was supposed to be the line that said a null is stored.

Proposed resolution:

27.7.2.2.3 [istream::extractors], paragraph 7, change the last list item from:

> A null byte (`charT()`) in the next position, which may be the first position if no characters were extracted.

to become a new paragraph which reads:

> Operator>> then stores a null byte (`charT()`) in the next position, which may be the first position if no characters were extracted.

### 69. Must elements of a vector be contiguous?

Section: 23.3.6 [vector] Status: TC1 Submitter: Andrew Koenig Opened: 1998-07-29 Last modified: 2015-04-08

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Discussion:

The issue is this: Must the elements of a vector be in contiguous memory?

(Please note that this is entirely separate from the question of whether a vector iterator is required to be a pointer; the answer to that question is clearly "no," as it would rule out debugging implementations)

Proposed resolution:

Add the following text to the end of 23.3.6 [vector], paragraph 1.

> The elements of a vector are stored contiguously, meaning that if v is a `vector<T, Allocator>` where T is some type other than `bool`, then it obeys the identity `&v[n] == &v[0] + n` for all `0 <= n < v.size()`.

Rationale:

The LWG feels that as a practical matter the answer is clearly "yes". There was considerable discussion as to the best way to express the concept of "contiguous", which is not directly defined in the standard. Discussion included:

- An operational definition similar to the above proposed resolution is already used for valarray (26.6.2.4 [valarray.access]).
- There is no need to explicitly consider a user-defined operator& because elements must be copyconstructible (23.2 [container.requirements] para 3) and copyconstructible (17.6.3.1 [utility.arg.requirements]) specifies requirements for operator&.
- There is no issue of one-past-the-end because of language rules.

### 70. Uncaught_exception() missing throw() specification

Section: 18.8 [support.exception], X [uncaught] Status: TC1 Submitter: Steve Clamage Opened: 1998-08-03 Last modified: 2015-04-08

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Discussion:

In article 3E04@pratique.fr, Valentin Bonnard writes:

uncaught_exception() doesn't have a throw specification.

It is intentional ? Does it means that one should be prepared to handle exceptions thrown from uncaught_exception() ?

uncaught_exception() is called in exception handling contexts where exception safety is very important.

Proposed resolution:

In 15.5.3 [except.uncaught], paragraph 1, 18.8 [support.exception], and X [uncaught], add "throw()" to uncaught_exception().

### 71. Do_get_monthname synopsis missing argument

Section: 22.4.5.1 [locale.time.get] Status: TC1 Submitter: Nathan Myers Opened: 1998-08-13 Last modified: 2015-04-08

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Discussion:

The locale facet member `time_get<>::do_get_monthname` is described in 22.4.5.1.2 [locale.time.get.virtuals] with five arguments, consistent with do_get_weekday and with its specified use by member get_monthname. However, in the synopsis, it is specified instead with four arguments. The missing argument is the "end" iterator value.

Proposed resolution:

In 22.4.5.1 [locale.time.get], add an "end" argument to the declaration of member do_monthname as follows:

      virtual iter_type do_get_monthname(iter_type s, iter_type end, ios_base&,
                                         ios_base::iostate& err, tm* t) const;

### 74. Garbled text for `codecvt::do_max_length`

Section: 22.4.1.4 [locale.codecvt] Status: TC1 Submitter: Matt Austern Opened: 1998-09-08 Last modified: 2015-04-08

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Discussion:

The text of `codecvt::do_max_length`'s "Returns" clause (22.2.1.5.2, paragraph 11) is garbled. It has unbalanced parentheses and a spurious n.

Proposed resolution:

Replace 22.4.1.4.2 [locale.codecvt.virtuals] paragraph 11 with the following:

> Returns: The maximum value that `do_length(state, from, from_end, 1)` can return for any valid range `[from, from_end)` and `stateT` value `state`. The specialization `codecvt<char, char, mbstate_t>::do_max_length()` returns 1.

### 75. Contradiction in `codecvt::length`'s argument types

Section: 22.4.1.4 [locale.codecvt] Status: TC1 Submitter: Matt Austern Opened: 1998-09-18 Last modified: 2015-04-08

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Discussion:

The class synopses for classes `codecvt<>` (22.2.1.5) and `codecvt_byname<>` (22.2.1.6) say that the first parameter of the member functions `length` and `do_length` is of type `const stateT&`. The member function descriptions, however (22.2.1.5.1, paragraph 6; 22.2.1.5.2, paragraph 9) say that the type is `stateT&`. Either the synopsis or the summary must be changed.

If (as I believe) the member function descriptions are correct, then we must also add text saying how `do_length` changes its `stateT` argument.

Proposed resolution:

In 22.4.1.4 [locale.codecvt], and also in 22.4.1.5 [locale.codecvt.byname], change the `stateT` argument type on both member `length()` and member `do_length()` from

> `const stateT&`

to

> `stateT&`

In 22.4.1.4.2 [locale.codecvt.virtuals], add to the definition for member `do_length` a paragraph:

> Effects: The effect on the `state` argument is ``as if'' it called `do_in(state, from, from_end, from, to, to+max, to)` for `to` pointing to a buffer of at least `max` elements.

### 76. Can a `codecvt` facet always convert one internal character at a time?

Section: 22.4.1.4 [locale.codecvt] Status: CD1 Submitter: Matt Austern Opened: 1998-09-25 Last modified: 2015-04-08

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Discussion:

This issue concerns the requirements on classes derived from `codecvt`, including user-defined classes. What are the restrictions on the conversion from external characters (e.g. `char`) to internal characters (e.g. `wchar_t`)? Or, alternatively, what assumptions about `codecvt` facets can the I/O library make?

The question is whether it's possible to convert from internal characters to external characters one internal character at a time, and whether, given a valid sequence of external characters, it's possible to pick off internal characters one at a time. Or, to put it differently: given a sequence of external characters and the corresponding sequence of internal characters, does a position in the internal sequence correspond to some position in the external sequence?

To make this concrete, suppose that `[first, last)` is a sequence of M external characters and that `[ifirst, ilast)` is the corresponding sequence of N internal characters, where N > 1. That is, `my_encoding.in()`, applied to `[first, last)`, yields `[ifirst, ilast)`. Now the question: does there necessarily exist a subsequence of external characters, `[first, last_1)`, such that the corresponding sequence of internal characters is the single character `*ifirst`?

(What a "no" answer would mean is that `my_encoding` translates sequences only as blocks. There's a sequence of M external characters that maps to a sequence of N internal characters, but that external sequence has no subsequence that maps to N-1 internal characters.)

Some of the wording in the standard, such as the description of `codecvt::do_max_length` (22.4.1.4.2 [locale.codecvt.virtuals], paragraph 11) and `basic_filebuf::underflow` (27.9.1.5 [filebuf.virtuals], paragraph 3) suggests that it must always be possible to pick off internal characters one at a time from a sequence of external characters. However, this is never explicitly stated one way or the other.

This issue seems (and is) quite technical, but it is important if we expect users to provide their own encoding facets. This is an area where the standard library calls user-supplied code, so a well-defined set of requirements for the user-supplied code is crucial. Users must be aware of the assumptions that the library makes. This issue affects positioning operations on `basic_filebuf`, unbuffered input, and several of `codecvt`'s member functions.

Proposed resolution:

Add the following text as a new paragraph, following 22.4.1.4.2 [locale.codecvt.virtuals] paragraph 2:

> A `codecvt` facet that is used by `basic_filebuf` (27.9 [file.streams]) must have the property that if

>         do_out(state, from, from_end, from_next, to, to_lim, to_next)

> would return `ok`, where `from != from_end`, then

>         do_out(state, from, from + 1, from_next, to, to_end, to_next)

> must also return `ok`, and that if

>         do_in(state, from, from_end, from_next, to, to_lim, to_next)

> would return `ok`, where `to != to_lim`, then

>         do_in(state, from, from_end, from_next, to, to + 1, to_next)

> must also return `ok`. [Footnote: Informally, this means that `basic_filebuf` assumes that the mapping from internal to external characters is 1 to N: a `codecvt` that is used by `basic_filebuf` must be able to translate characters one internal character at a time. --End Footnote]

[Redmond: Minor change in proposed resolution. Original proposed resolution talked about "success", with a parenthetical comment that success meant returning `ok`. New wording removes all talk about "success", and just talks about the return value.]

Rationale:

The proposed resoluion says that conversions can be performed one internal character at a time. This rules out some encodings that would otherwise be legal. The alternative answer would mean there would be some internal positions that do not correspond to any external file position.

An example of an encoding that this rules out is one where the `internT` and `externT` are of the same type, and where the internal sequence `c1 c2` corresponds to the external sequence `c2 c1`.

It was generally agreed that `basic_filebuf` relies on this property: it was designed under the assumption that the external-to-internal mapping is N-to-1, and it is not clear that `basic_filebuf` is implementable without that restriction.

The proposed resolution is expressed as a restriction on `codecvt` when used by `basic_filebuf`, rather than a blanket restriction on all `codecvt` facets, because `basic_filebuf` is the only other part of the library that uses `codecvt`. If a user wants to define a `codecvt` facet that implements a more general N-to-M mapping, there is no reason to prohibit it, so long as the user does not expect `basic_filebuf` to be able to use it.

### 78. Typo: event_call_back

Section: 27.5.3 [ios.base] Status: TC1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2015-04-08

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Discussion:

typo: event_call_back should be event_callback

Proposed resolution:

In the 27.5.3 [ios.base] synopsis change "event_call_back" to "event_callback".

### 79. Inconsistent declaration of polar()

Section: 26.4.1 [complex.syn], 26.4.7 [complex.value.ops] Status: TC1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2015-04-08

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Discussion:

In 26.4.1 [complex.syn] polar is declared as follows:

       template<class T> complex<T> polar(const T&, const T&);

In 26.4.7 [complex.value.ops] it is declared as follows:

       template<class T> complex<T> polar(const T& rho, const T& theta = 0);

Thus whether the second parameter is optional is not clear.

Proposed resolution:

In 26.4.1 [complex.syn] change:

       template<class T> complex<T> polar(const T&, const T&);

to:

       template<class T> complex<T> polar(const T& rho, const T& theta = 0);

### 80. Global Operators of complex declared twice

Section: 26.4.1 [complex.syn], 26.4.2 [complex] Status: TC1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2015-04-08

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Discussion:

Both 26.2.1 and 26.2.2 contain declarations of global operators for class complex. This redundancy should be removed.

Proposed resolution:

Reduce redundancy according to the general style of the standard.

### 83. String::npos vs. string::max_size()

Section: 21.4 [basic.string] Status: TC1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2015-04-08

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Duplicate of: 89

Discussion:

Many string member functions throw if size is getting or exceeding npos. However, I wonder why they don't throw if size is getting or exceeding max_size() instead of npos. May be npos is known at compile time, while max_size() is known at runtime. However, what happens if size exceeds max_size() but not npos, then? It seems the standard lacks some clarifications here.

Proposed resolution:

After 21.4 [basic.string] paragraph 4 ("The functions described in this clause...") add a new paragraph:

> For any string operation, if as a result of the operation, `size()` would exceed `max_size()` then the operation throws `length_error`.

Rationale:

The LWG believes length_error is the correct exception to throw.

### 86. String constructors don't describe exceptions

Section: 21.4.1 [string.require] Status: TC1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2015-04-08

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Discussion:

The constructor from a range:

    template<class InputIterator>
             basic_string(InputIterator begin, InputIterator end,
                          const Allocator& a = Allocator());

lacks a throws clause. However, I would expect that it throws according to the other constructors if the numbers of characters in the range equals npos (or exceeds max_size(), see above).

Proposed resolution:

In 21.4.1 [string.require], Strike throws paragraphs for constructors which say "Throws: length_error if n == npos."

Rationale:

Throws clauses for length_error if n == npos are no longer needed because they are subsumed by the general wording added by the resolution for issue 83.

### 90. Incorrect description of operator >> for strings

Section: 21.4.8.9 [string.io] Status: TC1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2015-04-08

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Discussion:

The effect of operator >> for strings contain the following item:

`isspace(c,getloc())` is true for the next available input character c.

Here `getloc()` has to be replaced by `is.getloc()`.

Proposed resolution:

In 21.4.8.9 [string.io] paragraph 1 Effects clause replace:

> `isspace(c,getloc())` is true for the next available input character c.

with:

> `isspace(c,is.getloc())` is true for the next available input character c.

### 91. Description of operator>> and getline() for string<> might cause endless loop

Section: 21.4.8.9 [string.io] Status: CD1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2015-04-08

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Discussion:

Operator >> and getline() for strings read until eof() in the input stream is true. However, this might never happen, if the stream can't read anymore without reaching EOF. So shouldn't it be changed into that it reads until !good() ?

Proposed resolution:

In 21.4.8.9 [string.io], paragraph 1, replace:

> Effects: Begins by constructing a sentry object k as if k were constructed by typename basic_istream<charT,traits>::sentry k( is). If bool( k) is true, it calls str.erase() and then extracts characters from is and appends them to str as if by calling str.append(1, c). If is.width() is greater than zero, the maximum number n of characters appended is is.width(); otherwise n is str.max_size(). Characters are extracted and appended until any of the following occurs:

with:

> Effects: Behaves as a formatted input function (27.7.2.2.1 [istream.formatted.reqmts]). After constructing a sentry object, if the sentry converts to true, calls str.erase() and then extracts characters from is and appends them to str as if by calling str.append(1,c). If is.width() is greater than zero, the maximum number n of characters appended is is.width(); otherwise n is str.max_size(). Characters are extracted and appended until any of the following occurs:

In 21.4.8.9 [string.io], paragraph 6, replace

> Effects: Begins by constructing a sentry object k as if by typename basic_istream<charT,traits>::sentry k( is, true). If bool( k) is true, it calls str.erase() and then extracts characters from is and appends them to str as if by calling str.append(1, c) until any of the following occurs:

with:

> Effects: Behaves as an unformatted input function (27.7.2.3 [istream.unformatted]), except that it does not affect the value returned by subsequent calls to basic_istream<>::gcount(). After constructing a sentry object, if the sentry converts to true, calls str.erase() and then extracts characters from is and appends them to str as if by calling str.append(1,c) until any of the following occurs:

[Redmond: Made changes in proposed resolution. `operator>>` should be a formatted input function, not an unformatted input function. `getline` should not be required to set `gcount`, since there is no mechanism for `gcount` to be set except by one of `basic_istream`'s member functions.]

[Curaçao: Nico agrees with proposed resolution.]

Rationale:

The real issue here is whether or not these string input functions get their characters from a streambuf, rather than by calling an istream's member functions, a streambuf signals failure either by returning eof or by throwing an exception; there are no other possibilities. The proposed resolution makes it clear that these two functions do get characters from a streambuf.

### 92. Incomplete Algorithm Requirements

Section: 25 [algorithms] Status: CD1 Submitter: Nico Josuttis Opened: 1998-09-29 Last modified: 2015-04-08

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Discussion:

The standard does not state, how often a function object is copied, called, or the order of calls inside an algorithm. This may lead to surprising/buggy behavior. Consider the following example:

    class Nth {    // function object that returns true for the nth element
      private:
        int nth;     // element to return true for
        int count;   // element counter
      public:
        Nth (int n) : nth(n), count(0) {
        }
        bool operator() (int) {
            return ++count == nth;
        }
    };
    ....
    // remove third element
        list<int>::iterator pos;
        pos = remove_if(coll.begin(),coll.end(),  // range
                        Nth(3)),                  // remove criterion
        coll.erase(pos,coll.end());

This call, in fact removes the 3rd AND the 6th element. This happens because the usual implementation of the algorithm copies the function object internally:

    template <class ForwIter, class Predicate>
    ForwIter std::remove_if(ForwIter beg, ForwIter end, Predicate op)
    {
        beg = find_if(beg, end, op);
        if (beg == end) {
            return beg;
        }
        else {
            ForwIter next = beg;
            return remove_copy_if(++next, end, beg, op);
        }
    }

The algorithm uses find_if() to find the first element that should be removed. However, it then uses a copy of the passed function object to process the resulting elements (if any). Here, Nth is used again and removes also the sixth element. This behavior compromises the advantage of function objects being able to have a state. Without any cost it could be avoided (just implement it directly instead of calling find_if()).

Proposed resolution:

Add a new paragraph following 25 [algorithms] paragraph 8:

> [Note: Unless otherwise specified, algorithms that take function objects as arguments are permitted to copy those function objects freely. Programmers for whom object identity is important should consider using a wrapper class that points to a noncopied implementation object, or some equivalent solution.]

[Dublin: Pete Becker felt that this may not be a defect, but rather something that programmers need to be educated about. There was discussion of adding wording to the effect that the number and order of calls to function objects, including predicates, not affect the behavior of the function object.]

[Pre-Kona: Nico comments: It seems the problem is that we don't have a clear statement of "predicate" in the standard. People including me seemed to think "a function returning a Boolean value and being able to be called by an STL algorithm or be used as sorting criterion or ... is a predicate". But a predicate has more requirements: It should never change its behavior due to a call or being copied. IMHO we have to state this in the standard. If you like, see section 8.1.4 of my library book for a detailed discussion.]

[Kona: Nico will provide wording to the effect that "unless otherwise specified, the number of copies of and calls to function objects by algorithms is unspecified". Consider placing in 25 [algorithms] after paragraph 9.]

[Santa Cruz: The standard doesn't currently guarantee that functions object won't be copied, and what isn't forbidden is allowed. It is believed (especially since implementations that were written in concert with the standard do make copies of function objects) that this was intentional. Thus, no normative change is needed. What we should put in is a non-normative note suggesting to programmers that if they want to guarantee the lack of copying they should use something like the `ref` wrapper.]

[Oxford: Matt provided wording.]

### 98. Input iterator requirements are badly written

Section: 24.2.3 [input.iterators] Status: CD1 Submitter: AFNOR Opened: 1998-10-07 Last modified: 2015-04-08

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Discussion:

Table 72 in 24.2.3 [input.iterators] specifies semantics for `*r++` of:

`{ T tmp = *r; ++r; return tmp; }`

There are two problems with this. First, the return type is specified to be "T", as opposed to something like "convertible to T". This is too specific: we want to allow *r++ to return an lvalue.

Second, writing the semantics in terms of code misleadingly suggests that the effects *r++ should precisely replicate the behavior of this code, including side effects. (Does this mean that *r++ should invoke the copy constructor exactly as many times as the sample code above would?) See issue 334 for a similar problem.

Proposed resolution:

In Table 72 in 24.2.3 [input.iterators], change the return type for `*r++` from `T` to "convertible to T".

Rationale:

This issue has two parts: the return type, and the number of times the copy constructor is invoked.

The LWG believes the the first part is a real issue. It's inappropriate for the return type to be specified so much more precisely for *r++ than it is for *r. In particular, if r is of (say) type `int*`, then *r++ isn't `int`, but `int&`.

The LWG does not believe that the number of times the copy constructor is invoked is a real issue. This can vary in any case, because of language rules on copy constructor elision. That's too much to read into these semantics clauses.

Additionally, as Dave Abrahams pointed out (c++std-lib-13703): since we're told (24.1/3) that forward iterators satisfy all the requirements of input iterators, we can't impose any requirements in the Input Iterator requirements table that forward iterators don't satisfy.

### 103. set::iterator is required to be modifiable, but this allows modification of keys

Section: 23.2.4 [associative.reqmts] Status: CD1 Submitter: AFNOR Opened: 1998-10-07 Last modified: 2015-04-08

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Discussion:

Set::iterator is described as implementation-defined with a reference to the container requirement; the container requirement says that const_iterator is an iterator pointing to const T and iterator an iterator pointing to T.

23.1.2 paragraph 2 implies that the keys should not be modified to break the ordering of elements. But that is not clearly specified. Especially considering that the current standard requires that iterator for associative containers be different from const_iterator. Set, for example, has the following:

`typedef implementation defined iterator;
// See _lib.container.requirements_`

23.2 [container.requirements] actually requires that iterator type pointing to T (table 65). Disallowing user modification of keys by changing the standard to require an iterator for associative container to be the same as const_iterator would be overkill since that will unnecessarily significantly restrict the usage of associative container. A class to be used as elements of set, for example, can no longer be modified easily without either redesigning the class (using mutable on fields that have nothing to do with ordering), or using const_cast, which defeats requiring iterator to be const_iterator. The proposed solution goes in line with trusting user knows what he is doing.

Other Options Evaluated:

Option A. In 23.2.4 [associative.reqmts], paragraph 2, after first sentence, and before "In addition,...", add one line:

> Modification of keys shall not change their strict weak ordering.

Option B. Add three new sentences to 23.2.4 [associative.reqmts]:

> At the end of paragraph 5: "Keys in an associative container are immutable." At the end of paragraph 6: "For associative containers where the value type is the same as the key type, both `iterator` and `const_iterator` are constant iterators. It is unspecified whether or not `iterator` and `const_iterator` are the same type."

Option C. To 23.2.4 [associative.reqmts], paragraph 3, which currently reads:

> The phrase ``equivalence of keys'' means the equivalence relation imposed by the comparison and not the operator== on keys. That is, two keys k1 and k2 in the same container are considered to be equivalent if for the comparison object comp, comp(k1, k2) == false && comp(k2, k1) == false.

add the following:

> For any two keys k1 and k2 in the same container, comp(k1, k2) shall return the same value whenever it is evaluated. [Note: If k2 is removed from the container and later reinserted, comp(k1, k2) must still return a consistent value but this value may be different than it was the first time k1 and k2 were in the same container. This is intended to allow usage like a string key that contains a filename, where comp compares file contents; if k2 is removed, the file is changed, and the same k2 (filename) is reinserted, comp(k1, k2) must again return a consistent value but this value may be different than it was the previous time k2 was in the container.]

Proposed resolution:

Add the following to 23.2.4 [associative.reqmts] at the indicated location:

> At the end of paragraph 3: "For any two keys k1 and k2 in the same container, calling comp(k1, k2) shall always return the same value."

> At the end of paragraph 5: "Keys in an associative container are immutable."

> At the end of paragraph 6: "For associative containers where the value type is the same as the key type, both `iterator` and `const_iterator` are constant iterators. It is unspecified whether or not `iterator` and `const_iterator` are the same type."

Rationale:

Several arguments were advanced for and against allowing set elements to be mutable as long as the ordering was not effected. The argument which swayed the LWG was one of safety; if elements were mutable, there would be no compile-time way to detect of a simple user oversight which caused ordering to be modified. There was a report that this had actually happened in practice, and had been painful to diagnose. If users need to modify elements, it is possible to use mutable members or const_cast.

Simply requiring that keys be immutable is not sufficient, because the comparison object may indirectly (via pointers) operate on values outside of the keys.

The types `iterator` and `const_iterator` are permitted to be different types to allow for potential future work in which some member functions might be overloaded between the two types. No such member functions exist now, and the LWG believes that user functionality will not be impaired by permitting the two types to be the same. A function that operates on both iterator types can be defined for `const_iterator` alone, and can rely on the automatic conversion from `iterator` to `const_iterator`.

[Tokyo: The LWG crafted the proposed resolution and rationale.]

### 106. Numeric library private members are implementation defined

Section: 26.6.5 [template.slice.array] Status: TC1 Submitter: AFNOR Opened: 1998-10-07 Last modified: 2015-04-08

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Discussion:

This is the only place in the whole standard where the implementation has to document something private.

Proposed resolution:

Remove the comment which says "// remainder implementation defined" from:

- 26.6.5 [template.slice.array]
- 26.6.7 [template.gslice.array]
- 26.6.8 [template.mask.array]
- 26.6.9 [template.indirect.array]

### 108. Lifetime of exception::what() return unspecified

Section: 18.7.1 [type.info] Status: TC1 Submitter: AFNOR Opened: 1998-10-07 Last modified: 2015-04-08

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Discussion:

In 18.6.1, paragraphs 8-9, the lifetime of the return value of exception::what() is left unspecified. This issue has implications with exception safety of exception handling: some exceptions should not throw bad_alloc.

Proposed resolution:

Add to 18.7.1 [type.info] paragraph 9 (exception::what notes clause) the sentence:

> The return value remains valid until the exception object from which it is obtained is destroyed or a non-const member function of the exception object is called.

Rationale:

If an exception object has non-const members, they may be used to set internal state that should affect the contents of the string returned by `what()`.

### 109. Missing binders for non-const sequence elements

Section: X [depr.lib.binders] Status: CD1 Submitter: Bjarne Stroustrup Opened: 1998-10-07 Last modified: 2015-04-08

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Discussion:

There are no versions of binders that apply to non-const elements of a sequence. This makes examples like for_each() using bind2nd() on page 521 of "The C++ Programming Language (3rd)" non-conforming. Suitable versions of the binders need to be added.

Further discussion from Nico:

What is probably meant here is shown in the following example:

    class Elem {
      public:
        void print (int i) const { }
        void modify (int i) { }
    };

    int main()
    {
        vector<Elem> coll(2);
        for_each (coll.begin(), coll.end(), bind2nd(mem_fun_ref(&Elem::print),42));    // OK
        for_each (coll.begin(), coll.end(), bind2nd(mem_fun_ref(&Elem::modify),42));   // ERROR
    }

The error results from the fact that bind2nd() passes its first argument (the argument of the sequence) as constant reference. See the following typical implementation:

>     template <class Operation>
>     class binder2nd
>       : public unary_function<typename Operation::first_argument_type,
>                               typename Operation::result_type> {
>     protected:
>       Operation op;
>       typename Operation::second_argument_type value;
>     public:
>       binder2nd(const Operation& o,
>                 const typename Operation::second_argument_type& v)
>           : op(o), value(v) {}

>      typename Operation::result_type
>       operator()(const typename Operation::first_argument_type& x) const {
>         return op(x, value);
>       }
>     };

The solution is to overload operator () of bind2nd for non-constant arguments:

>     template <class Operation>
>     class binder2nd
>       : public unary_function<typename Operation::first_argument_type,
>                               typename Operation::result_type> {
>     protected:
>       Operation op;
>       typename Operation::second_argument_type value;
>     public:
>       binder2nd(const Operation& o,
>                 const typename Operation::second_argument_type& v)
>           : op(o), value(v) {}

>      typename Operation::result_type
>       operator()(const typename Operation::first_argument_type& x) const {
>         return op(x, value);
>       }
>       typename Operation::result_type
>       operator()(typename Operation::first_argument_type& x) const {
>         return op(x, value);
>       }
>     };

Proposed resolution:

Howard believes there is a flaw in this resolution. See c++std-lib-9127. We may need to reopen this issue.

In X [depr.lib.binders] in the declaration of binder1st after:

> `typename Operation::result_type
> operator()(const typename Operation::second_argument_type& x) const;`

insert:

> `typename Operation::result_type
> operator()(typename Operation::second_argument_type& x) const;`

In X [depr.lib.binders] in the declaration of binder2nd after:

> `typename Operation::result_type
> operator()(const typename Operation::first_argument_type& x) const;`

insert:

> `typename Operation::result_type
> operator()(typename Operation::first_argument_type& x) const;`

[Kona: The LWG discussed this at some length.It was agreed that this is a mistake in the design, but there was no consensus on whether it was a defect in the Standard. Straw vote: NAD - 5. Accept proposed resolution - 3. Leave open - 6.]

[Copenhagen: It was generally agreed that this was a defect. Strap poll: NAD - 0. Accept proposed resolution - 10. Leave open - 1.]

### 110. istreambuf_iterator::equal not const

Section: 24.6.3 [istreambuf.iterator], 24.6.3.5 [istreambuf.iterator::equal] Status: TC1 Submitter: Nathan Myers Opened: 1998-10-15 Last modified: 2015-04-08

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Discussion:

Member istreambuf_iterator<>::equal is not declared "const", yet 24.6.3.6 [istreambuf.iterator::op==] says that operator==, which is const, calls it. This is contradictory.

Proposed resolution:

In 24.6.3 [istreambuf.iterator] and also in 24.6.3.5 [istreambuf.iterator::equal], replace:

>     bool equal(istreambuf_iterator& b);

with:

>     bool equal(const istreambuf_iterator& b) const;

### 112. Minor typo in `ostreambuf_iterator` constructor

Section: 24.6.4.1 [ostreambuf.iter.cons] Status: TC1 Submitter: Matt Austern Opened: 1998-10-20 Last modified: 2015-04-08

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Discussion:

The requires clause for `ostreambuf_iterator`'s constructor from an `ostream_type` (24.5.4.1, paragraph 1) reads "s is not null". However, s is a reference, and references can't be null.

Proposed resolution:

In 24.6.4.1 [ostreambuf.iter.cons]:

Move the current paragraph 1, which reads "Requires: s is not null.", from the first constructor to the second constructor.

Insert a new paragraph 1 Requires clause for the first constructor reading:

> Requires: `s.rdbuf()` is not null.

### 114. Placement forms example in error twice

Section: 18.6.1.3 [new.delete.placement] Status: TC1 Submitter: Steve Clamage Opened: 1998-10-28 Last modified: 2015-04-08

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Duplicate of: 196

Discussion:

Section 18.5.1.3 contains the following example:

    [Example: This can be useful for constructing an object at a known address:
            char place[sizeof(Something)];
            Something* p = new (place) Something();
     -end example]

First code line: "place" need not have any special alignment, and the following constructor could fail due to misaligned data.

Second code line: Aren't the parens on Something() incorrect? [Dublin: the LWG believes the () are correct.]

Examples are not normative, but nevertheless should not show code that is invalid or likely to fail.

Proposed resolution:

Replace the first line of code in the example in 18.6.1.3 [new.delete.placement] with:

>     void* place = operator new(sizeof(Something));

### 115. Typo in strstream constructors

Section: D.7.4.1 [depr.strstream.cons] Status: TC1 Submitter: Steve Clamage Opened: 1998-11-02 Last modified: 2015-04-08

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Discussion:

D.7.4.1 strstream constructors paragraph 2 says:

> Effects: Constructs an object of class strstream, initializing the base class with iostream(& sb) and initializing sb with one of the two constructors:

> - If mode&app==0, then s shall designate the first element of an array of n elements. The constructor is strstreambuf(s, n, s).

> - If mode&app==0, then s shall designate the first element of an array of n elements that contains an NTBS whose first element is designated by s. The constructor is strstreambuf(s, n, s+std::strlen(s)).

Notice the second condition is the same as the first. I think the second condition should be "If mode&app==app", or "mode&app!=0", meaning that the append bit is set.

Proposed resolution:

In D.7.3.1 [depr.ostrstream.cons] paragraph 2 and D.7.4.1 [depr.strstream.cons] paragraph 2, change the first condition to `(mode&app)==0` and the second condition to `(mode&app)!=0`.

### 117. `basic_ostream` uses nonexistent `num_put` member functions

Section: 27.7.3.6.2 [ostream.inserters.arithmetic] Status: CD1 Submitter: Matt Austern Opened: 1998-11-20 Last modified: 2015-04-08

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Discussion:

The effects clause for numeric inserters says that insertion of a value `x`, whose type is either `bool`, `short`, `unsigned short`, `int`, `unsigned int`, `long`, `unsigned long`, `float`, `double`, `long double`, or `const void*`, is delegated to `num_put`, and that insertion is performed as if through the following code fragment:

    bool failed = use_facet<
       num_put<charT,ostreambuf_iterator<charT,traits> >
       >(getloc()).put(*this, *this, fill(), val). failed();

This doesn't work, because `num_put<>`::put is only overloaded for the types `bool`, `long`, `unsigned long`, `double`, `long double`, and `const void*`. That is, the code fragment in the standard is incorrect (it is diagnosed as ambiguous at compile time) for the types `short`, `unsigned short`, `int`, `unsigned int`, and `float`.

We must either add new member functions to `num_put`, or else change the description in `ostream` so that it only calls functions that are actually there. I prefer the latter.

Proposed resolution:

Replace 27.6.2.5.2, paragraph 1 with the following:

> The classes num_get<> and num_put<> handle locale-dependent numeric formatting and parsing. These inserter functions use the imbued locale value to perform numeric formatting. When val is of type bool, long, unsigned long, double, long double, or const void*, the formatting conversion occurs as if it performed the following code fragment:

>     bool failed = use_facet<
>        num_put<charT,ostreambuf_iterator<charT,traits> >
>        >(getloc()).put(*this, *this, fill(), val). failed();

> When val is of type short the formatting conversion occurs as if it performed the following code fragment:

>     ios_base::fmtflags baseflags = ios_base::flags() & ios_base::basefield;
>     bool failed = use_facet<
>        num_put<charT,ostreambuf_iterator<charT,traits> >
>        >(getloc()).put(*this, *this, fill(),
>           baseflags == ios_base::oct || baseflags == ios_base::hex
>              ? static_cast<long>(static_cast<unsigned short>(val))
>              : static_cast<long>(val)). failed();

> When val is of type int the formatting conversion occurs as if it performed the following code fragment:

>     ios_base::fmtflags baseflags = ios_base::flags() & ios_base::basefield;
>     bool failed = use_facet<
>        num_put<charT,ostreambuf_iterator<charT,traits> >
>        >(getloc()).put(*this, *this, fill(),
>           baseflags == ios_base::oct || baseflags == ios_base::hex
>              ? static_cast<long>(static_cast<unsigned int>(val))
>              : static_cast<long>(val)). failed();

> When val is of type unsigned short or unsigned int the formatting conversion occurs as if it performed the following code fragment:

>     bool failed = use_facet<
>        num_put<charT,ostreambuf_iterator<charT,traits> >
>        >(getloc()).put(*this, *this, fill(), static_cast<unsigned long>(val)).
>     failed();

> When val is of type float the formatting conversion occurs as if it performed the following code fragment:

>     bool failed = use_facet<
>        num_put<charT,ostreambuf_iterator<charT,traits> >
>        >(getloc()).put(*this, *this, fill(), static_cast<double>(val)).
>     failed();

[post-Toronto: This differs from the previous proposed resolution; PJP provided the new wording. The differences are in signed short and int output.]

Rationale:

The original proposed resolution was to cast int and short to long, unsigned int and unsigned short to unsigned long, and float to double, thus ensuring that we don't try to use nonexistent num_put<> member functions. The current proposed resolution is more complicated, but gives more expected results for hex and octal output of signed short and signed int. (On a system with 16-bit short, for example, printing short(-1) in hex format should yield 0xffff.)

### 118. `basic_istream` uses nonexistent `num_get` member functions

Section: 27.7.2.2.2 [istream.formatted.arithmetic] Status: CD1 Submitter: Matt Austern Opened: 1998-11-20 Last modified: 2015-04-08

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Discussion:

Formatted input is defined for the types `short`, `unsigned short`, `int`, `unsigned int`, `long`, `unsigned long`, `float`, `double`, `long double`, `bool`, and `void*`. According to section 27.6.1.2.2, formatted input of a value `x` is done as if by the following code fragment:

    typedef num_get< charT,istreambuf_iterator<charT,traits> > numget;
    iostate err = 0;
    use_facet< numget >(loc).get(*this, 0, *this, err, val);
    setstate(err);

According to section 22.4.2.1.1 [facet.num.get.members], however, `num_get<>::get()` is only overloaded for the types `bool`, `long`, `unsigned short`, `unsigned int`, `unsigned long`, `unsigned long`, `float`, `double`, `long double`, and `void*`. Comparing the lists from the two sections, we find that 27.6.1.2.2 is using a nonexistent function for types `short` and `int`.

Proposed resolution:

In 27.7.2.2.2 [istream.formatted.arithmetic] Arithmetic Extractors, remove the two lines (1st and 3rd) which read:

>     operator>>(short& val);
>     ...
>     operator>>(int& val);

And add the following at the end of that section (27.6.1.2.2) :

>     operator>>(short& val);

> The conversion occurs as if performed by the following code fragment (using the same notation as for the preceding code fragment):

>       typedef num_get< charT,istreambuf_iterator<charT,traits> > numget;
>       iostate err = 0;
>       long lval;
>       use_facet< numget >(loc).get(*this, 0, *this, err, lval);
>             if (err == 0
>                     && (lval < numeric_limits<short>::min() || numeric_limits<short>::max() < lval))
>                     err = ios_base::failbit;
>       setstate(err);

>     operator>>(int& val);

> The conversion occurs as if performed by the following code fragment (using the same notation as for the preceding code fragment):

>       typedef num_get< charT,istreambuf_iterator<charT,traits> > numget;
>       iostate err = 0;
>       long lval;
>       use_facet< numget >(loc).get(*this, 0, *this, err, lval);
>             if (err == 0
>                     && (lval < numeric_limits<int>::min() || numeric_limits<int>::max() < lval))
>                     err = ios_base::failbit;
>       setstate(err);

[Post-Tokyo: PJP provided the above wording.]

### 119. Should virtual functions be allowed to strengthen the exception specification?

Section: 17.6.5.12 [res.on.exception.handling] Status: TC1 Submitter: Judy Ward Opened: 1998-12-15 Last modified: 2015-04-08

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Discussion:

Section 17.6.5.12 [res.on.exception.handling] states:

"An implementation may strengthen the exception-specification for a function by removing listed exceptions."

The problem is that if an implementation is allowed to do this for virtual functions, then a library user cannot write a class that portably derives from that class.

For example, this would not compile if ios_base::failure::~failure had an empty exception specification:

    #include <ios>
    #include <string>

    class D : public std::ios_base::failure {
    public:
            D(const std::string&);
            ~D(); // error - exception specification must be compatible with
                  // overridden virtual function ios_base::failure::~failure()
    };

Proposed resolution:

Change Section 17.6.5.12 [res.on.exception.handling] from:

"may strengthen the exception-specification for a function"

to:

"may strengthen the exception-specification for a non-virtual function".

### 120. Can an implementor add specializations?

Section: 17.6.4.3 [reserved.names] Status: CD1 Submitter: Judy Ward Opened: 1998-12-15 Last modified: 2015-04-08

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Discussion:

The original issue asked whether a library implementor could specialize standard library templates for built-in types. (This was an issue because users are permitted to explicitly instantiate standard library templates.)

Specializations are no longer a problem, because of the resolution to core issue 259. Under the proposed resolution, it will be legal for a translation unit to contain both a specialization and an explicit instantiation of the same template, provided that the specialization comes first. In such a case, the explicit instantiation will be ignored. Further discussion of library issue 120 assumes that the core 259 resolution will be adopted.

However, as noted in lib-7047, one piece of this issue still remains: what happens if a standard library implementor explicitly instantiates a standard library templates? It's illegal for a program to contain two different explicit instantiations of the same template for the same type in two different translation units (ODR violation), and the core working group doesn't believe it is practical to relax that restriction.

The issue, then, is: are users allowed to explicitly instantiate standard library templates for non-user defined types? The status quo answer is 'yes'. Changing it to 'no' would give library implementors more freedom.

This is an issue because, for performance reasons, library implementors often need to explicitly instantiate standard library templates. (for example, std::basic_string<char>) Does giving users freedom to explicitly instantiate standard library templates for non-user defined types make it impossible or painfully difficult for library implementors to do this?

John Spicer suggests, in lib-8957, that library implementors have a mechanism they can use for explicit instantiations that doesn't prevent users from performing their own explicit instantiations: put each explicit instantiation in its own object file. (Different solutions might be necessary for Unix DSOs or MS-Windows DLLs.) On some platforms, library implementors might not need to do anything special: the "undefined behavior" that results from having two different explicit instantiations might be harmless.

Proposed resolution:

Append to 17.6.4.3 [reserved.names] paragraph 1:

> A program may explicitly instantiate any templates in the standard library only if the declaration depends on the name of a user-defined type of external linkage and the instantiation meets the standard library requirements for the original template.

[Kona: changed the wording from "a user-defined name" to "the name of a user-defined type"]

Rationale:

The LWG considered another possible resolution:

> In light of the resolution to core issue 259, no normative changes in the library clauses are necessary. Add the following non-normative note to the end of 17.6.4.3 [reserved.names] paragraph 1:

> > [Note: A program may explicitly instantiate standard library templates, even when an explicit instantiation does not depend on a user-defined name. --end note]

The LWG rejected this because it was believed that it would make it unnecessarily difficult for library implementors to write high-quality implementations. A program may not include an explicit instantiation of the same template, for the same template arguments, in two different translation units. If users are allowed to provide explicit instantiations of Standard Library templates for built-in types, then library implementors aren't, at least not without nonportable tricks.

The most serious problem is a class template that has writeable static member variables. Unfortunately, such class templates are important and, in existing Standard Library implementations, are often explicitly specialized by library implementors: locale facets, which have a writeable static member variable `id`. If a user's explicit instantiation collided with the implementations explicit instantiation, iostream initialization could cause locales to be constructed in an inconsistent state.

One proposed implementation technique was for Standard Library implementors to provide explicit instantiations in separate object files, so that they would not be picked up by the linker when the user also provides an explicit instantiation. However, this technique only applies for Standard Library implementations that are packaged as static archives. Most Standard Library implementations nowadays are packaged as dynamic libraries, so this technique would not apply.

The Committee is now considering standardization of dynamic linking. If there are such changes in the future, it may be appropriate to revisit this issue later.

### 122. streambuf/wstreambuf description should not say they are specializations

Section: 27.6.3 [streambuf] Status: TC1 Submitter: Judy Ward Opened: 1998-12-15 Last modified: 2015-04-08

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Discussion:

Section 27.5.2 describes the streambuf classes this way:

> The class streambuf is a specialization of the template class basic_streambuf specialized for the type char.

> The class wstreambuf is a specialization of the template class basic_streambuf specialized for the type wchar_t.

This implies that these classes must be template specializations, not typedefs.

It doesn't seem this was intended, since Section 27.5 has them declared as typedefs.

Proposed resolution:

Remove 27.6.3 [streambuf] paragraphs 2 and 3 (the above two sentences).

Rationale:

The `streambuf` synopsis already has a declaration for the typedefs and that is sufficient.

### 123. Should valarray helper arrays fill functions be const?

Section: 26.6.5.4 [slice.arr.fill], 26.6.7.4 [gslice.array.fill], 26.6.8.4 [mask.array.fill], 26.6.9.4 [indirect.array.fill] Status: CD1 Submitter: Judy Ward Opened: 1998-12-15 Last modified: 2015-04-08

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Discussion:

One of the operator= in the valarray helper arrays is const and one is not. For example, look at slice_array. This operator= in Section 26.6.5.2 [slice.arr.assign] is const:

`void operator=(const valarray<T>&) const;`

but this one in Section 26.6.5.4 [slice.arr.fill] is not:

`void operator=(const T&);`

The description of the semantics for these two functions is similar.

Proposed resolution:

26.6.5 [template.slice.array] Template class slice_array

> In the class template definition for slice_array, replace the member function declaration

>           void operator=(const T&);

> with

>           void operator=(const T&) const;

26.6.5.4 [slice.arr.fill] slice_array fill function

> Change the function declaration

>           void operator=(const T&);

> to

>           void operator=(const T&) const;

26.6.7 [template.gslice.array] Template class gslice_array

> In the class template definition for gslice_array, replace the member function declaration

>           void operator=(const T&);

> with

>           void operator=(const T&) const;

26.6.7.4 [gslice.array.fill] gslice_array fill function

> Change the function declaration

>           void operator=(const T&);

> to

>           void operator=(const T&) const;

26.6.8 [template.mask.array] Template class mask_array

> In the class template definition for mask_array, replace the member function declaration

>           void operator=(const T&);

> with

>           void operator=(const T&) const;

26.6.8.4 [mask.array.fill] mask_array fill function

> Change the function declaration

>           void operator=(const T&);

> to

>           void operator=(const T&) const;

26.6.9 [template.indirect.array] Template class indirect_array

> In the class template definition for indirect_array, replace the member function declaration

>           void operator=(const T&);

> with

>           void operator=(const T&) const;

26.6.9.4 [indirect.array.fill] indirect_array fill function

> Change the function declaration

>           void operator=(const T&);

> to

>           void operator=(const T&) const;

[Redmond: Robert provided wording.]

Rationale:

There's no good reason for one version of operator= being const and another one not. Because of issue 253, this now matters: these functions are now callable in more circumstances. In many existing implementations, both versions are already const.

### 124. ctype_byname<charT>::do_scan_is & do_scan_not return type should be const charT*

Section: 22.4.1.2 [locale.ctype.byname] Status: TC1 Submitter: Judy Ward Opened: 1998-12-15 Last modified: 2015-04-08

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Discussion:

In Section 22.4.1.2 [locale.ctype.byname] ctype_byname<charT>::do_scan_is() and do_scan_not() are declared to return a const char* not a const charT*.

Proposed resolution:

Change Section 22.4.1.2 [locale.ctype.byname] `do_scan_is()` and `do_scan_not()` to return a `const charT*`.

### 125. valarray<T>::operator!() return type is inconsistent

Section: 26.6.2 [template.valarray] Status: TC1 Submitter: Judy Ward Opened: 1998-12-15 Last modified: 2015-04-08

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Discussion:

In Section 26.6.2 [template.valarray] valarray<T>::operator!() is declared to return a valarray<T>, but in Section 26.6.2.6 [valarray.unary] it is declared to return a valarray<bool>. The latter appears to be correct.

Proposed resolution:

Change in Section 26.6.2 [template.valarray] the declaration of `operator!()` so that the return type is `valarray<bool>`.

### 126. typos in Effects clause of ctype::do_narrow()

Section: 22.4.1.1.2 [locale.ctype.virtuals] Status: TC1 Submitter: Judy Ward Opened: 1998-12-15 Last modified: 2015-04-08

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Discussion:

Typos in 22.2.1.1.2 need to be fixed.

Proposed resolution:

In Section 22.4.1.1.2 [locale.ctype.virtuals] change:

       do_widen(do_narrow(c),0) == c

to:

       do_widen(do_narrow(c,0)) == c

and change:

       (is(M,c) || !ctc.is(M, do_narrow(c),dfault) )

to:

       (is(M,c) || !ctc.is(M, do_narrow(c,dfault)) )

### 127. auto_ptr<> conversion issues

Section: X [auto.ptr] Status: TC1 Submitter: Greg Colvin Opened: 1999-02-17 Last modified: 2015-04-08

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Discussion:

There are two problems with the current `auto_ptr` wording in the standard:

First, the `auto_ptr_ref` definition cannot be nested because `auto_ptr<Derived>::auto_ptr_ref` is unrelated to `auto_ptr<Base>::auto_ptr_ref`. Also submitted by Nathan Myers, with the same proposed resolution.

Second, there is no `auto_ptr` assignment operator taking an `auto_ptr_ref` argument.

I have discussed these problems with my proposal coauthor, Bill Gibbons, and with some compiler and library implementors, and we believe that these problems are not desired or desirable implications of the standard.

25 Aug 1999: The proposed resolution now reflects changes suggested by Dave Abrahams, with Greg Colvin's concurrence; 1) changed "assignment operator" to "public assignment operator", 2) changed effects to specify use of release(), 3) made the conversion to auto_ptr_ref const.

2 Feb 2000: Lisa Lippincott comments: [The resolution of] this issue states that the conversion from auto_ptr to auto_ptr_ref should be const. This is not acceptable, because it would allow initialization and assignment from _any_ const auto_ptr! It also introduces an implementation difficulty in writing this conversion function -- namely, somewhere along the line, a const_cast will be necessary to remove that const so that release() may be called. This may result in undefined behavior [7.1.5.1/4]. The conversion operator does not have to be const, because a non-const implicit object parameter may be bound to an rvalue [13.3.3.1.4/3] [13.3.1/5].

Tokyo: The LWG removed the following from the proposed resolution:

In 20.10.4 [meta.unary], paragraph 2, and 20.10.4.3 [meta.unary.prop], paragraph 2, make the conversion to auto_ptr_ref const:

>     template<class Y> operator auto_ptr_ref<Y>() const throw();

Proposed resolution:

In 20.10.4 [meta.unary], paragraph 2, move the `auto_ptr_ref` definition to namespace scope.

In 20.10.4 [meta.unary], paragraph 2, add a public assignment operator to the `auto_ptr` definition:

>     auto_ptr& operator=(auto_ptr_ref<X> r) throw();

Also add the assignment operator to 20.10.4.3 [meta.unary.prop]:

>     auto_ptr& operator=(auto_ptr_ref<X> r) throw()

> Effects: Calls `reset(p.release())` for the `auto_ptr p` that `r` holds a reference to.
> Returns: `*this`.

### 129. Need error indication from seekp() and seekg()

Section: 27.7.2.3 [istream.unformatted], 27.7.3.5 [ostream.seeks] Status: TC1 Submitter: Angelika Langer Opened: 1999-02-22 Last modified: 2015-04-08

View other active issues in [istream.unformatted].

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Discussion:

Currently, the standard does not specify how seekg() and seekp() indicate failure. They are not required to set failbit, and they can't return an error indication because they must return *this, i.e. the stream. Hence, it is undefined what happens if they fail. And they can fail, for instance, when a file stream is disconnected from the underlying file (is_open()==false) or when a wide character file stream must perform a state-dependent code conversion, etc.

The stream functions seekg() and seekp() should set failbit in the stream state in case of failure.

Proposed resolution:

Add to the Effects: clause of seekg() in 27.7.2.3 [istream.unformatted] and to the Effects: clause of seekp() in 27.7.3.5 [ostream.seeks]:

> In case of failure, the function calls `setstate(failbit)` (which may throw `ios_base::failure`).

Rationale:

Setting failbit is the usual error reporting mechanism for streams

### 130. Return type of container::erase(iterator) differs for associative containers

Section: 23.2.4 [associative.reqmts], 23.2.3 [sequence.reqmts] Status: CD1 Submitter: Andrew Koenig Opened: 1999-03-02 Last modified: 2015-04-08

View other active issues in [associative.reqmts].

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Duplicate of: 451

Discussion:

Table 67 (23.1.1) says that container::erase(iterator) returns an iterator. Table 69 (23.1.2) says that in addition to this requirement, associative containers also say that container::erase(iterator) returns void. That's not an addition; it's a change to the requirements, which has the effect of making associative containers fail to meet the requirements for containers.

Proposed resolution:

In 23.2.4 [associative.reqmts], in Table 69 Associative container requirements, change the return type of `a.erase(q)` from `void` to `iterator`. Change the assertion/not/pre/post-condition from "erases the element pointed to by `q`" to "erases the element pointed to by `q`. Returns an iterator pointing to the element immediately following q prior to the element being erased. If no such element exists, a.end() is returned."

In 23.2.4 [associative.reqmts], in Table 69 Associative container requirements, change the return type of `a.erase(q1, q2)` from `void` to `iterator`. Change the assertion/not/pre/post-condition from "erases the elements in the range `[q1, q2)`" to "erases the elements in the range `[q1, q2)`. Returns q2."

In 23.4.4 [map], in the `map` class synopsis; and in 23.4.5 [multimap], in the `multimap` class synopsis; and in 23.4.6 [set], in the `set` class synopsis; and in 23.4.7 [multiset], in the `multiset` class synopsis: change the signature of the first `erase` overload to

       iterator erase(iterator position);

and change the signature of the third `erase` overload to

      iterator erase(iterator first, iterator last);

[Pre-Kona: reopened at the request of Howard Hinnant]

[Post-Kona: the LWG agrees the return type should be `iterator`, not `void`. (Alex Stepanov agrees too.) Matt provided wording.]

[ Sydney: the proposed wording went in the right direction, but it wasn't good enough. We want to return an iterator from the range form of erase as well as the single-iterator form. Also, the wording is slightly different from the wording we have for sequences; there's no good reason for having a difference. Matt provided new wording, (reflected above) which we will review at the next meeting. ]

[ Redmond: formally voted into WP. ]

### 132. list::resize description uses random access iterators

Section: 23.3.5.3 [list.capacity] Status: TC1 Submitter: Howard Hinnant Opened: 1999-03-06 Last modified: 2015-04-08

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Discussion:

The description reads:

-1- Effects:

             if (sz > size())
               insert(end(), sz-size(), c);
             else if (sz < size())
               erase(begin()+sz, end());
             else
               ;                           //  do nothing

Obviously list::resize should not be specified in terms of random access iterators.

Proposed resolution:

Change 23.3.5.3 [list.capacity] paragraph 1 to:

Effects:

             if (sz > size())
               insert(end(), sz-size(), c);
             else if (sz < size())
             {
               iterator i = begin();
               advance(i, sz);
               erase(i, end());
             }

[Dublin: The LWG asked Howard to discuss exception safety offline with David Abrahams. They had a discussion and believe there is no issue of exception safety with the proposed resolution.]

### 133. map missing get_allocator()

Section: 23.4.4 [map] Status: TC1 Submitter: Howard Hinnant Opened: 1999-03-06 Last modified: 2015-04-08

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Discussion:

The title says it all.

Proposed resolution:

Insert in 23.4.4 [map], paragraph 2, after operator= in the map declaration:

        allocator_type get_allocator() const;

### 134. vector constructors over specified

Section: 23.3.6.2 [vector.cons] Status: TC1 Submitter: Howard Hinnant Opened: 1999-03-06 Last modified: 2015-04-08

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Discussion:

The complexity description says: "It does at most 2N calls to the copy constructor of T and logN reallocations if they are just input iterators ...".

This appears to be overly restrictive, dictating the precise memory/performance tradeoff for the implementor.

Proposed resolution:

Change 23.3.6.2 [vector.cons], paragraph 1 to:

-1- Complexity: The constructor template <class InputIterator> vector(InputIterator first, InputIterator last) makes only N calls to the copy constructor of T (where N is the distance between first and last) and no reallocations if iterators first and last are of forward, bidirectional, or random access categories. It makes order N calls to the copy constructor of T and order logN reallocations if they are just input iterators.

Rationale:

"at most 2N calls" is correct only if the growth factor is greater than or equal to 2.

### 136. seekp, seekg setting wrong streams?

Section: 27.7.2.3 [istream.unformatted] Status: CD1 Submitter: Howard Hinnant Opened: 1999-03-06 Last modified: 2015-04-08

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Discussion:

I may be misunderstanding the intent, but should not seekg set only the input stream and seekp set only the output stream? The description seems to say that each should set both input and output streams. If that's really the intent, I withdraw this proposal.

Proposed resolution:

In section 27.6.1.3 change:

    basic_istream<charT,traits>& seekg(pos_type pos);
    Effects: If fail() != true, executes rdbuf()->pubseekpos(pos).

To:

    basic_istream<charT,traits>& seekg(pos_type pos);
    Effects: If fail() != true, executes rdbuf()->pubseekpos(pos, ios_base::in).

In section 27.6.1.3 change:

    basic_istream<charT,traits>& seekg(off_type& off, ios_base::seekdir dir);
    Effects: If fail() != true, executes rdbuf()->pubseekoff(off, dir).

To:

    basic_istream<charT,traits>& seekg(off_type& off, ios_base::seekdir dir);
    Effects: If fail() != true, executes rdbuf()->pubseekoff(off, dir, ios_base::in).

In section 27.6.2.4, paragraph 2 change:

    -2- Effects: If fail() != true, executes rdbuf()->pubseekpos(pos).

To:

    -2- Effects: If fail() != true, executes rdbuf()->pubseekpos(pos, ios_base::out).

In section 27.6.2.4, paragraph 4 change:

    -4- Effects: If fail() != true, executes rdbuf()->pubseekoff(off, dir).

To:

    -4- Effects: If fail() != true, executes rdbuf()->pubseekoff(off, dir, ios_base::out).

[Dublin: Dietmar Kühl thinks this is probably correct, but would like the opinion of more iostream experts before taking action.]

[Tokyo: Reviewed by the LWG. PJP noted that although his docs are incorrect, his implementation already implements the Proposed Resolution.]

[Post-Tokyo: Matt Austern comments:
Is it a problem with basic_istream and basic_ostream, or is it a problem with basic_stringbuf? We could resolve the issue either by changing basic_istream and basic_ostream, or by changing basic_stringbuf. I prefer the latter change (or maybe both changes): I don't see any reason for the standard to require that std::stringbuf s(std::string("foo"), std::ios_base::in); s.pubseekoff(0, std::ios_base::beg); must fail.
This requirement is a bit weird. There's no similar requirement for basic_streambuf<>::seekpos, or for basic_filebuf<>::seekoff or basic_filebuf<>::seekpos.]

### 137. Do use_facet and has_facet look in the global locale?

Section: 22.3.1 [locale] Status: TC1 Submitter: Angelika Langer Opened: 1999-03-17 Last modified: 2015-04-08

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Discussion:

Section 22.3.1 [locale] says:

-4- In the call to use_facet<Facet>(loc), the type argument chooses a facet, making available all members of the named type. If Facet is not present in a locale (or, failing that, in the global locale), it throws the standard exception bad_cast. A C++ program can check if a locale implements a particular facet with the template function has_facet<Facet>().

This contradicts the specification given in section 22.3.2 [locale.global.templates]:
template <class Facet> const Facet& use_facet(const locale& loc);
-1- Get a reference to a facet of a locale.
-2- Returns: a reference to the corresponding facet of loc, if present.
-3- Throws: bad_cast if has_facet<Facet>(loc) is false.
-4- Notes: The reference returned remains valid at least as long as any copy of loc exists

Proposed resolution:

Remove the phrase "(or, failing that, in the global locale)" from section 22.1.1.

Rationale:

Needed for consistency with the way locales are handled elsewhere in the standard.

### 139. Optional sequence operation table description unclear

Section: 23.2.3 [sequence.reqmts] Status: TC1 Submitter: Andrew Koenig Opened: 1999-03-30 Last modified: 2015-04-08

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Discussion:

The sentence introducing the Optional sequence operation table (23.1.1 paragraph 12) has two problems:

A. It says ``The operations in table 68 are provided only for the containers for which they take constant time.''
That could be interpreted in two ways, one of them being ``Even though table 68 shows particular operations as being provided, implementations are free to omit them if they cannot implement them in constant time.''
B. That paragraph says nothing about amortized constant time, and it should.

Proposed resolution:

Replace the wording in 23.1.1 paragraph 12 which begins ``The operations in table 68 are provided only..." with:

> Table 68 lists sequence operations that are provided for some types of sequential containers but not others. An implementation shall provide these operations for all container types shown in the ``container'' column, and shall implement them so as to take amortized constant time.

### 141. basic_string::find_last_of, find_last_not_of say pos instead of xpos

Section: 21.4.6.4 [string::insert], 21.4.6.6 [string::replace] Status: TC1 Submitter: Arch Robison Opened: 1999-04-28 Last modified: 2015-04-08

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Discussion:

Sections 21.3.6.4 paragraph 1 and 21.3.6.6 paragraph 1 surely have misprints where they say:
— `xpos <= pos` and `pos < size();`

Surely the document meant to say ```xpos < size()`'' in both places.

[Judy Ward also sent in this issue for 21.3.6.4 with the same proposed resolution.]

Proposed resolution:

Change Sections 21.3.6.4 paragraph 1 and 21.3.6.6 paragraph 1, the line which says:
— `xpos <= pos` and `pos < size();
` to:
`
` — `xpos <= pos` and `xpos < size();`

### 142. lexicographical_compare complexity wrong

Section: 25.4.8 [alg.lex.comparison] Status: TC1 Submitter: Howard Hinnant Opened: 1999-06-20 Last modified: 2015-04-08

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Discussion:

The lexicographical_compare complexity is specified as:
"At most min((last1 - first1), (last2 - first2)) applications of the corresponding comparison."
The best I can do is twice that expensive.

Nicolai Josuttis comments in lib-6862: You mean, to check for equality you have to check both < and >? Yes, IMO you are right! (and Matt states this complexity in his book)

Proposed resolution:

Change 25.4.8 [alg.lex.comparison] complexity to:

> At most `2*min((last1 - first1), (last2 - first2))` applications of the corresponding comparison.

Change the example at the end of paragraph 3 to read:

> [Example:
> for ( ; first1 != last1 && first2 != last2 ; ++first1, ++first2) {
> if (*first1 < *first2) return true;
> if (*first2 < *first1) return false;
> }
> return first1 == last1 && first2 != last2;
> --end example]

### 144. Deque constructor complexity wrong

Section: 23.3.3.2 [deque.cons] Status: TC1 Submitter: Herb Sutter Opened: 1999-05-09 Last modified: 2015-04-08

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Discussion:

In 23.3.3.2 [deque.cons] paragraph 6, the deque ctor that takes an iterator range appears to have complexity requirements which are incorrect, and which contradict the complexity requirements for insert(). I suspect that the text in question, below, was taken from vector:

> Complexity: If the iterators first and last are forward iterators, bidirectional iterators, or random access iterators the constructor makes only N calls to the copy constructor, and performs no reallocations, where N is last - first.

The word "reallocations" does not really apply to deque. Further, all of the following appears to be spurious:

> It makes at most 2N calls to the copy constructor of T and log N reallocations if they are input iterators.1)

> 1) The complexity is greater in the case of input iterators because each element must be added individually: it is impossible to determine the distance between first abd last before doing the copying.

This makes perfect sense for vector, but not for deque. Why should deque gain an efficiency advantage from knowing in advance the number of elements to insert?

Proposed resolution:

In 23.3.3.2 [deque.cons] paragraph 6, replace the Complexity description, including the footnote, with the following text (which also corrects the "abd" typo):

> Complexity: Makes last - first calls to the copy constructor of T.

### 146. complex<T> Inserter and Extractor need sentries

Section: 26.4.6 [complex.ops] Status: TC1 Submitter: Angelika Langer Opened: 1999-05-12 Last modified: 2015-04-08

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Discussion:

The extractor for complex numbers is specified as:

> template<class T, class charT, class traits>
> basic_istream<charT, traits>&
> operator>>(basic_istream<charT, traits>& is, complex<T>& x);
> Effects: Extracts a complex number x of the form: u, (u), or (u,v), where u is the real part and v is the imaginary part (lib.istream.formatted).
> Requires: The input values be convertible to T. If bad input is encountered, calls is.setstate(ios::failbit) (which may throw ios::failure (lib.iostate.flags).
> Returns: is .

Is it intended that the extractor for complex numbers does not skip whitespace, unlike all other extractors in the standard library do? Shouldn't a sentry be used?
The inserter for complex numbers is specified as:

> template<class T, class charT, class traits>
> basic_ostream<charT, traits>&
> operator<<(basic_ostream<charT, traits>& o, const complex<T>& x);
> Effects: inserts the complex number x onto the stream o as if it were implemented as follows:
> template<class T, class charT, class traits>
> basic_ostream<charT, traits>&
> operator<<(basic_ostream<charT, traits>& o, const complex<T>& x)
> {
> basic_ostringstream<charT, traits> s;
> s.flags(o.flags());
> s.imbue(o.getloc());
> s.precision(o.precision());
> s << '(' << x.real() << "," << x.imag() << ')';
> return o << s.str();
> }

Is it intended that the inserter for complex numbers ignores the field width and does not do any padding? If, with the suggested implementation above, the field width were set in the stream then the opening parentheses would be adjusted, but the rest not, because the field width is reset to zero after each insertion.

I think that both operations should use sentries, for sake of consistency with the other inserters and extractors in the library. Regarding the issue of padding in the inserter, I don't know what the intent was.

Proposed resolution:

After 26.4.6 [complex.ops] paragraph 14 (operator>>), add a Notes clause:

> Notes: This extraction is performed as a series of simpler extractions. Therefore, the skipping of whitespace is specified to be the same for each of the simpler extractions.

Rationale:

For extractors, the note is added to make it clear that skipping whitespace follows an "all-or-none" rule.

For inserters, the LWG believes there is no defect; the standard is correct as written.

### 147. Library Intro refers to global functions that aren't global

Section: 17.6.5.4 [global.functions] Status: TC1 Submitter: Lois Goldthwaite Opened: 1999-06-04 Last modified: 2015-04-08

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Discussion:

The library had many global functions until 17.4.1.1 [lib.contents] paragraph 2 was added:

> All library entities except macros, operator new and operator delete are defined within the namespace std or namespaces nested within namespace std.

It appears "global function" was never updated in the following:

> 17.4.4.3 - Global functions [lib.global.functions]
> -1- It is unspecified whether any global functions in the C++ Standard Library are defined as inline (dcl.fct.spec).
> -2- A call to a global function signature described in Clauses lib.language.support through lib.input.output behaves the same as if the implementation declares no additional global function signatures.*
> [Footnote: A valid C++ program always calls the expected library global function. An implementation may also define additional global functions that would otherwise not be called by a valid C++ program. --- end footnote]
> -3- A global function cannot be declared by the implementation as taking additional default arguments.
> 17.4.4.4 - Member functions [lib.member.functions]
> -2- An implementation can declare additional non-virtual member function signatures within a class:

> > -- by adding arguments with default values to a member function signature; The same latitude does not extend to the implementation of virtual or global functions, however.

Proposed resolution:

Change "global" to "global or non-member" in:

> 17.4.4.3 [lib.global.functions] section title,
> 17.4.4.3 [lib.global.functions] para 1,
> 17.4.4.3 [lib.global.functions] para 2 in 2 places plus 2 places in the footnote,
> 17.4.4.3 [lib.global.functions] para 3,
> 17.4.4.4 [lib.member.functions] para 2

Rationale:

Because operator new and delete are global, the proposed resolution was changed from "non-member" to "global or non-member.

### 148. Functions in the example facet BoolNames should be const

Section: 22.4.8 [facets.examples] Status: TC1 Submitter: Jeremy Siek Opened: 1999-06-03 Last modified: 2015-04-08

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Discussion:

In 22.4.8 [facets.examples] paragraph 13, the do_truename() and do_falsename() functions in the example facet BoolNames should be const. The functions they are overriding in numpunct_byname<char> are const.

Proposed resolution:

In 22.4.8 [facets.examples] paragraph 13, insert "const" in two places:

> `string do_truename() const { return "Oui Oui!"; }
> string do_falsename() const { return "Mais Non!"; }`

### 149. Insert should return iterator to first element inserted

Section: 23.2.3 [sequence.reqmts] Status: C++11 Submitter: Andrew Koenig Opened: 1999-06-28 Last modified: 2015-04-08

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Discussion:

Suppose that c and c1 are sequential containers and i is an iterator that refers to an element of c. Then I can insert a copy of c1's elements into c ahead of element i by executing

>     c.insert(i, c1.begin(), c1.end());

If c is a vector, it is fairly easy for me to find out where the newly inserted elements are, even though i is now invalid:

>     size_t i_loc = i - c.begin();
>     c.insert(i, c1.begin(), c1.end());

and now the first inserted element is at c.begin()+i_loc and one past the last is at c.begin()+i_loc+c1.size().
But what if c is a list? I can still find the location of one past the last inserted element, because i is still valid. To find the location of the first inserted element, though, I must execute something like

>     for (size_t n = c1.size(); n; --n)
>        --i;

because i is now no longer a random-access iterator.
Alternatively, I might write something like

>     bool first = i == c.begin();
>     list<T>::iterator j = i;
>     if (!first) --j;
>     c.insert(i, c1.begin(), c1.end());
>     if (first)
>        j = c.begin();
>     else
>        ++j;

which, although wretched, requires less overhead.
But I think the right solution is to change the definition of insert so that instead of returning void, it returns an iterator that refers to the first element inserted, if any, and otherwise is a copy of its first argument.

[ Summit: ]

> Reopened by Alisdair.

[ Post Summit Alisdair adds: ]

> In addition to the original rationale for C++03, this change also gives a consistent interface for all container insert operations i.e. they all return an iterator to the (first) inserted item.

> Proposed wording provided.

[ 2009-07 Frankfurt ]

> Q: why isn't this change also proposed for associative containers?

> A: The returned iterator wouldn't necessarily point to a contiguous range.

> Moved to Ready.

Proposed resolution:

23.2.3 [sequence.reqmts] Table 83 change return type from `void` to `iterator` for the following rows:

> Table 83 — Sequence container requirements (in addition to container)
> Expression | Return type | Assertion/note pre-/post-condition
> --- | --- | ---
> `a.insert(p,n,t)` | `void iterator` | Inserts `n` copies of `t` before `p`.
> `a.insert(p,i,j)` | `void iterator` | Each iterator in the range `[i,j)` shall be dereferenced exactly once. pre: `i` and `j` are not iterators into `a`. Inserts copies of elements in `[i, j)` before `p`
> `a.insert(p,il)` | `void iterator` | `a.insert(p, il.begin(), il.end())`.

Add after p6 23.2.3 [sequence.reqmts]:

> -6- ...

> The iterator returned from `a.insert(p,n,t)` points to the copy of the first element inserted into `a`, or `p` if `n == 0`.

> The iterator returned from `a.insert(p,i,j)` points to the copy of the first element inserted into `a`, or `p` if `i == j`.

> The iterator returned from `a.insert(p,il)` points to the copy of the first element inserted into `a`, or `p` if `il` is empty.

p2 23.3.3 [deque] Update class definition, change return type from `void` to `iterator`:

>     void iterator insert(const_iterator position, size_type n, const T& x);
>     template <class InputIterator>
>       void iterator insert(const_iterator position, InputIterator first, InputIterator last);
>       void iterator insert(const_iterator position, initializer_list<T>);

23.3.3.4 [deque.modifiers] change return type from `void` to `iterator` on following declarations:

>       void iterator insert(const_iterator position, size_type n, const T& x);
>     template <class InputIterator>
>       void iterator insert(const_iterator position, InputIterator first, InputIterator last);

Add the following (missing) declaration

>     iterator insert(const_iterator position, initializer_list<T>);

23.3.4 [forwardlist] Update class definition, change return type from `void` to `iterator`:

>     void iterator insert_after(const_iterator position, initializer_list<T> il);
>     void iterator insert_after(const_iterator position, size_type n, const T& x);
>     template <class InputIterator>
>       void iterator insert_after(const_iterator position, InputIterator first, InputIterator last);

p8 23.3.4.5 [forwardlist.modifiers] change return type from `void` to `iterator`:

>     void iterator insert_after(const_iterator position, size_type n, const T& x);

Add paragraph:

> Returns: position.

p10 23.3.4.5 [forwardlist.modifiers] change return type from `void` to `iterator`:

>     template <class InputIterator>
>       void iterator insert_after(const_iterator position, InputIterator first, InputIterator last);

Add paragraph:

> Returns: position.

p12 23.3.4.5 [forwardlist.modifiers] change return type from `void` to `iterator` on following declarations:

>     void iterator insert_after(const_iterator position, initializer_list<T> il);

change return type from `void` to `iterator` on following declarations:

p2 23.3.5 [list] Update class definition, change return type from `void` to `iterator`:

>     void iterator insert(const_iterator position, size_type n, const T& x);

>     template <class InputIterator>
>     void iterator insert(const_iterator position, InputIterator first, InputIterator last);

>     void iterator insert(const_iterator position, initializer_list<T>);

23.3.5.4 [list.modifiers] change return type from `void` to `iterator` on following declarations:

>     void iterator insert(const_iterator position, size_type n, const T& x);

>     template <class InputIterator>
>       void iterator insert(const_iterator position, InputIterator first, InputIterator last);

Add the following (missing) declaration

>     iterator insert(const_iterator position, initializer_list<T>);

p2 23.3.6 [vector]

Update class definition, change return type from `void` to `iterator`:

>     void iterator insert(const_iterator position, T&& x);

>     void iterator insert(const_iterator position, size_type n, const T& x);

>     template <class InputIterator>
>       void iterator insert(const_iterator position, InputIterator first, InputIterator last);

>     void iterator insert(const_iterator position, initializer_list<T>);

23.3.6.5 [vector.modifiers] change return type from `void` to `iterator` on following declarations:

>     void iterator insert(const_iterator position, size_type n, const T& x);

>     template <class InputIterator>
>       void iterator insert(const_iterator position, InputIterator first, InputIterator last);

Add the following (missing) declaration

>     iterator insert(const_iterator position, initializer_list<T>);

p1 23.3.7 [vector.bool] Update class definition, change return type from `void` to `iterator`:

>     void iterator insert (const_iterator position, size_type n, const bool& x);

>     template <class InputIterator>
>       void iterator insert(const_iterator position, InputIterator first, InputIterator last);

>       void iterator insert(const_iterator position, initializer_list<bool> il);

p5 21.4 [basic.string] Update class definition, change return type from `void` to `iterator`:

>     void iterator insert(const_iterator p, size_type n, charT c);

>     template<class InputIterator>
>       void iterator insert(const_iterator p, InputIterator first, InputIterator last);

>     void iterator insert(const_iterator p, initializer_list<charT>);

p13 21.4.6.4 [string::insert] change return type from `void` to `iterator`:

>     void iterator insert(const_iterator p, size_type n, charT c);

Add paragraph:

> Returns: an iterator which refers to the copy of the first inserted character, or `p` if `n == 0`.

p15 21.4.6.4 [string::insert] change return type from `void` to `iterator`:

>     template<class InputIterator>
>       void iterator insert(const_iterator p, InputIterator first, InputIterator last);

Add paragraph:

> Returns: an iterator which refers to the copy of the first inserted character, or `p` if `first == last`.

p17 21.4.6.4 [string::insert] change return type from `void` to `iterator`:

>     void iterator insert(const_iterator p, initializer_list<charT> il);

Add paragraph:

> Returns: an iterator which refers to the copy of the first inserted character, or `p` if `il` is empty.

Rationale:

[ The following was the C++98/03 rationale and does not necessarily apply to the proposed resolution in the C++0X time frame: ]

> The LWG believes this was an intentional design decision and so is not a defect. It may be worth revisiting for the next standard.

### 150. Find_first_of says integer instead of iterator

Section: 25.2.7 [alg.find.first.of] Status: TC1 Submitter: Matt McClure Opened: 1999-06-30 Last modified: 2015-04-08

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Discussion:

Proposed resolution:

Change 25.2.7 [alg.find.first.of] paragraph 2 from:

> Returns: The first iterator i in the range [first1, last1) such that for some integer j in the range [first2, last2) ...

to:

> Returns: The first iterator i in the range [first1, last1) such that for some iterator j in the range [first2, last2) ...

### 151. Can't currently clear() empty container

Section: 23.2.3 [sequence.reqmts] Status: TC1 Submitter: Ed Brey Opened: 1999-06-21 Last modified: 2015-04-08

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Discussion:

For both sequences and associative containers, a.clear() has the semantics of erase(a.begin(),a.end()), which is undefined for an empty container since erase(q1,q2) requires that q1 be dereferenceable (23.1.1,3 and 23.1.2,7). When the container is empty, a.begin() is not dereferenceable.
The requirement that q1 be unconditionally dereferenceable causes many operations to be intuitively undefined, of which clearing an empty container is probably the most dire.
Since q1 and q2 are only referenced in the range [q1, q2), and [q1, q2) is required to be a valid range, stating that q1 and q2 must be iterators or certain kinds of iterators is unnecessary.

Proposed resolution:

In 23.1.1, paragraph 3, change:

> p and q2 denote valid iterators to a, q and q1 denote valid dereferenceable iterators to a, [q1, q2) denotes a valid range

to:

> p denotes a valid iterator to a, q denotes a valid dereferenceable iterator to a, [q1, q2) denotes a valid range in a

In 23.1.2, paragraph 7, change:

> p and q2 are valid iterators to a, q and q1 are valid dereferenceable iterators to a, [q1, q2) is a valid range

to

> p is a valid iterator to a, q is a valid dereferenceable iterator to a, [q1, q2) is a valid range into a

### 152. Typo in `scan_is()` semantics

Section: 22.4.1.1.2 [locale.ctype.virtuals] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2015-04-08

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Discussion:

The semantics of `scan_is()` (paragraphs 4 and 6) is not exactly described because there is no function `is()` which only takes a character as argument. Also, in the effects clause (paragraph 3), the semantic is also kept vague.

Proposed resolution:

In 22.4.1.1.2 [locale.ctype.virtuals] paragraphs 4 and 6, change the returns clause from:

> "... such that `is(*p)` would..."

to: "... such that `is(m, *p)` would...."

### 153. Typo in `narrow()` semantics

Section: 22.4.1.3.2 [facet.ctype.char.members] Status: CD1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2015-04-08

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Duplicate of: 207

Discussion:

The description of the array version of `narrow()` (in paragraph 11) is flawed: There is no member `do_narrow()` which takes only three arguments because in addition to the range a default character is needed.

Additionally, for both `widen` and `narrow` we have two signatures followed by a Returns clause that only addresses one of them.

Proposed resolution:

Change the returns clause in 22.4.1.3.2 [facet.ctype.char.members] paragraph 10 from:

Returns: do_widen(low, high, to).

to:

Returns: do_widen(c) or do_widen(low, high, to), respectively.

Change 22.4.1.3.2 [facet.ctype.char.members] paragraph 10 and 11 from:

            char        narrow(char c, char /*dfault*/) const;
            const char* narrow(const char* low, const char* high,
                               char /*dfault*/, char* to) const;

            Returns: do_narrow(low, high, to).

to:

            char        narrow(char c, char dfault) const;
            const char* narrow(const char* low, const char* high,
                               char dfault, char* to) const;

            Returns: do_narrow(c, dfault) or
                     do_narrow(low, high, dfault, to), respectively.

[Kona: 1) the problem occurs in additional places, 2) a user defined version could be different.]

[Post-Tokyo: Dietmar provided the above wording at the request of the LWG. He could find no other places the problem occurred. He asks for clarification of the Kona "a user defined version..." comment above. Perhaps it was a circuitous way of saying "dfault" needed to be uncommented?]

[Post-Toronto: the issues list maintainer has merged in the proposed resolution from issue 207, which addresses the same paragraphs.]

### 154. Missing `double` specifier for `do_get()`

Section: 22.4.2.1.2 [facet.num.get.virtuals] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2015-04-08

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Discussion:

The table in paragraph 7 for the length modifier does not list the length modifier `l` to be applied if the type is `double`. Thus, the standard asks the implementation to do undefined things when using `scanf()` (the missing length modifier for `scanf()` when scanning `double`s is actually a problem I found quite often in production code, too).

Proposed resolution:

In 22.4.2.1.2 [facet.num.get.virtuals], paragraph 7, add a row in the Length Modifier table to say that for `double` a length modifier `l` is to be used.

Rationale:

The standard makes an embarrassing beginner's mistake.

### 155. Typo in naming the class defining the class `Init`

Section: 27.4 [iostream.objects] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2015-04-08

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Discussion:

There are conflicting statements about where the class `Init` is defined. According to 27.4 [iostream.objects] paragraph 2 it is defined as `basic_ios::Init`, according to 27.5.3 [ios.base] it is defined as `ios_base::Init`.

Proposed resolution:

Change 27.4 [iostream.objects] paragraph 2 from "`basic_ios::Init"` to "`ios_base::Init"`.

Rationale:

Although not strictly wrong, the standard was misleading enough to warrant the change.

### 156. Typo in `imbue()` description

Section: 27.5.3.3 [ios.base.locales] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2015-04-08

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Discussion:

There is a small discrepancy between the declarations of `imbue()`: in 27.5.3 [ios.base] the argument is passed as `locale const&` (correct), in 27.5.3.3 [ios.base.locales] it is passed as `locale const` (wrong).

Proposed resolution:

In 27.5.3.3 [ios.base.locales] change the `imbue` argument from "`locale const" to "locale const&".`

### 158. Underspecified semantics for `setbuf()`

Section: 27.6.3.4.2 [streambuf.virt.buffer] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2015-04-08

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Discussion:

The default behavior of `setbuf()` is described only for the situation that `gptr() != 0 && gptr() != egptr()`: namely to do nothing. What has to be done in other situations is not described although there is actually only one reasonable approach, namely to do nothing, too.

Since changing the buffer would almost certainly mess up most buffer management of derived classes unless these classes do it themselves, the default behavior of `setbuf()` should always be to do nothing.

Proposed resolution:

Change 27.6.3.4.2 [streambuf.virt.buffer], paragraph 3, Default behavior, to: "Default behavior: Does nothing. Returns this."

### 159. Strange use of `underflow()`

Section: 27.6.3.4.3 [streambuf.virt.get] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2015-04-08

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Discussion:

The description of the meaning of the result of `showmanyc()` seems to be rather strange: It uses calls to `underflow()`. Using `underflow()` is strange because this function only reads the current character but does not extract it, `uflow()` would extract the current character. This should be fixed to use `sbumpc()` instead.

Proposed resolution:

Change 27.6.3.4.3 [streambuf.virt.get] paragraph 1, `showmanyc()`returns clause, by replacing the word "supplied" with the words "extracted from the stream".

### 160. Typo: Use of non-existing function `exception()`

Section: 27.7.2.1 [istream] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2015-04-08

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Discussion:

The paragraph 4 refers to the function `exception()` which is not defined. Probably, the referred function is `basic_ios<>::exceptions()`.

Proposed resolution:

In 27.7.2.1 [istream], 27.7.2.3 [istream.unformatted], paragraph 1, 27.7.3.1 [ostream], paragraph 3, and 27.7.3.6.1 [ostream.formatted.reqmts], paragraph 1, change "`exception()" to "exceptions()"`.

[Note to Editor: "exceptions" with an "s" is the correct spelling.]

### 161. Typo: `istream_iterator` vs. `istreambuf_iterator`

Section: 27.7.2.2.2 [istream.formatted.arithmetic] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2015-04-08

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Discussion:

The note in the second paragraph pretends that the first argument is an object of type `istream_iterator`. This is wrong: It is an object of type `istreambuf_iterator`.

Proposed resolution:

Change 27.7.2.2.2 [istream.formatted.arithmetic] from:

> The first argument provides an object of the istream_iterator class...

to

> The first argument provides an object of the istreambuf_iterator class...

### 164. do_put() has apparently unused fill argument

Section: 22.4.5.3.2 [locale.time.put.virtuals] Status: TC1 Submitter: Angelika Langer Opened: 1999-07-23 Last modified: 2015-04-08

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Discussion:

In 22.4.5.3.2 [locale.time.put.virtuals] the do_put() function is specified as taking a fill character as an argument, but the description of the function does not say whether the character is used at all and, if so, in which way. The same holds for any format control parameters that are accessible through the ios_base& argument, such as the adjustment or the field width. Is strftime() supposed to use the fill character in any way? In any case, the specification of time_put.do_put() looks inconsistent to me.
Is the signature of do_put() wrong, or is the effects clause incomplete?

Proposed resolution:

Add the following note after 22.4.5.3.2 [locale.time.put.virtuals] paragraph 2:

> [Note: the `fill` argument may be used in the implementation-defined formats, or by derivations. A space character is a reasonable default for this argument. --end Note]

Rationale:

The LWG felt that while the normative text was correct, users need some guidance on what to pass for the `fill` argument since the standard doesn't say how it's used.

### 165. `xsputn()`, `pubsync()` never called by `basic_ostream` members?

Section: 27.7.3.1 [ostream] Status: CD1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2015-04-08

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Discussion:

Paragraph 2 explicitly states that none of the `basic_ostream` functions falling into one of the groups "formatted output functions" and "unformatted output functions" calls any stream buffer function which might call a virtual function other than `overflow()`. Basically this is fine but this implies that `sputn()` (this function would call the virtual function `xsputn()`) is never called by any of the standard output functions. Is this really intended? At minimum it would be convenient to call `xsputn()` for strings... Also, the statement that `overflow()` is the only virtual member of `basic_streambuf` called is in conflict with the definition of `flush()` which calls `rdbuf()->pubsync()` and thereby the virtual function `sync()` (`flush()` is listed under "unformatted output functions").

In addition, I guess that the sentence starting with "They may use other public members of `basic_ostream` ..." probably was intended to start with "They may use other public members of `basic_streamuf`..." although the problem with the virtual members exists in both cases.

I see two obvious resolutions:

1. state in a footnote that this means that `xsputn()` will never be called by any ostream member and that this is intended.
2. relax the restriction and allow calling `overflow()` and `xsputn()`. Of course, the problem with `flush()` has to be resolved in some way.

Proposed resolution:

Change the last sentence of 27.6.2.1 (lib.ostream) paragraph 2 from:

> They may use other public members of basic_ostream except that they do not invoke any virtual members of rdbuf() except overflow().

to:

> They may use other public members of basic_ostream except that they shall not invoke any virtual members of rdbuf() except overflow(), xsputn(), and sync().

[Kona: the LWG believes this is a problem. Wish to ask Jerry or PJP why the standard is written this way.]

[Post-Tokyo: Dietmar supplied wording at the request of the LWG. He comments: The rules can be made a little bit more specific if necessary be explicitly spelling out what virtuals are allowed to be called from what functions and eg to state specifically that flush() is allowed to call sync() while other functions are not.]

### 167. Improper use of `traits_type::length()`

Section: 27.7.3.6.4 [ostream.inserters.character] Status: CD1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2015-04-08

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Discussion:

Paragraph 4 states that the length is determined using `traits::length(s)`. Unfortunately, this function is not defined for example if the character type is `wchar_t` and the type of `s` is `char const*`. Similar problems exist if the character type is `char` and the type of `s` is either `signed char const*` or `unsigned char const*`.

Proposed resolution:

Change 27.7.3.6.4 [ostream.inserters.character] paragraph 4 from:

> Effects: Behaves like an formatted inserter (as described in lib.ostream.formatted.reqmts) of out. After a sentry object is constructed it inserts characters. The number of characters starting at s to be inserted is traits::length(s). Padding is determined as described in lib.facet.num.put.virtuals. The traits::length(s) characters starting at s are widened using out.widen (lib.basic.ios.members). The widened characters and any required padding are inserted into out. Calls width(0).

to:

> Effects: Behaves like a formatted inserter (as described in lib.ostream.formatted.reqmts) of out. After a sentry object is constructed it inserts n characters starting at s, where n is the number that would be computed as if by:

> - traits::length(s) for the overload where the first argument is of type basic_ostream<charT, traits>& and the second is of type const charT*, and also for the overload where the first argument is of type basic_ostream<char, traits>& and the second is of type const char*.
> - std::char_traits<char>::length(s) for the overload where the first argument is of type basic_ostream<charT, traits>& and the second is of type const char*.
> - traits::length(reinterpret_cast<const char*>(s)) for the other two overloads.

> Padding is determined as described in lib.facet.num.put.virtuals. The n characters starting at s are widened using out.widen (lib.basic.ios.members). The widened characters and any required padding are inserted into out. Calls width(0).

[Santa Cruz: Matt supplied new wording]

[Kona: changed "where n is" to " where n is the number that would be computed as if by"]

Rationale:

We have five separate cases. In two of them we can use the user-supplied traits class without any fuss. In the other three we try to use something as close to that user-supplied class as possible. In two cases we've got a traits class that's appropriate for char and what we've got is a const signed char* or a const unsigned char*; that's close enough so we can just use a reinterpret cast, and continue to use the user-supplied traits class. Finally, there's one case where we just have to give up: where we've got a traits class for some arbitrary charT type, and we somehow have to deal with a const char*. There's nothing better to do but fall back to char_traits<char>

### 168. Typo: formatted vs. unformatted

Section: 27.7.3.7 [ostream.unformatted] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2015-04-08

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Discussion:

The first paragraph begins with a descriptions what has to be done in formatted output functions. Probably this is a typo and the paragraph really want to describe unformatted output functions...

Proposed resolution:

In 27.7.3.7 [ostream.unformatted] paragraph 1, the first and last sentences, change the word "formatted" to "unformatted":

> "Each unformatted output function begins ..."
> "... value specified for the unformatted output function."

### 169. Bad efficiency of `overflow()` mandated

Section: 27.8.2.4 [stringbuf.virtuals] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2015-04-08

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Discussion:

Paragraph 8, Notes, of this section seems to mandate an extremely inefficient way of buffer handling for `basic_stringbuf`, especially in view of the restriction that `basic_ostream` member functions are not allowed to use `xsputn()` (see 27.7.3.1 [ostream]): For each character to be inserted, a new buffer is to be created.

Of course, the resolution below requires some handling of simultaneous input and output since it is no longer possible to update `egptr()` whenever `epptr()` is changed. A possible solution is to handle this in `underflow()`.

Proposed resolution:

In 27.8.2.4 [stringbuf.virtuals] paragraph 8, Notes, insert the words "at least" as in the following:

> To make a write position available, the function reallocates (or initially allocates) an array object with a sufficient number of elements to hold the current array object (if any), plus at least one additional write position.

### 170. Inconsistent definition of `traits_type`

Section: 27.8.5 [stringstream] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2015-04-08

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Discussion:

The classes `basic_stringstream` (27.8.5 [stringstream]), `basic_istringstream` (27.8.3 [istringstream]), and `basic_ostringstream` (27.8.4 [ostringstream]) are inconsistent in their definition of the type `traits_type`: For `istringstream`, this type is defined, for the other two it is not. This should be consistent.

Proposed resolution:

Proposed resolution:

To the declarations of `basic_ostringstream` (27.8.4 [ostringstream]) and `basic_stringstream` (27.8.5 [stringstream]) add:

>     typedef traits traits_type;

### 171. Strange `seekpos()` semantics due to joint position

Section: 27.9.1.5 [filebuf.virtuals] Status: CD1 Submitter: Dietmar Kühl Opened: 1999-07-20 Last modified: 2015-04-08

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Discussion:

Overridden virtual functions, seekpos()

In 27.9.1.1 [filebuf] paragraph 3, it is stated that a joint input and output position is maintained by `basic_filebuf`. Still, the description of `seekpos()` seems to talk about different file positions. In particular, it is unclear (at least to me) what is supposed to happen to the output buffer (if there is one) if only the input position is changed. The standard seems to mandate that the output buffer is kept and processed as if there was no positioning of the output position (by changing the input position). Of course, this can be exactly what you want if the flag `ios_base::ate` is set. However, I think, the standard should say something like this:

- If `(which & mode) == 0` neither read nor write position is changed and the call fails. Otherwise, the joint read and write position is altered to correspond to `sp`.
- If there is an output buffer, the output sequences is updated and any unshift sequence is written before the position is altered.
- If there is an input buffer, the input sequence is updated after the position is altered.

Plus the appropriate error handling, that is...

Proposed resolution:

Change the unnumbered paragraph in 27.8.1.4 (lib.filebuf.virtuals) before paragraph 14 from:

> pos_type seekpos(pos_type sp, ios_base::openmode = ios_base::in | ios_base::out);

> Alters the file position, if possible, to correspond to the position stored in sp (as described below).

> - if (which&ios_base::in)!=0, set the file position to sp, then update the input sequence

> - if (which&ios_base::out)!=0, then update the output sequence, write any unshift sequence, and set the file position to sp.

to:

> pos_type seekpos(pos_type sp, ios_base::openmode = ios_base::in | ios_base::out);

> Alters the file position, if possible, to correspond to the position stored in sp (as described below). Altering the file position performs as follows:

> 1. if (om & ios_base::out)!=0, then update the output sequence and write any unshift sequence;

> 2. set the file position to sp;

> 3. if (om & ios_base::in)!=0, then update the input sequence;

> where om is the open mode passed to the last call to open(). The operation fails if is_open() returns false.

[Kona: Dietmar is working on a proposed resolution.]

[Post-Tokyo: Dietmar supplied the above wording.]

### 172. Inconsistent types for `basic_istream::ignore()`

Section: 27.7.2.3 [istream.unformatted] Status: TC1 Submitter: Greg Comeau, Dietmar Kühl Opened: 1999-07-23 Last modified: 2015-04-08

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Discussion:

In 27.7.2.1 [istream] the function `ignore()` gets an object of type `streamsize` as first argument. However, in 27.7.2.3 [istream.unformatted] paragraph 23 the first argument is of type `int.`

As far as I can see this is not really a contradiction because everything is consistent if `streamsize` is typedef to be `int`. However, this is almost certainly not what was intended. The same thing happened to `basic_filebuf::setbuf()`, as described in issue 173.

Darin Adler also submitted this issue, commenting: Either 27.6.1.1 should be modified to show a first parameter of type int, or 27.6.1.3 should be modified to show a first parameter of type streamsize and use numeric_limits<streamsize>::max.

Proposed resolution:

In 27.7.2.3 [istream.unformatted] paragraph 23 and 24, change both uses of `int` in the description of `ignore()` to `streamsize`.

### 173. Inconsistent types for `basic_filebuf::setbuf()`

Section: 27.9.1.5 [filebuf.virtuals] Status: TC1 Submitter: Greg Comeau, Dietmar Kühl Opened: 1999-07-23 Last modified: 2015-04-08

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Discussion:

In 27.9.1.1 [filebuf] the function `setbuf()` gets an object of type `streamsize` as second argument. However, in 27.9.1.5 [filebuf.virtuals] paragraph 9 the second argument is of type `int`.

As far as I can see this is not really a contradiction because everything is consistent if `streamsize` is typedef to be `int`. However, this is almost certainly not what was intended. The same thing happened to `basic_istream::ignore()`, as described in issue 172.

Proposed resolution:

In 27.9.1.5 [filebuf.virtuals] paragraph 9, change all uses of `int` in the description of `setbuf()` to `streamsize`.

### 174. Typo: `OFF_T` vs. `POS_T`

Section: D.6 [depr.ios.members] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-23 Last modified: 2015-04-08

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Discussion:

According to paragraph 1 of this section, `streampos` is the type `OFF_T`, the same type as `streamoff`. However, in paragraph 6 the `streampos` gets the type `POS_T`

Proposed resolution:

Change D.6 [depr.ios.members] paragraph 1 from "`typedef OFF_T streampos;`" to "`typedef POS_T streampos;`"

### 175. Ambiguity for `basic_streambuf::pubseekpos()` and a few other functions.

Section: D.6 [depr.ios.members] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-23 Last modified: 2015-04-08

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Discussion:

According to paragraph 8 of this section, the methods `basic_streambuf::pubseekpos()`, `basic_ifstream::open()`, and `basic_ofstream::open` "may" be overloaded by a version of this function taking the type `ios_base::open_mode` as last argument argument instead of `ios_base::openmode` (`ios_base::open_mode` is defined in this section to be an alias for one of the integral types). The clause specifies, that the last argument has a default argument in three cases. However, this generates an ambiguity with the overloaded version because now the arguments are absolutely identical if the last argument is not specified.

Proposed resolution:

In D.6 [depr.ios.members] paragraph 8, remove the default arguments for `basic_streambuf::pubseekpos()`, `basic_ifstream::open()`, and `basic_ofstream::open().`

### 176. `exceptions()` in `ios_base`...?

Section: D.6 [depr.ios.members] Status: TC1 Submitter: Dietmar Kühl Opened: 1999-07-23 Last modified: 2015-04-08

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Discussion:

The "overload" for the function `exceptions()` in paragraph 8 gives the impression that there is another function of this function defined in class `ios_base`. However, this is not the case. Thus, it is hard to tell how the semantics (paragraph 9) can be implemented: "Call the corresponding member function specified in clause 27 [input.output]."

Proposed resolution:

In D.6 [depr.ios.members] paragraph 8, move the declaration of the function `exceptions()`into class `basic_ios`.

### 179. Comparison of const_iterators to iterators doesn't work

Section: 23.2 [container.requirements] Status: CD1 Submitter: Judy Ward Opened: 1998-07-02 Last modified: 2015-04-08

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Discussion:

Currently the following will not compile on two well-known standard library implementations:

>     #include <set>
>     using namespace std;

>     void f(const set<int> &s)
>     {
>       set<int>::iterator i;
>       if (i==s.end()); // s.end() returns a const_iterator
>     }

The reason this doesn't compile is because operator== was implemented as a member function of the nested classes set:iterator and set::const_iterator, and there is no conversion from const_iterator to iterator. Surprisingly, (s.end() == i) does work, though, because of the conversion from iterator to const_iterator.

I don't see a requirement anywhere in the standard that this must work. Should there be one? If so, I think the requirement would need to be added to the tables in section 24.1.1. I'm not sure about the wording. If this requirement existed in the standard, I would think that implementors would have to make the comparison operators non-member functions.

This issues was also raised on comp.std.c++ by Darin Adler. The example given was:

>     bool check_equal(std::deque<int>::iterator i,
>     std::deque<int>::const_iterator ci)
>     {
>     return i == ci;
>     }

Comment from John Potter:

> In case nobody has noticed, accepting it will break reverse_iterator.

> The fix is to make the comparison operators templated on two types.

>         template <class Iterator1, class Iterator2>
>         bool operator== (reverse_iterator<Iterator1> const& x,
>                          reverse_iterator<Iterator2> const& y);

> Obviously: return x.base() == y.base();

> Currently, no reverse_iterator to const_reverse_iterator compares are valid.

> BTW, I think the issue is in support of bad code. Compares should be between two iterators of the same type. All std::algorithms require the begin and end iterators to be of the same type.

Proposed resolution:

Insert this paragraph after 23.2 [container.requirements] paragraph 7:

> In the expressions

>         i == j
>         i != j
>         i < j
>         i <= j
>         i >= j
>         i > j
>         i - j

> Where i and j denote objects of a container's iterator type, either or both may be replaced by an object of the container's const_iterator type referring to the same element with no change in semantics.

[post-Toronto: Judy supplied a proposed resolution saying that `iterator` and `const_iterator` could be freely mixed in iterator comparison and difference operations.]

[Redmond: Dave and Howard supplied a new proposed resolution which explicitly listed expressions; there was concern that the previous proposed resolution was too informal.]

Rationale:

The LWG believes it is clear that the above wording applies only to the nested types `X::iterator` and `X::const_iterator`, where `X` is a container. There is no requirement that `X::reverse_iterator` and `X::const_reverse_iterator` can be mixed. If mixing them is considered important, that's a separate issue. (Issue 280.)

### 180. Container member iterator arguments constness has unintended consequences

Section: 21.4 [basic.string] Status: CD1 Submitter: Dave Abrahams Opened: 1999-07-01 Last modified: 2015-04-08

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Discussion:

It is the constness of the container which should control whether it can be modified through a member function such as erase(), not the constness of the iterators. The iterators only serve to give positioning information.

Here's a simple and typical example problem which is currently very difficult or impossible to solve without the change proposed below.

Wrap a standard container C in a class W which allows clients to find and read (but not modify) a subrange of (C.begin(), C.end()]. The only modification clients are allowed to make to elements in this subrange is to erase them from C through the use of a member function of W.

[ post Bellevue, Alisdair adds: ]

> This issue was implemented by N2350 for everything but `basic_string`.

> Note that the specific example in this issue (`basic_string`) is the one place we forgot to amend in N2350, so we might open this issue for that single container?

[ Sophia Antipolis: ]

> This was a fix that was intended for all standard library containers, and has been done for other containers, but string was missed.

> The wording updated.

> We did not make the change in `replace`, because this change would affect the implementation because the string may be written into. This is an issue that should be taken up by concepts.

> We note that the supplied wording addresses the initializer list provided in N2679.

Proposed resolution:

Update the following signature in the `basic_string` class template definition in 21.4 [basic.string], p5:

>     namespace std {
>       template<class charT, class traits = char_traits<charT>,
>         class Allocator = allocator<charT> >
>       class basic_string {

>         ...

>         iterator insert(const_iterator p, charT c);
>         void insert(const_iterator p, size_type n, charT c);
>         template<class InputIterator>
>           void insert(const_iterator p, InputIterator first, InputIterator last);
>         void insert(const_iterator p, initializer_list<charT>);

>         ...

>         iterator erase(const_iterator const_position);
>         iterator erase(const_iterator first, const_iterator last);

>         ...

>       };
>     }

Update the following signatures in 21.4.6.4 [string::insert]:

>     iterator insert(const_iterator p, charT c);
>     void insert(const_iterator p, size_type n, charT c);
>     template<class InputIterator>
>       void insert(const_iterator p, InputIterator first, InputIterator last);
>     void insert(const_iterator p, initializer_list<charT>);

Update the following signatures in 21.4.6.5 [string::erase]:

>     iterator erase(const_iterator const_position);
>     iterator erase(const_iterator first, const_iterator last);

Rationale:

The issue was discussed at length. It was generally agreed that 1) There is no major technical argument against the change (although there is a minor argument that some obscure programs may break), and 2) Such a change would not break const correctness. The concerns about making the change were 1) it is user detectable (although only in boundary cases), 2) it changes a large number of signatures, and 3) it seems more of a design issue that an out-and-out defect.

The LWG believes that this issue should be considered as part of a general review of const issues for the next revision of the standard. Also see issue 200.

### 181. make_pair() unintended behavior

Section: 20.3 [pairs] Status: TC1 Submitter: Andrew Koenig Opened: 1999-08-03 Last modified: 2015-04-08

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Discussion:

The claim has surfaced in Usenet that expressions such as
`make_pair("abc", 3)`
are illegal, notwithstanding their use in examples, because template instantiation tries to bind the first template parameter to `const char (&)[4]`, which type is uncopyable.
I doubt anyone intended that behavior...

Proposed resolution:

In 20.2 [utility], paragraph 1 change the following declaration of make_pair():

>     template <class T1, class T2> pair<T1,T2> make_pair(const T1&, const T2&);

to:

>     template <class T1, class T2> pair<T1,T2> make_pair(T1, T2);

In 20.3 [pairs] paragraph 7 and the line before, change:

>     template <class T1, class T2>
>     pair<T1, T2> make_pair(const T1& x, const T2& y);

to:

>     template <class T1, class T2>
>     pair<T1, T2> make_pair(T1 x, T2 y);

and add the following footnote to the effects clause:

> According to 12.8 [class.copy], an implementation is permitted to not perform a copy of an argument, thus avoiding unnecessary copies.

Rationale:

Two potential fixes were suggested by Matt Austern and Dietmar Kühl, respectively, 1) overloading with array arguments, and 2) use of a reference_traits class with a specialization for arrays. Andy Koenig suggested changing to pass by value. In discussion, it appeared that this was a much smaller change to the standard that the other two suggestions, and any efficiency concerns were more than offset by the advantages of the solution. Two implementors reported that the proposed resolution passed their test suites.

### 182. Ambiguous references to size_t

Section: 17 [library] Status: CD1 Submitter: Al Stevens Opened: 1999-08-15 Last modified: 2015-04-08

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Discussion:

Many references to `size_t` throughout the document omit the `std::` namespace qualification.

For example, 17.6.4.6 [replacement.functions] paragraph 2:

>      operator new(size_t)
>      operator new(size_t, const std::nothrow_t&)
>      operator new[](size_t)
>      operator new[](size_t, const std::nothrow_t&)

Proposed resolution:

In 17.6.4.6 [replacement.functions] paragraph 2: replace:

> `- operator new(size_t)
> - operator new(size_t, const std::nothrow_t&)
> - operator new[](size_t)
> - operator new[](size_t, const std::nothrow_t&)`

by:

>     - operator new(std::size_t)
>     - operator new(std::size_t, const std::nothrow_t&)
>     - operator new[](std::size_t)
>     - operator new[](std::size_t, const std::nothrow_t&)

In [lib.allocator.requirements] 20.1.5, paragraph 4: replace:

> The typedef members pointer, const_pointer, size_type, and difference_type are required to be T*, T const*, size_t, and ptrdiff_t, respectively.

by:

> The typedef members pointer, const_pointer, size_type, and difference_type are required to be T*, T const*, std::size_t, and std::ptrdiff_t, respectively.

In [lib.allocator.members] 20.4.1.1, paragraphs 3 and 6: replace:

> 3 Notes: Uses ::operator new(size_t) (18.4.1).

> 6 Note: the storage is obtained by calling ::operator new(size_t), but it is unspecified when or how often this function is called. The use of hint is unspecified, but intended as an aid to locality if an implementation so desires.

by:

> 3 Notes: Uses ::operator new(std::size_t) (18.4.1).

> 6 Note: the storage is obtained by calling ::operator new(std::size_t), but it is unspecified when or how often this function is called. The use of hint is unspecified, but intended as an aid to locality if an implementation so desires.

In [lib.char.traits.require] 21.1.1, paragraph 1: replace:

> In Table 37, X denotes a Traits class defining types and functions for the character container type CharT; c and d denote values of type CharT; p and q denote values of type const CharT*; s denotes a value of type CharT*; n, i and j denote values of type size_t; e and f denote values of type X::int_type; pos denotes a value of type X::pos_type; and state denotes a value of type X::state_type.

by:

> In Table 37, X denotes a Traits class defining types and functions for the character container type CharT; c and d denote values of type CharT; p and q denote values of type const CharT*; s denotes a value of type CharT*; n, i and j denote values of type std::size_t; e and f denote values of type X::int_type; pos denotes a value of type X::pos_type; and state denotes a value of type X::state_type.

In [lib.char.traits.require] 21.1.1, table 37: replace the return type of X::length(p): "size_t" by "std::size_t".

In [lib.std.iterator.tags] 24.3.3, paragraph 2: replace:
typedef ptrdiff_t difference_type;
by:
typedef std::ptrdiff_t difference_type;

In [lib.locale.ctype] 22.2.1.1 put namespace std { ...} around the declaration of template <class charT> class ctype.
In [lib.iterator.traits] 24.3.1, paragraph 2 put namespace std { ...} around the declaration of:
template<class Iterator> struct iterator_traits
template<class T> struct iterator_traits<T*>
template<class T> struct iterator_traits<const T*>

Rationale:

The LWG believes correcting names like `size_t` and `ptrdiff_t` to `std::size_t` and `std::ptrdiff_t` to be essentially editorial. There there can't be another size_t or ptrdiff_t meant anyway because, according to 17.6.4.3.3 [extern.types],

> For each type T from the Standard C library, the types ::T and std::T are reserved to the implementation and, when defined, ::T shall be identical to std::T.

The issue is treated as a Defect Report to make explicit the Project Editor's authority to make this change.

[Post-Tokyo: Nico Josuttis provided the above wording at the request of the LWG.]

[Toronto: This is tangentially related to issue 229, but only tangentially: the intent of this issue is to address use of the name `size_t` in contexts outside of namespace std, such as in the description of `::operator new`. The proposed changes should be reviewed to make sure they are correct.]

[pre-Copenhagen: Nico has reviewed the changes and believes them to be correct.]

### 183. I/O stream manipulators don't work for wide character streams

Section: 27.7.4 [std.manip] Status: CD1 Submitter: Andy Sawyer Opened: 1999-07-07 Last modified: 2015-04-08

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Discussion:

27.7.4 [std.manip] paragraph 3 says (clause numbering added for exposition):

> Returns: An object s of unspecified type such that if [1] out is an (instance of) basic_ostream then the expression out<<s behaves as if f(s) were called, and if [2] in is an (instance of) basic_istream then the expression in>>s behaves as if f(s) were called. Where f can be defined as: ios_base& f(ios_base& str, ios_base::fmtflags mask) { // reset specified flags str.setf(ios_base::fmtflags(0), mask); return str; } [3] The expression out<<s has type ostream& and value out. [4] The expression in>>s has type istream& and value in.

Given the definitions [1] and [2] for out and in, surely [3] should read: "The expression out << s has type basic_ostream& ..." and [4] should read: "The expression in >> s has type basic_istream& ..."

If the wording in the standard is correct, I can see no way of implementing any of the manipulators so that they will work with wide character streams.

e.g. wcout << setbase( 16 );

Must have value 'wcout' (which makes sense) and type 'ostream&' (which doesn't).

The same "cut'n'paste" type also seems to occur in Paras 4,5,7 and 8. In addition, Para 6 [setfill] has a similar error, but relates only to ostreams.

I'd be happier if there was a better way of saying this, to make it clear that the value of the expression is "the same specialization of basic_ostream as out"&

Proposed resolution:

Replace section 27.7.4 [std.manip] except paragraph 1 with the following:

> 2- The type designated smanip in each of the following function descriptions is implementation-specified and may be different for each function.
> `smanip resetiosflags(ios_base::fmtflags mask);`
> -3- Returns: An object s of unspecified type such that if out is an instance of basic_ostream<charT,traits> then the expression out<<s behaves as if f(s, mask) were called, or if in is an instance of basic_istream<charT,traits> then the expression in>>s behaves as if f(s, mask) were called. The function f can be defined as:*
> [Footnote: The expression cin >> resetiosflags(ios_base::skipws) clears ios_base::skipws in the format flags stored in the basic_istream<charT,traits> object cin (the same as cin >> noskipws), and the expression cout << resetiosflags(ios_base::showbase) clears ios_base::showbase in the format flags stored in the basic_ostream<charT,traits> object cout (the same as cout << noshowbase). --- end footnote]
> `ios_base& f(ios_base& str, ios_base::fmtflags mask)
> {
> // reset specified flags
> str.setf(ios_base::fmtflags(0), mask);
> return str;
> }
> `
> The expression out<<s has type basic_ostream<charT,traits>& and value out. The expression in>>s has type basic_istream<charT,traits>& and value in.
> `smanip setiosflags(ios_base::fmtflags mask);`
> -4- Returns: An object s of unspecified type such that if out is an instance of basic_ostream<charT,traits> then the expression out<<s behaves as if f(s, mask) were called, or if in is an instance of basic_istream<charT,traits> then the expression in>>s behaves as if f(s, mask) were called. The function f can be defined as:
> `ios_base& f(ios_base& str, ios_base::fmtflags mask)
> {
> // set specified flags
> str.setf(mask);
> return str;
> }
> `
> The expression out<<s has type basic_ostream<charT,traits>& and value out. The expression in>>s has type basic_istream<charT,traits>& and value in.
> `smanip setbase(int base);`
> -5- Returns: An object s of unspecified type such that if out is an instance of basic_ostream<charT,traits> then the expression out<<s behaves as if f(s, base) were called, or if in is an instance of basic_istream<charT,traits> then the expression in>>s behaves as if f(s, base) were called. The function f can be defined as:
> `ios_base& f(ios_base& str, int base)
> {
> // set basefield
> str.setf(base == 8 ? ios_base::oct :
> base == 10 ? ios_base::dec :
> base == 16 ? ios_base::hex :
> ios_base::fmtflags(0), ios_base::basefield);
> return str;
> }
> `
> The expression out<<s has type basic_ostream<charT,traits>& and value out. The expression in>>s has type basic_istream<charT,traits>& and value in.
> `smanip setfill(char_type c);
> `
> -6- Returns: An object s of unspecified type such that if out is (or is derived from) basic_ostream<charT,traits> and c has type charT then the expression out<<s behaves as if f(s, c) were called. The function f can be defined as:
> `template<class charT, class traits>
> basic_ios<charT,traits>& f(basic_ios<charT,traits>& str, charT c)
> {
> // set fill character
> str.fill(c);
> return str;
> }
> `
> The expression out<<s has type basic_ostream<charT,traits>& and value out.
> `smanip setprecision(int n);`
> -7- Returns: An object s of unspecified type such that if out is an instance of basic_ostream<charT,traits> then the expression out<<s behaves as if f(s, n) were called, or if in is an instance of basic_istream<charT,traits> then the expression in>>s behaves as if f(s, n) were called. The function f can be defined as:
> `ios_base& f(ios_base& str, int n)
> {
> // set precision
> str.precision(n);
> return str;
> }
> `
> The expression out<<s has type basic_ostream<charT,traits>& and value out. The expression in>>s has type basic_istream<charT,traits>& and value in
> .
> `smanip setw(int n);
> `
> -8- Returns: An object s of unspecified type such that if out is an instance of basic_ostream<charT,traits> then the expression out<<s behaves as if f(s, n) were called, or if in is an instance of basic_istream<charT,traits> then the expression in>>s behaves as if f(s, n) were called. The function f can be defined as:
> `ios_base& f(ios_base& str, int n)
> {
> // set width
> str.width(n);
> return str;
> }
> `
> The expression out<<s has type basic_ostream<charT,traits>& and value out. The expression in>>s has type basic_istream<charT,traits>& and value in.

[Kona: Andy Sawyer and Beman Dawes will work to improve the wording of the proposed resolution.]

[Tokyo - The LWG noted that issue 216 involves the same paragraphs.]

[Post-Tokyo: The issues list maintainer combined the proposed resolution of this issue with the proposed resolution for issue 216 as they both involved the same paragraphs, and were so intertwined that dealing with them separately appear fraught with error. The full text was supplied by Bill Plauger; it was cross checked against changes supplied by Andy Sawyer. It should be further checked by the LWG.]

### 184. numeric_limits<bool> wording problems

Section: 18.3.2.7 [numeric.special] Status: CD1 Submitter: Gabriel Dos Reis Opened: 1999-07-21 Last modified: 2015-04-08

View all other issues in [numeric.special].

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Discussion:

bools are defined by the standard to be of integer types, as per 3.9.1 [basic.fundamental] paragraph 7. However "integer types" seems to have a special meaning for the author of 18.2. The net effect is an unclear and confusing specification for numeric_limits<bool> as evidenced below.

18.2.1.2/7 says numeric_limits<>::digits is, for built-in integer types, the number of non-sign bits in the representation.

4.5/4 states that a bool promotes to int ; whereas 4.12/1 says any non zero arithmetical value converts to true.

I don't think it makes sense at all to require numeric_limits<bool>::digits and numeric_limits<bool>::digits10 to be meaningful.

The standard defines what constitutes a signed (resp. unsigned) integer types. It doesn't categorize bool as being signed or unsigned. And the set of values of bool type has only two elements.

I don't think it makes sense to require numeric_limits<bool>::is_signed to be meaningful.

18.2.1.2/18 for numeric_limits<integer_type>::radix says:

> For integer types, specifies the base of the representation.186)

This disposition is at best misleading and confusing for the standard requires a "pure binary numeration system" for integer types as per 3.9.1/7

The footnote 186) says: "Distinguishes types with base other than 2 (e.g BCD)." This also erroneous as the standard never defines any integer types with base representation other than 2.

Furthermore, numeric_limits<bool>::is_modulo and numeric_limits<bool>::is_signed have similar problems.

Proposed resolution:

Append to the end of 18.3.2.7 [numeric.special]:

> The specialization for bool shall be provided as follows:

>         namespace std {
>            template<> class numeric_limits<bool> {
>            public:
>              static const bool is_specialized = true;
>              static bool min() throw() { return false; }
>              static bool max() throw() { return true; }

>              static const int  digits = 1;
>              static const int  digits10 = 0;
>              static const bool is_signed = false;
>              static const bool is_integer = true;
>              static const bool is_exact = true;
>              static const int  radix = 2;
>              static bool epsilon() throw() { return 0; }
>              static bool round_error() throw() { return 0; }

>              static const int  min_exponent = 0;
>              static const int  min_exponent10 = 0;
>              static const int  max_exponent = 0;
>              static const int  max_exponent10 = 0;

>              static const bool has_infinity = false;
>              static const bool has_quiet_NaN = false;
>              static const bool has_signaling_NaN = false;
>              static const float_denorm_style has_denorm = denorm_absent;
>              static const bool has_denorm_loss = false;
>              static bool infinity() throw() { return 0; }
>              static bool quiet_NaN() throw() { return 0; }
>              static bool signaling_NaN() throw() { return 0; }
>              static bool denorm_min() throw() { return 0; }

>              static const bool is_iec559 = false;
>              static const bool is_bounded = true;
>              static const bool is_modulo = false;

>              static const bool traps = false;
>              static const bool tinyness_before = false;
>              static const float_round_style round_style = round_toward_zero;
>            };
>          }

[Tokyo: The LWG desires wording that specifies exact values rather than more general wording in the original proposed resolution.]

[Post-Tokyo: At the request of the LWG in Tokyo, Nico Josuttis provided the above wording.]

### 185. Questionable use of term "inline"

Section: 20.9 [function.objects] Status: CD1 Submitter: UK Panel Opened: 1999-07-26 Last modified: 2015-04-08

View all other issues in [function.objects].

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Discussion:

Paragraph 4 of 20.9 [function.objects] says:

> [Example: To negate every element of a: transform(a.begin(), a.end(), a.begin(), negate<double>()); The corresponding functions will inline the addition and the negation. end example]

(Note: The "addition" referred to in the above is in para 3) we can find no other wording, except this (non-normative) example which suggests that any "inlining" will take place in this case.

Indeed both:

> 17.4.4.3 Global Functions [lib.global.functions] 1 It is unspecified whether any global functions in the C++ Standard Library are defined as inline (7.1.2).

and

> 17.4.4.4 Member Functions [lib.member.functions] 1 It is unspecified whether any member functions in the C++ Standard Library are defined as inline (7.1.2).

take care to state that this may indeed NOT be the case.

Thus the example "mandates" behavior that is explicitly not required elsewhere.

Proposed resolution:

In 20.9 [function.objects] paragraph 1, remove the sentence:

> They are important for the effective use of the library.

Remove 20.9 [function.objects] paragraph 2, which reads:

> Using function objects together with function templates increases the expressive power of the library as well as making the resulting code much more efficient.

In 20.9 [function.objects] paragraph 4, remove the sentence:

> The corresponding functions will inline the addition and the negation.

[Kona: The LWG agreed there was a defect.]

[Tokyo: The LWG crafted the proposed resolution.]

### 186. bitset::set() second parameter should be bool

Section: 20.6.2 [bitset.members] Status: CD1 Submitter: Darin Adler Opened: 1999-08-13 Last modified: 2015-04-08

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Discussion:

In section 20.6.2 [bitset.members], paragraph 13 defines the bitset::set operation to take a second parameter of type int. The function tests whether this value is non-zero to determine whether to set the bit to true or false. The type of this second parameter should be bool. For one thing, the intent is to specify a Boolean value. For another, the result type from test() is bool. In addition, it's possible to slice an integer that's larger than an int. This can't happen with bool, since conversion to bool has the semantic of translating 0 to false and any non-zero value to true.

Proposed resolution:

In 20.6 [template.bitset] Para 1 Replace:

>     bitset<N>& set(size_t pos, int val = true );

With:

>     bitset<N>& set(size_t pos, bool val = true );

In 20.6.2 [bitset.members] Para 12(.5) Replace:

>     bitset<N>& set(size_t pos, int val = 1 );

With:

>     bitset<N>& set(size_t pos, bool val = true );

[Kona: The LWG agrees with the description. Andy Sawyer will work on better P/R wording.]

[Post-Tokyo: Andy provided the above wording.]

Rationale:

`bool` is a better choice. It is believed that binary compatibility is not an issue, because this member function is usually implemented as `inline`, and because it is already the case that users cannot rely on the type of a pointer to a nonvirtual member of a standard library class.

### 187. iter_swap underspecified

Section: 25.3.3 [alg.swap] Status: CD1 Submitter: Andrew Koenig Opened: 1999-08-14 Last modified: 2015-04-08

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Discussion:

The description of iter_swap in 25.2.2 paragraph 7,says that it ``exchanges the values'' of the objects to which two iterators refer.
What it doesn't say is whether it does so using swap or using the assignment operator and copy constructor.
This question is an important one to answer, because swap is specialized to work efficiently for standard containers.
For example:

>     vector<int> v1, v2;
>     iter_swap(&v1, &v2);

Is this call to iter_swap equivalent to calling swap(v1, v2)? Or is it equivalent to

>     {
>     vector<int> temp = v1;
>     v1 = v2;
>     v2 = temp;
>     }

The first alternative is O(1); the second is O(n).

A LWG member, Dave Abrahams, comments:

> Not an objection necessarily, but I want to point out the cost of that requirement:

> > `iter_swap(list<T>::iterator, list<T>::iterator)`

> can currently be specialized to be more efficient than iter_swap(T*,T*) for many T (by using splicing). Your proposal would make that optimization illegal.

[Kona: The LWG notes the original need for iter_swap was proxy iterators which are no longer permitted.]

Proposed resolution:

Change the effect clause of iter_swap in 25.2.2 paragraph 7 from:

> Exchanges the values pointed to by the two iterators a and b.

to

> `swap(*a, *b)`.

Rationale:

It's useful to say just what `iter_swap` does. There may be some iterators for which we want to specialize `iter_swap`, but the fully general version should have a general specification.

Note that in the specific case of `list<T>::iterator`, iter_swap should not be specialized as suggested above. That would do much more than exchanging the two iterators' values: it would change predecessor/successor relationships, possibly moving the iterator from one list to another. That would surely be inappropriate.

### 189. setprecision() not specified correctly

Section: 27.5.3.2 [fmtflags.state] Status: TC1 Submitter: Andrew Koenig Opened: 1999-08-25 Last modified: 2015-04-08

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Discussion:

27.4.2.2 paragraph 9 claims that setprecision() sets the precision, and includes a parenthetical note saying that it is the number of digits after the decimal point.
This claim is not strictly correct. For example, in the default floating-point output format, setprecision sets the number of significant digits printed, not the number of digits after the decimal point.
I would like the committee to look at the definition carefully and correct the statement in 27.4.2.2

Proposed resolution:

Remove from 27.5.3.2 [fmtflags.state], paragraph 9, the text "(number of digits after the decimal point)".

### 193. Heap operations description incorrect

Section: 25.4.6 [alg.heap.operations] Status: TC1 Submitter: Markus Mauhart Opened: 1999-09-24 Last modified: 2015-04-08

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Duplicate of: 216

Discussion:

25.3.6 [lib.alg.heap.operations] states two key properties of a heap [a,b), the first of them is
`"(1) *a is the largest element"
I think this is incorrect and should be changed to the wording in the proposed resolution.

Actually there are two independent changes:

> A-"part of largest equivalence class" instead of "largest", cause 25.3 [lib.alg.sorting] asserts "strict weak ordering" for all its sub clauses.

> B-Take 'an oldest' from that equivalence class, otherwise the heap functions could not be used for a priority queue as explained in 23.2.3.2.2 [lib.priqueue.members] (where I assume that a "priority queue" respects priority AND time).

Proposed resolution:

Change 25.4.6 [alg.heap.operations] property (1) from:

> (1) *a is the largest element

to:

> (1) There is no element greater than `*a`

### 195. Should `basic_istream::sentry`'s constructor ever set eofbit?

Section: 27.7.2.1.3 [istream::sentry] Status: TC1 Submitter: Matt Austern Opened: 1999-10-13 Last modified: 2015-04-08

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Discussion:

Suppose that `is.flags() & ios_base::skipws` is nonzero. What should `basic_istream<>::sentry`'s constructor do if it reaches eof while skipping whitespace? 27.6.1.1.2/5 suggests it should set failbit. Should it set eofbit as well? The standard doesn't seem to answer that question.

On the one hand, nothing in 27.7.2.1.3 [istream::sentry] says that `basic_istream<>::sentry` should ever set eofbit. On the other hand, 27.7.2.1 [istream] paragraph 4 says that if extraction from a `streambuf` "returns `traits::eof()`, then the input function, except as explicitly noted otherwise, completes its actions and does `setstate(eofbit)"`. So the question comes down to whether `basic_istream<>::sentry`'s constructor is an input function.

Comments from Jerry Schwarz:

> It was always my intention that eofbit should be set any time that a virtual returned something to indicate eof, no matter what reason iostream code had for calling the virtual.

> The motivation for this is that I did not want to require streambufs to behave consistently if their virtuals are called after they have signaled eof.

> The classic case is a streambuf reading from a UNIX file. EOF isn't really a state for UNIX file descriptors. The convention is that a read on UNIX returns 0 bytes to indicate "EOF", but the file descriptor isn't shut down in any way and future reads do not necessarily also return 0 bytes. In particular, you can read from tty's on UNIX even after they have signaled "EOF". (It isn't always understood that a ^D on UNIX is not an EOF indicator, but an EOL indicator. By typing a "line" consisting solely of ^D you cause a read to return 0 bytes, and by convention this is interpreted as end of file.)

Proposed resolution:

Add a sentence to the end of 27.6.1.1.2 paragraph 2:

> If `is.rdbuf()->sbumpc()` or `is.rdbuf()->sgetc()` returns `traits::eof()`, the function calls `setstate(failbit | eofbit)` (which may throw `ios_base::failure`).

### 198. Validity of pointers and references unspecified after iterator destruction

Section: X [iterator.concepts] Status: CD1 Submitter: Beman Dawes Opened: 1999-11-03 Last modified: 2015-04-08

View all other issues in [iterator.concepts].

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Discussion:

Is a pointer or reference obtained from an iterator still valid after destruction of the iterator?

Is a pointer or reference obtained from an iterator still valid after the value of the iterator changes?

>     #include <iostream>
>     #include <vector>
>     #include <iterator>

>     int main()
>     {
>         typedef std::vector<int> vec_t;
>         vec_t v;
>         v.push_back( 1 );

>         // Is a pointer or reference obtained from an iterator still
>         // valid after destruction of the iterator?
>         int * p = &*v.begin();
>         std::cout << *p << '\n';  // OK?

>         // Is a pointer or reference obtained from an iterator still
>         // valid after the value of the iterator changes?
>         vec_t::iterator iter( v.begin() );
>         p = &*iter++;
>         std::cout << *p << '\n';  // OK?

>         return 0;
>     }

The standard doesn't appear to directly address these questions. The standard needs to be clarified. At least two real-world cases have been reported where library implementors wasted considerable effort because of the lack of clarity in the standard. The question is important because requiring pointers and references to remain valid has the effect for practical purposes of prohibiting iterators from pointing to cached rather than actual elements of containers.

The standard itself assumes that pointers and references obtained from an iterator are still valid after iterator destruction or change. The definition of reverse_iterator::operator*(), 24.5.1.3.3 [reverse.iter.conv], which returns a reference, defines effects:

>     Iterator tmp = current;
>     return *--tmp;

The definition of reverse_iterator::operator->(), 24.5.1.3.4 [reverse.iter.op.star], which returns a pointer, defines effects:

>     return &(operator*());

Because the standard itself assumes pointers and references remain valid after iterator destruction or change, the standard should say so explicitly. This will also reduce the chance of user code breaking unexpectedly when porting to a different standard library implementation.

Proposed resolution:

Add a new paragraph to X [iterator.concepts]:

> Destruction of an iterator may invalidate pointers and references previously obtained from that iterator.

Replace paragraph 1 of 24.5.1.3.3 [reverse.iter.conv] with:

> Effects:

>       this->tmp = current;
>       --this->tmp;
>       return *this->tmp;

> [Note: This operation must use an auxiliary member variable, rather than a temporary variable, to avoid returning a reference that persists beyond the lifetime of its associated iterator. (See X [iterator.concepts].) The name of this member variable is shown for exposition only. --end note]

[Post-Tokyo: The issue has been reformulated purely in terms of iterators.]

[Pre-Toronto: Steve Cleary pointed out the no-invalidation assumption by reverse_iterator. The issue and proposed resolution was reformulated yet again to reflect this reality.]

[Copenhagen: Steve Cleary pointed out that reverse_iterator assumes its underlying iterator has persistent pointers and references. Andy Koenig pointed out that it is possible to rewrite reverse_iterator so that it no longer makes such an assupmption. However, this issue is related to issue 299. If we decide it is intentional that `p[n]` may return by value instead of reference when `p` is a Random Access Iterator, other changes in reverse_iterator will be necessary.]

Rationale:

This issue has been discussed extensively. Note that it is not an issue about the behavior of predefined iterators. It is asking whether or not user-defined iterators are permitted to have transient pointers and references. Several people presented examples of useful user-defined iterators that have such a property; examples include a B-tree iterator, and an "iota iterator" that doesn't point to memory. Library implementors already seem to be able to cope with such iterators: they take pains to avoid forming references to memory that gets iterated past. The only place where this is a problem is `reverse_iterator`, so this issue changes `reverse_iterator` to make it work.

This resolution does not weaken any guarantees provided by predefined iterators like `list<int>::iterator`. Clause 23 should be reviewed to make sure that guarantees for predefined iterators are as strong as users expect.

### 199. What does `allocate(0)` return?

Section: 17.6.3.5 [allocator.requirements] Status: TC1 Submitter: Matt Austern Opened: 1999-11-19 Last modified: 2015-04-08

View other active issues in [allocator.requirements].

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Discussion:

Suppose that `A` is a class that conforms to the Allocator requirements of Table 32, and `a` is an object of class `A` What should be the return value of `a.allocate(0)`? Three reasonable possibilities: forbid the argument `0`, return a null pointer, or require that the return value be a unique non-null pointer.

Proposed resolution:

Add a note to the `allocate` row of Table 32: "[Note: If `n == 0`, the return value is unspecified. --end note]"

Rationale:

A key to understanding this issue is that the ultimate use of allocate() is to construct an iterator, and that iterator for zero length sequences must be the container's past-the-end representation. Since this already implies special case code, it would be over-specification to mandate the return value.

### 200. Forward iterator requirements don't allow constant iterators

Section: 24.2.5 [forward.iterators] Status: CD1 Submitter: Matt Austern Opened: 1999-11-19 Last modified: 2015-04-08

View all other issues in [forward.iterators].

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Discussion:

In table 74, the return type of the expression `*a` is given as `T&`, where `T` is the iterator's value type. For constant iterators, however, this is wrong. ("Value type" is never defined very precisely, but it is clear that the value type of, say, `std::list<int>::const_iterator` is supposed to be `int`, not `const int`.)

Proposed resolution:

In table 74, in the `*a` and `*r++` rows, change the return type from "`T&`" to "`T&` if `X` is mutable, otherwise `const T&`". In the `a->m` row, change the return type from "`U&`" to "`U&` if `X` is mutable, otherwise `const U&`".

[Tokyo: The LWG believes this is the tip of a larger iceberg; there are multiple const problems with the STL portion of the library and that these should be addressed as a single package. Note that issue 180 has already been declared NAD Future for that very reason.]

[Redmond: the LWG thinks this is separable from other constness issues. This issue is just cleanup; it clarifies language that was written before we had iterator_traits. Proposed resolution was modified: the original version only discussed *a. It was pointed out that we also need to worry about *r++ and a->m.]

### 201. Numeric limits terminology wrong

Section: 18.3.2 [limits] Status: CD1 Submitter: Stephen Cleary Opened: 1999-12-21 Last modified: 2015-04-08

View all other issues in [limits].

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Discussion:

In some places in this section, the terms "fundamental types" and "scalar types" are used when the term "arithmetic types" is intended. The current usage is incorrect because void is a fundamental type and pointers are scalar types, neither of which should have specializations of numeric_limits.

[Lillehammer: it remains true that numeric_limits is using imprecise language. However, none of the proposals for changed wording are clearer. A redesign of numeric_limits is needed, but this is more a task than an open issue.]

Proposed resolution:

Change 18.3 [support.limits] to:

> -1- The headers `<limits>`, `<climits>`, `<cfloat>`, and `<cinttypes>` supply characteristics of implementation-dependent fundamental arithmetic types (3.9.1).

Change 18.3.2 [limits] to:

> -1- The `numeric_limits` component provides a C++ program with information about various properties of the implementation's representation of the fundamental arithmetic types.

> -2- Specializations shall be provided for each fundamental arithmetic type, both floating point and integer, including `bool`. The member `is_specialized` shall be `true` for all such specializations of `numeric_limits`.

> -4- Non-fundamental arithmetic standard types, such as `complex<T>` (26.3.2), shall not have specializations.

Change 18.3.2.3 [numeric.limits] to:

> -1- The member `is_specialized` makes it possible to distinguish between fundamental types, which have specializations, and non-scalar types, which do not.

### 202. unique() effects unclear when predicate not an equivalence relation

Section: 25.3.9 [alg.unique] Status: CD1 Submitter: Andrew Koenig Opened: 2000-01-13 Last modified: 2015-04-08

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Discussion:

What should unique() do if you give it a predicate that is not an equivalence relation? There are at least two plausible answers:

> 1. You can't, because 25.2.8 says that it it "eliminates all but the first element from every consecutive group of equal elements..." and it wouldn't make sense to interpret "equal" as meaning anything but an equivalence relation. [It also doesn't make sense to interpret "equal" as meaning ==, because then there would never be any sense in giving a predicate as an argument at all.]

> 2. The word "equal" should be interpreted to mean whatever the predicate says, even if it is not an equivalence relation (and in particular, even if it is not transitive).

The example that raised this question is from Usenet:

>     int f[] = { 1, 3, 7, 1, 2 };
>     int* z = unique(f, f+5, greater<int>());

If one blindly applies the definition using the predicate greater<int>, and ignore the word "equal", you get:

> Eliminates all but the first element from every consecutive group of elements referred to by the iterator i in the range [first, last) for which *i > *(i - 1).

The first surprise is the order of the comparison. If we wanted to allow for the predicate not being an equivalence relation, then we should surely compare elements the other way: pred(*(i - 1), *i). If we do that, then the description would seem to say: "Break the sequence into subsequences whose elements are in strictly increasing order, and keep only the first element of each subsequence". So the result would be 1, 1, 2. If we take the description at its word, it would seem to call for strictly DEcreasing order, in which case the result should be 1, 3, 7, 2.
In fact, the SGI implementation of unique() does neither: It yields 1, 3, 7.

Proposed resolution:

Change 25.3.9 [alg.unique] paragraph 1 to:

> For a nonempty range, eliminates all but the first element from every consecutive group of equivalent elements referred to by the iterator `i` in the range [first+1, last) for which the following conditions hold: `*(i-1) == *i` or `pred(*(i-1), *i) != false`.

Also insert a new paragraph, paragraph 2a, that reads: "Requires: The comparison function must be an equivalence relation."

[Redmond: discussed arguments for and against requiring the comparison function to be an equivalence relation. Straw poll: 14-2-5. First number is to require that it be an equivalence relation, second number is to explicitly not require that it be an equivalence relation, third number is people who believe they need more time to consider the issue. A separate issue: Andy Sawyer pointed out that "i-1" is incorrect, since "i" can refer to the first iterator in the range. Matt provided wording to address this problem.]

[Curaçao: The LWG changed "... the range (first, last)..." to "... the range [first+1, last)..." for clarity. They considered this change close enough to editorial to not require another round of review.]

Rationale:

The LWG also considered an alternative resolution: change 25.3.9 [alg.unique] paragraph 1 to:

> For a nonempty range, eliminates all but the first element from every consecutive group of elements referred to by the iterator `i` in the range (first, last) for which the following conditions hold: `*(i-1) == *i` or `pred(*(i-1), *i) != false`.

Also insert a new paragraph, paragraph 1a, that reads: "Notes: The comparison function need not be an equivalence relation."

Informally: the proposed resolution imposes an explicit requirement that the comparison function be an equivalence relation. The alternative resolution does not, and it gives enough information so that the behavior of unique() for a non-equivalence relation is specified. Both resolutions are consistent with the behavior of existing implementations.

### 206. operator new(size_t, nothrow) may become unlinked to ordinary operator new if ordinary version replaced

Section: 18.6.1.1 [new.delete.single] Status: CD1 Submitter: Howard Hinnant Opened: 1999-08-29 Last modified: 2015-04-08

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Discussion:

As specified, the implementation of the nothrow version of operator new does not necessarily call the ordinary operator new, but may instead simply call the same underlying allocator and return a null pointer instead of throwing an exception in case of failure.

Such an implementation breaks code that replaces the ordinary version of new, but not the nothrow version. If the ordinary version of new/delete is replaced, and if the replaced delete is not compatible with pointers returned from the library versions of new, then when the replaced delete receives a pointer allocated by the library new(nothrow), crash follows.

The fix appears to be that the lib version of new(nothrow) must call the ordinary new. Thus when the ordinary new gets replaced, the lib version will call the replaced ordinary new and things will continue to work.

An alternative would be to have the ordinary new call new(nothrow). This seems sub-optimal to me as the ordinary version of new is the version most commonly replaced in practice. So one would still need to replace both ordinary and nothrow versions if one wanted to replace the ordinary version.

Another alternative is to put in clear text that if one version is replaced, then the other must also be replaced to maintain compatibility. Then the proposed resolution below would just be a quality of implementation issue. There is already such text in paragraph 7 (under the new(nothrow) version). But this nuance is easily missed if one reads only the paragraphs relating to the ordinary new.

N2158 has been written explaining the rationale for the proposed resolution below.

Proposed resolution:

Change 18.5.1.1 [new.delete.single]:

>     void* operator new(std::size_t size, const std::nothrow_t&) throw();

> > -5- Effects: Same as above, except that it is called by a placement version of a new-expression when a C++ program prefers a null pointer result as an error indication, instead of a `bad_alloc` exception.

> > -6- Replaceable: a C++ program may define a function with this function signature that displaces the default version defined by the C++ Standard library.

> > -7- Required behavior: Return a non-null pointer to suitably aligned storage (3.7.4), or else return a null pointer. This nothrow version of operator new returns a pointer obtained as if acquired from the (possibly replaced) ordinary version. This requirement is binding on a replacement version of this function.

> > -8- Default behavior:

> > - Calls `operator new(size)`.
> > - If the call to `operator new(size)` returns normally, returns the result of that call, else
> > - if the call to `operator new(size)` throws an exception, returns a null pointer.
> > - Executes a loop: Within the loop, the function first attempts to allocate the requested storage. Whether the attempt involves a call to the Standard C library function `malloc` is unspecified.
> > - Returns a pointer to the allocated storage if the attempt is successful. Otherwise, if the last argument to `set_new_handler()` was a null pointer, return a null pointer.
> > - Otherwise, the function calls the current new_handler (18.5.2.2). If the called function returns, the loop repeats.
> > - The loop terminates when an attempt to allocate the requested storage is successful or when a called new_handler function does not return. If the called new_handler function terminates by throwing a `bad_alloc exception`, the function returns a null pointer.

> > -9- [Example:

> > >     T* p1 = new T;                 // throws bad_alloc if it fails
> > >     T* p2 = new(nothrow) T;        // returns 0 if it fails

> > --end example]

>     void operator delete(void* ptr) throw();
>     void operator delete(void* ptr, const std::nothrow_t&) throw();

> > -10- Effects: The deallocation function (3.7.4.2) called by a delete-expression to render the value of `ptr` invalid.

> > -11- Replaceable: a C++ program may define a function with this function signature that displaces the default version defined by the C++ Standard library.

> > -12- Requires: the value of `ptr` is null or the value returned by an earlier call to the default (possibly replaced) `operator new(std::size_t)` or `operator new(std::size_t, const std::nothrow_t&)`.

> > -13- Default behavior:

> > - For a null value of `ptr`, do nothing.
> > - Any other value of `ptr` shall be a value returned earlier by a call to the default `operator new`, which was not invalidated by an intervening call to `operator delete(void*)` (17.4.3.7). For such a non-null value of `ptr`, reclaims storage allocated by the earlier call to the default `operator new`.

> > -14- Remarks: It is unspecified under what conditions part or all of such reclaimed storage is allocated by a subsequent call to `operator new` or any of `calloc`, `malloc`, or `realloc`, declared in `<cstdlib>`.

>     void operator delete(void* ptr, const std::nothrow_t&) throw();

> > -15- Effects: Same as above, except that it is called by the implementation when an exception propagates from a nothrow placement version of the new-expression (i.e. when the constructor throws an exception).

> > -16- Replaceable: a C++ program may define a function with this function signature that displaces the default version defined by the C++ Standard library.

> > -17- Requires: the value of `ptr` is null or the value returned by an earlier call to the (possibly replaced) `operator new(std::size_t)` or `operator new(std::size_t, const std::nothrow_t&)`.

> > -18- Default behavior: Calls `operator delete(ptr)`.

Change 18.5.1.2 [new.delete.array]

>     void* operator new[](std::size_t size, const std::nothrow_t&) throw();

> > -5- Effects: Same as above, except that it is called by a placement version of a new-expression when a C++ program prefers a null pointer result as an error indication, instead of a `bad_alloc` exception.

> > -6- Replaceable: a C++ program can define a function with this function signature that displaces the default version defined by the C++ Standard library.

> > -7- Required behavior: Same as for operator `new(std::size_t, const std::nothrow_t&)`. This nothrow version of operator `new[]` returns a pointer obtained as if acquired from the ordinary version. Return a non-null pointer to suitably aligned storage (3.7.4), or else return a null pointer. This nothrow version of operator new returns a pointer obtained as if acquired from the (possibly replaced) `operator new[](std::size_t size)`. This requirement is binding on a replacement version of this function.

> > -8- Default behavior: Returns `operator new(size, nothrow)`.

> > - Calls `operator new[](size)`.
> > - If the call to `operator new[](size)` returns normally, returns the result of that call, else
> > - if the call to `operator new[](size)` throws an exception, returns a null pointer.

>     void operator delete[](void* ptr) throw();
>     void operator delete[](void* ptr, const std::nothrow_t&) throw();

> > -9- Effects: The deallocation function (3.7.4.2) called by the array form of a delete-expression to render the value of `ptr` invalid.

> > -10- Replaceable: a C++ program can define a function with this function signature that displaces the default version defined by the C++ Standard library.

> > -11- Requires: the value of `ptr` is null or the value returned by an earlier call to `operator new[](std::size_t)` or `operator new[](std::size_t, const std::nothrow_t&)`.

> > -12- Default behavior: Calls `operator delete(ptr)` or `operator delete[](ptr, std::nothrow)` respectively.

Rationale:

Yes, they may become unlinked, and that is by design. If a user replaces one, the user should also replace the other.

[ Reopened due to a gcc conversation between Howard, Martin and Gaby. Forwarding or not is visible behavior to the client and it would be useful for the client to know which behavior it could depend on. ]

[ Batavia: Robert voiced serious reservations about backwards compatibility for his customers. ]

### 208. Unnecessary restriction on past-the-end iterators

Section: X [iterator.concepts] Status: TC1 Submitter: Stephen Cleary Opened: 2000-02-02 Last modified: 2015-04-08

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Discussion:

In 24.1 paragraph 5, it is stated ". . . Dereferenceable and past-the-end values are always non-singular."

This places an unnecessary restriction on past-the-end iterators for containers with forward iterators (for example, a singly-linked list). If the past-the-end value on such a container was a well-known singular value, it would still satisfy all forward iterator requirements.

Removing this restriction would allow, for example, a singly-linked list without a "footer" node.

This would have an impact on existing code that expects past-the-end iterators obtained from different (generic) containers being not equal.

Proposed resolution:

Change X [iterator.concepts] paragraph 5, the last sentence, from:

> Dereferenceable and past-the-end values are always non-singular.

to:

> Dereferenceable values are always non-singular.

Rationale:

For some kinds of containers, including singly linked lists and zero-length vectors, null pointers are perfectly reasonable past-the-end iterators. Null pointers are singular.

### 209. basic_string declarations inconsistent

Section: 21.4 [basic.string] Status: TC1 Submitter: Igor Stauder Opened: 2000-02-11 Last modified: 2015-04-08

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Discussion:

In Section 21.4 [basic.string] the basic_string member function declarations use a consistent style except for the following functions:

>     void push_back(const charT);
>     basic_string& assign(const basic_string&);
>     void swap(basic_string<charT,traits,Allocator>&);

- push_back, assign, swap: missing argument name
- push_back: use of const with charT (i.e. POD type passed by value not by reference - should be charT or const charT& )
- swap: redundant use of template parameters in argument basic_string<charT,traits,Allocator>&

Proposed resolution:

In Section 21.4 [basic.string] change the basic_string member function declarations push_back, assign, and swap to:

>     void push_back(charT c);

>     basic_string& assign(const basic_string& str);
>     void swap(basic_string& str);

Rationale:

Although the standard is in general not consistent in declaration style, the basic_string declarations are consistent other than the above. The LWG felt that this was sufficient reason to merit the change.

### 210. distance first and last confused

Section: 25 [algorithms] Status: TC1 Submitter: Lisa Lippincott Opened: 2000-02-15 Last modified: 2015-04-08

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Discussion:

In paragraph 9 of section 25 [algorithms], it is written:

> In the description of the algorithms operators + and - are used for some of the iterator categories for which they do not have to be defined. In these cases the semantics of [...] a-b is the same as of
> `return distance(a, b);`

Proposed resolution:

On the last line of paragraph 9 of section 25 [algorithms] change `"a-b"` to `"b-a".`

Rationale:

There are two ways to fix the defect; change the description to b-a or change the return to distance(b,a). The LWG preferred the former for consistency.

### 211. operator>>(istream&, string&) doesn't set failbit

Section: 21.4.8.9 [string.io] Status: TC1 Submitter: Scott Snyder Opened: 2000-02-04 Last modified: 2015-04-08

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Discussion:

The description of the stream extraction operator for std::string (section 21.3.7.9 [lib.string.io]) does not contain a requirement that failbit be set in the case that the operator fails to extract any characters from the input stream.

This implies that the typical construction

>     std::istream is;
>     std::string str;
>     ...
>     while (is >> str) ... ;

(which tests failbit) is not required to terminate at EOF.

Furthermore, this is inconsistent with other extraction operators, which do include this requirement. (See sections 27.7.2.2 [istream.formatted] and 27.7.2.3 [istream.unformatted]), where this requirement is present, either explicitly or implicitly, for the extraction operators. It is also present explicitly in the description of getline (istream&, string&, charT) in section 21.4.8.9 [string.io] paragraph 8.)

Proposed resolution:

Insert new paragraph after paragraph 2 in section 21.4.8.9 [string.io]:

> If the function extracts no characters, it calls is.setstate(ios::failbit) which may throw ios_base::failure (27.4.4.3).

### 212. Empty range behavior unclear for several algorithms

Section: 25.4.7 [alg.min.max] Status: TC1 Submitter: Nico Josuttis Opened: 2000-02-26 Last modified: 2015-04-08

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Discussion:

The standard doesn't specify what min_element() and max_element() shall return if the range is empty (first equals last). The usual implementations return last. This problem seems also apply to partition(), stable_partition(), next_permutation(), and prev_permutation().

Proposed resolution:

In 25.4.7 [alg.min.max] - Minimum and maximum, paragraphs 7 and 9, append: Returns last if first==last.

Rationale:

The LWG looked in some detail at all of the above mentioned algorithms, but believes that except for min_element() and max_element() it is already clear that last is returned if first == last.

### 214. set::find() missing const overload

Section: 23.4.6 [set], 23.4.7 [multiset] Status: CD1 Submitter: Judy Ward Opened: 2000-02-28 Last modified: 2015-04-08

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Duplicate of: 450

Discussion:

The specification for the associative container requirements in Table 69 state that the find member function should "return iterator; const_iterator for constant a". The map and multimap container descriptions have two overloaded versions of find, but set and multiset do not, all they have is:

>     iterator find(const key_type & x) const;

Proposed resolution:

Change the prototypes for find(), lower_bound(), upper_bound(), and equal_range() in section 23.4.6 [set] and section 23.4.7 [multiset] to each have two overloads:

>     iterator find(const key_type & x);
>     const_iterator find(const key_type & x) const;

>     iterator lower_bound(const key_type & x);
>     const_iterator lower_bound(const key_type & x) const;

>     iterator upper_bound(const key_type & x);
>     const_iterator upper_bound(const key_type & x) const;

>     pair<iterator, iterator> equal_range(const key_type & x);
>     pair<const_iterator, const_iterator> equal_range(const key_type & x) const;

[Tokyo: At the request of the LWG, Judy Ward provided wording extending the proposed resolution to lower_bound, upper_bound, and equal_range.]

### 217. Facets example (Classifying Japanese characters) contains errors

Section: 22.4.8 [facets.examples] Status: TC1 Submitter: Martin Sebor Opened: 2000-02-29 Last modified: 2015-04-08

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Discussion:

The example in 22.2.8, paragraph 11 contains the following errors:

1) The member function `My::JCtype::is_kanji()' is non-const; the function must be const in order for it to be callable on a const object (a reference to which which is what std::use_facet<>() returns).

2) In file filt.C, the definition of `JCtype::id' must be qualified with the name of the namespace `My'.

3) In the definition of `loc' and subsequently in the call to use_facet<>() in main(), the name of the facet is misspelled: it should read `My::JCtype' rather than `My::JCType'.

Proposed resolution:

Replace the "Classifying Japanese characters" example in 22.2.8, paragraph 11 with the following:

    #include <locale>

    namespace My {
        using namespace std;
        class JCtype : public locale::facet {
        public:
            static locale::id id;     //  required for use as a new locale facet
            bool is_kanji (wchar_t c) const;
            JCtype() {}
        protected:
            ~JCtype() {}
        };
    }

    //  file:  filt.C
    #include <iostream>
    #include <locale>
    #include "jctype"                 //  above
    std::locale::id My::JCtype::id;   //  the static  JCtype  member
    declared above.

    int main()
    {
        using namespace std;
        typedef ctype<wchar_t> wctype;
        locale loc(locale(""),              //  the user's preferred locale...
                   new My::JCtype);         //  and a new feature ...
        wchar_t c = use_facet<wctype>(loc).widen('!');
        if (!use_facet<My::JCtype>(loc).is_kanji(c))
            cout << "no it isn't!" << endl;
        return 0;
    }

### 220. ~ios_base() usage valid?

Section: 27.5.3.7 [ios.base.cons] Status: TC1 Submitter: Jonathan Schilling, Howard Hinnant Opened: 2000-03-13 Last modified: 2015-04-08

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Discussion:

The pre-conditions for the ios_base destructor are described in 27.4.2.7 paragraph 2:

> Effects: Destroys an object of class ios_base. Calls each registered callback pair (fn,index) (27.4.2.6) as (*fn)(erase_event,*this,index) at such time that any ios_base member function called from within fn has well defined results.

But what is not clear is: If no callback functions were ever registered, does it matter whether the ios_base members were ever initialized?

For instance, does this program have defined behavior:

>     #include <ios>

>     class D : public std::ios_base { };

>     int main() { D d; }

It seems that registration of a callback function would surely affect the state of an ios_base. That is, when you register a callback function with an ios_base, the ios_base must record that fact somehow.

But if after construction the ios_base is in an indeterminate state, and that state is not made determinate before the destructor is called, then how would the destructor know if any callbacks had indeed been registered? And if the number of callbacks that had been registered is indeterminate, then is not the behavior of the destructor undefined?

By comparison, the basic_ios class description in 27.4.4.1 paragraph 2 makes it explicit that destruction before initialization results in undefined behavior.

Proposed resolution:

Modify 27.4.2.7 paragraph 1 from

> Effects: Each ios_base member has an indeterminate value after construction.

to

> Effects: Each ios_base member has an indeterminate value after construction. These members must be initialized by calling basic_ios::init. If an ios_base object is destroyed before these initializations have taken place, the behavior is undefined.

### 221. num_get<>::do_get stage 2 processing broken

Section: 22.4.2.1.2 [facet.num.get.virtuals] Status: CD1 Submitter: Matt Austern Opened: 2000-03-14 Last modified: 2015-04-08

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Discussion:

Stage 2 processing of numeric conversion is broken.

Table 55 in 22.2.2.1.2 says that when basefield is 0 the integral conversion specifier is %i. A %i specifier determines a number's base by its prefix (0 for octal, 0x for hex), so the intention is clearly that a 0x prefix is allowed. Paragraph 8 in the same section, however, describes very precisely how characters are processed. (It must be done "as if" by a specified code fragment.) That description does not allow a 0x prefix to be recognized.

Very roughly, stage 2 processing reads a char_type ct. It converts ct to a char, not by using narrow but by looking it up in a translation table that was created by widening the string literal "0123456789abcdefABCDEF+-". The character "x" is not found in that table, so it can't be recognized by stage 2 processing.

Proposed resolution:

In 22.2.2.1.2 paragraph 8, replace the line:

>     static const char src[] = "0123456789abcdefABCDEF+-";

with the line:

>     static const char src[] = "0123456789abcdefxABCDEFX+-";

Rationale:

If we're using the technique of widening a string literal, the string literal must contain every character we wish to recognize. This technique has the consequence that alternate representations of digits will not be recognized. This design decision was made deliberately, with full knowledge of that limitation.

### 222. Are throw clauses necessary if a throw is already implied by the effects clause?

Section: 17.5.1.4 [structure.specifications] Status: TC1 Submitter: Judy Ward Opened: 2000-03-17 Last modified: 2015-04-08

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Discussion:

Section 21.3.6.8 describes the basic_string::compare function this way:

>     21.3.6.8 - basic_string::compare [lib.string::compare]

>     int compare(size_type pos1, size_type n1,
>                     const basic_string<charT,traits,Allocator>&  str ,
>                     size_type  pos2 , size_type  n2 ) const;

>     -4- Returns:

>         basic_string<charT,traits,Allocator>(*this,pos1,n1).compare(
>                      basic_string<charT,traits,Allocator>(str,pos2,n2)) .

and the constructor that's implicitly called by the above is defined to throw an out-of-range exception if pos > str.size(). See section 21.4.1 [string.require] paragraph 4.

On the other hand, the compare function descriptions themselves don't have "Throws: " clauses and according to 17.3.1.3, paragraph 3, elements that do not apply to a function are omitted.

So it seems there is an inconsistency in the standard -- are the "Effects" clauses correct, or are the "Throws" clauses missing?

Proposed resolution:

In 17.5.1.4 [structure.specifications] paragraph 3, the footnote 148 attached to the sentence "Descriptions of function semantics contain the following elements (as appropriate):", insert the word "further" so that the foot note reads:

> To save space, items that do not apply to a function are omitted. For example, if a function does not specify any further preconditions, there will be no "Requires" paragraph.

Rationale:

The standard is somewhat inconsistent, but a failure to note a throw condition in a throws clause does not grant permission not to throw. The inconsistent wording is in a footnote, and thus non-normative. The proposed resolution from the LWG clarifies the footnote.

### 223. reverse algorithm should use iter_swap rather than swap

Section: 25.3.10 [alg.reverse] Status: TC1 Submitter: Dave Abrahams Opened: 2000-03-21 Last modified: 2015-04-08

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Discussion:

Shouldn't the effects say "applies iter_swap to all pairs..."?

Proposed resolution:

In 25.3.10 [alg.reverse], replace:

> Effects: For each non-negative integer i <= (last - first)/2, applies swap to all pairs of iterators first + i, (last - i) - 1.

with:

> Effects: For each non-negative integer i <= (last - first)/2, applies iter_swap to all pairs of iterators first + i, (last - i) - 1.

### 224. clear() complexity for associative containers refers to undefined N

Section: 23.2.4 [associative.reqmts] Status: TC1 Submitter: Ed Brey Opened: 2000-03-23 Last modified: 2015-04-08

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Discussion:

In the associative container requirements table in 23.1.2 paragraph 7, a.clear() has complexity "log(size()) + N". However, the meaning of N is not defined.

Proposed resolution:

In the associative container requirements table in 23.1.2 paragraph 7, the complexity of a.clear(), change "log(size()) + N" to "linear in `size()`".

Rationale:

It's the "log(size())", not the "N", that is in error: there's no difference between O(N) and O(N + log(N)). The text in the standard is probably an incorrect cut-and-paste from the range version of `erase`.

### 225. std:: algorithms use of other unqualified algorithms

Section: 17.6.5.4 [global.functions] Status: CD1 Submitter: Dave Abrahams Opened: 2000-04-01 Last modified: 2015-04-08

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Discussion:

Are algorithms in std:: allowed to use other algorithms without qualification, so functions in user namespaces might be found through Koenig lookup?

For example, a popular standard library implementation includes this implementation of std::unique:

>     namespace std {
>         template <class _ForwardIter>
>         _ForwardIter unique(_ForwardIter __first, _ForwardIter __last) {
>           __first = adjacent_find(__first, __last);
>           return unique_copy(__first, __last, __first);
>         }
>         }

Imagine two users on opposite sides of town, each using unique on his own sequences bounded by my_iterators . User1 looks at his standard library implementation and says, "I know how to implement a more efficient unique_copy for my_iterators", and writes:

>     namespace user1 {
>         class my_iterator;
>         // faster version for my_iterator
>         my_iterator unique_copy(my_iterator, my_iterator, my_iterator);
>         }

user1::unique_copy() is selected by Koenig lookup, as he intended.

User2 has other needs, and writes:

>     namespace user2 {
>         class my_iterator;
>         // Returns true iff *c is a unique copy of *a and *b.
>         bool unique_copy(my_iterator a, my_iterator b, my_iterator c);
>         }

User2 is shocked to find later that his fully-qualified use of std::unique(user2::my_iterator, user2::my_iterator, user2::my_iterator) fails to compile (if he's lucky). Looking in the standard, he sees the following Effects clause for unique():

> Effects: Eliminates all but the first element from every consecutive group of equal elements referred to by the iterator i in the range [first, last) for which the following corresponding conditions hold: *i == *(i - 1) or pred(*i, *(i - 1)) != false

The standard gives user2 absolutely no reason to think he can interfere with std::unique by defining names in namespace user2. His standard library has been built with the template export feature, so he is unable to inspect the implementation. User1 eventually compiles his code with another compiler, and his version of unique_copy silently stops being called. Eventually, he realizes that he was depending on an implementation detail of his library and had no right to expect his unique_copy() to be called portably.

On the face of it, and given above scenario, it may seem obvious that the implementation of unique() shown is non-conforming because it uses unique_copy() rather than ::std::unique_copy(). Most standard library implementations, however, seem to disagree with this notion.

[Tokyo: Steve Adamczyk from the core working group indicates that "std::" is sufficient; leading "::" qualification is not required because any namespace qualification is sufficient to suppress Koenig lookup.]

Proposed resolution:

Add a paragraph and a note at the end of 17.6.5.4 [global.functions]:

> Unless otherwise specified, no global or non-member function in the standard library shall use a function from another namespace which is found through argument-dependent name lookup (3.4.2 [basic.lookup.argdep]).

> [Note: the phrase "unless otherwise specified" is intended to allow Koenig lookup in cases like that of ostream_iterators:
> Effects:

> > *out_stream << value;
> > if(delim != 0) *out_stream << delim;
> > return (*this);

> > --end note]

[Tokyo: The LWG agrees that this is a defect in the standard, but is as yet unsure if the proposed resolution is the best solution. Furthermore, the LWG believes that the same problem of unqualified library names applies to wording in the standard itself, and has opened issue 229 accordingly. Any resolution of issue 225 should be coordinated with the resolution of issue 229.]

[Toronto: The LWG is not sure if this is a defect in the standard. Most LWG members believe that an implementation of `std::unique` like the one quoted in this issue is already illegal, since, under certain circumstances, its semantics are not those specified in the standard. The standard's description of `unique` does not say that overloading `adjacent_find` should have any effect.]

[Curaçao: An LWG-subgroup spent an afternoon working on issues 225, 226, and 229. Their conclusion was that the issues should be separated into an LWG portion (Howard's paper, N1387=02-0045), and a EWG portion (Dave will write a proposal). The LWG and EWG had (separate) discussions of this plan the next day. The proposed resolution for this issue is in accordance with Howard's paper.]

Rationale:

It could be argued that this proposed isn't strictly necessary, that the Standard doesn't grant implementors license to write a standard function that behaves differently than specified in the Standard just because of an unrelated user-defined name in some other namespace. However, this is at worst a clarification. It is surely right that algorithsm shouldn't pick up random names, that user-defined names should have no effect unless otherwise specified. Issue 226 deals with the question of when it is appropriate for the standard to explicitly specify otherwise.

### 226. User supplied specializations or overloads of namespace std function templates

Section: 17.6.4.3 [reserved.names] Status: CD1 Submitter: Dave Abrahams Opened: 2000-04-01 Last modified: 2015-04-08

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Discussion:

The issues are:

1. How can a 3rd party library implementor (lib1) write a version of a standard algorithm which is specialized to work with his own class template?

2. How can another library implementor (lib2) write a generic algorithm which will take advantage of the specialized algorithm in lib1?

This appears to be the only viable answer under current language rules:

>     namespace lib1
>     {
>         // arbitrary-precision numbers using T as a basic unit
>         template <class T>
>         class big_num { //...
>         };

>         // defining this in namespace std is illegal (it would be an
>         // overload), so we hope users will rely on Koenig lookup
>         template <class T>
>         void swap(big_int<T>&, big_int<T>&);
>     }

>     #include <algorithm>
>     namespace lib2
>     {
>         template <class T>
>         void generic_sort(T* start, T* end)
>         {
>                 ...
>             // using-declaration required so we can work on built-in types
>             using std::swap;
>             // use Koenig lookup to find specialized algorithm if available
>             swap(*x, *y);
>         }
>     }

This answer has some drawbacks. First of all, it makes writing lib2 difficult and somewhat slippery. The implementor needs to remember to write the using-declaration, or generic_sort will fail to compile when T is a built-in type. The second drawback is that the use of this style in lib2 effectively "reserves" names in any namespace which defines types which may eventually be used with lib2. This may seem innocuous at first when applied to names like swap, but consider more ambiguous names like unique_copy() instead. It is easy to imagine the user wanting to define these names differently in his own namespace. A definition with semantics incompatible with the standard library could cause serious problems (see issue 225).

Why, you may ask, can't we just partially specialize std::swap()? It's because the language doesn't allow for partial specialization of function templates. If you write:

>     namespace std
>     {
>         template <class T>
>         void swap(lib1::big_int<T>&, lib1::big_int<T>&);
>     }

You have just overloaded std::swap, which is illegal under the current language rules. On the other hand, the following full specialization is legal:

>     namespace std
>     {
>         template <>
>         void swap(lib1::other_type&, lib1::other_type&);
>     }

This issue reflects concerns raised by the "Namespace issue with specialized swap" thread on comp.lang.c++.moderated. A similar set of concerns was earlier raised on the boost.org mailing list and the ACCU-general mailing list. Also see library reflector message c++std-lib-7354.

J. C. van Winkel points out (in c++std-lib-9565) another unexpected fact: it's impossible to output a container of std::pair's using copy and an ostream_iterator, as long as both pair-members are built-in or std:: types. That's because a user-defined operator<< for (for example) std::pair<const std::string, int> will not be found: lookup for operator<< will be performed only in namespace std. Opinions differed on whether or not this was a defect, and, if so, whether the defect is that something is wrong with user-defined functionality and std, or whether it's that the standard library does not provide an operator<< for std::pair<>.

Proposed resolution:

Adopt the wording proposed in Howard Hinnant's paper N1523=03-0106, "Proposed Resolution To LWG issues 225, 226, 229".

[Tokyo: Summary, "There is no conforming way to extend std::swap for user defined templates." The LWG agrees that there is a problem. Would like more information before proceeding. This may be a core issue. Core issue 229 has been opened to discuss the core aspects of this problem. It was also noted that submissions regarding this issue have been received from several sources, but too late to be integrated into the issues list. ]

[Post-Tokyo: A paper with several proposed resolutions, J16/00-0029==WG21/N1252, "Shades of namespace std functions " by Alan Griffiths, is in the Post-Tokyo mailing. It should be considered a part of this issue.]

[Toronto: Dave Abrahams and Peter Dimov have proposed a resolution that involves core changes: it would add partial specialization of function template. The Core Working Group is reluctant to add partial specialization of function templates. It is viewed as a large change, CWG believes that proposal presented leaves some syntactic issues unanswered; if the CWG does add partial specialization of function templates, it wishes to develop its own proposal. The LWG continues to believe that there is a serious problem: there is no good way for users to force the library to use user specializations of generic standard library functions, and in certain cases (e.g. transcendental functions called by `valarray` and `complex`) this is important. Koenig lookup isn't adequate, since names within the library must be qualified with `std` (see issue 225), specialization doesn't work (we don't have partial specialization of function templates), and users aren't permitted to add overloads within namespace std. ]

[Copenhagen: Discussed at length, with no consensus. Relevant papers in the pre-Copenhagen mailing: N1289, N1295, N1296. Discussion focused on four options. (1) Relax restrictions on overloads within namespace std. (2) Mandate that the standard library use unqualified calls for `swap` and possibly other functions. (3) Introduce helper class templates for `swap` and possibly other functions. (4) Introduce partial specialization of function templates. Every option had both support and opposition. Straw poll (first number is support, second is strongly opposed): (1) 6, 4; (2) 6, 7; (3) 3, 8; (4) 4, 4.]

[Redmond: Discussed, again no consensus. Herb presented an argument that a user who is defining a type `T` with an associated `swap` should not be expected to put that `swap` in namespace std, either by overloading or by partial specialization. The argument is that `swap` is part of `T`'s interface, and thus should to in the same namespace as `T` and only in that namespace. If we accept this argument, the consequence is that standard library functions should use unqualified call of `swap`. (And which other functions? Any?) A small group (Nathan, Howard, Jeremy, Dave, Matt, Walter, Marc) will try to put together a proposal before the next meeting.]

[Curaçao: An LWG-subgroup spent an afternoon working on issues 225, 226, and 229. Their conclusion was that the issues should be separated into an LWG portion (Howard's paper, N1387=02-0045), and a EWG portion (Dave will write a proposal). The LWG and EWG had (separate) discussions of this plan the next day. The proposed resolution is the one proposed by Howard.]

[Santa Cruz: the LWG agreed with the general direction of Howard's paper, N1387. (Roughly: Koenig lookup is disabled unless we say otherwise; this issue is about when we do say otherwise.) However, there were concerns about wording. Howard will provide new wording. Bill and Jeremy will review it.]

[Kona: Howard proposed the new wording. The LWG accepted his proposed resolution.]

Rationale:

Informally: introduce a Swappable concept, and specify that the value types of the iterators passed to certain standard algorithms (such as iter_swap, swap_ranges, reverse, rotate, and sort) conform to that concept. The Swappable concept will make it clear that these algorithms use unqualified lookup for the calls to `swap`. Also, in 26.6.3.3 [valarray.transcend] paragraph 1, state that the valarray transcendentals use unqualified lookup.

### 227. std::swap() should require CopyConstructible or DefaultConstructible arguments

Section: 25.3.3 [alg.swap] Status: TC1 Submitter: Dave Abrahams Opened: 2000-04-09 Last modified: 2015-04-08

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Discussion:

25.2.2 reads:

> `template<class T> void swap(T& a, T& b);`
> Requires: Type T is Assignable (_lib.container.requirements_).
> Effects: Exchanges values stored in two locations.

The only reasonable** generic implementation of swap requires construction of a new temporary copy of one of its arguments:

>     template<class T> void swap(T& a, T& b);
>       {
>           T tmp(a);
>           a = b;
>           b = tmp;
>       }

But a type which is only Assignable cannot be swapped by this implementation.

**Yes, there's also an unreasonable implementation which would require T to be DefaultConstructible instead of CopyConstructible. I don't think this is worthy of consideration:

>     template<class T> void swap(T& a, T& b);
>     {
>         T tmp;
>         tmp = a;
>         a = b;
>         b = tmp;
>     }

Proposed resolution:

Change 25.2.2 paragraph 1 from:

> Requires: Type T is Assignable (23.1).

to:

> Requires: Type T is CopyConstructible (20.1.3) and Assignable (23.1)

### 228. Incorrect specification of "..._byname" facets

Section: 22.4 [locale.categories] Status: CD1 Submitter: Dietmar Kühl Opened: 2000-04-20 Last modified: 2015-04-08

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Discussion:

The sections 22.4.1.2 [locale.ctype.byname], 22.4.1.5 [locale.codecvt.byname], sref ref="22.2.1.6", 22.4.3.2 [locale.numpunct.byname], 22.4.4.2 [locale.collate.byname], 22.4.5.4 [locale.time.put.byname], 22.4.6.4 [locale.moneypunct.byname], and 22.4.7.2 [locale.messages.byname] overspecify the definitions of the "..._byname" classes by listing a bunch of virtual functions. At the same time, no semantics of these functions are defined. Real implementations do not define these functions because the functional part of the facets is actually implemented in the corresponding base classes and the constructor of the "..._byname" version just provides suitable date used by these implementations. For example, the 'numpunct' methods just return values from a struct. The base class uses a statically initialized struct while the derived version reads the contents of this struct from a table. However, no virtual function is defined in 'numpunct_byname'.

For most classes this does not impose a problem but specifically for 'ctype' it does: The specialization for 'ctype_byname<char>' is required because otherwise the semantics would change due to the virtual functions defined in the general version for 'ctype_byname': In 'ctype<char>' the method 'do_is()' is not virtual but it is made virtual in both 'ctype<cT>' and 'ctype_byname<cT>'. Thus, a class derived from 'ctype_byname<char>' can tell whether this class is specialized or not under the current specification: Without the specialization, 'do_is()' is virtual while with specialization it is not virtual.

Proposed resolution:

Change section 22.2.1.2 (lib.locale.ctype.byname) to become:

         namespace std {
           template <class charT>
           class ctype_byname : public ctype<charT> {
           public:
             typedef ctype<charT>::mask mask;
             explicit ctype_byname(const char*, size_t refs = 0);
           protected:
            ~ctype_byname();             //  virtual
           };
         }

Change section 22.2.1.6 (lib.locale.codecvt.byname) to become:

        namespace std {
          template <class internT, class externT, class stateT>
          class codecvt_byname : public codecvt<internT, externT, stateT> {
          public:
           explicit codecvt_byname(const char*, size_t refs = 0);
          protected:
          ~codecvt_byname();             //  virtual
           };
         }

Change section 22.2.3.2 (lib.locale.numpunct.byname) to become:

         namespace std {
           template <class charT>
           class numpunct_byname : public numpunct<charT> {
         //  this class is specialized for  char  and  wchar_t.
           public:
             typedef charT                char_type;
             typedef basic_string<charT>  string_type;
             explicit numpunct_byname(const char*, size_t refs = 0);
           protected:
            ~numpunct_byname();          //  virtual
           };
         }

Change section 22.2.4.2 (lib.locale.collate.byname) to become:

         namespace std {
           template <class charT>
           class collate_byname : public collate<charT> {
           public:
             typedef basic_string<charT> string_type;
             explicit collate_byname(const char*, size_t refs = 0);
           protected:
            ~collate_byname();           //  virtual
           };
         }

Change section 22.2.5.2 (lib.locale.time.get.byname) to become:

         namespace std {
           template <class charT, class InputIterator = istreambuf_iterator<charT> >
           class time_get_byname : public time_get<charT, InputIterator> {
           public:
             typedef time_base::dateorder dateorder;
             typedef InputIterator        iter_type

             explicit time_get_byname(const char*, size_t refs = 0);
           protected:
            ~time_get_byname();          //  virtual
           };
         }

Change section 22.2.5.4 (lib.locale.time.put.byname) to become:

         namespace std {
           template <class charT, class OutputIterator = ostreambuf_iterator<charT> >
           class time_put_byname : public time_put<charT, OutputIterator>
           {
           public:
             typedef charT          char_type;
             typedef OutputIterator iter_type;

             explicit time_put_byname(const char*, size_t refs = 0);
           protected:
            ~time_put_byname();          //  virtual
           };
         }"

Change section 22.2.6.4 (lib.locale.moneypunct.byname) to become:

         namespace std {
           template <class charT, bool Intl = false>
           class moneypunct_byname : public moneypunct<charT, Intl> {
           public:
             typedef money_base::pattern pattern;
             typedef basic_string<charT> string_type;

             explicit moneypunct_byname(const char*, size_t refs = 0);
           protected:
            ~moneypunct_byname();        //  virtual
           };
         }

Change section 22.2.7.2 (lib.locale.messages.byname) to become:

         namespace std {
           template <class charT>
           class messages_byname : public messages<charT> {
           public:
             typedef messages_base::catalog catalog;
             typedef basic_string<charT>    string_type;

             explicit messages_byname(const char*, size_t refs = 0);
           protected:
            ~messages_byname();          //  virtual
           };
         }

Remove section 22.4.1.4 [locale.codecvt] completely (because in this case only those members are defined to be virtual which are defined to be virtual in 'ctype<cT>'.)

[Post-Tokyo: Dietmar Kühl submitted this issue at the request of the LWG to solve the underlying problems raised by issue 138.]

[Copenhagen: proposed resolution was revised slightly, to remove three last virtual functions from `messages_byname`.]

### 229. Unqualified references of other library entities

Section: 17.6.1.1 [contents] Status: CD1 Submitter: Steve Clamage Opened: 2000-04-19 Last modified: 2015-04-08

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Discussion:

Throughout the library chapters, the descriptions of library entities refer to other library entities without necessarily qualifying the names.

For example, section 25.2.2 "Swap" describes the effect of swap_ranges in terms of the unqualified name "swap". This section could reasonably be interpreted to mean that the library must be implemented so as to do a lookup of the unqualified name "swap", allowing users to override any ::std::swap function when Koenig lookup applies.

Although it would have been best to use explicit qualification with "::std::" throughout, too many lines in the standard would have to be adjusted to make that change in a Technical Corrigendum.

Issue 182, which addresses qualification of `size_t`, is a special case of this.

Proposed resolution:

To section 17.4.1.1 "Library contents" Add the following paragraph:

> Whenever a name x defined in the standard library is mentioned, the name x is assumed to be fully qualified as ::std::x, unless explicitly described otherwise. For example, if the Effects section for library function F is described as calling library function G, the function ::std::G is meant.

[Post-Tokyo: Steve Clamage submitted this issue at the request of the LWG to solve a problem in the standard itself similar to the problem within implementations of library identified by issue 225. Any resolution of issue 225 should be coordinated with the resolution of this issue.]

[post-Toronto: Howard is undecided about whether it is appropriate for all standard library function names referred to in other standard library functions to be explicitly qualified by `std`: it is common advice that users should define global functions that operate on their class in the same namespace as the class, and this requires argument-dependent lookup if those functions are intended to be called by library code. Several LWG members are concerned that valarray appears to require argument-dependent lookup, but that the wording may not be clear enough to fall under "unless explicitly described otherwise".]

[Curaçao: An LWG-subgroup spent an afternoon working on issues 225, 226, and 229. Their conclusion was that the issues should be separated into an LWG portion (Howard's paper, N1387=02-0045), and a EWG portion (Dave will write a proposal). The LWG and EWG had (separate) discussions of this plan the next day. This paper resolves issues 225 and 226. In light of that resolution, the proposed resolution for the current issue makes sense.]

### 230. Assignable specified without also specifying CopyConstructible

Section: 17 [library] Status: CD1 Submitter: Beman Dawes Opened: 2000-04-26 Last modified: 2015-04-08

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Discussion:

Issue 227 identified an instance (std::swap) where Assignable was specified without also specifying CopyConstructible. The LWG asked that the standard be searched to determine if the same defect existed elsewhere.

There are a number of places (see proposed resolution below) where Assignable is specified without also specifying CopyConstructible. There are also several cases where both are specified. For example, 26.5.1 [rand.req].

Proposed resolution:

In 23.2 [container.requirements] table 65 for value_type: change "T is Assignable" to "T is CopyConstructible and Assignable"

In 23.2.4 [associative.reqmts] table 69 X::key_type; change "Key is Assignable" to "Key is CopyConstructible and Assignable"

In 24.2.4 [output.iterators] paragraph 1, change:

> A class or a built-in type X satisfies the requirements of an output iterator if X is an Assignable type (23.1) and also the following expressions are valid, as shown in Table 73:

to:

> A class or a built-in type X satisfies the requirements of an output iterator if X is a CopyConstructible (20.1.3) and Assignable type (23.1) and also the following expressions are valid, as shown in Table 73:

[Post-Tokyo: Beman Dawes submitted this issue at the request of the LWG. He asks that the 25.3.5 [alg.replace] and 25.3.6 [alg.fill] changes be studied carefully, as it is not clear that CopyConstructible is really a requirement and may be overspecification.]

[Portions of the resolution for issue 230 have been superceded by the resolution of issue 276.]

Rationale:

The original proposed resolution also included changes to input iterator, fill, and replace. The LWG believes that those changes are not necessary. The LWG considered some blanket statement, where an Assignable type was also required to be Copy Constructible, but decided against this because fill and replace really don't require the Copy Constructible property.

### 231. Precision in iostream?

Section: 22.4.2.2.2 [facet.num.put.virtuals] Status: CD1 Submitter: James Kanze, Stephen Clamage Opened: 2000-04-25 Last modified: 2015-04-08

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Discussion:

What is the following program supposed to output?

    #include <iostream>

        int
        main()
        {
            std::cout.setf( std::ios::scientific , std::ios::floatfield ) ;
            std::cout.precision( 0 ) ;
            std::cout << 1.00 << '\n' ;
            return 0 ;
        }

From my C experience, I would expect "1e+00"; this is what `printf("%.0e" , 1.00 );` does. G++ outputs "1.000000e+00".

The only indication I can find in the standard is 22.2.2.2.2/11, where it says "For conversion from a floating-point type, if (flags & fixed) != 0 or if str.precision() > 0, then str.precision() is specified in the conversion specification." This is an obvious error, however, fixed is not a mask for a field, but a value that a multi-bit field may take -- the results of and'ing fmtflags with ios::fixed are not defined, at least not if ios::scientific has been set. G++'s behavior corresponds to what might happen if you do use (flags & fixed) != 0 with a typical implementation (floatfield == 3 << something, fixed == 1 << something, and scientific == 2 << something).

Presumably, the intent is either (flags & floatfield) != 0, or (flags & floatfield) == fixed; the first gives something more or less like the effect of precision in a printf floating point conversion. Only more or less, of course. In order to implement printf formatting correctly, you must know whether the precision was explicitly set or not. Say by initializing it to -1, instead of 6, and stating that for floating point conversions, if precision < -1, 6 will be used, for fixed point, if precision < -1, 1 will be used, etc. Plus, of course, if precision == 0 and flags & floatfield == 0, 1 should be = used. But it probably isn't necessary to emulate all of the anomalies of printf:-).

Proposed resolution:

Replace 22.4.2.2.2 [facet.num.put.virtuals], paragraph 11, with the following sentence:

> For conversion from a floating-point type, `str.precision()` is specified in the conversion specification.

Rationale:

The floatfield determines whether numbers are formatted as if with %f, %e, or %g. If the `fixed` bit is set, it's %f, if `scientific` it's %e, and if both bits are set, or neither, it's %g.

Turning to the C standard, a precision of 0 is meaningful for %f and %e. For %g, precision 0 is taken to be the same as precision 1.

The proposed resolution has the effect that if neither `fixed` nor `scientific` is set we'll be specifying a precision of 0, which will be internally turned into 1. There's no need to call it out as a special case.

The output of the above program will be "1e+00".

[Post-Curaçao: Howard provided improved wording covering the case where precision is 0 and mode is %g.]

### 232. "depends" poorly defined in 17.4.3.1

Section: 17.6.4.3 [reserved.names] Status: CD1 Submitter: Peter Dimov Opened: 2000-04-18 Last modified: 2015-04-08

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Discussion:

17.4.3.1/1 uses the term "depends" to limit the set of allowed specializations of standard templates to those that "depend on a user-defined name of external linkage."

This term, however, is not adequately defined, making it possible to construct a specialization that is, I believe, technically legal according to 17.4.3.1/1, but that specializes a standard template for a built-in type such as 'int'.

The following code demonstrates the problem:

>     #include <algorithm>

>     template<class T> struct X
>     {
>      typedef T type;
>     };

>     namespace std
>     {
>      template<> void swap(::X<int>::type& i, ::X<int>::type& j);
>     }

Proposed resolution:

Change "user-defined name" to "user-defined type".

Rationale:

This terminology is used in section 2.5.2 and 4.1.1 of The C++ Programming Language. It disallows the example in the issue, since the underlying type itself is not user-defined. The only possible problem I can see is for non-type templates, but there's no possible way for a user to come up with a specialization for bitset, for example, that might not have already been specialized by the implementor?

[Toronto: this may be related to issue 120.]

[post-Toronto: Judy provided the above proposed resolution and rationale.]

### 233. Insertion hints in associative containers

Section: 23.2.4 [associative.reqmts] Status: CD1 Submitter: Andrew Koenig Opened: 2000-04-30 Last modified: 2015-04-08

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Duplicate of: 192, 246

Discussion:

If `mm` is a multimap and `p` is an iterator into the multimap, then `mm.insert(p, x)` inserts `x` into `mm` with `p` as a hint as to where it should go. Table 69 claims that the execution time is amortized constant if the insert winds up taking place adjacent to `p`, but does not say when, if ever, this is guaranteed to happen. All it says it that `p` is a hint as to where to insert.

The question is whether there is any guarantee about the relationship between `p` and the insertion point, and, if so, what it is.

I believe the present state is that there is no guarantee: The user can supply `p`, and the implementation is allowed to disregard it entirely.

Additional comments from Nathan:
The vote [in Redmond] was on whether to elaborately specify the use of the hint, or to require behavior only if the value could be inserted adjacent to the hint. I would like to ensure that we have a chance to vote for a deterministic treatment: "before, if possible, otherwise after, otherwise anywhere appropriate", as an alternative to the proposed "before or after, if possible, otherwise [...]".

[Toronto: there was general agreement that this is a real defect: when inserting an element x into a multiset that already contains several copies of x, there is no way to know whether the hint will be used. The proposed resolution was that the new element should always be inserted as close to the hint as possible. So, for example, if there is a subsequence of equivalent values, then providing a.begin() as the hint means that the new element should be inserted before the subsequence even if a.begin() is far away. JC van Winkel supplied precise wording for this proposed resolution, and also for an alternative resolution in which hints are only used when they are adjacent to the insertion point.]

[Copenhagen: the LWG agreed to the original proposed resolution, in which an insertion hint would be used even when it is far from the insertion point. This was contingent on seeing a example implementation showing that it is possible to implement this requirement without loss of efficiency. John Potter provided such a example implementation.]

[Redmond: The LWG was reluctant to adopt the proposal that emerged from Copenhagen: it seemed excessively complicated, and went beyond fixing the defect that we identified in Toronto. PJP provided the new wording described in this issue. Nathan agrees that we shouldn't adopt the more detailed semantics, and notes: "we know that you can do it efficiently enough with a red-black tree, but there are other (perhaps better) balanced tree techniques that might differ enough to make the detailed semantics hard to satisfy."]

[Curaçao: Nathan should give us the alternative wording he suggests so the LWG can decide between the two options.]

[Lillehammer: The LWG previously rejected the more detailed semantics, because it seemed more loike a new feature than like defect fixing. We're now more sympathetic to it, but we (especially Bill) are still worried about performance. N1780 describes a naive algorithm, but it's not clear whether there is a non-naive implementation. Is it possible to implement this as efficently as the current version of insert?]

[Post Lillehammer: N1780 updated in post meeting mailing with feedback from Lillehammer with more information regarding performance. ]

[ Batavia: 1780 accepted with minor wording changes in the proposed wording (reflected in the proposed resolution below). Concerns about the performance of the algorithm were satisfactorily met by 1780. 371 already handles the stability of equal ranges and so that part of the resolution from 1780 is no longer needed (or reflected in the proposed wording below). ]

Proposed resolution:

Change the indicated rows of the "Associative container requirements" Table in 23.2.4 [associative.reqmts] to:

Associative container requirements
expression | return type | assertion/note pre/post-condition | complexity
--- | --- | --- | ---
`a_eq.insert(t)` | `iterator` | inserts `t` and returns the iterator pointing to the newly inserted element. If a range containing elements equivalent to `t` exists in `a_eq`, `t` is inserted at the end of that range. | logarithmic

`a.insert(p,t)`

`iterator`

inserts `t` if and only if there is no element with key equivalent to the key of `t` in containers with unique keys; always inserts `t` in containers with equivalent keys. always returns the iterator pointing to the element with key equivalent to the key of `t`. iterator `p` is a hint pointing to where the insert should start to search. `t` is inserted as close as possible to the position just prior to `p`.

logarithmic in general, but amortized constant if `t` is inserted right after before `p`.

### 234. Typos in allocator definition

Section: 20.7.9.1 [allocator.members] Status: CD1 Submitter: Dietmar Kühl Opened: 2000-04-24 Last modified: 2015-04-08

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Discussion:

In paragraphs 12 and 13 the effects of `construct()` and `destruct()` are described as returns but the functions actually return `void`.

Proposed resolution:

Substitute "Returns" by "Effect".

### 235. No specification of default ctor for reverse_iterator

Section: 24.5.1.1 [reverse.iterator] Status: CD1 Submitter: Dietmar Kühl Opened: 2000-04-24 Last modified: 2015-04-08

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Discussion:

The declaration of `reverse_iterator` lists a default constructor. However, no specification is given what this constructor should do.

Proposed resolution:

In section 24.5.1.3.1 [reverse.iter.cons] add the following paragraph:

> `reverse_iterator()`

> Default initializes `current`. Iterator operations applied to the resulting iterator have defined behavior if and only if the corresponding operations are defined on a default constructed iterator of type `Iterator`.

[pre-Copenhagen: Dietmar provide wording for proposed resolution.]

### 237. Undefined expression in complexity specification

Section: 23.3.3.2 [deque.cons] Status: CD1 Submitter: Dietmar Kühl Opened: 2000-04-24 Last modified: 2015-04-08

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Discussion:

The complexity specification in paragraph 6 says that the complexity is linear in `first - last`. Even if `operator-()` is defined on iterators this term is in general undefined because it would have to be `last - first`.

Proposed resolution:

Change paragraph 6 from

> Linear in first - last.

to become

> Linear in distance(first, last).

### 238. Contradictory results of stringbuf initialization.

Section: 27.8.2.1 [stringbuf.cons] Status: CD1 Submitter: Dietmar Kühl Opened: 2000-05-11 Last modified: 2015-04-08

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Discussion:

In 27.7.1.1 paragraph 4 the results of calling the constructor of 'basic_stringbuf' are said to be `str() == str`. This is fine that far but consider this code:

      std::basic_stringbuf<char> sbuf("hello, world", std::ios_base::openmode(0));
      std::cout << "'" << sbuf.str() << "'\n";

Paragraph 3 of 27.7.1.1 basically says that in this case neither the output sequence nor the input sequence is initialized and paragraph 2 of 27.7.1.2 basically says that `str()` either returns the input or the output sequence. None of them is initialized, ie. both are empty, in which case the return from `str()` is defined to be `basic_string<cT>()`.

However, probably only test cases in some testsuites will detect this "problem"...

Proposed resolution:

Remove 27.7.1.1 paragraph 4.

Rationale:

We could fix 27.7.1.1 paragraph 4, but there would be no point. If we fixed it, it would say just the same thing as text that's already in the standard.

### 239. Complexity of unique() and/or unique_copy incorrect

Section: 25.3.9 [alg.unique] Status: CD1 Submitter: Angelika Langer Opened: 2000-05-15 Last modified: 2015-04-08

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Discussion:

The complexity of unique and unique_copy are inconsistent with each other and inconsistent with the implementations. The standard specifies:

for unique():

> -3- Complexity: If the range (last - first) is not empty, exactly (last - first) - 1 applications of the corresponding predicate, otherwise no applications of the predicate.

for unique_copy():

> -7- Complexity: Exactly last - first applications of the corresponding predicate.

The implementations do it the other way round: unique() applies the predicate last-first times and unique_copy() applies it last-first-1 times.

As both algorithms use the predicate for pair-wise comparison of sequence elements I don't see a justification for unique_copy() applying the predicate last-first times, especially since it is not specified to which pair in the sequence the predicate is applied twice.

Proposed resolution:

Change both complexity sections in 25.3.9 [alg.unique] to:

> Complexity: For nonempty ranges, exactly last - first - 1 applications of the corresponding predicate.

### 240. Complexity of adjacent_find() is meaningless

Section: 25.2.8 [alg.adjacent.find] Status: CD1 Submitter: Angelika Langer Opened: 2000-05-15 Last modified: 2015-04-08

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Discussion:

The complexity section of adjacent_find is defective:

>     template <class ForwardIterator>
>     ForwardIterator adjacent_find(ForwardIterator first, ForwardIterator last
>                                   BinaryPredicate pred);

> -1- Returns: The first iterator i such that both i and i + 1 are in the range [first, last) for which the following corresponding conditions hold: *i == *(i + 1), pred(*i, *(i + 1)) != false. Returns last if no such iterator is found.

> -2- Complexity: Exactly find(first, last, value) - first applications of the corresponding predicate.

In the Complexity section, it is not defined what "value" is supposed to mean. My best guess is that "value" means an object for which one of the conditions pred(*i,value) or pred(value,*i) is true, where i is the iterator defined in the Returns section. However, the value type of the input sequence need not be equality-comparable and for this reason the term find(first, last, value) - first is meaningless.

A term such as find_if(first, last, bind2nd(pred,*i)) - first or find_if(first, last, bind1st(pred,*i)) - first might come closer to the intended specification. Binders can only be applied to function objects that have the function call operator declared const, which is not required of predicates because they can have non-const data members. For this reason, a specification using a binder could only be an "as-if" specification.

Proposed resolution:

Change the complexity section in 25.2.8 [alg.adjacent.find] to:

> For a nonempty range, exactly `min((i - first) + 1, (last - first) - 1)` applications of the corresponding predicate, where i is `adjacent_find`'s return value.

[Copenhagen: the original resolution specified an upper bound. The LWG preferred an exact count.]

### 241. Does unique_copy() require CopyConstructible and Assignable?

Section: 25.3.9 [alg.unique] Status: CD1 Submitter: Angelika Langer Opened: 2000-05-15 Last modified: 2015-04-08

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Discussion:

Some popular implementations of unique_copy() create temporary copies of values in the input sequence, at least if the input iterator is a pointer. Such an implementation is built on the assumption that the value type is CopyConstructible and Assignable.

It is common practice in the standard that algorithms explicitly specify any additional requirements that they impose on any of the types used by the algorithm. An example of an algorithm that creates temporary copies and correctly specifies the additional requirements is accumulate(), 26.5.1 [rand.req].

Since the specifications of unique() and unique_copy() do not require CopyConstructible and Assignable of the InputIterator's value type the above mentioned implementations are not standard-compliant. I cannot judge whether this is a defect in the standard or a defect in the implementations.

Proposed resolution:

In 25.2.8 change:

> -4- Requires: The ranges [first, last) and [result, result+(last-first)) shall not overlap.

to:

> -4- Requires: The ranges [first, last) and [result, result+(last-first)) shall not overlap. The expression *result = *first must be valid. If neither InputIterator nor OutputIterator meets the requirements of forward iterator then the value type of InputIterator must be copy constructible. Otherwise copy constructible is not required.

[Redmond: the original proposed resolution didn't impose an explicit requirement that the iterator's value type must be copy constructible, on the grounds that an input iterator's value type must always be copy constructible. Not everyone in the LWG thought that this requirement was clear from table 72. It has been suggested that it might be possible to implement `unique_copy` without requiring assignability, although current implementations do impose that requirement. Howard provided new wording.]

[ Curaçao: The LWG changed the PR editorially to specify "neither...nor...meet..." as clearer than "both...and...do not meet...". Change believed to be so minor as not to require re-review. ]

### 242. Side effects of function objects

Section: 25.3.4 [alg.transform], 26.5 [rand] Status: CD1 Submitter: Angelika Langer Opened: 2000-05-15 Last modified: 2015-04-08

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Discussion:

The algorithms transform(), accumulate(), inner_product(), partial_sum(), and adjacent_difference() require that the function object supplied to them shall not have any side effects.

The standard defines a side effect in 1.9 [intro.execution] as:

> -7- Accessing an object designated by a volatile lvalue (basic.lval), modifying an object, calling a library I/O function, or calling a function that does any of those operations are all side effects, which are changes in the state of the execution environment.

As a consequence, the function call operator of a function object supplied to any of the algorithms listed above cannot modify data members, cannot invoke any function that has a side effect, and cannot even create and modify temporary objects. It is difficult to imagine a function object that is still useful under these severe limitations. For instance, any non-trivial transformator supplied to transform() might involve creation and modification of temporaries, which is prohibited according to the current wording of the standard.

On the other hand, popular implementations of these algorithms exhibit uniform and predictable behavior when invoked with a side-effect-producing function objects. It looks like the strong requirement is not needed for efficient implementation of these algorithms.

The requirement of side-effect-free function objects could be replaced by a more relaxed basic requirement (which would hold for all function objects supplied to any algorithm in the standard library):

> A function objects supplied to an algorithm shall not invalidate any iterator or sequence that is used by the algorithm. Invalidation of the sequence includes destruction of the sorting order if the algorithm relies on the sorting order (see section 25.3 - Sorting and related operations [lib.alg.sorting]).

I can't judge whether it is intended that the function objects supplied to transform(), accumulate(), inner_product(), partial_sum(), or adjacent_difference() shall not modify sequence elements through dereferenced iterators.

It is debatable whether this issue is a defect or a change request. Since the consequences for user-supplied function objects are drastic and limit the usefulness of the algorithms significantly I would consider it a defect.

Proposed resolution:

Things to notice about these changes:

1. The fully-closed ("[]" as opposed to half-closed "[)" ranges are intentional. we want to prevent side-effects from invalidating the end iterators.
2. That has the unintentional side-effect of prohibiting modification of the end element as a side-effect. This could conceivably be significant in some cases.
3. The wording also prevents side-effects from modifying elements of the output sequence. I can't imagine why anyone would want to do this, but it is arguably a restriction that implementors don't need to place on users.
4. Lifting the restrictions imposed in #2 and #3 above is possible and simple, but would require more verbiage.

Change 25.2.3/2 from:

> -2- Requires: op and binary_op shall not have any side effects.

to:

> -2- Requires: in the ranges [first1, last1], [first2, first2 + (last1 - first1)] and [result, result + (last1- first1)], op and binary_op shall neither modify elements nor invalidate iterators or subranges. [Footnote: The use of fully closed ranges is intentional --end footnote]

Change 25.2.3/2 from:

> -2- Requires: op and binary_op shall not have any side effects.

to:

> -2- Requires: op and binary_op shall not invalidate iterators or subranges, or modify elements in the ranges [first1, last1], [first2, first2 + (last1 - first1)], and [result, result + (last1 - first1)]. [Footnote: The use of fully closed ranges is intentional --end footnote]

Change 26.4.1/2 from:

> -2- Requires: T must meet the requirements of CopyConstructible (lib.copyconstructible) and Assignable (lib.container.requirements) types. binary_op shall not cause side effects.

to:

> -2- Requires: T must meet the requirements of CopyConstructible (lib.copyconstructible) and Assignable (lib.container.requirements) types. In the range [first, last], binary_op shall neither modify elements nor invalidate iterators or subranges. [Footnote: The use of a fully closed range is intentional --end footnote]

Change 26.4.2/2 from:

> -2- Requires: T must meet the requirements of CopyConstructible (lib.copyconstructible) and Assignable (lib.container.requirements) types. binary_op1 and binary_op2 shall not cause side effects.

to:

> -2- Requires: T must meet the requirements of CopyConstructible (lib.copyconstructible) and Assignable (lib.container.requirements) types. In the ranges [first, last] and [first2, first2 + (last - first)], binary_op1 and binary_op2 shall neither modify elements nor invalidate iterators or subranges. [Footnote: The use of fully closed ranges is intentional --end footnote]

Change 26.4.3/4 from:

> -4- Requires: binary_op is expected not to have any side effects.

to:

> -4- Requires: In the ranges [first, last] and [result, result + (last - first)], binary_op shall neither modify elements nor invalidate iterators or subranges. [Footnote: The use of fully closed ranges is intentional --end footnote]

Change 26.4.4/2 from:

> -2- Requires: binary_op shall not have any side effects.

to:

> -2- Requires: In the ranges [first, last] and [result, result + (last - first)], binary_op shall neither modify elements nor invalidate iterators or subranges. [Footnote: The use of fully closed ranges is intentional --end footnote]

[Toronto: Dave Abrahams supplied wording.]

[Copenhagen: Proposed resolution was modified slightly. Matt added footnotes pointing out that the use of closed ranges was intentional.]

### 243. `get` and `getline` when sentry reports failure

Section: 27.7.2.3 [istream.unformatted] Status: CD1 Submitter: Martin Sebor Opened: 2000-05-15 Last modified: 2015-04-08

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Discussion:

basic_istream<>::get(), and basic_istream<>::getline(), are unclear with respect to the behavior and side-effects of the named functions in case of an error.

27.6.1.3, p1 states that "... If the sentry object returns true, when converted to a value of type bool, the function endeavors to obtain the requested input..." It is not clear from this (or the rest of the paragraph) what precisely the behavior should be when the sentry ctor exits by throwing an exception or when the sentry object returns false. In particular, what is the number of characters extracted that gcount() returns supposed to be?

27.6.1.3 p8 and p19 say about the effects of get() and getline(): "... In any case, it then stores a null character (using charT()) into the next successive location of the array." Is not clear whether this sentence applies if either of the conditions above holds (i.e., when sentry fails).

Proposed resolution:

Add to 27.6.1.3, p1 after the sentence

> "... If the sentry object returns true, when converted to a value of type bool, the function endeavors to obtain the requested input."

the following

> "Otherwise, if the sentry constructor exits by throwing an exception or if the sentry object returns false, when converted to a value of type bool, the function returns without attempting to obtain any input. In either case the number of extracted characters is set to 0; unformatted input functions taking a character array of non-zero size as an argument shall also store a null character (using charT()) in the first location of the array."

Rationale:

Although the general philosophy of the input functions is that the argument should not be modified upon failure, `getline` historically added a terminating null unconditionally. Most implementations still do that. Earlier versions of the draft standard had language that made this an unambiguous requirement; those words were moved to a place where their context made them less clear. See Jerry Schwarz's message c++std-lib-7618.

### 247. `vector`, `deque::insert` complexity

Section: 23.3.6.5 [vector.modifiers] Status: CD1 Submitter: Lisa Lippincott Opened: 2000-06-06 Last modified: 2015-04-08

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Discussion:

Paragraph 2 of 23.3.6.5 [vector.modifiers] describes the complexity of `vector::insert`:

> Complexity: If first and last are forward iterators, bidirectional iterators, or random access iterators, the complexity is linear in the number of elements in the range [first, last) plus the distance to the end of the vector. If they are input iterators, the complexity is proportional to the number of elements in the range [first, last) times the distance to the end of the vector.

First, this fails to address the non-iterator forms of `insert`.

Second, the complexity for input iterators misses an edge case -- it requires that an arbitrary number of elements can be added at the end of a `vector` in constant time.

I looked to see if `deque` had a similar problem, and was surprised to find that `deque` places no requirement on the complexity of inserting multiple elements (23.3.3.4 [deque.modifiers], paragraph 3):

> Complexity: In the worst case, inserting a single element into a deque takes time linear in the minimum of the distance from the insertion point to the beginning of the deque and the distance from the insertion point to the end of the deque. Inserting a single element either at the beginning or end of a deque always takes constant time and causes a single call to the copy constructor of T.

Proposed resolution:

Change Paragraph 2 of 23.3.6.5 [vector.modifiers] to

> Complexity: The complexity is linear in the number of elements inserted plus the distance to the end of the vector.

[For input iterators, one may achieve this complexity by first inserting at the end of the `vector`, and then using `rotate`.]

Change 23.3.3.4 [deque.modifiers], paragraph 3, to:

> Complexity: The complexity is linear in the number of elements inserted plus the shorter of the distances to the beginning and end of the deque. Inserting a single element at either the beginning or the end of a deque causes a single call to the copy constructor of T.

Rationale:

This is a real defect, and proposed resolution fixes it: some complexities aren't specified that should be. This proposed resolution does constrain deque implementations (it rules out the most naive possible implementations), but the LWG doesn't see a reason to permit that implementation.

### 248. time_get fails to set eofbit

Section: 22.4.5 [category.time] Status: CD1 Submitter: Martin Sebor Opened: 2000-06-22 Last modified: 2015-04-08

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Discussion:

There is no requirement that any of time_get member functions set ios::eofbit when they reach the end iterator while parsing their input. Since members of both the num_get and money_get facets are required to do so (22.2.2.1.2, and 22.2.6.1.2, respectively), time_get members should follow the same requirement for consistency.

Proposed resolution:

Add paragraph 2 to section 22.2.5.1 with the following text:

> If the end iterator is reached during parsing by any of the get() member functions, the member sets ios_base::eofbit in err.

Rationale:

Two alternative resolutions were proposed. The LWG chose this one because it was more consistent with the way eof is described for other input facets.

### 250. splicing invalidates iterators

Section: 23.3.5.5 [list.ops] Status: CD1 Submitter: Brian Parker Opened: 2000-07-14 Last modified: 2015-04-08

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Discussion:

Section 23.3.5.5 [list.ops] states that

      void splice(iterator position, list<T, Allocator>& x);

invalidates all iterators and references to list `x`.

This is unnecessary and defeats an important feature of splice. In fact, the SGI STL guarantees that iterators to `x` remain valid after `splice`.

Proposed resolution:

Add a footnote to 23.3.5.5 [list.ops], paragraph 1:

> [Footnote: As specified in [default.con.req], paragraphs 4-5, the semantics described in this clause applies only to the case where allocators compare equal. --end footnote]

In 23.3.5.5 [list.ops], replace paragraph 4 with:

> Effects: Inserts the contents of x before position and x becomes empty. Pointers and references to the moved elements of x now refer to those same elements but as members of *this. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into *this, not into x.

In 23.3.5.5 [list.ops], replace paragraph 7 with:

> Effects: Inserts an element pointed to by i from list x before position and removes the element from x. The result is unchanged if position == i or position == ++i. Pointers and references to *i continue to refer to this same element but as a member of *this. Iterators to *i (including i itself) continue to refer to the same element, but now behave as iterators into *this, not into x.

In 23.3.5.5 [list.ops], replace paragraph 12 with:

> Requires: [first, last) is a valid range in x. The result is undefined if position is an iterator in the range [first, last). Pointers and references to the moved elements of x now refer to those same elements but as members of *this. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into *this, not into x.

[pre-Copenhagen: Howard provided wording.]

Rationale:

The original proposed resolution said that iterators and references would remain "valid". The new proposed resolution clarifies what that means. Note that this only applies to the case of equal allocators. From [default.con.req] paragraph 4, the behavior of list when allocators compare nonequal is outside the scope of the standard.

### 251. basic_stringbuf missing allocator_type

Section: 27.8.2 [stringbuf] Status: CD1 Submitter: Martin Sebor Opened: 2000-07-28 Last modified: 2015-04-08

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Discussion:

The synopsis for the template class `basic_stringbuf` doesn't list a typedef for the template parameter `Allocator`. This makes it impossible to determine the type of the allocator at compile time. It's also inconsistent with all other template classes in the library that do provide a typedef for the `Allocator` parameter.

Proposed resolution:

Add to the synopses of the class templates basic_stringbuf (27.7.1), basic_istringstream (27.7.2), basic_ostringstream (27.7.3), and basic_stringstream (27.7.4) the typedef:

      typedef Allocator allocator_type;

### 252. missing casts/C-style casts used in iostreams

Section: 27.8 [string.streams] Status: CD1 Submitter: Martin Sebor Opened: 2000-07-28 Last modified: 2015-04-08

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Discussion:

27.7.2.2, p1 uses a C-style cast rather than the more appropriate const_cast<> in the Returns clause for basic_istringstream<>::rdbuf(). The same C-style cast is being used in 27.7.3.2, p1, D.7.2.2, p1, and D.7.3.2, p1, and perhaps elsewhere. 27.7.6, p1 and D.7.2.2, p1 are missing the cast altogether.

C-style casts have not been deprecated, so the first part of this issue is stylistic rather than a matter of correctness.

Proposed resolution:

In 27.7.2.2, p1 replace

      -1- Returns: (basic_stringbuf<charT,traits,Allocator>*)&sb.

with

      -1- Returns: const_cast<basic_stringbuf<charT,traits,Allocator>*>(&sb).

In 27.7.3.2, p1 replace

      -1- Returns: (basic_stringbuf<charT,traits,Allocator>*)&sb.

with

      -1- Returns: const_cast<basic_stringbuf<charT,traits,Allocator>*>(&sb).

In 27.7.6, p1, replace

      -1- Returns: &sb

with

      -1- Returns: const_cast<basic_stringbuf<charT,traits,Allocator>*>(&sb).

In D.7.2.2, p1 replace

      -2- Returns: &sb.

with

      -2- Returns: const_cast<strstreambuf*>(&sb).

### 253. valarray helper functions are almost entirely useless

Section: 26.6.2.2 [valarray.cons], 26.6.2.3 [valarray.assign] Status: CD1 Submitter: Robert Klarer Opened: 2000-07-31 Last modified: 2015-04-08

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Discussion:

This discussion is adapted from message c++std-lib-7056 posted November 11, 1999. I don't think that anyone can reasonably claim that the problem described below is NAD.

These valarray constructors can never be called:

       template <class T>
             valarray<T>::valarray(const slice_array<T> &);
       template <class T>
             valarray<T>::valarray(const gslice_array<T> &);
       template <class T>
             valarray<T>::valarray(const mask_array<T> &);
       template <class T>
             valarray<T>::valarray(const indirect_array<T> &);

Similarly, these valarray assignment operators cannot be called:

         template <class T>
         valarray<T> valarray<T>::operator=(const slice_array<T> &);
         template <class T>
         valarray<T> valarray<T>::operator=(const gslice_array<T> &);
         template <class T>
         valarray<T> valarray<T>::operator=(const mask_array<T> &);
         template <class T>
         valarray<T> valarray<T>::operator=(const indirect_array<T> &);

Please consider the following example:

       #include <valarray>
       using namespace std;

       int main()
       {
           valarray<double> va1(12);
           valarray<double> va2(va1[slice(1,4,3)]); // line 1
       }

Since the valarray va1 is non-const, the result of the sub-expression va1[slice(1,4,3)] at line 1 is an rvalue of type const std::slice_array<double>. This slice_array rvalue is then used to construct va2. The constructor that is used to construct va2 is declared like this:

         template <class T>
         valarray<T>::valarray(const slice_array<T> &);

Notice the constructor's const reference parameter. When the constructor is called, a slice_array must be bound to this reference. The rules for binding an rvalue to a const reference are in 8.5.3, paragraph 5 (see also 13.3.3.1.4). Specifically, paragraph 5 indicates that a second slice_array rvalue is constructed (in this case copy-constructed) from the first one; it is this second rvalue that is bound to the reference parameter. Paragraph 5 also requires that the constructor that is used for this purpose be callable, regardless of whether the second rvalue is elided. The copy-constructor in this case is not callable, however, because it is private. Therefore, the compiler should report an error.

Since slice_arrays are always rvalues, the valarray constructor that has a parameter of type const slice_array<T> & can never be called. The same reasoning applies to the three other constructors and the four assignment operators that are listed at the beginning of this post. Furthermore, since these functions cannot be called, the valarray helper classes are almost entirely useless.

Proposed resolution:

slice_array:

- Make the copy constructor and copy-assignment operator declarations public in the slice_array class template definition in 26.6.5 [template.slice.array]
- remove paragraph 3 of 26.6.5 [template.slice.array]
- remove the copy constructor declaration from [cons.slice.arr]
- change paragraph 1 of [cons.slice.arr] to read "This constructor is declared to be private. This constructor need not be defined."
- remove the first sentence of paragraph 1 of 26.6.5.2 [slice.arr.assign]
- Change the first three words of the second sentence of paragraph 1 of 26.6.5.2 [slice.arr.assign] to "These assignment operators have"

gslice_array:

- Make the copy constructor and copy-assignment operator declarations public in the gslice_array class template definition in 26.6.7 [template.gslice.array]
- remove the note in paragraph 3 of 26.6.7 [template.gslice.array]
- remove the copy constructor declaration from [gslice.array.cons]
- change paragraph 1 of [gslice.array.cons] to read "This constructor is declared to be private. This constructor need not be defined."
- remove the first sentence of paragraph 1 of 26.6.7.2 [gslice.array.assign]
- Change the first three words of the second sentence of paragraph 1 of 26.6.7.2 [gslice.array.assign] to "These assignment operators have"

mask_array:

- Make the copy constructor and copy-assignment operator declarations public in the mask_array class template definition in 26.6.8 [template.mask.array]
- remove the note in paragraph 2 of 26.6.8 [template.mask.array]
- remove the copy constructor declaration from [mask.array.cons]
- change paragraph 1 of [mask.array.cons] to read "This constructor is declared to be private. This constructor need not be defined."
- remove the first sentence of paragraph 1 of 26.6.8.2 [mask.array.assign]
- Change the first three words of the second sentence of paragraph 1 of 26.6.8.2 [mask.array.assign] to "These assignment operators have"

indirect_array:

- Make the copy constructor and copy-assignment operator declarations public in the indirect_array class definition in 26.6.9 [template.indirect.array]
- remove the note in paragraph 2 of 26.6.9 [template.indirect.array]
- remove the copy constructor declaration from [indirect.array.cons]
- change the descriptive text in [indirect.array.cons] to read "This constructor is declared to be private. This constructor need not be defined."
- remove the first sentence of paragraph 1 of 26.6.9.2 [indirect.array.assign]
- Change the first three words of the second sentence of paragraph 1 of 26.6.9.2 [indirect.array.assign] to "These assignment operators have"

[Proposed resolution was modified in Santa Cruz: explicitly make copy constructor and copy assignment operators public, instead of removing them.]

Rationale:

Keeping the valarray constructors private is untenable. Merely making valarray a friend of the helper classes isn't good enough, because access to the copy constructor is checked in the user's environment.

Making the assignment operator public is not strictly necessary to solve this problem. A majority of the LWG (straw poll: 13-4) believed we should make the assignment operators public, in addition to the copy constructors, for reasons of symmetry and user expectation.

### 254. Exception types in clause 19 are constructed from `std::string`

Section: 19.2 [std.exceptions], 27.5.3.1.1 [ios::failure] Status: CD1 Submitter: Dave Abrahams Opened: 2000-08-01 Last modified: 2015-04-08

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Discussion:

Many of the standard exception types which implementations are required to throw are constructed with a const std::string& parameter. For example:

         19.1.5  Class out_of_range                          [lib.out.of.range]
         namespace std {
           class out_of_range : public logic_error {
           public:
             explicit out_of_range(const string& what_arg);
           };
         }

       1 The class out_of_range defines the type of objects  thrown  as  excep-
         tions to report an argument value not in its expected range.

         out_of_range(const string& what_arg);

         Effects:
           Constructs an object of class out_of_range.
         Postcondition:
           strcmp(what(), what_arg.c_str()) == 0.

There are at least two problems with this:

1. A program which is low on memory may end up throwing std::bad_alloc instead of out_of_range because memory runs out while constructing the exception object.
2. An obvious implementation which stores a std::string data member may end up invoking terminate() during exception unwinding because the exception object allocates memory (or rather fails to) as it is being copied.

There may be no cure for (1) other than changing the interface to out_of_range, though one could reasonably argue that (1) is not a defect. Personally I don't care that much if out-of-memory is reported when I only have 20 bytes left, in the case when out_of_range would have been reported. People who use exception-specifications might care a lot, though.

There is a cure for (2), but it isn't completely obvious. I think a note for implementors should be made in the standard. Avoiding possible termination in this case shouldn't be left up to chance. The cure is to use a reference-counted "string" implementation in the exception object. I am not necessarily referring to a std::string here; any simple reference-counting scheme for a NTBS would do.

Further discussion, in email:

...I'm not so concerned about (1). After all, a library implementation can add const char* constructors as an extension, and users don't need to avail themselves of the standard exceptions, though this is a lame position to be forced into. FWIW, std::exception and std::bad_alloc don't require a temporary basic_string.

...I don't think the fixed-size buffer is a solution to the problem, strictly speaking, because you can't satisfy the postcondition
`strcmp(what(), what_arg.c_str()) == 0`
For all values of what_arg (i.e. very long values). That means that the only truly conforming solution requires a dynamic allocation.

Further discussion, from Redmond:

The most important progress we made at the Redmond meeting was realizing that there are two separable issues here: the const string& constructor, and the copy constructor. If a user writes something like `throw std::out_of_range("foo")`, the const string& constructor is invoked before anything gets thrown. The copy constructor is potentially invoked during stack unwinding.

The copy constructor is a more serious problem, becuase failure during stack unwinding invokes `terminate`. The copy constructor must be nothrow. Curaçao: Howard thinks this requirement may already be present.

The fundamental problem is that it's difficult to get the nothrow requirement to work well with the requirement that the exception objects store a string of unbounded size, particularly if you also try to make the const string& constructor nothrow. Options discussed include:

- Limit the size of a string that exception objects are required to throw: change the postconditions of 19.2.2 [domain.error] paragraph 3 and 19.2.6 [runtime.error] paragraph 3 to something like this: "strncmp(what(), what_arg._str(), N) == 0, where N is an implementation defined constant no smaller than 256".
- Allow the const string& constructor to throw, but not the copy constructor. It's the implementor's responsibility to get it right. (An implementor might use a simple refcount class.)
- Compromise between the two: an implementation is not allowed to throw if the string's length is less than some N, but, if it doesn't throw, the string must compare equal to the argument.
- Add a new constructor that takes a const char*

(Not all of these options are mutually exclusive.)

Proposed resolution:

Change 19.2.1 [logic.error]

>     namespace std {
>       class logic_error : public exception {
>       public:
>         explicit logic_error(const string& what_arg);
>         explicit logic_error(const char* what_arg);
>       };
>     }

> ...

> `logic_error(const char* what_arg);`

> > -4- Effects: Constructs an object of class `logic_error`.

> > -5- Postcondition: `strcmp(what(), what_arg) == 0`.

Change 19.2.2 [domain.error]

>     namespace std {
>       class domain_error : public logic_error {
>       public:
>         explicit domain_error(const string& what_arg);
>         explicit domain_error(const char* what_arg);
>       };
>     }

> ...

> `domain_error(const char* what_arg);`

> > -4- Effects: Constructs an object of class `domain_error`.

> > -5- Postcondition: `strcmp(what(), what_arg) == 0`.

Change 19.2.3 [invalid.argument]

>     namespace std {
>       class invalid_argument : public logic_error {
>       public:
>         explicit invalid_argument(const string& what_arg);
>         explicit invalid_argument(const char* what_arg);
>       };
>     }

> ...

> `invalid_argument(const char* what_arg);`

> > -4- Effects: Constructs an object of class `invalid_argument`.

> > -5- Postcondition: `strcmp(what(), what_arg) == 0`.

Change 19.2.4 [length.error]

>     namespace std {
>       class length_error : public logic_error {
>       public:
>         explicit length_error(const string& what_arg);
>         explicit length_error(const char* what_arg);
>       };
>     }

> ...

> `length_error(const char* what_arg);`

> > -4- Effects: Constructs an object of class `length_error`.

> > -5- Postcondition: `strcmp(what(), what_arg) == 0`.

Change 19.2.5 [out.of.range]

>     namespace std {
>       class out_of_range : public logic_error {
>       public:
>         explicit out_of_range(const string& what_arg);
>         explicit out_of_range(const char* what_arg);
>       };
>     }

> ...

> `out_of_range(const char* what_arg);`

> > -4- Effects: Constructs an object of class `out_of_range`.

> > -5- Postcondition: `strcmp(what(), what_arg) == 0`.

Change 19.2.6 [runtime.error]

>     namespace std {
>       class runtime_error : public exception {
>       public:
>         explicit runtime_error(const string& what_arg);
>         explicit runtime_error(const char* what_arg);
>       };
>     }

> ...

> `runtime_error(const char* what_arg);`

> > -4- Effects: Constructs an object of class `runtime_error`.

> > -5- Postcondition: `strcmp(what(), what_arg) == 0`.

Change 19.2.7 [range.error]

>     namespace std {
>       class range_error : public runtime_error {
>       public:
>         explicit range_error(const string& what_arg);
>         explicit range_error(const char* what_arg);
>       };
>     }

> ...

> `range_error(const char* what_arg);`

> > -4- Effects: Constructs an object of class `range_error`.

> > -5- Postcondition: `strcmp(what(), what_arg) == 0`.

Change 19.2.8 [overflow.error]

>     namespace std {
>       class overflow_error : public runtime_error {
>       public:
>         explicit overflow_error(const string& what_arg);
>         explicit overflow_error(const char* what_arg);
>       };
>     }

> ...

> `overflow_error(const char* what_arg);`

> > -4- Effects: Constructs an object of class `overflow_error`.

> > -5- Postcondition: `strcmp(what(), what_arg) == 0`.

Change 19.2.9 [underflow.error]

>     namespace std {
>       class underflow_error : public runtime_error {
>       public:
>         explicit underflow_error(const string& what_arg);
>         explicit underflow_error(const char* what_arg);
>       };
>     }

> ...

> `underflow_error(const char* what_arg);`

> > -4- Effects: Constructs an object of class `underflow_error`.

> > -5- Postcondition: `strcmp(what(), what_arg) == 0`.

Change 27.5.3.1.1 [ios::failure]

>     namespace std {
>       class ios_base::failure : public exception {
>       public:
>         explicit failure(const string& msg);
>         explicit failure(const char* msg);
>         virtual const char* what() const throw();
>     };
>     }

> ...

> `failure(const char* msg);`

> > -4- Effects: Constructs an object of class `failure`.

> > -5- Postcondition: `strcmp(what(), msg) == 0`.

Rationale:

Throwing a bad_alloc while trying to construct a message for another exception-derived class is not necessarily a bad thing. And the bad_alloc constructor already has a no throw spec on it (18.4.2.1).

Future:

All involved would like to see const char* constructors added, but this should probably be done for C++0X as opposed to a DR.

I believe the no throw specs currently decorating these functions could be improved by some kind of static no throw spec checking mechanism (in a future C++ language). As they stand, the copy constructors might fail via a call to unexpected. I think what is intended here is that the copy constructors can't fail.

[Pre-Sydney: reopened at the request of Howard Hinnant. Post-Redmond: James Kanze noticed that the copy constructors of exception-derived classes do not have nothrow clauses. Those classes have no copy constructors declared, meaning the compiler-generated implicit copy constructors are used, and those compiler-generated constructors might in principle throw anything.]

[ Batavia: Merged copy constructor and assignment operator spec into `exception` and added `ios::failure` into the proposed resolution. ]

[ Oxford: The proposed resolution simply addresses the issue of constructing the exception objects with `const char*` and string literals without the need to explicit include or construct a `std::string`. ]

### 256. typo in 27.4.4.2, p17: copy_event does not exist

Section: 27.5.5.3 [basic.ios.members] Status: CD1 Submitter: Martin Sebor Opened: 2000-08-21 Last modified: 2015-04-08

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Discussion:

27.4.4.2, p17 says

> -17- Before copying any parts of rhs, calls each registered callback pair (fn,index) as (*fn)(erase_event,*this,index). After all parts but exceptions() have been replaced, calls each callback pair that was copied from rhs as (*fn)(copy_event,*this,index).

The name copy_event isn't defined anywhere. The intended name was copyfmt_event.

Proposed resolution:

Replace copy_event with copyfmt_event in the named paragraph.

### 258. Missing allocator requirement

Section: 17.6.3.5 [allocator.requirements] Status: CD1 Submitter: Matt Austern Opened: 2000-08-22 Last modified: 2015-04-08

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Discussion:

From lib-7752:

I've been assuming (and probably everyone else has been assuming) that allocator instances have a particular property, and I don't think that property can be deduced from anything in Table 32.

I think we have to assume that allocator type conversion is a homomorphism. That is, if x1 and x2 are of type X, where X::value_type is T, and if type Y is X::template rebind<U>::other, then Y(x1) == Y(x2) if and only if x1 == x2.

Further discussion: Howard Hinnant writes, in lib-7757:

I think I can prove that this is not provable by Table 32. And I agree it needs to be true except for the "and only if". If x1 != x2, I see no reason why it can't be true that Y(x1) == Y(x2). Admittedly I can't think of a practical instance where this would happen, or be valuable. But I also don't see a need to add that extra restriction. I think we only need:

> if (x1 == x2) then Y(x1) == Y(x2)

If we decide that == on allocators is transitive, then I think I can prove the above. But I don't think == is necessarily transitive on allocators. That is:

Given x1 == x2 and x2 == x3, this does not mean x1 == x3.

Example:

> x1 can deallocate pointers from: x1, x2, x3
> x2 can deallocate pointers from: x1, x2, x4
> x3 can deallocate pointers from: x1, x3
> x4 can deallocate pointers from: x2, x4

> x1 == x2, and x2 == x4, but x1 != x4

[Toronto: LWG members offered multiple opinions. One opinion is that it should not be required that `x1 == x2` implies `Y(x1) == Y(x2)`, and that it should not even be required that `X(x1) == x1`. Another opinion is that the second line from the bottom in table 32 already implies the desired property. This issue should be considered in light of other issues related to allocator instances.]

Proposed resolution:

Accept proposed wording from N2436 part 3.

[Lillehammer: Same conclusion as before: this should be considered as part of an allocator redesign, not solved on its own.]

[ Batavia: An allocator redesign is not forthcoming and thus we fixed this one issue. ]

[ Toronto: Reopened at the request of the project editor (Pete) because the proposed wording did not fit within the indicated table. The intent of the resolution remains unchanged. Pablo to work with Pete on improved wording. ]

[ Kona (2007): The LWG adopted the proposed resolution of N2387 for this issue which was subsequently split out into a separate paper N2436 for the purposes of voting. The resolution in N2436 addresses this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 259. `basic_string::operator[]` and const correctness

Section: 21.4.4 [string.capacity] Status: CD1 Submitter: Chris Newton Opened: 2000-08-27 Last modified: 2015-04-08

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Discussion:

Paraphrased from a message that Chris Newton posted to comp.std.c++:

The standard's description of `basic_string<>::operator[]` seems to violate const correctness.

The standard (21.3.4/1) says that "If `pos < size()`, returns `data()[pos]`." The types don't work. The return value of `data()` is `const charT*`, but `operator[]` has a non-const version whose return type is `reference`.

Proposed resolution:

In section 21.3.4, paragraph 1, change "`data()[pos]`" to "`*(begin() + pos)`".

### 260. Inconsistent return type of `istream_iterator::operator++(int)`

Section: 24.6.1.2 [istream.iterator.ops] Status: CD1 Submitter: Martin Sebor Opened: 2000-08-27 Last modified: 2015-04-08

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Discussion:

The synopsis of istream_iterator::operator++(int) in 24.5.1 shows it as returning the iterator by value. 24.5.1.2, p5 shows the same operator as returning the iterator by reference. That's incorrect given the Effects clause below (since a temporary is returned). The `&' is probably just a typo.

Proposed resolution:

Change the declaration in 24.5.1.2, p5 from

     istream_iterator<T,charT,traits,Distance>& operator++(int);

to

     istream_iterator<T,charT,traits,Distance> operator++(int);

(that is, remove the `&').

### 261. Missing description of `istream_iterator::operator!=`

Section: 24.6.1.2 [istream.iterator.ops] Status: CD1 Submitter: Martin Sebor Opened: 2000-08-27 Last modified: 2015-04-08

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Discussion:

24.5.1, p3 lists the synopsis for

       template <class T, class charT, class traits, class Distance>
            bool operator!=(const istream_iterator<T,charT,traits,Distance>& x,
                            const istream_iterator<T,charT,traits,Distance>& y);

but there is no description of what the operator does (i.e., no Effects or Returns clause) in 24.5.1.2.

Proposed resolution:

Add paragraph 7 to the end of section 24.5.1.2 with the following text:

       template <class T, class charT, class traits, class Distance>
            bool operator!=(const istream_iterator<T,charT,traits,Distance>& x,
                            const istream_iterator<T,charT,traits,Distance>& y);

-7- Returns: !(x == y).

### 262. Bitmask operator ~ specified incorrectly

Section: 17.5.2.1.3 [bitmask.types] Status: CD1 Submitter: Beman Dawes Opened: 2000-09-03 Last modified: 2015-04-08

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Discussion:

The ~ operation should be applied after the cast to int_type.

Proposed resolution:

Change 17.3.2.1.2 [lib.bitmask.types] operator~ from:

       bitmask operator~ ( bitmask X )
         { return static_cast< bitmask>(static_cast<int_type>(~ X)); }

to:

       bitmask operator~ ( bitmask X )
         { return static_cast< bitmask>(~static_cast<int_type>(X)); }

### 263. Severe restriction on `basic_string` reference counting

Section: 21.4 [basic.string] Status: CD1 Submitter: Kevlin Henney Opened: 2000-09-04 Last modified: 2015-04-08

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Discussion:

The note in paragraph 6 suggests that the invalidation rules for references, pointers, and iterators in paragraph 5 permit a reference- counted implementation (actually, according to paragraph 6, they permit a "reference counted implementation", but this is a minor editorial fix).

However, the last sub-bullet is so worded as to make a reference-counted implementation unviable. In the following example none of the conditions for iterator invalidation are satisfied:

        // first example: "*******************" should be printed twice
        string original = "some arbitrary text", copy = original;
        const string & alias = original;

        string::const_iterator i = alias.begin(), e = alias.end();
        for(string::iterator j = original.begin(); j != original.end(); ++j)
            *j = '*';
        while(i != e)
            cout << *i++;
        cout << endl;
        cout << original << endl;

Similarly, in the following example:

        // second example: "some arbitrary text" should be printed out
        string original = "some arbitrary text", copy = original;
        const string & alias = original;

        string::const_iterator i = alias.begin();
        original.begin();
        while(i != alias.end())
            cout << *i++;

I have tested this on three string implementations, two of which were reference counted. The reference-counted implementations gave "surprising behavior" because they invalidated iterators on the first call to non-const begin since construction. The current wording does not permit such invalidation because it does not take into account the first call since construction, only the first call since various member and non-member function calls.

Proposed resolution:

Change the following sentence in 21.3 paragraph 5 from

> Subsequent to any of the above uses except the forms of insert() and erase() which return iterators, the first call to non-const member functions operator[](), at(), begin(), rbegin(), end(), or rend().

to

> Following construction or any of the above uses, except the forms of insert() and erase() that return iterators, the first call to non- const member functions operator[](), at(), begin(), rbegin(), end(), or rend().

### 264. Associative container `insert(i, j)` complexity requirements are not feasible.

Section: 23.2.4 [associative.reqmts] Status: CD1 Submitter: John Potter Opened: 2000-09-07 Last modified: 2015-04-08

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Duplicate of: 102

Discussion:

Table 69 requires linear time if [i, j) is sorted. Sorted is necessary but not sufficient. Consider inserting a sorted range of even integers into a set<int> containing the odd integers in the same range.

Related issue: 102

Proposed resolution:

In Table 69, in section 23.1.2, change the complexity clause for insertion of a range from "N log(size() + N) (N is the distance from i to j) in general; linear if [i, j) is sorted according to value_comp()" to "N log(size() + N), where N is the distance from i to j".

[Copenhagen: Minor fix in proposed resolution: fixed unbalanced parens in the revised wording.]

Rationale:

Testing for valid insertions could be less efficient than simply inserting the elements when the range is not both sorted and between two adjacent existing elements; this could be a QOI issue.

The LWG considered two other options: (a) specifying that the complexity was linear if [i, j) is sorted according to value_comp() and between two adjacent existing elements; or (b) changing to Klog(size() + N) + (N - K) (N is the distance from i to j and K is the number of elements which do not insert immediately after the previous element from [i, j) including the first). The LWG felt that, since we can't guarantee linear time complexity whenever the range to be inserted is sorted, it's more trouble than it's worth to say that it's linear in some special cases.

### 265. std::pair::pair() effects overly restrictive

Section: 20.3 [pairs] Status: CD1 Submitter: Martin Sebor Opened: 2000-09-11 Last modified: 2015-04-08

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Discussion:

I don't see any requirements on the types of the elements of the std::pair container in 20.2.2. From the descriptions of the member functions it appears that they must at least satisfy the requirements of 20.1.3 [lib.copyconstructible] and 20.1.4 [lib.default.con.req], and in the case of the [in]equality operators also the requirements of 20.1.1 [lib.equalitycomparable] and 20.1.2 [lib.lessthancomparable].

I believe that the the CopyConstructible requirement is unnecessary in the case of 20.2.2, p2.

Proposed resolution:

Change the Effects clause in 20.2.2, p2 from

> -2- Effects: Initializes its members as if implemented: `pair() : first(T1()), second(T2()) {}`

to

> -2- Effects: Initializes its members as if implemented: `pair() : first(), second() {}`

Rationale:

The existing specification of pair's constructor appears to be a historical artifact: there was concern that pair's members be properly zero-initialized when they are built-in types. At one time there was uncertainty about whether they would be zero-initialized if the default constructor was written the obvious way. This has been clarified by core issue 178, and there is no longer any doubt that the straightforward implementation is correct.

### 266. bad_exception::~bad_exception() missing Effects clause

Section: 18.8.2 [bad.exception] Status: CD1 Submitter: Martin Sebor Opened: 2000-09-24 Last modified: 2015-04-08

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Discussion:

The synopsis for std::bad_exception lists the function ~bad_exception() but there is no description of what the function does (the Effects clause is missing).

Proposed resolution:

Remove the destructor from the class synopses of `bad_alloc` (18.6.2.1 [bad.alloc]), `bad_cast` (18.7.2 [bad.cast]), `bad_typeid` (18.7.3 [bad.typeid]), and `bad_exception` (18.8.2 [bad.exception]).

Rationale:

This is a general problem with the exception classes in clause 18. The proposed resolution is to remove the destructors from the class synopses, rather than to document the destructors' behavior, because removing them is more consistent with how exception classes are described in clause 19.

### 268. Typo in locale synopsis

Section: 22.3.1 [locale] Status: CD1 Submitter: Martin Sebor Opened: 2000-10-05 Last modified: 2015-04-08

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Discussion:

The synopsis of the class std::locale in 22.1.1 contains two typos: the semicolons after the declarations of the default ctor locale::locale() and the copy ctor locale::locale(const locale&) are missing.

Proposed resolution:

Add the missing semicolons, i.e., change

        //  construct/copy/destroy:
            locale() throw()
            locale(const locale& other) throw()

in the synopsis in 22.1.1 to

        //  construct/copy/destroy:
            locale() throw();
            locale(const locale& other) throw();

### 270. Binary search requirements overly strict

Section: 25.4.3 [alg.binary.search] Status: CD1 Submitter: Matt Austern Opened: 2000-10-18 Last modified: 2015-04-08

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Duplicate of: 472

Discussion:

Each of the four binary search algorithms (lower_bound, upper_bound, equal_range, binary_search) has a form that allows the user to pass a comparison function object. According to 25.3, paragraph 2, that comparison function object has to be a strict weak ordering.

This requirement is slightly too strict. Suppose we are searching through a sequence containing objects of type X, where X is some large record with an integer key. We might reasonably want to look up a record by key, in which case we would want to write something like this:

        struct key_comp {
          bool operator()(const X& x, int n) const {
            return x.key() < n;
          }
        }

        std::lower_bound(first, last, 47, key_comp());

key_comp is not a strict weak ordering, but there is no reason to prohibit its use in lower_bound.

There's no difficulty in implementing lower_bound so that it allows the use of something like key_comp. (It will probably work unless an implementor takes special pains to forbid it.) What's difficult is formulating language in the standard to specify what kind of comparison function is acceptable. We need a notion that's slightly more general than that of a strict weak ordering, one that can encompass a comparison function that involves different types. Expressing that notion may be complicated.

Additional questions raised at the Toronto meeting:

- Do we really want to specify what ordering the implementor must use when calling the function object? The standard gives specific expressions when describing these algorithms, but it also says that other expressions (with different argument order) are equivalent.
- If we are specifying ordering, note that the standard uses both orderings when describing `equal_range`.
- Are we talking about requiring these algorithms to work properly when passed a binary function object whose two argument types are not the same, or are we talking about requirements when they are passed a binary function object with several overloaded versions of `operator()`?
- The definition of a strict weak ordering does not appear to give any guidance on issues of overloading; it only discusses expressions, and all of the values in these expressions are of the same type. Some clarification would seem to be in order.

Additional discussion from Copenhagen:

- It was generally agreed that there is a real defect here: if the predicate is merely required to be a Strict Weak Ordering, then it's possible to pass in a function object with an overloaded operator(), where the version that's actually called does something completely inappropriate. (Such as returning a random value.)
- An alternative formulation was presented in a paper distributed by David Abrahams at the meeting, "Binary Search with Heterogeneous Comparison", J16-01/0027 = WG21 N1313: Instead of viewing the predicate as a Strict Weak Ordering acting on a sorted sequence, view the predicate/value pair as something that partitions a sequence. This is almost equivalent to saying that we should view binary search as if we are given a unary predicate and a sequence, such that f(*p) is true for all p below a specific point and false for all p above it. The proposed resolution is based on that alternative formulation.

Proposed resolution:

Change 25.3 [lib.alg.sorting] paragraph 3 from:

> 3 For all algorithms that take Compare, there is a version that uses operator< instead. That is, comp(*i, *j) != false defaults to *i < *j != false. For the algorithms to work correctly, comp has to induce a strict weak ordering on the values.

to:

> 3 For all algorithms that take Compare, there is a version that uses operator< instead. That is, comp(*i, *j) != false defaults to *i < *j != false. For algorithms other than those described in lib.alg.binary.search (25.3.3) to work correctly, comp has to induce a strict weak ordering on the values.

Add the following paragraph after 25.3 [lib.alg.sorting] paragraph 5:

> -6- A sequence [start, finish) is partitioned with respect to an expression f(e) if there exists an integer n such that for all 0 <= i < distance(start, finish), f(*(begin+i)) is true if and only if i < n.

Change 25.3.3 [lib.alg.binary.search] paragraph 1 from:

> -1- All of the algorithms in this section are versions of binary search and assume that the sequence being searched is in order according to the implied or explicit comparison function. They work on non-random access iterators minimizing the number of comparisons, which will be logarithmic for all types of iterators. They are especially appropriate for random access iterators, because these algorithms do a logarithmic number of steps through the data structure. For non-random access iterators they execute a linear number of steps.

to:

> -1- All of the algorithms in this section are versions of binary search and assume that the sequence being searched is partitioned with respect to an expression formed by binding the search key to an argument of the implied or explicit comparison function. They work on non-random access iterators minimizing the number of comparisons, which will be logarithmic for all types of iterators. They are especially appropriate for random access iterators, because these algorithms do a logarithmic number of steps through the data structure. For non-random access iterators they execute a linear number of steps.

Change 25.3.3.1 [lib.lower.bound] paragraph 1 from:

> -1- Requires: Type T is LessThanComparable (lib.lessthancomparable).

to:

> -1- Requires: The elements e of [first, last) are partitioned with respect to the expression e < value or comp(e, value)

Remove 25.3.3.1 [lib.lower.bound] paragraph 2:

> -2- Effects: Finds the first position into which value can be inserted without violating the ordering.

Change 25.3.3.2 [lib.upper.bound] paragraph 1 from:

> -1- Requires: Type T is LessThanComparable (lib.lessthancomparable).

to:

> -1- Requires: The elements e of [first, last) are partitioned with respect to the expression !(value < e) or !comp(value, e)

Remove 25.3.3.2 [lib.upper.bound] paragraph 2:

> -2- Effects: Finds the furthermost position into which value can be inserted without violating the ordering.

Change 25.3.3.3 [lib.equal.range] paragraph 1 from:

> -1- Requires: Type T is LessThanComparable (lib.lessthancomparable).

to:

> -1- Requires: The elements e of [first, last) are partitioned with respect to the expressions e < value and !(value < e) or comp(e, value) and !comp(value, e). Also, for all elements e of [first, last), e < value implies !(value < e) or comp(e, value) implies !comp(value, e)

Change 25.3.3.3 [lib.equal.range] paragraph 2 from:

> -2- Effects: Finds the largest subrange [i, j) such that the value can be inserted at any iterator k in it without violating the ordering. k satisfies the corresponding conditions: !(*k < value) && !(value < *k) or comp(*k, value) == false && comp(value, *k) == false.

to:

       -2- Returns:
             make_pair(lower_bound(first, last, value),
                       upper_bound(first, last, value))
           or
             make_pair(lower_bound(first, last, value, comp),
                       upper_bound(first, last, value, comp))

Change 25.3.3.3 [lib.binary.search] paragraph 1 from:

> -1- Requires: Type T is LessThanComparable (lib.lessthancomparable).

to:

> -1- Requires: The elements e of [first, last) are partitioned with respect to the expressions e < value and !(value < e) or comp(e, value) and !comp(value, e). Also, for all elements e of [first, last), e < value implies !(value < e) or comp(e, value) implies !comp(value, e)

[Copenhagen: Dave Abrahams provided this wording]

[Redmond: Minor changes in wording. (Removed "non-negative", and changed the "other than those described in" wording.) Also, the LWG decided to accept the "optional" part.]

Rationale:

The proposed resolution reinterprets binary search. Instead of thinking about searching for a value in a sorted range, we view that as an important special case of a more general algorithm: searching for the partition point in a partitioned range.

We also add a guarantee that the old wording did not: we ensure that the upper bound is no earlier than the lower bound, that the pair returned by equal_range is a valid range, and that the first part of that pair is the lower bound.

### 271. basic_iostream missing typedefs

Section: 27.7.2.5 [iostreamclass] Status: CD1 Submitter: Martin Sebor Opened: 2000-11-02 Last modified: 2015-04-08

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Discussion:

Class template basic_iostream has no typedefs. The typedefs it inherits from its base classes can't be used, since (for example) basic_iostream<T>::traits_type is ambiguous.

Proposed resolution:

Add the following to basic_iostream's class synopsis in 27.7.2.5 [iostreamclass], immediately after `public`:

      // types:
      typedef charT                     char_type;
      typedef typename traits::int_type int_type;
      typedef typename traits::pos_type pos_type;
      typedef typename traits::off_type off_type;
      typedef traits                    traits_type;

### 272. Missing parentheses around subexpression

Section: 27.5.5.4 [iostate.flags] Status: CD1 Submitter: Martin Sebor Opened: 2000-11-02 Last modified: 2015-04-08

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Duplicate of: 569

Discussion:

27.4.4.3, p4 says about the postcondition of the function: If rdbuf()!=0 then state == rdstate(); otherwise rdstate()==state|ios_base::badbit.

The expression on the right-hand-side of the operator==() needs to be parenthesized in order for the whole expression to ever evaluate to anything but non-zero.

Proposed resolution:

Add parentheses like so: rdstate()==(state|ios_base::badbit).

### 273. Missing ios_base qualification on members of a dependent class

Section: 27 [input.output] Status: CD1 Submitter: Martin Sebor Opened: 2000-11-02 Last modified: 2015-04-08

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Discussion:

27.5.2.4.2, p4, and 27.8.1.6, p2, 27.8.1.7, p3, 27.8.1.9, p2, 27.8.1.10, p3 refer to in and/or out w/o ios_base:: qualification. That's incorrect since the names are members of a dependent base class (14.6.2 [temp.dep]) and thus not visible.

Proposed resolution:

Qualify the names with the name of the class of which they are members, i.e., ios_base.

### 274. a missing/impossible allocator requirement

Section: 17.6.3.5 [allocator.requirements] Status: CD1 Submitter: Martin Sebor Opened: 2000-11-02 Last modified: 2015-04-08

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Discussion:

I see that table 31 in 20.1.5, p3 allows T in std::allocator<T> to be of any type. But the synopsis in 20.4.1 calls for allocator<>::address() to be overloaded on reference and const_reference, which is ill-formed for all T = const U. In other words, this won't work:

template class std::allocator<const int>;

The obvious solution is to disallow specializations of allocators on const types. However, while containers' elements are required to be assignable (which rules out specializations on const T's), I think that allocators might perhaps be potentially useful for const values in other contexts. So if allocators are to allow const types a partial specialization of std::allocator<const T> would probably have to be provided.

Proposed resolution:

Change the text in row 1, column 2 of table 32 in 20.1.5, p3 from

> any type

to

> any non-const, non-reference type

[Redmond: previous proposed resolution was "any non-const, non-volatile, non-reference type". Got rid of the "non-volatile".]

Rationale:

Two resolutions were originally proposed: one that partially specialized std::allocator for const types, and one that said an allocator's value type may not be const. The LWG chose the second. The first wouldn't be appropriate, because allocators are intended for use by containers, and const value types don't work in containers. Encouraging the use of allocators with const value types would only lead to unsafe code.

The original text for proposed resolution 2 was modified so that it also forbids volatile types and reference types.

[Curaçao: LWG double checked and believes volatile is correctly excluded from the PR.]

### 275. Wrong type in num_get::get() overloads

Section: 22.4.2.1.1 [facet.num.get.members] Status: CD1 Submitter: Matt Austern Opened: 2000-11-02 Last modified: 2015-04-08

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Discussion:

In 22.2.2.1.1, we have a list of overloads for num_get<>::get(). There are eight overloads, all of which are identical except for the last parameter. The overloads are:

- long&
- unsigned short&
- unsigned int&
- unsigned long&
- short&
- double&
- long double&
- void*&

There is a similar list, in 22.2.2.1.2, of overloads for num_get<>::do_get(). In this list, the last parameter has the types:

- long&
- unsigned short&
- unsigned int&
- unsigned long&
- float&
- double&
- long double&
- void*&

These two lists are not identical. They should be, since `get` is supposed to call `do_get` with exactly the arguments it was given.

Proposed resolution:

In 22.4.2.1.1 [facet.num.get.members], change

      iter_type get(iter_type in, iter_type end, ios_base& str,
                    ios_base::iostate& err, short& val) const;

to

      iter_type get(iter_type in, iter_type end, ios_base& str,
                    ios_base::iostate& err, float& val) const;

### 276. Assignable requirement for container value type overly strict

Section: 23.2 [container.requirements] Status: CD1 Submitter: Peter Dimov Opened: 2000-11-07 Last modified: 2015-04-08

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Discussion:

23.1/3 states that the objects stored in a container must be Assignable. 23.4.4 [map], paragraph 2, states that map satisfies all requirements for a container, while in the same time defining value_type as pair<const Key, T> - a type that is not Assignable.

It should be noted that there exists a valid and non-contradictory interpretation of the current text. The wording in 23.1/3 avoids mentioning value_type, referring instead to "objects stored in a container." One might argue that map does not store objects of type map::value_type, but of map::mapped_type instead, and that the Assignable requirement applies to map::mapped_type, not map::value_type.

However, this makes map a special case (other containers store objects of type value_type) and the Assignable requirement is needlessly restrictive in general.

For example, the proposed resolution of active library issue 103 is to make set::iterator a constant iterator; this means that no set operations can exploit the fact that the stored objects are Assignable.

This is related to, but slightly broader than, closed issue 140.

Proposed resolution:

23.1/3: Strike the trailing part of the sentence:

> , and the additional requirements of Assignable types from 23.1/3

so that it reads:

> -3- The type of objects stored in these components must meet the requirements of CopyConstructible types (lib.copyconstructible).

23.1/4: Modify to make clear that this requirement is not for all containers. Change to:

> -4- Table 64 defines the Assignable requirement. Some containers require this property of the types to be stored in the container. T is the type used to instantiate the container. t is a value of T, and u is a value of (possibly const) T.

23.1, Table 65: in the first row, change "T is Assignable" to "T is CopyConstructible".

23.2.1/2: Add sentence for Assignable requirement. Change to:

> -2- A deque satisfies all of the requirements of a container and of a reversible container (given in tables in lib.container.requirements) and of a sequence, including the optional sequence requirements (lib.sequence.reqmts). In addition to the requirements on the stored object described in 23.1[lib.container.requirements], the stored object must also meet the requirements of Assignable. Descriptions are provided here only for operations on deque that are not described in one of these tables or for operations where there is additional semantic information.

23.2.2/2: Add Assignable requirement to specific methods of list. Change to:

> -2- A list satisfies all of the requirements of a container and of a reversible container (given in two tables in lib.container.requirements) and of a sequence, including most of the the optional sequence requirements (lib.sequence.reqmts). The exceptions are the operator[] and at member functions, which are not provided. [Footnote: These member functions are only provided by containers whose iterators are random access iterators. --- end foonote]

> list does not require the stored type T to be Assignable unless the following methods are instantiated: [Footnote: Implementors are permitted but not required to take advantage of T's Assignable properties for these methods. -- end foonote]

>          list<T,Allocator>& operator=(const list<T,Allocator>&  x );
>          template <class InputIterator>
>            void assign(InputIterator first, InputIterator last);
>          void assign(size_type n, const T& t);

> Descriptions are provided here only for operations on list that are not described in one of these tables or for operations where there is additional semantic information.

23.2.4/2: Add sentence for Assignable requirement. Change to:

> -2- A vector satisfies all of the requirements of a container and of a reversible container (given in two tables in lib.container.requirements) and of a sequence, including most of the optional sequence requirements (lib.sequence.reqmts). The exceptions are the push_front and pop_front member functions, which are not provided. In addition to the requirements on the stored object described in 23.1[lib.container.requirements], the stored object must also meet the requirements of Assignable. Descriptions are provided here only for operations on vector that are not described in one of these tables or for operations where there is additional semantic information.

Rationale:

list, set, multiset, map, multimap are able to store non-Assignables. However, there is some concern about `list<T>`: although in general there's no reason for T to be Assignable, some implementations of the member functions `operator=` and `assign` do rely on that requirement. The LWG does not want to forbid such implementations.

Note that the type stored in a standard container must still satisfy the requirements of the container's allocator; this rules out, for example, such types as "const int". See issue 274 for more details.

In principle we could also relax the "Assignable" requirement for individual `vector` member functions, such as `push_back`. However, the LWG did not see great value in such selective relaxation. Doing so would remove implementors' freedom to implement `vector::push_back` in terms of `vector::insert`.

### 278. What does iterator validity mean?

Section: 23.3.5.5 [list.ops] Status: CD1 Submitter: P.J. Plauger Opened: 2000-11-27 Last modified: 2015-04-08

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Discussion:

Section 23.3.5.5 [list.ops] states that

      void splice(iterator position, list<T, Allocator>& x);

invalidates all iterators and references to list `x`.

But what does the C++ Standard mean by "invalidate"? You can still dereference the iterator to a spliced list element, but you'd better not use it to delimit a range within the original list. For the latter operation, it has definitely lost some of its validity.

If we accept the proposed resolution to issue 250, then we'd better clarify that a "valid" iterator need no longer designate an element within the same container as it once did. We then have to clarify what we mean by invalidating a past-the-end iterator, as when a vector or string grows by reallocation. Clearly, such an iterator has a different kind of validity. Perhaps we should introduce separate terms for the two kinds of "validity."

Proposed resolution:

Add the following text to the end of section X [iterator.concepts], after paragraph 5:

> An invalid iterator is an iterator that may be singular. [Footnote: This definition applies to pointers, since pointers are iterators. The effect of dereferencing an iterator that has been invalidated is undefined.]

[post-Copenhagen: Matt provided wording.]

[Redmond: General agreement with the intent, some objections to the wording. Dave provided new wording.]

Rationale:

This resolution simply defines a term that the Standard uses but never defines, "invalid", in terms of a term that is defined, "singular".

Why do we say "may be singular", instead of "is singular"? That's becuase a valid iterator is one that is known to be nonsingular. Invalidating an iterator means changing it in such a way that it's no longer known to be nonsingular. An example: inserting an element into the middle of a vector is correctly said to invalidate all iterators pointing into the vector. That doesn't necessarily mean they all become singular.

### 280. Comparison of reverse_iterator to const reverse_iterator

Section: 24.5.1 [reverse.iterators] Status: CD1 Submitter: Steve Cleary Opened: 2000-11-27 Last modified: 2015-04-08

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Discussion:

This came from an email from Steve Cleary to Fergus in reference to issue 179. The library working group briefly discussed this in Toronto and believed it should be a separate issue. There was also some reservations about whether this was a worthwhile problem to fix.

Steve said: "Fixing reverse_iterator. std::reverse_iterator can (and should) be changed to preserve these additional requirements." He also said in email that it can be done without breaking user's code: "If you take a look at my suggested solution, reverse_iterator doesn't have to take two parameters; there is no danger of breaking existing code, except someone taking the address of one of the reverse_iterator global operator functions, and I have to doubt if anyone has ever done that. . . But, just in case they have, you can leave the old global functions in as well -- they won't interfere with the two-template-argument functions. With that, I don't see how any user code could break."

Proposed resolution:

Section: 24.5.1.1 [reverse.iterator] add/change the following declarations:

      A) Add a templated assignment operator, after the same manner
            as the templated copy constructor, i.e.:

      template < class U >
      reverse_iterator < Iterator >& operator=(const reverse_iterator< U >& u);

      B) Make all global functions (except the operator+) have
      two template parameters instead of one, that is, for
      operator ==, !=, <, >, <=, >=, - replace:

           template < class Iterator >
           typename reverse_iterator< Iterator >::difference_type operator-(
                     const reverse_iterator< Iterator >& x,
                     const reverse_iterator< Iterator >& y);

      with:

          template < class Iterator1, class Iterator2 >
          typename reverse_iterator < Iterator1 >::difference_type operator-(
                     const reverse_iterator < Iterator1 > & x,
                     const reverse_iterator < Iterator2 > & y);

Also make the addition/changes for these signatures in 24.5.1.3 [reverse.iter.ops].

[ Copenhagen: The LWG is concerned that the proposed resolution introduces new overloads. Experience shows that introducing overloads is always risky, and that it would be inappropriate to make this change without implementation experience. It may be desirable to provide this feature in a different way. ]

[ Lillehammer: We now have implementation experience, and agree that this solution is safe and correct. ]

### 281. std::min() and max() requirements overly restrictive

Section: 25.4.7 [alg.min.max] Status: CD1 Submitter: Martin Sebor Opened: 2000-12-02 Last modified: 2015-04-08

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Duplicate of: 486

Discussion:

The requirements in 25.3.7, p1 and 4 call for T to satisfy the requirements of `LessThanComparable` ( [lessthancomparable]) and `CopyConstructible` (17.6.3.1 [utility.arg.requirements]). Since the functions take and return their arguments and result by const reference, I believe the `CopyConstructible` requirement is unnecessary.

Proposed resolution:

Remove the `CopyConstructible` requirement. Specifically, replace 25.3.7, p1 with

-1- Requires: Type T is `LessThanComparable` ( [lessthancomparable]).

and replace 25.3.7, p4 with

-4- Requires: Type T is `LessThanComparable` ( [lessthancomparable]).

### 282. What types does numpunct grouping refer to?

Section: 22.4.2.2.2 [facet.num.put.virtuals] Status: CD1 Submitter: Howard Hinnant Opened: 2000-12-05 Last modified: 2015-04-08

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Discussion:

Paragraph 16 mistakenly singles out integral types for inserting thousands_sep() characters. This conflicts with the syntax for floating point numbers described under 22.2.3.1/2.

Proposed resolution:

Change paragraph 16 from:

> For integral types, punct.thousands_sep() characters are inserted into the sequence as determined by the value returned by punct.do_grouping() using the method described in 22.4.3.1.2 [facet.numpunct.virtuals].

To:

> For arithmetic types, punct.thousands_sep() characters are inserted into the sequence as determined by the value returned by punct.do_grouping() using the method described in 22.4.3.1.2 [facet.numpunct.virtuals].

[ Copenhagen: Opinions were divided about whether this is actually an inconsistency, but at best it seems to have been unintentional. This is only an issue for floating-point output: The standard is unambiguous that implementations must parse thousands_sep characters when performing floating-point. The standard is also unambiguous that this requirement does not apply to the "C" locale. ]

[ A survey of existing practice is needed; it is believed that some implementations do insert thousands_sep characters for floating-point output and others fail to insert thousands_sep characters for floating-point input even though this is unambiguously required by the standard. ]

[Post-Curaçao: the above proposed resolution is the consensus of Howard, Bill, Pete, Benjamin, Nathan, Dietmar, Boris, and Martin.]

### 283. std::replace() requirement incorrect/insufficient

Section: 25.3.5 [alg.replace] Status: CD1 Submitter: Martin Sebor Opened: 2000-12-15 Last modified: 2015-04-08

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Duplicate of: 483

Discussion:

(revision of the further discussion) There are a number of problems with the requires clauses for the algorithms in 25.1 and 25.2. The requires clause of each algorithm should describe the necessary and sufficient requirements on the inputs to the algorithm such that the algorithm compiles and runs properly. Many of the requires clauses fail to do this. Here is a summary of the kinds of mistakes:

1. Use of EqualityComparable, which only puts requirements on a single type, when in fact an equality operator is required between two different types, typically either T and the iterator's value type or between the value types of two different iterators.
2. Use of Assignable for T when in fact what was needed is Assignable for the value_type of the iterator, and convertability from T to the value_type of the iterator. Or for output iterators, the requirement should be that T is writable to the iterator (output iterators do not have value types).

Here is the list of algorithms that contain mistakes:

- 25.1.2 std::find
- 25.1.6 std::count
- 25.1.8 std::equal
- 25.1.9 std::search, std::search_n
- 25.2.4 std::replace, std::replace_copy
- 25.2.5 std::fill
- 25.2.7 std::remove, std::remove_copy

Also, in the requirements for EqualityComparable, the requirement that the operator be defined for const objects is lacking.

Proposed resolution:

20.1.1 Change p1 from

In Table 28, `T` is a type to be supplied by a C++ program instantiating a template, `a`, `b`, and `c` are values of type `T`.

to

In Table 28, `T` is a type to be supplied by a C++ program instantiating a template, `a`, `b`, and `c` are values of type `const T`.

25 Between p8 and p9

Add the following sentence:

When the description of an algorithm gives an expression such as `*first == value` for a condition, it is required that the expression evaluate to either true or false in boolean contexts.

25.1.2 Change p1 by deleting the requires clause.

25.1.6 Change p1 by deleting the requires clause.

25.1.9

Change p4 from

-4- Requires: Type `T` is `EqualityComparable` (20.1.1), type Size is convertible to integral type (4.7.12.3).

to

-4- Requires: The type `Size` is convertible to integral type (4.7.12.3).

25.2.4 Change p1 from

-1- Requires: Type `T` is `Assignable` (23.1 ) (and, for `replace()`, `EqualityComparable` (20.1.1 )).

to

-1- Requires: The expression `*first = new_value` must be valid.

and change p4 from

-4- Requires: Type `T` is `Assignable` (23.1) (and, for `replace_copy()`, `EqualityComparable` (20.1.1)). The ranges `[first, last)` and `[result, result + (last - first))` shall not overlap.

to

-4- Requires: The results of the expressions `*first` and `new_value` must be writable to the result output iterator. The ranges `[first, last)` and `[result, result + (last - first))` shall not overlap.

25.2.5 Change p1 from

-1- Requires: Type `T` is `Assignable` (23.1). The type `Size` is convertible to an integral type (4.7.12.3).

to

-1- Requires: The expression `value` must be is writable to the output iterator. The type `Size` is convertible to an integral type (4.7.12.3).

25.2.7 Change p1 from

-1- Requires: Type `T` is `EqualityComparable` (20.1.1).

to

-1- Requires: The value type of the iterator must be `Assignable` (23.1).

Rationale:

The general idea of the proposed solution is to remove the faulty requires clauses and let the returns and effects clauses speak for themselves. That is, the returns clauses contain expressions that must be valid, and therefore already imply the correct requirements. In addition, a sentence is added at the beginning of chapter 25 saying that expressions given as conditions must evaluate to true or false in a boolean context. An alternative would be to say that the type of these condition expressions must be literally bool, but that would be imposing a greater restriction that what the standard currently says (which is convertible to bool).

### 284. unportable example in 20.3.7, p6

Section: 20.9.6 [comparisons] Status: CD1 Submitter: Martin Sebor Opened: 2000-12-26 Last modified: 2015-04-08

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Discussion:

The example in 20.9.6 [comparisons], p6 shows how to use the C library function `strcmp()` with the function pointer adapter `ptr_fun()`. But since it's unspecified whether the C library functions have `extern "C"` or `extern "C++"` linkage [17.6.2.3 [using.linkage]], and since function pointers with different the language linkage specifications (7.5 [dcl.link]) are incompatible, whether this example is well-formed is unspecified.

Proposed resolution:

Change 20.9.6 [comparisons] paragraph 6 from:

> [Example:

>         replace_if(v.begin(), v.end(), not1(bind2nd(ptr_fun(strcmp), "C")), "C++");

> replaces each `C` with `C++` in sequence `v`.

to:

> [Example:

>         int compare(const char*, const char*);
>         replace_if(v.begin(), v.end(),
>                    not1(bind2nd(ptr_fun(compare), "abc")), "def");

> replaces each `abc` with `def` in sequence `v`.

Also, remove footnote 215 in that same paragraph.

[Copenhagen: Minor change in the proposed resolution. Since this issue deals in part with C and C++ linkage, it was believed to be too confusing for the strings in the example to be "C" and "C++". ]

[Redmond: More minor changes. Got rid of the footnote (which seems to make a sweeping normative requirement, even though footnotes aren't normative), and changed the sentence after the footnote so that it corresponds to the new code fragment.]

### 285. minor editorial errors in fstream ctors

Section: 27.9.1.7 [ifstream.cons] Status: CD1 Submitter: Martin Sebor Opened: 2000-12-31 Last modified: 2015-04-08

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Discussion:

27.9.1.7 [ifstream.cons], p2, 27.9.1.11 [ofstream.cons], p2, and 27.9.1.15 [fstream.cons], p2 say about the effects of each constructor:

... If that function returns a null pointer, calls `setstate(failbit)` (which may throw `ios_base::failure`).

The parenthetical note doesn't apply since the ctors cannot throw an exception due to the requirement in 27.5.5.2 [basic.ios.cons], p3 that `exceptions()` be initialized to `ios_base::goodbit`.

Proposed resolution:

Strike the parenthetical note from the Effects clause in each of the paragraphs mentioned above.

### 286. <cstdlib> requirements missing size_t typedef

Section: 25.5 [alg.c.library] Status: CD1 Submitter: Judy Ward Opened: 2000-12-30 Last modified: 2015-04-08

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Discussion:

The <cstdlib> header file contains prototypes for bsearch and qsort (C++ Standard section 25.4 paragraphs 3 and 4) and other prototypes (C++ Standard section 21.4 paragraph 1 table 49) that require the typedef size_t. Yet size_t is not listed in the <cstdlib> synopsis table 78 in section 25.4.

Proposed resolution:

Add the type size_t to Table 78 (section 25.4) and add the type size_t <cstdlib> to Table 97 (section C.2).

Rationale:

Since size_t is in <stdlib.h>, it must also be in <cstdlib>.

### 288. <cerrno> requirements missing macro EILSEQ

Section: 19.4 [errno] Status: CD1 Submitter: Judy Ward Opened: 2000-12-30 Last modified: 2015-04-08

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Discussion:

ISO/IEC 9899:1990/Amendment1:1994 Section 4.3 States: "The list of macros defined in <errno.h> is adjusted to include a new macro, EILSEQ"

ISO/IEC 14882:1998(E) section 19.3 does not refer to the above amendment.

Proposed resolution:

Update Table 26 (section 19.3) "Header <cerrno> synopsis" and Table 95 (section C.2) "Standard Macros" to include EILSEQ.

### 291. Underspecification of set algorithms

Section: 25.4.5 [alg.set.operations] Status: CD1 Submitter: Matt Austern Opened: 2001-01-03 Last modified: 2015-04-08

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Discussion:

The standard library contains four algorithms that compute set operations on sorted ranges: `set_union`, `set_intersection`, `set_difference`, and `set_symmetric_difference`. Each of these algorithms takes two sorted ranges as inputs, and writes the output of the appropriate set operation to an output range. The elements in the output range are sorted.

The ordinary mathematical definitions are generalized so that they apply to ranges containing multiple copies of a given element. Two elements are considered to be "the same" if, according to an ordering relation provided by the user, neither one is less than the other. So, for example, if one input range contains five copies of an element and another contains three, the output range of `set_union` will contain five copies, the output range of `set_intersection` will contain three, the output range of `set_difference` will contain two, and the output range of `set_symmetric_difference` will contain two.

Because two elements can be "the same" for the purposes of these set algorithms, without being identical in other respects (consider, for example, strings under case-insensitive comparison), this raises a number of unanswered questions:

- If we're copying an element that's present in both of the input ranges, which one do we copy it from?
- If there are n copies of an element in the relevant input range, and the output range will contain fewer copies (say m) which ones do we choose? The first m, or the last m, or something else?
- Are these operations stable? That is, does a run of equivalent elements appear in the output range in the same order as as it appeared in the input range(s)?

The standard should either answer these questions, or explicitly say that the answers are unspecified. I prefer the former option, since, as far as I know, all existing implementations behave the same way.

Proposed resolution:

Add the following to the end of 25.4.5.2 [set.union] paragraph 5:

> If [first1, last1) contains m elements that are equivalent to each other and [first2, last2) contains n elements that are equivalent to them, then max(m, n) of these elements will be copied to the output range: all m of these elements from [first1, last1), and the last max(n-m, 0) of them from [first2, last2), in that order.

Add the following to the end of 25.4.5.3 [set.intersection] paragraph 5:

> If [first1, last1) contains m elements that are equivalent to each other and [first2, last2) contains n elements that are equivalent to them, the first min(m, n) of those elements from [first1, last1) are copied to the output range.

Add a new paragraph, Notes, after 25.4.5.4 [set.difference] paragraph 4:

> If [first1, last1) contains m elements that are equivalent to each other and [first2, last2) contains n elements that are equivalent to them, the last max(m-n, 0) elements from [first1, last1) are copied to the output range.

Add a new paragraph, Notes, after 25.4.5.5 [set.symmetric.difference] paragraph 4:

> If [first1, last1) contains m elements that are equivalent to each other and [first2, last2) contains n elements that are equivalent to them, then |m - n| of those elements will be copied to the output range: the last m - n of these elements from [first1, last1) if m > n, and the last n - m of these elements from [first2, last2) if m < n.

[Santa Cruz: it's believed that this language is clearer than what's in the Standard. However, it's also believed that the Standard may already make these guarantees (although not quite in these words). Bill and Howard will check and see whether they think that some or all of these changes may be redundant. If so, we may close this issue as NAD.]

Rationale:

For simple cases, these descriptions are equivalent to what's already in the Standard. For more complicated cases, they describe the behavior of existing implementations.

### 292. effects of a.copyfmt (a)

Section: 27.5.5.3 [basic.ios.members] Status: CD1 Submitter: Martin Sebor Opened: 2001-01-05 Last modified: 2015-04-08

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Discussion:

The Effects clause of the member function `copyfmt()` in 27.4.4.2, p15 doesn't consider the case where the left-hand side argument is identical to the argument on the right-hand side, that is `(this == &rhs)`. If the two arguments are identical there is no need to copy any of the data members or call any callbacks registered with `register_callback()`. Also, as Howard Hinnant points out in message c++std-lib-8149 it appears to be incorrect to allow the object to fire `erase_event` followed by `copyfmt_event` since the callback handling the latter event may inadvertently attempt to access memory freed by the former.

Proposed resolution:

Change the Effects clause in 27.4.4.2, p15 from

> -15- Effects:Assigns to the member objects of `*this` the corresponding member objects of `rhs`, except that...

to

> -15- Effects:If `(this == &rhs)` does nothing. Otherwise assigns to the member objects of `*this` the corresponding member objects of `rhs`, except that...

### 294. User defined macros and standard headers

Section: 17.6.4.3.1 [macro.names] Status: CD1 Submitter: James Kanze Opened: 2001-01-11 Last modified: 2015-04-08

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Discussion:

Paragraph 2 of 17.6.4.3.1 [macro.names] reads: "A translation unit that includes a header shall not contain any macros that define names declared in that header." As I read this, it would mean that the following program is legal:

      #define npos 3.14
      #include <sstream>

since npos is not defined in <sstream>. It is, however, defined in <string>, and it is hard to imagine an implementation in which <sstream> didn't include <string>.

I think that this phrase was probably formulated before it was decided that a standard header may freely include other standard headers. The phrase would be perfectly appropriate for C, for example. In light of 17.6.5.2 [res.on.headers] paragraph 1, however, it isn't stringent enough.

Proposed resolution:

For 17.6.4.3.1 [macro.names], replace the current wording, which reads:

> Each name defined as a macro in a header is reserved to the implementation for any use if the translation unit includes the header.168)

> A translation unit that includes a header shall not contain any macros that define names declared or defined in that header. Nor shall such a translation unit define macros for names lexically identical to keywords.

> 168) It is not permissible to remove a library macro definition by using the #undef directive.

with the wording:

> A translation unit that includes a standard library header shall not #define or #undef names declared in any standard library header.

> A translation unit shall not #define or #undef names lexically identical to keywords.

[Lillehammer: Beman provided new wording]

### 295. Is abs defined in <cmath>?

Section: 26.8 [c.math] Status: CD1 Submitter: Jens Maurer Opened: 2001-01-12 Last modified: 2015-04-08

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Discussion:

Table 80 lists the contents of the <cmath> header. It does not list `abs()`. However, 26.5, paragraph 6, which lists added signatures present in <cmath>, does say that several overloads of `abs()` should be defined in <cmath>.

Proposed resolution:

Add `abs` to Table 80. Also, remove the parenthetical list of functions "(abs(), div(), rand(), srand())" from 26.6 [numarray], paragraph 1.

[Copenhagen: Modified proposed resolution so that it also gets rid of that vestigial list of functions in paragraph 1.]

Rationale:

All this DR does is fix a typo; it's uncontroversial. A separate question is whether we're doing the right thing in putting some overloads in <cmath> that we aren't also putting in <cstdlib>. That's issue 323.

### 296. Missing descriptions and requirements of pair operators

Section: 20.3 [pairs] Status: C++11 Submitter: Martin Sebor Opened: 2001-01-14 Last modified: 2015-04-08

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Discussion:

The synopsis of the header `<utility>` in 20.2 [utility] lists the complete set of equality and relational operators for `pair` but the section describing the template and the operators only describes `operator==()` and `operator<()`, and it fails to mention any requirements on the template arguments. The remaining operators are not mentioned at all.

[ 2009-09-27 Alisdair reopens. ]

> The issue is a lack of wording specifying the semantics of `std::pair` relational operators. The rationale is that this is covered by catch-all wording in the relops component, and that as relops directly precedes `pair` in the document this is an easy connection to make.

> Reading the current working paper I make two observations:

> 1. relops no longer immediately precedes `pair` in the order of specification. However, even if it did, there is a lot of `pair` specification itself between the (apparently) unrelated relops and the relational operators for `pair`. (The catch-all still requires `operator==` and `operator<` to be specified explicitly)
> 2. No other library component relies on the catch-all clause. The following all explicitly document all six relational operators, usually in a manner that could have deferred to the relops clause.

> >     tuple
> >     unique_ptr
> >     duration
> >     time_point
> >     basic_string
> >     queue
> >     stack
> >     move_iterator
> >     reverse_iterator
> >     regex submatch
> >     thread::id

> The container components provide their own (equivalent) definition in 23.2.1 [container.requirements.general] Table 90 -- Container requirements and do so do not defer to relops.

> `Shared_ptr` explicitly documents `operator!=` and does not supply the other 3 missing operators (`>`,`>=`,`<=`) so does not meet the reqirements of the relops clause.

> `Weak_ptr` only supports `operator<` so would not be covered by relops.

> At the very least I would request a note pointing to the relops clause we rely on to provide this definition. If this route is taken, I would recommend reducing many of the above listed clauses to a similar note rather than providing redundant specification.

> My preference would be to supply the 4 missing specifications consistent with the rest of the library.

[ 2009-10-11 Daniel opens 1233 which deals with the same issue as it pertains to `unique_ptr`. ]

[ 2009-10 Santa Cruz: ]

> Move to Ready

Proposed resolution:

After p20 20.3 [pairs] add:

>     template <class T1, class T2>
>     bool operator!=(const pair<T1,T2>& x, const pair<T1,T2>& y);

> > Returns: `!(x==y)`

>     template <class T1, class T2>
>     bool operator> (const pair<T1,T2>& x, const pair<T1,T2>& y);

> > Returns: `y < x`

>     template <class T1, class T2>
>     bool operator>=(const pair<T1,T2>& x, const pair<T1,T2>& y);

> > Returns: `!(x < y)`

>     template <class T1, class T2>
>     bool operator<=(const pair<T1,T2>& x, const pair<T1,T2>& y);

> > Returns: `!(y < x)`

Rationale:

20.2.1 [operators] paragraph 10 already specifies the semantics. That paragraph says that, if declarations of operator!=, operator>, operator<=, and operator>= appear without definitions, they are defined as specified in 20.2.1 [operators]. There should be no user confusion, since that paragraph happens to immediately precede the specification of `pair`.

### 297. const_mem_fun_t<>::argument_type should be const T*

Section: 20.9.7 [logical.operations] Status: CD1 Submitter: Martin Sebor Opened: 2001-01-06 Last modified: 2015-04-08

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Discussion:

The class templates `const_mem_fun_t` in 20.5.8, p8 and `const_mem_fun1_t` in 20.5.8, p9 derive from `unary_function<T*, S>`, and `binary_function<T*, A, S>`, respectively. Consequently, their `argument_type`, and `first_argument_type` members, respectively, are both defined to be `T*` (non-const). However, their function call member operator takes a `const T*` argument. It is my opinion that `argument_type` should be `const T*` instead, so that one can easily refer to it in generic code. The example below derived from existing code fails to compile due to the discrepancy:

`template <class T>`
`void foo (typename T::argument_type arg) // #1`
`{`
`typename T::result_type (T::*pf) (typename T::argument_type) const = // #2`
`&T::operator();`
`}`

`struct X { /* ... */ };`

`int main ()`
`{`
`const X x;`
`foo<std::const_mem_fun_t<void, X> >(&x); // #3`
`}`

#1 `foo()` takes a plain unqualified `X*` as an argument
#2 the type of the pointer is incompatible with the type of the member function
#3 the address of a constant being passed to a function taking a non-const `X*`

Proposed resolution:

Replace the top portion of the definition of the class template const_mem_fun_t in 20.5.8, p8

`template <class S, class T> class const_mem_fun_t`
`: public unary_function<T*, S> {`

with

`template <class S, class T> class const_mem_fun_t`
`: public unary_function<const T*, S> {`

Also replace the top portion of the definition of the class template const_mem_fun1_t in 20.5.8, p9

`template <class S, class T, class A> class const_mem_fun1_t`
`: public binary_function<T*, A, S> {`

with

`template <class S, class T, class A> class const_mem_fun1_t`
`: public binary_function<const T*, A, S> {`

Rationale:

This is simply a contradiction: the `argument_type` typedef, and the argument type itself, are not the same.

### 298. ::operator delete[] requirement incorrect/insufficient

Section: 18.6.1.2 [new.delete.array] Status: CD1 Submitter: John A. Pedretti Opened: 2001-01-10 Last modified: 2015-04-08

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Discussion:

The default behavior of `operator delete[]` described in 18.5.1.2, p12 - namely that for non-null value of ptr, the operator reclaims storage allocated by the earlier call to the default `operator new[]` - is not correct in all cases. Since the specified `operator new[]` default behavior is to call `operator new` (18.5.1.2, p4, p8), which can be replaced, along with `operator delete`, by the user, to implement their own memory management, the specified default behavior of`operator delete[]` must be to call `operator delete`.

Proposed resolution:

Change 18.5.1.2, p12 from

> -12- Default behavior:

> - For a null value of `ptr` , does nothing.
> - Any other value of `ptr` shall be a value returned earlier by a call to the default `operator new[](std::size_t)`. [Footnote: The value must not have been invalidated by an intervening call to `operator delete[](void*)` (17.6.4.9 [res.on.arguments]). --- end footnote] For such a non-null value of `ptr` , reclaims storage allocated by the earlier call to the default `operator new[]`.

to

> -12- Default behavior: Calls `operator delete(`ptr) or `operator delete(ptr, std::nothrow)` respectively.

and expunge paragraph 13.

### 300. list::merge() specification incomplete

Section: 23.3.5.5 [list.ops] Status: CD1 Submitter: John Pedretti Opened: 2001-01-23 Last modified: 2015-04-08

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Discussion:

The "Effects" clause for list::merge() (23.3.5.5 [list.ops], p23) appears to be incomplete: it doesn't cover the case where the argument list is identical to *this (i.e., this == &x). The requirement in the note in p24 (below) is that x be empty after the merge which is surely unintended in this case.

Proposed resolution:

In 23.3.5.5 [list.ops], replace paragraps 23-25 with:

> 23 Effects: if (&x == this) does nothing; otherwise, merges the two sorted ranges [begin(), end()) and [x.begin(), x.end()). The result is a range in which the elements will be sorted in non-decreasing order according to the ordering defined by comp; that is, for every iterator i in the range other than the first, the condition comp(*i, *(i - 1)) will be false.

> 24 Notes: Stable: if (&x != this), then for equivalent elements in the two original ranges, the elements from the original range [begin(), end()) always precede the elements from the original range [x.begin(), x.end()). If (&x != this) the range [x.begin(), x.end()) is empty after the merge.

> 25 Complexity: At most size() + x.size() - 1 applications of comp if (&x ! = this); otherwise, no applications of comp are performed. If an exception is thrown other than by a comparison there are no effects.

[Copenhagen: The original proposed resolution did not fix all of the problems in 23.3.5.5 [list.ops], p22-25. Three different paragraphs (23, 24, 25) describe the effects of `merge`. Changing p23, without changing the other two, appears to introduce contradictions. Additionally, "merges the argument list into the list" is excessively vague.]

[Post-Curaçao: Robert Klarer provided new wording.]

### 301. basic_string template ctor effects clause omits allocator argument

Section: 21.4.1 [string.require] Status: CD1 Submitter: Martin Sebor Opened: 2001-01-27 Last modified: 2015-04-08

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Discussion:

The effects clause for the basic_string template ctor in 21.3.1, p15 leaves out the third argument of type Allocator. I believe this to be a mistake.

Proposed resolution:

Replace

> -15- Effects: If `InputIterator` is an integral type, equivalent to

> > `basic_string(static_cast<size_type>(begin), static_cast<value_type>(end))`

with

> -15- Effects: If `InputIterator` is an integral type, equivalent to

> > `basic_string(static_cast<size_type>(begin), static_cast<value_type>(end), a)`

### 303. Bitset input operator underspecified

Section: 20.6.4 [bitset.operators] Status: CD1 Submitter: Matt Austern Opened: 2001-02-05 Last modified: 2015-04-08

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Discussion:

In 23.3.5.3, we are told that `bitset`'s input operator "Extracts up to N (single-byte) characters from is.", where is is a stream of type `basic_istream<charT, traits>`.

The standard does not say what it means to extract single byte characters from a stream whose character type, `charT`, is in general not a single-byte character type. Existing implementations differ.

A reasonable solution will probably involve `widen()` and/or `narrow()`, since they are the supplied mechanism for converting a single character between `char` and arbitrary `charT`.

Narrowing the input characters is not the same as widening the literals `'0'` and `'1'`, because there may be some locales in which more than one wide character maps to the narrow character `'0'`. Narrowing means that alternate representations may be used for bitset input, widening means that they may not be.

Note that for numeric input, `num_get<>` (22.2.2.1.2/8) compares input characters to widened version of narrow character literals.

From Pete Becker, in c++std-lib-8224:

> Different writing systems can have different representations for the digits that represent 0 and 1. For example, in the Unicode representation of the Devanagari script (used in many of the Indic languages) the digit 0 is 0x0966, and the digit 1 is 0x0967. Calling narrow would translate those into '0' and '1'. But Unicode also provides the ASCII values 0x0030 and 0x0031 for for the Latin representations of '0' and '1', as well as code points for the same numeric values in several other scripts (Tamil has no character for 0, but does have the digits 1-9), and any of these values would also be narrowed to '0' and '1'.

> ...

> It's fairly common to intermix both native and Latin representations of numbers in a document. So I think the rule has to be that if a wide character represents a digit whose value is 0 then the bit should be cleared; if it represents a digit whose value is 1 then the bit should be set; otherwise throw an exception. So in a Devanagari locale, both 0x0966 and 0x0030 would clear the bit, and both 0x0967 and 0x0031 would set it. Widen can't do that. It would pick one of those two values, and exclude the other one.

From Jens Maurer, in c++std-lib-8233:

> Whatever we decide, I would find it most surprising if bitset conversion worked differently from int conversion with regard to alternate local representations of numbers.

> Thus, I think the options are:

> - Have a new defect issue for 22.2.2.1.2/8 so that it will require the use of narrow().
> - Have a defect issue for bitset() which describes clearly that widen() is to be used.

Proposed resolution:

Replace the first two sentences of paragraph 5 with:

> Extracts up to N characters from is. Stores these characters in a temporary object str of type `basic_string<charT, traits>`, then evaluates the expression `x = bitset<N>(str)`.

Replace the third bullet item in paragraph 5 with:

- the next input character is neither `is.widen(0)` nor `is.widen(1)` (in which case the input character is not extracted).

Rationale:

Input for `bitset` should work the same way as numeric input. Using `widen` does mean that alternative digit representations will not be recognized, but this was a known consequence of the design choice.

### 305. Default behavior of codecvt<wchar_t, char, mbstate_t>::length()

Section: 22.4.1.5 [locale.codecvt.byname] Status: CD1 Submitter: Howard Hinnant Opened: 2001-01-24 Last modified: 2015-04-08

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Discussion:

22.2.1.5/3 introduces codecvt in part with:

> codecvt<wchar_t,char,mbstate_t> converts between the native character sets for tiny and wide characters. Instantiations on mbstate_t perform conversion between encodings known to the library implementor.

But 22.2.1.5.2/10 describes do_length in part with:

> ... codecvt<wchar_t, char, mbstate_t> ... return(s) the lesser of max and (from_end-from).

The semantics of do_in and do_length are linked. What one does must be consistent with what the other does. 22.2.1.5/3 leads me to believe that the vendor is allowed to choose the algorithm that codecvt<wchar_t,char,mbstate_t>::do_in performs so that it makes his customers happy on a given platform. But 22.2.1.5.2/10 explicitly says what codecvt<wchar_t,char,mbstate_t>::do_length must return. And thus indirectly specifies the algorithm that codecvt<wchar_t,char,mbstate_t>::do_in must perform. I believe that this is not what was intended and is a defect.

Discussion from the -lib reflector:
This proposal would have the effect of making the semantics of all of the virtual functions in `codecvt<wchar_t, char, mbstate_t>` implementation specified. Is that what we want, or do we want to mandate specific behavior for the base class virtuals and leave the implementation specified behavior for the codecvt_byname derived class? The tradeoff is that former allows implementors to write a base class that actually does something useful, while the latter gives users a way to get known and specified---albeit useless---behavior, and is consistent with the way the standard handles other facets. It is not clear what the original intention was.

Nathan has suggest a compromise: a character that is a widened version of the characters in the basic execution character set must be converted to a one-byte sequence, but there is no such requirement for characters that are not part of the basic execution character set.

Proposed resolution:

Change 22.2.1.5.2/5 from:

The instantiations required in Table 51 (lib.locale.category), namely codecvt<wchar_t,char,mbstate_t> and codecvt<char,char,mbstate_t>, store no characters. Stores no more than (to_limit-to) destination elements. It always leaves the to_next pointer pointing one beyond the last element successfully stored.

to:

Stores no more than (to_limit-to) destination elements, and leaves the to_next pointer pointing one beyond the last element successfully stored. codecvt<char,char,mbstate_t> stores no characters.

Change 22.2.1.5.2/10 from:

> -10- Returns: (from_next-from) where from_next is the largest value in the range [from,from_end] such that the sequence of values in the range [from,from_next) represents max or fewer valid complete characters of type internT. The instantiations required in Table 51 (21.1.1.1.1), namely codecvt<wchar_t, char, mbstate_t> and codecvt<char, char, mbstate_t>, return the lesser of max and (from_end-from).

to:

> -10- Returns: (from_next-from) where from_next is the largest value in the range [from,from_end] such that the sequence of values in the range [from,from_next) represents max or fewer valid complete characters of type internT. The instantiation codecvt<char, char, mbstate_t> returns the lesser of max and (from_end-from).

[Redmond: Nathan suggested an alternative resolution: same as above, but require that, in the default encoding, a character from the basic execution character set would map to a single external character. The straw poll was 8-1 in favor of the proposed resolution.]

Rationale:

The default encoding should be whatever users of a given platform would expect to be the most natural. This varies from platform to platform. In many cases there is a preexisting C library, and users would expect the default encoding to be whatever C uses in the default "C" locale. We could impose a guarantee like the one Nathan suggested (a character from the basic execution character set must map to a single external character), but this would rule out important encodings that are in common use: it would rule out JIS, for example, and it would rule out a fixed-width encoding of UCS-4.

[Curaçao: fixed rationale typo at the request of Ichiro Koshida; "shift-JIS" changed to "JIS".]

### 306. offsetof macro and non-POD types

Section: 18.2 [support.types] Status: CD1 Submitter: Steve Clamage Opened: 2001-02-21 Last modified: 2015-04-08

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Discussion:

Spliced together from reflector messages c++std-lib-8294 and -8295:

18.1, paragraph 5, reads: "The macro `offsetof` accepts a restricted set of type arguments in this International Standard. type shall be a POD structure or a POD union (clause 9). The result of applying the offsetof macro to a field that is a static data member or a function member is undefined."

For the POD requirement, it doesn't say "no diagnostic required" or "undefined behavior". I read 1.4 [intro.compliance], paragraph 1, to mean that a diagnostic is required. It's not clear whether this requirement was intended. While it's possible to provide such a diagnostic, the extra complication doesn't seem to add any value.

Proposed resolution:

Change 18.1, paragraph 5, to "If type is not a POD structure or a POD union the results are undefined."

[Copenhagen: straw poll was 7-4 in favor. It was generally agreed that requiring a diagnostic was inadvertent, but some LWG members thought that diagnostics should be required whenever possible.]

### 307. Lack of reference typedefs in container adaptors

Section: 23.3.5 [list] Status: CD1 Submitter: Howard Hinnant Opened: 2001-03-13 Last modified: 2015-04-08

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Discussion:

From reflector message c++std-lib-8330. See also lib-8317.

The standard is currently inconsistent in 23.3.5.3 [list.capacity] paragraph 1 and 23.3.5.4 [list.modifiers] paragraph 1. 23.2.3.3/1, for example, says:

> -1- Any sequence supporting operations back(), push_back() and pop_back() can be used to instantiate stack. In particular, vector (lib.vector), list (lib.list) and deque (lib.deque) can be used.

But this is false: vector<bool> can not be used, because the container adaptors return a T& rather than using the underlying container's reference type.

This is a contradiction that can be fixed by:

1. Modifying these paragraphs to say that vector<bool> is an exception.
2. Removing the vector<bool> specialization.
3. Changing the return types of stack and priority_queue to use reference typedef's.

I propose 3. This does not preclude option 2 if we choose to do it later (see issue 96); the issues are independent. Option 3 offers a small step towards support for proxied containers. This small step fixes a current contradiction, is easy for vendors to implement, is already implemented in at least one popular lib, and does not break any code.

Proposed resolution:

Summary: Add reference and const_reference typedefs to queue, priority_queue and stack. Change return types of "value_type&" to "reference". Change return types of "const value_type&" to "const_reference". Details:

Change 23.2.3.1/1 from:

      namespace std {
        template <class T, class Container = deque<T> >
        class queue {
        public:
          typedef typename Container::value_type            value_type;
          typedef typename Container::size_type             size_type;
          typedef          Container                        container_type;
        protected:
          Container c;

        public:
          explicit queue(const Container& = Container());

          bool      empty() const             { return c.empty(); }
          size_type size()  const             { return c.size(); }
          value_type&       front()           { return c.front(); }
          const value_type& front() const     { return c.front(); }
          value_type&       back()            { return c.back(); }
          const value_type& back() const      { return c.back(); }
          void push(const value_type& x)      { c.push_back(x); }
          void pop()                          { c.pop_front(); }
        };

to:

      namespace std {
        template <class T, class Container = deque<T> >
        class queue {
        public:
          typedef typename Container::value_type            value_type;
          typedef typename Container::reference             reference;
          typedef typename Container::const_reference       const_reference;
          typedef typename Container::value_type            value_type;
          typedef typename Container::size_type             size_type;
          typedef          Container                        container_type;
        protected:
          Container c;

        public:
          explicit queue(const Container& = Container());

          bool      empty() const             { return c.empty(); }
          size_type size()  const             { return c.size(); }
          reference         front()           { return c.front(); }
          const_reference   front() const     { return c.front(); }
          reference         back()            { return c.back(); }
          const_reference   back() const      { return c.back(); }
          void push(const value_type& x)      { c.push_back(x); }
          void pop()                          { c.pop_front(); }
        };

Change 23.2.3.2/1 from:

      namespace std {
        template <class T, class Container = vector<T>,
                  class Compare = less<typename Container::value_type> >
        class priority_queue {
        public:
          typedef typename Container::value_type            value_type;
          typedef typename Container::size_type             size_type;
          typedef          Container                        container_type;
        protected:
          Container c;
          Compare comp;

        public:
          explicit priority_queue(const Compare& x = Compare(),
                                  const Container& = Container());
          template <class InputIterator>
            priority_queue(InputIterator first, InputIterator last,
                           const Compare& x = Compare(),
                           const Container& = Container());

          bool      empty() const       { return c.empty(); }
          size_type size()  const       { return c.size(); }
          const value_type& top() const { return c.front(); }
          void push(const value_type& x);
          void pop();
        };
                                      //  no equality is provided
      }

to:

      namespace std {
        template <class T, class Container = vector<T>,
                  class Compare = less<typename Container::value_type> >
        class priority_queue {
        public:
          typedef typename Container::value_type            value_type;
          typedef typename Container::reference             reference;
          typedef typename Container::const_reference       const_reference;
          typedef typename Container::size_type             size_type;
          typedef          Container                        container_type;
        protected:
          Container c;
          Compare comp;

        public:
          explicit priority_queue(const Compare& x = Compare(),
                                  const Container& = Container());
          template <class InputIterator>
            priority_queue(InputIterator first, InputIterator last,
                           const Compare& x = Compare(),
                           const Container& = Container());

          bool      empty() const       { return c.empty(); }
          size_type size()  const       { return c.size(); }
          const_reference   top() const { return c.front(); }
          void push(const value_type& x);
          void pop();
        };
                                      //  no equality is provided
      }

And change 23.2.3.3/1 from:

      namespace std {
        template <class T, class Container = deque<T> >
        class stack {
        public:
          typedef typename Container::value_type            value_type;
          typedef typename Container::size_type             size_type;
          typedef          Container                        container_type;
        protected:
          Container c;

        public:
          explicit stack(const Container& = Container());

          bool      empty() const             { return c.empty(); }
          size_type size()  const             { return c.size(); }
          value_type&       top()             { return c.back(); }
          const value_type& top() const       { return c.back(); }
          void push(const value_type& x)      { c.push_back(x); }
          void pop()                          { c.pop_back(); }
        };

        template <class T, class Container>
          bool operator==(const stack<T, Container>& x,
                          const stack<T, Container>& y);
        template <class T, class Container>
          bool operator< (const stack<T, Container>& x,
                          const stack<T, Container>& y);
        template <class T, class Container>
          bool operator!=(const stack<T, Container>& x,
                          const stack<T, Container>& y);
        template <class T, class Container>
          bool operator> (const stack<T, Container>& x,
                          const stack<T, Container>& y);
        template <class T, class Container>
          bool operator>=(const stack<T, Container>& x,
                          const stack<T, Container>& y);
        template <class T, class Container>
          bool operator<=(const stack<T, Container>& x,
                          const stack<T, Container>& y);
      }

to:

      namespace std {
        template <class T, class Container = deque<T> >
        class stack {
        public:
          typedef typename Container::value_type            value_type;
          typedef typename Container::reference             reference;
          typedef typename Container::const_reference       const_reference;
          typedef typename Container::size_type             size_type;
          typedef          Container                        container_type;
        protected:
          Container c;

        public:
          explicit stack(const Container& = Container());

          bool      empty() const             { return c.empty(); }
          size_type size()  const             { return c.size(); }
          reference         top()             { return c.back(); }
          const_reference   top() const       { return c.back(); }
          void push(const value_type& x)      { c.push_back(x); }
          void pop()                          { c.pop_back(); }
        };

        template <class T, class Container>
          bool operator==(const stack<T, Container>& x,
                          const stack<T, Container>& y);
        template <class T, class Container>
          bool operator< (const stack<T, Container>& x,
                          const stack<T, Container>& y);
        template <class T, class Container>
          bool operator!=(const stack<T, Container>& x,
                          const stack<T, Container>& y);
        template <class T, class Container>
          bool operator> (const stack<T, Container>& x,
                          const stack<T, Container>& y);
        template <class T, class Container>
          bool operator>=(const stack<T, Container>& x,
                          const stack<T, Container>& y);
        template <class T, class Container>
          bool operator<=(const stack<T, Container>& x,
                          const stack<T, Container>& y);
      }

[Copenhagen: This change was discussed before the IS was released and it was deliberately not adopted. Nevertheless, the LWG believes (straw poll: 10-2) that it is a genuine defect.]

### 308. Table 82 mentions unrelated headers

Section: 27 [input.output] Status: CD1 Submitter: Martin Sebor Opened: 2001-03-15 Last modified: 2015-04-08

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Discussion:

Table 82 in section 27 mentions the header <cstdlib> for String streams (27.8 [string.streams]) and the headers <cstdio> and <cwchar> for File streams (27.9 [file.streams]). It's not clear why these headers are mentioned in this context since they do not define any of the library entities described by the subclauses. According to 17.6.1.1 [contents], only such headers are to be listed in the summary.

Proposed resolution:

Remove <cstdlib> and <cwchar> from Table 82.

[Copenhagen: changed the proposed resolution slightly. The original proposed resolution also said to remove <cstdio> from Table 82. However, <cstdio> is mentioned several times within section 27.9 [file.streams], including 27.9.2 [c.files].]

### 310. Is errno a macro?

Section: 17.6.1.2 [headers], 19.4 [errno] Status: CD1 Submitter: Steve Clamage Opened: 2001-03-21 Last modified: 2015-04-08

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Discussion:

Exactly how should errno be declared in a conforming C++ header?

The C standard says in 7.1.4 that it is unspecified whether errno is a macro or an identifier with external linkage. In some implementations it can be either, depending on compile-time options. (E.g., on Solaris in multi-threading mode, errno is a macro that expands to a function call, but is an extern int otherwise. "Unspecified" allows such variability.)

The C++ standard:

- 17.4.1.2 says in a note that errno must be macro in C. (false)
- 17.4.3.1.3 footnote 166 says errno is reserved as an external name (true), and implies that it is an identifier.
- 19.3 simply lists errno as a macro (by what reasoning?) and goes on to say that the contents of of C++ <errno.h> are the same as in C, begging the question.
- C.2, table 95 lists errno as a macro, without comment.

I find no other references to errno.

We should either explicitly say that errno must be a macro, even though it need not be a macro in C, or else explicitly leave it unspecified. We also need to say something about namespace std. A user who includes <cerrno> needs to know whether to write `errno`, or `::errno`, or `std::errno`, or else <cerrno> is useless.

Two acceptable fixes:

- errno must be a macro. This is trivially satisfied by adding
  #define errno (::std::errno)
  to the headers if errno is not already a macro. You then always write errno without any scope qualification, and it always expands to a correct reference. Since it is always a macro, you know to avoid using errno as a local identifer.

- errno is in the global namespace. This fix is inferior, because ::errno is not guaranteed to be well-formed.

[ This issue was first raised in 1999, but it slipped through the cracks. ]

Proposed resolution:

Change the Note in section 17.4.1.2p5 from

> Note: the names defined as macros in C include the following: assert, errno, offsetof, setjmp, va_arg, va_end, and va_start.

to

> Note: the names defined as macros in C include the following: assert, offsetof, setjmp, va_arg, va_end, and va_start.

In section 19.3, change paragraph 2 from

> The contents are the same as the Standard C library header <errno.h>.

to

> The contents are the same as the Standard C library header <errno.h>, except that errno shall be defined as a macro.

Rationale:

C++ must not leave it up to the implementation to decide whether or not a name is a macro; it must explicitly specify exactly which names are required to be macros. The only one that really works is for it to be a macro.

[Curaçao: additional rationale added.]

### 311. Incorrect wording in basic_ostream class synopsis

Section: 27.7.3.1 [ostream] Status: CD1 Submitter: Andy Sawyer Opened: 2001-03-21 Last modified: 2015-04-08

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Discussion:

In 27.7.3.1 [ostream], the synopsis of class basic_ostream says:

      // partial specializationss
      template<class traits>
        basic_ostream<char,traits>& operator<<( basic_ostream<char,traits>&,
                                                const char * );

Problems:

- Too many 's's at the end of "specializationss"
- This is an overload, not a partial specialization

Proposed resolution:

In the synopsis in 27.7.3.1 [ostream], remove the // partial specializationss comment. Also remove the same comment (correctly spelled, but still incorrect) from the synopsis in 27.7.3.6.4 [ostream.inserters.character].

[ Pre-Redmond: added 27.7.3.6.4 [ostream.inserters.character] because of Martin's comment in c++std-lib-8939. ]

### 312. Table 27 is missing headers

Section: 20 [utilities] Status: CD1 Submitter: Martin Sebor Opened: 2001-03-29 Last modified: 2015-04-08

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Discussion:

Table 27 in section 20 lists the header <memory> (only) for Memory (lib.memory) but neglects to mention the headers <cstdlib> and <cstring> that are discussed in 20.10.6 [meta.rel].

Proposed resolution:

Add <cstdlib> and <cstring> to Table 27, in the same row as <memory>.

### 315. Bad "range" in list::unique complexity

Section: 23.3.5.5 [list.ops] Status: CD1 Submitter: Andy Sawyer Opened: 2001-05-01 Last modified: 2015-04-08

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Discussion:

23.3.5.5 [list.ops], Para 21 describes the complexity of list::unique as: "If the range (last - first) is not empty, exactly (last - first) -1 applications of the corresponding predicate, otherwise no applications of the predicate)".

"(last - first)" is not a range.

Proposed resolution:

Change the "range" from (last - first) to [first, last).

### 316. Vague text in Table 69

Section: 23.2.4 [associative.reqmts] Status: CD1 Submitter: Martin Sebor Opened: 2001-05-04 Last modified: 2015-04-08

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Discussion:

Table 69 says this about a_uniq.insert(t):

> inserts t if and only if there is no element in the container with key equivalent to the key of t. The bool component of the returned pair indicates whether the insertion takes place and the iterator component of the pair points to the element with key equivalent to the key of t.

The description should be more specific about exactly how the bool component indicates whether the insertion takes place.

Proposed resolution:

Change the text in question to

> ...The bool component of the returned pair is true if and only if the insertion takes place...

### 317. Instantiation vs. specialization of facets

Section: 22 [localization] Status: CD1 Submitter: Martin Sebor Opened: 2001-05-04 Last modified: 2015-04-08

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Discussion:

The localization section of the standard refers to specializations of the facet templates as instantiations even though the required facets are typically specialized rather than explicitly (or implicitly) instantiated. In the case of ctype<char> and ctype_byname<char> (and the wchar_t versions), these facets are actually required to be specialized. The terminology should be corrected to make it clear that the standard doesn't mandate explicit instantiation (the term specialization encompasses both explicit instantiations and specializations).

Proposed resolution:

In the following paragraphs, replace all occurrences of the word instantiation or instantiations with specialization or specializations, respectively:

> 22.1.1.1.1, p4, Table 52, 22.2.1.1, p2, 22.2.1.5, p3, 22.2.1.5.1, p5, 22.2.1.5.2, p10, 22.2.2, p2, 22.2.3.1, p1, 22.2.3.1.2, p1, p2 and p3, 22.2.4.1, p1, 22.2.4.1.2, p1, 22,2,5, p1, 22,2,6, p2, 22.2.6.3.2, p7, and Footnote 242.

And change the text in 22.1.1.1.1, p4 from

> An implementation is required to provide those instantiations for facet templates identified as members of a category, and for those shown in Table 52:

to

> An implementation is required to provide those specializations...

[Nathan will review these changes, and will look for places where explicit specialization is necessary.]

Rationale:

This is a simple matter of outdated language. The language to describe templates was clarified during the standardization process, but the wording in clause 22 was never updated to reflect that change.

### 318. Misleading comment in definition of numpunct_byname

Section: 22.4.3.2 [locale.numpunct.byname] Status: CD1 Submitter: Martin Sebor Opened: 2001-05-12 Last modified: 2015-04-08

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Discussion:

The definition of the numpunct_byname template contains the following comment:

        namespace std {
            template <class charT>
            class numpunct_byname : public numpunct<charT> {
        // this class is specialized for char and wchar_t.
            ...

There is no documentation of the specializations and it seems conceivable that an implementation will not explicitly specialize the template at all, but simply provide the primary template.

Proposed resolution:

Remove the comment from the text in 22.2.3.2 and from the proposed resolution of library issue 228.

### 319. Storage allocation wording confuses "Required behavior", "Requires"

Section: 18.6.1.1 [new.delete.single], 18.6.1.2 [new.delete.array] Status: CD1 Submitter: Beman Dawes Opened: 2001-05-15 Last modified: 2015-04-08

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Discussion:

The standard specifies 17.5.1.4 [structure.specifications] that "Required behavior" elements describe "the semantics of a function definition provided by either the implementation or a C++ program."

The standard specifies 17.5.1.4 [structure.specifications] that "Requires" elements describe "the preconditions for calling the function."

In the sections noted below, the current wording specifies "Required Behavior" for what are actually preconditions, and thus should be specified as "Requires".

Proposed resolution:

In 18.6.1.1 [new.delete.single] Para 12 Change:

> Required behavior: accept a value of ptr that is null or that was returned by an earlier call ...

to:

> Requires: the value of ptr is null or the value returned by an earlier call ...

In 18.6.1.2 [new.delete.array] Para 11 Change:

> Required behavior: accept a value of ptr that is null or that was returned by an earlier call ...

to:

> Requires: the value of ptr is null or the value returned by an earlier call ...

### 320. list::assign overspecified

Section: 23.3.5.2 [list.cons] Status: CD1 Submitter: Howard Hinnant Opened: 2001-05-17 Last modified: 2015-04-08

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Discussion:

Section 23.3.5.2 [list.cons], paragraphs 6-8 specify that list assign (both forms) have the "effects" of a call to erase followed by a call to insert.

I would like to document that implementers have the freedom to implement assign by other methods, as long as the end result is the same and the exception guarantee is as good or better than the basic guarantee.

The motivation for this is to use T's assignment operator to recycle existing nodes in the list instead of erasing them and reallocating them with new values. It is also worth noting that, with careful coding, most common cases of assign (everything but assignment with true input iterators) can elevate the exception safety to strong if T's assignment has a nothrow guarantee (with no extra memory cost). Metrowerks does this. However I do not propose that this subtlety be standardized. It is a QoI issue.

Existing practise: Metrowerks and SGI recycle nodes, Dinkumware and Rogue Wave don't.

Proposed resolution:

Change 23.3.5.2 [list.cons]/7 from:

> Effects:

>        erase(begin(), end());
>        insert(begin(), first, last);

to:

> Effects: Replaces the contents of the list with the range [first, last).

In 23.2.3 [sequence.reqmts], in Table 67 (sequence requirements), add two new rows:

          a.assign(i,j)     void      pre: i,j are not iterators into a.
                                      Replaces elements in a with a copy
                                      of [i, j).

          a.assign(n,t)     void      pre: t is not a reference into a.
                                      Replaces elements in a with n copies
                                      of t.

Change 23.3.5.2 [list.cons]/8 from:

> Effects:

>        erase(begin(), end());
>        insert(begin(), n, t);

to:

> Effects: Replaces the contents of the list with n copies of t.

[Redmond: Proposed resolution was changed slightly. Previous version made explicit statement about exception safety, which wasn't consistent with the way exception safety is expressed elsewhere. Also, the change in the sequence requirements is new. Without that change, the proposed resolution would have required that assignment of a subrange would have to work. That too would have been overspecification; it would effectively mandate that assignment use a temporary. Howard provided wording. ]

[Curaçao: Made editorial improvement in wording; changed "Replaces elements in a with copies of elements in [i, j)." with "Replaces the elements of a with a copy of [i, j)." Changes not deemed serious enough to requre rereview.]

### 321. Typo in num_get

Section: 22.4.2.1.2 [facet.num.get.virtuals] Status: CD1 Submitter: Kevin Djang Opened: 2001-05-17 Last modified: 2015-04-08

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Discussion:

Section 22.2.2.1.2 at p7 states that "A length specifier is added to the conversion function, if needed, as indicated in Table 56." However, Table 56 uses the term "length modifier", not "length specifier".

Proposed resolution:

In 22.2.2.1.2 at p7, change the text "A length specifier is added ..." to be "A length modifier is added ..."

Rationale:

C uses the term "length modifier". We should be consistent.

### 322. iterator and const_iterator should have the same value type

Section: 23.2 [container.requirements] Status: CD1 Submitter: Matt Austern Opened: 2001-05-17 Last modified: 2015-04-08

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Discussion:

It's widely assumed that, if X is a container, iterator_traits<X::iterator>::value_type and iterator_traits<X::const_iterator>::value_type should both be X::value_type. However, this is nowhere stated. The language in Table 65 is not precise about the iterators' value types (it predates iterator_traits), and could even be interpreted as saying that iterator_traits<X::const_iterator>::value_type should be "const X::value_type".

Related issue: 279.

Proposed resolution:

In Table 65 ("Container Requirements"), change the return type for X::iterator to "iterator type whose value type is T". Change the return type for X::const_iterator to "constant iterator type whose value type is T".

Rationale:

This belongs as a container requirement, rather than an iterator requirement, because the whole notion of iterator/const_iterator pairs is specific to containers' iterator.

It is existing practice that (for example) iterator_traits<list<int>::const_iterator>::value_type is "int", rather than "const int". This is consistent with the way that const pointers are handled: the standard already requires that iterator_traits<const int*>::value_type is int.

### 324. Do output iterators have value types?

Section: 24.2.4 [output.iterators] Status: CD1 Submitter: Dave Abrahams Opened: 2001-06-07 Last modified: 2015-04-08

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Discussion:

Table 73 suggests that output iterators have value types. It requires the expression "*a = t". Additionally, although Table 73 never lists "a = t" or "X(a) = t" in the "expressions" column, it contains a note saying that "a = t" and "X(a) = t" have equivalent (but nowhere specified!) semantics.

According to 24.1/9, t is supposed to be "a value of value type T":

> In the following sections, a and b denote values of X, n denotes a value of the difference type Distance, u, tmp, and m denote identifiers, r denotes a value of X&, t denotes a value of value type T.

Two other parts of the standard that are relevant to whether output iterators have value types:

- 24.1/1 says "All iterators i support the expression *i, resulting in a value of some class, enumeration, or built-in type T, called the value type of the iterator".
- 24.3.1/1, which says "In the case of an output iterator, the types iterator_traits<Iterator>::difference_type iterator_traits<Iterator>::value_type are both defined as void."

The first of these passages suggests that "*i" is supposed to return a useful value, which contradicts the note in 24.1.2/2 saying that the only valid use of "*i" for output iterators is in an expression of the form "*i = t". The second of these passages appears to contradict Table 73, because it suggests that "*i"'s return value should be void. The second passage is also broken in the case of a an iterator type, like non-const pointers, that satisfies both the output iterator requirements and the forward iterator requirements.

What should the standard say about `*i`'s return value when i is an output iterator, and what should it say about that t is in the expression "*i = t"? Finally, should the standard say anything about output iterators' pointer and reference types?

Proposed resolution:

24.1 p1, change

> All iterators `i` support the expression `*i`, resulting in a value of some class, enumeration, or built-in type `T`, called the value type of the iterator.

to

> All input iterators `i` support the expression `*i`, resulting in a value of some class, enumeration, or built-in type `T`, called the value type of the iterator. All output iterators support the expression `*i = o` where `o` is a value of some type that is in the set of types that are writable to the particular iterator type of `i`.

24.1 p9, add

> `o` denotes a value of some type that is writable to the output iterator.

Table 73, change

>     *a = t

to

>     *r = o

and change

>     *r++ = t

to

>     *r++ = o

[post-Redmond: Jeremy provided wording]

Rationale:

The LWG considered two options: change all of the language that seems to imply that output iterators have value types, thus making it clear that output iterators have no value types, or else define value types for output iterator consistently. The LWG chose the former option, because it seems clear that output iterators were never intended to have value types. This was a deliberate design decision, and any language suggesting otherwise is simply a mistake.

A future revision of the standard may wish to revisit this design decision.

### 325. Misleading text in moneypunct<>::do_grouping

Section: 22.4.6.3.2 [locale.moneypunct.virtuals] Status: CD1 Submitter: Martin Sebor Opened: 2001-07-02 Last modified: 2015-04-08

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Discussion:

The Returns clause in 22.2.6.3.2, p3 says about moneypunct<charT>::do_grouping()

> Returns: A pattern defined identically as the result of numpunct<charT>::do_grouping().241)

Footnote 241 then reads

> This is most commonly the value "\003" (not "3").

The returns clause seems to imply that the two member functions must return an identical value which in reality may or may not be true, since the facets are usually implemented in terms of struct std::lconv and return the value of the grouping and mon_grouping, respectively. The footnote also implies that the member function of the moneypunct facet (rather than the overridden virtual functions in moneypunct_byname) most commonly return "\003", which contradicts the C standard which specifies the value of "" for the (most common) C locale.

Proposed resolution:

Replace the text in Returns clause in 22.2.6.3.2, p3 with the following:

> Returns: A pattern defined identically as, but not necessarily equal to, the result of numpunct<charT>::do_grouping().241)

and replace the text in Footnote 241 with the following:

> To specify grouping by 3s the value is "\003", not "3".

Rationale:

The fundamental problem is that the description of the locale facet virtuals serves two purposes: describing the behavior of the base class, and describing the meaning of and constraints on the behavior in arbitrary derived classes. The new wording makes that separation a little bit clearer. The footnote (which is nonnormative) is not supposed to say what the grouping is in the "C" locale or in any other locale. It is just a reminder that the values are interpreted as small integers, not ASCII characters.

### 327. Typo in time_get facet in table 52

Section: 22.3.1.1.1 [locale.category] Status: CD1 Submitter: Tiki Wan Opened: 2001-07-06 Last modified: 2015-04-08

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Duplicate of: 447

Discussion:

The `wchar_t` versions of `time_get` and `time_get_byname` are listed incorrectly in table 52, required instantiations. In both cases the second template parameter is given as OutputIterator. It should instead be InputIterator, since these are input facets.

Proposed resolution:

In table 52, required instantiations, in 22.3.1.1.1 [locale.category], change

        time_get<wchar_t, OutputIterator>
        time_get_byname<wchar_t, OutputIterator>

to

        time_get<wchar_t, InputIterator>
        time_get_byname<wchar_t, InputIterator>

[Redmond: Very minor change in proposed resolution. Original had a typo, wchart instead of wchar_t.]

### 328. Bad sprintf format modifier in money_put<>::do_put()

Section: 22.4.6.2.2 [locale.money.put.virtuals] Status: CD1 Submitter: Martin Sebor Opened: 2001-07-07 Last modified: 2015-04-08

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Discussion:

The sprintf format string , "%.01f" (that's the digit one), in the description of the do_put() member functions of the money_put facet in 22.2.6.2.2, p1 is incorrect. First, the f format specifier is wrong for values of type long double, and second, the precision of 01 doesn't seem to make sense. What was most likely intended was "%.0Lf"., that is a precision of zero followed by the L length modifier.

Proposed resolution:

Change the format string to "%.0Lf".

Rationale:

Fixes an obvious typo

### 329. vector capacity, reserve and reallocation

Section: 23.3.6.3 [vector.capacity], 23.3.6.5 [vector.modifiers] Status: CD1 Submitter: Anthony Williams Opened: 2001-07-13 Last modified: 2015-04-08

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Discussion:

There is an apparent contradiction about which circumstances can cause a reallocation of a vector in Section 23.3.6.3 [vector.capacity] and section 23.3.6.5 [vector.modifiers].

23.3.6.3 [vector.capacity],p5 says:

> Notes: Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence. It is guaranteed that no reallocation takes place during insertions that happen after a call to reserve() until the time when an insertion would make the size of the vector greater than the size specified in the most recent call to reserve().

Which implies if I do

      std::vector<int> vec;
      vec.reserve(23);
      vec.reserve(0);
      vec.insert(vec.end(),1);

then the implementation may reallocate the vector for the insert, as the size specified in the previous call to reserve was zero.

However, the previous paragraphs (23.3.6.3 [vector.capacity], p1-2) state:

> (capacity) Returns: The total number of elements the vector can hold without requiring reallocation

> ...After reserve(), capacity() is greater or equal to the argument of reserve if reallocation happens; and equal to the previous value of capacity() otherwise...

This implies that vec.capacity() is still 23, and so the insert() should not require a reallocation, as vec.size() is 0. This is backed up by 23.3.6.5 [vector.modifiers], p1:

> (insert) Notes: Causes reallocation if the new size is greater than the old capacity.

Though this doesn't rule out reallocation if the new size is less than the old capacity, I think the intent is clear.

Proposed resolution:

Change the wording of 23.3.6.3 [vector.capacity] paragraph 5 to:

> Notes: Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence. It is guaranteed that no reallocation takes place during insertions that happen after a call to reserve() until the time when an insertion would make the size of the vector greater than the value of capacity().

[Redmond: original proposed resolution was modified slightly. In the original, the guarantee was that there would be no reallocation until the size would be greater than the value of capacity() after the most recent call to reserve(). The LWG did not believe that the "after the most recent call to reserve()" added any useful information.]

Rationale:

There was general agreement that, when reserve() is called twice in succession and the argument to the second invocation is smaller than the argument to the first, the intent was for the second invocation to have no effect. Wording implying that such cases have an effect on reallocation guarantees was inadvertant.

### 331. bad declaration of destructor for ios_base::failure

Section: 27.5.3.1.1 [ios::failure] Status: CD1 Submitter: PremAnand M. Rao Opened: 2001-08-23 Last modified: 2015-04-08

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Discussion:

With the change in 17.6.5.12 [res.on.exception.handling] to state "An implementation may strengthen the exception-specification for a non-virtual function by removing listed exceptions." (issue 119) and the following declaration of ~failure() in ios_base::failure

        namespace std {
           class ios_base::failure : public exception {
           public:
               ...
               virtual ~failure();
               ...
           };
         }

the class failure cannot be implemented since in 18.7.1 [type.info] the destructor of class exception has an empty exception specification:

        namespace std {
           class exception {
           public:
             ...
             virtual ~exception() throw();
             ...
           };
         }

Proposed resolution:

Remove the declaration of ~failure().

Rationale:

The proposed resolution is consistent with the way that destructors of other classes derived from `exception` are handled.

### 333. does endl imply synchronization with the device?

Section: 27.7.3.8 [ostream.manip] Status: CD1 Submitter: PremAnand M. Rao Opened: 2001-08-27 Last modified: 2015-04-08

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Discussion:

A footnote in 27.7.3.8 [ostream.manip] states:

> [Footnote: The effect of executing cout << endl is to insert a newline character in the output sequence controlled by cout, then synchronize it with any external file with which it might be associated. --- end foonote]

Does the term "file" here refer to the external device? This leads to some implementation ambiguity on systems with fully buffered files where a newline does not cause a flush to the device.

Choosing to sync with the device leads to significant performance penalties for each call to endl, while not sync-ing leads to errors under special circumstances.

I could not find any other statement that explicitly defined the behavior one way or the other.

Proposed resolution:

Remove footnote 300 from section 27.7.3.8 [ostream.manip].

Rationale:

We already have normative text saying what `endl` does: it inserts a newline character and calls `flush`. This footnote is at best redundant, at worst (as this issue says) misleading, because it appears to make promises about what `flush` does.

### 334. map::operator[] specification forces inefficient implementation

Section: 23.4.4.3 [map.access] Status: CD1 Submitter: Andrea Griffini Opened: 2001-09-02 Last modified: 2015-04-08

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Discussion:

The current standard describes map::operator[] using a code example. That code example is however quite inefficient because it requires several useless copies of both the passed key_type value and of default constructed mapped_type instances. My opinion is that was not meant by the comitee to require all those temporary copies.

Currently map::operator[] behaviour is specified as:

      Returns:
        (*((insert(make_pair(x, T()))).first)).second.

This specification however uses make_pair that is a template function of which parameters in this case will be deduced being of type const key_type& and const T&. This will create a pair<key_type,T> that isn't the correct type expected by map::insert so another copy will be required using the template conversion constructor available in pair to build the required pair<const key_type,T> instance.

If we consider calling of key_type copy constructor and mapped_type default constructor and copy constructor as observable behaviour (as I think we should) then the standard is in this place requiring two copies of a key_type element plus a default construction and two copy construction of a mapped_type (supposing the addressed element is already present in the map; otherwise at least another copy construction for each type).

A simple (half) solution would be replacing the description with:

      Returns:
        (*((insert(value_type(x, T()))).first)).second.

This will remove the wrong typed pair construction that requires one extra copy of both key and value.

However still the using of map::insert requires temporary objects while the operation, from a logical point of view, doesn't require any.

I think that a better solution would be leaving free an implementer to use a different approach than map::insert that, because of its interface, forces default constructed temporaries and copies in this case. The best solution in my opinion would be just requiring map::operator[] to return a reference to the mapped_type part of the contained element creating a default element with the specified key if no such an element is already present in the container. Also a logarithmic complexity requirement should be specified for the operation.

This would allow library implementers to write alternative implementations not using map::insert and reaching optimal performance in both cases of the addressed element being present or absent from the map (no temporaries at all and just the creation of a new pair inside the container if the element isn't present). Some implementer has already taken this option but I think that the current wording of the standard rules that as non-conforming.

Proposed resolution:

Replace 23.4.4.3 [map.access] paragraph 1 with

> -1- Effects: If there is no key equivalent to x in the map, inserts value_type(x, T()) into the map.

> -2- Returns: A reference to the mapped_type corresponding to x in *this.

> -3- Complexity: logarithmic.

[This is the second option mentioned above. Howard provided wording. We may also wish to have a blanket statement somewhere in clause 17 saying that we do not intend the semantics of sample code fragments to be interpreted as specifing exactly how many copies are made. See issue 98 for a similar problem.]

Rationale:

This is the second solution described above; as noted, it is consistent with existing practice.

Note that we now need to specify the complexity explicitly, because we are no longer defining `operator[]` in terms of `insert`.

### 335. minor issue with char_traits, table 37

Section: 21.2.1 [char.traits.require] Status: CD1 Submitter: Andy Sawyer Opened: 2001-09-06 Last modified: 2015-04-08

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Discussion:

Table 37, in 21.2.1 [char.traits.require], descibes char_traits::assign as:

      X::assign(c,d)   assigns c = d.

And para 1 says:

> [...] c and d denote values of type CharT [...]

Naturally, if c and d are values, then the assignment is (effectively) meaningless. It's clearly intended that (in the case of assign, at least), 'c' is intended to be a reference type.

I did a quick survey of the four implementations I happened to have lying around, and sure enough they all have signatures:

        assign( charT&, const charT& );

(or the equivalent). It's also described this way in Nico's book. (Not to mention the synopses of char_traits<char> in 21.1.3.1 and char_traits<wchar_t> in 21.1.3.2...)

Proposed resolution:

Add the following to 21.1.1 para 1:

> r denotes an lvalue of CharT

and change the description of assign in the table to:

      X::assign(r,d)   assigns r = d

### 336. Clause 17 lack of references to deprecated headers

Section: 17 [library] Status: CD1 Submitter: Detlef Vollmann Opened: 2001-09-05 Last modified: 2015-04-08

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Discussion:

From c++std-edit-873:

17.6.1.2 [headers], Table 11. In this table, the header <strstream> is missing.

This shows a general problem: The whole clause 17 refers quite often to clauses 18 through 27, but D.7 is also a part of the standard library (though a deprecated one).

Proposed resolution:

To 17.6.1.2 [headers] Table 11, C++ Library Headers, add "<strstream>".

In the following places, change "clauses 17 through 27" to "clauses 17 through 27 and Annex D":

- 1.2 [intro.refs] Normative references/1/footnote 1
- 1.3 [intro.defs] Definitions/1
- 7 [dcl.dcl] Library introduction/9
- 17.5 [description] Method of description (Informative)/1
- 17.5.2.1.4 [character.seq] Character sequences/1/bullet 2
- 17.5.2.2 [functions.within.classes] Functions within classes/1
- 17.5.2.3 [objects.within.classes] Private members/1/(2 places)
- 17.6 [requirements] Library-wide requirements/1
- 17.6.1.2 [headers] Headers/4
- 17.6.4.6 [replacement.functions] Replacement functions/1
- 17.6.5.4 [global.functions] Global or non-member functions/2
- 17.6.5.10 [protection.within.classes] Protection within classes/1

### 337. replace_copy_if's template parameter should be InputIterator

Section: 25.3.5 [alg.replace] Status: CD1 Submitter: Detlef Vollmann Opened: 2001-09-07 Last modified: 2015-04-08

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Discussion:

From c++std-edit-876:

In section 25.3.5 [alg.replace] before p4: The name of the first parameter of template replace_copy_if should be "InputIterator" instead of "Iterator". According to 17.5.2.1 [type.descriptions] p1 the parameter name conveys real normative meaning.

Proposed resolution:

Change `Iterator` to `InputIterator`.

### 338. is whitespace allowed between `-' and a digit?

Section: 22.4 [locale.categories] Status: CD1 Submitter: Martin Sebor Opened: 2001-09-17 Last modified: 2015-04-08

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Discussion:

From Stage 2 processing in 22.4.2.1.2 [facet.num.get.virtuals], p8 and 9 (the original text or the text corrected by the proposed resolution of issue 221) it seems clear that no whitespace is allowed within a number, but 22.4.3.1 [locale.numpunct], p2, which gives the format for integer and floating point values, says that whitespace is optional between a plusminus and a sign.

The text needs to be clarified to either consistently allow or disallow whitespace between a plusminus and a sign. It might be worthwhile to consider the fact that the C library stdio facility does not permit whitespace embedded in numbers and neither does the C or C++ core language (the syntax of integer-literals is given in 2.13.2 [lex.icon], that of floating-point-literals in 2.13.4 [lex.fcon] of the C++ standard).

Proposed resolution:

Change the first part of 22.4.3.1 [locale.numpunct] paragraph 2 from:

> The syntax for number formats is as follows, where `digit` represents the radix set specified by the `fmtflags` argument value, `whitespace` is as determined by the facet `ctype<charT>` (22.2.1.1), and `thousands-sep` and `decimal-point` are the results of corresponding `numpunct<charT>` members. Integer values have the format:

>       integer   ::= [sign] units
>       sign      ::= plusminus [whitespace]
>       plusminus ::= '+' | '-'
>       units     ::= digits [thousands-sep units]
>       digits    ::= digit [digits]

to:

> The syntax for number formats is as follows, where `digit` represents the radix set specified by the `fmtflags` argument value, and `thousands-sep` and `decimal-point` are the results of corresponding `numpunct<charT>` members. Integer values have the format:

>       integer   ::= [sign] units
>       sign      ::= plusminus
>       plusminus ::= '+' | '-'
>       units     ::= digits [thousands-sep units]
>       digits    ::= digit [digits]

Rationale:

It's not clear whether the format described in 22.4.3.1 [locale.numpunct] paragraph 2 has any normative weight: nothing in the standard says how, or whether, it's used. However, there's no reason for it to differ gratuitously from the very specific description of numeric processing in 22.4.2.1.2 [facet.num.get.virtuals]. The proposed resolution removes all mention of "whitespace" from that format.

### 339. definition of bitmask type restricted to clause 27

Section: 22.4.1 [category.ctype], 17.5.2.1.3 [bitmask.types] Status: CD1 Submitter: Martin Sebor Opened: 2001-09-17 Last modified: 2015-04-08

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Discussion:

The ctype_category::mask type is declared to be an enum in 22.4.1 [category.ctype] with p1 then stating that it is a bitmask type, most likely referring to the definition of bitmask type in 17.5.2.1.3 [bitmask.types], p1. However, the said definition only applies to clause 27, making the reference in 22.2.1 somewhat dubious.

Proposed resolution:

Clarify 17.3.2.1.2, p1 by changing the current text from

> Several types defined in clause 27 are bitmask types. Each bitmask type can be implemented as an enumerated type that overloads certain operators, as an integer type, or as a bitset (20.6 [template.bitset]).

to read

> Several types defined in clauses lib.language.support through lib.input.output and Annex D are bitmask types. Each bitmask type can be implemented as an enumerated type that overloads certain operators, as an integer type, or as a bitset (lib.template.bitset).

Additionally, change the definition in 22.2.1 to adopt the same convention as in clause 27 by replacing the existing text with the following (note, in particluar, the cross-reference to 17.3.2.1.2 in 22.2.1, p1):

> 22.2.1 The ctype category [lib.category.ctype]

>     namespace std {
>         class ctype_base {
>         public:
>             typedef T mask;

>             // numeric values are for exposition only.
>             static const mask space = 1 << 0;
>             static const mask print = 1 << 1;
>             static const mask cntrl = 1 << 2;
>             static const mask upper = 1 << 3;
>             static const mask lower = 1 << 4;
>             static const mask alpha = 1 << 5;
>             static const mask digit = 1 << 6;
>             static const mask punct = 1 << 7;
>             static const mask xdigit = 1 << 8;
>             static const mask alnum = alpha | digit;
>             static const mask graph = alnum | punct;
>         };
>     }

> The type `mask` is a bitmask type (17.5.2.1.3 [bitmask.types]).

[Curaçao: The LWG notes that T above should be bold-italics to be consistent with the rest of the standard.]

### 340. interpretation of `has_facet<Facet>(loc)`

Section: 22.3.1.1.1 [locale.category] Status: CD1 Submitter: Martin Sebor Opened: 2001-09-18 Last modified: 2015-04-08

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Discussion:

It's unclear whether 22.1.1.1.1, p3 says that `has_facet<Facet>(loc)` returns true for any `Facet` from Table 51 or whether it includes Table 52 as well:

> For any locale `loc` either constructed, or returned by locale::classic(), and any facet `Facet` that is a member of a standard category, `has_facet<Facet>(loc)` is true. Each locale member function which takes a `locale::category` argument operates on the corresponding set of facets.

It seems that it comes down to which facets are considered to be members of a standard category. Intuitively, I would classify all the facets in Table 52 as members of their respective standard categories, but there are an unbounded set of them...

The paragraph implies that, for instance, `has_facet<num_put<C, OutputIterator> >(loc)` must always return true. I don't think that's possible. If it were, then `use_facet<num_put<C, OutputIterator> >(loc)` would have to return a reference to a distinct object for each valid specialization of `num_put<C, OutputIteratory>`, which is clearly impossible.

On the other hand, if none of the facets in Table 52 is a member of a standard category then none of the locale member functions that operate on entire categories of facets will work properly.

It seems that what p3 should mention that it's required (permitted?) to hold only for specializations of `Facet` from Table 52 on `C` from the set { `char`, `wchar_t` }, and `InputIterator` and `OutputIterator` from the set of { {i,o}`streambuf_iterator`<{`char`,`wchar_t`}`>` }.

Proposed resolution:

In 22.3.1.1.1 [locale.category], paragraph 3, change "that is a member of a standard category" to "shown in Table 51".

Rationale:

The facets in Table 52 are an unbounded set. Locales should not be required to contain an infinite number of facets.

It's not necessary to talk about which values of InputIterator and OutputIterator must be supported. Table 51 already contains a complete list of the ones we need.

### 341. Vector reallocation and swap

Section: 23.3.6.3 [vector.capacity] Status: CD1 Submitter: Anthony Williams Opened: 2001-09-27 Last modified: 2015-04-08

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Discussion:

It is a common idiom to reduce the capacity of a vector by swapping it with an empty one:

      std::vector<SomeType> vec;
      // fill vec with data
      std::vector<SomeType>().swap(vec);
      // vec is now empty, with minimal capacity

However, the wording of 23.3.6.3 [vector.capacity]paragraph 5 prevents the capacity of a vector being reduced, following a call to reserve(). This invalidates the idiom, as swap() is thus prevented from reducing the capacity. The proposed wording for issue 329 does not affect this. Consequently, the example above requires the temporary to be expanded to cater for the contents of vec, and the contents be copied across. This is a linear-time operation.

However, the container requirements state that swap must have constant complexity (23.2 [container.requirements] note to table 65).

This is an important issue, as reallocation affects the validity of references and iterators.

If the wording of 23.2.4.2p5 is taken to be the desired intent, then references and iterators remain valid after a call to swap, if they refer to an element before the new end() of the vector into which they originally pointed, in which case they refer to the element at the same index position. Iterators and references that referred to an element whose index position was beyond the new end of the vector are invalidated.

If the note to table 65 is taken as the desired intent, then there are two possibilities with regard to iterators and references:

1. All Iterators and references into both vectors are invalidated.
2. Iterators and references into either vector remain valid, and remain pointing to the same element. Consequently iterators and references that referred to one vector now refer to the other, and vice-versa.

Proposed resolution:

Add a new paragraph after 23.3.6.3 [vector.capacity] paragraph 5:

>       void swap(vector<T,Allocator>& x);

> Effects: Exchanges the contents and capacity() of `*this` with that of `x`.

> Complexity: Constant time.

[This solves the problem reported for this issue. We may also have a problem with a circular definition of swap() for other containers.]

Rationale:

swap should be constant time. The clear intent is that it should just do pointer twiddling, and that it should exchange all properties of the two vectors, including their reallocation guarantees.

### 343. Unspecified library header dependencies

Section: 17 [library] Status: Resolved Submitter: Martin Sebor Opened: 2001-10-09 Last modified: 2015-04-08

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Discussion:

The synopses of the C++ library headers clearly show which names are required to be defined in each header. Since in order to implement the classes and templates defined in these headers declarations of other templates (but not necessarily their definitions) are typically necessary the standard in 17.4.4, p1 permits library implementers to include any headers needed to implement the definitions in each header.

For instance, although it is not explicitly specified in the synopsis of `<string>`, at the point of definition of the `std::basic_string` template the declaration of the `std::allocator` template must be in scope. All current implementations simply include `<memory>` from within `<string>`, either directly or indirectly, to bring the declaration of `std::allocator` into scope.

Additionally, however, some implementation also include `<istream>` and `<ostream>` at the top of `<string>` to bring the declarations of `std::basic_istream` and `std::basic_ostream` into scope (which are needed in order to implement the string inserter and extractor operators (21.3.7.9 [lib.string.io])). Other implementations only include `<iosfwd>`, since strictly speaking, only the declarations and not the full definitions are necessary.

Obviously, it is possible to implement `<string>` without actually providing the full definitions of all the templates `std::basic_string` uses (`std::allocator`, `std::basic_istream`, and `std::basic_ostream`). Furthermore, not only is it possible, doing so is likely to have a positive effect on compile-time efficiency.

But while it may seem perfectly reasonable to expect a program that uses the `std::basic_string` insertion and extraction operators to also explicitly include `<istream>` or `<ostream>`, respectively, it doesn't seem reasonable to also expect it to explicitly include `<memory>`. Since what's reasonable and what isn't is highly subjective one would expect the standard to specify what can and what cannot be assumed. Unfortunately, that isn't the case.

The examples below demonstrate the issue.

Example 1:

It is not clear whether the following program is complete:

    #include <string>

    extern std::basic_ostream<char> &strm;

    int main () {
        strm << std::string ("Hello, World!\n");
    }

or whether one must explicitly include `<memory>` or `<ostream>` (or both) in addition to `<string>` in order for the program to compile.

Example 2:

Similarly, it is unclear whether the following program is complete:

    #include <istream>

    extern std::basic_iostream<char> &strm;

    int main () {
        strm << "Hello, World!\n";
    }

or whether one needs to explicitly include `<ostream>`, and perhaps even other headers containing the definitions of other required templates:

    #include <ios>
    #include <istream>
    #include <ostream>
    #include <streambuf>

    extern std::basic_iostream<char> &strm;

    int main () {
        strm << "Hello, World!\n";
    }

Example 3:

Likewise, it seems unclear whether the program below is complete:

    #include <iterator>

    bool foo (std::istream_iterator<int> a, std::istream_iterator<int> b)
    {
        return a == b;
    }

    int main () { }

or whether one should be required to include `<istream>`.

There are many more examples that demonstrate this lack of a requirement. I believe that in a good number of cases it would be unreasonable to require that a program explicitly include all the headers necessary for a particular template to be specialized, but I think that there are cases such as some of those above where it would be desirable to allow implementations to include only as much as necessary and not more.

[ post Bellevue: ]

> Position taken in prior reviews is that the idea of a table of header dependencies is a good one. Our view is that a full paper is needed to do justice to this, and we've made that recommendation to the issue author.

[ 2009-07 Frankfurt ]

> Resolved. Handled by LWG 1178.

Proposed resolution:

For every C++ library header, supply a minimum set of other C++ library headers that are required to be included by that header. The proposed list is below (C++ headers for C Library Facilities, table 12 in 17.4.1.2, p3, are omitted):

    +------------+--------------------+
    | C++ header |required to include |
    +============+====================+
    |<algorithm> |                    |
    +------------+--------------------+
    |<bitset>    |                    |
    +------------+--------------------+
    |<complex>   |                    |
    +------------+--------------------+
    |<deque>     |<memory>            |
    +------------+--------------------+
    |<exception> |                    |
    +------------+--------------------+
    |<fstream>   |<ios>               |
    +------------+--------------------+
    |<functional>|                    |
    +------------+--------------------+
    |<iomanip>   |<ios>               |
    +------------+--------------------+
    |<ios>       |<streambuf>         |
    +------------+--------------------+
    |<iosfwd>    |                    |
    +------------+--------------------+
    |<iostream>  |<istream>, <ostream>|
    +------------+--------------------+
    |<istream>   |<ios>               |
    +------------+--------------------+
    |<iterator>  |                    |
    +------------+--------------------+
    |<limits>    |                    |
    +------------+--------------------+
    |<list>      |<memory>            |
    +------------+--------------------+
    |<locale>    |                    |
    +------------+--------------------+
    |<map>       |<memory>            |
    +------------+--------------------+
    |<memory>    |                    |
    +------------+--------------------+
    |<new>       |<exception>         |
    +------------+--------------------+
    |<numeric>   |                    |
    +------------+--------------------+
    |<ostream>   |<ios>               |
    +------------+--------------------+
    |<queue>     |<deque>             |
    +------------+--------------------+
    |<set>       |<memory>            |
    +------------+--------------------+
    |<sstream>   |<ios>, <string>     |
    +------------+--------------------+
    |<stack>     |<deque>             |
    +------------+--------------------+
    |<stdexcept> |                    |
    +------------+--------------------+
    |<streambuf> |<ios>               |
    +------------+--------------------+
    |<string>    |<memory>            |
    +------------+--------------------+
    |<strstream> |                    |
    +------------+--------------------+
    |<typeinfo>  |<exception>         |
    +------------+--------------------+
    |<utility>   |                    |
    +------------+--------------------+
    |<valarray>  |                    |
    +------------+--------------------+
    |<vector>    |<memory>            |
    +------------+--------------------+

Rationale:

The portability problem is real. A program that works correctly on one implementation might fail on another, because of different header dependencies. This problem was understood before the standard was completed, and it was a conscious design choice.

One possible way to deal with this, as a library extension, would be an `<all>` header.

Hinnant: It's time we dealt with this issue for C++0X. Reopened.

### 345. type tm in <cwchar>

Section: 21.8 [c.strings] Status: CD1 Submitter: Clark Nelson Opened: 2001-10-19 Last modified: 2015-04-08

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Discussion:

C99, and presumably amendment 1 to C90, specify that <wchar.h> declares struct tm as an incomplete type. However, table 48 in 21.8 [c.strings] does not mention the type tm as being declared in <cwchar>. Is this omission intentional or accidental?

Proposed resolution:

In section 21.8 [c.strings], add "tm" to table 48.

### 346. Some iterator member functions should be const

Section: X [iterator.concepts] Status: CD1 Submitter: Jeremy Siek Opened: 2001-10-20 Last modified: 2015-04-08

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Discussion:

Iterator member functions and operators that do not change the state of the iterator should be defined as const member functions or as functions that take iterators either by const reference or by value. The standard does not explicitly state which functions should be const. Since this a fairly common mistake, the following changes are suggested to make this explicit.

The tables almost indicate constness properly through naming: r for non-const and a,b for const iterators. The following changes make this more explicit and also fix a couple problems.

Proposed resolution:

In X [iterator.concepts] Change the first section of p9 from "In the following sections, a and b denote values of X..." to "In the following sections, a and b denote values of type const X...".

In Table 73, change

        a->m   U&         ...

to

        a->m   const U&   ...
        r->m   U&         ...

In Table 73 expression column, change

        *a = t

to

        *r = t

[Redmond: The container requirements should be reviewed to see if the same problem appears there.]

### 347. locale::category and bitmask requirements

Section: 22.3.1.1.1 [locale.category] Status: CD1 Submitter: P.J. Plauger, Nathan Myers Opened: 2001-10-23 Last modified: 2015-04-08

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Discussion:

In 22.3.1.1.1 [locale.category] paragraph 1, the category members are described as bitmask elements. In fact, the bitmask requirements in 17.5.2.1.3 [bitmask.types] don't seem quite right: `none` and `all` are bitmask constants, not bitmask elements.

In particular, the requirements for `none` interact poorly with the requirement that the LC_* constants from the C library must be recognizable as C++ locale category constants. LC_* values should not be mixed with these values to make category values.

We have two options for the proposed resolution. Informally: option 1 removes the requirement that LC_* values be recognized as category arguments. Option 2 changes the category type so that this requirement is implementable, by allowing `none` to be some value such as 0x1000 instead of 0.

Nathan writes: "I believe my proposed resolution [Option 2] merely re-expresses the status quo more clearly, without introducing any changes beyond resolving the DR.

Proposed resolution:

Replace the first two paragraphs of 22.3.1.1 [locale.types] with:

>         typedef int category;

> Valid category values include the `locale` member bitmask elements `collate`, `ctype`, `monetary`, `numeric`, `time`, and `messages`, each of which represents a single locale category. In addition, `locale` member bitmask constant `none` is defined as zero and represents no category. And locale member bitmask constant `all` is defined such that the expression

>         (collate | ctype | monetary | numeric | time | messages | all) == all

> is `true`, and represents the union of all categories. Further the expression `(X | Y)`, where `X` and `Y` each represent a single category, represents the union of the two categories.

> `locale` member functions expecting a `category` argument require one of the `category` values defined above, or the union of two or more such values. Such a `category` argument identifies a set of locale categories. Each locale category, in turn, identifies a set of locale facets, including at least those shown in Table 51:

[Curaçao: need input from locale experts.]

Rationale:

The LWG considered, and rejected, an alternate proposal (described as "Option 2" in the discussion). The main reason for rejecting it was that library implementors were concerened about implementation difficult, given that getting a C++ library to work smoothly with a separately written C library is already a delicate business. Some library implementers were also concerned about the issue of adding extra locale categories.

> Option 2:
> Replace the first paragraph of 22.3.1.1 [locale.types] with:

> > Valid category values include the enumerated values. In addition, the result of applying commutative operators | and & to any two valid values is valid, and results in the setwise union and intersection, respectively, of the argument categories. The values `all` and `none` are defined such that for any valid value `cat`, the expressions `(cat | all == all)`, `(cat & all == cat)`, `(cat | none == cat)` and `(cat & none == none)` are true. For non-equal values `cat1` and `cat2` of the remaining enumerated values, `(cat1 & cat2 == none)` is true. For any valid categories `cat1` and `cat2`, the result of `(cat1 & ~cat2)` is valid, and equals the setwise union of those categories found in `cat1` but not found in `cat2`. [Footnote: it is not required that `all` equal the setwise union of the other enumerated values; implementations may add extra categories.]

### 349. Minor typographical error in ostream_iterator

Section: 24.6.2 [ostream.iterator] Status: CD1 Submitter: Andy Sawyer Opened: 2001-10-24 Last modified: 2015-04-08

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Discussion:

24.5.2 [lib.ostream.iterator] states:

        [...]

        private:
        // basic_ostream<charT,traits>* out_stream; exposition only
        // const char* delim; exposition only

Whilst it's clearly marked "exposition only", I suspect 'delim' should be of type 'const charT*'.

Proposed resolution:

In 24.6.2 [ostream.iterator], replace `const char* delim` with `const charT* delim`.

### 352. missing fpos requirements

Section: 21.2.2 [char.traits.typedefs] Status: CD1 Submitter: Martin Sebor Opened: 2001-12-02 Last modified: 2015-04-08

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Discussion:

(1) There are no requirements on the `stateT` template parameter of `fpos` listed in 27.4.3. The interface appears to require that the type be at least Assignable and CopyConstructible (27.4.3.1, p1), and I think also DefaultConstructible (to implement the operations in Table 88).

21.1.2, p3, however, only requires that `char_traits<charT>::state_type` meet the requirements of CopyConstructible types.

(2) Additionally, the `stateT` template argument has no corresponding typedef in fpos which might make it difficult to use in generic code.

Proposed resolution:

Modify 21.1.2, p4 from

Requires: `state_type` shall meet the requirements of CopyConstructible types (20.1.3).

Requires: state_type shall meet the requirements of Assignable (23.1, p4), CopyConstructible (20.1.3), and DefaultConstructible (20.1.4) types.

Rationale:

The LWG feels this is two issues, as indicated above. The first is a defect---std::basic_fstream is unimplementable without these additional requirements---and the proposed resolution fixes it. The second is questionable; who would use that typedef? The class template fpos is used only in a very few places, all of which know the state type already. Unless motivation is provided, the second should be considered NAD.

### 353. `std::pair` missing template assignment

Section: 20.3 [pairs] Status: Resolved Submitter: Martin Sebor Opened: 2001-12-02 Last modified: 2015-04-08

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Discussion:

The class template `std::pair` defines a template ctor (20.2.2, p4) but no template assignment operator. This may lead to inefficient code since assigning an object of `pair<C, D>` to `pair<A, B>` where the types `C` and `D` are distinct from but convertible to `A` and `B`, respectively, results in a call to the template copy ctor to construct an unnamed temporary of type `pair<A, B>` followed by an ordinary (perhaps implicitly defined) assignment operator, instead of just a straight assignment.

Proposed resolution:

Add the following declaration to the definition of `std::pair`:

        template<class U, class V>
        pair& operator=(const pair<U, V> &p);

And also add a paragraph describing the effects of the function template to the end of 20.2.2:

        template<class U, class V>
        pair& operator=(const pair<U, V> &p);

Effects: `first = p.first;` `second = p.second;` Returns: `*this`

[Curaçao: There is no indication this is was anything other than a design decision, and thus NAD. May be appropriate for a future standard.]

[ Pre Bellevue: It was recognized that this was taken care of by N1856, and thus moved from NAD Future to NAD Editorial Resolved. ]

### 354. Associative container lower/upper bound requirements

Section: 23.2.4 [associative.reqmts] Status: CD1 Submitter: Hans Aberg Opened: 2001-12-17 Last modified: 2015-04-08

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Discussion:

Discussions in the thread "Associative container lower/upper bound requirements" on comp.std.c++ suggests that there is a defect in the C++ standard, Table 69 of section 23.1.2, "Associative containers", [lib.associative.reqmts]. It currently says:

> a.find(k): returns an iterator pointing to an element with the key equivalent to k, or a.end() if such an element is not found.

> a.lower_bound(k): returns an iterator pointing to the first element with key not less than k.

> a.upper_bound(k): returns an iterator pointing to the first element with key greater than k.

We have "or a.end() if such an element is not found" for `find`, but not for `upper_bound` or `lower_bound`. As the text stands, one would be forced to insert a new element into the container and return an iterator to that in case the sought iterator does not exist, which does not seem to be the intention (and not possible with the "const" versions).

Proposed resolution:

Change Table 69 of section 23.2.4 [associative.reqmts] indicated entries to:

> a.lower_bound(k): returns an iterator pointing to the first element with key not less than k, or a.end() if such an element is not found.

> a.upper_bound(k): returns an iterator pointing to the first element with key greater than k, or a.end() if such an element is not found.

[Curaçao: LWG reviewed PR.]

### 355. Operational semantics for a.back()

Section: 23.2.3 [sequence.reqmts] Status: CD1 Submitter: Yaroslav Mironov Opened: 2002-01-23 Last modified: 2015-04-08

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Discussion:

Table 68 "Optional Sequence Operations" in 23.1.1/12 specifies operational semantics for "a.back()" as "*--a.end()", which may be ill-formed [because calling operator-- on a temporary (the return) of a built-in type is ill-formed], provided a.end() returns a simple pointer rvalue (this is almost always the case for std::vector::end(), for example). Thus, the specification is not only incorrect, it demonstrates a dangerous construct: "--a.end()" may successfully compile and run as intended, but after changing the type of the container or the mode of compilation it may produce compile-time error.

Proposed resolution:

Change the specification in table 68 "Optional Sequence Operations" in 23.1.1/12 for "a.back()" from

>     *--a.end()

to

>       { iterator tmp = a.end(); --tmp; return *tmp; }

and the specification for "a.pop_back()" from

>     a.erase(--a.end())

to

>       { iterator tmp = a.end(); --tmp; a.erase(tmp); }

[Curaçao: LWG changed PR from "{ X::iterator tmp = a.end(); return *--tmp; }" to "*a.rbegin()", and from "{ X::iterator tmp = a.end(); a.erase(--tmp); }" to "a.erase(rbegin())".]

[There is a second possible defect; table 68 "Optional Sequence Operations" in the "Operational Semantics" column uses operations present only in the "Reversible Container" requirements, yet there is no stated dependency between these separate requirements tables. Ask in Santa Cruz if the LWG would like a new issue opened.]

[Santa Cruz: the proposed resolution is even worse than what's in the current standard: erase is undefined for reverse iterator. If we're going to make the change, we need to define a temporary and use operator--. Additionally, we don't know how prevalent this is: do we need to make this change in more than one place? Martin has volunteered to review the standard and see if this problem occurs elsewhere.]

[Oxford: Matt provided new wording to address the concerns raised in Santa Cruz. It does not appear that this problem appears anywhere else in clauses 23 or 24.]

[Kona: In definition of operational semantics of back(), change "*tmp" to "return *tmp;"]

### 358. interpreting `thousands_sep` after a `decimal_point`

Section: 22.4.2.1.2 [facet.num.get.virtuals] Status: CD1 Submitter: Martin Sebor Opened: 2002-03-12 Last modified: 2015-04-08

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Discussion:

I don't think `thousands_sep` is being treated correctly after decimal_point has been seen. Since grouping applies only to the integral part of the number, the first such occurrence should, IMO, terminate Stage 2. (If it does not terminate it, then 22.2.2.1.2, p12 and 22.2.3.1.2, p3 need to explain how `thousands_sep` is to be interpreted in the fractional part of a number.)

The easiest change I can think of that resolves this issue would be something like below.

Proposed resolution:

Change the first sentence of 22.2.2.1.2, p9 from

> If discard is true then the position of the character is remembered, but the character is otherwise ignored. If it is not discarded, then a check is made to determine if c is allowed as the next character of an input field of the conversion specifier returned by stage 1. If so it is accumulated.

to

> If `discard` is true, then if `'.'` has not yet been accumulated, then the position of the character is remembered, but the character is otherwise ignored. Otherwise, if `'.'` has already been accumulated, the character is discarded and Stage 2 terminates. ...

Rationale:

We believe this reflects the intent of the Standard. Thousands sep characters after the decimal point are not useful in any locale. Some formatting conventions do group digits that follow the decimal point, but they usually introduce a different grouping character instead of reusing the thousand sep character. If we want to add support for such conventions, we need to do so explicitly.

### 359. num_put<>::do_put (..., bool) undocumented

Section: 22.4.2.2.1 [facet.num.put.members] Status: CD1 Submitter: Martin Sebor Opened: 2002-03-12 Last modified: 2015-04-08

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Discussion:

22.2.2.2.1, p1:

        iter_type put (iter_type out, ios_base& str, char_type fill,
                       bool val) const;
        ...

        1   Returns: do_put (out, str, fill, val).

AFAICS, the behavior of do_put (..., bool) is not documented anywhere, however, 22.2.2.2.2, p23:

>     iter_type put (iter_type out, ios_base& str, char_type fill,
>                    bool val) const;

> Effects: If (str.flags() & ios_base::boolalpha) == 0 then do out = do_put(out, str, fill, (int)val) Otherwise do

>                  string_type s =
>                      val ? use_facet<ctype<charT> >(loc).truename()
>                          : use_facet<ctype<charT> >(loc).falsename();

> and then insert the characters of s into out. out.

This means that the bool overload of `do_put()` will never be called, which contradicts the first paragraph. Perhaps the declaration should read `do_put()`, and not `put()`?

Note also that there is no Returns clause for this function, which should probably be corrected, just as should the second occurrence of "out." in the text.

I think the least invasive change to fix it would be something like the following:

Proposed resolution:

In 22.4.2.2.2 [facet.num.put.virtuals], just above paragraph 1, remove the `bool` overload.

In 22.4.2.2.2 [facet.num.put.virtuals], p23, make the following changes

> Replace `put()` with `do_put()` in the declaration of the member function.

> Change the Effects clause to a Returns clause (to avoid the requirement to call `do_put(..., int)` from `do_put (..., bool))` like so:

> 23 Returns: If `(str.flags() & ios_base::boolalpha) == 0` then `do_put (out, str, fill, (long)val)` Otherwise the function obtains a string `s` as if by

>                  string_type s =
>                     val ? use_facet<ctype<charT> >(loc).truename()
>                         : use_facet<ctype<charT> >(loc).falsename();

> and then inserts each character `c` of s into out via `*out++ = c` and returns `out`.

Rationale:

This fixes a couple of obvious typos, and also fixes what appears to be a requirement of gratuitous inefficiency.

### 360. locale mandates inefficient implementation

Section: 22.3.1 [locale] Status: CD1 Submitter: Martin Sebor Opened: 2002-03-12 Last modified: 2015-04-08

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Discussion:

22.1.1, p7 (copied below) allows iostream formatters and extractors to make assumptions about the values returned from facet members. However, such assumptions are apparently not guaranteed to hold in other cases (e.g., when the facet members are being called directly rather than as a result of iostream calls, or between successive calls to the same iostream functions with no interevening calls to `imbue()`, or even when the facet member functions are called from other member functions of other facets). This restriction prevents locale from being implemented efficiently.

Proposed resolution:

Change the first sentence in 22.1.1, p7 from

> In successive calls to a locale facet member function during a call to an iostream inserter or extractor or a streambuf member function, the returned result shall be identical. [Note: This implies that such results may safely be reused without calling the locale facet member function again, and that member functions of iostream classes cannot safely call `imbue()` themselves, except as specified elsewhere. --end note]

to

> In successive calls to a locale facet member function on a facet object installed in the same locale, the returned result shall be identical. ...

Rationale:

This change is reasonable becuase it clarifies the intent of this part of the standard.

### 362. bind1st/bind2nd type safety

Section: X [depr.lib.binders] Status: CD1 Submitter: Andrew Demkin Opened: 2002-04-26 Last modified: 2015-04-08

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Discussion:

The definition of bind1st() (X [depr.lib.binders]) can result in the construction of an unsafe binding between incompatible pointer types. For example, given a function whose first parameter type is 'pointer to T', it's possible without error to bind an argument of type 'pointer to U' when U does not derive from T:

       foo(T*, int);

       struct T {};
       struct U {};

       U u;

       int* p;
       int* q;

       for_each(p, q, bind1st(ptr_fun(foo), &u));    // unsafe binding

The definition of bind1st() includes a functional-style conversion to map its argument to the expected argument type of the bound function (see below):

      typename Operation::first_argument_type(x)

A functional-style conversion (X [depr.lib.binders]) is defined to be semantically equivalent to an explicit cast expression (X [depr.lib.binders]), which may (according to 5.4, paragraph 5) be interpreted as a reinterpret_cast, thus masking the error.

The problem and proposed change also apply to X [depr.lib.binders].

Proposed resolution:

Add this sentence to the end of X [depr.lib.binders]/1: "Binders `bind1st` and `bind2nd` are deprecated in favor of `std::tr1::bind`."

(Notes to editor: (1) when and if tr1::bind is incorporated into the standard, "std::tr1::bind" should be changed to "std::bind". (2) 20.5.6 should probably be moved to Annex D.

Rationale:

There is no point in fixing bind1st and bind2nd. tr1::bind is a superior solution. It solves this problem and others.

### 363. Missing exception specification in 27.4.2.1.1

Section: 27.5.3.1.1 [ios::failure] Status: CD1 Submitter: Walter Brown and Marc Paterno Opened: 2002-05-20 Last modified: 2015-04-08

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Discussion:

The destructor of ios_base::failure should have an empty throw specification, because the destructor of its base class, exception, is declared in this way.

Proposed resolution:

Change the destructor to

      virtual ~failure() throw();

Rationale:

Fixes an obvious glitch. This is almost editorial.

### 364. Inconsistent wording in 27.5.2.4.2

Section: 27.6.3.4.2 [streambuf.virt.buffer] Status: CD1 Submitter: Walter Brown, Marc Paterno Opened: 2002-05-10 Last modified: 2015-04-08

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Discussion:

27.6.3.4.2 [streambuf.virt.buffer] paragraph 1 is inconsistent with the Effects clause for seekoff.

Proposed resolution:

Make this paragraph, the Effects clause for setbuf, consistent in wording with the Effects clause for seekoff in paragraph 3 by amending paragraph 1 to indicate the purpose of setbuf:

Original text:

> 1 Effects: Performs an operation that is defined separately for each class derived from basic_streambuf in this clause (27.7.1.3, 27.8.1.4).

Proposed text:

> 1 Effects: Influences stream buffering in a way that is defined separately for each class derived from basic_streambuf in this clause (27.7.1.3, 27.8.1.4).

Rationale:

The LWG doesn't believe there is any normative difference between the existing wording and what's in the proposed resolution, but the change may make the intent clearer.

### 365. Lack of const-qualification in clause 27

Section: 27 [input.output] Status: CD1 Submitter: Walter Brown, Marc Paterno Opened: 2002-05-10 Last modified: 2015-04-08

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Discussion:

Some stream and streambuf member functions are declared non-const, even thought they appear only to report information rather than to change an object's logical state. They should be declared const. See document N1360 for details and rationale.

The list of member functions under discussion: `in_avail`, `showmanyc`, `tellg`, `tellp`, `is_open`.

Related issue: 73

Proposed resolution:

In 27.8.1.5, 27.8.1.7, 27.8.1.8, 27.8.1.10, 27.8.1.11, and 27.8.1.13

Replace

      bool is_open();

with

      bool is_open() const;

Rationale:

Of the changes proposed in N1360, the only one that is safe is changing the filestreams' is_open to const. The LWG believed that this was NAD the first time it considered this issue (issue 73), but now thinks otherwise. The corresponding streambuf member function, after all,is already const.

The other proposed changes are less safe, because some streambuf functions that appear merely to report a value do actually perform mutating operations. It's not even clear that they should be considered "logically const", because streambuf has two interfaces, a public one and a protected one. These functions may, and often do, change the state as exposed by the protected interface, even if the state exposed by the public interface is unchanged.

Note that implementers can make this change in a binary compatible way by providing both overloads; this would be a conforming extension.

### 369. io stream objects and static ctors

Section: 27.4 [iostream.objects] Status: CD1 Submitter: Ruslan Abdikeev Opened: 2002-07-08 Last modified: 2015-04-08

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Discussion:

Is it safe to use standard iostream objects from constructors of static objects? Are standard iostream objects constructed and are their associations established at that time?

Surpisingly enough, Standard does NOT require that.

27.3/2 [lib.iostream.objects] guarantees that standard iostream objects are constructed and their associations are established before the body of main() begins execution. It also refers to ios_base::Init class as the panacea for constructors of static objects.

However, there's nothing in 27.3 [lib.iostream.objects], in 27.4.2 [lib.ios.base], and in 27.4.2.1.6 [lib.ios::Init], that would require implementations to allow access to standard iostream objects from constructors of static objects.

Details:

Core text refers to some magic object ios_base::Init, which will be discussed below:

> "The [standard iostream] objects are constructed, and their associations are established at some time prior to or during first time an object of class basic_ios<charT,traits>::Init is constructed, and in any case before the body of main begins execution." (27.3/2 [lib.iostream.objects])

The first non-normative footnote encourages implementations to initialize standard iostream objects earlier than required.

However, the second non-normative footnote makes an explicit and unsupported claim:

> "Constructors and destructors for static objects can access these [standard iostream] objects to read input from stdin or write output to stdout or stderr." (27.3/2 footnote 265 [lib.iostream.objects])

The only bit of magic is related to that ios_base::Init class. AFAIK, the rationale behind ios_base::Init was to bring an instance of this class to each translation unit which #included <iostream> or related header. Such an inclusion would support the claim of footnote quoted above, because in order to use some standard iostream object it is necessary to #include <iostream>.

However, while Standard explicitly describes ios_base::Init as an appropriate class for doing the trick, I failed to found a mention of an _instance_ of ios_base::Init in Standard.

Proposed resolution:

Add to 27.4 [iostream.objects], p2, immediately before the last sentence of the paragraph, the following two sentences:

> If a translation unit includes <iostream>, or explicitly constructs an ios_base::Init object, these stream objects shall be constructed before dynamic initialization of non-local objects defined later in that translation unit, and these stream objects shall be destroyed after the destruction of dynamically initialized non-local objects defined later in that translation unit.

[Lillehammer: Matt provided wording.]

[Mont Tremblant: Matt provided revised wording.]

Rationale:

The original proposed resolution unconditionally required implementations to define an ios_base::Init object of some implementation-defined name in the header <iostream>. That's an overspecification. First, defining the object may be unnecessary and even detrimental to performance if an implementation can guarantee that the 8 standard iostream objects will be initialized before any other user-defined object in a program. Second, there is no need to require implementations to document the name of the object.

The new proposed resolution gives users guidance on what they need to do to ensure that stream objects are constructed during startup.

### 370. Minor error in basic_istream::get

Section: 27.7.2.3 [istream.unformatted] Status: CD1 Submitter: Ray Lischner Opened: 2002-07-15 Last modified: 2015-04-08

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Discussion:

Defect report for description of basic_istream::get (section 27.7.2.3 [istream.unformatted]), paragraph 15. The description for the get function with the following signature:

      basic_istream<charT,traits>& get(basic_streambuf<char_type,traits>&
      sb);

is incorrect. It reads

> Effects: Calls get(s,n,widen('\n'))

which I believe should be:

> Effects: Calls get(sb,widen('\n'))

Proposed resolution:

Change the Effects paragraph to:

> Effects: Calls get(sb,this->widen('\n'))

[Pre-Oxford: Minor correction from Howard: replaced 'widen' with 'this->widen'.]

Rationale:

Fixes an obvious typo.

### 371. Stability of multiset and multimap member functions

Section: 23.2 [container.requirements] Status: CD1 Submitter: Frank Compagner Opened: 2002-07-20 Last modified: 2015-04-08

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Discussion:

The requirements for multiset and multimap containers (23.1 [lib.containers.requirements], 23.1.2 [lib.associative.reqmnts], 23.3.2 [lib.multimap] and 23.3.4 [lib.multiset]) make no mention of the stability of the required (mutating) member functions. It appears the standard allows these functions to reorder equivalent elements of the container at will, yet the pervasive red-black tree implementation appears to provide stable behaviour.

This is of most concern when considering the behaviour of erase(). A stability requirement would guarantee the correct working of the following 'idiom' that removes elements based on a certain predicate function.

      multimap<int, int> m;
      multimap<int, int>::iterator i = m.begin();
      while (i != m.end()) {
          if (pred(i))
              m.erase (i++);
          else
              ++i;
      }

Although clause 23.1.2/8 guarantees that i remains a valid iterator througout this loop, absence of the stability requirement could potentially result in elements being skipped. This would make this code incorrect, and, furthermore, means that there is no way of erasing these elements without iterating first over the entire container, and second over the elements to be erased. This would be unfortunate, and have a negative impact on both performance and code simplicity.

If the stability requirement is intended, it should be made explicit (probably through an extra paragraph in clause 23.1.2).

If it turns out stability cannot be guaranteed, i'd argue that a remark or footnote is called for (also somewhere in clause 23.1.2) to warn against relying on stable behaviour (as demonstrated by the code above). If most implementations will display stable behaviour, any problems emerging on an implementation without stable behaviour will be hard to track down by users. This would also make the need for an erase_if() member function that much greater.

This issue is somewhat related to LWG issue 130.

Proposed resolution:

Add the following to the end of 23.2.4 [associative.reqmts] paragraph 4: "For `multiset` and `multimap`, `insert`and `erase` are stable: they preserve the relative ordering of equivalent elements.

[Lillehammer: Matt provided wording]

[Joe Gottman points out that the provided wording does not address multimap and multiset. N1780 also addresses this issue and suggests wording.]

[Mont Tremblant: Changed set and map to multiset and multimap.]

Rationale:

The LWG agrees that this guarantee is necessary for common user idioms to work, and that all existing implementations provide this property. Note that this resolution guarantees stability for multimap and multiset, not for all associative containers in general.

### 373. Are basic_istream and basic_ostream to use (exceptions()&badbit) != 0 ?

Section: 27.7.2.2.1 [istream.formatted.reqmts], 27.7.3.6.1 [ostream.formatted.reqmts] Status: CD1 Submitter: Keith Baker Opened: 2002-07-23 Last modified: 2015-04-08

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Discussion:

In 27.7.2.2.1 [istream.formatted.reqmts] and 27.7.3.6.1 [ostream.formatted.reqmts] (exception()&badbit) != 0 is used in testing for rethrow, yet exception() is the constructor to class std::exception in 18.7.1 [type.info] that has no return type. Should member function exceptions() found in 27.5.5 [ios] be used instead?

Proposed resolution:

In 27.7.2.2.1 [istream.formatted.reqmts] and 27.7.3.6.1 [ostream.formatted.reqmts], change "(exception()&badbit) != 0" to "(exceptions()&badbit) != 0".

Rationale:

Fixes an obvious typo.

### 375. basic_ios should be ios_base in 27.7.1.3

Section: 27.8.2.4 [stringbuf.virtuals] Status: CD1 Submitter: Ray Lischner Opened: 2002-08-14 Last modified: 2015-04-08

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Discussion:

In Section 27.8.2.4 [stringbuf.virtuals]: Table 90, Table 91, and paragraph 14 all contain references to "basic_ios::" which should be "ios_base::".

Proposed resolution:

Change all references to "basic_ios" in Table 90, Table 91, and paragraph 14 to "ios_base".

Rationale:

Fixes an obvious typo.

### 376. basic_streambuf semantics

Section: 27.8.2.4 [stringbuf.virtuals] Status: CD1 Submitter: Ray Lischner Opened: 2002-08-14 Last modified: 2015-04-08

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Discussion:

In Section 27.8.2.4 [stringbuf.virtuals], Table 90, the implication is that the four conditions should be mutually exclusive, but they are not. The first two cases, as written, are subcases of the third.

As written, it is unclear what should be the result if cases 1 and 2 are both true, but case 3 is false.

Proposed resolution:

Rewrite these conditions as:

> (which & (ios_base::in|ios_base::out)) == ios_base::in

> (which & (ios_base::in|ios_base::out)) == ios_base::out

> (which & (ios_base::in|ios_base::out)) == (ios_base::in|ios_base::out) and way == either ios_base::beg or ios_base::end

> Otherwise

Rationale:

It's clear what we wanted to say, we just failed to say it. This fixes it.

### 379. nonsensical ctype::do_widen() requirement

Section: 22.4.1.1.2 [locale.ctype.virtuals] Status: CD1 Submitter: Martin Sebor Opened: 2002-09-06 Last modified: 2015-04-08

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Discussion:

The last sentence in 22.2.1.1.2, p11 below doesn't seem to make sense.

      charT do_widen (char c) const;

      -11- Effects: Applies the simplest reasonable transformation from
           a char value or sequence of char values to the corresponding
           charT value or values. The only characters for which unique
           transformations are required are those in the basic source
           character set (2.2). For any named ctype category with a
           ctype<charT> facet ctw and valid ctype_base::mask value
           M (is(M, c) || !ctw.is(M, do_widen(c))) is true.

Shouldn't the last sentence instead read

           For any named ctype category with a ctype<char> facet ctc
           and valid ctype_base::mask value M
           (ctc.is(M, c) || !is(M, do_widen(c))) is true.

I.e., if the narrow character c is not a member of a class of characters then neither is the widened form of c. (To paraphrase footnote 224.)

Proposed resolution:

Replace the last sentence of 22.4.1.1.2 [locale.ctype.virtuals], p11 with the following text:

           For any named ctype category with a ctype<char> facet ctc
           and valid ctype_base::mask value M,
           (ctc.is(M, c) || !is(M, do_widen(c))) is true.

[Kona: Minor edit. Added a comma after the M for clarity.]

Rationale:

The LWG believes this is just a typo, and that this is the correct fix.

### 380. typos in codecvt tables 53 and 54

Section: 22.4.1.5 [locale.codecvt.byname] Status: CD1 Submitter: Martin Sebor Opened: 2002-09-06 Last modified: 2015-04-08

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Discussion:

Tables 53 and 54 in 22.4.1.5 [locale.codecvt.byname] are both titled "convert result values," when surely "do_in/do_out result values" must have been intended for Table 53 and "do_unshift result values" for Table 54.

Table 54, row 3 says that the meaning of partial is "more characters needed to be supplied to complete termination." The function is not supplied any characters, it is given a buffer which it fills with characters or, more precisely, destination elements (i.e., an escape sequence). So partial means that space for more than (to_limit - to) destination elements was needed to terminate a sequence given the value of state.

Proposed resolution:

Change the title of Table 53 to "do_in/do_out result values" and the title of Table 54 to "do_unshift result values."

Change the text in Table 54, row 3 (the partial row), under the heading Meaning, to "space for more than (to_limit - to) destination elements was needed to terminate a sequence given the value of state."

### 381. detection of invalid mbstate_t in codecvt

Section: 22.4.1.5 [locale.codecvt.byname] Status: CD1 Submitter: Martin Sebor Opened: 2002-09-06 Last modified: 2015-04-08

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Discussion:

All but one codecvt member functions that take a state_type argument list as one of their preconditions that the state_type argument have a valid value. However, according to 22.2.1.5.2, p6, codecvt::do_unshift() is the only codecvt member that is supposed to return error if the state_type object is invalid.

It seems to me that the treatment of state_type by all codecvt member functions should be the same and the current requirements should be changed. Since the detection of invalid state_type values may be difficult in general or computationally expensive in some specific cases, I propose the following:

Proposed resolution:

Add a new paragraph before 22.2.1.5.2, p5, and after the function declaration below

        result do_unshift(stateT& state,
        externT* to, externT* to_limit, externT*& to_next) const;

as follows:

        Requires: (to <= to_end) well defined and true; state initialized,
        if at the beginning of a sequence, or else equal to the result of
        converting the preceding characters in the sequence.

and change the text in Table 54, row 4, the error row, under the heading Meaning, from

        state has invalid value

to

        an unspecified error has occurred

Rationale:

The intent is that implementations should not be required to detect invalid state values; such a requirement appears nowhere else. An invalid state value is a precondition violation, i.e. undefined behavior. Implementations that do choose to detect invalid state values, or that choose to detect any other kind of error, may return error as an indication.

### 383. Bidirectional iterator assertion typo

Section: 24.2.6 [bidirectional.iterators] Status: CD1 Submitter: ysapir (submitted via comp.std.c++) Opened: 2002-10-17 Last modified: 2015-04-08

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Discussion:

Following a discussion on the boost list regarding end iterators and the possibility of performing operator--() on them, it seems to me that there is a typo in the standard. This typo has nothing to do with that discussion.

I have checked this newsgroup, as well as attempted a search of the Active/Defect/Closed Issues List on the site for the words "s is derefer" so I believe this has not been proposed before. Furthermore, the "Lists by Index" mentions only DR 299 on section 24.1.4, and DR 299 is not related to this issue.

The standard makes the following assertion on bidirectional iterators, in section 24.1.4 [lib.bidirectional.iterators], Table 75:

                             operational  assertion/note
    expression  return type   semantics    pre/post-condition

    --r          X&                        pre: there exists s such
                                           that r == ++s.
                                           post: s is dereferenceable.
                                           --(++r) == r.
                                           --r == --s implies r == s.
                                           &r == &--r.

(See http://lists.boost.org/Archives/boost/2002/10/37636.php.)

In particular, "s is dereferenceable" seems to be in error. It seems that the intention was to say "r is dereferenceable".

If it were to say "r is dereferenceable" it would make perfect sense. Since s must be dereferenceable prior to operator++, then the natural result of operator-- (to undo operator++) would be to make r dereferenceable. Furthermore, without other assertions, and basing only on precondition and postconditions, we could not otherwise know this. So it is also interesting information.

Proposed resolution:

Change the guarantee to "postcondition: r is dereferenceable."

Rationale:

Fixes an obvious typo

### 384. equal_range has unimplementable runtime complexity

Section: 25.4.3.3 [equal.range] Status: CD1 Submitter: Hans Bos Opened: 2002-10-18 Last modified: 2015-04-08

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Discussion:

Section 25.4.3.3 [equal.range] states that at most 2 * log(last - first) + 1 comparisons are allowed for equal_range.

It is not possible to implement equal_range with these constraints.

In a range of one element as in:

        int x = 1;
        equal_range(&x, &x + 1, 1)

it is easy to see that at least 2 comparison operations are needed.

For this case at most 2 * log(1) + 1 = 1 comparison is allowed.

I have checked a few libraries and they all use the same (nonconforming) algorithm for equal_range that has a complexity of

         2* log(distance(first, last)) + 2.

I guess this is the algorithm that the standard assumes for equal_range.

It is easy to see that 2 * log(distance) + 2 comparisons are enough since equal range can be implemented with lower_bound and upper_bound (both log(distance) + 1).

I think it is better to require something like 2log(distance) + O(1) (or even logarithmic as multiset::equal_range). Then an implementation has more room to optimize for certain cases (e.g. have log(distance) characteristics when at most match is found in the range but 2log(distance) + 4 for the worst case).

Proposed resolution:

In 25.4.3.1 [lower.bound]/4, change `log(last - first) + 1` to `log2(last - first) + O(1)`.

In 25.4.3.2 [upper.bound]/4, change `log(last - first) + 1` to `log2(last - first) + O(1)`.

In 25.4.3.3 [equal.range]/4, change `2*log(last - first) + 1` to `2*log2(last - first) + O(1)`.

[Matt provided wording]

Rationale:

The LWG considered just saying O(log n) for all three, but decided that threw away too much valuable information. The fact that lower_bound is twice as fast as equal_range is important. However, it's better to allow an arbitrary additive constant than to specify an exact count. An exact count would have to involve `floor` or `ceil`. It would be too easy to get this wrong, and don't provide any substantial value for users.

### 386. Reverse iterator's operator[] has impossible return type

Section: 24.5.1.3.11 [reverse.iter.op-=] Status: CD1 Submitter: Matt Austern Opened: 2002-10-23 Last modified: 2015-04-08

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Discussion:

In 24.5.1.3.11 [reverse.iter.op-=], `reverse_iterator<>::operator[]` is specified as having a return type of `reverse_iterator::reference`, which is the same as `iterator_traits<Iterator>::reference`. (Where `Iterator` is the underlying iterator type.)

The trouble is that `Iterator`'s own operator[] doesn't necessarily have a return type of `iterator_traits<Iterator>::reference`. Its return type is merely required to be convertible to `Iterator`'s value type. The return type specified for reverse_iterator's operator[] would thus appear to be impossible.

With the resolution of issue 299, the type of `a[n]` will continue to be required (for random access iterators) to be convertible to the value type, and also `a[n] = t` will be a valid expression. Implementations of `reverse_iterator` will likely need to return a proxy from `operator[]` to meet these requirements. As mentioned in the comment from Dave Abrahams, the simplest way to specify that `reverse_iterator` meet this requirement to just mandate it and leave the return type of `operator[]` unspecified.

Proposed resolution:

In 24.5.1.2 [reverse.iter.requirements] change:

>     reference operator[](difference_type n) const;

to:

>     unspecified operator[](difference_type n) const; // see 24.2.7 [random.access.iterators]

[ Comments from Dave Abrahams: IMO we should resolve 386 by just saying that the return type of reverse_iterator's operator[] is unspecified, allowing the random access iterator requirements to impose an appropriate return type. If we accept 299's proposed resolution (and I think we should), the return type will be readable and writable, which is about as good as we can do. ]

### 387. `std::complex` over-encapsulated

Section: 26.4 [complex.numbers] Status: CD1 Submitter: Gabriel Dos Reis Opened: 2002-11-08 Last modified: 2015-04-08

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Discussion:

The absence of explicit description of `std::complex<T>` layout makes it imposible to reuse existing software developed in traditional languages like Fortran or C with unambigous and commonly accepted layout assumptions. There ought to be a way for practitioners to predict with confidence the layout of `std::complex<T>` whenever `T` is a numerical datatype. The absence of ways to access individual parts of a `std::complex<T>` object as lvalues unduly promotes severe pessimizations. For example, the only way to change, independently, the real and imaginary parts is to write something like

    complex<T> z;
    // ...
    // set the real part to r
    z = complex<T>(r, z.imag());
    // ...
    // set the imaginary part to i
    z = complex<T>(z.real(), i);

At this point, it seems appropriate to recall that a complex number is, in effect, just a pair of numbers with no particular invariant to maintain. Existing practice in numerical computations has it that a complex number datatype is usually represented by Cartesian coordinates. Therefore the over-encapsulation put in the specification of `std::complex<>` is not justified.

Proposed resolution:

Add the following requirements to 26.4 [complex.numbers] as 26.3/4:

> If `z` is an lvalue expression of type cv `std::complex<T>` then

> - the expression `reinterpret_cast<cv T(&)[2]>(z)` is well-formed; and
> - `reinterpret_cast<cv T(&)[2]>(z)[0]` designates the real part of `z`; and
> - `reinterpret_cast<cv T(&)[2]>(z)[1]` designates the imaginary part of `z`.

> Moreover, if `a` is an expression of pointer type cv `complex<T>*` and the expression `a[i]` is well-defined for an integer expression `i` then:

> - `reinterpret_cast<cv T*>(a)[2*i]` designates the real part of `a[i]`; and
> - `reinterpret_cast<cv T*>(a)[2*i+1]` designates the imaginary part of `a[i]`.

In 26.4.2 [complex] and 26.4.3 [complex.special] add the following member functions (changing `T` to concrete types as appropriate for the specializations).

>     void real(T);
>     void imag(T);

Add to 26.4.4 [complex.members]

>     T real() const;

> > Returns: the value of the real component

>     void real(T val);

> > Assigns `val` to the real component.

>     T imag() const;

> > Returns: the value of the imaginary component

>     void imag(T val);

> > Assigns `val` to the imaginary component.

[Kona: The layout guarantee is absolutely necessary for C compatibility. However, there was disagreement about the other part of this proposal: retrieving elements of the complex number as lvalues. An alternative: continue to have `real()` and `imag()` return rvalues, but add `set_real()` and `set_imag()`. Straw poll: return lvalues - 2, add setter functions - 5. Related issue: do we want reinterpret_cast as the interface for converting a complex to an array of two reals, or do we want to provide a more explicit way of doing it? Howard will try to resolve this issue for the next meeting.]

[pre-Sydney: Howard summarized the options in n1589.]

[ Bellevue: ]

> Second half of proposed wording replaced and moved to Ready.

[ Pre-Sophia Antipolis, Howard adds: ]

> Added the members to 26.4.3 [complex.special] and changed from Ready to Review.

[ Post-Sophia Antipolis: ]

> Moved from WP back to Ready so that the "and 26.4.3 [complex.special]" in the proposed resolution can be officially applied.

Rationale:

The LWG believes that C99 compatibility would be enough justification for this change even without other considerations. All existing implementations already have the layout proposed here.

### 389. Const overload of valarray::operator[] returns by value

Section: 26.6.2.4 [valarray.access] Status: CD1 Submitter: Gabriel Dos Reis Opened: 2002-11-08 Last modified: 2015-04-08

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Duplicate of: 77

Discussion:

Consider the following program:

        #include <iostream>
        #include <ostream>
        #include <vector>
        #include <valarray>
        #include <algorithm>
        #include <iterator>
        template<typename Array>
        void print(const Array& a)
        {
        using namespace std;
        typedef typename Array::value_type T;
        copy(&a[0], &a[0] + a.size(),
        ostream_iterator<T>(std::cout, " "));
        }
        template<typename T, unsigned N>
        unsigned size(T(&)[N]) { return N; }
        int main()
        {
        double array[] = { 0.89, 9.3, 7, 6.23 };
        std::vector<double> v(array, array + size(array));
        std::valarray<double> w(array, size(array));
        print(v); // #1
        std::cout << std::endl;
        print(w); // #2
        std::cout << std::endl;
        }

While the call numbered #1 succeeds, the call numbered #2 fails because the const version of the member function valarray<T>::operator[](size_t) returns a value instead of a const-reference. That seems to be so for no apparent reason, no benefit. Not only does that defeats users' expectation but it also does hinder existing software (written either in C or Fortran) integration within programs written in C++. There is no reason why subscripting an expression of type valarray<T> that is const-qualified should not return a const T&.

Proposed resolution:

In the class synopsis in 26.6.2 [template.valarray], and in 26.6.2.4 [valarray.access] just above paragraph 1, change

      T operator[](size_t const);

to

      const T& operator[](size_t const);

[Kona: fixed a minor typo: put semicolon at the end of the line wehre it belongs.]

Rationale:

Return by value seems to serve no purpose. Valaray was explicitly designed to have a specified layout so that it could easily be integrated with libraries in other languages, and return by value defeats that purpose. It is believed that this change will have no impact on allowable optimizations.

### 391. non-member functions specified as const

Section: 22.3.3.2 [conversions] Status: CD1 Submitter: James Kanze Opened: 2002-12-10 Last modified: 2015-04-08

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Discussion:

The specifications of toupper and tolower both specify the functions as const, althought they are not member functions, and are not specified as const in the header file synopsis in section 22.3 [locales].

Proposed resolution:

In 22.3.3.2 [conversions], remove `const` from the function declarations of std::toupper and std::tolower

Rationale:

Fixes an obvious typo

### 395. inconsistencies in the definitions of rand() and random_shuffle()

Section: 26.8 [c.math] Status: CD1 Submitter: James Kanze Opened: 2003-01-03 Last modified: 2015-04-08

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Discussion:

In 26.8 [c.math], the C++ standard refers to the C standard for the definition of rand(); in the C standard, it is written that "The implementation shall behave as if no library function calls the rand function."

In 25.3.12 [alg.random.shuffle], there is no specification as to how the two parameter version of the function generates its random value. I believe that all current implementations in fact call rand() (in contradiction with the requirement avove); if an implementation does not call rand(), there is the question of how whatever random generator it does use is seeded. Something is missing.

Proposed resolution:

In [lib.c.math], add a paragraph specifying that the C definition of rand shal be modified to say that "Unless otherwise specified, the implementation shall behave as if no library function calls the rand function."

In [lib.alg.random.shuffle], add a sentence to the effect that "In the two argument form of the function, the underlying source of random numbers is implementation defined. [Note: in particular, an implementation is permitted to use `rand`.]

Rationale:

The original proposed resolution proposed requiring the two-argument from of `random_shuffle` to use `rand`. We don't want to do that, because some existing implementations already use something else: gcc uses `lrand48`, for example. Using `rand` presents a problem if the number of elements in the sequence is greater than RAND_MAX.

### 396. what are characters zero and one

Section: 20.6.1 [bitset.cons] Status: CD1 Submitter: Martin Sebor Opened: 2003-01-05 Last modified: 2015-04-08

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Discussion:

23.3.5.1, p6 [lib.bitset.cons] talks about a generic character having the value of 0 or 1 but there is no definition of what that means for charT other than char and wchar_t. And even for those two types, the values 0 and 1 are not actually what is intended -- the values '0' and '1' are. This, along with the converse problem in the description of to_string() in 23.3.5.2, p33, looks like a defect remotely related to DR 303.

http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#303

    23.3.5.1:
      -6-  An element of the constructed string has value zero if the
           corresponding character in str, beginning at position pos,
           is 0. Otherwise, the element has the value one.

    23.3.5.2:
      -33-  Effects: Constructs a string object of the appropriate
            type and initializes it to a string of length N characters.
            Each character is determined by the value of its
            corresponding bit position in *this. Character position N
            ?- 1 corresponds to bit position zero. Subsequent decreasing
            character positions correspond to increasing bit positions.
            Bit value zero becomes the character 0, bit value one becomes
            the character 1.

Also note the typo in 23.3.5.1, p6: the object under construction is a bitset, not a string.

[ Sophia Antipolis: ]

> We note that `bitset` has been moved from section 23 to section 20, by another issue (842) previously resolved at this meeting.

> Disposition: move to ready.

> We request that Howard submit a separate issue regarding the three `to_string` overloads.

Proposed resolution:

Change the constructor's function declaration immediately before 20.6.1 [bitset.cons] p3 to:

        template <class charT, class traits, class Allocator>
        explicit
        bitset(const basic_string<charT, traits, Allocator>& str,
               typename basic_string<charT, traits, Allocator>::size_type pos = 0,
               typename basic_string<charT, traits, Allocator>::size_type n =
                 basic_string<charT, traits, Allocator>::npos,
               charT zero = charT('0'), charT one = charT('1'))

Change the first two sentences of 20.6.1 [bitset.cons] p6 to: "An element of the constructed string has value 0 if the corresponding character in str, beginning at position pos, is zero. Otherwise, the element has the value 1.

Change the text of the second sentence in 23.3.5.1, p5 to read: "The function then throws invalid_argument if any of the rlen characters in str beginning at position pos is other than zero or one. The function uses traits::eq() to compare the character values."

Change the declaration of the `to_string` member function immediately before 20.6.2 [bitset.members] p33 to:

        template <class charT, class traits, class Allocator>
        basic_string<charT, traits, Allocator>
        to_string(charT zero = charT('0'), charT one = charT('1')) const;

Change the last sentence of 20.6.2 [bitset.members] p33 to: "Bit value 0 becomes the character `zero`, bit value 1 becomes the character `one`.

Change 20.6.4 [bitset.operators] p8 to:

Returns:

      os << x.template to_string<charT,traits,allocator<charT> >(
          use_facet<ctype<charT> >(os.getloc()).widen('0'),
          use_facet<ctype<charT> >(os.getloc()).widen('1'));

Rationale:

There is a real problem here: we need the character values of '0' and '1', and we have no way to get them since strings don't have imbued locales. In principle the "right" solution would be to provide an extra object, either a ctype facet or a full locale, which would be used to widen '0' and '1'. However, there was some discomfort about using such a heavyweight mechanism. The proposed resolution allows those users who care about this issue to get it right.

We fix the inserter to use the new arguments. Note that we already fixed the analogous problem with the extractor in issue 303.

[ post Bellevue: ]

> We are happy with the resolution as proposed, and we move this to Ready.

[ Howard adds: ]

> The proposed wording neglects the 3 newer `to_string` overloads.

### 400. redundant type cast in lib.allocator.members

Section: 20.7.9.1 [allocator.members] Status: CD1 Submitter: Markus Mauhart Opened: 2003-02-27 Last modified: 2015-04-08

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Discussion:

20.7.9.1 [allocator.members] allocator members, contains the following 3 lines:

      12 Returns: new((void *) p) T( val)
         void destroy(pointer p);
      13 Returns: ((T*) p)->~T()

The type cast "(T*) p" in the last line is redundant cause we know that std::allocator<T>::pointer is a typedef for T*.

Proposed resolution:

Replace "((T*) p)" with "p".

Rationale:

Just a typo, this is really editorial.

### 401. incorrect type casts in table 32 in lib.allocator.requirements

Section: 17.6.3.5 [allocator.requirements] Status: CD1 Submitter: Markus Mauhart Opened: 2003-02-27 Last modified: 2015-04-08

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Discussion:

I think that in par2 of [default.con.req] the last two lines of table 32 contain two incorrect type casts. The lines are ...

      a.construct(p,t)   Effect: new((void*)p) T(t)
      a.destroy(p)       Effect: ((T*)p)?->~T()

.... with the prerequisits coming from the preceding two paragraphs, especially from table 31:

      alloc<T>             a     ;// an allocator for T
      alloc<T>::pointer    p     ;// random access iterator
                                  // (may be different from T*)
      alloc<T>::reference  r = *p;// T&
      T const&             t     ;

For that two type casts ("(void*)p" and "(T*)p") to be well-formed this would require then conversions to T* and void* for all alloc<T>::pointer, so it would implicitely introduce extra requirements for alloc<T>::pointer, additionally to the only current requirement (being a random access iterator).

Proposed resolution:

Accept proposed wording from N2436 part 1.

Note: Actually I would prefer to replace "((T*)p)?->dtor_name" with "p?->dtor_name", but AFAICS this is not possible cause of an omission in 13.5.6 [over.ref] (for which I have filed another DR on 29.11.2002).

[Kona: The LWG thinks this is somewhere on the border between Open and NAD. The intend is clear: `construct` constructs an object at the location p. It's reading too much into the description to think that literally calling `new` is required. Tweaking this description is low priority until we can do a thorough review of allocators, and, in particular, allocators with non-default pointer types.]

[ Batavia: Proposed resolution changed to less code and more description. ]

[ post Oxford: This would be rendered NAD Editorial by acceptance of N2257. ]

[ Kona (2007): The LWG adopted the proposed resolution of N2387 for this issue which was subsequently split out into a separate paper N2436 for the purposes of voting. The resolution in N2436 addresses this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 402. wrong new expression in [some_]allocator::construct

Section: 17.6.3.5 [allocator.requirements], 20.7.9.1 [allocator.members] Status: CD1 Submitter: Markus Mauhart Opened: 2003-02-27 Last modified: 2015-04-08

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Discussion:

This applies to the new expression that is contained in both par12 of 20.7.9.1 [allocator.members] and in par2 (table 32) of [default.con.req]. I think this new expression is wrong, involving unintended side effects.

20.7.9.1 [allocator.members] contains the following 3 lines:

      11 Returns: the largest value N for which the call allocate(N,0) might succeed.
         void construct(pointer p, const_reference val);
      12 Returns: new((void *) p) T( val)

[default.con.req] in table 32 has the following line:

      a.construct(p,t)   Effect: new((void*)p) T(t)

.... with the prerequisits coming from the preceding two paragraphs, especially from table 31:

      alloc<T>             a     ;// an allocator for T
      alloc<T>::pointer    p     ;// random access iterator
                                  // (may be different from T*)
      alloc<T>::reference  r = *p;// T&
      T const&             t     ;

Cause of using "new" but not "::new", any existing "T::operator new" function will hide the global placement new function. When there is no "T::operator new" with adequate signature, every_alloc<T>::construct(..) is ill-formed, and most std::container<T,every_alloc<T>> use it; a workaround would be adding placement new and delete functions with adequate signature and semantic to class T, but class T might come from another party. Maybe even worse is the case when T has placement new and delete functions with adequate signature but with "unknown" semantic: I dont like to speculate about it, but whoever implements any_container<T,any_alloc> and wants to use construct(..) probably must think about it.

Proposed resolution:

Replace "new" with "::new" in both cases.

### 403. basic_string::swap should not throw exceptions

Section: 21.4.6.8 [string::swap] Status: CD1 Submitter: Beman Dawes Opened: 2003-03-25 Last modified: 2015-04-08

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Discussion:

std::basic_string, 21.4 [basic.string] paragraph 2 says that basic_string "conforms to the requirements of a Sequence, as specified in (23.1.1)." The sequence requirements specified in (23.1.1) to not include any prohibition on swap members throwing exceptions.

Section 23.2 [container.requirements] paragraph 10 does limit conditions under which exceptions may be thrown, but applies only to "all container types defined in this clause" and so excludes basic_string::swap because it is defined elsewhere.

Eric Niebler points out that 21.4 [basic.string] paragraph 5 explicitly permits basic_string::swap to invalidates iterators, which is disallowed by 23.2 [container.requirements] paragraph 10. Thus the standard would be contradictory if it were read or extended to read as having basic_string meet 23.2 [container.requirements] paragraph 10 requirements.

Yet several LWG members have expressed the belief that the original intent was that basic_string::swap should not throw exceptions as specified by 23.2 [container.requirements] paragraph 10, and that the standard is unclear on this issue. The complexity of basic_string::swap is specified as "constant time", indicating the intent was to avoid copying (which could cause a bad_alloc or other exception). An important use of swap is to ensure that exceptions are not thrown in exception-safe code.

Note: There remains long standing concern over whether or not it is possible to reasonably meet the 23.2 [container.requirements] paragraph 10 swap requirements when allocators are unequal. The specification of basic_string::swap exception requirements is in no way intended to address, prejudice, or otherwise impact that concern.

Proposed resolution:

In 21.4.6.8 [string::swap], add a throws clause:

Throws: Shall not throw exceptions.

### 404. May a replacement allocation function be declared inline?

Section: 17.6.4.6 [replacement.functions], 18.6.1 [new.delete] Status: CD1 Submitter: Matt Austern Opened: 2003-04-24 Last modified: 2015-04-08

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Discussion:

The eight basic dynamic memory allocation functions (single-object and array versions of ::operator new and ::operator delete, in the ordinary and nothrow forms) are replaceable. A C++ program may provide an alternative definition for any of them, which will be used in preference to the implementation's definition.

Three different parts of the standard mention requirements on replacement functions: 17.6.4.6 [replacement.functions], 18.6.1.1 [new.delete.single] and 18.6.1.2 [new.delete.array], and 3.7.3 [basic.stc.auto].

None of these three places say whether a replacement function may be declared inline. 18.6.1.1 [new.delete.single] paragraph 2 specifies a signature for the replacement function, but that's not enough: the `inline` specifier is not part of a function's signature. One might also reason from 7.1.2 [dcl.fct.spec] paragraph 2, which requires that "an inline function shall be defined in every translation unit in which it is used," but this may not be quite specific enough either. We should either explicitly allow or explicitly forbid inline replacement memory allocation functions.

Proposed resolution:

Add a new sentence to the end of 17.6.4.6 [replacement.functions] paragraph 3: "The program's definitions shall not be specified as `inline`. No diagnostic is required."

[Kona: added "no diagnostic is required"]

Rationale:

The fact that `inline` isn't mentioned appears to have been nothing more than an oversight. Existing implementations do not permit inline functions as replacement memory allocation functions. Providing this functionality would be difficult in some cases, and is believed to be of limited value.

### 405. qsort and POD

Section: 25.5 [alg.c.library] Status: CD1 Submitter: Ray Lischner Opened: 2003-04-08 Last modified: 2015-04-08

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Discussion:

Section 25.5 [alg.c.library] describes bsearch and qsort, from the C standard library. Paragraph 4 does not list any restrictions on qsort, but it should limit the base parameter to point to POD. Presumably, qsort sorts the array by copying bytes, which requires POD.

Proposed resolution:

In 25.5 [alg.c.library] paragraph 4, just after the declarations and before the nonnormative note, add these words: "both of which have the same behavior as the original declaration. The behavior is undefined unless the objects in the array pointed to by base are of POD type."

[Something along these lines is clearly necessary. Matt provided wording.]

### 406. vector::insert(s) exception safety

Section: 23.3.6.5 [vector.modifiers] Status: CD1 Submitter: Dave Abrahams Opened: 2003-04-27 Last modified: 2015-04-08

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Discussion:

There is a possible defect in the standard: the standard text was never intended to prevent arbitrary ForwardIterators, whose operations may throw exceptions, from being passed, and it also wasn't intended to require a temporary buffer in the case where ForwardIterators were passed (and I think most implementations don't use one). As is, the standard appears to impose requirements that aren't met by any existing implementation.

Proposed resolution:

Replace 23.3.6.5 [vector.modifiers] paragraph 1 with:

> 1- Notes: Causes reallocation if the new size is greater than the old capacity. If no reallocation happens, all the iterators and references before the insertion point remain valid. If an exception is thrown other than by the copy constructor or assignment operator of T or by any InputIterator operation there are no effects.

[We probably need to say something similar for deque.]

### 407. Can singular iterators be destroyed?

Section: X [iterator.concepts] Status: CD1 Submitter: Nathan Myers Opened: 2003-06-03 Last modified: 2015-04-08

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Discussion:

Clause X [iterator.concepts], paragraph 5, says that the only expression that is defined for a singular iterator is "an assignment of a non-singular value to an iterator that holds a singular value". This means that destroying a singular iterator (e.g. letting an automatic variable go out of scope) is technically undefined behavior. This seems overly strict, and probably unintentional.

Proposed resolution:

Change the sentence in question to "... the only exceptions are destroying an iterator that holds a singular value, or the assignment of a non-singular value to an iterator that holds a singular value."

### 409. Closing an fstream should clear error state

Section: 27.9.1.9 [ifstream.members], 27.9.1.13 [ofstream.members] Status: CD1 Submitter: Nathan Myers Opened: 2003-06-03 Last modified: 2015-04-08

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Discussion:

A strict reading of 27.9.1 [fstreams] shows that opening or closing a basic_[io]fstream does not affect the error bits. This means, for example, that if you read through a file up to EOF, and then close the stream and reopen it at the beginning of the file, the EOF bit in the stream's error state is still set. This is counterintuitive.

The LWG considered this issue once before, as issue 22, and put in a footnote to clarify that the strict reading was indeed correct. We did that because we believed the standard was unambiguous and consistent, and that we should not make architectural changes in a TC. Now that we're working on a new revision of the language, those considerations no longer apply.

Proposed resolution:

Change 27.9.1.9 [ifstream.members], para. 3 from:

> Calls rdbuf()->open(s,mode|in). If that function returns a null pointer, calls setstate(failbit) (which may throw ios_base::failure [Footnote: (lib.iostate.flags)].

to:

> Calls rdbuf()->open(s,mode|in). If that function returns a null pointer, calls setstate(failbit) (which may throw ios_base::failure [Footnote: (lib.iostate.flags)), else calls clear().

Change 27.9.1.13 [ofstream.members], para. 3 from:

> Calls rdbuf()->open(s,mode|out). If that function returns a null pointer, calls setstate(failbit) (which may throw ios_base::failure [Footnote: (lib.iostate.flags)).

to:

> Calls rdbuf()->open(s,mode|out). If that function returns a null pointer, calls setstate(failbit) (which may throw ios_base::failure [Footnote: (lib.iostate.flags)), else calls clear().

Change 27.9.1.17 [fstream.members], para. 3 from:

> Calls rdbuf()->open(s,mode), If that function returns a null pointer, calls setstate(failbit), (which may throw ios_base::failure). (lib.iostate.flags) )

to:

> Calls rdbuf()->open(s,mode), If that function returns a null pointer, calls setstate(failbit), (which may throw ios_base::failure). (lib.iostate.flags) ), else calls clear().

[Kona: the LWG agrees this is a good idea. Post-Kona: Bill provided wording. He suggests having open, not close, clear the error flags.]

[Post-Sydney: Howard provided a new proposed resolution. The old one didn't make sense because it proposed to fix this at the level of basic_filebuf, which doesn't have access to the stream's error state. Howard's proposed resolution fixes this at the level of the three fstream class template instead.]

### 410. Missing semantics for stack and queue comparison operators

Section: 23.3.5.2 [list.cons], 23.3.5.4 [list.modifiers] Status: CD1 Submitter: Hans Bos Opened: 2003-06-07 Last modified: 2015-04-08

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Discussion:

Sections 23.3.5.2 [list.cons] and 23.3.5.4 [list.modifiers] list comparison operators (==, !=, <, <=, >, =>) for queue and stack. Only the semantics for queue::operator== (23.3.5.2 [list.cons] par2) and queue::operator< (23.3.5.2 [list.cons] par3) are defined.

Proposed resolution:

Add the following new paragraphs after 23.3.5.2 [list.cons] paragraph 3:

>       operator!=

> Returns: `x.c != y.c`

>       operator>

> Returns: `x.c > y.c`

>       operator<=

> Returns: `x.c <= y.c`

>       operator>=

> Returns: `x.c >= y.c`

Add the following paragraphs at the end of 23.3.5.4 [list.modifiers]:

>       operator==

> Returns: `x.c == y.c`

>       operator<

> Returns: `x.c < y.c`

>       operator!=

> Returns: `x.c != y.c`

>       operator>

> Returns: `x.c > y.c`

>       operator<=

> Returns: `x.c <= y.c`

>       operator>=

> Returns: `x.c >= y.c`

[Kona: Matt provided wording.]

Rationale:

There isn't any real doubt about what these operators are supposed to do, but we ought to spell it out.

### 411. Wrong names of set member functions

Section: 25.4.5 [alg.set.operations] Status: CD1 Submitter: Daniel Frey Opened: 2003-07-09 Last modified: 2015-04-08

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Discussion:

25.4.5 [alg.set.operations] paragraph 1 reads: "The semantics of the set operations are generalized to multisets in a standard way by defining union() to contain the maximum number of occurrences of every element, intersection() to contain the minimum, and so on."

This is wrong. The name of the functions are set_union() and set_intersection(), not union() and intersection().

Proposed resolution:

Change that sentence to use the correct names.

### 412. Typo in 27.4.4.3

Section: 27.5.5.4 [iostate.flags] Status: CD1 Submitter: Martin Sebor Opened: 2003-07-10 Last modified: 2015-04-08

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Duplicate of: 429

Discussion:

The Effects clause in 27.5.5.4 [iostate.flags] paragraph 5 says that the function only throws if the respective bits are already set prior to the function call. That's obviously not the intent. The typo ought to be corrected and the text reworded as: "If (state & exceptions()) == 0, returns. ..."

Proposed resolution:

In 27.5.5.4 [iostate.flags] paragraph 5, replace "If (rdstate() & exceptions()) == 0" with "If ((state | (rdbuf() ? goodbit : badbit)) & exceptions()) == 0".

[Kona: the original proposed resolution wasn't quite right. We really do mean rdstate(); the ambiguity is that the wording in the standard doesn't make it clear whether we mean rdstate() before setting the new state, or rdsate() after setting it. We intend the latter, of course. Post-Kona: Martin provided wording.]

### 413. Proposed resolution to LDR#64 still wrong

Section: 27.7.2.2.3 [istream::extractors] Status: CD1 Submitter: Bo Persson Opened: 2003-07-13 Last modified: 2015-04-08

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Discussion:

The second sentence of the proposed resolution says:

"If it inserted no characters because it caught an exception thrown while extracting characters from sb and ..."

However, we are not extracting from sb, but extracting from the basic_istream (*this) and inserting into sb. I can't really tell if "extracting" or "sb" is a typo.

[ Sydney: Definitely a real issue. We are, indeed, extracting characters from an istream and not from sb. The problem was there in the FDIS and wasn't fixed by issue 64. Probably what was intended was to have *this instead of sb. We're talking about the exception flag state of a basic_istream object, and there's only one basic_istream object in this discussion, so that would be a consistent interpretation. (But we need to be careful: the exception policy of this member function must be consistent with that of other extractors.) PJP will provide wording. ]

Proposed resolution:

Change the sentence from:

> If it inserted no characters because it caught an exception thrown while extracting characters from sb and failbit is on in exceptions(), then the caught exception is rethrown.

to:

> If it inserted no characters because it caught an exception thrown while extracting characters from *this and failbit is on in exceptions(), then the caught exception is rethrown.

### 414. Which iterators are invalidated by v.erase()?

Section: 23.3.6.5 [vector.modifiers] Status: CD1 Submitter: Matt Austern Opened: 2003-08-19 Last modified: 2015-04-08

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Discussion:

Consider the following code fragment:

>     int A[8] = { 1,3,5,7,9,8,4,2 };
>     std::vector<int> v(A, A+8);

>     std::vector<int>::iterator i1 = v.begin() + 3;
>     std::vector<int>::iterator i2 = v.begin() + 4;
>     v.erase(i1);

Which iterators are invalidated by `v.erase(i1)`: i1, i2, both, or neither?

On all existing implementations that I know of, the status of i1 and i2 is the same: both of them will be iterators that point to some elements of the vector (albeit not the same elements they did before). You won't get a crash if you use them. Depending on exactly what you mean by "invalidate", you might say that neither one has been invalidated because they still point to something, or you might say that both have been invalidated because in both cases the elements they point to have been changed out from under the iterator.

The standard doesn't say either of those things. It says that erase invalidates all iterators and references "after the point of the erase". This doesn't include i1, since it's at the point of the erase instead of after it. I can't think of any sensible definition of invalidation by which one can say that i2 is invalidated but i1 isn't.

(This issue is important if you try to reason about iterator validity based only on the guarantees in the standard, rather than reasoning from typical implementation techniques. Strict debugging modes, which some programmers find useful, do not use typical implementation techniques.)

Proposed resolution:

In 23.3.6.5 [vector.modifiers] paragraph 3, change "Invalidates all the iterators and references after the point of the erase" to "Invalidates iterators and references at or after the point of the erase".

Rationale:

I believe this was essentially a typographical error, and that it was taken for granted that erasing an element invalidates iterators that point to it. The effects clause in question treats iterators and references in parallel, and it would seem counterintuitive to say that a reference to an erased value remains valid.

### 415. behavior of std::ws

Section: 27.7.2.4 [istream.manip] Status: CD1 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2015-04-08

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Discussion:

According to 27.6.1.4, the ws() manipulator is not required to construct the sentry object. The manipulator is also not a member function so the text in 27.6.1, p1 through 4 that describes the exception policy for istream member functions does not apply. That seems inconsistent with the rest of extractors and all the other input functions (i.e., ws will not cause a tied stream to be flushed before extraction, it doesn't check the stream's exceptions or catch exceptions thrown during input, and it doesn't affect the stream's gcount).

Proposed resolution:

Add to 27.7.2.4 [istream.manip], immediately before the first sentence of paragraph 1, the following text:

> Behaves as an unformatted input function (as described in 27.6.1.3, paragraph 1), except that it does not count the number of characters extracted and does not affect the value returned by subsequent calls to is.gcount(). After constructing a sentry object...

[Post-Kona: Martin provided wording]

### 416. definitions of XXX_MIN and XXX_MAX macros in climits

Section: 18.3.3 [c.limits] Status: CD1 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2015-04-08

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Discussion:

Given two overloads of the function foo(), one taking an argument of type int and the other taking a long, which one will the call foo(LONG_MAX) resolve to? The expected answer should be foo(long), but whether that is true depends on the #defintion of the LONG_MAX macro, specifically its type. This issue is about the fact that the type of these macros is not actually required to be the same as the the type each respective limit.
Section 18.2.2 of the C++ Standard does not specify the exact types of the XXX_MIN and XXX_MAX macros #defined in the <climits> and <limits.h> headers such as INT_MAX and LONG_MAX and instead defers to the C standard.
Section 5.2.4.2.1, p1 of the C standard specifies that "The values [of these constants] shall be replaced by constant expressions suitable for use in #if preprocessing directives. Moreover, except for CHAR_BIT and MB_LEN_MAX, the following shall be replaced by expressions that have the same type as would an expression that is an object of the corresponding type converted according to the integer promotions."
The "corresponding type converted according to the integer promotions" for LONG_MAX is, according to 6.4.4.1, p5 of the C standard, the type of long converted to the first of the following set of types that can represent it: int, long int, long long int. So on an implementation where (sizeof(long) == sizeof(int)) this type is actually int, while on an implementation where (sizeof(long) > sizeof(int)) holds this type will be long.
This is not an issue in C since the type of the macro cannot be detected by any conforming C program, but it presents a portability problem in C++ where the actual type is easily detectable by overload resolution.

[Kona: the LWG does not believe this is a defect. The C macro definitions are what they are; we've got a better mechanism, `std::numeric_limits`, that is specified more precisely than the C limit macros. At most we should add a nonnormative note recommending that users who care about the exact types of limit quantities should use <limits> instead of <climits>.]

Proposed resolution:

Change 18.3.3 [c.limits], paragraph 2:

> -2- The contents are the same as the Standard C library header `<limits.h>`. [Note: The types of the macros in `<climits>` are not guaranteed to match the type to which they refer.--end note]

### 419. istream extractors not setting `failbit` if `eofbit` is already set

Section: 27.7.2.1.3 [istream::sentry] Status: C++11 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2015-04-08

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Discussion:

27.7.2.1.3 [istream::sentry], p2 says that `istream::sentry` ctor prepares for input if `is.good()` is true. p4 then goes on to say that the ctor sets the `sentry::ok_` member to true if the stream state is good after any preparation. 27.7.2.2.1 [istream.formatted.reqmts], p1 then says that a formatted input function endeavors to obtain the requested input if the sentry's `operator bool()` returns true. Given these requirements, no formatted extractor should ever set `failbit` if the initial stream `rdstate() == eofbit`. That is contrary to the behavior of all implementations I tested. The program below prints out

>     eof = 1, fail = 0
>     eof = 1, fail = 1

on all of them.

    #include <sstream>
    #include <cstdio>

    int main()
    {
        std::istringstream strm ("1");

        int i = 0;

        strm >> i;

        std::printf ("eof = %d, fail = %d\n",
                     !!strm.eof (), !!strm.fail ());

        strm >> i;

        std::printf ("eof = %d, fail = %d\n",
                     !!strm.eof (), !!strm.fail ());
    }

Comments from Jerry Schwarz (c++std-lib-11373):
Jerry Schwarz wrote:
I don't know where (if anywhere) it says it in the standard, but the formatted extractors are supposed to set `failbit` if they don't extract any characters. If they didn't then simple loops like
while (cin >> x);
would loop forever.
Further comments from Martin Sebor:
The question is which part of the extraction should prevent this from happening by setting `failbit` when `eofbit` is already set. It could either be the sentry object or the extractor. It seems that most implementations have chosen to set `failbit` in the sentry [...] so that's the text that will need to be corrected.

Pre Berlin: This issue is related to 342. If the sentry sets `failbit` when it finds `eofbit` already set, then you can never seek away from the end of stream.

Kona: Possibly NAD. If `eofbit` is set then `good()` will return false. We then set ok to false. We believe that the sentry's constructor should always set `failbit` when ok is false, and we also think the standard already says that. Possibly it could be clearer.

[ 2009-07 Frankfurt ]

> Moved to Ready.

Proposed resolution:

Change 27.7.2.1.3 [istream::sentry], p2 to:

>     explicit sentry(basic_istream<charT,traits>& is , bool noskipws = false);

> -2- Effects: If `is.good()` is `true` `false`, calls `is.setstate(failbit)`. Otherwise prepares for formatted or unformatted input. ...

### 420. is std::FILE a complete type?

Section: 27.9.1 [fstreams] Status: CD1 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2015-04-08

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Discussion:

7.19.1, p2, of C99 requires that the FILE type only be declared in <stdio.h>. None of the (implementation-defined) members of the struct is mentioned anywhere for obvious reasons.

C++ says in 27.8.1, p2 that FILE is a type that's defined in <cstdio>. Is it really the intent that FILE be a complete type or is an implementation allowed to just declare it without providing a full definition?

Proposed resolution:

In the first sentence of 27.9.1 [fstreams] paragraph 2, change "defined" to "declared".

Rationale:

We don't want to impose any restrictions beyond what the C standard already says. We don't want to make anything implementation defined, because that imposes new requirements in implementations.

### 422. explicit specializations of member functions of class templates

Section: 17.6.4.3 [reserved.names] Status: CD1 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2015-04-08

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Discussion:

It has been suggested that 17.4.3.1, p1 may or may not allow programs to explicitly specialize members of standard templates on user-defined types. The answer to the question might have an impact where library requirements are given using the "as if" rule. I.e., if programs are allowed to specialize member functions they will be able to detect an implementation's strict conformance to Effects clauses that describe the behavior of the function in terms of the other member function (the one explicitly specialized by the program) by relying on the "as if" rule.

Proposed resolution:

Add the following sentence to 17.6.4.3 [reserved.names], p1:

> It is undefined for a C++ program to add declarations or definitions to namespace std or namespaces within namespace `std` unless otherwise specified. A program may add template specializations for any standard library template to namespace `std`. Such a specialization (complete or partial) of a standard library template results in undefined behavior unless the declaration depends on a user-defined type of external linkage and unless the specialization meets the standard library requirements for the original template.168) A program has undefined behavior if it declares

> - an explicit specialization of any member function of a standard library class template, or
> - an explicit specialization of any member function template of a standard library class or class template, or
> - an explicit or partial specialization of any member class template of a standard library class or class template.

> A program may explicitly instantiate any templates in the standard library only if the declaration depends on the name of a user-defined type of external linkage and the instantiation meets the standard library requirements for the original template.

[Kona: straw poll was 6-1 that user programs should not be allowed to specialize individual member functions of standard library class templates, and that doing so invokes undefined behavior. Post-Kona: Martin provided wording.]

[Sydney: The LWG agrees that the standard shouldn't permit users to specialize individual member functions unless they specialize the whole class, but we're not sure these words say what we want them to; they could be read as prohibiting the specialization of any standard library class templates. We need to consult with CWG to make sure we use the right wording.]

### 425. return value of std::get_temporary_buffer

Section: 20.7.11 [temporary.buffer] Status: CD1 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2015-04-08

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Discussion:

The standard is not clear about the requirements on the value returned from a call to get_temporary_buffer(0). In particular, it fails to specify whether the call should return a distinct pointer each time it is called (like operator new), or whether the value is unspecified (as if returned by malloc). The standard also fails to mention what the required behavior is when the argument is less than 0.

Proposed resolution:

Change 20.10.3 [meta.help] paragraph 2 from "...or a pair of 0 values if no storage can be obtained" to "...or a pair of 0 values if no storage can be obtained or if n <= 0."

[Kona: Matt provided wording]

### 426. search_n(), fill_n(), and generate_n() with negative n

Section: 25.2.13 [alg.search], 25.3.6 [alg.fill], 25.3.7 [alg.generate] Status: CD1 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2015-04-08

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Discussion:

The complexity requirements for these function templates are incorrect (or don't even make sense) for negative n:

25.1.9, p7 (search_n):
Complexity: At most (last1 - first1) * count applications of the corresponding predicate.

25.2.5, p3 (fill_n):
Complexity: Exactly last - first (or n) assignments.

25.2.6, p3 (generate_n):
Complexity: Exactly last - first (or n) assignments.

In addition, the Requirements or the Effects clauses for the latter two templates don't say anything about the behavior when n is negative.

Proposed resolution:

Change 25.1.9, p7 to

> Complexity: At most (last1 - first1) * count applications of the corresponding predicate if count is positive, or 0 otherwise.

Change 25.2.5, p2 to

> Effects: Assigns value through all the iterators in the range [first, last), or [first, first + n) if n is positive, none otherwise.

Change 25.2.5, p3 to:

> Complexity: Exactly last - first (or n if n is positive, or 0 otherwise) assignments.

Change 25.2.6, p1 to (notice the correction for the misspelled "through"):

> Effects: Invokes the function object genand assigns the return value of gen through all the iterators in the range [first, last), or [first, first + n) if n is positive, or [first, first) otherwise.

Change 25.2.6, p3 to:

> Complexity: Exactly last - first (or n if n is positive, or 0 otherwise) assignments.

Rationale:

Informally, we want to say that whenever we see a negative number we treat it the same as if it were zero. We believe the above changes do that (although they may not be the minimal way of saying so). The LWG considered and rejected the alternative of saying that negative numbers are undefined behavior.

### 427. Stage 2 and rationale of DR 221

Section: 22.4.2.1.2 [facet.num.get.virtuals] Status: C++11 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2015-04-08

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Discussion:

The requirements specified in Stage 2 and reiterated in the rationale of DR 221 (and echoed again in DR 303) specify that num_get<charT>:: do_get() compares characters on the stream against the widened elements of "012...abc...ABCX+-"

An implementation is required to allow programs to instantiate the num_get template on any charT that satisfies the requirements on a user-defined character type. These requirements do not include the ability of the character type to be equality comparable (the char_traits template must be used to perform tests for equality). Hence, the num_get template cannot be implemented to support any arbitrary character type. The num_get template must either make the assumption that the character type is equality-comparable (as some popular implementations do), or it may use char_traits<charT> to do the comparisons (some other popular implementations do that). This diversity of approaches makes it difficult to write portable programs that attempt to instantiate the num_get template on user-defined types.

[Kona: the heart of the problem is that we're theoretically supposed to use traits classes for all fundamental character operations like assignment and comparison, but facets don't have traits parameters. This is a fundamental design flaw and it appears all over the place, not just in this one place. It's not clear what the correct solution is, but a thorough review of facets and traits is in order. The LWG considered and rejected the possibility of changing numeric facets to use narrowing instead of widening. This may be a good idea for other reasons (see issue 459), but it doesn't solve the problem raised by this issue. Whether we use widen or narrow the `num_get` facet still has no idea which traits class the user wants to use for the comparison, because only streams, not facets, are passed traits classes. The standard does not require that two different traits classes with the same `char_type` must necessarily have the same behavior.]

Informally, one possibility: require that some of the basic character operations, such as `eq`, `lt`, and `assign`, must behave the same way for all traits classes with the same `char_type`. If we accept that limitation on traits classes, then the facet could reasonably be required to use `char_traits<charT>`.

[ 2009-07 Frankfurt ]

> There was general agreement that the standard only needs to specify the behavior when the character type is char or wchar_t.

> Beman: we don't need to worry about C++1x because there is a non-zero possibility that we would have a replacement facility for iostreams that would solve these problems.

> We need to change the following sentence in [locale.category], paragraph 6 to specify that C is char and wchar_t:

> "A template formal parameter with name C represents the set of all possible specializations on a parameter that satisfies the requirements for a character on which any member of the iostream components can be instantiated."

> We also need to specify in 27 that the basic character operations, such as eq, lt, and assign use std::char_traits.

> Daniel volunteered to provide wording.

[ 2009-09-19 Daniel provided wording. ]

[ 2009-10 Santa Cruz: ]

> Leave as Open. Alisdair and/or Tom will provide wording based on discussions. We want to clearly state that streams and locales work just on `char` and `wchar_t` (except where otherwise specified).

[ 2010-02-06 Tom updated the proposed wording. ]

> [ The original proposed wording is preserved here: ]

> > 1. Change 22.3.1.1.1 [locale.category]/6:

> >    > [..] A template formal parameter with name `C` represents the set of all possible specializations on a `char` or `wchar_t` parameter that satisfies the requirements for a character on which any of the iostream components can be instantiated. [..]

> > 2. Add the following sentence to the end of 22.4.2 [category.numeric]/2:

> >    > [..] These specializations refer to [..], and also for the `ctype<>` facet to perform character classification. Implementations are encouraged but not required to use the `char_traits<charT>` functions for all comparisons and assignments of characters of type `charT` that do not belong to the set of required specializations.

> > 3. Change 22.4.2.1.2 [facet.num.get.virtuals]/3:

> >    > Stage 2: If `in==end` then stage 2 terminates. Otherwise a `charT` is taken from `in` and local variables are initialized as if by

> >    > >     char_type ct = *in;
> >    > >     using tr = char_traits<char_type>;
> >    > >     const char_type* pos = tr::find(atoms, sizeof(src) - 1, ct);
> >    > >     char c = src[find(atoms, atoms + sizeof(src) - 1, ct) - atoms
> >    > >                  pos ? pos - atoms : sizeof(src) - 1];
> >    > >     if (tr::eq(ct, ct == use_facet<numpunct<charT>(loc).decimal_point()))
> >    > >         c = '.';
> >    > >     bool discard =
> >    > >         tr::eq(ct, ct == use_facet<numpunct<charT>(loc).thousands_sep())
> >    > >         && use_facet<numpunct<charT> >(loc).grouping().length() != 0;

> >    > where the values `src` and `atoms` are defined as if by: [..]

> >    [Remark of the author: I considered to replace the initialization "`char_type ct = *in;`" by the sequence "`char_type ct; tr::assign(ct, *in);`", but decided against it, because it is a copy-initialization context, not an assignment]

> > 4. Add the following sentence to the end of 22.4.5 [category.time]/1:

> >    > [..] Their members use [..] , to determine formatting details. Implementations are encouraged but not required to use the `char_traits<charT>` functions for all comparisons and assignments of characters of type `charT` that do not belong to the set of required specializations.

> > 5. Change 22.4.5.1.1 [locale.time.get.members]/8 bullet 4:

> >    - The next element of `fmt` is equal to `'%'` For the next element `c` of `fmt char_traits<char_type>::eq(c, use_facet<ctype<char_type>>(f.getloc()).widen('%')) == true`, [..]

> > 6. Add the following sentence to the end of 22.4.6 [category.monetary]/2:

> >    > Their members use [..] to determine formatting details. Implementations are encouraged but not required to use the `char_traits<charT>` functions for all comparisons and assignments of characters of type `charT` that do not belong to the set of required specializations.

> > 7. Change 22.4.6.1.2 [locale.money.get.virtuals]/4:

> >    > [..] The value `units` is produced as if by:

> >    > >     for (int i = 0; i < n; ++i)
> >    > >       buf2[i] = src[char_traits<charT>::find(atoms, atoms+sizeof(src), buf1[i]) - atoms];
> >    > >     buf2[n] = 0;
> >    > >     sscanf(buf2, "%Lf", &units);

> > 8. Change 22.4.6.2.2 [locale.money.put.virtuals]/1:

> >    > [..] for character buffers `buf1` and `buf2`. If for the first character `c` in `digits` or `buf2` is equal to `ct.widen('-')``char_traits<charT>::eq(c, ct.widen('-')) == true`, [..]

> > 9. Add a footnote to the first sentence of 27.7.2.2.2 [istream.formatted.arithmetic]/1:

> >    > As in the case of the inserters, these extractors depend on the locale's `num_get<>` (22.4.2.1) object to perform parsing the input stream data.(footnote) [..]

> >    > footnote) If the traits of the input stream has different semantics for `lt()`, `eq()`, and `assign()` than `char_traits<char_type>`, this may give surprising results.

> > 10. Add a footnote to the second sentence of 27.7.3.6.2 [ostream.inserters.arithmetic]/1:

> >     > Effects: The classes `num_get<>` and `num_put<>` handle locale-dependent numeric formatting and parsing. These inserter functions use the imbued locale value to perform numeric formatting.(footnote) [..]

> >     > footnote) If the traits of the output stream has different semantics for `lt()`, `eq()`, and `assign()` than `char_traits<char_type>`, this may give surprising results.

> > 11. Add a footnote after the first sentence of 27.7.5 [ext.manip]/4:

> >     > Returns: An object of unspecified type such that if in is an object of type `basic_istream<charT, traits>` then the expression `in >> get_money(mon, intl)` behaves as if it called `f(in, mon, intl)`, where the function `f` is defined as:(footnote) [..]

> >     > footnote) If the traits of the input stream has different semantics for `lt()`, `eq()`, and `assign()` than `char_traits<char_type>`, this may give surprising results.

> > 12. Add a footnote after the first sentence of 27.7.5 [ext.manip]/5:

> >     > Returns: An object of unspecified type such that if `out` is an object of type `basic_ostream<charT, traits>` then the expression `out << put_money(mon, intl)` behaves as a formatted input function that calls `f(out, mon, intl)`, where the function `f` is defined as:(footnote) [..]

> >     > footnote) If the traits of the output stream has different semantics for `lt()`, `eq()`, and `assign()` than `char_traits<char_type>`, this may give surprising results.

> > 13. 13) Add a footnote after the first sentence of 27.7.5 [ext.manip]/8:

> >     > Returns: An object of unspecified type such that if `in` is an object of type b`asic_istream<charT, traits>` then the expression `in >>get_time(tmb, fmt)` behaves as if it called `f(in, tmb, fmt)`, where the function `f` is defined as:(footnote) [..]

> >     > footnote) If the traits of the input stream has different semantics for `lt()`, `eq()`, and `assign()` than `char_traits<char_type>`, this may give surprising results.

> > 14. Add a footnote after the first sentence of 27.7.5 [ext.manip]/10:

> >     > Returns: An object of unspecified type such that if `out` is an object of type `basic_ostream<charT, traits>` then the expression `out <<put_time(tmb, fmt)` behaves as if it called `f(out, tmb, fmt)`, where the function `f` is defined as:(footnote) [..]

> >     > footnote) If the traits of the output stream has different semantics for `lt()`, `eq()`, and `assign()` than `char_traits<char_type>`, this may give surprising results.

[ 2010 Pittsburgh: ]

> Moved to Ready with only two of the bullets. The original wording is preserved here:

> > 1. Change 22.3.1.1.1 [locale.category]/6:

> >    > [..] A template formal parameter with name `C` represents the set of all possible specializations on a of types containing `char`, `wchar_t`, and any other implementation-defined character type parameter that satisfies the requirements for a character on which any of the iostream components can be instantiated. [..]

> > 2. Add the following sentence to the end of 22.4.2 [category.numeric]/2:

> >    > [..] These specializations refer to [..], and also for the `ctype<>` facet to perform character classification. [Note: Implementations are encouraged but not required to use the `char_traits<charT>` functions for all comparisons and assignments of characters of type `charT` that do not belong to the set of required specializations - end note].

> > 3. Change 22.4.2.1.2 [facet.num.get.virtuals]/3:

> >    > Stage 2: If `in==end` then stage 2 terminates. Otherwise a `charT` is taken from `in` and local variables are initialized as if by

> >    > >     char_type ct = *in;
> >    > >     using tr = char_traits<char_type>;
> >    > >     const char_type* pos = tr::find(atoms, sizeof(src) - 1, ct);
> >    > >     char c = src[find(atoms, atoms + sizeof(src) - 1, ct) - atoms
> >    > >                  pos ? pos - atoms : sizeof(src) - 1];
> >    > >     if (tr::eq(ct, ct == use_facet<numpunct<charT>(loc).decimal_point()))
> >    > >         c = '.';
> >    > >     bool discard =
> >    > >         tr::eq(ct, ct == use_facet<numpunct<charT>(loc).thousands_sep())
> >    > >         && use_facet<numpunct<charT> >(loc).grouping().length() != 0;

> >    > where the values `src` and `atoms` are defined as if by: [..]

> >    [Remark of the author: I considered to replace the initialization "`char_type ct = *in;`" by the sequence "`char_type ct; tr::assign(ct, *in);`", but decided against it, because it is a copy-initialization context, not an assignment]

> > 4. Add the following sentence to the end of 22.4.5 [category.time]/1:

> >    > [..] Their members use [..] , to determine formatting details. [Note: Implementations are encouraged but not required to use the `char_traits<charT>` functions for all comparisons and assignments of characters of type `charT` that do not belong to the set of required specializations - end note].

> > 5. Change 22.4.5.1.1 [locale.time.get.members]/8 bullet 4:

> >    - The next element of `fmt` is equal to `'%'` For the next element `c` of `fmt char_traits<char_type>::eq(c, use_facet<ctype<char_type>>(f.getloc()).widen('%')) == true`, [..]

> > 6. Add the following sentence to the end of 22.4.6 [category.monetary]/2:

> >    > Their members use [..] to determine formatting details. [Note: Implementations are encouraged but not required to use the `char_traits<charT>` functions for all comparisons and assignments of characters of type `charT` that do not belong to the set of required specializations - end note].

> > 7. Change 22.4.6.1.2 [locale.money.get.virtuals]/4:

> >    > [..] The value `units` is produced as if by:

> >    > >     for (int i = 0; i < n; ++i)
> >    > >       buf2[i] = src[char_traits<charT>::find(atoms, atoms+sizeof(src), buf1[i]) - atoms];
> >    > >     buf2[n] = 0;
> >    > >     sscanf(buf2, "%Lf", &units);

> > 8. Change 22.4.6.2.2 [locale.money.put.virtuals]/1:

> >    > [..] for character buffers `buf1` and `buf2`. If for the first character `c` in `digits` or `buf2` is equal to `ct.widen('-')``char_traits<charT>::eq(c, ct.widen('-')) == true`, [..]

> > 9. Add a new paragraph after the first paragraph of 27.2.2 [iostreams.limits.pos]/1:

> >    > In the classes of clause 27, a template formal parameter with name `charT` represents one of the set of types containing `char`, `wchar_t`, and any other implementation-defined character type that satisfies the requirements for a character on which any of the iostream components can be instantiated.

> > 10. Add a footnote to the first sentence of 27.7.2.2.2 [istream.formatted.arithmetic]/1:

> >     > As in the case of the inserters, these extractors depend on the locale's `num_get<>` (22.4.2.1) object to perform parsing the input stream data.(footnote) [..]

> >     > footnote) If the traits of the input stream has different semantics for `lt()`, `eq()`, and `assign()` than `char_traits<char_type>`, this may give surprising results.

> > 11. Add a footnote to the second sentence of 27.7.3.6.2 [ostream.inserters.arithmetic]/1:

> >     > Effects: The classes `num_get<>` and `num_put<>` handle locale-dependent numeric formatting and parsing. These inserter functions use the imbued locale value to perform numeric formatting.(footnote) [..]

> >     > footnote) If the traits of the output stream has different semantics for `lt()`, `eq()`, and `assign()` than `char_traits<char_type>`, this may give surprising results.

> > 12. Add a footnote after the first sentence of 27.7.5 [ext.manip]/4:

> >     > Returns: An object of unspecified type such that if in is an object of type `basic_istream<charT, traits>` then the expression `in >> get_money(mon, intl)` behaves as if it called `f(in, mon, intl)`, where the function `f` is defined as:(footnote) [..]

> >     > footnote) If the traits of the input stream has different semantics for `lt()`, `eq()`, and `assign()` than `char_traits<char_type>`, this may give surprising results.

> > 13. Add a footnote after the first sentence of 27.7.5 [ext.manip]/5:

> >     > Returns: An object of unspecified type such that if `out` is an object of type `basic_ostream<charT, traits>` then the expression `out << put_money(mon, intl)` behaves as a formatted input function that calls `f(out, mon, intl)`, where the function `f` is defined as:(footnote) [..]

> >     > footnote) If the traits of the output stream has different semantics for `lt()`, `eq()`, and `assign()` than `char_traits<char_type>`, this may give surprising results.

> > 14. Add a footnote after the first sentence of 27.7.5 [ext.manip]/8:

> >     > Returns: An object of unspecified type such that if `in` is an object of type b`asic_istream<charT, traits>` then the expression `in >>get_time(tmb, fmt)` behaves as if it called `f(in, tmb, fmt)`, where the function `f` is defined as:(footnote) [..]

> >     > footnote) If the traits of the input stream has different semantics for `lt()`, `eq()`, and `assign()` than `char_traits<char_type>`, this may give surprising results.

> > 15. Add a footnote after the first sentence of 27.7.5 [ext.manip]/10:

> >     > Returns: An object of unspecified type such that if `out` is an object of type `basic_ostream<charT, traits>` then the expression `out <<put_time(tmb, fmt)` behaves as if it called `f(out, tmb, fmt)`, where the function `f` is defined as:(footnote) [..]

> >     > footnote) If the traits of the output stream has different semantics for `lt()`, `eq()`, and `assign()` than `char_traits<char_type>`, this may give surprising results.

Proposed resolution:

1. Change 22.3.1.1.1 [locale.category]/6:

   > [..] A template formal parameter with name `C` represents the set of all possible specializations on a of types containing `char`, `wchar_t`, and any other implementation-defined character type parameter that satisfies the requirements for a character on which any of the iostream components can be instantiated. [..]

2. Add a new paragraph after the first paragraph of 27.2.2 [iostreams.limits.pos]/1:

   > In the classes of clause 27, a template formal parameter with name `charT` represents one of the set of types containing `char`, `wchar_t`, and any other implementation-defined character type that satisfies the requirements for a character on which any of the iostream components can be instantiated.

### 428. string::erase(iterator) validity

Section: 21.4.6.5 [string::erase] Status: CD1 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2015-04-08

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Discussion:

23.1.1, p3 along with Table 67 specify as a prerequisite for a.erase(q) that q must be a valid dereferenceable iterator into the sequence a.

However, 21.3.5.5, p5 describing string::erase(p) only requires that p be a valid iterator.

This may be interepreted as a relaxation of the general requirement, which is most likely not the intent.

Proposed resolution:

Remove 21.4.6.5 [string::erase] paragraph 5.

Rationale:

The LWG considered two options: changing the string requirements to match the general container requirements, or just removing the erroneous string requirements altogether. The LWG chose the latter option, on the grounds that duplicating text always risks the possibility that it might be duplicated incorrectly.

### 430. `valarray` subset operations

Section: 26.6.2.5 [valarray.sub] Status: C++11 Submitter: Martin Sebor Opened: 2003-09-18 Last modified: 2015-04-08

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Discussion:

The standard fails to specify the behavior of valarray::operator[](slice) and other valarray subset operations when they are passed an "invalid" slice object, i.e., either a slice that doesn't make sense at all (e.g., slice (0, 1, 0) or one that doesn't specify a valid subset of the valarray object (e.g., slice (2, 1, 1) for a valarray of size 1).

[Kona: the LWG believes that invalid slices should invoke undefined behavior. Valarrays are supposed to be designed for high performance, so we don't want to require specific checking. We need wording to express this decision.]

[ Bellevue: ]

> Please note that the standard also fails to specify the behavior of slice_array and gslice_array in the valid case. Bill Plauger will endeavor to provide revised wording for `slice_array` and `gslice_array`.

[ post-Bellevue: Bill provided wording. ]

[ 2009-07 Frankfurt ]

> Move to Ready.

[ 2009-11-04 Pete opens: ]

> The resolution to LWG issue 430 has not been applied — there have been changes to the underlying text, and the resolution needs to be reworked.

[ 2010-03-09 Matt updated wording. ]

[ 2010 Pittsburgh: Moved to Ready for Pittsburgh. ]

Proposed resolution:

Replace 26.6.2.5 [valarray.sub], with the following:

> The member operator is overloaded to provide several ways to select sequences of elements from among those controlled by `*this`. Each of these operations returns a subset of the array. The const-qualified versions return this subset as a new `valarray`. The non-const versions return a class template object which has reference semantics to the original array, working in conjunction with various overloads of `operator=` (and other assigning operators) to allow selective replacement (slicing) of the controlled sequence. In each case the selected element(s) must exist.

>     valarray<T> operator[](slice slicearr) const;

> > This function returns an object of class `valarray<T>` containing those elements of the controlled sequence designated by `slicearr`. [Example:

> > >     valarray<char> v0("abcdefghijklmnop", 16);
> > >     valarray<char> v1("ABCDE", 5);
> > >     v0[slice(2, 5, 3)] = v1;
> > >     // v0 == valarray<char>("abAdeBghCjkDmnEp", 16)

> > end example]

>     valarray<T> operator[](slice slicearr);

> > This function selects those elements of the controlled sequence designated by `slicearr`. [Example:

> > >     valarray<char> v0("abcdefghijklmnop", 16);
> > >     valarray<char> v1("ABCDE", 5);
> > >     v0[slice(2, 5, 3)] = v1;
> > >     // v0 == valarray<char>("abAdeBghCjkDmnEp", 16)

> > end example]

>     valarray<T> operator[](const gslice& gslicearr) const;

> > This function returns an object of class `valarray<T>` containing those elements of the controlled sequence designated by `gslicearr`. [Example:

> > >     valarray<char> v0("abcdefghijklmnop", 16);
> > >     const size_t lv[] = {2, 3};
> > >     const size_t dv[] = {7, 2};
> > >     const valarray<size_t> len(lv, 2), str(dv, 2);
> > >     // v0[gslice(3, len, str)] returns
> > >     // valarray<char>("dfhkmo", 6)

> > end example]

>     gslice_array<T> operator[](const gslice& gslicearr);

> > This function selects those elements of the controlled sequence designated by `gslicearr`. [Example:

> > >     valarray<char> v0("abcdefghijklmnop", 16);
> > >     valarray<char> v1("ABCDEF", 6);
> > >     const size_t lv[] = {2, 3};
> > >     const size_t dv[] = {7, 2};
> > >     const valarray<size_t> len(lv, 2), str(dv, 2);
> > >     v0[gslice(3, len, str)] = v1;
> > >     // v0 == valarray<char>("abcAeBgCijDlEnFp", 16)

> > end example]

>     valarray<T> operator[](const valarray<bool>& boolarr) const;

> > This function returns an object of class `valarray<T>` containing those elements of the controlled sequence designated by `boolarr`. [Example:

> > >     valarray<char> v0("abcdefghijklmnop", 16);
> > >     const bool vb[] = {false, false, true, true, false, true};
> > >     // v0[valarray<bool>(vb, 6)] returns
> > >     // valarray<char>("cdf", 3)

> > end example]

>     mask_array<T> operator[](const valarray<bool>& boolarr);

> > This function selects those elements of the controlled sequence designated by `boolarr`. [Example:

> > >     valarray<char> v0("abcdefghijklmnop", 16);
> > >     valarray<char> v1("ABC", 3);
> > >     const bool vb[] = {false, false, true, true, false, true};
> > >     v0[valarray<bool>(vb, 6)] = v1;
> > >     // v0 == valarray<char>("abABeCghijklmnop", 16)

> > end example]

>     valarray<T> operator[](const valarray<size_t>& indarr) const;

> > This function returns an object of class `valarray<T>` containing those elements of the controlled sequence designated by `indarr`. [Example:

> > >     valarray<char> v0("abcdefghijklmnop", 16);
> > >     const size_t vi[] = {7, 5, 2, 3, 8};
> > >     // v0[valarray<size_t>(vi, 5)] returns
> > >     // valarray<char>("hfcdi", 5)

> > end example]

>     indirect_array<T> operator[](const valarray<size_t>& indarr);

> > This function selects those elements of the controlled sequence designated by `indarr`. [Example:

> > >     valarray<char> v0("abcdefghijklmnop", 16);
> > >     valarray<char> v1("ABCDE", 5);
> > >     const size_t vi[] = {7, 5, 2, 3, 8};
> > >     v0[valarray<size_t>(vi, 5)] = v1;
> > >     // v0 == valarray<char>("abCDeBgAEjklmnop", 16)

> > end example]

### 431. Swapping containers with unequal allocators

Section: 17.6.3.5 [allocator.requirements], 25 [algorithms] Status: Resolved Submitter: Matt Austern Opened: 2003-09-20 Last modified: 2015-04-08

View other active issues in [allocator.requirements].

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Discussion:

Clause 17.6.3.5 [allocator.requirements] paragraph 4 says that implementations are permitted to supply containers that are unable to cope with allocator instances and that container implementations may assume that all instances of an allocator type compare equal. We gave implementers this latitude as a temporary hack, and eventually we want to get rid of it. What happens when we're dealing with allocators that don't compare equal?

In particular: suppose that `v1` and `v2` are both objects of type `vector<int, my_alloc>` and that `v1.get_allocator() != v2.get_allocator()`. What happens if we write `v1.swap(v2)`? Informally, three possibilities:

1. This operation is illegal. Perhaps we could say that an implementation is required to check and to throw an exception, or perhaps we could say it's undefined behavior.

2. The operation performs a slow swap (i.e. using three invocations of `operator=`, leaving each allocator with its original container. This would be an O(N) operation.

3. The operation swaps both the vectors' contents and their allocators. This would be an O(1) operation. That is:

>         my_alloc a1(...);
>         my_alloc a2(...);
>         assert(a1 != a2);

>         vector<int, my_alloc> v1(a1);
>         vector<int, my_alloc> v2(a2);
>         assert(a1 == v1.get_allocator());
>         assert(a2 == v2.get_allocator());

>         v1.swap(v2);
>         assert(a1 == v2.get_allocator());
>         assert(a2 == v1.get_allocator());

[Kona: This is part of a general problem. We need a paper saying how to deal with unequal allocators in general.]

[pre-Sydney: Howard argues for option 3 in N1599. ]

[ 2007-01-12, Howard: This issue will now tend to come up more often with move constructors and move assignment operators. For containers, these members transfer resources (i.e. the allocated memory) just like swap. ]

[ Batavia: There is agreement to overload the container `swap` on the allocator's Swappable requirement using concepts. If the allocator supports Swappable, then container's swap will swap allocators, else it will perform a "slow swap" using copy construction and copy assignment. ]

[ 2009-04-28 Pablo adds: ]

> Fixed in N2525. I argued for marking this Tentatively-Ready right after Bellevue, but there was a concern that N2525 would break in the presence of the RVO. (That breakage had nothing to do with swap, but never-the-less). I addressed that breakage in in N2840 (Summit) by means of a non-normative reference:

> > [Note: in situations where the copy constructor for a container is elided, this function is not called. The behavior in these cases is as if `select_on_container_copy_construction` returned `x` — end note]

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Addressed by N2982.

Proposed resolution:

### 432. stringbuf::overflow() makes only one write position available

Section: 27.8.2.4 [stringbuf.virtuals] Status: CD1 Submitter: Christian W Brock Opened: 2003-09-24 Last modified: 2015-04-08

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Discussion:

27.7.1.3 par 8 says:

> Notes: The function can make a write position available only if ( mode & ios_base::out) != 0. To make a write position available, the function reallocates (or initially allocates) an array object with a sufficient number of elements to hold the current array object (if any), plus one additional write position. If ( mode & ios_base::in) != 0, the function alters the read end pointer egptr() to point just past the new write position (as does the write end pointer epptr()).

The sentences "plus one additional write position." and especially "(as does the write end pointer epptr())" COULD by interpreted (and is interpreted by at least my library vendor) as:

> post-condition: epptr() == pptr()+1

This WOULD force sputc() to call the virtual overflow() each time.

The proposed change also affects Defect Report 169.

Proposed resolution:

27.7.1.1/2 Change:

> 2- Notes: The function allocates no array object.

to:

> 2- Postcondition: str() == "".

27.7.1.1/3 Change:

> -3- Effects: Constructs an object of class basic_stringbuf, initializing the base class with basic_streambuf() (lib.streambuf.cons), and initializing mode with which . Then copies the content of str into the basic_stringbuf underlying character sequence and initializes the input and output sequences according to which. If which & ios_base::out is true, initializes the output sequence with the underlying sequence. If which & ios_base::in is true, initializes the input sequence with the underlying sequence.

to:

> -3- Effects: Constructs an object of class basic_stringbuf, initializing the base class with basic_streambuf() (lib.streambuf.cons), and initializing mode with which. Then copies the content of str into the basic_stringbuf underlying character sequence. If which & ios_base::out is true, initializes the output sequence such that pbase() points to the first underlying character, epptr() points one past the last underlying character, and if (which & ios_base::ate) is true, pptr() is set equal to epptr() else pptr() is set equal to pbase(). If which & ios_base::in is true, initializes the input sequence such that eback() and gptr() point to the first underlying character and egptr() points one past the last underlying character.

27.7.1.2/1 Change:

> -1- Returns: A basic_string object whose content is equal to the basic_stringbuf underlying character sequence. If the buffer is only created in input mode, the underlying character sequence is equal to the input sequence; otherwise, it is equal to the output sequence. In case of an empty underlying character sequence, the function returns basic_string<charT,traits,Allocator>().

to:

> -1- Returns: A basic_string object whose content is equal to the basic_stringbuf underlying character sequence. If the basic_stringbuf was created only in input mode, the resultant basic_string contains the character sequence in the range [eback(), egptr()). If the basic_stringbuf was created with (which & ios_base::out) being true then the resultant basic_string contains the character sequence in the range [pbase(), high_mark) where high_mark represents the position one past the highest initialized character in the buffer. Characters can be initialized either through writing to the stream, or by constructing the basic_stringbuf with a basic_string, or by calling the str(basic_string) member function. In the case of calling the str(basic_string) member function, all characters initialized prior to the call are now considered uninitialized (except for those characters re-initialized by the new basic_string). Otherwise the basic_stringbuf has been created in neither input nor output mode and a zero length basic_string is returned.

27.7.1.2/2 Change:

> -2- Effects: If the basic_stringbuf's underlying character sequence is not empty, deallocates it. Then copies the content of s into the basic_stringbuf underlying character sequence and initializes the input and output sequences according to the mode stored when creating the basic_stringbuf object. If (mode&ios_base::out) is true, then initializes the output sequence with the underlying sequence. If (mode&ios_base::in) is true, then initializes the input sequence with the underlying sequence.

to:

> -2- Effects: Copies the content of s into the basic_stringbuf underlying character sequence. If mode & ios_base::out is true, initializes the output sequence such that pbase() points to the first underlying character, epptr() points one past the last underlying character, and if (mode & ios_base::ate) is true, pptr() is set equal to epptr() else pptr() is set equal to pbase(). If mode & ios_base::in is true, initializes the input sequence such that eback() and gptr() point to the first underlying character and egptr() points one past the last underlying character.

Remove 27.2.1.2/3. (Same rationale as issue 238: incorrect and unnecessary.)

27.7.1.3/1 Change:

> 1- Returns: If the input sequence has a read position available, returns traits::to_int_type(*gptr()). Otherwise, returns traits::eof().

to:

> 1- Returns: If the input sequence has a read position available, returns traits::to_int_type(*gptr()). Otherwise, returns traits::eof(). Any character in the underlying buffer which has been initialized is considered to be part of the input sequence.

27.7.1.3/9 Change:

> -9- Notes: The function can make a write position available only if ( mode & ios_base::out) != 0. To make a write position available, the function reallocates (or initially allocates) an array object with a sufficient number of elements to hold the current array object (if any), plus one additional write position. If ( mode & ios_base::in) != 0, the function alters the read end pointer egptr() to point just past the new write position (as does the write end pointer epptr()).

to:

> -9- The function can make a write position available only if ( mode & ios_base::out) != 0. To make a write position available, the function reallocates (or initially allocates) an array object with a sufficient number of elements to hold the current array object (if any), plus one additional write position. If ( mode & ios_base::in) != 0, the function alters the read end pointer egptr() to point just past the new write position.

27.7.1.3/12 Change:

> -12- _ If (newoff + off) < 0, or (xend - xbeg) < (newoff + off), the positioning operation fails. Otherwise, the function assigns xbeg + newoff + off to the next pointer xnext .

to:

> -12- _ If (newoff + off) < 0, or if (newoff + off) refers to an uninitialized character (as defined in 27.8.2.3 [stringbuf.members] paragraph 1), the positioning operation fails. Otherwise, the function assigns xbeg + newoff + off to the next pointer xnext .

[post-Kona: Howard provided wording. At Kona the LWG agreed that something along these lines was a good idea, but the original proposed resolution didn't say enough about the effect of various member functions on the underlying character sequences.]

Rationale:

The current basic_stringbuf description is over-constrained in such a way as to prohibit vendors from making this the high-performance in-memory stream it was meant to be. The fundamental problem is that the pointers: eback(), gptr(), egptr(), pbase(), pptr(), epptr() are observable from a derived client, and the current description restricts the range [pbase(), epptr()) from being grown geometrically. This change allows, but does not require, geometric growth of this range.

Backwards compatibility issues: These changes will break code that derives from basic_stringbuf, observes epptr(), and depends upon [pbase(), epptr()) growing by one character on each call to overflow() (i.e. test suites). Otherwise there are no backwards compatibility issues.

27.7.1.1/2: The non-normative note is non-binding, and if it were binding, would be over specification. The recommended change focuses on the important observable fact.

27.7.1.1/3: This change does two things: 1. It describes exactly what must happen in terms of the sequences. The terms "input sequence" and "output sequence" are not well defined. 2. It introduces a common extension: open with app or ate mode. I concur with issue 238 that paragraph 4 is both wrong and unnecessary.

27.7.1.2/1: This change is the crux of the efficiency issue. The resultant basic_string is not dependent upon epptr(), and thus implementors are free to grow the underlying buffer geometrically during overflow() *and* place epptr() at the end of that buffer.

27.7.1.2/2: Made consistent with the proposed 27.7.1.1/3.

27.7.1.3/1: Clarifies that characters written to the stream beyond the initially specified string are available for reading in an i/o basic_streambuf.

27.7.1.3/9: Made normative by removing "Notes:", and removed the trailing parenthetical comment concerning epptr().

27.7.1.3/12: Restricting the positioning to [xbeg, xend) is no longer allowable since [pbase(), epptr()) may now contain uninitialized characters. Positioning is only allowable over the initialized range.

### 434. bitset::to_string() hard to use

Section: 20.6.2 [bitset.members] Status: CD1 Submitter: Martin Sebor Opened: 2003-10-15 Last modified: 2015-04-08

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Discussion:

It has been pointed out a number of times that the bitset to_string() member function template is tedious to use since callers must explicitly specify the entire template argument list (3 arguments). At least two implementations provide a number of overloads of this template to make it easier to use.

Proposed resolution:

In order to allow callers to specify no template arguments at all, just the first one (charT), or the first 2 (charT and traits), in addition to all three template arguments, add the following three overloads to both the interface (declarations only) of the class template bitset as well as to section 23.3.5.2, immediately after p34, the Returns clause of the existing to_string() member function template:

        template <class charT, class traits>
        basic_string<charT, traits, allocator<charT> >
        to_string () const;

        -34.1- Returns: to_string<charT, traits, allocator<charT> >().

        template <class charT>
        basic_string<charT, char_traits<charT>, allocator<charT> >
        to_string () const;

        -34.2- Returns: to_string<charT, char_traits<charT>, allocator<charT> >().

        basic_string<char, char_traits<char>, allocator<char> >
        to_string () const;

        -34.3- Returns: to_string<char, char_traits<char>, allocator<char> >().

[Kona: the LWG agrees that this is an improvement over the status quo. Dietmar thought about an alternative using a proxy object but now believes that the proposed resolution above is the right choice. ]

### 435. bug in DR 25

Section: 21.4.8.9 [string.io] Status: CD1 Submitter: Martin Sebor Opened: 2003-10-15 Last modified: 2015-04-08

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Discussion:

It has been pointed out that the proposed resolution in DR 25 may not be quite up to snuff:
http://gcc.gnu.org/ml/libstdc++/2003-09/msg00147.html http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#25

It looks like Petur is right. The complete corrected text is copied below. I think we may have have been confused by the reference to 22.2.2.2.2 and the subsequent description of `n' which actually talks about the second argument to sputn(), not about the number of fill characters to pad with.

So the question is: was the original text correct? If the intent was to follow classic iostreams then it most likely wasn't, since setting width() to less than the length of the string doesn't truncate it on output. This is also the behavior of most implementations (except for SGI's standard iostreams where the operator does truncate).

Proposed resolution:

Change the text in 21.3.7.9, p4 from

> If bool(k) is true, inserts characters as if by calling os.rdbuf()->sputn(str.data(), n), padding as described in stage 3 of lib.facet.num.put.virtuals, where n is the larger of os.width() and str.size();

to

> If bool(k) is true, determines padding as described in lib.facet.num.put.virtuals, and then inserts the resulting sequence of characters `seq` as if by calling `os.rdbuf()->sputn(seq, n)`, where `n` is the larger of `os.width()` and `str.size()`;

[Kona: it appears that neither the original wording, DR25, nor the proposed resolution, is quite what we want. We want to say that the string will be output, padded to os.width() if necessary. We don't want to duplicate the padding rules in clause 22, because they're complicated, but we need to be careful because they weren't quite written with quite this case in mind. We need to say what the character sequence is, and then defer to clause 22. Post-Kona: Benjamin provided wording.]

### 436. are cv-qualified facet types valid facets?

Section: 22.3.1.1.2 [locale.facet] Status: CD1 Submitter: Martin Sebor Opened: 2003-10-15 Last modified: 2015-04-08

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Discussion:

Is "const std::ctype<char>" a valid template argument to has_facet, use_facet, and the locale template ctor? And if so, does it designate the same Facet as the non-const "std::ctype<char>?" What about "volatile std::ctype<char>?" Different implementations behave differently: some fail to compile, others accept such types but behave inconsistently.

Proposed resolution:

Change 22.1.1.1.2, p1 to read:

Template parameters in this clause which are required to be facets are those named Facet in declarations. A program that passes a type that is not a facet, or a type that refers to volatile-qualified facet, as an (explicit or deduced) template parameter to a locale function expecting a facet, is ill-formed. A const-qualified facet is a valid template argument to any locale function that expects a Facet template parameter.

[Kona: changed the last sentence from a footnote to normative text.]

### 438. Ambiguity in the "do the right thing" clause

Section: 23.2.3 [sequence.reqmts] Status: CD1 Submitter: Howard Hinnant Opened: 2003-10-20 Last modified: 2015-04-08

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Discussion:

Section 23.2.3 [sequence.reqmts], paragraphs 9-11, fixed up the problem noticed with statements like:

    vector<int> v(10, 1);

The intent of the above statement was to construct with:

    vector(size_type, const value_type&);

but early implementations failed to compile as they bound to:

    template <class InputIterator>
    vector(InputIterator f, InputIterator l);

instead.

Paragraphs 9-11 say that if InputIterator is an integral type, then the member template constructor will have the same effect as:

    vector<static_cast<size_type>(f), static_cast<value_type>(l));

(and similarly for the other member template functions of sequences).

There is also a note that describes one implementation technique:

> One way that sequence implementors can satisfy this requirement is to specialize the member template for every integral type.

This might look something like:

>     template <class T>
>     struct vector
>     {
>          typedef unsigned size_type;

>          explicit vector(size_type) {}
>          vector(size_type, const T&) {}

>          template <class I>
>          vector(I, I);

>          // ...
>     };

>     template <class T>
>     template <class I>
>     vector<T>::vector(I, I) { ... }

>     template <>
>     template <>
>     vector<int>::vector(int, int) { ... }

>     template <>
>     template <>
>     vector<int>::vector(unsigned, unsigned) { ... }

>     //  ...

Label this solution 'A'.

The standard also says:

> Less cumbersome implementation techniques also exist.

A popular technique is to not specialize as above, but instead catch every call with the member template, detect the type of InputIterator, and then redirect to the correct logic. Something like:

>     template <class T>
>     template <class I>
>     vector<T>::vector(I f, I l)
>     {
>          choose_init(f, l, int2type<is_integral<I>::value>());
>     }

>     template <class T>
>     template <class I>
>     vector<T>::choose_init(I f, I l, int2type<false>)
>     {
>         // construct with iterators
>     }

>     template <class T>
>     template <class I>
>     vector<T>::choose_init(I f, I l, int2type<true>)
>     {
>         size_type sz = static_cast<size_type>(f);
>         value_type v = static_cast<value_type>(l);
>         // construct with sz,v
>     }

Label this solution 'B'.

Both of these solutions solve the case the standard specifically mentions:

    vector<int> v(10, 1);  // ok, vector size 10, initialized to 1

However, (and here is the problem), the two solutions have different behavior in some cases where the value_type of the sequence is not an integral type. For example consider:

>          pair<char, char>                     p('a', 'b');
>          vector<vector<pair<char, char> > >   d('a', 'b');

The second line of this snippet is likely an error. Solution A catches the error and refuses to compile. The reason is that there is no specialization of the member template constructor that looks like:

    template <>
    template <>
    vector<vector<pair<char, char> > >::vector(char, char) { ... }

So the expression binds to the unspecialized member template constructor, and then fails (compile time) because char is not an InputIterator.

Solution B compiles the above example though. 'a' is casted to an unsigned integral type and used to size the outer vector. 'b' is static casted to the inner vector using it's explicit constructor:

    explicit vector(size_type n);

and so you end up with a static_cast<size_type>('a') by static_cast<size_type>('b') matrix.

It is certainly possible that this is what the coder intended. But the explicit qualifier on the inner vector has been thwarted at any rate.

The standard is not clear whether the expression:

         vector<vector<pair<char, char> > >   d('a', 'b');

(and similar expressions) are:

1. undefined behavior.
2. illegal and must be rejected.
3. legal and must be accepted.

My preference is listed in the order presented.

There are still other techniques for implementing the requirements of paragraphs 9-11, namely the "restricted template technique" (e.g. enable_if). This technique is the most compact and easy way of coding the requirements, and has the behavior of #2 (rejects the above expression).

Choosing 1 would allow all implementation techniques I'm aware of. Choosing 2 would allow only solution 'A' and the enable_if technique. Choosing 3 would allow only solution 'B'.

Possible wording for a future standard if we wanted to actively reject the expression above would be to change "static_cast" in paragraphs 9-11 to "implicit_cast" where that is defined by:

>     template <class T, class U>
>     inline
>     T implicit_cast(const U& u)
>     {
>          return u;
>     }

Proposed resolution:

Replace 23.2.3 [sequence.reqmts] paragraphs 9 - 11 with:

For every sequence defined in this clause and in clause lib.strings:

- If the constructor

             template <class InputIterator>
             X(InputIterator f, InputIterator l,
               const allocator_type& a = allocator_type())

  is called with a type InputIterator that does not qualify as an input iterator, then the constructor will behave as if the overloaded constructor:

             X(size_type, const value_type& = value_type(),
               const allocator_type& = allocator_type())

  were called instead, with the arguments static_cast<size_type>(f), l and a, respectively.

- If the member functions of the forms:

             template <class InputIterator>          //  such as  insert()
             rt fx1(iterator p, InputIterator f, InputIterator l);

             template <class InputIterator>          //  such as  append(), assign()
             rt fx2(InputIterator f, InputIterator l);

             template <class InputIterator>          //  such as  replace()
             rt fx3(iterator i1, iterator i2, InputIterator f, InputIterator l);

  are called with a type InputIterator that does not qualify as an input iterator, then these functions will behave as if the overloaded member functions:

             rt fx1(iterator, size_type, const value_type&);

             rt fx2(size_type, const value_type&);

             rt fx3(iterator, iterator, size_type, const value_type&);

  were called instead, with the same arguments.

In the previous paragraph the alternative binding will fail if f is not implicitly convertible to X::size_type or if l is not implicitly convertible to X::value_type.

The extent to which an implementation determines that a type cannot be an input iterator is unspecified, except that as a minimum integral types shall not qualify as input iterators.

[ Kona: agreed that the current standard requires `v('a', 'b')` to be accepted, and also agreed that this is surprising behavior. The LWG considered several options, including something like implicit_cast, which doesn't appear to be quite what we want. We considered Howards three options: allow acceptance or rejection, require rejection as a compile time error, and require acceptance. By straw poll (1-6-1), we chose to require a compile time error. Post-Kona: Howard provided wording. ]

[ Sydney: The LWG agreed with this general direction, but there was some discomfort with the wording in the original proposed resolution. Howard submitted new wording, and we will review this again in Redmond. ]

[Redmond: one very small change in wording: the first argument is cast to size_t. This fixes the problem of something like `vector<vector<int> >(5, 5)`, where int is not implicitly convertible to the value type.]

Rationale:

The proposed resolution fixes:

      vector<int> v(10, 1);

since as integral types 10 and 1 must be disqualified as input iterators and therefore the (size,value) constructor is called (as if).

The proposed resolution breaks:

      vector<vector<T> > v(10, 1);

because the integral type 1 is not *implicitly* convertible to vector<T>. The wording above requires a diagnostic.

The proposed resolution leaves the behavior of the following code unspecified.

      struct A
      {
        operator int () const {return 10;}
      };

      struct B
      {
        B(A) {}
      };

      vector<B> v(A(), A());

The implementation may or may not detect that A is not an input iterator and employee the (size,value) constructor. Note though that in the above example if the B(A) constructor is qualified explicit, then the implementation must reject the constructor as A is no longer implicitly convertible to B.

### 441. Is fpos::state const?

Section: 27.5.4 [fpos] Status: CD1 Submitter: Vincent Leloup Opened: 2003-11-17 Last modified: 2015-04-08

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Discussion:

In section 27.5.4.1 [fpos.members] fpos<stateT>::state() is declared non const, but in section 27.5.4 [fpos] it is declared const.

Proposed resolution:

In section 27.5.4.1 [fpos.members], change the declaration of `fpos<stateT>::state()` to const.

### 442. sentry::operator bool() inconsistent signature

Section: 27.7.3.4 [ostream::sentry] Status: CD1 Submitter: Vincent Leloup Opened: 2003-11-18 Last modified: 2015-04-08

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Discussion:

In section 27.7.3.4 [ostream::sentry] paragraph 4, in description part basic_ostream<charT, traits>::sentry::operator bool() is declared as non const, but in section 27.6.2.3, in synopsis it is declared const.

Proposed resolution:

In section 27.7.3.4 [ostream::sentry] paragraph 4, change the declaration of `sentry::operator bool()` to const.

### 443. filebuf::close() inconsistent use of EOF

Section: 27.9.1.4 [filebuf.members] Status: CD1 Submitter: Vincent Leloup Opened: 2003-11-20 Last modified: 2015-04-08

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Discussion:

In section 27.9.1.4 [filebuf.members] par6, in effects description of basic_filebuf<charT, traits>::close(), overflow(EOF) is used twice; should be overflow(traits::eof()).

Proposed resolution:

Change overflow(EOF) to overflow(traits::eof()).

### 444. Bad use of casts in fstream

Section: 27.9.1 [fstreams] Status: CD1 Submitter: Vincent Leloup Opened: 2003-11-20 Last modified: 2015-04-08

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Discussion:

27.9.1.9 [ifstream.members] p1, 27.9.1.13 [ofstream.members] p1, 27.9.1.17 [fstream.members] p1 seems have same problem as exposed in LWG issue 252.

Proposed resolution:

[Sydney: Genuine defect. 27.8.1.13 needs a cast to cast away constness. The other two places are stylistic: we could change the C-style casts to const_cast. Post-Sydney: Howard provided wording. ]

Change 27.8.1.7/1 from:

> Returns: (basic_filebuf<charT,traits>*)&sb.

to:

> Returns: const_cast<basic_filebuf<charT,traits>*>(&sb).

Change 27.8.1.10/1 from:

> Returns: (basic_filebuf<charT,traits>*)&sb.

to:

> Returns: const_cast<basic_filebuf<charT,traits>*>(&sb).

Change 27.8.1.13/1 from:

> Returns: &sb.

to:

> Returns: const_cast<basic_filebuf<charT,traits>*>(&sb).

### 445. iterator_traits::reference unspecified for some iterator categories

Section: 24.4.1 [iterator.traits] Status: CD1 Submitter: Dave Abrahams Opened: 2003-12-09 Last modified: 2015-04-08

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Discussion:

The standard places no restrictions at all on the reference type of input, output, or forward iterators (for forward iterators it only specifies that *x must be value_type& and doesn't mention the reference type). Bidirectional iterators' reference type is restricted only by implication, since the base iterator's reference type is used as the return type of reverse_iterator's operator*, which must be T& in order to be a conforming forward iterator.

Here's what I think we ought to be able to expect from an input or forward iterator's reference type R, where a is an iterator and V is its value_type

- *a is convertible to R
- R is convertible to V
- static_cast<V>(static_cast<R>(*a)) is equivalent to static_cast<V>(*a)

A mutable forward iterator ought to satisfy, for x of type V:

          { R r = *a; r = x; } is equivalent to *a = x;

I think these requirements capture existing container iterators (including vector<bool>'s), but render istream_iterator invalid; its reference type would have to be changed to a constant reference.

(Jeremy Siek) During the discussion in Sydney, it was felt that a simpler long term solution for this was needed. The solution proposed was to require `reference` to be the same type as `*a` and `pointer` to be the same type as `a->`. Most iterators in the Standard Library already meet this requirement. Some iterators are output iterators, and do not need to meet the requirement, and others are only specified through the general iterator requirements (which will change with this resolution). The sole case where there is an explicit definition of the reference type that will need to change is `istreambuf_iterator` which returns `charT` from `operator*` but has a reference type of `charT&`. We propose changing the reference type of `istreambuf_iterator` to `charT`.

The other option for resolving the issue with `pointer`, mentioned in the note below, is to remove `pointer` altogether. I prefer placing requirements on `pointer` to removing it for two reasons. First, `pointer` will become useful for implementing iterator adaptors and in particular, `reverse_iterator` will become more well defined. Second, removing `pointer` is a rather drastic and publicly-visible action to take.

The proposed resolution technically enlarges the requirements for iterators, which means there are existing iterators (such as `istreambuf_iterator`, and potentially some programmer-defined iterators) that will no longer meet the requirements. Will this break existing code? The scenario in which it would is if an algorithm implementation (say in the Standard Library) is changed to rely on `iterator_traits::reference`, and then is used with one of the iterators that do not have an appropriately defined `iterator_traits::reference`.

The proposed resolution makes one other subtle change. Previously, it was required that output iterators have a `difference_type` and `value_type` of `void`, which means that a forward iterator could not be an output iterator. This is clearly a mistake, so I've changed the wording to say that those types may be `void`.

Proposed resolution:

In 24.4.1 [iterator.traits], after:

> be defined as the iterator's difference type, value type and iterator category, respectively.

add

> In addition, the types

>     iterator_traits<Iterator>::reference
>     iterator_traits<Iterator>::pointer

> must be defined as the iterator's reference and pointer types, that is, the same type as the type of `*a` and `a->`, respectively.

In 24.4.1 [iterator.traits], change:

> In the case of an output iterator, the types

>     iterator_traits<Iterator>::difference_type
>     iterator_traits<Iterator>::value_type

> are both defined as `void`.

to:

> In the case of an output iterator, the types

>     iterator_traits<Iterator>::difference_type
>     iterator_traits<Iterator>::value_type
>     iterator_traits<Iterator>::reference
>     iterator_traits<Iterator>::pointer

> may be defined as `void`.

In 24.6.3 [istreambuf.iterator], change:

>     typename traits::off_type, charT*, charT&>

to:

>     typename traits::off_type, charT*, charT>

[ Redmond: there was concern in Sydney that this might not be the only place where things were underspecified and needed to be changed. Jeremy reviewed iterators in the standard and confirmed that nothing else needed to be changed. ]

### 448. Random Access Iterators over abstract classes

Section: 24.2.7 [random.access.iterators] Status: CD1 Submitter: Dave Abrahams Opened: 2004-01-07 Last modified: 2015-04-08

View all other issues in [random.access.iterators].

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Discussion:

Table 76, the random access iterator requirement table, says that the return type of a[n] must be "convertible to T". When an iterator's value_type T is an abstract class, nothing is convertible to T. Surely this isn't an intended restriction?

Proposed resolution:

Change the return type to "convertible to T const&".

### 449. Library Issue 306 Goes Too Far

Section: 18.2 [support.types] Status: CD1 Submitter: Pete Becker Opened: 2004-01-15 Last modified: 2015-04-08

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Discussion:

Original text:

> The macro offsetof accepts a restricted set of type arguments in this International Standard. type shall be a POD structure or a POD union (clause 9). The result of applying the offsetof macro to a field that is a static data member or a function member is undefined."

Revised text:

> "If type is not a POD structure or a POD union the results are undefined."

Looks to me like the revised text should have replaced only the second sentence. It doesn't make sense standing alone.

Proposed resolution:

Change 18.1, paragraph 5, to:

> The macro offsetof accepts a restricted set of type arguments in this International Standard. If type is not a POD structure or a POD union the results are undefined. The result of applying the offsetof macro to a field that is a static data member or a function member is undefined."

### 453. basic_stringbuf::seekoff need not always fail for an empty stream

Section: 27.8.2.4 [stringbuf.virtuals] Status: CD1 Submitter: Bill Plauger Opened: 2004-01-30 Last modified: 2015-04-08

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Discussion:

      pos_type basic_stringbuf::seekoff(off_type, ios_base::seekdir,
                                        ios_base::openmode);

is obliged to fail if nothing has been inserted into the stream. This is unnecessary and undesirable. It should be permissible to seek to an effective offset of zero.

[ Sydney: Agreed that this is an annoying problem: seeking to zero should be legal. Bill will provide wording. ]

Proposed resolution:

Change the sentence from:

> For a sequence to be positioned, if its next pointer (either gptr() or pptr()) is a null pointer, the positioning operation fails.

to:

> For a sequence to be positioned, if its next pointer (either gptr() or pptr()) is a null pointer and the new offset newoff is nonzero, the positioning operation fails.

### 455. cerr::tie() and wcerr::tie() are overspecified

Section: 27.4 [iostream.objects] Status: CD1 Submitter: Bill Plauger Opened: 2004-01-30 Last modified: 2015-04-08

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Discussion:

Both cerr::tie() and wcerr::tie() are obliged to be null at program startup. This is overspecification and overkill. It is both traditional and useful to tie cerr to cout, to ensure that standard output is drained whenever an error message is written. This behavior should at least be permitted if not required. Same for wcerr::tie().

Proposed resolution:

Add to the description of cerr:

> After the object cerr is initialized, cerr.tie() returns &cout. Its state is otherwise the same as required for basic_ios<char>::init (lib.basic.ios.cons).

Add to the description of wcerr:

> After the object wcerr is initialized, wcerr.tie() returns &wcout. Its state is otherwise the same as required for basic_ios<wchar_t>::init (lib.basic.ios.cons).

[Sydney: straw poll (3-1): we should require, not just permit, cout and cerr to be tied on startup. Pre-Redmond: Bill will provide wording.]

### 456. Traditional C header files are overspecified

Section: 17.6.1.2 [headers] Status: CD1 Submitter: Bill Plauger Opened: 2004-01-30 Last modified: 2015-04-08

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Discussion:

The C++ Standard effectively requires that the traditional C headers (of the form <xxx.h>) be defined in terms of the newer C++ headers (of the form <cxxx>). Clauses 17.4.1.2/4 and D.5 combine to require that:

- Including the header <cxxx> declares a C name in namespace std.
- Including the header <xxx.h> declares a C name in namespace std (effectively by including <cxxx>), then imports it into the global namespace with an individual using declaration.

The rules were left in this form despited repeated and heated objections from several compiler vendors. The C headers are often beyond the direct control of C++ implementors. In some organizations, it's all they can do to get a few #ifdef __cplusplus tests added. Third-party library vendors can perhaps wrap the C headers. But neither of these approaches supports the drastic restructuring required by the C++ Standard. As a result, it is still widespread practice to ignore this conformance requirement, nearly seven years after the committee last debated this topic. Instead, what is often implemented is:

- Including the header <xxx.h> declares a C name in the global namespace.
- Including the header <cxxx> declares a C name in the global namespace (effectively by including <xxx.h>), then imports it into namespace std with an individual using declaration.

The practical benefit for implementors with the second approach is that they can use existing C library headers, as they are pretty much obliged to do. The practical cost for programmers facing a mix of implementations is that they have to assume weaker rules:

- If you want to assuredly declare a C name in the global namespace, include <xxx.h>. You may or may not also get the declaration in namespace std.
- If you want to assuredly declare a C name in namespace std, include <cxxx>. You may or may not also get the declaration in the global namespace.

There also exists the possibility of subtle differences due to Koenig lookup, but there are so few non-builtin types defined in the C headers that I've yet to see an example of any real problems in this area.

It is worth observing that the rate at which programmers fall afoul of these differences has remained small, at least as measured by newsgroup postings and our own bug reports. (By an overwhelming margin, the commonest problem is still that programmers include <string> and can't understand why the typename string isn't defined -- this a decade after the committee invented namespace std, nominally for the benefit of all programmers.)

We should accept the fact that we made a serious mistake and rectify it, however belatedly, by explicitly allowing either of the two schemes for declaring C names in headers.

[Sydney: This issue has been debated many times, and will certainly have to be discussed in full committee before any action can be taken. However, the preliminary sentiment of the LWG was in favor of the change. (6 yes, 0 no, 2 abstain) Robert Klarer suggests that we might also want to undeprecate the C-style `.h` headers.]

Proposed resolution:

Add to 17.6.1.2 [headers], para. 4:

> Except as noted in clauses 18 through 27 and Annex D, the contents of each header cname shall be the same as that of the corresponding header name.h, as specified in ISO/IEC 9899:1990 Programming Languages C (Clause 7), or ISO/IEC:1990 Programming Languages-C AMENDMENT 1: C Integrity, (Clause 7), as appropriate, as if by inclusion. In the C++ Standard Library, however, the declarations and definitions (except for names which are defined as macros in C) are within namespace scope (3.3.5) of the namespace std. It is unspecified whether these names are first declared within the global namespace scope and are then injected into namespace std by explicit using-declarations (7.3.3 [namespace.udecl]).

Change D.5 [depr.c.headers], para. 2-3:

> -2- Every C header, each of which has a name of the form name.h, behaves as if each name placed in the Standard library namespace by the corresponding cname header is also placed within the global namespace scope. of the namespace `std` and is followed by an explicit using-declaration (7.3.3 [namespace.udecl]). It is unspecified whether these names are first declared or defined within namespace scope (3.3.6 [basic.scope.namespace]) of the namespace `std` and are then injected into the global namespace scope by explicit using-declarations (7.3.3 [namespace.udecl]).

> -3- [Example: The header `<cstdlib>` assuredly provides its declarations and definitions within the namespace `std`. It may also provide these names within the global namespace. The header `<stdlib.h>` makes these available also in assuredly provides the same declarations and definitions within the global namespace, much as in the C Standard. It may also provide these names within the namespace `std`. -- end example]

### 457. bitset constructor: incorrect number of initialized bits

Section: 20.6.1 [bitset.cons] Status: CD1 Submitter: Dag Henriksson Opened: 2004-01-30 Last modified: 2015-04-08

View all other issues in [bitset.cons].

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Discussion:

The constructor from unsigned long says it initializes "the first M bit positions to the corresponding bit values in val. M is the smaller of N and the value CHAR_BIT * sizeof(unsigned long)."

Object-representation vs. value-representation strikes again. CHAR_BIT * sizeof (unsigned long) does not give us the number of bits an unsigned long uses to hold the value. Thus, the first M bit position above is not guaranteed to have any corresponding bit values in val.

Proposed resolution:

In 20.6.1 [bitset.cons] paragraph 2, change "M is the smaller of N and the value CHAR_BIT * sizeof (unsigned long). (249)" to "`M` is the smaller of `N` and the number of bits in the value representation (section 3.9 [basic.types]) of `unsigned long`."

### 460. Default modes missing from basic_fstream member specifications

Section: 27.9.1 [fstreams] Status: CD1 Submitter: Ben Hutchings Opened: 2004-04-01 Last modified: 2015-04-08

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Discussion:

The second parameters of the non-default constructor and of the open member function for basic_fstream, named "mode", are optional according to the class declaration in 27.8.1.11 [lib.fstream]. The specifications of these members in 27.8.1.12 [lib.fstream.cons] and 27.8.1.13 lib.fstream.members] disagree with this, though the constructor declaration has the "explicit" function-specifier implying that it is intended to be callable with one argument.

Proposed resolution:

In 27.9.1.15 [fstream.cons], change

      explicit basic_fstream(const char* s, ios_base::openmode mode);

to

      explicit basic_fstream(const char* s,
                             ios_base::openmode mode = ios_base::in|ios_base::out);

In 27.9.1.17 [fstream.members], change

      void open(const char*s, ios_base::openmode mode);

to

      void open(const char*s,
                ios_base::openmode mode = ios_base::in|ios_base::out);

### 461. time_get hard or impossible to implement

Section: 22.4.5.1.2 [locale.time.get.virtuals] Status: CD1 Submitter: Bill Plauger Opened: 2004-03-23 Last modified: 2015-04-08

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Discussion:

Template time_get currently contains difficult, if not impossible, requirements for do_date_order, do_get_time, and do_get_date. All require the implementation to scan a field generated by the %x or %X conversion specifier in strftime. Yes, do_date_order can always return no_order, but that doesn't help the other functions. The problem is that %x can be nearly anything, and it can vary widely with locales. It's horribly onerous to have to parse "third sunday after Michaelmas in the year of our Lord two thousand and three," but that's what we currently ask of do_get_date. More practically, it leads some people to think that if %x produces 10.2.04, we should know to look for dots as separators. Still not easy.

Note that this is the opposite effect from the intent stated in the footnote earlier in this subclause:

> "In other words, user confirmation is required for reliable parsing of user-entered dates and times, but machine-generated formats can be parsed reliably. This allows parsers to be aggressive about interpreting user variations on standard formats."

We should give both implementers and users an easier and more reliable alternative: provide a (short) list of alternative delimiters and say what the default date order is for no_order. For backward compatibility, and maximum latitude, we can permit an implementation to parse whatever %x or %X generates, but we shouldn't require it.

Proposed resolution:

In the description:

    iter_type do_get_time(iter_type s, iter_type end, ios_base& str,
            ios_base::iostate& err, tm* t) const;

2 Effects: Reads characters starting at suntil it has extracted those struct tm members, and remaining format characters, used by time_put<>::put to produce the format specified by 'X', or until it encounters an error or end of sequence.

change: 'X'

to: "%H:%M:%S"

Change

    iter_type do_get_date(iter_type s, iter_type end, ios_base& str,
            ios_base::iostate& err, tm* t) const;

    4 Effects: Reads characters starting at s until it has extracted those
    struct tm members, and remaining format characters, used by
    time_put<>::put to produce the format specified by 'x', or until it
    encounters an error.

to

    iter_type do_get_date(iter_type s, iter_type end, ios_base& str,
            ios_base::iostate& err, tm* t) const;

4 Effects: Reads characters starting at s until it has extracted those struct tm members, and remaining format characters, used by time_put<>::put to produce one of the following formats, or until it encounters an error. The format depends on the value returned by date_order() as follows:

            date_order()  format

            no_order      "%m/%d/%y"
            dmy           "%d/%m/%y"
            mdy           "%m/%d/%y"
            ymd           "%y/%m/%d"
            ydm           "%y/%d/%m"

An implementation may also accept additional implementation-defined formats.

[Redmond: agreed that this is a real problem. The solution is probably to match C99's parsing rules. Bill provided wording. ]

### 464. Suggestion for new member functions in standard containers

Section: 23.3.6 [vector], 23.4.4 [map] Status: CD1 Submitter: Thorsten Ottosen Opened: 2004-05-12 Last modified: 2015-04-08

View all other issues in [vector].

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Discussion:

To add slightly more convenience to vector<T> and map<Key,T> we should consider to add

1. add vector<T>::data() member (const and non-const version) semantics: if( empty() ) return 0; else return buffer_;
2. add map<Key,T>::at( const Key& k ) member (const and non-const version) semantics: iterator i = find( k ); if( i != end() ) return *i; else throw range_error();

Rationale:

- To obtain a pointer to the vector's buffer, one must use either operator[]() (which can give undefined behavior for empty vectors) or at() (which will then throw if the vector is empty).
- tr1::array<T,sz> already has a data() member
- e cannot use operator[]() when T is not DefaultDonstructible
- Neither when the map is const.
- when we want to make sure we don't add an element accidently
- when it should be considered an error if a key is not in the map

Proposed resolution:

In 23.3.6 [vector], add the following to the `vector` synopsis after "element access" and before "modifiers":

      // [lib.vector.data] data access
      pointer       data();
      const_pointer data() const;

Add a new subsection of 23.3.6 [vector]:

> 23.2.4.x `vector` data access

>        pointer       data();
>        const_pointer data() const;

> Returns: A pointer such that [data(), data() + size()) is a valid range. For a non-empty vector, data() == &front().

> Complexity: Constant time.

> Throws: Nothing.

In 23.4.4 [map], add the following to the `map` synopsis immediately after the line for operator[]:

      T&       at(const key_type& x);
      const T& at(const key_type& x) const;

Add the following to 23.4.4.3 [map.access]:

>       T&       at(const key_type& x);
>       const T& at(const key_type& x) const;

> Returns: A reference to the element whose key is equivalent to x, if such an element is present in the map.

> Throws: `out_of_range` if no such element is present.

Rationale:

Neither of these additions provides any new functionality but the LWG agreed that they are convenient, especially for novices. The exception type chosen for `at`, `std::out_of_range`, was chosen to match `vector::at`.

### 465. Contents of <ciso646>

Section: 17.6.1.2 [headers] Status: CD1 Submitter: Steve Clamage Opened: 2004-06-03 Last modified: 2015-04-08

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Discussion:

C header <iso646.h> defines macros for some operators, such as not_eq for !=.

Section 17.6.1.2 [headers] "Headers" says that except as noted in clauses 18 through 27, the <cname> C++ header contents are the same as the C header <name.h>. In particular, table 12 lists <ciso646> as a C++ header.

I don't find any other mention of <ciso646>, or any mention of <iso646.h>, in clauses 17 thorough 27. That implies that the contents of <ciso646> are the same as C header <iso646.h>.

Annex C (informative, not normative) in [diff.header.iso646.h] C.2.2.2 "Header <iso646.h>" says that the alternative tokens are not defined as macros in <ciso646>, but does not mention the contents of <iso646.h>.

I don't find any normative text to support C.2.2.2.

Proposed resolution:

Add to section 17.4.1.2 Headers [lib.headers] a new paragraph after paragraph 6 (the one about functions must be functions):

> Identifiers that are keywords or operators in C++ shall not be defined as macros in C++ standard library headers. [Footnote:In particular, including the standard header <iso646.h> or <ciso646> has no effect.

[post-Redmond: Steve provided wording.]

### 467. char_traits::lt(), compare(), and memcmp()

Section: 21.2.3.1 [char.traits.specializations.char] Status: CD1 Submitter: Martin Sebor Opened: 2004-06-28 Last modified: 2015-04-08

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Discussion:

Table 37 describes the requirements on Traits::compare() in terms of those on Traits::lt(). 21.1.3.1, p6 requires char_traits<char>::lt() to yield the same result as operator<(char, char).

Most, if not all, implementations of char_traits<char>::compare() call memcmp() for efficiency. However, the C standard requires both memcmp() and strcmp() to interpret characters under comparison as unsigned, regardless of the signedness of char. As a result, all these char_traits implementations fail to meet the requirement imposed by Table 37 on compare() when char is signed.

Read email thread starting with c++std-lib-13499 for more.

Proposed resolution:

Change 21.1.3.1, p6 from

> The two-argument members assign, eq, and lt are defined identically to the built-in operators =, ==, and < respectively.

to

> The two-argument member assign is defined identically to the built-in operator =. The two argument members eq and lt are defined identically to the built-in operators == and < for type unsigned char.

[Redmond: The LWG agreed with this general direction, but we also need to change `eq` to be consistent with this change. Post-Redmond: Martin provided wording.]

### 468. unexpected consequences of ios_base::operator void*()

Section: 27.5.5.4 [iostate.flags] Status: CD1 Submitter: Martin Sebor Opened: 2004-06-28 Last modified: 2015-04-08

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Discussion:

The program below is required to compile but when run it typically produces unexpected results due to the user-defined conversion from std::cout or any object derived from basic_ios to void*.

        #include <cassert>
        #include <iostream>

        int main ()
        {
            assert (std::cin.tie () == std::cout);
            // calls std::cout.ios::operator void*()
        }

Proposed resolution:

Replace std::basic_ios<charT, traits>::operator void*() with another conversion operator to some unspecified type that is guaranteed not to be convertible to any other type except for bool (a pointer-to-member might be one such suitable type). In addition, make it clear that the pointer type need not be a pointer to a complete type and when non-null, the value need not be valid.

Specifically, change in [lib.ios] the signature of

        operator void*() const;

to

        operator unspecified-bool-type() const;

and change [lib.iostate.flags], p1 from

        operator void*() const;

to

    operator unspecified-bool-type() const;

         -1- Returns: if fail() then a value that will evaluate false in a
          boolean context; otherwise a value that will evaluate true in a
          boolean context. The value type returned shall not be
          convertible to int.

         -2- [Note: This conversion can be used in contexts where a bool
          is expected (e.g., an if condition); however, implicit
          conversions (e.g., to int) that can occur with bool are not
          allowed, eliminating some sources of user error. One possible
          implementation choice for this type is pointer-to-member.  - end
          note]

[Redmond: 5-4 straw poll in favor of doing this.]

[Lillehammer: Doug provided revised wording for "unspecified-bool-type".]

### 469. vector<bool> ill-formed relational operators

Section: 23.3.6 [vector] Status: CD1 Submitter: Martin Sebor Opened: 2004-06-28 Last modified: 2015-04-08

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Discussion:

The overloads of relational operators for vector<bool> specified in [lib.vector.bool] are redundant (they are semantically identical to those provided for the vector primary template) and may even be diagnosed as ill-formed (refer to Daveed Vandevoorde's explanation in c++std-lib-13647).

Proposed resolution:

Remove all overloads of overloads of relational operators for vector<bool> from [lib.vector.bool].

### 471. result of `what()` implementation-defined

Section: 18.8.1 [exception] Status: C++11 Submitter: Martin Sebor Opened: 2004-06-28 Last modified: 2015-04-08

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Discussion:

[lib.exception] specifies the following:

        exception (const exception&) throw();
        exception& operator= (const exception&) throw();

        -4- Effects: Copies an exception object.
        -5- Notes: The effects of calling what() after assignment
            are implementation-defined.

First, does the Note only apply to the assignment operator? If so, what are the effects of calling what() on a copy of an object? Is the returned pointer supposed to point to an identical copy of the NTBS returned by what() called on the original object or not?

Second, is this Note intended to extend to all the derived classes in section 19? I.e., does the standard provide any guarantee for the effects of what() called on a copy of any of the derived class described in section 19?

Finally, if the answer to the first question is no, I believe it constitutes a defect since throwing an exception object typically implies invoking the copy ctor on the object. If the answer is yes, then I believe the standard ought to be clarified to spell out exactly what the effects are on the copy (i.e., after the copy ctor was called).

[Redmond: Yes, this is fuzzy. The issue of derived classes is fuzzy too.]

[ Batavia: Howard provided wording. ]

[ Bellevue: ]

> Eric concerned this is unimplementable, due to nothrow guarantees. Suggested implementation would involve reference counting.

> Is the implied reference counting subtle enough to call out a note on implementation? Probably not.

> If reference counting required, could we tighten specification further to require same pointer value? Probably an overspecification, especially if exception classes defer evalutation of final string to calls to what().

> Remember issue moved open and not resolved at Batavia, but cannot remember who objected to canvas a disenting opinion - please speak up if you disagree while reading these minutes!

> Move to Ready as we are accepting words unmodified.

[ Sophia Antipolis: ]

> The issue was pulled from Ready. It needs to make clear that only homogenous copying is intended to be supported, not coping from a derived to a base.

[ Batavia (2009-05): ]

> Howard supplied the following replacement wording for paragraph 7 of the proposed resolution:

> > -7- Postcondition: `what()` shall return the same NTBS as would be obtained by using `static_cast` to cast the rhs to the same types as the lhs and then calling `what()` on that possibly sliced object.

> Pete asks what "the same NTBS" means.

[ 2009-07-30 Niels adds: ]

> Further discussion in the thread starting with c++std-lib-24512.

[ 2009-09-24 Niels provided updated wording: ]

> I think the resolution should at least guarantee that the result of `what()` is independent of whether the compiler does copy-elision. And for any class derived from `std::excepion` that has a constructor that allows specifying a `what_arg`, it should make sure that the text of a user-provided `what_arg` is preserved, when the object is copied. Note that all the implementations I've tested already appear to satisfy the proposed resolution, including MSVC 2008 SP1, Apache stdcxx-4.2.1, GCC 4.1.2, GCC 4.3.2, and CodeGear C++ 6.13.

> The proposed resolution was updated with help from Daniel Krügler; the update aims to clarify that the proposed postcondition only applies to homogeneous copying.

[ 2009-10 Santa Cruz: ]

> Moved to Ready after inserting "publicly accessible" in two places.

Proposed resolution:

Change 18.8.1 [exception] to:

> -1- The class `exception` defines the base class for the types of objects thrown as exceptions by C++ standard library components, and certain expressions, to report errors detected during program execution.

> Each standard library class `T` that derives from class `exception` shall have a publicly accessible copy constructor and a publicly accessible copy assignment operator that do not exit with an exception. These member functions shall preserve the following postcondition: If two objects lhs and rhs both have dynamic type `T`, and lhs is a copy of rhs, then `strcmp(lhs.what(), rhs.what()) == 0`.

> ...

>     exception(const exception& rhs) throw();
>     exception& operator=(const exception& rhs) throw();

> > -4- Effects: Copies an exception object.

> > -5- Remarks: The effects of calling `what()` after assignment are implementation-defined.

> > -5- Postcondition: If `*this` and rhs both have dynamic type `exception` then `strcmp(what(), rhs.what()) == 0`.

### 473. underspecified `ctype` calls

Section: 22.4.1.1 [locale.ctype] Status: C++11 Submitter: Martin Sebor Opened: 2004-07-01 Last modified: 2015-04-08

View all issues with C++11 status.

Discussion:

Most `ctype` member functions come in two forms: one that operates on a single character at a time and another form that operates on a range of characters. Both forms are typically described by a single Effects and/or Returns clause.

The Returns clause of each of the single-character non-virtual forms suggests that the function calls the corresponding single character virtual function, and that the array form calls the corresponding virtual array form. Neither of the two forms of each virtual member function is required to be implemented in terms of the other.

There are three problems:

1. One is that while the standard does suggest that each non-virtual member function calls the corresponding form of the virtual function, it doesn't actually explicitly require it.

Implementations that cache results from some of the virtual member functions for some or all values of their arguments might want to call the array form from the non-array form the first time to fill the cache and avoid any or most subsequent virtual calls. Programs that rely on each form of the virtual function being called from the corresponding non-virtual function will see unexpected behavior when using such implementations.

2. The second problem is that either form of each of the virtual functions can be overridden by a user-defined function in a derived class to return a value that is different from the one produced by the virtual function of the alternate form that has not been overriden.

Thus, it might be possible for, say, `ctype::widen(c)` to return one value, while for `ctype::widen(&c, &c + 1, &wc)` to set `wc` to another value. This is almost certainly not intended. Both forms of every function should be required to return the same result for the same character, otherwise the same program using an implementation that calls one form of the functions will behave differently than when using another implementation that calls the other form of the function "under the hood."

3. The last problem is that the standard text fails to specify whether one form of any of the virtual functions is permitted to be implemented in terms of the other form or not, and if so, whether it is required or permitted to call the overridden virtual function or not.

Thus, a program that overrides one of the virtual functions so that it calls the other form which then calls the base member might end up in an infinite loop if the called form of the base implementation of the function in turn calls the other form.

Lillehammer: Part of this isn't a real problem. We already talk about caching. 22.1.1/6 But part is a real problem. `ctype` virtuals may call each other, so users don't know which ones to override to avoid avoid infinite loops.

This is a problem for all facet virtuals, not just `ctype` virtuals, so we probably want a blanket statement in clause 22 for all facets. The LWG is leaning toward a blanket prohibition, that a facet's virtuals may never call each other. We might want to do that in clause 27 too, for that matter. A review is necessary. Bill will provide wording.

[ 2009-07 Frankfurt, Howard provided wording directed by consensus. ]

[ 2009-10 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

Add paragraph 3 to 22.4 [locale.categories]:

> -3- Within this clause it is unspecified if one virtual function calls another virtual function.

Rationale:

We are explicitly not addressing bullet item #2, thus giving implementors more latitude. Users will have to override both virtual functions, not just one.

### 474. confusing Footnote 297

Section: 27.7.3.6.4 [ostream.inserters.character] Status: CD1 Submitter: Martin Sebor Opened: 2004-07-01 Last modified: 2015-04-08

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Discussion:

I think Footnote 297 is confused. The paragraph it applies to seems quite clear in that widen() is only called if the object is not a char stream (i.e., not basic_ostream<char>), so it's irrelevant what the value of widen(c) is otherwise.

Proposed resolution:

I propose to strike the Footnote.

### 475. May the function object passed to for_each modify the elements of the iterated sequence?

Section: 25.2.4 [alg.foreach] Status: CD1 Submitter: Stephan T. Lavavej, Jaakko Jarvi Opened: 2004-07-09 Last modified: 2015-04-08

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Discussion:

It is not clear whether the function object passed to for_each is allowed to modify the elements of the sequence being iterated over.

for_each is classified without explanation in [lib.alg.nonmodifying], "25.1 Non-modifying sequence operations". 'Non-modifying sequence operation' is never defined.

25(5) says: "If an algorithm's Effects section says that a value pointed to by any iterator passed as an argument is modified, then that algorithm has an additional type requirement: The type of that argument shall satisfy the requirements of a mutable iterator (24.1)."

for_each's Effects section does not mention whether arguments can be modified:

> "Effects: Applies f to the result of dereferencing every iterator in the range [first, last), starting from first and proceeding to last - 1."

Every other algorithm in [lib.alg.nonmodifying] is "really" non-modifying in the sense that neither the algorithms themselves nor the function objects passed to the algorithms may modify the sequences or elements in any way. This DR affects only for_each.

We suspect that for_each's classification in "non-modifying sequence operations" means that the algorithm itself does not inherently modify the sequence or the elements in the sequence, but that the function object passed to it may modify the elements it operates on.

The original STL document by Stepanov and Lee explicitly prohibited the function object from modifying its argument. The "obvious" implementation of for_each found in several standard library implementations, however, does not impose this restriction. As a result, we suspect that the use of for_each with function objects that modify their arguments is wide-spread. If the restriction was reinstated, all such code would become non-conforming. Further, none of the other algorithms in the Standard could serve the purpose of for_each (transform does not guarantee the order in which its function object is called).

We suggest that the standard be clarified to explicitly allow the function object passed to for_each modify its argument.

Proposed resolution:

Add a nonnormative note to the Effects in 25.2.4 [alg.foreach]: If the type of 'first' satisfies the requirements of a mutable iterator, 'f' may apply nonconstant functions through the dereferenced iterators passed to it.

Rationale:

The LWG believes that nothing in the standard prohibits function objects that modify the sequence elements. The problem is that for_each is in a secion entitled "nonmutating algorithms", and the title may be confusing. A nonnormative note should clarify that.

### 478. Should forward iterator requirements table have a line for r->m?

Section: 24.2.5 [forward.iterators] Status: CD1 Submitter: Dave Abrahams Opened: 2004-07-11 Last modified: 2015-04-08

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Duplicate of: 477

Discussion:

The Forward Iterator requirements table contains the following:

     expression  return type         operational  precondition
                                      semantics
      ==========  ==================  ===========  ==========================
      a->m        U& if X is mutable, (*a).m       pre: (*a).m is well-defined.
                  otherwise const U&

      r->m        U&                  (*r).m       pre: (*r).m is well-defined.

The second line may be unnecessary. Paragraph 11 of [lib.iterator.requirements] says:

> In the following sections, a and b denote values of type const X, n denotes a value of the difference type Distance, u, tmp, and m denote identifiers, r denotes a value of X&, t denotes a value of value type T, o denotes a value of some type that is writable to the output iterator.

Because operators can be overloaded on an iterator's const-ness, the current requirements allow iterators to make many of the operations specified using the identifiers a and b invalid for non-const iterators.

Related issue: 477

Proposed resolution:

Remove the "r->m" line from the Forward Iterator requirements table. Change

> "const X"

to

> "X or const X"

in paragraph 11 of [lib.iterator.requirements].

Rationale:

This is a defect because it constrains an lvalue to returning a modifiable lvalue.

### 482. Swapping pairs

Section: 20.3 [pairs], 20.4 [tuple] Status: Resolved Submitter: Andrew Koenig Opened: 2004-09-14 Last modified: 2015-04-08

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Discussion:

(Based on recent comp.std.c++ discussion)

Pair (and tuple) should specialize std::swap to work in terms of std::swap on their components. For example, there's no obvious reason why swapping two objects of type pair<vector<int>, list<double> > should not take O(1).

[Lillehammer: We agree it should be swappable. Howard will provide wording.]

[ Post Oxford: We got `swap` for `pair` but accidently missed `tuple`. `tuple::swap` is being tracked by 522. ]

Proposed resolution:

Wording provided in N1856.

Rationale:

Recommend NAD Resolved, fixed by N1856.

### 485. output iterator insufficiently constrained

Section: 24.2.4 [output.iterators] Status: Resolved Submitter: Chris Jefferson Opened: 2004-10-13 Last modified: 2015-04-08

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Discussion:

The note on 24.1.2 Output iterators insufficiently limits what can be performed on output iterators. While it requires that each iterator is progressed through only once and that each iterator is written to only once, it does not require the following things:

Note: Here it is assumed that `x` is an output iterator of type `X` which has not yet been assigned to.

a) That each value of the output iterator is written to: The standard allows: `++x; ++x; ++x;`

b) That assignments to the output iterator are made in order `X a(x); ++a; *a=1; *x=2;` is allowed

c) Chains of output iterators cannot be constructed: `X a(x); ++a; X b(a); ++b; X c(b); ++c;` is allowed, and under the current wording (I believe) `x,a,b,c` could be written to in any order.

I do not believe this was the intension of the standard?

[Lillehammer: Real issue. There are lots of constraints we intended but didn't specify. Should be solved as part of iterator redesign.]

[ 2009-07 Frankfurt ]

> Bill provided wording according to consensus.

[ 2009-07-21 Alisdair requests change from Review to Open. See thread starting with c++std-lib-24459 for discussion. ]

[ 2009-10 Santa Cruz: ]

> Modified wording. Set to Review.

[ 2009-10 Santa Cruz: ]

> Move to Ready after looking at again in a larger group in Santa Cruz.

[ 2010 Pittsburgh: ]

> Moved to NAD Editorial Resolved. Rationale added below.

Rationale:

Solved by N3066.

Proposed resolution:

Change Table 101 — Output iterator requirements in 24.2.4 [output.iterators]:

> Table 101 — Output iterator requirements
> Expression | Return type | Operational semantics | Assertion/note pre-/post-condition
> --- | --- | --- | ---
> `X(a)` |  |  | `a = t` is equivalent to `X(a) = t`. note: a destructor is assumed.
> `X u(a);` `X u = a;` |  |  |
> `*r = o` | result is not used |  | Post: `r` is not required to be dereferenceable. `r` is incrementable.
> `++r` | `X&` |  | `&r == &++r` Post: `r` is dereferenceable, unless otherwise specified. `r` is not required to be incrementable.
> `r++` | convertible to `const X&` | `{X tmp = r; ++r; return tmp;}` | Post: `r` is dereferenceable, unless otherwise specified. `r` is not required to be incrementable.
> `*r++ = o;` | result is not used |  |

### 488. rotate throws away useful information

Section: 25.3.11 [alg.rotate] Status: CD1 Submitter: Howard Hinnant Opened: 2004-11-22 Last modified: 2015-04-08

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Discussion:

rotate takes 3 iterators: first, middle and last which point into a sequence, and rearranges the sequence such that the subrange [middle, last) is now at the beginning of the sequence and the subrange [first, middle) follows. The return type is void.

In many use cases of rotate, the client needs to know where the subrange [first, middle) starts after the rotate is performed. This might look like:

      rotate(first, middle, last);
      Iterator i = advance(first, distance(middle, last));

Unless the iterators are random access, the computation to find the start of the subrange [first, middle) has linear complexity. However, it is not difficult for rotate to return this information with negligible additional computation expense. So the client could code:

      Iterator i = rotate(first, middle, last);

and the resulting program becomes significantly more efficient.

While the backwards compatibility hit with this change is not zero, it is very small (similar to that of lwg 130), and there is a significant benefit to the change.

Proposed resolution:

In 25 [algorithms] p2, change:

>       template<class ForwardIterator>
>         void ForwardIterator rotate(ForwardIterator first, ForwardIterator middle,
>                     ForwardIterator last);

In 25.3.11 [alg.rotate], change:

>       template<class ForwardIterator>
>         void ForwardIterator rotate(ForwardIterator first, ForwardIterator middle,
>                     ForwardIterator last);

In 25.3.11 [alg.rotate] insert a new paragraph after p1:

> Returns: `first + (last - middle)`.

[ The LWG agrees with this idea, but has one quibble: we want to make sure not to give the impression that the function "advance" is actually called, just that the nth iterator is returned. (Calling advance is observable behavior, since users can specialize it for their own iterators.) Howard will provide wording. ]

[Howard provided wording for mid-meeting-mailing Jun. 2005.]

[ Toronto: moved to Ready. ]

### 495. Clause 22 template parameter requirements

Section: 22 [localization] Status: CD1 Submitter: Beman Dawes Opened: 2005-01-10 Last modified: 2015-04-08

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Discussion:

It appears that there are no requirements specified for many of the template parameters in clause 22. It looks like this issue has never come up, except perhaps for Facet.

Clause 22 isn't even listed in 17.3.2.1 [lib.type.descriptions], either, which is the wording that allows requirements on template parameters to be identified by name.

So one issue is that 17.3.2.1 [lib.type.descriptions] Should be changed to cover clause 22. A better change, which will cover us in the future, would be to say that it applies to all the library clauses. Then if a template gets added to any library clause we are covered.

charT, InputIterator, and other names with requirements defined elsewhere are fine, assuming the 17.3.2.1 [lib.type.descriptions] fix. But there are a few template arguments names which I don't think have requirements given elsewhere:

- internT and externT. The fix is to add wording saying that internT and externT must meet the same requirements as template arguments named charT.
- stateT. I'm not sure about this one. There already is some wording, but it seems a bit vague.
- Intl. [lib.locale.moneypunct.byname] The fix for this one is to rename "Intl" to "International". The name is important because other text identifies the requirements for the name International but not for Intl.

Proposed resolution:

Change 17.5.2.1 [type.descriptions], paragraph 1, from:

> The Requirements subclauses may describe names that are used to specify constraints on template arguments.153) These names are used in clauses 20, 23, 25, and 26 to describe the types that may be supplied as arguments by a C++ program when instantiating template components from the library.

to:

> The Requirements subclauses may describe names that are used to specify constraints on template arguments.153) These names are used in library clauses to describe the types that may be supplied as arguments by a C++ program when instantiating template components from the library.

In the front matter of class 22, locales, add:

> Template parameter types internT and externT shall meet the requirements of charT (described in 21 [strings]).

Rationale:

Again, a blanket clause isn't blanket enough. Also, we've got a couple of names that we don't have blanket requirement statements for. The only issue is what to do about stateT. This wording is thin, but probably adequate.

### 496. Illegal use of "T" in vector<bool>

Section: 23.3.6 [vector] Status: CD1 Submitter: richard@ex-parrot.com Opened: 2005-02-10 Last modified: 2015-04-08

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Discussion:

In the synopsis of the std::vector<bool> specialisation in 23.3.6 [vector], the non-template assign() function has the signature

      void assign( size_type n, const T& t );

The type, T, is not defined in this context.

Proposed resolution:

Replace "T" with "value_type".

### 497. meaning of numeric_limits::traps for floating point types

Section: 18.3.2.4 [numeric.limits.members] Status: CD1 Submitter: Martin Sebor Opened: 2005-03-02 Last modified: 2015-04-08

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Discussion:

18.2.1.2, p59 says this much about the traps member of numeric_limits:

> static const bool traps;
> -59- true if trapping is implemented for the type.204)
> Footnote 204: Required by LIA-1.

It's not clear what is meant by "is implemented" here.

In the context of floating point numbers it seems reasonable to expect to be able to use traps to determine whether a program can "safely" use infinity(), quiet_NaN(), etc., in arithmetic expressions, that is without causing a trap (i.e., on UNIX without having to worry about getting a signal). When traps is true, I would expect any of the operations in section 7 of IEEE 754 to cause a trap (and my program to get a SIGFPE). So, for example, on Alpha, I would expect traps to be true by default (unless I compiled my program with the -ieee option), false by default on most other popular architectures, including IA64, MIPS, PA-RISC, PPC, SPARC, and x86 which require traps to be explicitly enabled by the program.

Another possible interpretation of p59 is that traps should be true on any implementation that supports traps regardless of whether they are enabled by default or not. I don't think such an interpretation makes the traps member very useful, even though that is how traps is implemented on several platforms. It is also the only way to implement traps on platforms that allow programs to enable and disable trapping at runtime.

Proposed resolution:

Change p59 to read:

> True if, at program startup, there exists a value of the type that would cause an arithmetic operation using that value to trap.

Rationale:

Real issue, since trapping can be turned on and off. Unclear what a static query can say about a dynamic issue. The real advice we should give users is to use cfenv for these sorts of queries. But this new proposed resolution is at least consistent and slightly better than nothing.

### 498. Requirements for partition() and stable_partition() too strong

Section: 25.3.13 [alg.partitions] Status: C++11 Submitter: Sean Parent, Joe Gottman Opened: 2005-05-04 Last modified: 2015-04-08

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Discussion:

Problem: The iterator requirements for partition() and stable_partition() [25.2.12] are listed as BidirectionalIterator, however, there are efficient algorithms for these functions that only require ForwardIterator that have been known since before the standard existed. The SGI implementation includes these (see http://www.sgi.com/tech/stl/partition.html and http://www.sgi.com/tech/stl/stable_partition.html).

[ 2009-04-30 Alisdair adds: ]

> Now we have concepts this is easier to express!

> Proposed resolution:

> Add the following signature to:

> Header `<algorithm>` synopsis [algorithms.syn]
> p3 Partitions 25.3.13 [alg.partitions]

> >      template<ForwardIterator Iter, Predicate<auto, Iter::value_type> Pred>
> >        requires ShuffleIterator<Iter>
> >              && CopyConstructible<Pred>
> >        Iter partition(Iter first, Iter last, Pred pred);

> Update p3 Partitions 25.3.13 [alg.partitions]:

> > Complexity: At most `(last - first)/2` swaps. Exactly `last - first` applications of the predicate are done. If `Iter` satisfies `BidirectionalIterator`, at most `(last - first)/2` swaps. Exactly `last - first` applications of the predicate are done.

> > If `Iter` merely satisfied `ForwardIterator` at most `(last - first)` swaps are done. Exactly `(last - first)` applications of the predicate are done.

> [Editorial note: I looked for existing precedent in how we might call out distinct overloads overloads from a set of constrained templates, but there is not much existing practice to lean on. advance/distance were the only algorithms I could find, and that wording is no clearer.]

[ 2009-07 Frankfurt ]

> Hinnant: if you want to partition your std::forward_list, you'll need partition() to accept ForwardIterators.

> No objection to Ready.

> Move to Ready.

Proposed resolution:

Change 25.2.12 from

>     template<class BidirectionalIterator, class Predicate>
>     BidirectionalIterator partition(BidirectionalIterato r first,
>                                     BidirectionalIterator last,
>                                     Predicate pred);

to

>     template<class ForwardIterator, class Predicate>
>     ForwardIterator partition(ForwardIterator first,
>                               ForwardIterator last,
>                               Predicate pred);

Change the complexity from

> At most (last - first)/2 swaps are done. Exactly (last - first) applications of the predicate are done.

to

> If ForwardIterator is a bidirectional_iterator, at most (last - first)/2 swaps are done; otherwise at most (last - first) swaps are done. Exactly (last - first) applications of the predicate are done.

Rationale:

Partition is a "foundation" algorithm useful in many contexts (like sorting as just one example) - my motivation for extending it to include forward iterators is foward_list - without this extension you can't partition an foward_list (without writing your own partition). Holes like this in the standard library weaken the argument for generic programming (ideally I'd be able to provide a library that would refine std::partition() to other concepts without fear of conflicting with other libraries doing the same - but that is a digression). I consider the fact that partition isn't defined to work for ForwardIterator a minor embarrassment.

[Mont Tremblant: Moved to Open, request motivation and use cases by next meeting. Sean provided further rationale by post-meeting mailing.]

### 505. Result_type in random distribution requirements

Section: 26.5.1 [rand.req], TR1 5.1.1 [tr.rand.req] Status: CD1 Submitter: Walter Brown Opened: 2005-07-03 Last modified: 2015-04-08

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Discussion:

Table 17: Random distribution requirements

Row 1 requires that each random distribution provide a nested type "input_type"; this type denotes the type of the values that the distribution consumes.

Inspection of all distributions in [tr.rand.dist] reveals that each distribution provides a second typedef ("result_type") that denotes the type of the values the distribution produces when called.

Proposed resolution:

It seems to me that this is also a requirement for all distributions and should therefore be indicated as such via a new second row to this table 17:

X::result_type | T | --- | compile-time

[ Berlin: Voted to WP. N1932 adopts the proposed resolution: see Table 5 row 1. ]

### 507. Missing requirement for variate_generator::operator()

Section: 26.5 [rand], TR1 5.1.3 [tr.rand.var] Status: CD1 Submitter: Walter Brown Opened: 2005-07-03 Last modified: 2015-04-08

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Discussion:

Paragraph 11 of [tr.rand.var] equires that the member template

>     template<class T> result_type operator() (T value);

return

>     distribution()(e, value)

However, not all distributions have an operator() with a corresponding signature.

[ Berlin: As a working group we voted in favor of N1932 which makes this moot: variate_generator has been eliminated. Then in full committee we voted to give this issue WP status (mistakenly). ]

Proposed resolution:

We therefore recommend that we insert the following precondition before paragraph 11:

> Precondition: `distribution().operator()(e,value)` is well-formed.

### 508. Bad parameters for ranlux64_base_01

Section: 26.5.5 [rand.predef], TR1 5.1.5 [tr.rand.predef] Status: CD1 Submitter: Walter Brown Opened: 2005-07-03 Last modified: 2015-04-08

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Discussion:

The fifth of these engines with predefined parameters, ranlux64_base_01, appears to have an unintentional error for which there is a simple correction. The two pre-defined subtract_with_carry_01 engines are given as:

>     typedef subtract_with_carry_01<float,  24, 10, 24> ranlux_base_01;
>     typedef subtract_with_carry_01<double, 48, 10, 24> ranlux64_base_01;

We demonstrate below that ranlux64_base_01 fails to meet the intent of the random number generation proposal, but that the simple correction to

>     typedef subtract_with_carry_01<double, 48,  5, 12> ranlux64_base_01;

does meet the intent of defining well-known good parameterizations.

The ranlux64_base_01 engine as presented fails to meet the intent for predefined engines, stated in proposal N1398 (section E):

> In order to make good random numbers available to a large number of library users, this proposal not only defines generic random-number engines, but also provides a number of predefined well-known good parameterizations for those.

The predefined ranlux_base_01 engine has been proven [1,2,3] to have a very long period and so meets this criterion. This property makes it suitable for use in the excellent discard_block engines defined subsequently. The proof of long period relies on the fact (proven in [1]) that 2**(w*r) - 2**(w*s) + 1 is prime (w, r, and s are template parameters to subtract_with_carry_01, as defined in [tr.rand.eng.sub1]).

The ranlux64_base_01 engine as presented in [tr.rand.predef] uses w=48, r=24, s=10. For these numbers, the combination 2**(w*r)-2**(w*s)+1 is non-prime (though explicit factorization would be a challenge). In consequence, while it is certainly possible for some seeding states that this engine would have a very long period, it is not at all "well-known" that this is the case. The intent in the N1398 proposal involved the base of the ranlux64 engine, which finds heavy use in the physics community. This is isomorphic to the predefined ranlux_base_01, but exploits the ability of double variables to hold (at least) 48 bits of mantissa, to deliver 48 random bits at a time rather than 24.

Proposed resolution:

To achieve this intended behavior, the correct template parameteriztion would be:

>     typedef subtract_with_carry_01<double, 48, 5, 12> ranlux64_base_01;

The sequence of mantissa bits delivered by this is isomorphic (treating each double as having the bits of two floats) to that delivered by ranlux_base_01.

References:

1. F. James, Comput. Phys. Commun. 60(1990) 329
2. G. Marsaglia and A. Zaman, Ann. Appl. Prob 1(1991) 462
3. M. Luscher, Comput. Phys. Commun. 79(1994) 100-110

[ Berlin: Voted to WP. N1932 adopts the proposed resolution in 26.3.5, just above paragraph 5. ]

### 518. Are insert and erase stable for unordered_multiset and unordered_multimap?

Section: 23.2.5 [unord.req], TR1 6.3.1 [tr.unord.req] Status: CD1 Submitter: Matt Austern Opened: 2005-07-03 Last modified: 2015-04-08

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Discussion:

Issue 371 deals with stability of multiset/multimap under insert and erase (i.e. do they preserve the relative order in ranges of equal elements). The same issue applies to unordered_multiset and unordered_multimap.

[ Moved to open (from review): There is no resolution. ]

[ Toronto: We have a resolution now. Moved to Review. Some concern was noted as to whether this conflicted with existing practice or not. An additional concern was in specifying (partial) ordering for an unordered container. ]

Proposed resolution:

Wording for the proposed resolution is taken from the equivalent text for associative containers.

Change 23.2.5 [unord.req], Unordered associative containers, paragraph 6 to:

> An unordered associative container supports unique keys if it may contain at most one element for each key. Otherwise, it supports equivalent keys. `unordered_set` and `unordered_map` support unique keys. `unordered_multiset` and `unordered_multimap` support equivalent keys. In containers that support equivalent keys, elements with equivalent keys are adjacent to each other. For `unordered_multiset` and `unordered_multimap`,`insert` and `erase` preserve the relative ordering of equivalent elements.

Change 23.2.5 [unord.req], Unordered associative containers, paragraph 8 to:

> The elements of an unordered associative container are organized into buckets. Keys with the same hash code appear in the same bucket. The number of buckets is automatically increased as elements are added to an unordered associative container, so that the average number of elements per bucket is kept below a bound. Rehashing invalidates iterators, changes ordering between elements, and changes which buckets elements appear in, but does not invalidate pointers or references to elements. For `unordered_multiset` and `unordered_multimap`, rehashing preserves the relative ordering of equivalent elements.

### 519. Data() undocumented

Section: 23.3.2 [array], TR1 6.2.2 [tr.array.array] Status: CD1 Submitter: Pete Becker Opened: 2005-07-03 Last modified: 2015-04-08

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Discussion:

`array<>::data()` is present in the class synopsis, but not documented.

Proposed resolution:

Add a new section, after 6.2.2.3:

>     T*       data()
>     const T* data() const;

Returns: `elems`.

Change 6.2.2.4/2 to:

> In the case where `N == 0`, `begin() == end()`. The return value of `data()` is unspecified.

### 520. Result_of and pointers to data members

Section: 20.9.10 [func.bind], TR1 3.6 [tr.func.bind] Status: CD1 Submitter: Pete Becker Opened: 2005-07-03 Last modified: 2015-04-08

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Discussion:

In the original proposal for binders, the return type of bind() when called with a pointer to member data as it's callable object was defined to be mem_fn(ptr); when Peter Dimov and I unified the descriptions of the TR1 function objects we hoisted the descriptions of return types into the INVOKE pseudo-function and into result_of. Unfortunately, we left pointer to member data out of result_of, so bind doesn't have any specified behavior when called with a pointer to member data.

Proposed resolution:

[ Pete and Peter will provide wording. ]

In 20.5.4 [lib.func.ret] ([tr.func.ret]) p3 add the following bullet after bullet 2:

1. […]
2. […]
3. If `F` is a member data pointer type `R T::*`, `type` shall be `cv R&` when `T1` is `cv U1&`, `R` otherwise.

[ Peter provided wording. ]

### 521. Garbled requirements for argument_type in reference_wrapper

Section: 20.9.4 [refwrap], TR1 2.1.2 [tr.util.refwrp.refwrp] Status: CD1 Submitter: Pete Becker Opened: 2005-07-03 Last modified: 2015-04-08

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Discussion:

2.1.2/3, second bullet item currently says that reference_wrapper<T> is derived from unary_function<T, R> if T is:

> a pointer to member function type with cv-qualifier cv and no arguments; the type T1 is cv T* and R is the return type of the pointer to member function;

The type of T1 can't be cv T*, 'cause that's a pointer to a pointer to member function. It should be a pointer to the class that T is a pointer to member of. Like this:

> a pointer to a member function R T0::f() cv (where cv represents the member function's cv-qualifiers); the type T1 is cv T0*

Similarly, bullet item 2 in 2.1.2/4 should be:

> a pointer to a member function R T0::f(T2) cv (where cv represents the member function's cv-qualifiers); the type T1 is cv T0*

Proposed resolution:

Change bullet item 2 in 2.1.2/3:

> - a pointer to member function type with cv-qualifier `cv` and no arguments; the type `T1` is `cv T*` and `R` is the return type of the pointer to member function `R T0::f() cv` (where `cv` represents the member function's cv-qualifiers); the type `T1` is `cv T0*`

Change bullet item 2 in 2.1.2/4:

> - a pointer to member function with cv-qualifier `cv` and taking one argument of type `T2`; the type `T1` is `cv T*` and `R` is the return type of the pointer to member function `R T0::f(T2) cv` (where `cv` represents the member function's cv-qualifiers); the type `T1` is `cv T0*`

### 522. Tuple doesn't define swap

Section: 20.4 [tuple], TR1 6.1 [tr.tuple] Status: CD1 Submitter: Andy Koenig Opened: 2005-07-03 Last modified: 2015-04-08

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Discussion:

Tuple doesn't define swap(). It should.

[ Berlin: Doug to provide wording. ]

[ Batavia: Howard to provide wording. ]

[ Toronto: Howard to provide wording (really this time). ]

[ Bellevue: Alisdair provided wording. ]

Proposed resolution:

Add these signatures to 20.4 [tuple]

>     template <class... Types>
>       void swap(tuple<Types...>& x, tuple<Types...>& y);
>     template <class... Types>
>       void swap(tuple<Types...>&& x, tuple<Types...>& y);
>     template <class... Types>
>       void swap(tuple<Types...>& x, tuple<Types...>&& y);

Add this signature to 20.4.2 [tuple.tuple]

>     void swap(tuple&&);

Add the following two sections to the end of the tuple clauses

> 20.3.1.7 tuple swap [tuple.swap]

>     void swap(tuple&& rhs);

> > Requires: Each type in `Types` shall be `Swappable`.

> > Effects: Calls `swap` for each element in `*this` and its corresponding element in `rhs`.

> > Throws: Nothing, unless one of the element-wise `swap` calls throw an exception.

> 20.3.1.8 tuple specialized algorithms [tuple.special]

>     template <class... Types>
>       void swap(tuple<Types...>& x, tuple<Types...>& y);
>     template <class... Types>
>       void swap(tuple<Types...>&& x, tuple<Types...>& y);
>     template <class... Types>
>       void swap(tuple<Types...>& x, tuple<Types...>&& y);

> > Effects: x.swap(y)

### 524. regex named character classes and case-insensitivity don't mix

Section: 28 [re] Status: CD1 Submitter: Eric Niebler Opened: 2005-07-01 Last modified: 2015-04-08

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Discussion:

This defect is also being discussed on the Boost developers list. The full discussion can be found here: http://lists.boost.org/boost/2005/07/29546.php

-- Begin original message --

Also, I may have found another issue, closely related to the one under discussion. It regards case-insensitive matching of named character classes. The regex_traits<> provides two functions for working with named char classes: lookup_classname and isctype. To match a char class such as [[:alpha:]], you pass "alpha" to lookup_classname and get a bitmask. Later, you pass a char and the bitmask to isctype and get a bool yes/no answer.

But how does case-insensitivity work in this scenario? Suppose we're doing a case-insensitive match on [[:lower:]]. It should behave as if it were [[:lower:][:upper:]], right? But there doesn't seem to be enough smarts in the regex_traits interface to do this.

Imagine I write a traits class which recognizes [[:fubar:]], and the "fubar" char class happens to be case-sensitive. How is the regex engine to know that? And how should it do a case-insensitive match of a character against the [[:fubar:]] char class? John, can you confirm this is a legitimate problem?

I see two options:

1) Add a bool icase parameter to lookup_classname. Then, lookup_classname( "upper", true ) will know to return lower|upper instead of just upper.

2) Add a isctype_nocase function

I prefer (1) because the extra computation happens at the time the pattern is compiled rather than when it is executed.

-- End original message --

For what it's worth, John has also expressed his preference for option (1) above.

Proposed resolution:

Adopt the proposed resolution in N2409.

[ Kona (2007): The LWG adopted the proposed resolution of N2409 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 525. type traits definitions not clear

Section: 20.10.4 [meta.unary], TR1 4.5 [tr.meta.unary] Status: Resolved Submitter: Robert Klarer Opened: 2005-07-11 Last modified: 2015-04-08

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Discussion:

It is not completely clear how the primary type traits deal with cv-qualified types. And several of the secondary type traits seem to be lacking a definition.

[ Berlin: Howard to provide wording. ]

Proposed resolution:

Wording provided in N2028. A revision (N2157) provides more detail for motivation.

Rationale:

Solved by revision (N2157) in the WP.

### 527. `tr1::bind` has lost its Throws clause

Section: 20.9.10.3 [func.bind.bind], TR1 3.6.3 [tr.func.bind.bind] Status: CD1 Submitter: Peter Dimov Opened: 2005-10-01 Last modified: 2015-04-08

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Discussion:

The original bind proposal gives the guarantee that `tr1::bind(f, t1, ..., tN)` does not throw when the copy constructors of `f, t1, ..., tN` don't.

This guarantee is not present in the final version of TR1.

I'm pretty certain that we never removed it on purpose. Editorial omission? :-)

[ Berlin: not quite editorial, needs proposed wording. ]

[ Batavia: Doug to translate wording to variadic templates. ]

[ Toronto: We agree but aren't quite happy with the wording. The "t"'s no longer refer to anything. Alan to provide improved wording. ]

[ Pre-Bellevue: Alisdair provided wording. ]

TR1 proposed resolution:

> In TR1 3.6.3 [tr.func.bind.bind], add a new paragraph after p2:

> > Throws: Nothing unless one of the copy constructors of `f, t1, t2, ..., tN` throws an exception.

> Add a new paragraph after p4:

> > Throws: nothing unless one of the copy constructors of `f, t1, t2, ..., tN` throws an exception.

Proposed resolution:

In 20.9.10.3 [func.bind.bind], add a new paragraph after p2:

> Throws: Nothing unless the copy constructor of `F` or of one of the types in the `BoundArgs...` pack expansion throws an exception.

In 20.9.10.3 [func.bind.bind], add a new paragraph after p4:

> Throws: Nothing unless the copy constructor of `F` or of one of the types in the `BoundArgs...` pack expansion throws an exception.

### 530. Must elements of a string be contiguous?

Section: 21.4 [basic.string] Status: CD1 Submitter: Matt Austern Opened: 2005-11-15 Last modified: 2015-04-08

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Discussion:

Issue 69, which was incorporated into C++03, mandated that the elements of a vector must be stored in contiguous memory. Should the same also apply to `basic_string`?

We almost require contiguity already. Clause 23.4.7 [multiset] defines `operator[]` as `data()[pos]`. What's missing is a similar guarantee if we access the string's elements via the iterator interface.

Given the existence of `data()`, and the definition of `operator[]` and `at` in terms of `data`, I don't believe it's possible to write a useful and standard- conforming `basic_string` that isn't contiguous. I'm not aware of any non-contiguous implementation. We should just require it.

Proposed resolution:

Add the following text to the end of 21.4 [basic.string], paragraph 2.

> The characters in a string are stored contiguously, meaning that if `s` is a `basic_string<charT, Allocator>`, then it obeys the identity `&*(s.begin() + n) == &*s.begin() + n` for all `0 <= n < s.size()`.

Rationale:

Not standardizing this existing practice does not give implementors more freedom. We thought it might a decade ago. But the vendors have spoken both with their implementations, and with their voice at the LWG meetings. The implementations are going to be contiguous no matter what the standard says. So the standard might as well give string clients more design choices.

### 531. array forms of unformatted input functions

Section: 27.7.2.3 [istream.unformatted] Status: CD1 Submitter: Martin Sebor Opened: 2005-11-23 Last modified: 2015-04-08

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Discussion:

The array forms of unformatted input functions don't seem to have well-defined semantics for zero-element arrays in a couple of cases. The affected ones (`istream::get()` and `istream::getline()`) are supposed to terminate when `(n - 1)` characters are stored, which obviously can never be true when `(n == 0)` holds to start with. See c++std-lib-16071.

Proposed resolution:

I suggest changing 27.6.1.3, p7 (`istream::get()`), bullet 1 to read:

- `(n < 1)` is true or `(n - 1)` characters are stored;

Change 27.6.1.3, p9:

> If the function stores no characters, it calls `setstate(failbit)` (which may throw `ios_base::failure` (27.4.4.3)). In any case, if `(n > 0)` is true it then stores a null character into the next successive location of the array.

and similarly p17 (`istream::getline()`), bullet 3 to:

- `(n < 1)` is true or `(n - 1)` characters are stored (in which case the function calls `setstate(failbit)`).

In addition, to clarify that `istream::getline()` must not store the terminating NUL character unless the the array has non-zero size, Robert Klarer suggests in c++std-lib-16082 to change 27.6.1.3, p20 to read:

> In any case, provided `(n > 0)` is true, it then stores a null character (using charT()) into the next successive location of the array.

[ post-Redmond: Pete noticed that the current resolution for `get` requires writing to out of bounds memory when `n == 0`. Martin provided fix. ]

### 533. typo in 2.2.3.10/1

Section: 20.8.2.2.10 [util.smartptr.getdeleter], TR1 2.2.3.10 [tr.util.smartptr.getdeleter] Status: CD1 Submitter: Paolo Carlini Opened: 2005-11-09 Last modified: 2015-04-08

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Discussion:

I'm seeing something that looks like a typo. The Return of `get_deleter` says:

> If `*this` owns a deleter `d`...

but `get_deleter` is a free function!

Proposed resolution:

Therefore, I think should be:

> If `*this p` owns a deleter `d`...

### 534. Missing basic_string members

Section: 21.4 [basic.string] Status: CD1 Submitter: Alisdair Meredith Opened: 2005-11-16 Last modified: 2015-04-08

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Discussion:

OK, we all know std::basic_string is bloated and already has way too many members. However, I propose it is missing 3 useful members that are often expected by users believing it is a close approximation of the container concept. All 3 are listed in table 71 as 'optional'

i/ pop_back.

This is the one I feel most strongly about, as I only just discovered it was missing as we are switching to a more conforming standard library <g>

I find it particularly inconsistent to support push_back, but not pop_back.

ii/ back.

There are certainly cases where I want to examine the last character of a string before deciding to append, or to trim trailing path separators from directory names etc. *rbegin() somehow feels inelegant.

iii/ front

This one I don't feel strongly about, but if I can get the first two, this one feels that it should be added as a 'me too' for consistency.

I believe this would be similarly useful to the data() member recently added to vector, or at() member added to the maps.

Proposed resolution:

Add the following members to definition of class template basic_string, 21.3p7

>     void pop_back ()

>     const charT & front() const
>     charT & front()

>     const charT & back() const
>     charT & back()

Add the following paragraphs to basic_string description

21.3.4p5

>     const charT & front() const
>     charT & front()

> Precondition: `!empty()`

> Effects: Equivalent to `operator[](0)`.

21.3.4p6

>     const charT & back() const
>     charT & back()

> Precondition: `!empty()`

> Effects: Equivalent to `operator[]( size() - 1)`.

21.3.5.5p10

>     void pop_back ()

> Precondition: `!empty()`

> Effects: Equivalent to `erase( size() - 1, 1 )`.

Update Table 71: (optional sequence operations) Add basic_string to the list of containers for the following operations.

>     a.front()
>     a.back()
>     a.push_back()
>     a.pop_back()
>     a[n]

[ Berlin: Has support. Alisdair provided wording. ]

### 535. std::string::swap specification poorly worded

Section: 21.4.6.8 [string::swap] Status: CD1 Submitter: Beman Dawes Opened: 2005-12-14 Last modified: 2015-04-08

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Discussion:

std::string::swap currently says for effects and postcondition:

> Effects: Swaps the contents of the two strings.

> Postcondition: `*this` contains the characters that were in `s`, `s` contains the characters that were in `*this`.

Specifying both Effects and Postcondition seems redundant, and the postcondition needs to be made stronger. Users would be unhappy if the characters were not in the same order after the swap.

Proposed resolution:

> Effects: Swaps the contents of the two strings.

> Postcondition: `*this` contains the same sequence of characters that were was in `s`, `s` contains the same sequence of characters that were was in `*this`.

### 537. Typos in the signatures in 27.6.1.3/42-43 and 27.6.2.4

Section: 27.7.2.3 [istream.unformatted] Status: CD1 Submitter: Paolo Carlini Opened: 2006-02-12 Last modified: 2015-04-08

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Discussion:

In the most recent working draft, I'm still seeing:

>     seekg(off_type& off, ios_base::seekdir dir)

and

>     seekp(pos_type& pos)

>     seekp(off_type& off, ios_base::seekdir dir)

that is, by reference off and pos arguments.

Proposed resolution:

After 27.6.1.3p42 change:

>     basic_istream<charT,traits>& seekg(off_type& off, ios_base::seekdir dir);

After 27.6.2.4p1 change:

>     basic_ostream<charT,traits>& seekp(pos_type& pos);

After 27.6.2.4p3 change:

>     basic_ostream<charT,traits>& seekp(off_type& off, ios_base::seekdir dir);

### 538. 241 again: Does unique_copy() require CopyConstructible and Assignable?

Section: 25.3.9 [alg.unique] Status: CD1 Submitter: Howard Hinnant Opened: 2006-02-09 Last modified: 2015-04-08

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Discussion:

I believe I botched the resolution of 241 "Does unique_copy() require CopyConstructible and Assignable?" which now has WP status.

This talks about `unique_copy` requirements and currently reads:

> -5- Requires: The ranges `[first, last)` and `[result, result+(last-first))` shall not overlap. The expression `*result = *first` shall be valid. If neither `InputIterator` nor `OutputIterator` meets the requirements of forward iterator then the value type of `InputIterator` must be CopyConstructible (20.1.3). Otherwise CopyConstructible is not required.

The problem (which Paolo discovered) is that when the iterators are at their most restrictive (`InputIterator`, `OutputIterator`), then we want `InputIterator::value_type` to be both `CopyConstructible` and `CopyAssignable` (for the most efficient implementation). However this proposed resolution only makes it clear that it is `CopyConstructible`, and that one can assign from `*first` to `*result`. This latter requirement does not necessarily imply that you can:

>     *first = *first;

Proposed resolution:

> -5- Requires: The ranges `[first, last)` and `[result, result+(last-first))` shall not overlap. The expression `*result = *first` shall be valid. If neither `InputIterator` nor `OutputIterator` meets the requirements of forward iterator then the value type `value_type` of `InputIterator` must be CopyConstructible (20.1.3) and Assignable. Otherwise CopyConstructible is not required.

### 539. `partial_sum` and `adjacent_difference` should mention requirements

Section: 26.7.4 [partial.sum] Status: C++11 Submitter: Marc Schoolderman Opened: 2006-02-06 Last modified: 2015-04-08

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Discussion:

There are some problems in the definition of `partial_sum` and `adjacent_difference` in 26.4 [lib.numeric.ops]

Unlike `accumulate` and `inner_product`, these functions are not parametrized on a "type T", instead, 26.4.3 [lib.partial.sum] simply specifies the effects clause as;

> Assigns to every element referred to by iterator `i` in the range `[result,result + (last - first))` a value correspondingly equal to

> >     ((...(* first + *( first + 1)) + ...) + *( first + ( i - result )))

And similarly for BinaryOperation. Using just this definition, it seems logical to expect that:

>     char i_array[4] = { 100, 100, 100, 100 };
>     int  o_array[4];

>     std::partial_sum(i_array, i_array+4, o_array);

Is equivalent to

>     int o_array[4] = { 100, 100+100, 100+100+100, 100+100+100+100 };

i.e. 100, 200, 300, 400, with addition happening in the `result type`, `int`.

Yet all implementations I have tested produce 100, -56, 44, -112, because they are using an accumulator of the `InputIterator`'s `value_type`, which in this case is `char`, not `int`.

The issue becomes more noticeable when the result of the expression `*i + *(i+1)` or `binary_op(*i, *i-1)` can't be converted to the `value_type`. In a contrived example:

>     enum not_int { x = 1, y = 2 };
>     ...
>     not_int e_array[4] = { x, x, y, y };
>     std::partial_sum(e_array, e_array+4, o_array);

Is it the intent that the operations happen in the `input type`, or in the `result type`?

If the intent is that operations happen in the `result type`, something like this should be added to the "Requires" clause of 26.4.3/4 [lib.partial.sum]:

> The type of `*i + *(i+1)` or `binary_op(*i, *(i+1))` shall meet the requirements of `CopyConstructible` (20.1.3) and `Assignable` (23.1) types.

(As also required for `T` in 26.4.1 [lib.accumulate] and 26.4.2 [lib.inner.product].)

The "auto initializer" feature proposed in N1894 is not required to implement `partial_sum` this way. The 'narrowing' behaviour can still be obtained by using the `std::plus<>` function object.

If the intent is that operations happen in the `input type`, then something like this should be added instead;

> The type of *first shall meet the requirements of `CopyConstructible` (20.1.3) and `Assignable` (23.1) types. The result of `*i + *(i+1)` or `binary_op(*i, *(i+1))` shall be convertible to this type.

The 'widening' behaviour can then be obtained by writing a custom proxy iterator, which is somewhat involved.

In both cases, the semantics should probably be clarified.

26.4.4 [lib.adjacent.difference] is similarly underspecified, although all implementations seem to perform operations in the 'result' type:

>     unsigned char i_array[4] = { 4, 3, 2, 1 };
>     int o_array[4];

>     std::adjacent_difference(i_array, i_array+4, o_array);

o_array is 4, -1, -1, -1 as expected, not 4, 255, 255, 255.

In any case, `adjacent_difference` doesn't mention the requirements on the `value_type`; it can be brought in line with the rest of 26.4 [lib.numeric.ops] by adding the following to 26.4.4/2 [lib.adjacent.difference]:

> The type of `*first` shall meet the requirements of `CopyConstructible` (20.1.3) and `Assignable` (23.1) types."

[ Berlin: Giving output iterator's value_types very controversial. Suggestion of adding signatures to allow user to specify "accumulator". ]

[ Bellevue: ]

> The intent of the algorithms is to perform their calculations using the type of the input iterator. Proposed wording provided.

[ Sophia Antipolis: ]

> We did not agree that the proposed resolution was correct. For example, when the arguments are types `(float*, float*, double*)`, the highest-quality solution would use double as the type of the accumulator. If the intent of the wording is to require that the type of the accumulator must be the `input_iterator`'s `value_type`, the wording should specify it.

[ 2009-05-09 Alisdair adds: ]

> Now that we have the facility, the 'best' accumulator type could probably be deduced as:

> >     std::common_type<InIter::value_type, OutIter::reference>::type

> This type would then have additional requirements of constructability and incrementability/assignability.

> If this extracting an accumulator type from a pair/set of iterators (with additional requirements on that type) is a problem for multiple functions, it might be worth extracting into a SharedAccumulator concept or similar.

> I'll go no further in writing up wording now, until the group gives a clearer indication of preferred direction.

[ 2009-07 Frankfurt ]

> The proposed resolution isn't quite right. For example, "the type of `*first`" should be changed to "`iterator::value_type`" or similar. Daniel volunteered to correct the wording.

[ 2009-07-29 Daniel corrected wording. ]

[ 2009-10 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

1. Change 26.7.4 [partial.sum]/1 as indicated:

   > Effects: Let `VT` be `InputIterator`'s value type. For a nonempty range, initializes an accumulator `acc` of type `VT` with `*first` and performs `*result = acc`. For every iterator `i` in `[first + 1, last)` in order, `acc` is then modified by `acc = acc + *i` or `acc = binary_op(acc, *i)` and is assigned to `*(result + (i - first))`. Assigns to every element referred to by iterator `i` in the range `[result,result + (last - first))` a value correspondingly equal to

   > >     ((...(*first + *(first + 1)) + ...) + *(first + (i - result)))

   > or

   > >     binary_op(binary_op(...,
   > >        binary_op(*first, *(first + 1)),...), *(first + (i - result)))

2. Change 26.7.4 [partial.sum]/3 as indicated:

   > Complexity: Exactly `max((last - first) - 1, 0)` applications of `binary_op` the binary operation.

3. Change 26.7.4 [partial.sum]/4 as indicated:

   > Requires: `VT` shall be constructible from the type of `*first`, the result of `acc + *i` or `binary_op(acc, *i)` shall be implicitly convertible to `VT`, and the result of the expression `acc` shall be writable to the `result` output iterator. In the ranges `[first,last]` and `[result,result + (last - first)]` [..]

4. Change 26.7.5 [adjacent.difference]/1 as indicated:

   > Effects: Let `VT` be `InputIterator`'s value type. For a nonempty range, initializes an accumulator `acc` of type `VT` with `*first` and performs `*result = acc`. For every iterator `i` in `[first + 1, last)` in order, initializes a value `val` of type `VT` with `*i`, assigns the result of `val - acc` or `binary_op(val, acc)` to `*(result + (i - first))` and modifies `acc = std::move(val)`. Assigns to every element referred to by iterator `i` in the range `[result + 1, result + (last - first))` a value correspondingly equal to

   > >     *(first + (i - result)) - *(first + (i - result) - 1)

   > or

   > >     binary_op(*(first + (i - result)), *(first + (i - result) - 1)).

   > result gets the value of *first.

5. Change 26.7.5 [adjacent.difference]/2 as indicated:

   > Requires: `VT` shall be `MoveAssignable` ([moveassignable]) and shall be constructible from the type of `*first`. The result of the expression `acc` and the result of the expression `val - acc` or `binary_op(val, acc)` shall be writable to the `result` output iterator. In the ranges `[first,last]` [..]

6. Change 26.7.5 [adjacent.difference]/5 as indicated:

   > Complexity: Exactly `max((last - first) - 1, 0)` applications of `binary_op` the binary operation.

### 540. shared_ptr<void>::operator*()

Section: 20.8.2.2.5 [util.smartptr.shared.obs], TR1 2.2.3.5 [tr.util.smartptr.shared.obs] Status: CD1 Submitter: Martin Sebor Opened: 2005-10-15 Last modified: 2015-04-08

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Discussion:

I'm trying to reconcile the note in tr.util.smartptr.shared.obs, p6 that talks about the operator*() member function of shared_ptr:

> Notes: When T is void, attempting to instantiate this member function renders the program ill-formed. [Note: Instantiating shared_ptr<void> does not necessarily result in instantiating this member function. --end note]

with the requirement in temp.inst, p1:

> The implicit instantiation of a class template specialization causes the implicit instantiation of the declarations, but not of the definitions...

I assume that what the note is really trying to say is that "instantiating shared_ptr<void> *must not* result in instantiating this member function." That is, that this function must not be declared a member of shared_ptr<void>. Is my interpretation correct?

Proposed resolution:

Change 2.2.3.5p6

> -6- Notes: When `T` is `void`, attempting to instantiate this member function renders the program ill-formed. [Note: Instantiating `shared_ptr<void>` does not necessarily result in instantiating this member function. --end note] it is unspecified whether this member function is declared or not, and if so, what its return type is, except that the declaration (although not necessarily the definition) of the function shall be well-formed.

### 541. shared_ptr template assignment and void

Section: 20.8.2.2 [util.smartptr.shared], TR1 2.2.3 [tr.util.smartptr.shared] Status: CD1 Submitter: Martin Sebor Opened: 2005-10-16 Last modified: 2015-04-08

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Discussion:

Is the void specialization of the template assignment operator taking a shared_ptr<void> as an argument supposed be well-formed?

I.e., is this snippet well-formed:

>     shared_ptr<void> p;
>     p.operator=<void>(p);

Gcc complains about auto_ptr<void>::operator*() returning a reference to void. I suspect it's because shared_ptr has two template assignment operators, one of which takes auto_ptr, and the auto_ptr template gets implicitly instantiated in the process of overload resolution.

The only way I see around it is to do the same trick with auto_ptr<void> operator*() as with the same operator in shared_ptr<void>.

PS Strangely enough, the EDG front end doesn't mind the code, even though in a small test case (below) I can reproduce the error with it as well.

>     template <class T>
>     struct A { T& operator*() { return *(T*)0; } };

>     template <class T>
>     struct B {
>         void operator= (const B&) { }
>         template <class U>
>         void operator= (const B<U>&) { }
>         template <class U>
>         void operator= (const A<U>&) { }
>     };

>     int main ()
>     {
>         B<void> b;
>         b.operator=<void>(b);
>     }

Proposed resolution:

In [lib.memory] change:

>     template<class X> class auto_ptr;
>     template<> class auto_ptr<void>;

In [lib.auto.ptr]/2 add the following before the last closing brace:

>     template<> class auto_ptr<void>
>     {
>     public:
>         typedef void element_type;
>     };

### 542. shared_ptr observers

Section: 20.8.2.2.5 [util.smartptr.shared.obs], TR1 2.2.3.5 [tr.util.smartptr.shared.obs] Status: CD1 Submitter: Martin Sebor Opened: 2005-10-18 Last modified: 2015-04-08

View all other issues in [util.smartptr.shared.obs].

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Discussion:

Peter Dimov wrote: To: C++ libraries mailing list Message c++std-lib-15614 [...] The intent is for both use_count() and unique() to work in a threaded environment. They are intrinsically prone to race conditions, but they never return garbage.

This is a crucial piece of information that I really wish were captured in the text. Having this in a non-normative note would have made everything crystal clear to me and probably stopped me from ever starting this discussion :) Instead, the sentence in p12 "use only for debugging and testing purposes, not for production code" very strongly suggests that implementations can and even are encouraged to return garbage (when threads are involved) for performance reasons.

How about adding an informative note along these lines:

> Note: Implementations are encouraged to provide well-defined behavior for use_count() and unique() even in the presence of multiple threads.

I don't necessarily insist on the exact wording, just that we capture the intent.

Proposed resolution:

Change 20.8.2.2.5 [util.smartptr.shared.obs] p12:

> [Note: `use_count()` is not necessarily efficient. Use only for debugging and testing purposes, not for production code. --end note]

Change 20.8.2.3.5 [util.smartptr.weak.obs] p3:

> [Note: `use_count()` is not necessarily efficient. Use only for debugging and testing purposes, not for production code. --end note]

### 543. valarray slice default constructor

Section: 26.6.4 [class.slice] Status: CD1 Submitter: Howard Hinnant Opened: 2005-11-03 Last modified: 2015-04-08

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Discussion:

If one explicitly constructs a slice or glice with the default constructor, does the standard require this slice to have any usable state? It says "creates a slice which specifies no elements", which could be interpreted two ways:

1. There are no elements to which the slice refers (i.e. undefined).
2. The slice specifies an array with no elements in it (i.e. defined).

Here is a bit of code to illustrate:

>     #include <iostream>
>     #include <valarray>

>     int main()
>     {
>         std::valarray<int> v(10);
>         std::valarray<int> v2 = v[std::slice()];
>         std::cout << "v[slice()].size() = " << v2.size() << '\n';
>     }

Is the behavior undefined? Or should the output be:

>     v[slice()].size() = 0

There is a similar question and wording for gslice at 26.3.6.1p1.

Proposed resolution:

[Martin suggests removing the second sentence in 26.6.4.2 [cons.slice] as well.]

Change 26.6.4.2 [cons.slice]:

> 1 - The default constructor for `slice` creates a `slice` which specifies no elements. The default constructor is equivalent to `slice(0, 0, 0)`. A default constructor is provided only to permit the declaration of arrays of slices. The constructor with arguments for a slice takes a start, length, and stride parameter.

Change 26.6.6.2 [gslice.cons]:

> 1 - The default constructor creates a `gslice` which specifies no elements. The default constructor is equivalent to `gslice(0, valarray<size_t>(), valarray<size_t>())`. The constructor with arguments builds a `gslice` based on a specification of start, lengths, and strides, as explained in the previous section.

### 545. When is a deleter deleted?

Section: 20.8.2.2.10 [util.smartptr.getdeleter], TR1 2.2.3.2 [tr.util.smartptr.shared.dest] Status: CD1 Submitter: Matt Austern Opened: 2006-01-10 Last modified: 2015-04-08

View all other issues in [util.smartptr.getdeleter].

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Discussion:

The description of ~shared_ptr doesn't say when the shared_ptr's deleter, if any, is destroyed. In principle there are two possibilities: it is destroyed unconditionally whenever ~shared_ptr is executed (which, from an implementation standpoint, means that the deleter is copied whenever the shared_ptr is copied), or it is destroyed immediately after the owned pointer is destroyed (which, from an implementation standpoint, means that the deleter object is shared between instances). We should say which it is.

Proposed resolution:

Add after the first sentence of 20.8.2.2.10 [util.smartptr.getdeleter]/1:

> The returned pointer remains valid as long as there exists a `shared_ptr` instance that owns `d`.

> [Note: it is unspecified whether the pointer remains valid longer than that. This can happen if the implementation doesn't destroy the deleter until all `weak_ptr` instances in the ownership group are destroyed. -- end note]

### 550. What should the return type of `pow(float,int)` be?

Section: 26.8 [c.math] Status: CD1 Submitter: Howard Hinnant Opened: 2006-01-12 Last modified: 2015-04-08

View other active issues in [c.math].

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Discussion:

Assuming we adopt the C compatibility package from C99 what should be the return type of the following signature be:

>     ?  pow(float, int);

C++03 says that the return type should be `float`. TR1 and C90/99 say the return type should be `double`. This can put clients into a situation where C++03 provides answers that are not as high quality as C90/C99/TR1. For example:

>     #include <math.h>

>     int main()
>     {
>         float x = 2080703.375F;
>         double y = pow(x, 2);
>     }

Assuming an IEEE 32 bit float and IEEE 64 bit double, C90/C99/TR1 all suggest:

>     y = 4329326534736.390625

which is exactly right. While C++98/C++03 demands:

>     y = 4329326510080.

which is only approximately right.

I recommend that C++0X adopt the mixed mode arithmetic already adopted by Fortran, C and TR1 and make the return type of `pow(float,int)` be `double`.

[ Kona (2007): Other functions that are affected by this issue include `ldexp`, `scalbln`, and `scalbn`. We also believe that there is a typo in 26.7/10: `float nexttoward(float, long double);` [sic] should be `float nexttoward(float, float);` Proposed Disposition: Review (the proposed resolution appears above, rather than below, the heading "Proposed resolution") ]

[Howard, post Kona:]

> Unfortunately I strongly disagree with a part of the resolution from Kona. I am moving from New to Open instead of to Review because I do not believe we have consensus on the intent of the resolution.

> This issue does not include `ldexp`, `scalbln`, and `scalbn` because the second integral parameter in each of these signatures (from C99) is not a generic parameter according to C99 7.22p2. The corresponding C++ overloads are intended (as far as I know) to correspond directly to C99's definition of generic parameter.

> For similar reasons, I do not believe that the second `long double` parameter of `nexttoward`, nor the return type of this function, is in error. I believe the correct signature is:

> >     float nexttoward(float, long double);

> which is what both the C++0X working paper and C99 state (as far as I currently understand).

> This is really only about `pow(float, int)`. And this is because C++98 took one route (with `pow` only) and C99 took another (with many math functions in `<tgmath.h>`. The proposed resolution basically says: C++98 got it wrong and C99 got it right; let's go with C99.

[ Bellevue: ]

> This signature was not picked up from C99. Instead, if one types `pow(2.0f,2)`, the promotion rules will invoke "double pow(double, double)", which generally gives special treatment for integral exponents, preserving full accuracy of the result. New proposed wording provided.

Proposed resolution:

Change 26.8 [c.math] p10:

> The added signatures are:

> >     ...
> >     float pow(float, int);
> >     ...
> >     double pow(double, int);
> >     ...
> >     long double pow(long double, int);

### 551. <ccomplex>

Section: X [cmplxh], TR1 8.3 [tr.c99.cmplxh] Status: CD1 Submitter: Howard Hinnant Opened: 2006-01-23 Last modified: 2015-04-08

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Discussion:

Previously xxx.h was parsable by C++. But in the case of C99's <complex.h> it isn't. Otherwise we could model it just like <string.h>, <cstring>, <string>:

- <string> : C++ API in namespace std
- <cstring> : C API in namespace std
- <string.h> : C API in global namespace

In the case of C's complex, the C API won't compile in C++. So we have:

- <complex> : C++ API in namespace std
- <ccomplex> : ?
- <complex.h> : ?

The ? can't refer to the C API. TR1 currently says:

- <complex> : C++ API in namespace std
- <ccomplex> : C++ API in namespace std
- <complex.h> : C++ API in global namespace

Proposed resolution:

Change 26.3.11 [cmplxh]:

> The header behaves as if it includes the header `<ccomplex>`., and provides sufficient using declarations to declare in the global namespace all function and type names declared or defined in the neader `<complex>`. [Note: `<complex.h>` does not promote any interface into the global namespace as there is no C interface to promote. --end note]

### 552. random_shuffle and its generator

Section: 25.3.12 [alg.random.shuffle] Status: CD1 Submitter: Martin Sebor Opened: 2006-01-25 Last modified: 2015-04-08

View all other issues in [alg.random.shuffle].

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Discussion:

...is specified to shuffle its range by calling swap but not how (or even that) it's supposed to use the RandomNumberGenerator argument passed to it.

Shouldn't we require that the generator object actually be used by the algorithm to obtain a series of random numbers and specify how many times its operator() should be invoked by the algorithm?

See N2391 and N2423 for some further discussion.

Proposed resolution:

Adopt the proposed resolution in N2423.

[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 556. Is `Compare` a `BinaryPredicate`?

Section: 25.4 [alg.sorting] Status: C++11 Submitter: Martin Sebor Opened: 2006-02-05 Last modified: 2015-04-08

View all other issues in [alg.sorting].

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Discussion:

In 25, p8 we allow BinaryPredicates to return a type that's convertible to bool but need not actually be bool. That allows predicates to return things like proxies and requires that implementations be careful about what kinds of expressions they use the result of the predicate in (e.g., the expression in if (!pred(a, b)) need not be well-formed since the negation operator may be inaccessible or return a type that's not convertible to bool).

Here's the text for reference:

> ...if an algorithm takes BinaryPredicate binary_pred as its argument and first1 and first2 as its iterator arguments, it should work correctly in the construct if (binary_pred(*first1, first2)){...}.

In 25.3, p2 we require that the Compare function object return true of false, which would seem to preclude such proxies. The relevant text is here:

> Compare is used as a function object which returns true if the first argument is less than the second, and false otherwise...

[ Portland: Jack to define "convertible to bool" such that short circuiting isn't destroyed. ]

[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]

[ 2009-10 Santa Cruz: ]

> Move to Review once wording received. Stefanus to send proposed wording.

[ 2009-10 Santa Cruz: ]

> Move to Review once wording received. Stefanus to send proposed wording.

[ 2009-10-24 Stefanus supplied wording. ]

> Move to Review once wording received. Stefanus to send proposed wording. Old proposed wording here:

> > I think we could fix this by rewording 25.3, p2 to read somthing like:

> > > -2- `Compare` is used as a function object which returns `true` if the first argument a `BinaryPredicate`. The return value of the function call operator applied to an object of type `Compare`, when converted to type `bool`, yields `true` if the first argument of the call is less than the second, and `false` otherwise. `Compare comp` is used throughout for algorithms assuming an ordering relation. It is assumed that `comp` will not apply any non-constant function through the dereferenced iterator.

[ 2010-01-17: ]

> Howard expresses concern that the current direction of the proposed wording outlaws expressions such as:

> >     if (!comp(x, y))

> Daniel provides wording which addresses that concern.

> The previous wording is saved here:

> > Change 25.4 [alg.sorting] p2:

> > > `Compare` is used as a function object. The return value of the function call operator applied to an object of type Compare, when converted to type bool, yields true if the first argument of the call which returns `true` if the first argument is less than the second, and `false` otherwise. `Compare comp` is used throughout for algorithms assuming an ordering relation. It is assumed that `comp` will not apply any non-constant function through the dereferenced iterator.

[ 2010-01-22 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

1. Change 25.1 [algorithms.general]/7+8 as indicated. [This change is recommended to bring the return value requirements of `BinaryPredicate` and `Compare` in sync.]

   > 7 The `Predicate` parameter is used whenever an algorithm expects a function object that when applied to the result of dereferencing the corresponding iterator returns a value testable as `true`. In other words, if an algorithm takes `Predicate pred` as its argument and `first` as its iterator argument, it should work correctly in the construct if `(pred(*first)){...}` `pred(*first)` contextually converted to `bool` (4 [conv]). The function object `pred` shall not apply any nonconstant function through the dereferenced iterator. This function object may be a pointer to function, or an object of a type with an appropriate function call operator.

   > 8 The `BinaryPredicate` parameter is used whenever an algorithm expects a function object that when applied to the result of dereferencing two corresponding iterators or to dereferencing an iterator and type `T` when `T` is part of the signature returns a value testable as `true`. In other words, if an algorithm takes `BinaryPredicate` `binary_pred` as its argument and `first1` and `first2` as its iterator arguments, it should work correctly in the construct `if (binary_pred(*first1, *first2)){...}` `binary_pred(*first1, *first2)` contextually converted to `bool` (4 [conv]). `BinaryPredicate` always takes the first iterator type as its first argument, that is, in those cases when `T value` is part of the signature, it should work correctly in the context of `if (binary_pred(*first1, value)){...}` construct `binary_pred(*first1, value)` contextually converted to `bool` (4 [conv]). `binary_pred` shall not apply any non-constant function through the dereferenced iterators.

2. Change 25.4 [alg.sorting]/2 as indicated:

   > 2 `Compare` is used as a function object type (20.9 [function.objects]). The return value of the function call operation applied to an object of type `Compare`, when contextually converted to type `bool` (4 [conv]), yields `true` if the first argument of the call which returns `true` if the first argument is less than the second, and `false` otherwise. `Compare comp` is used throughout for algorithms assuming an ordering relation. It is assumed that `comp` will not apply any non-constant function through the dereferenced iterator.

### 559. numeric_limits<const T>

Section: 18.3.2 [limits] Status: CD1 Submitter: Martin Sebor Opened: 2006-02-19 Last modified: 2015-04-08

View all other issues in [limits].

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Discussion:

18.3.2 [limits], p2 requires implementations to provide specializations of the `numeric_limits` template for each scalar type. While this could be interepreted to include cv-qualified forms of such types such an interepretation is not reflected in the synopsis of the `<limits>` header.

The absence of specializations of the template on cv-qualified forms of fundamental types makes `numeric_limits` difficult to use in generic code where the constness (or volatility) of a type is not always immediately apparent. In such contexts, the primary template ends up being instantiated instead of the provided specialization, typically yielding unexpected behavior.

Require that specializations of `numeric_limits` on cv-qualified fundamental types have the same semantics as those on the unqualifed forms of the same types.

Proposed resolution:

Add to the synopsis of the `<limits>` header, immediately below the declaration of the primary template, the following:

    template <class T> class numeric_limits<const T>;
    template <class T> class numeric_limits<volatile T>;
    template <class T> class numeric_limits<const volatile T>;

Add a new paragraph to the end of 18.3.2.3 [numeric.limits], with the following text:

-new-para- The value of each member of a `numeric_limits` specialization on a cv-qualified T is equal to the value of the same member of `numeric_limits<T>`.

[ Portland: Martin will clarify that user-defined types get cv-specializations automatically. ]

### 561. inserter overly generic

Section: 24.5.2.6.5 [inserter] Status: CD1 Submitter: Howard Hinnant Opened: 2006-02-21 Last modified: 2015-04-08

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Discussion:

The declaration of `std::inserter` is:

>     template <class Container, class Iterator>
>     insert_iterator<Container>
>     inserter(Container& x, Iterator i);

The template parameter `Iterator` in this function is completely unrelated to the template parameter `Container` when it doesn't need to be. This causes the code to be overly generic. That is, any type at all can be deduced as `Iterator`, whether or not it makes sense. Now the same is true of `Container`. However, for every free (unconstrained) template parameter one has in a signature, the opportunity for a mistaken binding grows geometrically.

It would be much better if `inserter` had the following signature instead:

>     template <class Container>
>     insert_iterator<Container>
>     inserter(Container& x, typename Container::iterator i);

Now there is only one free template parameter. And the second argument to `inserter` must be implicitly convertible to the container's iterator, else the call will not be a viable overload (allowing other functions in the overload set to take precedence). Furthermore, the first parameter must have a nested type named `iterator`, or again the binding to `std::inserter` is not viable. Contrast this with the current situation where any type can bind to `Container` or `Iterator` and those types need not be anything closely related to containers or iterators.

This can adversely impact well written code. Consider:

>     #include <iterator>
>     #include <string>

>     namespace my
>     {

>     template <class String>
>     struct my_type {};

>     struct my_container
>     {
>     template <class String>
>     void push_back(const my_type<String>&);
>     };

>     template <class String>
>     void inserter(const my_type<String>& m, my_container& c) {c.push_back(m);}

>     }  // my

>     int main()
>     {
>         my::my_container c;
>         my::my_type<std::string> m;
>         inserter(m, c);
>     }

Today this code fails because the call to `inserter` binds to `std::inserter` instead of to `my::inserter`. However with the proposed change `std::inserter` will no longer be a viable function which leaves only `my::inserter` in the overload resolution set. Everything works as the client intends.

To make matters a little more insidious, the above example works today if you simply change the first argument to an rvalue:

>         inserter(my::my_type(), c);

It will also work if instantiated with some string type other than `std::string` (or any other `std` type). It will also work if `<iterator>` happens to not get included.

And it will fail again for such inocuous reaons as `my_type` or `my_container` privately deriving from any `std` type.

It seems unfortunate that such simple changes in the client's code can result in such radically differing behavior.

Proposed resolution:

Change 24.2:

> 24.2 Header `<iterator>` synopsis

> >     ...
> >     template <class Container, class Iterator>
> >        insert_iterator<Container> inserter(Container& x, Iterator typename Container::iterator i);
> >     ...

Change 24.4.2.5:

> 24.4.2.5 Class template `insert_iterator`

> >     ...
> >     template <class Container, class Iterator>
> >        insert_iterator<Container> inserter(Container& x, Iterator typename Container::iterator i);
> >     ...

Change 24.4.2.6.5:

> 24.4.2.6.5 `inserter`

>     template <class Container, class Inserter>
>        insert_iterator<Container> inserter(Container& x, Inserter typename Container::iterator i);

> > -1- Returns: `insert_iterator<Container>(x,typename Container::iterator(i))`.

[ Kona (2007): This issue will probably be addressed as a part of the concepts overhaul of the library anyway, but the proposed resolution is correct in the absence of concepts. Proposed Disposition: Ready ]

### 562. stringbuf ctor inefficient

Section: 27.8 [string.streams] Status: CD1 Submitter: Martin Sebor Opened: 2006-02-23 Last modified: 2015-04-08

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Discussion:

For better efficiency, the requirement on the stringbuf ctor that takes a string argument should be loosened up to let it set `epptr()` beyond just one past the last initialized character just like `overflow()` has been changed to be allowed to do (see issue 432). That way the first call to `sputc()` on an object won't necessarily cause a call to `overflow`. The corresponding change should be made to the string overload of the `str()` member function.

Proposed resolution:

Change 27.7.1.1, p3 of the Working Draft, N1804, as follows:

>     explicit basic_stringbuf(const basic_string<charT,traits,Allocator>& str,
>                    ios_base::openmode which = ios_base::in | ios_base::out);

> -3- Effects: Constructs an object of class `basic_stringbuf`, initializing the base class with `basic_streambuf()` (27.5.2.1), and initializing `mode` with `which`. Then calls `str(s)`. copies the content of str into the `basic_stringbuf` underlying character sequence. If `which & ios_base::out` is true, initializes the output sequence such that `pbase()` points to the first underlying character, `epptr()` points one past the last underlying character, and `pptr()` is equal to `epptr()` if `which & ios_base::ate` is true, otherwise `pptr()` is equal to `pbase()`. If `which & ios_base::in` is true, initializes the input sequence such that `eback()` and `gptr()` point to the first underlying character and `egptr()` points one past the last underlying character.

Change the Effects clause of the `str()` in 27.7.1.2, p2 to read:

> -2- Effects: Copies the contents of `s` into the `basic_stringbuf` underlying character sequence and initializes the input and output sequences according to `mode`. If `mode & ios_base::out` is true, initializes the output sequence such that `pbase()` points to the first underlying character, `epptr()` points one past the last underlying character, and `pptr()` is equal to `epptr()` if `mode & ios_base::in` is true, otherwise `pptr()` is equal to `pbase()`. If `mode & ios_base::in` is true, initializes the input sequence such that `eback()` and `gptr()` point to the first underlying character and `egptr()` points one past the last underlying character.

> -3- Postconditions: If `mode & ios_base::out` is true, `pbase()` points to the first underlying character and `(epptr() >= pbase() + s.size())` holds; in addition, if `mode & ios_base::in` is true, `(pptr() == pbase() + s.data())` holds, otherwise `(pptr() == pbase())` is true. If `mode & ios_base::in` is true, `eback()` points to the first underlying character, and `(gptr() == eback())` and `(egptr() == eback() + s.size())` hold.

[ Kona (2007) Moved to Ready. ]

### 563. stringbuf seeking from end

Section: 27.8.2.4 [stringbuf.virtuals] Status: CD1 Submitter: Martin Sebor Opened: 2006-02-23 Last modified: 2015-04-08

View all other issues in [stringbuf.virtuals].

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Discussion:

According to Table 92 (unchanged by issue 432), when `(way == end)` the `newoff` value in out mode is computed as the difference between `epptr()` and `pbase()`.

This value isn't meaningful unless the value of `epptr()` can be precisely controlled by a program. That used to be possible until we accepted the resolution of issue 432, but since then the requirements on `overflow()` have been relaxed to allow it to make more than 1 write position available (i.e., by setting `epptr()` to some unspecified value past `pptr()`). So after the first call to `overflow()` positioning the output sequence relative to end will have unspecified results.

In addition, in `in|out` mode, since `(egptr() == epptr())` need not hold, there are two different possible values for `newoff`: `epptr() - pbase()` and `egptr() - eback()`.

Proposed resolution:

Change the `newoff` column in the last row of Table 94 to read:

> the end high mark pointer minus the beginning pointer (`xend high_mark - xbeg`).

[ Kona (2007) Moved to Ready. ]

### 564. `stringbuf` seekpos underspecified

Section: 27.8.2.4 [stringbuf.virtuals] Status: C++11 Submitter: Martin Sebor Opened: 2006-02-23 Last modified: 2015-04-08

View all other issues in [stringbuf.virtuals].

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Discussion:

The effects of the `seekpos()` member function of `basic_stringbuf` simply say that the function positions the input and/or output sequences but fail to spell out exactly how. This is in contrast to the detail in which `seekoff()` is described.

[ 2009-07 Frankfurt ]

> Move to Ready.

Proposed resolution:

Change 27.7.1.3, p13 to read:

> -13- Effects: Equivalent to `seekoff(off_type(sp), ios_base::beg, which)`. Alters the stream position within the controlled sequences, if possible, to correspond to the stream position stored in `sp` (as described below).

> - If `(which & ios_base::in) != 0`, positions the input sequence.
> - If `(which & ios_base::out) != 0`, positions the output sequence.
> - If `sp` is an invalid stream position, or if the function positions neither sequence, the positioning operation fails. If `sp` has not been obtained by a previous successful call to one of the positioning functions (`seekoff`, `seekpos`, `tellg`, `tellp`) the effect is undefined.

[ Kona (2007): A `pos_type` is a position in a stream by definition, so there is no ambiguity as to what it means. Proposed Disposition: NAD ]

[ Post-Kona Martin adds: I'm afraid I disagree with the Kona '07 rationale for marking it NAD. The only text that describes precisely what it means to position the input or output sequence is in `seekoff()`. The `seekpos()` Effects clause is inadequate in comparison and the proposed resolution plugs the hole by specifying `seekpos()` in terms of `seekoff()`. ]

### 565. `xsputn` inefficient

Section: 27.6.3.4.5 [streambuf.virt.put] Status: C++11 Submitter: Martin Sebor Opened: 2006-02-23 Last modified: 2015-04-08

View all issues with C++11 status.

Discussion:

`streambuf::xsputn()` is specified to have the effect of "writing up to `n` characters to the output sequence as if by repeated calls to `sputc(c)`."

Since `sputc()` is required to call `overflow()` when `(pptr() == epptr())` is true, strictly speaking `xsputn()` should do the same. However, doing so would be suboptimal in some interesting cases, such as in unbuffered mode or when the buffer is `basic_stringbuf`.

Assuming calling `overflow()` is not really intended to be required and the wording is simply meant to describe the general effect of appending to the end of the sequence it would be worthwhile to mention in `xsputn()` that the function is not actually required to cause a call to `overflow()`.

[ 2009-07 Frankfurt ]

> Move to Ready.

Proposed resolution:

Add the following sentence to the `xsputn()` Effects clause in 27.5.2.4.5, p1 (N1804):

> -1- Effects: Writes up to `n` characters to the output sequence as if by repeated calls to `sputc(c)`. The characters written are obtained from successive elements of the array whose first element is designated by `s`. Writing stops when either `n` characters have been written or a call to `sputc(c)` would return `traits::eof()`. It is uspecified whether the function calls `overflow()` when `(pptr() == epptr())` becomes true or whether it achieves the same effects by other means.

In addition, I suggest to add a footnote to this function with the same text as Footnote 292 to make it extra clear that derived classes are permitted to override `xsputn()` for efficiency.

[ Kona (2007): We want to permit a `streambuf` that streams output directly to a device without making calls to `sputc` or `overflow`. We believe that has always been the intention of the committee. We believe that the proposed wording doesn't accomplish that. Proposed Disposition: Open ]

### 566. array forms of unformatted input function undefined for zero-element arrays

Section: 27.7.2.3 [istream.unformatted] Status: CD1 Submitter: Martin Sebor Opened: 2006-02-23 Last modified: 2015-04-08

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Discussion:

The array forms of unformatted input functions don't have well-defined semantics for zero-element arrays in a couple of cases. The affected ones (`istream::get()` and `getline()`) are supposed to terminate when `(n - 1)` characters are stored, which obviously can never be true when `(n == 0)` to start with.

Proposed resolution:

I propose the following changes (references are relative to the Working Draft (document N1804).

Change 27.6.1.3, p8 (`istream::get()`), bullet 1 as follows:

> if `(n < 1)` is true or `(n - 1)` characters are stored;

Similarly, change 27.6.1.3, p18 (`istream::getline()`), bullet 3 as follows:

> `(n < 1)` is true or `(n - 1)` characters are stored (in which case the function calls `setstate(failbit)`).

Finally, change p21 as follows:

> In any case, provided `(n > 0)` is true, it then stores a null character (using charT()) into the next successive location of the array.

### 567. streambuf inserter and extractor should be unformatted

Section: 27.7 [iostream.format] Status: CD1 Submitter: Martin Sebor Opened: 2006-02-25 Last modified: 2015-04-08

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Discussion:

Issue 60 explicitly made the extractor and inserter operators that take a `basic_streambuf*` argument formatted input and output functions, respectively. I believe that's wrong, certainly in the case of the extractor, since formatted functions begin by extracting and discarding whitespace. The extractor should not discard any characters.

Proposed resolution:

I propose to change each operator to behave as unformatted input and output function, respectively. The changes below are relative to the working draft document number N1804.

Specifically, change 27.6.1.2.3, p14 as follows:

> Effects: Behaves as an unformatted input function (as described in 27.6.1.2.1 27.6.1.3, paragraph 1).

And change 27.6.2.5.3, p7 as follows:

> Effects: Behaves as an unformatted output function (as described in 27.6.2.5.1 27.6.2.6, paragraph 1).

[ Kona (2007): Proposed Disposition: Ready ]

### 574. DR 369 Contradicts Text

Section: 27.4 [iostream.objects] Status: CD1 Submitter: Pete Becker Opened: 2006-04-18 Last modified: 2015-04-08

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Discussion:

lib.iostream.objects requires that the standard stream objects are never destroyed, and it requires that they be destroyed.

DR 369 adds words to say that we really mean for `ios_base::Init` objects to force construction of standard stream objects. It ends, though, with the phrase "these stream objects shall be destroyed after the destruction of dynamically ...". However, the rule for destruction is stated in the standard: "The objects are not destroyed during program execution."

Proposed resolution:

Change 27.4 [iostream.objects]/1:

> -2- The objects are constructed and the associations are established at some time prior to or during the first time an object of class `ios_base::Init` is constructed, and in any case before the body of main begins execution.290) The objects are not destroyed during program execution.291) If a translation unit includes `<iostream>` or explicitly constructs an `ios_base::Init` object, these stream objects shall be constructed before dynamic initialization of non-local objects defined later in that translation unit, and these stream objects shall be destroyed after the destruction of dynamically initialized non-local objects defined later in that translation unit.

[ Kona (2007): From 27.4 [iostream.objects]/2, strike the words "...and these stream objects shall be destroyed after the destruction of dynamically initialized non-local objects defined later in that translation unit." Proposed Disposition: Review ]

### 575. the specification of ~shared_ptr is MT-unfriendly, makes implementation assumptions

Section: 20.8.2.2.2 [util.smartptr.shared.dest], TR1 2.2.3.2 [tr.util.smartptr.shared.dest] Status: CD1 Submitter: Peter Dimov Opened: 2006-04-23 Last modified: 2015-04-08

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Discussion:

[tr.util.smartptr.shared.dest] says in its second bullet:

"If *this shares ownership with another shared_ptr instance (use_count() > 1), decrements that instance's use count."

The problem with this formulation is that it presupposes the existence of an "use count" variable that can be decremented and that is part of the state of a shared_ptr instance (because of the "that instance's use count".)

This is contrary to the spirit of the rest of the specification that carefully avoids to require an use count variable. Instead, use_count() is specified to return a value, a number of instances.

In multithreaded code, the usual implicit assumption is that a shared variable should not be accessed by more than one thread without explicit synchronization, and by introducing the concept of an "use count" variable, the current wording implies that two shared_ptr instances that share ownership cannot be destroyed simultaneously.

In addition, if we allow the interpretation that an use count variable is part of shared_ptr's state, this would lead to other undesirable consequences WRT multiple threads. For example,

>     p1 = p2;

would now visibly modify the state of p2, a "write" operation, requiring a lock.

Proposed resolution:

Change the first two bullets of [lib.util.smartptr.shared.dest]/1 to:

> - If `*this` is empty or shares ownership with another `shared_ptr` instance (`use_count() > 1`), there are no side effects.
> - If `*this` shares ownership with another `shared_ptr` instance (`use_count() > 1`), decrements that instance's use count.

Add the following paragraph after [lib.util.smartptr.shared.dest]/1:

> [Note: since the destruction of `*this` decreases the number of instances in `*this`'s ownership group by one, all `shared_ptr` instances that share ownership with `*this` will report an `use_count()` that is one lower than its previous value after `*this` is destroyed. --end note]

### 576. find_first_of is overconstrained

Section: 25.2.7 [alg.find.first.of] Status: CD1 Submitter: Doug Gregor Opened: 2006-04-25 Last modified: 2015-04-08

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Discussion:

In 25.1.4 Find First [lib.alg.find.first], the two iterator type parameters to find_first_of are specified to require Forward Iterators, as follows:

>     template<class ForwardIterator1, class ForwardIterator2>
>       ForwardIterator1
>       find_first_of(ForwardIterator1 first1, ForwardIterator1 last1,
>                             ForwardIterator2 first2, ForwardIterator2 last2);
>     template<class ForwardIterator1, class ForwardIterator2,
>                       class BinaryPredicate>
>     ForwardIterator1
>       find_first_of(ForwardIterator1 first1, ForwardIterator1 last1,
>                              ForwardIterator2 first2, ForwardIterator2 last2,
>                             BinaryPredicate pred);

However, ForwardIterator1 need not actually be a Forward Iterator; an Input Iterator suffices, because we do not need the multi-pass property of the Forward Iterator or a true reference.

Proposed resolution:

Change the declarations of `find_first_of` to:

>     template<class ForwardIterator1InputIterator1, class ForwardIterator2>
>       ForwardIterator1InputIterator1
>       find_first_of(ForwardIterator1InputIterator1 first1, ForwardIterator1InputIterator1 last1,
>                             ForwardIterator2 first2, ForwardIterator2 last2);
>     template<class ForwardIterator1InputIterator1, class ForwardIterator2,
>                       class BinaryPredicate>
>     ForwardIterator1InputIterator1
>       find_first_of(ForwardIterator1InputIterator1 first1, ForwardIterator1InputIterator1 last1,
>                              ForwardIterator2 first2, ForwardIterator2 last2,
>                             BinaryPredicate pred);

### 577. upper_bound(first, last, ...) cannot return last

Section: 25.4.3.2 [upper.bound] Status: CD1 Submitter: Seungbeom Kim Opened: 2006-05-03 Last modified: 2015-04-08

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Discussion:

ISO/IEC 14882:2003 says:

> 25.3.3.2 upper_bound

> Returns: The furthermost iterator `i` in the range `[first, last)` such that for any iterator `j` in the range `[first, i)` the following corresponding conditions hold: `!(value < *j)` or `comp(value, *j) == false`.

From the description above, upper_bound cannot return last, since it's not in the interval [first, last). This seems to be a typo, because if value is greater than or equal to any other values in the range, or if the range is empty, returning last seems to be the intended behaviour. The corresponding interval for lower_bound is also [first, last].

Proposed resolution:

Change [lib.upper.bound]:

> Returns: The furthermost iterator `i` in the range `[first, last)]` such that for any iterator `j` in the range `[first, i)` the following corresponding conditions hold: `!(value < *j)` or `comp(value, *j) == false`.

### 578. purpose of hint to allocator::allocate()

Section: 20.7.9.1 [allocator.members] Status: CD1 Submitter: Martin Sebor Opened: 2006-05-17 Last modified: 2015-04-08

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Discussion:

The description of the allocator member function `allocate()` requires that the hint argument be either 0 or a value previously returned from `allocate()`. Footnote 227 further suggests that containers may pass the address of an adjacent element as this argument.

I believe that either the footnote is wrong or the normative requirement that the argument be a value previously returned from a call to `allocate()` is wrong. The latter is supported by the resolution to issue 20-004 proposed in c++std-lib-3736 by Nathan Myers. In addition, the hint is an ordinary void* and not the `pointer` type returned by `allocate()`, with the two types potentially being incompatible and the requirement impossible to satisfy.

See also c++std-lib-14323 for some more context on where this came up (again).

Proposed resolution:

Remove the requirement in 20.6.1.1, p4 that the hint be a value previously returned from `allocate()`. Specifically, change the paragraph as follows:

Requires: hint either 0 or previously obtained from member `allocate` and not yet passed to member `deallocate`. The value hint may be used by an implementation to help improve performance 223). [Note: The value hint may be used by an implementation to help improve performance. -- end note]

> [Footnote: 223)In a container member function, the address of an adjacent element is often a good choice to pass for this argument.

### 581. `flush()` not unformatted function

Section: 27.7.3.7 [ostream.unformatted] Status: CD1 Submitter: Martin Sebor Opened: 2006-06-14 Last modified: 2015-04-08

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Discussion:

The resolution of issue 60 changed `basic_ostream::flush()` so as not to require it to behave as an unformatted output function. That has at least two in my opinion problematic consequences:

First, `flush()` now calls `rdbuf()->pubsync()` unconditionally, without regard to the state of the stream. I can't think of any reason why `flush()` should behave differently from the vast majority of stream functions in this respect.

Second, `flush()` is not required to catch exceptions from `pubsync()` or set `badbit` in response to such events. That doesn't seem right either, as most other stream functions do so.

Proposed resolution:

I propose to revert the resolution of issue 60 with respect to `flush()`. Specifically, I propose to change 27.6.2.6, p7 as follows:

Effects: Behaves as an unformatted output function (as described in 27.6.2.6, paragraph 1). If `rdbuf()` is not a null pointer, constructs a sentry object. If this object returns `true` when converted to a value of type bool the function calls `rdbuf()->pubsync()`. If that function returns -1 calls `setstate(badbit)` (which may throw `ios_base::failure` (27.4.4.3)). Otherwise, if the sentry object returns `false`, does nothing.Does not behave as an unformatted output function (as described in 27.6.2.6, paragraph 1).

[ Kona (2007): Proposed Disposition: Ready ]

### 586. string inserter not a formatted function

Section: 21.4.8.9 [string.io] Status: CD1 Submitter: Martin Sebor Opened: 2006-06-22 Last modified: 2015-04-08

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Discussion:

Section and paragraph numbers in this paper are relative to the working draft document number N2009 from 4/21/2006.

The `basic_string` extractor in 21.3.7.9, p1 is clearly required to behave as a formatted input function, as is the `std::getline()` overload for string described in p7.

However, the `basic_string` inserter described in p5 of the same section has no such requirement. This has implications on how the operator responds to exceptions thrown from `xsputn()` (formatted output functions are required to set `badbit` and swallow the exception unless `badbit` is also set in `exceptions()`; the string inserter doesn't have any such requirement).

I don't see anything in the spec for the string inserter that would justify requiring it to treat exceptions differently from all other similar operators. (If it did, I think it should be made this explicit by saying that the operator "does not behave as a formatted output function" as has been made customary by the adoption of the resolution of issue 60).

Proposed resolution:

I propose to change the Effects clause in 21.3.7.9, p5, as follows:

> Effects: Begins by constructing a sentry object k as if k were constructed by typename `basic_ostream<charT, traits>::sentry k (os)`. If `bool(k)` is `true`, Behaves as a formatted output function (27.6.2.5.1). After constructing a `sentry` object, if this object returns `true` when converted to a value of type `bool`, determines padding as described in 22.2.2.2.2, then inserts the resulting sequence of characters `seq` as if by calling `os.rdbuf()->sputn(seq , n)`, where `n` is the larger of `os.width()` and `str.size()`; then calls `os.width(0)`. If the call to sputn fails, calls `os.setstate(ios_base::failbit)`.

This proposed resilution assumes the resolution of issue 394 (i.e., that all formatted output functions are required to set `ios_base::badbit` in response to any kind of streambuf failure), and implicitly assumes that a return value of `sputn(seq, n)` other than `n` indicates a failure.

### 589. Requirements on iterators of member template functions of containers

Section: 23.2 [container.requirements] Status: CD1 Submitter: Peter Dimov Opened: 2006-08-02 Last modified: 2015-04-08

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Duplicate of: 536

Discussion:

There appears to be no requirements on the InputIterators used in sequences in 23.1.1 in terms of their value_type, and the requirements in 23.1.2 appear to be overly strict (requires InputIterator::value_type be the same type as the container's value_type).

Proposed resolution:

Change 23.1.1 p3:

> In Tables 82 and 83, `X` denotes a sequence class, `a` denotes a value of `X`, `i` and `j` denote iterators satisfying input iterator requirements and refer to elements implicitly convertible to `value_type`, `[i, j)` denotes a valid range, `n` denotes a value of `X::size_type`, `p` denotes a valid iterator to `a`, `q` denotes a valid dereferenceable iterator to `a`, `[q1, q2)` denotes a valid range in `a`, and `t` denotes a value of `X::value_type`.

Change 23.1.2 p7:

> In Table 84, `X` is an associative container class, `a` is a value of `X`, `a_uniq` is a value of `X` when `X` supports unique keys, and `a_eq` is a value of `X` when `X` supports multiple keys, `i` and `j` satisfy input iterator requirements and refer to elements of implicitly convertible to `value_type`, `[i, j)` is a valid range, `p` is a valid iterator to `a`, `q` is a valid dereferenceable iterator to `a`, `[q1, q2)` is a valid range in `a`, `t` is a value of `X::value_type`, `k` is a value of `X::key_type` and `c` is a value of type `X::key_compare`.

Rationale:

Concepts will probably come in and rewrite this section anyway. But just in case it is easy to fix this up as a safety net and as a clear statement of intent.

### 593. __STDC_CONSTANT_MACROS

Section: 18.4 [cstdint], TR1 8.22 [tr.c99.cstdint] Status: CD1 Submitter: Walter Brown Opened: 2006-08-28 Last modified: 2015-04-08

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Discussion:

Clause 18.3 of the current Working Paper (N2009) deals with the new C++ headers <cstdint> and <stdint.h>. These are of course based on the C99 header <stdint.h>, and were part of TR1.

Per 18.3.1/1, these headers define a number of macros and function macros. While the WP does not mention __STDC_CONSTANT_MACROS in this context, C99 footnotes do mention __STDC_CONSTANT_MACROS. Further, 18.3.1/2 states that "The header defines all ... macros the same as C99 subclause 7.18."

Therefore, if I wish to have the above-referenced macros and function macros defined, must I #define __STDC_CONSTANT_MACROS before I #include <cstdint>, or does the C++ header define these macros/function macros unconditionally?

Proposed resolution:

To put this issue to rest for C++0X, I propose the following addition to 18.3.1/2 of the Working Paper N2009:

> [Note: The macros defined by <cstdint> are provided unconditionally: in particular, the symbols __STDC_LIMIT_MACROS and __STDC_CONSTANT_MACROS (mentioned in C99 footnotes 219, 220, and 222) play no role in C++. --end note]

### 594. Disadvantages of defining `Swappable` in terms of `CopyConstructible` and `Assignable`

Section: 17.6.3.1 [utility.arg.requirements] Status: Resolved Submitter: Niels Dekker Opened: 2006-11-02 Last modified: 2015-04-08

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Discussion:

It seems undesirable to define the `Swappable` requirement in terms of `CopyConstructible` and `Assignable` requirements. And likewise, once the `MoveConstructible` and `MoveAssignable` requirements (N1860) have made it into the Working Draft, it seems undesirable to define the `Swappable` requirement in terms of those requirements. Instead, it appears preferable to have the `Swappable` requirement defined exclusively in terms of the existence of an appropriate swap function.

Section 20.1.4 [lib.swappable] of the current Working Draft (N2009) says:

> The Swappable requirement is met by satisfying one or more of the following conditions:

> - T is Swappable if T satisfies the CopyConstructible requirements (20.1.3) and the Assignable requirements (23.1);
> - T is Swappable if a namespace scope function named swap exists in the same namespace as the definition of T, such that the expression swap(t,u) is valid and has the semantics described in Table 33.

I can think of three disadvantages of this definition:

1. If a client's type T satisfies the first condition (T is both CopyConstructible and Assignable), the client cannot stop T from satisfying the Swappable requirement without stopping T from satisfying the first condition.

   A client might want to stop T from satisfying the Swappable requirement, because swapping by means of copy construction and assignment might throw an exception, and she might find a throwing swap unacceptable for her type. On the other hand, she might not feel the need to fully implement her own swap function for this type. In this case she would want to be able to simply prevent algorithms that would swap objects of type T from being used, e.g., by declaring a swap function for T, and leaving this function purposely undefined. This would trigger a link error, if an attempt would be made to use such an algorithm for this type. For most standard library implementations, this practice would indeed have the effect of stopping T from satisfying the Swappable requirement.

2. A client's type T that does not satisfy the first condition can not be made Swappable by providing a specialization of std::swap for T.

   While I'm aware about the fact that people have mixed feelings about providing a specialization of std::swap, it is well-defined to do so. It sounds rather counter-intuitive to say that T is not Swappable, if it has a valid and semantically correct specialization of std::swap. Also in practice, providing such a specialization will have the same effect as satisfying the Swappable requirement.

3. For a client's type T that satisfies both conditions of the Swappable requirement, it is not specified which of the two conditions prevails. After reading section 20.1.4 [lib.swappable], one might wonder whether objects of T will be swapped by doing copy construction and assignments, or by calling the swap function of T.

   I'm aware that the intention of the Draft is to prefer calling the swap function of T over doing copy construction and assignments. Still in my opinion, it would be better to make this clear in the wording of the definition of Swappable.

I would like to have the Swappable requirement defined in such a way that the following code fragment will correctly swap two objects of a type T, if and only if T is Swappable:

       using std::swap;
       swap(t, u);  // t and u are of type T.

This is also the way Scott Meyers recommends calling a swap function, in Effective C++, Third Edition, item 25.

Most aspects of this issue have been dealt with in a discussion on comp.std.c++ about the Swappable requirement, from 13 September to 4 October 2006, including valuable input by David Abrahams, Pete Becker, Greg Herlihy, Howard Hinnant and others.

[ San Francisco: ]

> Recommend NAD Resolved. Solved by N2774.

[ 2009-07 Frankfurt ]

> Moved to Open. Waiting for non-concepts draft.

[ 2009-11-08 Howard adds: ]

> This issue is very closely related to 742.

[ 2010-02-03 Sean Hunt adds: ]

> While reading N3000, I independently came across Issue 594. Having seen that it's an issue under discussion, I think the proposed wording needs fixing to something more like "...function call swap(t,u) that includes std::swap in its overload set is valid...", because "...is valid within the namespace std..." does not allow other libraries to simply use the Swappable requirement by referring to the standard's definition, since they cannot actually perform any calls within std.

> This wording I suggested would also make overloads visible in the same scope as the `using std::swap` valid for Swappable requirements; a more complex wording limiting the non-ADL overload set to std::swap might be required.

[ 2010 Pittsburgh: ]

> Moved to NAD Editorial. Rationale added.

Rationale:

Solved by N3048.

Proposed resolution:

Change section 20.1.4 [lib.swappable] as follows:

> The Swappable requirement is met by satisfying one or more of the following conditions: the following condition:

> - T is Swappable if T satisfies the CopyConstructible requirements (20.1.3) and the Assignable requirements (23.1);
> - T is Swappable if a namespace scope function named swap exists in the same namespace as the definition of T, such that the expression swap(t,u) is valid and has the semantics described in Table 33. T is Swappable if an unqualified function call swap(t,u) is valid within the namespace std, and has the semantics described in Table 33.

### 595. TR1/C++0x: `fabs(complex<T>)` redundant / wrongly specified

Section: 26.4.7 [complex.value.ops] Status: CD1 Submitter: Stefan Große Pawig Opened: 2006-09-24 Last modified: 2015-04-08

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Discussion:

TR1 introduced, in the C compatibility chapter, the function `fabs(complex<T>)`:

>     ----- SNIP -----
>     8.1.1 Synopsis                                [tr.c99.cmplx.syn]

>       namespace std {
>       namespace tr1 {
>     [...]
>       template<class T> complex<T> fabs(const complex<T>& x);
>       } // namespace tr1
>       } // namespace std

>     [...]

>     8.1.8 Function fabs                          [tr.c99.cmplx.fabs]

>     1 Effects: Behaves the same as C99 function cabs, defined in
>       subclause 7.3.8.1.
>     ----- SNIP -----

The current C++0X draft document (n2009.pdf) adopted this definition in chapter 26.3.1 (under the comment // 26.3.7 values) and 26.3.7/7.

But in C99 (ISO/IEC 9899:1999 as well as the 9899:TC2 draft document n1124), the referenced subclause reads

>     ----- SNIP -----
>     7.3.8.1 The cabs functions

>       Synopsis

>     1 #include <complex.h>
>       double cabs(double complex z);
>       float cabsf(float complex z);
>       long double cabsl(long double z);

>       Description

>     2 The cabs functions compute the complex absolute value (also called
>       norm, modulus, or magnitude) of z.

>       Returns

>     3 The cabs functions return the complex absolute value.
>     ----- SNIP -----

Note that the return type of the cabs*() functions is not a complex type. Thus, they are equivalent to the already well established template<class T> T abs(const complex<T>& x); (26.2.7/2 in ISO/IEC 14882:1998, 26.3.7/2 in the current draft document n2009.pdf).

So either the return value of fabs() is specified wrongly, or fabs() does not behave the same as C99's cabs*().

Possible Resolutions

This depends on the intention behind the introduction of fabs().

If the intention was to provide a /complex/ valued function that calculates the magnitude of its argument, this should be explicitly specified. In TR1, the categorization under "C compatibility" is definitely wrong, since C99 does not provide such a complex valued function.

Also, it remains questionable if such a complex valued function is really needed, since complex<T> supports construction and assignment from real valued arguments. There is no difference in observable behaviour between

>       complex<double> x, y;
>       y = fabs(x);
>       complex<double> z(fabs(x));

and

>       complex<double> x, y;
>       y = abs(x);
>       complex<double> z(abs(x));

If on the other hand the intention was to provide the intended functionality of C99, fabs() should be either declared deprecated or (for C++0X) removed from the standard, since the functionality is already provided by the corresponding overloads of abs().

[ Bellevue: ]

> Bill believes that `abs()` is a suitable overload. We should remove `fabs()`.

Proposed resolution:

Change the synopsis in 26.4.1 [complex.syn]:

>     template<class T> complex<T> fabs(const complex<T>&);

Remove 26.4.7 [complex.value.ops], p7:

>     template<class T> complex<T> fabs(const complex<T>& x);

> > -7- Effects: Behaves the same as C99 function `cabs`, defined in subclause 7.3.8.1.

[ Kona (2007): Change the return type of `fabs(complex)` to `T`. Proposed Disposition: Ready ]

### 596. 27.8.1.3 Table 112 omits "a+" and "a+b" modes

Section: 27.9.1.4 [filebuf.members] Status: CD1 Submitter: Thomas Plum Opened: 2006-09-26 Last modified: 2015-04-08

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Discussion:

In testing 27.9.1.4 [filebuf.members], Table 112 (in the latest N2009 draft), we invoke

>        ostr.open("somename", ios_base::out | ios_base::in | ios_base::app)

and we expect the open to fail, because out|in|app is not listed in Table 92, and just before the table we see very specific words:

> If mode is not some combination of flags shown in the table then the open fails.

But the corresponding table in the C standard, 7.19.5.3, provides two modes "a+" and "a+b", to which the C++ modes out|in|app and out|in|app|binary would presumably apply.

We would like to argue that the intent of Table 112 was to match the semantics of 7.19.5.3 and that the omission of "a+" and "a+b" was unintentional. (Otherwise there would be valid and useful behaviors available in C file I/O which are unavailable using C++, for no valid functional reason.)

We further request that the missing modes be explicitly restored to the WP, for inclusion in C++0x.

[ Martin adds: ]

...besides "a+" and "a+b" the C++ table is also missing a row for a lone app bit which in at least two current implementation as well as in Classic Iostreams corresponds to the C stdio "a" mode and has been traditionally documented as implying ios::out. Which means the table should also have a row for in|app meaning the same thing as "a+" already proposed in the issue.

Proposed resolution:

Add to the table "File open modes" in 27.9.1.4 [filebuf.members]:

> File open modes
> `ios_base` Flag combination | `stdio` equivalent
> --- | ---
> `binary` | `in` | `out` | `trunc` | `app` | ``
> |  | `+` |  |  | `"w"`
> |  | `+` |  | `+` | `"a"`
> |  |  |  | `+` | `"a"`
> |  | `+` | `+` |  | `"w"`
> | `+` |  |  |  | `"r"`
> | `+` | `+` |  |  | `"r+"`
> | `+` | `+` | `+` |  | `"w+"`
> | `+` | `+` |  | `+` | `"a+"`
> | `+` |  |  | `+` | `"a+"`
> `+` |  | `+` |  |  | `"wb"`
> `+` |  | `+` |  | `+` | `"ab"`
> `+` |  |  |  | `+` | `"ab"`
> `+` |  | `+` | `+` |  | `"wb"`
> `+` | `+` |  |  |  | `"rb"`
> `+` | `+` | `+` |  |  | `"r+b"`
> `+` | `+` | `+` | `+` |  | `"w+b"`
> `+` | `+` | `+` |  | `+` | `"a+b"`
> `+` | `+` |  |  | `+` | `"a+b"`

[ Kona (2007) Added proposed wording and moved to Review. ]

### 598. Decimal: Conversion to integral should truncate, not round.

Section: TRDecimal 3.2 [trdec.types.types] Status: TRDec Submitter: Daniel Krugler Opened: 2006-05-28 Last modified: 2015-04-08

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Discussion:

In a private email, Daniel writes:

> I would like to ask, what where the reason for the decision to define the semantics of the integral conversion of the decimal types, namely

>     "operator long long() const;

>          Returns: Returns the result of the
>     conversion of *this to the type long long, as if
>     performed by the expression llrounddXX(*this)."

> where XX stands for either 32, 64, or 128, corresponding to the proper decimal type. The exact meaning of llrounddXX is not given in that paper, so I compared it to the corresponding definition given in C99, 2nd edition (ISO 9899), which says in 7.12.9.7 p. 2:

> "The lround and llround functions round their argument to the nearest integer value, rounding halfway cases away from zero, regardless of the current rounding direction. [..]"

> Now considering the fact that integral conversion of the usual floating-point types ("4.9 Floating-integral conversions") has truncation semantic I wonder why this conversion behaviour has not been transferred for the decimal types.

Robert comments:

Also, there is a further error in the Returns: clause for converting `decimal::decimal128` to `long long`. It currently calls `llroundd64`, not `llroundd128`.

Proposed resolution:

Change the Returns: clause in 3.2.2.4 to:

> Returns: Returns the result of the conversion of `*this` to the type `long long`, as if performed by the expression `llroundd32(*this)` while the decimal rounding direction mode [3.5.2] `FE_DEC_TOWARD_ZERO` is in effect.

Change the Returns: clause in 3.2.3.4 to:

> Returns: Returns the result of the conversion of `*this` to the type `long long`, as if performed by the expression `llroundd64(*this)` while the decimal rounding direction mode [3.5.2] `FE_DEC_TOWARD_ZERO` is in effect.

Change the Returns: clause in 3.2.4.4 to:

> Returns: Returns the result of the conversion of `*this` to the type `long long`, as if performed by the expression `llroundd64(*this)` `llroundd128(*this)` while the decimal rounding direction mode [3.5.2] `FE_DEC_TOWARD_ZERO` is in effect.

### 599. Decimal: Say "octets" instead of "bytes."

Section: TRDecimal 3.1 [trdec.types.encodings] Status: TRDec Submitter: Daniel Krugler Opened: 2006-05-28 Last modified: 2015-04-08

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Discussion:

Daniel writes in a private email:

> - 3.1 'Decimal type encodings' says in its note:

>     "this implies that
>     sizeof(std::decimal::decimal32) == 4,
>     sizeof(std::decimal::decimal64) == 8, and
>     sizeof(std::decimal::decimal128) == 16."

> This is a wrong assertion, because the definition of 'byte' in 1.7 'The C+ + memory model' of ISO 14882 (2nd edition) does not specify that a byte must be necessarily 8 bits large, which would be necessary to compare with the specified bit sizes of the types decimal32, decimal64, and decimal128.

Proposed resolution:

Change 3.1 as follows:

> The three decimal encoding formats defined in IEEE-754R correspond to the three decimal floating types as follows:

> - decimal32 is a decimal32 number, which is encoded in four consecutive bytes octets (32 bits)
> - decimal64 is a decimal64 number, which is encoded in eight consecutive bytes octets (64 bits)
> - decimal128 is a decimal128 number, which is encoded in 16 consecutive bytes octets (128 bits)

> [Note: this implies that `sizeof(std::decimal::decimal32) == 4`, `sizeof(std::decimal::decimal64) == 8`, and `sizeof(std::decimal::decimal128) == 16`. --end note]

### 600. Decimal: Wrong parameters for wcstod* functions

Section: TRDecimal 3.9 [trdec.types.cwchar] Status: TRDec Submitter: Daniel Krugler Opened: 2006-05-28 Last modified: 2015-04-08

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Discussion:

Daniel writes:

> - 3.9.1 'Additions to <cwchar>' provides wrong signatures to the wcstod32, wcstod64, and wcstod128 functions ([the parameters have type pointer-to-] char instead of wchar_t).

Proposed resolution:

Change "3.9.1 Additions to `<cwchar>` synopsis" to:

           namespace std {
           namespace decimal {
             // 3.9.2 wcstod functions:
             decimal32  wcstod32  (const char wchar_t * nptr, char wchar_t ** endptr);
             decimal64  wcstod64  (const char wchar_t * nptr, char wchar_t ** endptr);
             decimal128 wcstod128 (const char wchar_t * nptr, char wchar_t ** endptr);
           }
           }

### 601. Decimal: numeric_limits typos

Section: TRDecimal 3.3 [trdec.types.limits] Status: TRDec Submitter: Daniel Krugler Opened: 2006-05-28 Last modified: 2015-04-08

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Discussion:

Daniel writes in a private email:

> - 3.3 'Additions to header <limits>' contains two errors in the specialisation of numeric_limits<decimal::decimal128>:

> 1. The static member max() returns DEC128_MIN, this should be DEC128_MAX.
> 2. The static member digits is assigned to 384, this should be 34 (Probably mixed up with the max. exponent for decimal::decimal64).

Proposed resolution:

In "3.3 Additions to header `<limits>`" change numeric_limits<decimal::decimal128> as follows:

            template<> class numeric_limits<decimal::decimal128> {
              public:
                static const bool is_specialized = true;

                static decimal::decimal128 min() throw() { return DEC128_MIN; }
                static decimal::decimal128 max() throw() { return DEC128_MIN; DEC128_MAX; }

                static const int digits       = 384 34;
                /* ... */

### 602. Decimal: "generic floating type" not defined.

Section: TRDecimal 3 [trdec.types] Status: TRDec Submitter: Daniel Krügler Opened: 2006-05-28 Last modified: 2015-04-08

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Discussion:

The document uses the term "generic floating types," but defines it nowhere.

Proposed resolution:

Change the first paragraph of "3 Decimal floating-point types" as follows:

> This Technical Report introduces three decimal floating-point types, named decimal32, decimal64, and decimal128. The set of values of type decimal32 is a subset of the set of values of type decimal64; the set of values of the type decimal64 is a subset of the set of values of the type decimal128. Support for decimal128 is optional. These types supplement the Standard C++ types `float`, `double`, and `long double`, which are collectively described as the basic floating types.

### 603. Decimal: Trivially simplifying decimal classes.

Section: TRDecimal 3 [trdec.types] Status: TRDec Submitter: Martin Sebor Opened: 2006-05-28 Last modified: 2015-04-08

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Discussion:

In c++std-lib-17198, Martin writes:

> Each of the three classes proposed in the paper (decimal32, decimal64, and decimal128) explicitly declares and specifies the semantics of its copy constructor, copy assignment operator, and destructor. Since the semantics of all three functions are identical to the trivial versions implicitly generated by the compiler in the absence of any declarations it is safe to drop them from the spec. This change would make the proposed classes consistent with other similar classes already in the standard (e.g., std::complex).

Proposed resolution:

Change "3.2.2 Class `decimal32`" as follows:

          namespace std {
          namespace decimal {
            class decimal32 {
              public:
                // 3.2.2.1 construct/copy/destroy:
                decimal32();
                decimal32(const decimal32 & d32);
                decimal32 & operator=(const decimal32 & d32);
                ~decimal32();
                /* ... */

Change "3.2.2.1 construct/copy/destroy" as follows:

            decimal32();

        Effects: Constructs an object of type decimal32 with the value 0;

            decimal32(const decimal32 & d32);
            decimal32 & operator=(const decimal32 & d32);

        Effects: Copies an object of type decimal32.

            ~decimal32();

        Effects: Destroys an object of type decimal32.

Change "3.2.3 Class `decimal64`" as follows:

          namespace std {
          namespace decimal {
            class decimal64 {
              public:
                // 3.2.3.1 construct/copy/destroy:
                decimal64();
                decimal64(const decimal64 & d64);
                decimal64 & operator=(const decimal64 & d64);
                ~decimal64();
                /* ... */

Change "3.2.3.1 construct/copy/destroy" as follows:

            decimal64();

        Effects: Constructs an object of type decimal64 with the value 0;

            decimal64(const decimal64 & d64);
            decimal64 & operator=(const decimal64 & d64);

        Effects: Copies an object of type decimal64.

            ~decimal64();

        Effects: Destroys an object of type decimal64.

Change "3.2.4 Class `decimal128`" as follows:

          namespace std {
          namespace decimal {
            class decimal128 {
              public:
                // 3.2.4.1 construct/copy/destroy:
                decimal128();
                decimal128(const decimal128 & d128);
                decimal128 & operator=(const decimal128 & d128);
                ~decimal128();
                /* ... */

Change "3.2.4.1 construct/copy/destroy" as follows:

            decimal128();

        Effects: Constructs an object of type decimal128 with the value 0;

            decimal128(const decimal128 & d128);
            decimal128 & operator=(const decimal128 & d128);

        Effects: Copies an object of type decimal128.

            ~decimal128();

        Effects: Destroys an object of type decimal128.

### 604. Decimal: Storing a reference to a facet unsafe.

Section: TRDecimal 3 [trdec.types] Status: TRDec Submitter: Martin Sebor Opened: 2006-05-28 Last modified: 2015-04-08

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Discussion:

In c++std-lib-17197, Martin writes:

> The extended_num_get and extended_num_put facets are designed to store a reference to a num_get or num_put facet which the extended facets delegate the parsing and formatting of types other than decimal. One form of the extended facet's ctor (the default ctor and the size_t overload) obtains the reference from the global C++ locale while the other form takes this reference as an argument.

> The problem with storing a reference to a facet in another object (as opposed to storing the locale object in which the facet is installed) is that doing so bypasses the reference counting mechanism designed to prevent a facet that is still being referenced (i.e., one that is still installed in some locale) from being destroyed when another locale that contains it is destroyed. Separating a facet reference from the locale it comes from van make it cumbersome (and in some cases might even make it impossible) for programs to prevent invalidating the reference. (The danger of this design is highlighted in the paper.)

> This problem could be easily avoided by having the extended facets store a copy of the locale from which they would extract the base facet either at construction time or when needed. To make it possible, the forms of ctors of the extended facets that take a reference to the base facet would need to be changed to take a locale argument instead.

Proposed resolution:

1. Change the `extended_num_get` synopsis in 3.10.2 as follows:

                extended_num_get(const std::num_get<charT, InputIterator> std::locale & b, size_t refs = 0);

                /* ... */

                // const std::num_get<charT, InputIterator> & base;        exposition only
                // std::locale baseloc;                                    exposition only

2. Change the description of the above constructor in 3.10.2.1:

                extended_num_get(const std::num_get<charT, InputIterator> std::locale & b, size_t refs = 0);

> Effects: Constructs an `extended_num_get` facet as if by:

>            extended_num_get(const std::num_get<charT, InputIterator> std::locale & b, size_t refs = 0)
>                     : facet(refs), baseloc(b)
>                     { /* ... */ }

> Notes: Care must be taken by the implementation to ensure that the lifetime of the facet referenced by base exceeds that of the resulting `extended_num_get` facet.

3. Change the Returns: clause for `do_get(iter_type, iter_type, ios_base &, ios_base::iostate &, bool &) const`, et al to

> Returns: `base std::use_facet<std::num_get<charT, InputIterator> >(baseloc).get(in, end, str, err, val)`.

4. Change the `extended_num_put` synopsis in 3.10.3 as follows:

                extended_num_put(const std::num_put<charT, OutputIterator> std::locale & b, size_t refs = 0);

                /* ... */

                // const std::num_put<charT, OutputIterator> & base;       exposition only
                // std::locale baseloc;                                    exposition only

5. Change the description of the above constructor in 3.10.3.1:

                extended_num_put(const std::num_put<charT, OutputIterator> std::locale & b, size_t refs = 0);

> Effects: Constructs an `extended_num_put` facet as if by:

>            extended_num_put(const std::num_put<charT, OutputIterator> std::locale & b, size_t refs = 0)
>                     : facet(refs), baseloc(b)
>                     { /* ... */ }

> Notes: Care must be taken by the implementation to ensure that the lifetime of the facet referenced by base exceeds that of the resulting `extended_num_put` facet.

6. Change the Returns: clause for `do_put(iter_type, ios_base &, char_type, bool &) const`, et al to

> Returns: `base std::use_facet<std::num_put<charT, OutputIterator> >(baseloc).put(s, f, fill, val)`.

[ Redmond: We would prefer to rename "extended" to "decimal". ]

### 605. Decimal: <decfloat.h> doesn't live here anymore.

Section: TRDecimal 3.4 [trdec.types.cdecfloat] Status: TRDec Submitter: Robert Klarer Opened: 2006-10-17 Last modified: 2015-04-08

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Discussion:

In Berlin, WG14 decided to drop the <decfloat.h> header. The contents of that header have been moved into <float.h>. For the sake of C compatibility, we should make corresponding changes.

Proposed resolution:

1. Change the heading of subclause 3.4, "Headers `<cdecfloat>` and `<decfloat.h>`" to "Additions to headers `<cfloat>` and `<float.h>`."

2. Change the text of subclause 3.4 as follows:

> The standard C++ headers `<cfloat>` and `<float.h>` define characteristics of the floating-point types `float`, `double`, and `long double`. Their contents remain unchanged by this Technical Report.

> Headers `<cdecfloat>` and `<decfloat.h>` define characteristics of the decimal floating-point types `decimal32`, `decimal64`, and `decimal128`. As well, `<decfloat.h>` defines the convenience typedefs `_Decimal32`, `_Decimal64`, and `_Decimal128`, for compatibilty with the C programming language.

> The header `<cfloat>` is described in [tr.c99.cfloat]. The header `<float.h>` is described in [tr.c99.floath]. These headers are extended by this Technical Report to define characteristics of the decimal floating-point types `decimal32`, `decimal64`, and `decimal128`. As well, `<float.h>` is extended to define the convenience typedefs `_Decimal32`, `_Decimal64`, and `_Decimal128` for compatibility with the C programming language.

3. Change the heading of subclause 3.4.1, "Header `<cdecfloat>` synopsis" to "Additions to header `<cfloat>` synopsis."

4. Change the heading of subclause 3.4.2, "Header `<decfloat.h>` synopsis" to "Additions to header `<float.h>` synopsis."

5. Change the contents of 3.4.2 as follows:

          #include <cdecfloat>

          // C-compatibility convenience typedefs:

          typedef std::decimal::decimal32  _Decimal32;
          typedef std::decimal::decimal64  _Decimal64;
          typedef std::decimal::decimal128 _Decimal128;

### 607. Concern about short seed vectors

Section: 26.5.7.1 [rand.util.seedseq] Status: CD1 Submitter: Charles Karney Opened: 2006-10-26 Last modified: 2015-04-08

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Discussion:

Short seed vectors of 32-bit quantities all result in different states. However this is not true of seed vectors of 16-bit (or smaller) quantities. For example these two seeds

>     unsigned short seed = {1, 2, 3};
>     unsigned short seed = {1, 2, 3, 0};

both pack to

>     unsigned seed = {0x20001, 0x3};

yielding the same state.

See N2391 and N2423 for some further discussion.

Proposed resolution:

Adopt the proposed resolution in N2423.

[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 608. Unclear seed_seq construction details

Section: 26.5.7.1 [rand.util.seedseq] Status: CD1 Submitter: Charles Karney Opened: 2006-10-26 Last modified: 2015-04-08

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Discussion:

In 26.4.7.1 [rand.util.seedseq] /6, the order of packing the inputs into b and the treatment of signed quantities is unclear. Better to spell it out.

See N2391 and N2423 for some further discussion.

Proposed resolution:

Adopt the proposed resolution in N2423.

[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 609. missing static const

Section: 26.5.4.3 [rand.adapt.ibits], TR1 5.1 [tr.rand] Status: CD1 Submitter: Walter E. Brown Opened: 2006-11-02 Last modified: 2015-04-08

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Discussion:

In preparing N2111, an error on my part resulted in the omission of the following line from the template synopsis in the cited section:

>     static const size_t word_size = w;

(This same constant is found, for example, in 26.4.3.3 [rand.eng.sub].)

Proposed resolution:

Add the above declaration as the first line after the comment in [rand.adapt.ibits] p4:

>     // engine characteristics
>     static const size_t word_size = w;

and accept my apologies for the oversight.

### 610. Suggested non-normative note for C++0x

Section: 20.9.12.2.1 [func.wrap.func.con], TR1 3.7.2.1 [tr.func.wrap.func.con] Status: CD1 Submitter: Scott Meyers Opened: 2006-11-02 Last modified: 2015-04-08

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Discussion:

My suggestion is that implementers of both tr1::function and its official C++0x successor be explicitly encouraged (but not required) to optimize for the cases mentioned above, i.e., function pointers and small function objects. They could do this by using a small internal buffer akin to the buffer used by implementations of the small string optimization. (That would make this the small functor optimization -- SFO :-}) The form of this encouragement could be a note in the standard akin to footnote 214 of the current standard.

Dave Abrahams notes:

"shall not throw exceptions" should really be "nothing," both to be more grammatical and to be consistent with existing wording in the standard.

Doug Gregor comments: I think this is a good idea. Currently, implementations of tr1::function are required to have non-throwing constructors and assignment operators when the target function object is a function pointer or a reference_wrapper. The common case, however, is for a tr1::function to store either an empty function object or a member pointer + an object pointer.

The function implementation in the upcoming Boost 1.34.0 uses the "SFO", so that the function objects for typical bind expressions like

>     bind(&X::f, this, _1, _2, _3)

do not require heap allocation when stored in a boost::function. I believe Dinkumware's implementation also performs this optimization.

Proposed resolution:

Revise 20.5.14.2.1 p6 [func.wrap.func.con] to add a note as follows:

> Throws: shall not throw exceptions if `f`'s target is a function pointer or a function object passed via `reference_wrapper`. Otherwise, may throw `bad_alloc` or any exception thrown by the copy constructor of the stored function object.

> Note: Implementations are encouraged to avoid the use of dynamically allocated memory for "small" function objects, e.g., where `f`'s target is an object holding only a pointer or reference to an object and a member function pointer (a "bound member function").

### 611. Standard library templates and incomplete types

Section: 17.6.4.8 [res.on.functions] Status: CD1 Submitter: Nicola Musatti Opened: 2006-11-13 Last modified: 2015-04-08

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Discussion:

In the latest available draft standard (N2134) § 17.4.3.6 [res.on.functions] states:

> -1- In certain cases (replacement functions, handler functions, operations on types used to instantiate standard library template components), the C++ Standard Library depends on components supplied by a C++ program. If these components do not meet their requirements, the Standard places no requirements on the implementation.

> -2- In particular, the effects are undefined in the following cases:

> [...]

> - if an incomplete type (3.9) is used as a template argument when instantiating a template component.

This is contradicted by § 20.6.6.2/2 [util.smartptr.shared] which states:

> [...]

> The template parameter `T` of `shared_ptr` may be an incomplete type.

Proposed resolution:

Modify the last bullet of § 17.4.3.6/2 [res.on.functions] to allow for exceptions:

> - if an incomplete type (3.9) is used as a template argument when instantiating a template component, unless specifically allowed for the component.

### 612. numeric_limits::is_modulo insufficiently defined

Section: 18.3.2.4 [numeric.limits.members] Status: CD1 Submitter: Chris Jefferson Opened: 2006-11-10 Last modified: 2015-04-08

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Discussion:

18.2.1.2 55 states that "A type is modulo if it is possible to add two positive numbers together and have a result that wraps around to a third number that is less". This seems insufficient for the following reasons:

1. Doesn't define what that value received is.
2. Doesn't state the result is repeatable
3. Doesn't require that doing addition, subtraction and other operations on all values is defined behaviour.

[ Batavia: Related to N2144. Pete: is there an ISO definition of modulo? Underflow on signed behavior is undefined. ]

[ Bellevue: accept resolution, move to ready status. Does this mandate that is_modulo be true on platforms for which int happens to b modulo? A: the standard already seems to require that. ]

Proposed resolution:

Suggest 18.3.2.4 [numeric.limits.members], paragraph 57 is amended to:

> A type is modulo if, it is possible to add two positive numbers and have a result that wraps around to a third number that is less. given any operation involving +,- or * on values of that type whose value would fall outside the range `[min(), max()]`, then the value returned differs from the true value by an integer multiple of `(max() - min() + 1)`.

### 613. max_digits10 missing from numeric_limits

Section: 18.3.2.7 [numeric.special] Status: CD1 Submitter: Bo Persson Opened: 2006-11-20 Last modified: 2015-04-08

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Discussion:

Section 18.3.2.7 [numeric.special] starts out by saying that "All members shall be provided for all specializations."

Then it goes on to show specializations for float and bool, where one member is missing (max_digits10).

Maarten Kronenburg adds:

I agree, just adding the comment that the exact number of decimal digits is digits * ln(radix) / ln(10), where probably this real number is rounded downward for digits10, and rounded upward for max_digits10 (when radix=10, then digits10=max_digits10). Why not add this exact definition also to the standard, so the user knows what these numbers exactly mean.

Howard adds:

For reference, here are the correct formulas from N1822:

>     digits10 = floor((digits-1) * log10(2))
>     max_digits10 = ceil((1 + digits) * log10(2))

We are also missing a statement regarding for what specializations this member has meaning.

Proposed resolution:

Change and add after 18.3.2.4 [numeric.limits.members], p11:

>     static const int max_digits10;

> > -11- Number of base 10 digits required to ensure that values which differ by only one epsilon are always differentiated.

> > -12- Meaningful for all floating point types.

Change 18.3.2.7 [numeric.special], p2:

>     template<> class numeric_limits<float> {
>     public:
>       static const bool is_specialized = true;
>       ...
>       static const int digits10 = 6;
>       static const int max_digits10 = 9;
>       ...

Change 18.3.2.7 [numeric.special], p3:

>     template<> class numeric_limits<bool> {
>     public:
>       static const bool is_specialized = true;
>       ...
>       static const int digits10 = 0;
>       static const int max_digits10 = 0;
>       ...

### 616. missing 'typename' in ctype_byname

Section: 22.4.1.2 [locale.ctype.byname] Status: CD1 Submitter: Bo Persson Opened: 2006-12-16 Last modified: 2015-04-08

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Discussion:

Section 22.2.1.2 defines the ctype_byname class template. It contains the line

>     typedef ctype<charT>::mask   mask;

Proposed resolution:

as this is a dependent type, it should obviously be

>     typedef typename ctype<charT>::mask   mask;

### 618. valarray::cshift() effects on empty array

Section: 26.6.2.8 [valarray.members] Status: CD1 Submitter: Gabriel Dos Reis Opened: 2007-01-10 Last modified: 2015-04-08

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Discussion:

I would respectfully request an issue be opened with the intention to clarify the wording for `size() == 0` for `cshift`.

Proposed resolution:

Change 26.6.2.8 [valarray.members], paragraph 10:

>     valarray<T> cshift(int n) const;

> > This function returns an object of class `valarray<T>`, of length `size()`, each of whose elements `I` is `(*this)[(I + n ) % size()]`. Thus, if element zero is taken as the leftmost element, a positive value of n shifts the elements circularly left n places. that is a circular shift of `*this`. If element zero is taken as the leftmost element, a non-negative value of n shifts the elements circularly left n places and a negative value of n shifts the elements circularly right -n places.

Rationale:

We do not believe that there is any real ambiguity about what happens when `size() == 0`, but we do believe that spelling this out as a C++ expression causes more trouble that it solves. The expression is certainly wrong when `n < 0`, since the sign of % with negative arguments is implementation defined.

[ Kona (2007) Changed proposed wording, added rationale and set to Review. ]

### 619. Longjmp wording problem

Section: 18.10 [support.runtime] Status: CD1 Submitter: Lawrence Crowl Opened: 2007-01-12 Last modified: 2015-04-08

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Discussion:

The wording for `longjmp` is confusing.

18.10 [support.runtime] -4- Other runtime support

> The function signature `longjmp(jmp_buf jbuf, int val)` has more restricted behavior in this International Standard. If any automatic objects would be destroyed by a thrown exception transferring control to another (destination) point in the program, then a call to `longjmp(jbuf, val)` that the throw point that transfers control to the same (destination) point has undefined behavior.

Someone at Google thinks that should say "then a call to `longjmp(jbuf, val)` *at* the throw point that transfers control".

Bill Gibbons thinks it should say something like "If any automatic objects would be destroyed by an exception thrown at the point of the longjmp and caught only at the point of the setjmp, the behavior is undefined."

Proposed resolution:

In general, accept Bill Gibbons' recommendation, but add "call" to indicate that the undefined behavior comes from the dynamic call, not from its presence in the code. In 18.10 [support.runtime] paragraph 4, change

> The function signature `longjmp(jmp_buf jbuf, int val)` has more restricted behavior in this International Standard. If any automatic objects would be destroyed by a thrown exception transferring control to another (destination) point in the program, then a call to `longjmp(jbuf, val)` that the throw point that transfers control to the same (destination) point has undefined behavior. A `setjmp`/`longjmp` call pair has undefined behavior if replacing the `setjmp` and `longjmp` by `catch` and `throw` would destroy any automatic objects.

### 620. valid uses of empty valarrays

Section: 26.6.2.2 [valarray.cons] Status: CD1 Submitter: Martin Sebor Opened: 2007-01-20 Last modified: 2015-04-08

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Discussion:

The Effects clause for the default `valarray` ctor suggests that it is possible to increase the size of an empty `valarray` object by calling other non-const member functions of the class besides `resize()`. However, such an interpretation would be contradicted by the requirement on the copy assignment operator (and apparently also that on the computed assignments) that the assigned arrays be the same size. See the reflector discussion starting with c++std-lib-17871.

In addition, Footnote 280 uses some questionable normative language.

Proposed resolution:

Reword the Effects clause and Footnote 280 as follows (26.6.2.2 [valarray.cons]):

> `valarray();`

> Effects: Constructs an object of class `valarray<T>`,279) which has zero length until it is passed into a library function as a modifiable lvalue or through a non-constant this pointer.280)

> Postcondition: `size() == 0`.

> Footnote 280: This default constructor is essential, since arrays of `valarray` are likely to prove useful. There shall also be a way to change the size of an array after initialization; this is supplied by the semantics may be useful. The length of an empty array can be increased after initialization by means of the `resize()` member function.

### 621. non-const copy assignment operators of helper arrays

Section: 26.6 [numarray] Status: CD1 Submitter: Martin Sebor Opened: 2007-01-20 Last modified: 2015-04-08

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Discussion:

The computed and "fill" assignment operators of `valarray` helper array class templates (`slice_array`, `gslice_array`, `mask_array`, and `indirect_array`) are const member functions of each class template (the latter by the resolution of 123 since they have reference semantics and thus do not affect the state of the object on which they are called. However, the copy assignment operators of these class templates, which also have reference semantics, are non-const. The absence of constness opens the door to speculation about whether they really are intended to have reference semantics (existing implementations vary widely).

Pre-Kona, Martin adds:

I realized that adding the const qualifier to the functions as I suggested would break the const correctness of the classes. A few possible solutions come to mind:

1. Add the const qualifier to the return types of these functions.
2. Change the return type of all the functions to void to match the signatures of all the other assignment operators these classes define.
3. Prohibit the copy assignment of these classes by declaring the copy assignment operators private (as is done and documented by some implementations).

Proposed resolution:

Declare the copy assignment operators of all four helper array class templates const.

Specifically, make the following edits:

Change the signature in 26.6.5 [template.slice.array] and 26.6.5.2 [slice.arr.assign] as follows:

>     const slice_array& operator= (const slice_array&) const;

Change the signature in 26.6.7 [template.gslice.array] and 26.6.7.2 [gslice.array.assign] as follows:

>     const gslice_array& operator= (const gslice_array&) const;

Change the signature in 26.6.8 [template.mask.array] and 26.6.8.2 [mask.array.assign] as follows:

>     const mask_array& operator= (const mask_array&) const;

Change the signature in 26.6.9 [template.indirect.array] and 26.6.9.2 [indirect.array.assign] as follows:

>     const indirect_array& operator= (const indirect_array&) const;

[ Kona (2007) Added const qualification to the return types and set to Ready. ]

### 622. behavior of `filebuf` dtor and `close` on error

Section: 27.9.1.17 [fstream.members] Status: CD1 Submitter: Martin Sebor Opened: 2007-01-20 Last modified: 2015-04-08

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Discussion:

`basic_filebuf` dtor is specified to have the following straightforward effects:

> Effects: Destroys an object of class `basic_filebuf`. Calls `close()`.

`close()` does a lot of potentially complicated processing, including calling `overflow()` to write out the termination sequence (to bring the output sequence to its initial shift state). Since any of the functions called during the processing can throw an exception, what should the effects of an exception be on the dtor? Should the dtor catch and swallow it or should it propagate it to the caller? The text doesn't seem to provide any guidance in this regard other than the general restriction on throwing (but not propagating) exceptions from destructors of library classes in 17.6.5.12 [res.on.exception.handling].

Further, the last thing `close()` is specified to do is call `fclose()` to close the `FILE` pointer. The last sentence of the Effects clause reads:

> ... If any of the calls to `overflow` or `std::fclose` fails then `close` fails.

This suggests that `close()` might be required to call `fclose()` if and only if none of the calls to `overflow()` fails, and avoid closing the `FILE` otherwise. This way, if `overflow()` failed to flush out the data, the caller would have the opportunity to try to flush it again (perhaps after trying to deal with whatever problem may have caused the failure), rather than losing it outright.

On the other hand, the function's Postcondition specifies that `is_open() == false`, which suggests that it should call `fclose()` unconditionally. However, since Postcondition clauses are specified for many functions in the standard, including constructors where they obviously cannot apply after an exception, it's not clear whether this Postcondition clause is intended to apply even after an exception.

It might be worth noting that the traditional behavior (Classic Iostreams `fstream::close()` and C `fclose()`) is to close the `FILE` unconditionally, regardless of errors.

[ See 397 and 418 for related issues. ]

Proposed resolution:

After discussing this on the reflector (see the thread starting with c++std-lib-17650) we propose that `close()` be clarified to match the traditional behavior, that is to close the `FILE` unconditionally, even after errors or exceptions. In addition, we propose the dtor description be amended so as to explicitly require it to catch and swallow any exceptions thrown by `close()`.

Specifically, we propose to make the following edits in 27.9.1.4 [filebuf.members]:

>     basic_filebuf<charT,traits>* close();

> Effects: If `is_open() == false`, returns a null pointer. If a put area exists, calls `overflow(traits::eof())` to flush characters. If the last virtual member function called on `*this` (between `underflow`, `overflow`, `seekoff`, and `seekpos`) was `overflow` then calls `a_codecvt.unshift` (possibly several times) to determine a termination sequence, inserts those characters and calls `overflow(traits::eof())` again. Finally, regardless of whether any of the preceding calls fails or throws an exception, the function it closes the file ("as if" by calling `std::fclose(file)`).334) If any of the calls made by the function to `overflow` or, including `std::fclose`, fails then `close` fails by returning a null pointer. If one of these calls throws an exception, the exception is caught and rethrown after closing the file.

And to make the following edits in 27.9.1.2 [filebuf.cons].

>     virtual ~basic_filebuf();

> Effects: Destroys an object of class `basic_filebuf<charT,traits>`. Calls `close()`. If an exception occurs during the destruction of the object, including the call to `close()`, the exception is caught but not rethrown (see 17.6.5.12 [res.on.exception.handling]).

### 623. `pubimbue` forbidden to call `imbue`

Section: 27.2.1 [iostream.limits.imbue] Status: CD1 Submitter: Martin Sebor Opened: 2007-01-20 Last modified: 2015-04-08

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Discussion:

27.2.1 [iostream.limits.imbue] specifies that "no function described in clause 27 except for `ios_base::imbue` causes any instance of `basic_ios::imbue` or `basic_streambuf::imbue` to be called."

That contradicts the Effects clause for `basic_streambuf::pubimbue()` which requires the function to do just that: call `basic_streambuf::imbue()`.

Proposed resolution:

To fix this, rephrase the sentence above to allow `pubimbue` to do what it was designed to do. Specifically. change 27.2.1 [iostream.limits.imbue], p1 to read:

> No function described in clause 27 except for `ios_base::imbue` and `basic_filebuf::pubimbue` causes any instance of `basic_ios::imbue` or `basic_streambuf::imbue` to be called. ...

### 624. `valarray` assignment and arrays of unequal length

Section: 26.6.2.3 [valarray.assign] Status: CD1 Submitter: Martin Sebor Opened: 2007-01-20 Last modified: 2015-04-08

View all other issues in [valarray.assign].

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Discussion:

The behavior of the `valarray` copy assignment operator is defined only when both sides have the same number of elements and the spec is explicit about assignments of arrays of unequal lengths having undefined behavior.

However, the generalized subscripting assignment operators overloaded on `slice_array` et al (26.6.2.3 [valarray.assign]) don't have any such restriction, leading the reader to believe that the behavior of these overloads is well defined regardless of the lengths of the arguments.

For example, based on the reading of the spec the behavior of the snippet below can be expected to be well-defined:

        const std::slice from_0_to_3 (0, 3, 1);   // refers to elements 0, 1, 2
        const std::valarray<int> a (1, 3);        // a = { 1, 1, 1 }
        std::valarray<int>       b (2, 4);        // b = { 2, 2, 2, 2 }

        b = a [from_0_to_3];

In practice, `b` may end up being `{ 1, 1, 1 }`, `{ 1, 1, 1, 2 }`, or anything else, indicating that existing implementations vary.

Quoting from Section 3.4, Assignment operators, of Al Vermeulen's Proposal for Standard C++ Array Classes (see c++std-lib-704; N0308):

> ...if the size of the array on the right hand side of the equal sign differs from the size of the array on the left, a run time error occurs. How this error is handled is implementation dependent; for compilers which support it, throwing an exception would be reasonable.

And see more history in N0280.

It has been argued in discussions on the committee's reflector that the semantics of all `valarray` assignment operators should be permitted to be undefined unless the length of the arrays being assigned is the same as the length of the one being assigned from. See the thread starting at c++std-lib-17786.

In order to reflect such views, the standard must specify that the size of the array referred to by the argument of the assignment must match the size of the array under assignment, for example by adding a Requires clause to 26.6.2.3 [valarray.assign] as follows:

> Requires: The length of the array to which the argument refers equals `size()`.

Note that it's far from clear that such leeway is necessary in order to implement `valarray` efficiently.

Proposed resolution:

Insert new paragraph into 26.6.2.3 [valarray.assign]:

>     valarray<T>& operator=(const slice_array<T>&);
>     valarray<T>& operator=(const gslice_array<T>&);
>     valarray<T>& operator=(const mask_array<T>&);
>     valarray<T>& operator=(const indirect_array<T>&);

> > Requires: The length of the array to which the argument refers equals `size()`.

> > These operators allow the results of a generalized subscripting operation to be assigned directly to a `valarray`.

### 625. Mixed up Effects and Returns clauses

Section: 17 [library] Status: Resolved Submitter: Martin Sebor Opened: 2007-01-20 Last modified: 2015-04-08

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Duplicate of: 895

Discussion:

Many member functions of `basic_string` are overloaded, with some of the overloads taking a `string` argument, others `value_type*`, others `size_type`, and others still `iterators`. Often, the requirements on one of the overloads are expressed in the form of Effects, Throws, and in the Working Paper (N2134) also Remark clauses, while those on the rest of the overloads via a reference to this overload and using a Returns clause.

The difference between the two forms of specification is that per 17.5.1.4 [structure.specifications], p3, an Effects clause specifies "actions performed by the functions," i.e., its observable effects, while a Returns clause is "a description of the return value(s) of a function" that does not impose any requirements on the function's observable effects.

Since only Notes are explicitly defined to be informative and all other paragraphs are explicitly defined to be normative, like Effects and Returns, the new Remark clauses also impose normative requirements.

So by this strict reading of the standard there are some member functions of `basic_string` that are required to throw an exception under some conditions or use specific traits members while many other otherwise equivalent overloads, while obliged to return the same values, aren't required to follow the exact same requirements with regards to the observable effects.

Here's an example of this problem that was precipitated by the change from informative Notes to normative Remarks (presumably made to address 424):

In the Working Paper, `find(string, size_type)` contains a Remark clause (which is just a Note in the current standard) requiring it to use `traits::eq()`.

`find(const charT *s, size_type pos)` is specified to return `find(string(s), pos)` by a Returns clause and so it is not required to use `traits::eq()`. However, the Working Paper has replaced the original informative Note about the function using `traits::length()` with a normative requirement in the form of a Remark. Calling `traits::length()` may be suboptimal, for example when the argument is a very long array whose initial substring doesn't appear anywhere in `*this`.

Here's another similar example, one that existed even prior to the introduction of Remarks:

`insert(size_type pos, string, size_type, size_type)` is required to throw `out_of_range` if `pos > size()`.

`insert(size_type pos, string str)` is specified to return `insert(pos, str, 0, npos)` by a Returns clause and so its effects when `pos > size()` are strictly speaking unspecified.

I believe a careful review of the current Effects and Returns clauses is needed in order to identify all such problematic cases. In addition, a review of the Working Paper should be done to make sure that the newly introduced normative Remark clauses do not impose any undesirable normative requirements in place of the original informative Notes.

[ Batavia: Alan and Pete to work. ]

[ Bellevue: Marked as NAD Editorial. ]

[ Post-Sophia Antipolis: Martin indicates there is still work to be done on this issue. Reopened. ]

[ Batavia (2009-05): ]

> Tom proposes we say that, unless specified otherwise, it is always the caller's responsibility to verify that supplied arguments meet the called function's requirements. If further semantics are specified (e.g., that the function throws under certain conditions), then it is up to the implementer to check those conditions. Alan feels strongly that our current use of Requires in this context is confusing, especially now that `requires` is a new keyword.

[ 2009-07 Frankfurt ]

> Move to Tentatively NAD.

[ 2009 Santa Cruz: ]

> Move to Open. Martin will work on proposed wording.

[ 2010 Pittsburgh: ]

> Moved to NAD Editorial, solved by revision to N3021.

Rationale:

Solved by revision to N3021.

Proposed resolution:

### 628. Inconsistent definition of basic_regex constructor

Section: 28.8 [re.regex] Status: CD1 Submitter: Bo Persson Opened: 2007-01-23 Last modified: 2015-04-08

View all other issues in [re.regex].

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Discussion:

Section 28.8 [re.regex] lists a constructor

>     template<class InputIterator>
>     basic_regex(InputIterator first, InputIterator last,
>                            flag_type f = regex_constants::ECMAScript);

However, in section 28.8.2 [re.regex.construct], this constructor takes a pair of `ForwardIterator`.

Proposed resolution:

Change 28.8.2 [re.regex.construct]:

>     template <class ForwardIterator InputIterator>
>       basic_regex(ForwardIterator InputIterator first, ForwardIterator InputIterator last,
>                   flag_type f = regex_constants::ECMAScript);

### 629. `complex<T>` insertion and locale dependence

Section: 26.4.6 [complex.ops] Status: CD1 Submitter: Gabriel Dos Reis Opened: 2007-01-28 Last modified: 2015-04-08

View all other issues in [complex.ops].

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Discussion:

is there an issue opened for (0,3) as complex number with the French local? With the English local, the above parses as an imaginery complex number. With the French locale it parses as a real complex number.

Further notes/ideas from the lib-reflector, messages 17982-17984:

> Add additional entries in `num_punct` to cover the complex separator (French would be ';').

> Insert a space before the comma, which should eliminate the ambiguity.

> Solve the problem for ordered sequences in general, perhaps with a dedicated facet. Then complex should use that solution.

[ Bellevue: ]

> After much discussion, we agreed on the following: Add a footnote:

> [In a locale in which comma is being used as a decimal point character, inserting "showbase" into the output stream forces all outputs to show an explicit decimal point character; then all inserted complex sequences will extract unambiguously.]

> And move this to READY status.

[ Pre-Sophia Antipolis, Howard adds: ]

> Changed "showbase" to "showpoint" and changed from Ready to Review.

[ Post-Sophia Antipolis: ]

> I neglected to pull this issue from the formal motions page after the "showbase" to "showpoint" change. In Sophia Antipolis this change was reviewed by the LWG and the issue was set to Ready. We subsequently voted the footnote into the WP with "showbase".

> I'm changing from WP back to Ready to pick up the "showbase" to "showpoint" change.

Proposed resolution:

Add a footnote to 26.4.6 [complex.ops] p16:

> [In a locale in which comma is being used as a decimal point character, inserting `showpoint` into the output stream forces all outputs to show an explicit decimal point character; then all inserted complex sequences will extract unambiguously.]

### 630. arrays of `valarray`

Section: 26.6.2.2 [valarray.cons] Status: C++11 Submitter: Martin Sebor Opened: 2007-01-28 Last modified: 2015-04-08

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Discussion:

Section 26.2 [numeric.requirements], p1 suggests that a `valarray` specialization on a type `T` that satisfies the requirements enumerated in the paragraph is itself a valid type on which `valarray` may be instantiated (Footnote 269 makes this clear). I.e., `valarray<valarray<T> >` is valid as long as `T` is valid. However, since implementations of `valarray` are permitted to initialize storage allocated by the class by invoking the default ctor of `T` followed by the copy assignment operator, such implementations of `valarray` wouldn't work with (perhaps user-defined) specializations of `valarray` whose assignment operator had undefined behavior when the size of its argument didn't match the size of `*this`. By "wouldn't work" I mean that it would be impossible to resize such an array of arrays by calling the `resize()` member function on it if the function used the copy assignment operator after constructing all elements using the default ctor (e.g., by invoking `new value_type[N]`) to obtain default-initialized storage) as it's permitted to do.

Stated more generally, the problem is that `valarray<valarray<T> >::resize(size_t)` isn't required or guaranteed to have well-defined semantics for every type `T` that satisfies all requirements in 26.2 [numeric.requirements].

I believe this problem was introduced by the adoption of the resolution outlined in N0857, Assignment of valarrays, from 1996. The copy assignment operator of the original numerical array classes proposed in N0280, as well as the one proposed in N0308 (both from 1993), had well-defined semantics for arrays of unequal size (the latter explicitly only when `*this` was empty; assignment of non empty arrays of unequal size was a runtime error).

The justification for the change given in N0857 was the "loss of performance [deemed] only significant for very simple operations on small arrays or for architectures with very few registers."

Since tiny arrays on a limited subset of hardware architectures are likely to be an exceedingly rare case (despite the continued popularity of x86) I propose to revert the resolution and make the behavior of all `valarray` assignment operators well-defined even for non-conformal arrays (i.e., arrays of unequal size). I have implemented this change and measured no significant degradation in performance in the common case (non-empty arrays of equal size). I have measured a 50% (and in some cases even greater) speedup in the case of assignments to empty arrays versus calling `resize()` first followed by an invocation of the copy assignment operator.

[ Bellevue: ]

> If no proposed wording by June meeting, this issue should be closed NAD.

[ 2009-07 Frankfurt ]

> Move resolution 1 to Ready.

> Howard: second resolution has been commented out (made invisible). Can be brought back on demand.

Proposed resolution:

Change 26.6.2.3 [valarray.assign], p1 as follows:

> `valarray<T>& operator=(const valarray<T>& x);`

> -1- Each element of the `*this` array is assigned the value of the corresponding element of the argument array. The resulting behavior is undefined if When the length of the argument array is not equal to the length of the *this array. resizes `*this` to make the two arrays the same length, as if by calling `resize(x.size())`, before performing the assignment.

And add a new paragraph just below paragraph 1 with the following text:

> -2- Postcondition: `size() == x.size()`.

Also add the following paragraph to 26.6.2.3 [valarray.assign], immediately after p4:

> -?- When the length, `N` of the array referred to by the argument is not equal to the length of `*this`, the operator resizes `*this` to make the two arrays the same length, as if by calling `resize(N)`, before performing the assignment.

[ pre-Sophia Antipolis, Martin adds the following compromise wording, but prefers the original proposed resolution: ]

[ Kona (2007): Gaby to propose wording for an alternative resolution in which you can assign to a `valarray` of size 0, but not to any other `valarray` whose size is unequal to the right hand side of the assignment. ]

### 634. `allocator.address()` doesn't work for types overloading `operator&`

Section: 20.7.9.1 [allocator.members] Status: CD1 Submitter: Howard Hinnant Opened: 2007-02-07 Last modified: 2015-04-08

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Duplicate of: 350

Discussion:

20.7.9.1 [allocator.members] says:

>     pointer address(reference x) const;

> > -1- Returns: `&x`.

20.7.9.1 [allocator.members] defines `CopyConstructible` which currently not only defines the semantics of copy construction, but also restricts what an overloaded `operator&` may do. I believe proposals are in the works (such as concepts and rvalue reference) to decouple these two requirements. Indeed it is not evident that we should disallow overloading `operator&` to return something other than the address of `*this`.

An example of when you want to overload `operator&` to return something other than the object's address is proxy references such as `vector<bool>` (or its replacement, currently code-named `bit_vector`). Taking the address of such a proxy reference should logically yield a proxy pointer, which when dereferenced, yields a copy of the original proxy reference again.

On the other hand, some code truly needs the address of an object, and not a proxy (typically for determining the identity of an object compared to a reference object). boost has long recognized this dilemma and solved it with `boost::addressof`. It appears to me that this would be useful functionality for the default allocator. Adopting this definition for `allocator::address` would free the standard of requiring anything special from types which overload `operator&`. Issue 580 is expected to make use of `allocator::address` mandatory for containers.

Proposed resolution:

Change 20.7.9.1 [allocator.members]:

>     pointer address(reference x) const;

> > -1- Returns: `&x`. The actual address of object referenced by x, even in the presence of an overloaded `operator&`.

>     const_pointer address(address(const_reference x) const;

> > -2- Returns: `&x`. The actual address of object referenced by x, even in the presence of an overloaded `operator&`.

[ post Oxford: This would be rendered NAD Editorial by acceptance of N2257. ]

[ Kona (2007): The LWG adopted the proposed resolution of N2387 for this issue which was subsequently split out into a separate paper N2436 for the purposes of voting. The resolution in N2436 addresses this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 635. domain of `allocator::address`

Section: 17.6.3.5 [allocator.requirements] Status: Resolved Submitter: Howard Hinnant Opened: 2007-02-08 Last modified: 2015-04-08

View other active issues in [allocator.requirements].

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Discussion:

The table of allocator requirements in 17.6.3.5 [allocator.requirements] describes `allocator::address` as:

>     a.address(r)
>     a.address(s)

where `r` and `s` are described as:

> a value of type `X::reference` obtained by the expression `*p`.

and `p` is

> a value of type `X::pointer`, obtained by calling `a1.allocate`, where `a1 == a`

This all implies that to get the address of some value of type `T` that value must have been allocated by this allocator or a copy of it.

However sometimes container code needs to compare the address of an external value of type `T` with an internal value. For example `list::remove(const T& t)` may want to compare the address of the external value `t` with that of a value stored within the list. Similarly `vector` or `deque insert` may want to make similar comparisons (to check for self-referencing calls).

Mandating that `allocator::address` can only be called for values which the allocator allocated seems overly restrictive.

[ post San Francisco: ]

> Pablo recommends NAD Editorial, solved by N2768.

[ 2009-04-28 Pablo adds: ]

> Tentatively-ready NAD Editorial as fixed by N2768.

[ 2009-07 Frankfurt ]

> Fixed by N2768.

[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Addressed by N2982.

Proposed resolution:

Change 17.6.3.5 [allocator.requirements]:

> `r` : a value of type `X::reference` obtained by the expression *p.

> `s` : a value of type `X::const_reference` obtained by the expression `*q` or by conversion from a value `r`.

[ post Oxford: This would be rendered NAD Editorial by acceptance of N2257. ]

[ Kona (2007): This issue is section 8 of N2387. There was some discussion of it but no resolution to this issue was recorded. Moved to Open. ]

### 638. `deque` end invalidation during erase

Section: 23.3.3.4 [deque.modifiers] Status: CD1 Submitter: Steve LoBasso Opened: 2007-02-17 Last modified: 2015-04-08

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Discussion:

The standard states at 23.3.3.4 [deque.modifiers]/4:

>     deque erase(...)

> Effects: ... An erase at either end of the deque invalidates only the iterators and the references to the erased elements.

This does not state that iterators to end will be invalidated. It needs to be amended in such a way as to account for end invalidation.

Something like:

> Any time the last element is erased, iterators to end are invalidated.

This would handle situations like:

>     erase(begin(), end())
>     erase(end() - 1)
>     pop_back()
>     resize(n, ...) where n < size()
>     pop_front() with size() == 1

[ Post Kona, Steve LoBasso notes: ]

> My only issue with the proposed resolution is that it might not be clear that `pop_front()` [where `size() == 1`] can invalidate past-the-end iterators.

Proposed resolution:

Change 23.3.3.4 [deque.modifiers], p4:

>     iterator erase(const_iterator position);
>     iterator erase(const_iterator first, const_iterator last);

> > -4- Effects: An erase in the middle of the `deque` invalidates all the iterators and references to elements of the `deque` and the past-the-end iterator. An erase at either end of the `deque` invalidates only the iterators and the references to the erased elements, except that erasing at the end also invalidates the past-the-end iterator.

[ Kona (2007): Proposed wording added and moved to Review. ]

[ Bellevue: ]

> Note that there is existing code that relies on iterators not being invalidated, but there are also existing implementations that do invalidate iterators. Thus, such code is not portable in any case. There is a `pop_front()` note, which should possibly be a separate issue. Mike Spertus to evaluate and, if need be, file an issue.

### 640. 27.6.2.5.2 does not handle `(unsigned) long long`

Section: 27.7.3.6.2 [ostream.inserters.arithmetic] Status: CD1 Submitter: Daniel Krügler Opened: 2007-02-17 Last modified: 2015-04-08

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Discussion:

The arithmetic inserters are described in 27.7.3.6.2 [ostream.inserters.arithmetic]. Although the section starts with a listing of the inserters including the new ones:

>     operator<<(long long val );
>     operator<<(unsigned long long val );

the text in paragraph 1, which describes the corresponding effects of the inserters, depending on the actual type of val, does not handle the types `long long` and `unsigned long long`.

[ Alisdair: In addition to the (unsigned) long long problem, that whole paragraph misses any reference to extended integral types supplied by the implementation - one of the additions by core a couple of working papers back. ]

Proposed resolution:

In 27.7.3.6.2 [ostream.inserters.arithmetic]/1 change the third sentence

> When val is of type `bool`, `long`, `unsigned long`, long long, unsigned long long, `double`, `long double`, or `const void*`, the formatting conversion occurs as if it performed the following code fragment:

### 643. Impossible "as if" clauses

Section: 27.9.1.1 [filebuf], 22.4.2.2.2 [facet.num.put.virtuals] Status: CD1 Submitter: Daniel Krügler Opened: 2007-02-20 Last modified: 2015-04-08

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Discussion:

The current standard 14882:2003(E) as well as N2134 have the following defects:

27.9.1.1 [filebuf]/5 says:

> In order to support file I/O and multibyte/wide character conversion, conversions are performed using members of a facet, referred to as `a_codecvt` in following sections, obtained "as if" by

> >     codecvt<charT,char,typename traits::state_type> a_codecvt =
> >       use_facet<codecvt<charT,char,typename traits::state_type> >(getloc());

`use_facet` returns a `const facet` reference and no facet is copyconstructible, so the codecvt construction should fail to compile.

A similar issue arises in 22.4.2.2.2 [facet.num.put.virtuals]/15 for `num_punct`.

Proposed resolution:

In 27.9.1.1 [filebuf]/5 change the "as if" code

>     const codecvt<charT,char,typename traits::state_type>& a_codecvt =
>       use_facet<codecvt<charT,char,typename traits::state_type> >(getloc());

In 22.4.2.2.2 [facet.num.put.virtuals]/15 (This is para 5 in N2134) change

> A local variable `punct` is initialized via

> >     const numpunct<charT>& punct = use_facet< numpunct<charT> >(str.getloc() );

(Please note also the additional provided trailing semicolon)

### 646. const incorrect match_result members

Section: 28.10.5 [re.results.form] Status: CD1 Submitter: Daniel Krügler Opened: 2007-02-26 Last modified: 2015-04-08

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Discussion:

28.10.5 [re.results.form] (root and para 3) in N2134 defines the two function template members format as non-const functions, although they are declared as const in 28.10 [re.results]/3.

Proposed resolution:

Add the missing `const` specifier to both `format` overloads described in section 28.10.5 [re.results.form].

### 650. regex_token_iterator and const correctness

Section: 28.12.2 [re.tokiter] Status: CD1 Submitter: Daniel Krügler Opened: 2007-03-05 Last modified: 2015-04-08

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Discussion:

Both the class definition of regex_token_iterator (28.12.2 [re.tokiter]/6) and the latter member specifications (28.12.2.2 [re.tokiter.comp]/1+2) declare both comparison operators as non-const functions. Furtheron, both dereference operators are unexpectedly also declared as non-const in 28.12.2 [re.tokiter]/6 as well as in (28.12.2.3 [re.tokiter.deref]/1+2).

Proposed resolution:

1) In (28.12.2 [re.tokiter]/6) change the current declarations

>     bool operator==(const regex_token_iterator&) const;
>     bool operator!=(const regex_token_iterator&) const;
>     const value_type& operator*() const;
>     const value_type* operator->() const;

2) In 28.12.2.2 [re.tokiter.comp] change the following declarations

>     bool operator==(const regex_token_iterator& right) const;
>     bool operator!=(const regex_token_iterator& right) const;

3) In 28.12.2.3 [re.tokiter.deref] change the following declarations

>     const value_type& operator*() const;
>     const value_type* operator->() const;

[ Kona (2007): The LWG adopted the proposed resolution of N2409 for this issue (which is to adopt the proposed wording in this issue). The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 651. Missing preconditions for regex_token_iterator c'tors

Section: 28.12.2.1 [re.tokiter.cnstr] Status: CD1 Submitter: Daniel Krügler Opened: 2007-03-05 Last modified: 2015-04-08

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Discussion:

The text provided in 28.12.2.1 [re.tokiter.cnstr]/2+3 describes the effects of the three non-default constructors of class template regex_token_iterator but is does not clarify which values are legal values for submatch/submatches. This becomes an issue, if one takes 28.12.2 [re.tokiter]/9 into account, which explains the notion of a "current match" by saying:

> The current match is `(*position).prefix()` if `subs[N] == -1`, or `(*position)[subs[N]]` for any other value of `subs[N]`.

It's not clear to me, whether other negative values except -1 are legal arguments or not - it seems they are not.

Proposed resolution:

Add the following precondition paragraph just before the current 28.12.2.1 [re.tokiter.cnstr]/2:

> Requires: Each of the initialization values of `subs` must be `>= -1`.

[ Kona (2007): The LWG adopted the proposed resolution of N2409 for this issue (which is to adopt the proposed wording in this issue). The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 652. regex_iterator and const correctness

Section: 28.12.1 [re.regiter] Status: CD1 Submitter: Daniel Krügler Opened: 2007-03-05 Last modified: 2015-04-08

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Discussion:

Both the class definition of regex_iterator (28.12.1 [re.regiter]/1) and the latter member specification (28.12.1.2 [re.regiter.comp]/1+2) declare both comparison operators as non-const functions. Furtheron, both dereference operators are unexpectedly also declared as non-const in 28.12.1 [re.regiter]/1 as well as in (28.12.1.3 [re.regiter.deref]/1+2).

Proposed resolution:

1) In (28.12.1 [re.regiter]/1) change the current declarations

>     bool operator==(const regex_iterator&) const;
>     bool operator!=(const regex_iterator&) const;
>     const value_type& operator*() const;
>     const value_type* operator->() const;

2) In 28.12.1.3 [re.regiter.deref] change the following declarations

>     const value_type& operator*() const;
>     const value_type* operator->() const;

3) In 28.12.1.2 [re.regiter.comp] change the following declarations

>     bool operator==(const regex_iterator& right) const;
>     bool operator!=(const regex_iterator& right) const;

[ Kona (2007): The LWG adopted the proposed resolution of N2409 for this issue (which is to adopt the proposed wording in this issue). The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 654. Missing IO roundtrip for random number engines

Section: 26.5.1.4 [rand.req.eng] Status: CD1 Submitter: Daniel Krügler Opened: 2007-03-08 Last modified: 2015-04-08

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Discussion:

Table 98 and para 5 in 26.5.1.4 [rand.req.eng] specify the IO insertion and extraction semantic of random number engines. It can be shown, v.i., that the specification of the extractor cannot guarantee to fulfill the requirement from para 5:

> If a textual representation written via os << x was subsequently read via is >> v, then x == v provided that there have been no intervening invocations of x or of v.

The problem is, that the extraction process described in table 98 misses to specify that it will initially set the if.fmtflags to ios_base::dec, see table 104:

> dec: converts integer input or generates integer output in decimal base

Proof: The following small program demonstrates the violation of requirements (exception safety not fulfilled):

>     #include <cassert>
>     #include <ostream>
>     #include <iostream>
>     #include <iomanip>
>     #include <sstream>

>     class RanNumEngine {
>       int state;
>     public:
>       RanNumEngine() : state(42) {}

>       bool operator==(RanNumEngine other) const {
>           return state == other.state;
>       }

>       template <typename Ch, typename Tr>
>       friend std::basic_ostream<Ch, Tr>& operator<<(std::basic_ostream<Ch, Tr>& os, RanNumEngine engine) {
>         Ch old = os.fill(os.widen(' ')); // Sets space character
>         std::ios_base::fmtflags f = os.flags();
>         os << std::dec << std::left << engine.state; // Adds ios_base::dec|ios_base::left
>         os.fill(old); // Undo
>         os.flags(f);
>         return os;
>       }

>       template <typename Ch, typename Tr>
>       friend std::basic_istream<Ch, Tr>& operator>>(std::basic_istream<Ch, Tr>& is, RanNumEngine& engine) {
>            // Uncomment only for the fix.

>         //std::ios_base::fmtflags f = is.flags();
>         //is >> std::dec;
>         is >> engine.state;
>         //is.flags(f);
>         return is;
>       }
>     };

>     int main() {
>         std::stringstream s;
>         s << std::setfill('#'); // No problem
>             s << std::oct; // Yikes!
>             // Here starts para 5 requirements:
>         RanNumEngine x;
>         s << x;
>         RanNumEngine v;
>         s >> v;
>         assert(x == v); // Fails: 42 == 34
>     }

A second, minor issue seems to be, that the insertion description from table 98 unnecessarily requires the addition of ios_base::fixed (which only influences floating-point numbers). Its not entirely clear to me whether the proposed standard does require that the state of random number engines is stored in integral types or not, but I have the impression that this is the indent, see e.g. p. 3

> The specification of each random number engine defines the size of its state in multiples of the size of its result_type.

If other types than integrals are supported, then I wonder why no requirements are specified for the precision of the stream.

See N2391 and N2423 for some further discussion.

Proposed resolution:

Adopt the proposed resolution in N2423.

[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 655. Signature of generate_canonical not useful

Section: 26.5.7.2 [rand.util.canonical] Status: CD1 Submitter: Daniel Krügler Opened: 2007-03-08 Last modified: 2015-04-08

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Discussion:

In 26.5.2 [rand.synopsis] we have the declaration

>     template<class RealType, class UniformRandomNumberGenerator,
>       size_t bits>
>     result_type generate_canonical(UniformRandomNumberGenerator& g);

Besides the "result_type" issue (already recognized by Bo Persson at Sun, 11 Feb 2007 05:26:47 GMT in this group) it's clear, that the template parameter order is not reasonably choosen: Obviously one always needs to specify all three parameters, although usually only two are required, namely the result type RealType and the wanted bits, because UniformRandomNumberGenerator can usually be deduced.

See N2391 and N2423 for some further discussion.

Proposed resolution:

Adopt the proposed resolution in N2423.

[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 658. Two unspecified function comparators in [function.objects]

Section: 20.9 [function.objects] Status: Resolved Submitter: Daniel Krügler Opened: 2007-03-19 Last modified: 2015-04-08

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Discussion:

The header `<functional>` synopsis in 20.9 [function.objects] contains the following two free comparison operator templates for the `function` class template

>     template<class Function1, class Function2>
>     void operator==(const function<Function1>&, const function<Function2>&);
>     template<class Function1, class Function2>
>     void operator!=(const function<Function1>&, const function<Function2>&);

which are nowhere described. I assume that they are relicts before the corresponding two private and undefined member templates in the function template (see 20.9.12.2 [func.wrap.func] and [func.wrap.func.undef]) have been introduced. The original free function templates should be removed, because using an undefined entity would lead to an ODR violation of the user.

Proposed resolution:

Remove the above mentioned two function templates from the header `<functional>` synopsis (20.9 [function.objects])

>     template<class Function1, class Function2>
>     void operator==(const function<Function1>&, const function<Function2>&);
>     template<class Function1, class Function2>
>     void operator!=(const function<Function1>&, const function<Function2>&);

Rationale:

Fixed by N2292 Standard Library Applications for Deleted Functions.

### 659. `istreambuf_iterator` should have an `operator->()`

Section: 24.6.3 [istreambuf.iterator] Status: C++11 Submitter: Niels Dekker Opened: 2007-03-25 Last modified: 2015-04-08

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Discussion:

Greg Herlihy has clearly demonstrated that a user defined input iterator should have an `operator->()`, even if its value type is a built-in type (comp.std.c++, "Re: Should any iterator have an `operator->()` in C++0x?", March 2007). And as Howard Hinnant remarked in the same thread that the input iterator `istreambuf_iterator` doesn't have one, this must be a defect!

Based on Greg's example, the following code demonstrates the issue:

     #include <iostream>
     #include <fstream>
     #include <streambuf>

     typedef char C;
     int main ()
     {
       std::ifstream s("filename", std::ios::in);
       std::istreambuf_iterator<char> i(s);

       (*i).~C();  // This is well-formed...
       i->~C();  // ... so this should be supported!
     }

Of course, `operator->` is also needed when the `value_type` of `istreambuf_iterator` is a class.

The `operator->` could be implemented in various ways. For instance, by storing the current value inside the iterator, and returning its address. Or by returning a proxy, like `operator_arrow_proxy`, from http://www.boost.org/boost/iterator/iterator_facade.hpp

I hope that the resolution of this issue will contribute to getting a clear and consistent definition of iterator concepts.

[ Kona (2007): The proposed resolution is inconsistent because the return type of `istreambuf_iterator::operator->()` is specified to be `pointer`, but the proposed text also states that "`operator->` may return a proxy." ]

[ Niels Dekker (mailed to Howard Hinnant): ]

> The proposed resolution does not seem inconsistent to me. `istreambuf_iterator::operator->()` should have `istreambuf_iterator::pointer` as return type, and this return type may in fact be a proxy.

> AFAIK, the resolution of 445 ("`iterator_traits::reference` unspecified for some iterator categories") implies that for any iterator class `Iter`, the return type of `operator->()` is `Iter::pointer`, by definition. I don't think `Iter::pointer` needs to be a raw pointer.

> Still I wouldn't mind if the text "`operator->` may return a proxy" would be removed from the resolution. I think it's up to the library implementation, how to implement `istreambuf_iterator::operator->()`. As longs as it behaves as expected: `i->m` should have the same effect as `(*i).m`. Even for an explicit destructor call, `i->~C()`. The main issue is just: `istreambuf_iterator` should have an `operator->()`!

[ 2009-04-30 Alisdair adds: ]

> Note that `operator->` is now a requirement in the `InputIterator` concept, so this issue cannot be ignored or existing valid programs will break when compiled with an 0x library.

[ 2009-05-29 Alisdair adds: ]

> I agree with the observation that in principle the type 'pointer' may be a proxy, and the words highlighting this are redundant.

> However, in the current draught `pointer` is required to be exactly '`charT *`' by the derivation from `std::iterator`. At a minimum, the 4th parameter of this base class template should become unspecified. That permits the introduction of a proxy as a nested class in some further undocumented (not even exposition-only) base.

> It also permits the `istream_iterator` approach where the cached value is stored in the iterator itself, and the iterator serves as its own proxy for post-increment `operator++` - removing the need for the existing exposition-only nested class `proxy`.

> Note that the current `proxy` class also has exactly the right properties to serve as the pointer `proxy` too. This is likely to be a common case where an `InputIterator` does not hold internal state but delegates to another class.

> Proposed Resolution:

> In addition to the current proposal:

> 24.6.3 [istreambuf.iterator]

> >     template<class charT, class traits = char_traits<charT> >
> >     class istreambuf_iterator
> >       : public iterator<input_iterator_tag, charT,
> >                         typename traits::off_type, charT* unspecified, charT> {

[ 2009-07 Frankfurt ]

> Move the additional part into the proposed resolution, and wrap the descriptive text in a Note.

> [Howard: done.]

> Move to Ready.

Proposed resolution:

Add to the synopsis in 24.6.3 [istreambuf.iterator]:

>     charT operator*() const;
>     pointer operator->() const;
>     istreambuf_iterator<charT,traits>& operator++();

24.6.3 [istreambuf.iterator]

>     template<class charT, class traits = char_traits<charT> >
>     class istreambuf_iterator
>       : public iterator<input_iterator_tag, charT,
>                         typename traits::off_type, charT* unspecified, charT> {

Change 24.6.3 [istreambuf.iterator], p1:

> The class template `istreambuf_iterator` reads successive characters from the `streambuf` for which it was constructed. `operator*` provides access to the current input character, if any. [Note: `operator->` may return a proxy. — end note] Each time `operator++` is evaluated, the iterator advances to the next input character. If the end of stream is reached (`streambuf_type::sgetc()` returns `traits::eof()`), the iterator becomes equal to the end of stream iterator value. The default constructor `istreambuf_iterator()` and the constructor `istreambuf_iterator(0)` both construct an end of stream iterator object suitable for use as an end-of-range.

### 660. Missing Bitwise Operations

Section: 20.9 [function.objects] Status: CD1 Submitter: Beman Dawes Opened: 2007-04-02 Last modified: 2015-04-08

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Discussion:

Section 20.9 [function.objects] provides function objects for some unary and binary operations, but others are missing. In a LWG reflector discussion, beginning with c++std-lib-18078, pros and cons of adding some of the missing operations were discussed. Bjarne Stroustrup commented "Why standardize what isn't used? Yes, I see the chicken and egg problems here, but it would be nice to see a couple of genuine uses before making additions."

A number of libraries, including Rogue Wave, GNU, Adobe ASL, and Boost, have already added these functions, either publicly or for internal use. For example, Doug Gregor commented: "Boost will also add ... (|, &, ^) in 1.35.0, because we need those function objects to represent various parallel collective operations (reductions, prefix reductions, etc.) in the new Message Passing Interface (MPI) library."

Because the bitwise operators have the strongest use cases, the proposed resolution is limited to them.

Proposed resolution:

To 20.9 [function.objects], Function objects, paragraph 2, add to the header <functional> synopsis:

>     template <class T> struct bit_and;
>     template <class T> struct bit_or;
>     template <class T> struct bit_xor;

At a location in clause 20 to be determined by the Project Editor, add:

> The library provides basic function object classes for all of the bitwise operators in the language ([expr.bit.and], [expr.or], [exp.xor]).

>     template <class T> struct bit_and : binary_function<T,T,T> {
>       T operator()(const T& x , const T& y ) const;
>     };

> > `operator()` returns`x & y` .

>     template <class T> struct bit_or : binary_function<T,T,T> {
>       T operator()(const T& x , const T& y ) const;
>     };

> > `operator()` returns `x | y` .

>     template <class T> struct bit_xor : binary_function<T,T,T> {
>       T operator()(const T& x , const T& y ) const;
>     };

> > `operator()` returns `x ^ y` .

### 661. New 27.6.1.2.2 changes make special extractions useless

Section: 27.7.2.2.2 [istream.formatted.arithmetic] Status: CD1 Submitter: Daniel Krügler Opened: 2007-04-01 Last modified: 2015-04-08

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Discussion:

To the more drastic changes of 27.7.2.2.2 [istream.formatted.arithmetic] in the current draft N2134 belong the explicit description of the extraction of the types short and int in terms of as-if code fragments.

1. The corresponding as-if extractions in paragraph 2 and 3 will never result in a change of the operator>> argument val, because the contents of the local variable lval is in no case written into val. Furtheron both fragments need a currently missing parentheses in the beginning of the if-statement to be valid C++.
2. I would like to ask whether the omission of a similar explicit extraction of unsigned short and unsigned int in terms of long - compared to their corresponding new insertions, as described in 27.7.3.6.2 [ostream.inserters.arithmetic], is a deliberate decision or an oversight.

Proposed resolution:

1. In 27.7.2.2.2 [istream.formatted.arithmetic]/2 change the current as-if code fragment

   >     typedef num_get<charT,istreambuf_iterator<charT,traits> > numget;
   >     iostate err = 0;
   >     long lval;
   >     use_facet<numget>(loc).get(*this, 0, *this, err, lval );
   >     if (err == 0) {
   >       && if (lval < numeric_limits<short>::min() || numeric_limits<short>::max() < lval))
   >           err = ios_base::failbit;
   >       else
   >         val = static_cast<short>(lval);
   >     }
   >     setstate(err);

   Similarily in 27.7.2.2.2 [istream.formatted.arithmetic]/3 change the current as-if fragment

   >     typedef num_get<charT,istreambuf_iterator<charT,traits> > numget;
   >     iostate err = 0;
   >     long lval;
   >     use_facet<numget>(loc).get(*this, 0, *this, err, lval );
   >     if (err == 0) {
   >       && if (lval < numeric_limits<int>::min() || numeric_limits<int>::max() < lval))
   >           err = ios_base::failbit;
   >       else
   >         val = static_cast<int>(lval);
   >     }
   >     setstate(err);

2. ---

[ Kona (2007): Note to the editor: the name lval in the call to `use_facet` is incorrectly italicized in the code fragments corresponding to `operator>>(short &)` and `operator >>(int &)`. Also, val -- which appears twice on the line with the `static_cast` in the proposed resolution -- should be italicized. Also, in response to part two of the issue: this is deliberate. ]

### 664. `do_unshift` for `codecvt<char, char, mbstate_t>`

Section: 22.4.1.4.2 [locale.codecvt.virtuals] Status: CD1 Submitter: Thomas Plum Opened: 2007-04-16 Last modified: 2015-04-08

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Discussion:

22.4.1.4.2 [locale.codecvt.virtuals], para 7 says (regarding `do_unshift`):

> Effects: Places characters starting at to that should be appended to terminate a sequence when the current `stateT` is given by `state`.237) Stores no more than `(to_limit - to)` destination elements, and leaves the `to_next` pointer pointing one beyond the last element successfully stored. `codecvt<char, char, mbstate_t>` stores no characters.

The following objection has been raised:

> Since the C++ Standard permits a nontrivial conversion for the required instantiations of `codecvt`, it is overly restrictive to say that `do_unshift` must store no characters and return `noconv`.

[Plum ref _222152Y50]

Proposed resolution:

Change 22.4.1.4.2 [locale.codecvt.virtuals], p7:

> Effects: Places characters starting at to that should be appended to terminate a sequence when the current `stateT` is given by state.237) Stores no more than (to_limit -to) destination elements, and leaves the to_next pointer pointing one beyond the last element successfully stored. `codecvt<char, char, mbstate_t>` stores no characters.

### 665. `do_unshift` return value

Section: 22.4.1.4.2 [locale.codecvt.virtuals] Status: CD1 Submitter: Thomas Plum Opened: 2007-04-16 Last modified: 2015-04-08

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Discussion:

22.4.1.4.2 [locale.codecvt.virtuals], para 8 says:

> `codecvt<char,char,mbstate_t>`, returns `noconv`.

The following objection has been raised:

> Despite what the C++ Standard says, `unshift` can't always return `noconv` for the default facets, since they can be nontrivial. At least one implementation does whatever the C functions do.

[Plum ref _222152Y62]

Proposed resolution:

Change 22.4.1.4.2 [locale.codecvt.virtuals], p8:

> Returns: An enumeration value, as summarized in Table 76:

> ...

> `codecvt<char,char,mbstate_t>`, returns `noconv`.

### 666. `moneypunct::do_curr_symbol()`

Section: 22.4.6.3.2 [locale.moneypunct.virtuals] Status: CD1 Submitter: Thomas Plum Opened: 2007-04-16 Last modified: 2015-04-08

View all other issues in [locale.moneypunct.virtuals].

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Discussion:

22.4.6.3.2 [locale.moneypunct.virtuals], para 4 footnote 257 says

> 257) For international specializations (second template parameter `true`) this is always four characters long, usually three letters and a space.

The following objection has been raised:

> The international currency symbol is whatever the underlying locale says it is, not necessarily four characters long.

[Plum ref _222632Y41]

Proposed resolution:

Change footnote 253 in 22.4.6.3.2 [locale.moneypunct.virtuals]:

> 253) For international specializations (second template parameter `true`) this is always typically four characters long, usually three letters and a space.

### 671. precision of `hexfloat`

Section: 22.4.2.2.2 [facet.num.put.virtuals] Status: C++11 Submitter: John Salmon Opened: 2007-04-20 Last modified: 2015-04-08

View all other issues in [facet.num.put.virtuals].

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Discussion:

I am trying to understand how TR1 supports hex float (%a) output.

As far as I can tell, it does so via the following:

8.15 Additions to header `<locale>` [tr.c99.locale]

In subclause 22.4.2.2.2 [facet.num.put.virtuals], Table 58 Floating-point conversions, after the line:
`floatfield == ios_base::scientific %E`

add the two lines:

>     floatfield == ios_base::fixed | ios_base::scientific && !uppercase %a
>     floatfield == ios_base::fixed | ios_base::scientific %A 2

[Note: The additional requirements on print and scan functions, later in this clause, ensure that the print functions generate hexadecimal floating-point fields with a %a or %A conversion specifier, and that the scan functions match hexadecimal floating-point fields with a %g conversion specifier. end note]

Following the thread, in 22.4.2.2.2 [facet.num.put.virtuals], we find:

For conversion from a floating-point type, if `(flags & fixed) != 0` or if `str.precision() > 0`, then `str.precision()` is specified in the conversion specification.

This would seem to imply that when `floatfield == fixed|scientific`, the precision of the conversion specifier is to be taken from `str.precision()`. Is this really what's intended? I sincerely hope that I'm either missing something or this is an oversight. Please tell me that the committee did not intend to mandate that hex floats (and doubles) should by default be printed as if by %.6a.

[ Howard: I think the fundamental issue we overlooked was that with %f, %e, %g, the default precision was always 6. With %a the default precision is not 6, it is infinity. So for the first time, we need to distinguish between the default value of precision, and the precision value 6. ]

[ 2009-07 Frankfurt ]

> Leave this open for Robert and Daniel to work on.

> Straw poll: Disposition?

> - Default is %.6a (i.e. NAD): 2
> - Always %a (no precision): 6
> - precision(-1) == %a: 3

> Daniel and Robert have direction to write up wording for the "always %a" solution.

> [ 2009-07-15 Robert provided wording. ]

[ 2009-10 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

Change 22.4.2.2.2 [facet.num.put.virtuals], Stage 1, under p5 (near the end of Stage 1):

> For conversion from a floating-point type, `str.precision()` is specified as precision in the conversion specification if `floatfield != (ios_base::fixed | ios_base::scientific)`, else no precision is specified.

[ Kona (2007): Robert volunteers to propose wording. ]

### 672. Swappable requirements need updating

Section: 17.6.3.1 [utility.arg.requirements] Status: CD1 Submitter: Howard Hinnant Opened: 2007-05-04 Last modified: 2015-04-08

View all other issues in [utility.arg.requirements].

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Discussion:

The current `Swappable` is:

> Table 37: `Swappable` requirements [swappable]
> expression | return type | post-condition
> --- | --- | ---
> `swap(s,t)` | `void` | `t` has the value originally held by `u`, and `u` has the value originally held by `t`

> The Swappable requirement is met by satisfying one or more of the following conditions:
> - `T` is Swappable if `T` satisfies the `CopyConstructible` requirements (Table 34) and the `CopyAssignable` requirements (Table 36);
> - `T` is Swappable if a namespace scope function named `swap` exists in the same namespace as the definition of `T`, such that the expression `swap(t,u)` is valid and has the semantics described in this table.

With the passage of rvalue reference into the language, `Swappable` needs to be updated to require only `MoveConstructible` and `MoveAssignable`. This is a minimum.

Additionally we may want to support proxy references such that the following code is acceptable:

>     namespace Mine {

>     template <class T>
>     struct proxy {...};

>     template <class T>
>     struct proxied_iterator
>     {
>        typedef T value_type;
>        typedef proxy<T> reference;
>        reference operator*() const;
>        ...
>     };

>     struct A
>     {
>        // heavy type, has an optimized swap, maybe isn't even copyable or movable, just swappable
>        void swap(A&);
>        ...
>     };

>     void swap(A&, A&);
>     void swap(proxy<A>, A&);
>     void swap(A&, proxy<A>);
>     void swap(proxy<A>, proxy<A>);

>     }  // Mine

>     ...

>     Mine::proxied_iterator<Mine::A> i(...)
>     Mine::A a;
>     swap(*i1, a);

I.e. here is a call to `swap` which the user enables swapping between a proxy to a class and the class itself. We do not need to anything in terms of implementation except not block their way with overly constrained concepts. That is, the `Swappable` concept should be expanded to allow swapping between two different types for the case that one is binding to a user-defined `swap`.

Proposed resolution:

Change 17.6.3.1 [utility.arg.requirements]:

> -1- The template definitions in the C++ Standard Library refer to various named requirements whose details are set out in tables 31-38. In these tables, `T` is a type to be supplied by a C++ program instantiating a template; `a`, `b`, and `c` are values of type `const T`; `s` and `t` are modifiable lvalues of type `T`; `u` is a value of type (possibly `const`) `T`; and `rv` is a non-`const` rvalue of type `T`.

> Table 37: `Swappable` requirements [swappable]
> expression | return type | post-condition
> --- | --- | ---
> `swap(s,t)` | `void` | `t` has the value originally held by `u`, and `u` has the value originally held by `t`

> The `Swappable` requirement is met by satisfying one or more of the following conditions:
> - `T` is `Swappable` if `T` satisfies the `CopyConstructible` MoveConstructible requirements (Table 34 33) and the `CopyAssignable` MoveAssignable requirements (Table 36 35);
> - `T` is `Swappable` if a namespace scope function named `swap` exists in the same namespace as the definition of `T`, such that the expression `swap(t,u)` is valid and has the semantics described in this table.

[ Kona (2007): We like the change to the `Swappable` requirements to use move semantics. The issue relating to the support of proxies is separable from the one relating to move semantics, and it's bigger than just swap. We'd like to address only the move semantics changes under this issue, and open a separated issue (742) to handle proxies. Also, there may be a third issue, in that the current definition of `Swappable` does not permit rvalues to be operands to a swap operation, and Howard's proposed resolution would allow the right-most operand to be an rvalue, but it would not allow the left-most operand to be an rvalue (some swap functions in the library have been overloaded to permit left operands to swap to be rvalues). ]

### 673. `unique_ptr` update

Section: 20.8.1 [unique.ptr] Status: CD1 Submitter: Howard Hinnant Opened: 2007-05-04 Last modified: 2015-04-08

View all other issues in [unique.ptr].

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Discussion:

Since the publication of N1856 there have been a few small but significant advances which should be included into `unique_ptr`. There exists a example implementation for all of these changes.

- Even though `unique_ptr<void>` is not a valid use case (unlike for `shared_ptr<void>`), unexpected cases to crop up which require the instantiation of the interface of `unique_ptr<void>` even if it is never used. For example see 541 for how this accidently happened to `auto_ptr`. I believe the most robust way to protect `unique_ptr` against this type of failure is to augment the return type of `unique_ptr<T>:operator*()` with `add_lvalue_reference<T>::type`. This means that given an instantiated `unique_ptr<void>` the act of dereferencing it will simply return `void` instead of causing a compile time failure. This is simpler than creating a `unique_ptr<void>` specialization which isn't robust in the face of `cv-`qualified `void` types.

  This resolution also supports instantiations such as `unique_ptr<void, free_deleter>` which could be very useful to the client.

- Efforts have been made to better support containers and smart pointers in shared memory contexts. One of the key hurdles in such support is not assuming that a pointer type is actually a `T*`. This can easily be accomplished for `unique_ptr` by having the deleter define the pointer type: `D::pointer`. Furthermore this type can easily be defaulted to `T*` should the deleter `D` choose not to define a pointer type (example implementation here). This change has no run time overhead. It has no interface overhead on authors of custom delter types. It simply allows (but not requires) authors of custom deleter types to define a smart pointer for the storage type of `unique_ptr` if they find such functionality useful. `std::default_delete` is an example of a deleter which defaults `pointer` to `T*` by simply ignoring this issue and not including a `pointer typedef`.

- When the deleter type is a function pointer then it is unsafe to construct a `unique_ptr` without specifying the function pointer in the constructor. This case is easy to check for with a `static_assert` assuring that the deleter is not a pointer type in those constructors which do not accept deleters.

  >     unique_ptr<A, void(*)(void*)> p(new A);  // error, no function given to delete the pointer!

[ Kona (2007): We don't like the solution given to the first bullet in light of concepts. The second bullet solves the problem of supporting fancy pointers for one library component only. The full LWG needs to decide whether to solve the problem of supporting fancy pointers piecemeal, or whether a paper addressing the whole library is needed. We think that the third bullet is correct. ]

[ Post Kona: Howard adds example user code related to the first bullet: ]

>     void legacy_code(void*, std::size_t);

>     void foo(std::size_t N)
>     {
>         std::unique_ptr<void, void(*)(void*)> ptr(std::malloc(N), std::free);
>         legacy_code(ptr.get(), N);
>     }   // unique_ptr used for exception safety purposes

I.e. `unique_ptr<void>` is a useful tool that we don't want to disable with concepts. The only part of `unique_ptr<void>` we want to disable (with concepts or by other means) are the two member functions:

>     T& operator*() const;
>     T* operator->() const;

Proposed resolution:

[ I am grateful for the generous aid of Peter Dimov and Ion Gaztañaga in helping formulate and review the proposed resolutions below. ]

- Change 20.8.1.2 [unique.ptr.single]:

  >     template <class T, class D = default_delete<T>> class unique_ptr {
  >        ...
  >        T& typename add_lvalue_reference<T>::type operator*() const;
  >        ...
  >     };

  Change 20.8.1.2.4 [unique.ptr.single.observers]:

  >     T& typename add_lvalue_reference<T>::type operator*() const;

- Change 20.8.1.2 [unique.ptr.single]:

  >     template <class T, class D = default_delete<T>> class unique_ptr {
  >     public:
  >       typedef implementation (see description below) pointer;
  >        ...
  >        explicit unique_ptr(T* pointer p);
  >        ...
  >        unique_ptr(T* pointer p, implementation defined (see description below) d);
  >        unique_ptr(T* pointer p, implementation defined (see description below) d);
  >        ...
  >        T* pointer operator->() const;
  >        T* pointer get() const;
  >        ...
  >        T* pointer release();
  >        void reset(T* pointer p = 0 pointer());
  >     };

  -3- If the type `remove_reference<D>::type::pointer` exists, then `unique_ptr<T, D>::pointer` is a typedef to `remove_reference<D>::type::pointer`. Otherwise `unique_ptr<T, D>::pointer` is a typedef to `T*`. The type `unique_ptr<T, D>::pointer` shall be `CopyConstructible` and `CopyAssignable`.

  Change 20.8.1.2.1 [unique.ptr.single.ctor]:

  >     unique_ptr(T* pointer p);
  >     ...
  >     unique_ptr(T* pointer p, implementation defined d);
  >     unique_ptr(T* pointer p, implementation defined d);
  >     ...
  >     unique_ptr(T* pointer p, const A& d);
  >     unique_ptr(T* pointer p, A&& d);
  >     ...
  >     unique_ptr(T* pointer p, A& d);
  >     unique_ptr(T* pointer p, A&& d);
  >     ...
  >     unique_ptr(T* pointer p, const A& d);
  >     unique_ptr(T* pointer p, const A&& d);
  >     ...

  -23- Requires: If `D` is not a reference type, construction of the deleter `D` from an rvalue of type `E` must shall be well formed and not throw an exception. If `D` is a reference type, then `E` must shall be the same type as `D` (diagnostic required). `U*` `unique_ptr<U,E>::pointer` must shall be implicitly convertible to `T*` pointer.

  -25- Postconditions: `get() == value u.get()` had before the construction, modulo any required offset adjustments resulting from the cast from `U*` `unique_ptr<U,E>::pointer` to `T*` pointer. `get_deleter()` returns a reference to the internally stored deleter which was constructed from `u.get_deleter()`.

  Change 20.8.1.2.3 [unique.ptr.single.asgn]:

  > -8- Requires: Assignment of the deleter `D` from an rvalue `D` must shall not throw an exception. `U*` `unique_ptr<U,E>::pointer` must shall be implicitly convertible to `T*` pointer.

  Change 20.8.1.2.4 [unique.ptr.single.observers]:

  >     T* pointer operator->() const;

  > ...

  >     T* pointer get() const;

  Change 20.8.1.2.5 [unique.ptr.single.modifiers]:

  >     T* pointer release();

  > ...

  >     void reset(T* pointer p = 0 pointer());

  Change 20.8.1.3 [unique.ptr.runtime]:

  >     template <class T, class D> class unique_ptr<T[], D> {
  >     public:
  >       typedef implementation pointer;
  >        ...
  >        explicit unique_ptr(T* pointer p);
  >        ...
  >        unique_ptr(T* pointer p, implementation defined d);
  >        unique_ptr(T* pointer p, implementation defined d);
  >        ...
  >        T* pointer get() const;
  >        ...
  >        T* pointer release();
  >        void reset(T* pointer p = 0 pointer());
  >     };

  Change 20.8.1.3.1 [unique.ptr.runtime.ctor]:

  >     unique_ptr(T* pointer p);
  >     unique_ptr(T* pointer p, implementation defined d);
  >     unique_ptr(T* pointer p, implementation defined d);

  > These constructors behave the same as in the primary template except that they do not accept pointer types which are convertible to `T*` `pointer`. [Note: One implementation technique is to create private templated overloads of these members. -- end note]

  Change 20.8.1.3.4 [unique.ptr.runtime.modifiers]:

  >     void reset(T* pointer p = 0 pointer());

  > -1- Requires: Does not accept pointer types which are convertible to `T*` `pointer` (diagnostic required). [Note: One implementation technique is to create a private templated overload. -- end note]

- Change 20.8.1.2.1 [unique.ptr.single.ctor]:

  >     unique_ptr();

  > > Requires: `D` must shall be default constructible, and that construction must shall not throw an exception. `D` must shall not be a reference type or pointer type (diagnostic required).

  >     unique_ptr(T* pointer p);

  > > Requires: The expression `D()(p)` must shall be well formed. The default constructor of `D` must shall not throw an exception. `D` must shall not be a reference type or pointer type (diagnostic required).

### 674. `shared_ptr` interface changes for consistency with N1856

Section: 20.8.2.2 [util.smartptr.shared] Status: CD1 Submitter: Peter Dimov Opened: 2007-05-05 Last modified: 2015-04-08

View other active issues in [util.smartptr.shared].

View all other issues in [util.smartptr.shared].

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Discussion:

N1856 does not propose any changes to `shared_ptr`. It needs to be updated to use a rvalue reference where appropriate and to interoperate with `unique_ptr` as it does with `auto_ptr`.

Proposed resolution:

Change 20.8.2.2 [util.smartptr.shared] as follows:

>     template<class Y> explicit shared_ptr(auto_ptr<Y>&&& r);
>     template<class Y, class D> explicit shared_ptr(const unique_ptr<Y,D>& r) = delete;
>     template<class Y, class D> explicit shared_ptr(unique_ptr<Y,D>&& r);
>     ...
>     template<class Y> shared_ptr& operator=(auto_ptr<Y>&&& r);
>     template<class Y, class D> shared_ptr& operator=(const unique_ptr<Y,D>& r) = delete;
>     template<class Y, class D> shared_ptr& operator=(unique_ptr<Y,D>&& r);

Change 20.8.2.2.1 [util.smartptr.shared.const] as follows:

>     template<class Y> shared_ptr(auto_ptr<Y>&&& r);

Add to 20.8.2.2.1 [util.smartptr.shared.const]:

>     template<class Y, class D> shared_ptr(unique_ptr<Y, D>&& r);

> > Effects: Equivalent to `shared_ptr( r.release(), r.get_deleter() )` when `D` is not a reference type, `shared_ptr( r.release(), ref( r.get_deleter() ) )` otherwise.

> > Exception safety: If an exception is thrown, the constructor has no effect.

Change 20.8.2.2.3 [util.smartptr.shared.assign] as follows:

>     template<class Y> shared_ptr& operator=(auto_ptr<Y>&&& r);

Add to 20.8.2.2.3 [util.smartptr.shared.assign]:

>     template<class Y, class D> shared_ptr& operator=(unique_ptr<Y,D>&& r);

> > -4- Effects: Equivalent to `shared_ptr(std::move(r)).swap(*this)`.

> > -5- Returns: `*this`.

[ Kona (2007): We may need to open an issue (743) to deal with the question of whether `shared_ptr` needs an rvalue `swap`. ]

### 675. Move assignment of containers

Section: 23.2 [container.requirements] Status: CD1 Submitter: Howard Hinnant Opened: 2007-05-05 Last modified: 2015-04-08

View other active issues in [container.requirements].

View all other issues in [container.requirements].

View all issues with CD1 status.

Discussion:

James Hopkin pointed out to me that if `vector<T>` move assignment is O(1) (just a `swap`) then containers such as `vector<shared_ptr<ostream>>` might have the wrong semantics under move assignment when the source is not truly an rvalue, but a moved-from lvalue (destructors could run late).

>     vector<shared_ptr<ostream>> v1;
>     vector<shared_ptr<ostream>> v2;
>     ...
>     v1 = v2;               // #1
>     v1 = std::move(v2);    // #2

Move semantics means not caring what happens to the source (`v2` in this example). It doesn't mean not caring what happens to the target (`v1`). In the above example both assignments should have the same effect on `v1`. Any non-shared `ostream`'s `v1` owns before the assignment should be closed, whether `v1` is undergoing copy assignment or move assignment.

This implies that the semantics of move assignment of a generic container should be `clear, swap` instead of just swap. An alternative which could achieve the same effect would be to move assign each element. In either case, the complexity of move assignment needs to be relaxed to `O(v1.size())`.

The performance hit of this change is not nearly as drastic as it sounds. In practice, the target of a move assignment has always just been move constructed or move assigned from. Therefore under `clear, swap` semantics (in this common use case) we are still achieving O(1) complexity.

Proposed resolution:

Change 23.2 [container.requirements]:

> Table 89: Container requirements
> expression | return type | operational semantics | assertion/note pre/post-condition | complexity
> --- | --- | --- | --- | ---
> `a = rv;` | `X&` | All existing elements of `a` are either move assigned or destructed | `a` shall be equal to the value that `rv` had before this construction | (Note C) linear

> Notes: the algorithms `swap()`, `equal()` and `lexicographical_compare()` are defined in clause 25. Those entries marked "(Note A)" should have constant complexity. Those entries marked "(Note B)" have constant complexity unless `allocator_propagate_never<X::allocator_type>::value` is `true`, in which case they have linear complexity. Those entries marked "(Note C)" have constant complexity if `a.get_allocator() == rv.get_allocator()` or if either `allocator_propagate_on_move_assignment<X::allocator_type>::value` is `true` or `allocator_propagate_on_copy_assignment<X::allocator_type>::value` is `true` and linear complexity otherwise.

[ post Bellevue Howard adds: ]

> This issue was voted to WP in Bellevue, but accidently got stepped on by N2525 which was voted to WP simulataneously. Moving back to Open for the purpose of getting the wording right. The intent of this issue and N2525 are not in conflict.

[ post Sophia Antipolis Howard updated proposed wording: ]

### 676. Moving the unordered containers

Section: 23.5 [unord] Status: C++11 Submitter: Howard Hinnant Opened: 2007-05-05 Last modified: 2015-04-08

View all other issues in [unord].

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Discussion:

Move semantics are missing from the `unordered` containers. The proposed resolution below adds move-support consistent with N1858 and the current working draft.

The current proposed resolution simply lists the requirements for each function. These might better be hoisted into the requirements table for unordered associative containers. Futhermore a mild reorganization of the container requirements could well be in order. This defect report is purposefully ignoring these larger issues and just focusing on getting the unordered containers "moved".

[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]

[ 2009-10-17 Removed rvalue-swaps from wording. ]

[ 2009-10 Santa Cruz: ]

> Move to Review. Alisdair will review proposed wording.

[ 2009-10-29 Daniel updates wording. ]

[ 2010-01-26 Alisdair updates wording. ]

[ 2010-02-10 Howard updates wording to reference the unordered container requirements table (modified by 704) as much as possible. ]

[ Voted to WP in Bellevue. ]

[ post Bellevue, Pete notes: ]

> Please remind people who are reviewing issues to check that the text modifications match the current draft. Issue 676, for example, adds two overloads for unordered_map::insert taking a hint. One takes a const_iterator and returns a const_iterator, and the other takes an iterator and returns an iterator. This was correct at the time the issue was written, but was changed in Toronto so there is only one hint overload, taking a const_iterator and returning an iterator.

> This issue is not ready. In addition to the relatively minor signature problem I mentioned earlier, it puts requirements in the wrong places. Instead of duplicating requirements throughout the template specifications, it should put them in the front matter that talks about requirements for unordered containers in general. This presentation problem is editorial, but I'm not willing to do the extensive rewrite that it requires. Please put it back into Open status.

[ 2010-02-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010-02-24 Pete moved to Open: ]

> The descriptions of the semantics of the added `insert` functions belong in the requirements table. That's where the rest of the `insert` functions are.

[ 2010 Pittsburgh: ]

> Move issue 676 to Ready for Pittsburgh. Nico to send Howard an issue for the broader problem.

Rationale:

[ San Francisco: ]

> Solved by N2776.

[ Rationale is obsolete. ]

Proposed resolution:

`unordered_map`

Change 23.5.4 [unord.map]:

>     class unordered_map
>     {
>         ...
>         unordered_map(const unordered_map&);
>         unordered_map(unordered_map&&);
>         unordered_map(const Allocator&);
>         unordered_map(const unordered_map&, const Allocator&);
>         unordered_map(unordered_map&&, const Allocator&);
>         ...
>         unordered_map& operator=(const unordered_map&);
>         unordered_map& operator=(unordered_map&&);
>         ...
>         // modifiers
>         ...
>         std::pair<iterator, bool> insert(const value_type& obj);
>         template <class P> pair<iterator, bool> insert(P&& obj);
>         iterator       insert(const_iterator hint, const value_type& obj);
>         template <class P> iterator       insert(const_iterator hint, P&& obj);
>         ...
>         mapped_type& operator[](const key_type& k);
>         mapped_type& operator[](key_type&& k);
>         ...
>     };

Add to 23.5.4.3 [unord.map.elem]:

>     mapped_type& operator[](const key_type& k);

> > ...

> > Requires: `key_type` shall be `CopyConstructible` and `mapped_type` shall be `DefaultConstructible`.

> > Complexity: Average case `O(1)`, worst case `O(size())`.

>     mapped_type& operator[](key_type&& k);

> > Requires: `key_type` shall be `MoveConstructible` and `mapped_type` shall be `DefaultConstructible`.

> > Effects: If the `unordered_map` does not already contain an element whose key is equivalent to `k` , inserts the value `value_type(std::move(k), mapped_type())`.

> > Returns: A reference to `x.second`, where `x` is the (unique) element whose key is equivalent to `k`.

> > Complexity: Average case `O(1)`, worst case `O(size())`.

Add new section [unord.map.modifiers]:

>     template <class P>
>       pair<iterator, bool> insert(P&& x);

> > Requires: `value_type` is constructible from `std::forward<P>(x)`.

> > Effects: Inserts `x` converted to `value_type` if and only if there is no element in the container with key equivalent to the key of `value_type(x)`.

> > Returns: The `bool` component of the returned `pair` indicates whether the insertion takes place, and the iterator component points to the element with key equivalent to the key of `value_type(x)`.

> > Complexity: Average case `O(1)`, worst case `O(size())`.

> > Remarks: `P` shall be implicitly convertible to `value_type`, else this signature shall not participate in overload resolution.

>     template <class P>
>       iterator insert(const_iterator hint, P&& x);

> > Requires: `value_type` is constructible from `std::forward<P>(x)`.

> > Effects: Inserts `x` converted to `value_type` if and only if there is no element in the container with key equivalent to the key of `value_type(x)`. The iterator `hint` is a hint pointing to where the search should start. Implementations are permitted to ignore the hint.

> > Returns: An iterator pointing to the element with key equivalent to the key of `value_type(x)`.

> > Complexity: Average case `O(1)`, worst case `O(size())`.

> > Remarks: `P` shall be implicitly convertible to `value_type`, else this signature shall not participate in overload resolution.

`unordered_multimap`

Change 23.5.5 [unord.multimap]:

>     class unordered_multimap
>     {
>         ...
>         unordered_multimap(const unordered_multimap&);
>         unordered_multimap(unordered_multimap&&);
>         unordered_multimap(const Allocator&);
>         unordered_multimap(const unordered_multimap&, const Allocator&);
>         unordered_multimap(unordered_multimap&&, const Allocator&);
>         ...
>         unordered_multimap& operator=(const unordered_multimap&);
>         unordered_multimap& operator=(unordered_multimap&&);
>         ...
>         // modifiers
>         ...
>         iterator insert(const value_type& obj);
>         template <class P> iterator insert(P&& obj);
>         iterator       insert(const_iterator hint, const value_type& obj);
>         template <class P> iterator       insert(const_iterator hint, P&& obj);
>         ...
>     };

Add new section [unord.multimap.modifiers]:

>     template <class P>
>       iterator insert(P&& x);

> > Requires: `value_type` is constructible from `std::forward<P>(x)`.

> > Effects: Inserts `x` converted to `value_type`.

> > Returns: An iterator pointing to the element with key equivalent to the key of `value_type(x)`.

> > Complexity: Average case `O(1)`, worst case `O(size())`.

> > Remarks: `P` shall be implicitly convertible to `value_type`, else this signature shall not participate in overload resolution.

>     template <class P>
>       iterator insert(const_iterator hint, P&& x);

> > Requires: `value_type` is constructible from `std::forward<P>(x)`.

> > Effects: Inserts `x` converted to `value_type` if and only if there is no element in the container with key equivalent to the key of `value_type(x)`. The iterator `hint` is a hint pointing to where the search should start. Implementations are permitted to ignore the hint.

> > Returns: An iterator pointing to the element with key equivalent to the key of `value_type(x)`.

> > Complexity: Average case `O(1)`, worst case `O(size())`.

> > Remarks: `P` shall be implicitly convertible to `value_type`, else this signature shall not participate in overload resolution.

`unordered_set`

Change 23.5.6 [unord.set]:

>     class unordered_set
>     {
>         ...
>         unordered_set(const unordered_set&);
>         unordered_set(unordered_set&&);
>         unordered_set(const Allocator&);
>         unordered_set(const unordered_set&, const Allocator&);
>         unordered_set(unordered_set&&, const Allocator&);
>         ...
>         unordered_set& operator=(const unordered_set&);
>         unordered_set& operator=(unordered_set&&);
>         ...
>         // modifiers
>         ...
>         std::pair<iterator, bool> insert(const value_type& obj);
>         pair<iterator, bool> insert(value_type&& obj);
>         iterator       insert(const_iterator hint, const value_type& obj);
>         iterator       insert(const_iterator hint, value_type&& obj);
>         ...
>     };

`unordered_multiset`

Change 23.5.7 [unord.multiset]:

>     class unordered_multiset
>     {
>         ...
>         unordered_multiset(const unordered_multiset&);
>         unordered_multiset(unordered_multiset&&);
>         unordered_multiset(const Allocator&);
>         unordered_multiset(const unordered_multiset&, const Allocator&);
>         unordered_multiset(unordered_multiset&&, const Allocator&);
>         ...
>         unordered_multiset& operator=(const unordered_multiset&);
>         unordered_multiset& operator=(unordered_multiset&&);
>         ...
>         // modifiers
>         ...
>         iterator insert(const value_type& obj);
>         iterator insert(value_type&& obj);
>         iterator       insert(const_iterator hint, const value_type& obj);
>         iterator       insert(const_iterator hint, value_type&& obj);
>         ...
>     };

### 677. Weaknesses in seed_seq::randomize [rand.util.seedseq]

Section: 26.5.7.1 [rand.util.seedseq] Status: CD1 Submitter: Charles Karney Opened: 2007-05-15 Last modified: 2015-04-08

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Discussion:

`seed_seq::randomize` provides a mechanism for initializing random number engines which ideally would yield "distant" states when given "close" seeds. The algorithm for `seed_seq::randomize` given in the current Working Draft for C++, N2284 (2007-05-08), has 3 weaknesses

1. Collisions in state. Because of the way the state is initialized, seeds of different lengths may result in the same state. The current version of seed_seq has the following properties:

   - For a given `s <= n`, each of the 2^(32s) seed vectors results in a distinct state.

   The proposed algorithm (below) has the considerably stronger properties:

   - All of the `(2^(32n)-1)/(2^32-1)` seed vectors of lengths `s < n` result in distinct states.
   - All of the `2^(32n)` seed vectors of length `s == n` result in distinct states.

2. Poor mixing of `v'`s entropy into the state. Consider `v.size() == n` and hold `v[n/2]` thru `v[n-1]` fixed while varying `v[0]` thru `v[n/2-1]`, a total of `2^(16n)` possibilities. Because of the simple recursion used in `seed_seq`, `begin[n/2]` thru `begin[n-1]` can take on only 2^64 possible states.

   The proposed algorithm uses a more complex recursion which results in much better mixing.

3. `seed_seq::randomize` is undefined for `v.size() == 0`. The proposed algorithm remedies this.

The current algorithm for `seed_seq::randomize` is adapted by me from the initialization procedure for the Mersenne Twister by Makoto Matsumoto and Takuji Nishimura. The weakness (2) given above was communicated to me by Matsumoto last year.

The proposed replacement for `seed_seq::randomize` is due to Mutsuo Saito, a student of Matsumoto, and is given in the implementation of the SIMD-oriented Fast Mersenne Twister random number generator SFMT. http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/SFMT/index.html http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/SFMT/SFMT-src-1.2.tar.gz

See Mutsuo Saito, An Application of Finite Field: Design and Implementation of 128-bit Instruction-Based Fast Pseudorandom Number Generator, Master's Thesis, Dept. of Math., Hiroshima University (Feb. 2007) http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/SFMT/M062821.pdf

One change has been made here, namely to treat the case of small `n` (setting `t = (n-1)/2` for `n < 7`).

Since `seed_seq` was introduced relatively recently there is little cost in making this incompatible improvement to it.

See N2391 and N2423 for some further discussion.

Proposed resolution:

Adopt the proposed resolution in N2423.

[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 678. Changes for [rand.req.eng]

Section: 26.5.1.4 [rand.req.eng] Status: CD1 Submitter: Charles Karney Opened: 2007-05-15 Last modified: 2015-04-08

View all other issues in [rand.req.eng].

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Discussion:

Section 26.5.1.4 [rand.req.eng] Random number engine requirements:

This change follows naturally from the proposed change to `seed_seq::randomize` in 677.

In table 104 the description of `X(q)` contains a special treatment of the case `q.size() == 0`. This is undesirable for 4 reasons:

1. It replicates the functionality provided by `X()`.
2. It leads to the possibility of a collision in the state provided by some other `X(q)` with `q.size() > 0`.
3. It is inconsistent with the description of the `X(q)` in paragraphs 26.5.3.1 [rand.eng.lcong] p5, 26.5.3.2 [rand.eng.mers] p8, and 26.5.3.3 [rand.eng.sub] p10 where there is no special treatment of `q.size() == 0`.
4. The proposed replacement for `seed_seq::randomize` given above allows for the case `q.size() == 0`.

See N2391 and N2423 for some further discussion.

Proposed resolution:

Adopt the proposed resolution in N2423.

[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 679. resize parameter by value

Section: 23.3 [sequences] Status: CD1 Submitter: Howard Hinnant Opened: 2007-06-11 Last modified: 2015-04-08

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Discussion:

The C++98 standard specifies that one member function alone of the containers passes its parameter (`T`) by value instead of by const reference:

>     void resize(size_type sz, T c = T());

This fact has been discussed / debated repeatedly over the years, the first time being even before C++98 was ratified. The rationale for passing this parameter by value has been:

> So that self referencing statements are guaranteed to work, for example:

> >     v.resize(v.size() + 1, v[0]);

However this rationale is not convincing as the signature for `push_back` is:

>     void push_back(const T& x);

And `push_back` has similar semantics to `resize` (append). And `push_back` must also work in the self referencing case:

>     v.push_back(v[0]);  // must work

The problem with passing `T` by value is that it can be significantly more expensive than passing by reference. The converse is also true, however when it is true it is usually far less dramatic (e.g. for scalar types).

Even with move semantics available, passing this parameter by value can be expensive. Consider for example `vector<vector<int>>`:

>     std::vector<int> x(1000);
>     std::vector<std::vector<int>> v;
>     ...
>     v.resize(v.size()+1, x);

In the pass-by-value case, `x` is copied once to the parameter of `resize`. And then internally, since the code can not know at compile time by how much `resize` is growing the `vector`, `x` is usually copied (not moved) a second time from `resize`'s parameter into its proper place within the `vector`.

With pass-by-const-reference, the `x` in the above example need be copied only once. In this case, `x` has an expensive copy constructor and so any copies that can be saved represents a significant savings.

If we can be efficient for `push_back`, we should be efficient for `resize` as well. The resize taking a reference parameter has been coded and shipped in the CodeWarrior library with no reports of problems which I am aware of.

Proposed resolution:

Change 23.3.3 [deque], p2:

>     class deque {
>        ...
>        void resize(size_type sz, const T& c);

Change 23.3.3.3 [deque.capacity], p3:

>     void resize(size_type sz, const T& c);

Change 23.3.5 [list], p2:

>     class list {
>        ...
>        void resize(size_type sz, const T& c);

Change 23.3.5.3 [list.capacity], p3:

>     void resize(size_type sz, const T& c);

Change 23.3.6 [vector], p2:

>     class vector {
>        ...
>        void resize(size_type sz, const T& c);

Change 23.3.6.3 [vector.capacity], p11:

>     void resize(size_type sz, const T& c);

### 680. move_iterator operator-> return

Section: 24.5.3.1 [move.iterator] Status: CD1 Submitter: Howard Hinnant Opened: 2007-06-11 Last modified: 2015-04-08

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Discussion:

`move_iterator`'s `operator->` return type `pointer` does not consistently match the type which is returned in the description in 24.5.3.3.5 [move.iter.op.ref].

>     template <class Iterator>
>     class move_iterator {
>     public:
>         ...
>         typedef typename iterator_traits<Iterator>::pointer pointer;
>         ...
>         pointer operator->() const {return current;}
>         ...
>     private:
>         Iterator current; // exposition only
>     };

There are two possible fixes.

1. `pointer operator->() const {return &*current;}`
2. `typedef Iterator pointer;`

The first solution is the one chosen by `reverse_iterator`. A potential disadvantage of this is it may not work well with iterators which return a proxy on dereference and that proxy has overloaded `operator&()`. Proxy references often need to overloaad `operator&()` to return a proxy pointer. That proxy pointer may or may not be the same type as the iterator's `pointer` type.

By simply returning the `Iterator` and taking advantage of the fact that the language forwards calls to `operator->` automatically until it finds a non-class type, the second solution avoids the issue of an overloaded `operator&()` entirely.

Proposed resolution:

Change the synopsis in 24.5.3.1 [move.iterator]:

>     typedef typename iterator_traits<Iterator>::pointer pointer;

### 681. Operator functions impossible to compare are defined in [re.submatch.op]

Section: 28.9.2 [re.submatch.op] Status: CD1 Submitter: Nozomu Katoo Opened: 2007-05-27 Last modified: 2015-04-08

View all other issues in [re.submatch.op].

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Discussion:

In 28.9.2 [re.submatch.op] of N2284, operator functions numbered 31-42 seem impossible to compare. E.g.:

>     template <class BiIter>
>        bool operator==(typename iterator_traits<BiIter>::value_type const& lhs,
>                         const sub_match<BiIter>& rhs);

> > -31- Returns: `lhs == rhs.str()`.

When `char*` is used as `BiIter`, `iterator_traits<BiIter>::value_type` would be `char`, so that `lhs == rhs.str()` ends up comparing a `char` value and an object of `std::basic_string<char>`. However, the behaviour of comparison between these two types is not defined in 21.4.8 [string.nonmembers] of N2284. This applies when `wchar_t*` is used as `BiIter`.

Proposed resolution:

Adopt the proposed resolution in N2409.

[ Kona (2007): The LWG adopted the proposed resolution of N2409 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 682. basic_regex ctor takes InputIterator or ForwardIterator?

Section: 28.8.2 [re.regex.construct] Status: CD1 Submitter: Eric Niebler Opened: 2007-06-03 Last modified: 2015-04-08

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Discussion:

Looking at N2284, 28.8 [re.regex], p3 `basic_regex` class template synopsis shows this constructor:

>     template <class InputIterator>
>          basic_regex(InputIterator first, InputIterator last,
>                      flag_type f = regex_constants::ECMAScript);

In 28.8.2 [re.regex.construct], p15, the constructor appears with this signature:

>     template <class ForwardIterator>
>          basic_regex(ForwardIterator first, ForwardIterator last,
>                      flag_type f = regex_constants::ECMAScript);

`ForwardIterator` is probably correct, so the synopsis is wrong.

[ John adds: ]

> I think either could be implemented? Although an input iterator would probably require an internal copy of the string being made.

> I have no strong feelings either way, although I think my original intent was `InputIterator`.

Proposed resolution:

Adopt the proposed resolution in N2409.

[ Kona (2007): The LWG adopted the proposed resolution of N2409 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 685. reverse_iterator/move_iterator difference has invalid signatures

Section: 24.5.1.3.19 [reverse.iter.opdiff], 24.5.3.3.14 [move.iter.nonmember] Status: CD1 Submitter: Bo Persson Opened: 2007-06-10 Last modified: 2015-04-08

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Discussion:

In C++03 the difference between two `reverse_iterators`

>     ri1 - ri2

is possible to compute only if both iterators have the same base iterator. The result type is the `difference_type` of the base iterator.

In the current draft, the operator is defined as 24.5.1.3.19 [reverse.iter.opdiff]

>     template<class Iterator1, class Iterator2>
>     typename reverse_iterator<Iterator>::difference_type
>        operator-(const reverse_iterator<Iterator1>& x,
>                         const reverse_iterator<Iterator2>& y);

The return type is the same as the C++03 one, based on the no longer present `Iterator` template parameter.

Besides being slightly invalid, should this operator work only when `Iterator1` and `Iterator2` has the same `difference_type`? Or should the implementation choose one of them? Which one?

The same problem now also appears in `operator-()` for `move_iterator` 24.5.3.3.14 [move.iter.nonmember].

Proposed resolution:

Change the synopsis in 24.5.1.1 [reverse.iterator]:

>     template <class Iterator1, class Iterator2>
>       typename reverse_iterator<Iterator>::difference_type auto operator-(
>         const reverse_iterator<Iterator1>& x,
>         const reverse_iterator<Iterator2>& y) -> decltype(y.current - x.current);

Change 24.5.1.3.19 [reverse.iter.opdiff]:

>     template <class Iterator1, class Iterator2>
>       typename reverse_iterator<Iterator>::difference_type auto operator-(
>         const reverse_iterator<Iterator1>& x,
>         const reverse_iterator<Iterator2>& y) -> decltype(y.current - x.current);

> > Returns: `y.current - x.current`.

Change the synopsis in 24.5.3.1 [move.iterator]:

>     template <class Iterator1, class Iterator2>
>       typename move_iterator<Iterator>::difference_type auto operator-(
>         const move_iterator<Iterator1>& x,
>         const move_iterator<Iterator2>& y) -> decltype(x.base() - y.base());

Change 24.5.3.3.14 [move.iter.nonmember]:

>     template <class Iterator1, class Iterator2>
>       typename move_iterator<Iterator>::difference_type auto operator-(
>         const move_iterator<Iterator1>& x,
>         const move_iterator<Iterator2>& y) -> decltype(x.base() - y.base());

> > Returns: `x.base() - y.base()`.

[ Pre Bellevue: This issue needs to wait until the `auto -> return` language feature goes in. ]

### 687. shared_ptr conversion constructor not constrained

Section: 20.8.2.2.1 [util.smartptr.shared.const], 20.8.2.3.1 [util.smartptr.weak.const] Status: CD1 Submitter: Peter Dimov Opened: 2007-05-10 Last modified: 2015-04-08

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Discussion:

Since all conversions from `shared_ptr<T>` to `shared_ptr<U>` have the same rank regardless of the relationship between `T` and `U`, reasonable user code that works with raw pointers fails with `shared_ptr`:

>     void f( shared_ptr<void> );
>     void f( shared_ptr<int> );

>     int main()
>     {
>       f( shared_ptr<double>() ); // ambiguous
>     }

Now that we officially have `enable_if`, we can constrain the constructor and the corresponding assignment operator to only participate in the overload resolution when the pointer types are compatible.

Proposed resolution:

In 20.8.2.2.1 [util.smartptr.shared.const], change:

> -14- Requires: For the second constructor The second constructor shall not participate in the overload resolution unless `Y*` shall be is implicitly convertible to `T*`.

In 20.8.2.3.1 [util.smartptr.weak.const], change:

>     template<class Y> weak_ptr(shared_ptr<Y> const& r);
>     weak_ptr(weak_ptr const& r);
>     template<class Y> weak_ptr(weak_ptr<Y> const& r);
>     weak_ptr(weak_ptr const& r);
>     template<class Y> weak_ptr(weak_ptr<Y> const& r);
>     template<class Y> weak_ptr(shared_ptr<Y> const& r);

> > -4- Requires: For t The second and third constructors, shall not participate in the overload resolution unless `Y*` shall be is implicitly convertible to `T*`.

### 688. reference_wrapper, cref unsafe, allow binding to rvalues

Section: 20.9.4.1 [refwrap.const] Status: C++11 Submitter: Peter Dimov Opened: 2007-05-10 Last modified: 2015-04-08

View all other issues in [refwrap.const].

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Discussion:

A `reference_wrapper` can be constructed from an rvalue, either by using the constructor, or via `cref` (and `ref` in some corner cases). This leads to a dangling reference being stored into the `reference_wrapper` object. Now that we have a mechanism to detect an rvalue, we can fix them to disallow this source of undefined behavior.

Also please see the thread starting at c++std-lib-17398 for some good discussion on this subject.

[ 2009-05-09 Alisdair adds: ]

> Now that `ref/cref` are constained that `T` must be an `ObjectType`, I do not believe there is any risk of binding `ref` to a temporary (which would rely on deducing `T` to be an rvalue reference type)

> However, the problem for `cref` remains, so I recommend retaining that deleted overload.

[ 2009-05-10 Howard adds: ]

> Without:

> >     template <class T> void ref(const T&& t) = delete;

> I believe this program will compile:

> >     #include <functional>

> >     struct A {};

> >     const A source() {return A();}

> >     int main()
> >     {
> >        std::reference_wrapper<const A> r = std::ref(source());
> >     }

> I.e. in:

> >     template <ObjectType T> reference_wrapper<T> ref(T& t);

> this:

> >     ref(source())

> deduces `T` as `const A`, and so:

> >     ref(const A& t)

> will bind to a temporary (tested with a pre-concepts rvalue-ref enabled compiler).

> Therefore I think we still need the ref-protection. I respectfully disagree with Alisdair's comment and am in favor of the proposed wording as it stands. Also, CWG 606 (noted below) has now been "favorably" resolved.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

In 20.9 [function.objects], add the following two signatures to the synopsis:

>     template <class T> void ref(const T&& t) = delete;
>     template <class T> void cref(const T&& t) = delete;

[ N2292 addresses the first part of the resolution but not the second. ]

[ Bellevue: Doug noticed problems with the current wording. ]

[ post Bellevue: Howard and Peter provided revised wording. ]

[ This resolution depends on a "favorable" resolution of CWG 606: that is, the "special deduction rule" is disabled with the const T&& pattern. ]

### 689. reference_wrapper constructor overly constrained

Section: 20.9.4.1 [refwrap.const] Status: CD1 Submitter: Peter Dimov Opened: 2007-05-10 Last modified: 2015-04-08

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Discussion:

The constructor of `reference_wrapper` is currently `explicit`. The primary motivation behind this is the safety problem with respect to rvalues, which is addressed by the proposed resolution of the previous issue. Therefore we should consider relaxing the requirements on the constructor since requests for the implicit conversion keep resurfacing.

Also please see the thread starting at c++std-lib-17398 for some good discussion on this subject.

Proposed resolution:

Remove the `explicit` from the constructor of `reference_wrapper`. If the proposed resolution of the previous issue is accepted, remove the `explicit` from the `T&&` constructor as well to keep them in sync.

### 691. const_local_iterator cbegin, cend missing from TR1

Section: 23.5 [unord], TR1 6.3 [tr.hash] Status: CD1 Submitter: Joaquín M López Muñoz Opened: 2007-06-14 Last modified: 2015-04-08

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Discussion:

The last version of TR1 does not include the following member functions for unordered containers:

>     const_local_iterator cbegin(size_type n) const;
>     const_local_iterator cend(size_type n) const;

which looks like an oversight to me. I've checked th TR1 issues lists and the latest working draft of the C++0x std (N2284) and haven't found any mention to these menfuns or to their absence.

Is this really an oversight, or am I missing something?

Proposed resolution:

Add the following two rows to table 93 (unordered associative container requirements) in section 23.2.5 [unord.req]:

> Unordered associative container requirements (in addition to container)
> expression | return type | assertion/note pre/post-condition | complexity
> --- | --- | --- | ---
> `b.cbegin(n)` | `const_local_iterator` | `n` shall be in the range `[0, bucket_count())`. Note: `[b.cbegin(n), b.cend(n))` is a valid range containing all of the elements in the `n`th bucket. | Constant
> `b.cend(n)` | `const_local_iterator` | `n` shall be in the range `[0, bucket_count())`. | Constant

Add to the synopsis in 23.5.4 [unord.map]:

>     const_local_iterator cbegin(size_type n) const;
>     const_local_iterator cend(size_type n) const;

Add to the synopsis in 23.5.5 [unord.multimap]:

>     const_local_iterator cbegin(size_type n) const;
>     const_local_iterator cend(size_type n) const;

Add to the synopsis in 23.5.6 [unord.set]:

>     const_local_iterator cbegin(size_type n) const;
>     const_local_iterator cend(size_type n) const;

Add to the synopsis in 23.5.7 [unord.multiset]:

>     const_local_iterator cbegin(size_type n) const;
>     const_local_iterator cend(size_type n) const;

### 692. `get_money` and `put_money` should be formatted I/O functions

Section: 27.7.5 [ext.manip] Status: CD1 Submitter: Martin Sebor Opened: 2007-06-22 Last modified: 2015-04-08

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Discussion:

In a private email Bill Plauger notes:

> I believe that the function that implements `get_money` [from N2072] should behave as a formatted input function, and the function that implements `put_money` should behave as a formatted output function. This has implications regarding the skipping of whitespace and the handling of errors, among other things.

> The words don't say that right now and I'm far from convinced that such a change is editorial.

Martin's response:

> I agree that the manipulators should handle exceptions the same way as formatted I/O functions do. The text in N2072 assumes so but the Returns clause explicitly omits exception handling for the sake of brevity. The spec should be clarified to that effect.

> As for dealing with whitespace, I also agree it would make sense for the extractors and inserters involving the new manipulators to treat it the same way as formatted I/O.

Proposed resolution:

Add a new paragraph immediately above p4 of 27.7.5 [ext.manip] with the following text:

> Effects: The expression `in >> get_money(mon, intl)` described below behaves as a formatted input function (as described in 27.7.2.2.1 [istream.formatted.reqmts]).

Also change p4 of 27.7.5 [ext.manip] as follows:

> Returns: An object `s` of unspecified type such that if `in` is an object of type `basic_istream<charT, traits>` then the expression `in >> get_money(mon, intl)` behaves as a formatted input function that calls `f(in, mon, intl)` were called. The function `f` can be defined as...

[ post Bellevue: ]

> We recommend moving immediately to Review. We've looked at the issue and have a consensus that the proposed resolution is correct, but want an iostream expert to sign off. Alisdair has taken the action item to putt this up on the reflector for possible movement by Howard to Tenatively Ready.

### 693. `std::bitset::all()` missing

Section: 20.6 [template.bitset] Status: CD1 Submitter: Martin Sebor Opened: 2007-06-22 Last modified: 2015-04-08

View other active issues in [template.bitset].

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Discussion:

The `bitset` class template provides the member function `any()` to determine whether an object of the type has any bits set, and the member function `none()` to determine whether all of an object's bits are clear. However, the template does not provide a corresponding function to discover whether a `bitset` object has all its bits set. While it is possible, even easy, to obtain this information by comparing the result of `count()` with the result of `size()` for equality (i.e., via `b.count() == b.size()`) the operation is less efficient than a member function designed specifically for that purpose could be. (`count()` must count all non-zero bits in a `bitset` a word at a time while `all()` could stop counting as soon as it encountered the first word with a zero bit).

Proposed resolution:

Add a declaration of the new member function `all()` to the defintion of the `bitset` template in 20.6 [template.bitset], p1, right above the declaration of `any()` as shown below:

>     bool operator!=(const bitset<N>& rhs) const;
>     bool test(size_t pos) const;
>     bool all() const;
>     bool any() const;
>     bool none() const;

Add a description of the new member function to the end of 20.6.2 [bitset.members] with the following text:

> `bool all() const;`

> > Returns: `count() == size()`.

In addition, change the description of `any()` and `none()` for consistency with `all()` as follows:

> `bool any() const;`

> > Returns: `true` if any bit in `*this` is one `count() != 0`.

> `bool none() const;`

> > Returns: `true` if no bit in `*this` is one `count() == 0`.

### 694. `std::bitset` and `long long`

Section: 20.6 [template.bitset] Status: CD1 Submitter: Martin Sebor Opened: 2007-06-22 Last modified: 2015-04-08

View other active issues in [template.bitset].

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Discussion:

Objects of the `bitset` class template specializations can be constructed from and explicitly converted to values of the widest C++ integer type, `unsigned long`. With the introduction of `long long` into the language the template should be enhanced to make it possible to interoperate with values of this type as well, or perhaps `uintmax_t`. See c++std-lib-18274 for a brief discussion in support of this change.

Proposed resolution:

For simplicity, instead of adding overloads for `unsigned long long` and dealing with possible ambiguities in the spec, replace the `bitset` ctor that takes an `unsigned long` argument with one taking `unsigned long long` in the definition of the template as shown below. (The standard permits implementations to add overloads on other integer types or employ template tricks to achieve the same effect provided they don't cause ambiguities or changes in behavior.)

>     // [bitset.cons] constructors:
>     bitset();
>     bitset(unsigned long long val);
>     template<class charT, class traits, class Allocator>
>     explicit bitset(
>                     const basic_string<charT,traits,Allocator>& str,
>                     typename basic_string<charT,traits,Allocator>::size_type pos = 0,
>                     typename basic_string<charT,traits,Allocator>::size_type n =
>                         basic_string<charT,traits,Allocator>::npos);

Make a corresponding change in 20.6.1 [bitset.cons], p2:

> `bitset(unsigned long long val);`

> > Effects: Constructs an object of class bitset<N>, initializing the first `M` bit positions to the corresponding bit values in `val`. `M` is the smaller of `N` and the number of bits in the value representation (section [basic.types]) of `unsigned long long`. If `M < N` is `true`, the remaining bit positions are initialized to zero.

Additionally, introduce a new member function `to_ullong()` to make it possible to convert `bitset` to values of the new type. Add the following declaration to the definition of the template, immediate after the declaration of `to_ulong()` in 20.6 [template.bitset], p1, as shown below:

>     // element access:
>     bool operator[](size_t pos) const; // for b[i];
>     reference operator[](size_t pos); // for b[i];
>     unsigned long to_ulong() const;
>     unsigned long long to_ullong() const;
>     template <class charT, class traits, class Allocator>
>     basic_string<charT, traits, Allocator> to_string() const;

And add a description of the new member function to 20.6.2 [bitset.members], below the description of the existing `to_ulong()` (if possible), with the following text:

> `unsigned long long to_ullong() const;`

> > Throws: `overflow_error` if the integral value `x` corresponding to the bits in `*this` cannot be represented as type `unsigned long long`.

> > Returns: `x`.

### 695. ctype<char>::classic_table() not accessible

Section: 22.4.1.3 [facet.ctype.special] Status: CD1 Submitter: Martin Sebor Opened: 2007-06-22 Last modified: 2015-04-08

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Discussion:

The `ctype<char>::classic_table()` static member function returns a pointer to an array of const `ctype_base::mask` objects (enums) that contains `ctype<char>::table_size` elements. The table describes the properties of the character set in the "C" locale (i.e., whether a character at an index given by its value is alpha, digit, punct, etc.), and is typically used to initialize the `ctype<char>` facet in the classic "C" locale (the protected `ctype<char>` member function `table()` then returns the same value as `classic_table()`).

However, while `ctype<char>::table_size` (the size of the table) is a public static const member of the `ctype<char>` specialization, the `classic_table()` static member function is protected. That makes getting at the classic data less than convenient (i.e., one has to create a whole derived class just to get at the masks array). It makes little sense to expose the size of the table in the public interface while making the table itself protected, especially when the table is a constant object.

The same argument can be made for the non-static protected member function `table()`.

Proposed resolution:

Make the `ctype<char>::classic_table()` and `ctype<char>::table()` member functions public by moving their declarations into the public section of the definition of specialization in 22.4.1.3 [facet.ctype.special] as shown below:

>       static locale::id id;
>       static const size_t table_size = IMPLEMENTATION_DEFINED;
>     protected:
>       const mask* table() const throw();
>       static const mask* classic_table() throw();
>     protected:

>     ~ctype(); // virtual
>     virtual char do_toupper(char c) const;

### 696. `istream::operator>>(int&)` broken

Section: 27.7.2.2.2 [istream.formatted.arithmetic] Status: C++11 Submitter: Martin Sebor Opened: 2007-06-23 Last modified: 2015-04-08

View all other issues in [istream.formatted.arithmetic].

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Discussion:

From message c++std-lib-17897:

The code shown in 27.7.2.2.2 [istream.formatted.arithmetic] as the "as if" implementation of the two arithmetic extractors that don't have a corresponding `num_get` interface (i.e., the `short` and `int` overloads) is subtly buggy in how it deals with `EOF`, overflow, and other similar conditions (in addition to containing a few typos).

One problem is that if `num_get::get()` reaches the EOF after reading in an otherwise valid value that exceeds the limits of the narrower type (but not `LONG_MIN` or `LONG_MAX`), it will set `err` to `eofbit`. Because of the if condition testing for `(err == 0)`, the extractor won't set `failbit` (and presumably, return a bogus value to the caller).

Another problem with the code is that it never actually sets the argument to the extracted value. It can't happen after the call to `setstate()` since the function may throw, so we need to show when and how it's done (we can't just punt as say: "it happens afterwards"). However, it turns out that showing how it's done isn't quite so easy since the argument is normally left unchanged by the facet on error except when the error is due to a misplaced thousands separator, which causes `failbit` to be set but doesn't prevent the facet from storing the value.

[ Batavia (2009-05): ]

> We believe this part of the Standard has been recently adjusted and that this issue was addressed during that rewrite.

> Move to NAD.

[ 2009-05-28 Howard adds: ]

> I've moved this issue from Tentatively NAD to Open.

> The current wording of N2857 in 22.4.2.1.2 [facet.num.get.virtuals] p3, stage 3 appears to indicate that in parsing arithmetic types, the value is always set, but sometimes in addition to setting `failbit`.

> - If there is a range error, the value is set to min or max, else
> - if there is a conversion error, the value is set to 0, else
> - if there is a grouping error, the value is set to whatever it would be if grouping were ignored, else
> - the value is set to its error-free result.

> However there is a contradictory sentence in 22.4.2.1.2 [facet.num.get.virtuals] p1.

> 27.7.2.2.2 [istream.formatted.arithmetic] should mimic the behavior of 22.4.2.1.2 [facet.num.get.virtuals] (whatever we decide that behavior is) for `int` and `short`, and currently does not. I believe that the correct code fragment should look like:

> >     typedef num_get<charT,istreambuf_iterator<charT,traits> > numget;
> >     iostate err = ios_base::goodbit;
> >     long lval;
> >     use_facet<numget>(loc).get(*this, 0, *this, err, lval);
> >     if (lval < numeric_limits<int>::min())
> >     {
> >       err |= ios_base::failbit;
> >       val = numeric_limits<int>::min();
> >     }
> >     else if (lval > numeric_limits<int>::max())
> >     {
> >       err |= ios_base::failbit;
> >       val = numeric_limits<int>::max();
> >     }
> >     else
> >       val = static_cast<int>(lval);
> >     setstate(err);

[ 2009-07 Frankfurt ]

> Move to Ready.

Proposed resolution:

Change 22.4.2.1.2 [facet.num.get.virtuals], p1:

> -1- Effects: Reads characters from `in`, interpreting them according to `str.flags()`, `use_facet<ctype<charT> >(loc)`, and `use_facet< numpunct<charT> >(loc)`, where `loc` is `str.getloc()`. If an error occurs, `val` is unchanged; otherwise it is set to the resulting value.

Change 27.7.2.2.2 [istream.formatted.arithmetic], p2 and p3:

>     operator>>(short& val);

> > -2- The conversion occurs as if performed by the following code fragment (using the same notation as for the preceding code fragment):

> > >     typedef num_get<charT,istreambuf_iterator<charT,traits> > numget;
> > >     iostate err = iostate_base::goodbit;
> > >     long lval;
> > >     use_facet<numget>(loc).get(*this, 0, *this, err, lval);
> > >     if (err != 0)
> > >       ;
> > >     else if (lval < numeric_limits<short>::min()
> > >       || numeric_limits<short>::max() < lval)
> > >          err = ios_base::failbit;
> > >     if (lval < numeric_limits<short>::min())
> > >     {
> > >       err |= ios_base::failbit;
> > >       val = numeric_limits<short>::min();
> > >     }
> > >     else if (lval > numeric_limits<short>::max())
> > >     {
> > >       err |= ios_base::failbit;
> > >       val = numeric_limits<short>::max();
> > >     }
> > >     else
> > >       val = static_cast<short>(lval);
> > >     setstate(err);

>     operator>>(int& val);

> > -3- The conversion occurs as if performed by the following code fragment (using the same notation as for the preceding code fragment):

> > >     typedef num_get<charT,istreambuf_iterator<charT,traits> > numget;
> > >     iostate err = iostate_base::goodbit;
> > >     long lval;
> > >     use_facet<numget>(loc).get(*this, 0, *this, err, lval);
> > >     if (err != 0)
> > >       ;
> > >     else if (lval < numeric_limits<int>::min()
> > >       || numeric_limits<int>::max() < lval)
> > >          err = ios_base::failbit;
> > >     if (lval < numeric_limits<int>::min())
> > >     {
> > >       err |= ios_base::failbit;
> > >       val = numeric_limits<int>::min();
> > >     }
> > >     else if (lval > numeric_limits<int>::max())
> > >     {
> > >       err |= ios_base::failbit;
> > >       val = numeric_limits<int>::max();
> > >     }
> > >     else
> > >       val = static_cast<int>(lval);
> > >     setstate(err);

### 697. New `<system_error>` header leads to name clashes

Section: 19.5 [syserr] Status: Resolved Submitter: Daniel Krügler Opened: 2007-06-24 Last modified: 2015-04-08

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Discussion:

The most recent state of N2241 as well as the current draft N2284 (section 19.5 [syserr], p.2) proposes a new enumeration type `posix_errno` immediatly in the namespace `std`. One of the enumerators has the name `invalid_argument`, or fully qualified: `std::invalid_argument`. This name clashes with the exception type `std::invalid_argument`, see 19.2 [std.exceptions]/p.3. This clash makes e.g. the following snippet invalid:

>     #include <system_error>
>     #include <stdexcept>

>     void foo() { throw std::invalid_argument("Don't call us - we call you!"); }

I propose that this enumeration type (and probably the remaining parts of `<system_error>` as well) should be moved into one additional inner namespace, e.g. `sys` or `system` to reduce foreseeable future clashes due to the great number of members that `std::posix_errno` already contains (Btw.: Why has the already proposed `std::sys` sub-namespace from N2066 been rejected?). A further clash candidate seems to be `std::protocol_error` (a reasonable name for an exception related to a std network library, I guess).

Another possible resolution would rely on the proposed strongly typed enums, as described in N2213. But maybe the forbidden implicit conversion to integral types would make these enumerators less attractive in this special case?

Proposed resolution:

Fixed by issue 7 of N2422.

### 698. `system_error` needs `const char*` constructors

Section: 19.5.6.1 [syserr.syserr.overview] Status: CD1 Submitter: Daniel Krügler Opened: 2007-06-24 Last modified: 2015-04-08

View all issues with CD1 status.

Discussion:

In 19.5.6.1 [syserr.syserr.overview] we have the class definition of `std::system_error`. In contrast to all exception classes, which are constructible with a `what_arg string` (see 19.2 [std.exceptions], or `ios_base::failure` in 27.5.3.1.1 [ios::failure]), only overloads with with `const string&` are possible. For consistency with the re-designed remaining exception classes this class should also provide c'tors which accept a const `char* what_arg` string.

Please note that this proposed addition makes sense even considering the given implementation hint for `what()`, because `what_arg` is required to be set as `what_arg` of the base class `runtime_error`, which now has the additional c'tor overload accepting a `const char*`.

Proposed resolution:

This proposed wording assumes issue 832 has been accepted and applied to the working paper.

Change 19.5.6.1 [syserr.syserr.overview] Class system_error overview, as indicated:

>     public:
>       system_error(error_code ec, const string& what_arg);
>       system_error(error_code ec, const char* what_arg);
>       system_error(error_code ec);
>       system_error(int ev, const error_category* ecat,
>           const string& what_arg);
>       system_error(int ev, const error_category* ecat,
>           const char* what_arg);
>       system_error(int ev, const error_category* ecat);

To 19.5.6.2 [syserr.syserr.members] Class system_error members add:

>     system_error(error_code ec, const char* what_arg);

> > Effects: Constructs an object of class `system_error`.

> > Postconditions: `code() == ec` and `strcmp(runtime_error::what(), what_arg) == 0`.

>     system_error(int ev, const error_category* ecat, const char* what_arg);

> > Effects: Constructs an object of class `system_error`.

> > Postconditions: `code() == error_code(ev, ecat)` and `strcmp(runtime_error::what(), what_arg) == 0`.

### 699. N2111 changes min/max

Section: 26.5 [rand] Status: CD1 Submitter: P.J. Plauger Opened: 2007-07-01 Last modified: 2015-04-08

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Discussion:

N2111 changes `min/max` in several places in random from member functions to static data members. I believe this introduces a needless backward compatibility problem between C++0X and TR1. I'd like us to find new names for the static data members, or perhaps change `min/max` to `constexpr`s in C++0X.

See N2391 and N2423 for some further discussion.

Proposed resolution:

Adopt the proposed resolution in N2423.

[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 700. N1856 defines struct `identity`

Section: 20.2.4 [forward] Status: CD1 Submitter: P.J. Plauger Opened: 2007-07-01 Last modified: 2015-04-08

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Discussion:

N1856 defines struct `identity` in `<utility>` which clashes with the traditional definition of struct `identity` in `<functional>` (not standard, but a common extension from old STL). Be nice if we could avoid this name clash for backward compatibility.

Proposed resolution:

Change 20.2.4 [forward]:

>     template <class T> struct identity
>     {
>         typedef T type;
>         const T& operator()(const T& x) const;
>     };

> >     const T& operator()(const T& x) const;

> > > Returns: `x`.

### 703. `map::at()` need a complexity specification

Section: 23.4.4.3 [map.access] Status: CD1 Submitter: Joe Gottman Opened: 2007-07-03 Last modified: 2015-04-08

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Discussion:

`map::at()` need a complexity specification.

Proposed resolution:

Add the following to the specification of `map::at()`, 23.4.4.3 [map.access]:

> Complexity: logarithmic.

### 704. `MoveAssignable` requirement for container value type overly strict

Section: 23.2 [container.requirements] Status: C++11 Submitter: Howard Hinnant Opened: 2007-05-20 Last modified: 2015-04-08

View other active issues in [container.requirements].

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Discussion:

The move-related changes inadvertently overwrote the intent of 276. Issue 276 removed the requirement of `CopyAssignable` from most of the member functions of node-based containers. But the move-related changes unnecessarily introduced the `MoveAssignable` requirement for those members which used to require `CopyAssignable`.

We also discussed (c++std-lib-18722) the possibility of dropping `MoveAssignable` from some of the sequence requirements. Additionally the in-place construction work may further reduce requirements. For purposes of an easy reference, here are the minimum sequence requirements as I currently understand them. Those items in requirements table in the working draft which do not appear below have been purposefully omitted for brevity as they do not have any requirements of this nature. Some items which do not have any requirements of this nature are included below just to confirm that they were not omitted by mistake.

Container Requirements
`X u(a)` | `value_type` must be `CopyConstructible`
`X u(rv)` | `array` requires `value_type` to be `CopyConstructible`
`a = u` | Sequences require `value_type` to be `CopyConstructible` and `CopyAssignable`. Associative containers require `value_type` to be `CopyConstructible`.
`a = rv` | `array` requires `value_type` to be `CopyAssignable`. Sequences containers with `propagate_on_container_move_assignment == false` allocators require `value_type` to be `MoveConstructible` and `MoveAssignable`. Associative containers with `propagate_on_container_move_assignment == false` allocators require `value_type` to be `MoveConstructible`.
`swap(a,u)` | `array` requires `value_type` to be `Swappable`.

Sequence Requirements
`X(n)` | `value_type` must be `DefaultConstructible`
`X(n, t)` | `value_type` must be `CopyConstructible`
`X(i, j)` | Sequences require `value_type` to be constructible from `*i`. Additionally if input_iterators are used, `vector` and `deque` require `MoveContructible` and `MoveAssignable`.
`a.insert(p, t)` | The `value_type` must be `CopyConstructible`. The sequences `vector` and `deque` also require the `value_type` to be `CopyAssignable`.
`a.insert(p, rv)` | The `value_type` must be `MoveConstructible`. The sequences `vector` and `deque` also require the `value_type` to be `MoveAssignable`.
`a.insert(p, n, t)` | The `value_type` must be `CopyConstructible`. The sequences `vector` and `deque` also require the `value_type` to be `CopyAssignable`.

`a.insert(p, i, j)`

If the iterators return an lvalue the `value_type` must be `CopyConstructible`. The sequences `vector` and `deque` also require the `value_type` to be `CopyAssignable` when the iterators return an lvalue. If the iterators return an rvalue the `value_type` must be `MoveConstructible`. The sequences `vector` and `deque` also require the `value_type` to be `MoveAssignable` when the iterators return an rvalue.

`a.erase(p)` | The sequences `vector` and `deque` require the `value_type` to be `MoveAssignable`.
`a.erase(q1, q2)` | The sequences `vector` and `deque` require the `value_type` to be `MoveAssignable`.
`a.clear()` |
`a.assign(i, j)` | If the iterators return an lvalue the `value_type` must be `CopyConstructible` and `CopyAssignable`. If the iterators return an rvalue the `value_type` must be `MoveConstructible` and `MoveAssignable`.
`a.assign(n, t)` | The `value_type` must be `CopyConstructible` and `CopyAssignable`.
`a.resize(n)` | The `value_type` must be `DefaultConstructible`. The sequence `vector` also requires the `value_type` to be `MoveConstructible`.
`a.resize(n, t)` | The `value_type` must be `CopyConstructible`.

Optional Sequence Requirements
`a.front()` |
`a.back()` |
`a.push_front(t)` | The `value_type` must be `CopyConstructible`.
`a.push_front(rv)` | The `value_type` must be `MoveConstructible`.
`a.push_back(t)` | The `value_type` must be `CopyConstructible`.
`a.push_back(rv)` | The `value_type` must be `MoveConstructible`.
`a.pop_front()` |
`a.pop_back()` |
`a[n]` |
`a.at[n]` |

Associative Container Requirements
`X(i, j)` | If the iterators return an lvalue the `value_type` must be `CopyConstructible`. If the iterators return an rvalue the `value_type` must be `MoveConstructible`.
`a_uniq.insert(t)` | The `value_type` must be `CopyConstructible`.
`a_uniq.insert(rv)` | The `key_type` and the `mapped_type` (if it exists) must be `MoveConstructible`.
`a_eq.insert(t)` | The `value_type` must be `CopyConstructible`.
`a_eq.insert(rv)` | The `key_type` and the `mapped_type` (if it exists) must be `MoveConstructible`.
`a.insert(p, t)` | The `value_type` must be `CopyConstructible`.
`a.insert(p, rv)` | The `key_type` and the `mapped_type` (if it exists) must be `MoveConstructible`.
`a.insert(i, j)` | If the iterators return an lvalue the `value_type` must be `CopyConstructible`. If the iterators return an rvalue the `key_type` and the `mapped_type` (if it exists) must be `MoveConstructible`..

Unordered Associative Container Requirements
`X(i, j, n, hf, eq)` | If the iterators return an lvalue the `value_type` must be `CopyConstructible`. If the iterators return an rvalue the `value_type` must be `MoveConstructible`.
`a_uniq.insert(t)` | The `value_type` must be `CopyConstructible`.
`a_uniq.insert(rv)` | The `key_type` and the `mapped_type` (if it exists) must be `MoveConstructible`.
`a_eq.insert(t)` | The `value_type` must be `CopyConstructible`.
`a_eq.insert(rv)` | The `key_type` and the `mapped_type` (if it exists) must be `MoveConstructible`.
`a.insert(p, t)` | The `value_type` must be `CopyConstructible`.
`a.insert(p, rv)` | The `key_type` and the `mapped_type` (if it exists) must be `MoveConstructible`.
`a.insert(i, j)` | If the iterators return an lvalue the `value_type` must be `CopyConstructible`. If the iterators return an rvalue the `key_type` and the `mapped_type` (if it exists) must be `MoveConstructible`..

Miscellaneous Requirements
`map[lvalue-key]` | The `key_type` must be `CopyConstructible`. The `mapped_type` must be `DefaultConstructible` and `MoveConstructible`.
`map[rvalue-key]` | The `key_type` must be `MoveConstructible`. The `mapped_type` must be `DefaultConstructible` and `MoveConstructible`.

[ Kona (2007): Howard and Alan to update requirements table in issue with emplace signatures. ]

[ Bellevue: This should be handled as part of the concepts work. ]

[ 2009-07-20 Reopened by Howard: ]

> This is one of the issues that was "solved by concepts" and is now no longer solved.

> In a nutshell, concepts adopted the "minimum requirements" philosophy outlined in the discussion of this issue, and enforced it. My strong suggestion is that we translate the concepts specification into documentation for the containers.

> What this means for vendors is that they will have to implement container members being careful to only use those characteristics of a type that the concepts specification formally allowed. Note that I am not talking about `enable_if`'ing everything. I am simply suggesting that (for example) we tell the vendor he can't call `T's` copy constructor or move constructor within the `emplace` member function, etc.

> What this means for customers is that they will be able to use types within C++03 containers which are sometimes not CopyConstructible, and sometimes not even MoveConstructible, etc.

[ 2009-10 Santa Cruz: ]

> Leave open. Howard to provide wording.

[ 2010-02-06 Howard provides wording. ]

[ 2010-02-08 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010-02-10 Howard opened. I neglected to reduce the requirements on value_type for the insert function of the ordered and unordered associative containers when the argument is an rvalue. Fixed it. ]

[ 2010-02-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010-03-08 Nico opens: ]

> I took the task to see whether 868 is covered by 704 already. However, by doing that I have the impression that 704 is a big mistake.

> Take e.g. the second change of 868:

> > Change 23.3.3.2 [deque.cons] para 5:

> > > Effects: Constructs a `deque` with `n` default constructed elements.

> > where "default constructed" should be replaced by "value-initialized". This is the constructor out of a number of elements:

> > >     ContType c(num)

> > 704 says:

> > > Remove the entire section 23.3.3.2 [deque.cons].

> > > > [ This section is already specified by the requirements tables. ]

> > BUT, there is no requirement table that lists this constructor at all, which means that we would lose the entire specification of this function !!!

> > In fact, I found with further investigation, if we follow 704 to remove 23.3.2.1 we

> > - have no semantics for `ContType c(num)`
> > - have no complexity and no allocator specification for `ContType c(num,val)`
> > - have no semantics for `ContType c(num,val,alloc)`
> > - - have no complexity and no allocator specification for `ContType c(beg,end)`
> > - - have no semantics for `ContType c(beg,end,alloc)`
> > - - have different wording (which might or might not give the same guarantees) for the `assign` functions

> > because all these guarantees are given in the removed section but nowhere else (as far as I saw).

> > Looks to me that 704 need a significant review before we take that change, because chances are high that there are similar flaws in other proposed changes there (provided I am not missing anything).

[ 2010 Pittsburgh: ]

> Removed the parts from the proposed wording that removed existing sections, and set to Ready for Pittsburgh.

Rationale:

[ post San Francisco: ]

> Solved by N2776.

This rationale is obsolete.

Proposed resolution:

Change 23.2.1 [container.requirements.general]/4:

> 4 In Tables 91 and 92, `X` denotes a container class containing objects of type `T`, `a` and `b` denote values of type `X`, `u` denotes an identifier, `r` denotes an lvalue or a const rvalue a non-const value of type `X`, and `rv` denotes a non-const rvalue of type `X`.

Change the following rows in Table 91 — Container requirements 23.2.1 [container.requirements.general]:

> Table 91 — Container requirements
> Expression | Return type | Assertion/note pre-/post-condition | Complexity
> --- | --- | --- | ---
> `X::value_type` | `T` | Requires: `T` is `Destructible`. | compile time

Change 23.2.1 [container.requirements.general]/10:

> Unless otherwise specified (see 23.2.4.1, 23.2.5.1, 23.3.2.3, and 23.3.6.4) all container types defined in this Clause meet the following additional requirements:

> - …

> - no `erase()`, `clear()`, `pop_back()` or `pop_front()` function throws an exception.

> - …

Insert a new paragraph prior to 23.2.1 [container.requirements.general]/14:

> The descriptions of the requirements of the type `T` in this section use the terms `CopyConstructible`, `MoveConstructible`, constructible from `*i`, and constructible from `args`. These terms are equivalent to the following expression using the appropriate arguments:

> >     allocator_traits<allocator_type>::construct(x.get_allocator(), q, args...);

> where `x` is a non-const lvalue of some container type `X` and `q` has type `X::value_type*`.

> [Example: The container is going to move construct a `T`, so will call:

> >     allocator_traits<allocator_type>::construct(get_allocator(), q, std::move(t));

> The default implementation of construct will call:

> >     ::new (q) T(std::forward<T>(t)); // where forward is the same as move here, cast to rvalue

> But the allocator author may override the above definition of `construct` and do the construction of `T` by some other means. — end example]

> 14 ...

Add to 23.2.1 [container.requirements.general]/14:

> 14 In Table 93, `X` denotes an allocator-aware container class with a `value_type` of `T` using allocator of type `A`, `u` denotes a variable, `a` and `b` denote non-const lvalues of type `X`, `t` denotes an lvalue or a const rvalue of type `X`, `rv` denotes a non-const rvalue of type `X`, `m` is a value of type `A`, and `Q` is an allocator type.

Change or add the following rows in Table 93 — Allocator-aware container requirements in 23.2.1 [container.requirements.general]:

> Table 93 — Allocator-aware container requirements
> Expression | Return type | Assertion/note pre-/post-condition | Complexity
> --- | --- | --- | ---
> `X(t, m) X u(t, m);` |  | Requires: `T` is `CopyConstructible`. post: `u == t`, `get_allocator() == m` | linear
> `X(rv, m) X u(rv, m);` |  | Requires: `T` is `MoveConstructible`. post: `u` shall have the same elements, or copies of the elements, that `rv` had before this construction, `get_allocator() == m` | constant if `m == rv.get_allocator()`, otherwise linear
> `a = t` | `X&` | Requires: `T` is `CopyConstructible` and `CopyAssignable` post: `a == t`. | linear
> `a = rv` | `X&` | Requires: If `allocator_traits< allocator_type > ::propagate_on_container_move_assignment ::value` is `false`, `T` is `MoveConstructible` and `MoveAssignable`. All existing elements of `a` are either move assigned to or destroyed. `a` shall be equal to the value that `rv` had before this assignment | linear
> `a.swap(b);` | `void` | exchanges the contents of `a` and `b` | constant

Change the following rows in Table 94 — Sequence container requirements (in addition to container) in 23.2.3 [sequence.reqmts]:

> Table 94 — Sequence container requirements (in addition to container)
> Expression | Return type | Assertion/note pre-/post-condition
> --- | --- | ---

> `X(i, j)
> X a(i, j)`

> Requires: If the iterator's dereference operation returns an lvalue or a const rvalue, `T` shall be `CopyConstructible`. `T` shall be constructible from `*i`.
> If the iterator does not meet the forward iterator requirements (24.2.5 [forward.iterators]), then `vector` also requires `T` to be `MoveConstructible`.
> Each iterator in the range `[i,j)` shall be dereferenced exactly once.
> post: `size() ==` distance between `i` and `j`
> Constructs a sequence container equal to the range `[i, j)`

> `a = il;` | `X&` | Requires: `T` is `CopyConstructible` and `CopyAssignable`. `a = X(il);` Assigns the range `[il.begin(), il.end())` into `a`. All existing elements of `a` are either assigned or destroyed. r Returns `*this;`
> `a.emplace(p, args);` | `iterator` | Requires: `ConstructibleAsElement<A, T, Args>`. `T` is constructible from `args`. `vector` and `deque` also require `T` to be `MoveConstructible` and `MoveAssignable`. Inserts an object of type `T` constructed with `std::forward<Args>(args)...` before `p`.
> `a.insert(p, t);` | `iterator` | Requires: `ConstructibleAsElement<A, T, Args>` and `T` shall be `CopyAssignable`. `T` shall be `CopyConstructible`. `vector` and `deque` also require `T` to be `CopyAssignable`. Inserts a copy `t` before `p`.
> `a.insert(p, rv);` | `iterator` | Requires: `ConstructibleAsElement<A, T, T&&>` and `T` shall be `MoveAssignable`. `T` shall be `MoveConstructible`. `vector` and `deque` also require `T` to be `MoveAssignable`. Inserts a copy `rv` before `p`.

> `a.insert(p, i, j)`

> `iterator`

> Requires: If the iterator's dereference operation returns an lvalue or a const rvalue, `T` shall be `CopyConstructible`. `T` shall be constructible from `*i`.
> If the iterator does not meet the forward iterator requirements (24.2.5 [forward.iterators]), then `vector` also requires `T` to be `MoveConstructible` and `MoveAssignable`.
> Each iterator in the range `[i,j)` shall be dereferenced exactly once.
> pre: `i` and `j` are not iterators into `a`.
> Inserts copies of elements in `[i, j)` before `p`

> `a.erase(q);` | `iterator` | Requires: `T` and `T` shall be `MoveAssignable`. `vector` and `deque` require `T` to be `MoveAssignable`. Erases the element pointed to by `q`.
> `a.erase(q1, q2);` | `iterator` | Requires: `T` and `T` shall be `MoveAssignable`. `vector` and `deque` require `T` to be `MoveAssignable`. Erases the elements in the range `[q1, q2)`.
> `a.clear();` | `void` | `erase(begin(), end())` Destroys all elements in `a`. Invalidates all references, pointers, and iterators referring to the elements of `a` and may invalidate the past-the-end iterator. post: `size() == 0 a.empty() == true`

> `a.assign(i, j)`

> `void`

> Requires: If the iterator's dereference operation returns an lvalue or a const rvalue, `T` shall be `CopyConstructible` and `CopyAssignable`. `T` shall be constructible and assignable from `*i`. If the iterator does not meet the forward iterator requirements (24.2.5 [forward.iterators]), then `vector` also requires `T` to be `MoveConstructible`.
> Each iterator in the range `[i,j)` shall be dereferenced exactly once.
> pre: `i`, `j` are not iterators into `a`.
> Replaces elements in `a` with a copy of `[i, j)`.

Change the following rows in Table 95 — Optional sequence container operations in 23.2.3 [sequence.reqmts]:

> Table 95 — Optional sequence container operations
> Expression | Return type | Operational semantics | Container
> --- | --- | --- | ---
> `a.emplace_front(args)` | `void` | `a.emplace(a.begin(), std::forward<Args>(args)...)` Prepends an object of type `T` constructed with `std::forward<Args>(args)...`. Requires: `ConstructibleAsElement<A, T, Args>` `T` shall be constructible from `args`. | `list`, `deque`, `forward_list`
> `a.emplace_back(args)` | `void` | `a.emplace(a.end(), std::forward<Args>(args)...)` Appends an object of type `T` constructed with `std::forward<Args>(args)...`. Requires: `ConstructibleAsElement<A, T, Args>` `T` shall be constructible from `args`. `vector` also requires `T` to be `MoveConstructible`. | `list`, `deque`, `vector`
> `a.push_front(t)` | `void` | `a.insert(a.begin(), t)` Prepends a copy of `t`. Requires: `ConstructibleAsElement<A, T, T>` and `T` shall be `CopyAssignable`. `T` shall be `CopyConstructible`. | `list`, `deque`, `forward_list`
> `a.push_front(rv)` | `void` | `a.insert(a.begin(), t)` Prepends a copy of `rv`. Requires: `ConstructibleAsElement<A, T, T&&>` and `T` shall be `MoveAssignable`. `T` shall be `MoveConstructible`. | `list`, `deque`, `forward_list`
> `a.push_back(t)` | `void` | `a.insert(a.end(), t)` Appends a copy of `t`. Requires: `ConstructibleAsElement<A, T, T>` and `T` shall be `CopyAssignable`. `T` shall be `CopyConstructible`. | `vector`, `list`, `deque`, `basic_string`
> `a.push_back(rv)` | `void` | `a.insert(a.end(), t)` Appends a copy of `rv`. Requires: `ConstructibleAsElement<A, T, T&&>` and `T` shall be `MoveAssignable`. `T` shall be `MoveConstructible`. | `vector`, `list`, `deque`, `basic_string`
> `a.pop_front()` | `void` | `a.erase(a.begin())` Destroys the first element. Requires: `a.empty()` shall be `false`. | `list`, `deque`, `forward_list`
> `a.pop_back()` | `void` | `{ iterator tmp = a.end(); --tmp; a.erase(tmp); }` Destroys the last element. Requires: `a.empty()` shall be `false`. | `vector`, `list`, `deque`, `basic_string`

Insert a new paragraph prior to 23.2.4 [associative.reqmts]/7, and edit paragraph 7:

> The associative containers meet all of the requirements of Allocator-aware containers (23.2.1 [container.requirements.general]), except for the containers `map` and `multimap`, the requirements placed on `value_type` in Table 93 apply instead directly to `key_type` and `mapped_type`. [Note: For example `key_type` and `mapped_type` are sometimes required to be `CopyAssignable` even though the `value_type` (`pair<const key_type, mapped_type>`) is not `CopyAssignable`. — end note]

> 7 In Table 96, `X` denotes an associative container class, a denotes a value of `X`, `a_uniq` denotes a value of `X` when `X` supports unique keys, `a_eq` denotes a value of `X` when `X` supports multiple keys, `u` denotes an identifier, `r` denotes an lvalue or a const rvalue of type `X`, `rv` denotes a non-const rvalue of type `X`, `i` and `j` satisfy input iterator requirements and refer to elements implicitly convertible to `value_type`, `[i,j)` denotes a valid range, `p` denotes a valid const iterator to `a`, `q` denotes a valid dereferenceable const iterator to `a`, `[q1, q2)` denotes a valid range of const iterators in `a`, `il` designates an object of type `initializer_list<value_type>`, `t` denotes a value of `X::value_type`, `k` denotes a value of `X::key_type` and `c` denotes a value of type `X::key_compare`. `A` denotes the storage allocator used by `X`, if any, or `std::allocator<X::value_type>` otherwise, and `m` denotes an allocator of a type convertible to `A`.

Change or add the following rows in Table 96 — Associative container requirements (in addition to container) in 23.2.4 [associative.reqmts]:

> Table 96 — Associative container requirements (in addition to container)
> Expression | Return type | Assertion/note pre-/post-condition | Complexity
> --- | --- | --- | ---
> `X::key_type` | `Key` | Requires: `Key` is `CopyConstructible` and `CopyAssignable` `Destructible` | compile time
> `X::mapped_type` (`map` and `multimap` only) | `T` | Requires: `T` is `Destructible` | compile time
> `X(c) X a(c);` |  | Requires: `ConstructibleAsElement<A, key_compare, key_compare>`. `key_compare` is `CopyConstructible`. Constructs an empty container. Uses a copy of `c` as a comparison object. | constant
> `X() X a;` |  | Requires: `ConstructibleAsElement<A, key_compare, key_compare>`. `key_compare` is `DefaultConstructible`. Constructs an empty container. Uses `Compare()` as a comparison object. | constant

> `X(i, j, c)
> X a(i, j, c);`

> Requires: `ConstructibleAsElement<A, key_compare, key_compare>`.
> `key_compare` is `CopyConstructible`. `value_type` shall be constructible from `*i`.
> Constructs an empty container ans inserts elements from the range `[i, j)` into it; uses `c` as a comparison object.

> `N` log `N` in general (`N` is the distance from `i` to `j`); linear if `[i, j)` is sorted with `value_comp()`

> `X(i, j) X a(i, j);` |  | Requires: `ConstructibleAsElement<A, key_compare, key_compare>`. `value_type` shall be constructible from `*i`. `key_compare` is `DefaultConstructible`. Same as above, but uses `Compare()` as a comparison object. | same as above
> `a = il` | `X&` | `a = X(il); return *this;` Requires: `T` is `CopyConstructible` and `CopyAssignable`. Assigns the range `[il.begin(), il.end())` into `a`. All existing elements of `a` are either assigned or destroyed. | Same as ``a = X(il)``. `N` log `N` in general (`N` is `il.size()` added to the existing size of `a`); linear if `[il.begin(), il.end())` is sorted with `value_comp()`

> `a_uniq.emplace(args)`

> `pair<iterator, bool>`

> Requires: `T` shall be constructible from `args`
> inserts a `T` object `t` constructed with `std::forward<Args>(args)...` if and only if there is no element in the container with key equivalent to the key of `t`. The `bool` component of the returned pair is true if and only if the insertion takes place, and the iterator component of the pair points to the element with key equivalent to the key of `t`.

> logarithmic

> `a_eq.emplace(args)` | `iterator` | Requires: `T` shall be constructible from `args` inserts a `T` object `t` constructed with `std::forward<Args>(args)...` and returns the iterator pointing to the newly inserted element. | logarithmic

> `a_uniq.insert(t)`

> `pair<iterator, bool>`

> Requires: `T` shall be `MoveConstructible` if `t` is a non-const rvalue expression, else `T` shall be `CopyConstructible`.
> inserts `t` if and only if there is no element in the container with key equivalent to the key of `t`. The `bool` component of the returned pair is true if and only if the insertion takes place, and the iterator component of the pair points to the element with key equivalent to the key of `t`.

> logarithmic

> `a_eq.insert(t)` | `iterator` | Requires: `T` shall be `MoveConstructible` if `t` is a non-const rvalue expression, else `T` shall be `CopyConstructible`. inserts `t` and returns the iterator pointing to the newly inserted element. If a range containing elements equivalent to `t` exists in `a_eq`, `t` is inserted at the end of that range. | logarithmic

> `a.insert(p, t)`

> `iterator`

> Requires: `T` shall be `MoveConstructible` if `t` is a non-const rvalue expression, else `T` shall be `CopyConstructible`.
> inserts `t` if and only if there is no element with key equivalent to the key of `t` in containers with unique keys; always inserts `t` in containers with equivalent keys; always returns the iterator pointing to the element with key equivalent to the key of `t`. `t` is inserted as close as possible to the position just prior to `p`.

> logarithmic in general, but amortized constant if `t` is inserted right before `p`.

> `a.insert(i, j)` | `void` | Requires: `T` shall be constructible from `*i`. pre: `i`, `j` are not iterators into `a`. inserts each element from the range `[i,j)` if and only if there is no element with key equivalent to the key of that element in containers with unique keys; always inserts that element in containers with equivalent keys. | N log(size() + N ) (N is the distance from i to j)

Insert a new paragraph prior to 23.2.5 [unord.req]/9:

> The unordered associative containers meet all of the requirements of Allocator-aware containers (23.2.1 [container.requirements.general]), except for the containers `unordered_map` and `unordered_multimap`, the requirements placed on `value_type` in Table 93 apply instead directly to `key_type` and `mapped_type`. [Note: For example `key_type` and `mapped_type` are sometimes required to be `CopyAssignable` even though the `value_type` (`pair<const key_type, mapped_type>`) is not `CopyAssignable`. — end note]

> 9 ...

Change or add the following rows in Table 98 — Unordered associative container requirements (in addition to container) in 23.2.5 [unord.req]:

> Table 98 — Unordered associative container requirements (in addition to container)
> Expression | Return type | Assertion/note pre-/post-condition | Complexity
> --- | --- | --- | ---
> `X::key_type` | `Key` | Requires: `Key` shall be `CopyAssignable` and `CopyConstructible` `Destructible` | compile time
> `X::mapped_type` (`unordered_map` and `unordered_multimap` only) | `T` | Requires:`T` is `Destructible` | compile time
> `X(n, hf, eq) X a(n, hf, eq)` | `X` | Requires: `hasher` and `key_equal` are `CopyConstructible`. Constructs an empty container with at least `n` buckets, using `hf` as the hash function and `eq` as the key equality predicate. | `O(N)`
> `X(n, hf) X a(n, hf)` | `X` | Requires: `hasher` is `CopyConstructible` and `key_equal` is `DefaultConstructible`. Constructs an empty container with at least `n` buckets, using `hf` as the hash function and `key_equal()` as the key equality predicate. | `O(N)`
> `X(n) X a(n)` | `X` | Requires: `hasher` and `key_equal` are `DefaultConstructible`. Constructs an empty container with at least `n` buckets, using `hasher()` as the hash function and `key_equal()` as the key equality predicate. | `O(N)`
> `X() X a` | `X` | Requires: `hasher` and `key_equal` are `DefaultConstructible`. Constructs an empty container an unspecified number of buckets, using `hasher()` as the hash function and `key_equal()` as the key equality predicate. | constant
> `X(i, j, n, hf, eq) X a(i, j, n, hf, eq)` | `X` | Requires: `value_type` is constructible from `*i`. `hasher` and `key_equal` are `CopyConstructible`. Constructs an empty container with at least `n` buckets, using `hf` as the hash function and `eq` as the key equality predicate, and inserts elements from `[i, j)` into it. | Average case `O(N)` (`N` is `distance(i, j)`), worst case `O(N2)`

> `X(i, j, n, hf)
> X a(i, j, n, hf)`

> `X`

> Requires: `value_type` is constructible from `*i`. `hasher` is `CopyConstructible` and `key_equal` is `DefaultConstructible`.
> Constructs an empty container with at least `n` buckets, using `hf` as the hash function and `key_equal()` as the key equality predicate, and inserts elements from `[i, j)` into it.

> Average case `O(N)` (`N` is `distance(i, j)`), worst case `O(N2)`

> `X(i, j, n) X a(i, j, n)` | `X` | Requires: `value_type` is constructible from `*i`. `hasher` and `key_equal` are `DefaultConstructible`. Constructs an empty container with at least `n` buckets, using `hasher()` as the hash function and `key_equal()` as the key equality predicate, and inserts elements from `[i, j)` into it. | Average case `O(N)` (`N` is `distance(i, j)`), worst case `O(N2)`
> `X(i, j) X a(i, j)` | `X` | Requires: `value_type` is constructible from `*i`. `hasher` and `key_equal` are `DefaultConstructible`. Constructs an empty container with an unspecified number of buckets, using `hasher()` as the hash function and `key_equal()` as the key equality predicate, and inserts elements from `[i, j)` into it. | Average case `O(N)` (`N` is `distance(i, j)`), worst case `O(N2)`
> `X(b) X a(b)` | `X` | Copy constructor. In addition to the contained elements requirements of Table 93 (23.2.1 [container.requirements.general]), copies the hash function, predicate, and maximum load factor. | Average case linear in `b.size()`, worst case quadratic.
> `a = b` | `X&` | Copy assignment operator. In addition to the contained elements requirements of Table 93 (23.2.1 [container.requirements.general]), copies the hash function, predicate, and maximum load factor. | Average case linear in `b.size()`, worst case quadratic.
> `a = il` | `X&` | `a = X(il); return *this;` Requires: `T` is `CopyConstructible` and `CopyAssignable`. Assigns the range `[il.begin(), il.end())` into `a`. All existing elements of `a` are either assigned or destroyed. | Average case linear in `il.size()`, worst case quadratic.

> `a_uniq.emplace(args)`

> `pair<iterator, bool>`

> Requires: `T` shall be constructible from `args`
> inserts a `T` object `t` constructed with `std::forward<Args>(args)...` if and only if there is no element in the container with key equivalent to the key of `t`. The `bool` component of the returned pair is true if and only if the insertion takes place, and the iterator component of the pair points to the element with key equivalent to the key of `t`.

> Average case O(1), worst case O(`a_uniq.size()`).

> `a_eq.emplace(args)` | `iterator` | Requires: `T` shall be constructible from `args` inserts a `T` object `t` constructed with `std::forward<Args>(args)...` and returns the iterator pointing to the newly inserted element. | Average case O(1), worst case O(`a_eq.size()`).

> `a.emplace_hint(p, args)`

> `iterator`

> Requires: `T` shall be constructible from `args`
> equivalent to `a.emplace( std::forward<Args>(args)...)`. Return value is an iterator pointing to the element with the key equivalent to the newly inserted element. The `const_iterator p` is a hint pointing to where the search should start. Implementations are permitted to ignore the hint.

> Average case O(1), worst case O(`a.size()`).

> `a_uniq.insert(t)`

> `pair<iterator, bool>`

> Requires: `T` shall be `MoveConstructible` if `t` is a non-const rvalue expression, else `T` shall be `CopyConstructible`.
> Inserts `t` if and only if there is no element in the container with key equivalent to the key of `t`. The `bool` component of the returned pair indicates whether the insertion takes place, and the iterator component points to the element with key equivalent to the key of `t`.

> Average case O(1), worst case O(`a_uniq.size()`).

> `a_eq.insert(t)` | `iterator` | Requires: `T` shall be `MoveConstructible` if `t` is a non-const rvalue expression, else `T` shall be `CopyConstructible`. Inserts `t`, and returns an iterator pointing to the newly inserted element. | Average case O(1), worst case O(`a_uniq.size()`).

> `a.insert(q, t)`

> `iterator`

> Requires: `T` shall be `MoveConstructible` if `t` is a non-const rvalue expression, else `T` shall be `CopyConstructible`.
> Equivalent to `a.insert(t)`. Return value is an iterator pointing to the element with the key equivalent to that of `t`. The iterator `q` is a hint pointing to where the search should start. Implementations are permitted to ignore the hint.

> Average case O(1), worst case O(`a_uniq.size()`).

> `a.insert(i, j)` | `void` | Requires: `T` shall be constructible from `*i`. Pre: `i` and `j` are not iterators in `a`. Equivalent to `a.insert(t)` for each element in `[i,j)`. | Average case O(`N`), where `N` is `distance(i, j)`. Worst case O(`N * a.size()`).

Change 23.3.4 [forwardlist]/2:

> 2 A `forward_list` satisfies all of the requirements of a container (table 91), except that the `size()` member function is not provided. A `forward_list` also satisfies all of the requirements of an allocator-aware container (table 93). And `forward_list` provides the `assign` member functions as specified in Table 94, Sequence container requirements, and several of the optional sequence container requirements (Table 95). Descriptions are provided here only for operations on `forward_list` that are not described in that table or for operations where there is additional semantic information.

Add a new paragraph after 23.3.4.5 [forwardlist.modifiers]/23:

>     void clear();

> > 23 Effects: Erases all elements in the range `[begin(),end())`.

> > Remarks: Does not invalidate past-the-end iterators.

Change 23.3.6.3 [vector.capacity]/13:

>     void resize(size_type sz, const T& c);

> > 13 Requires: `T` shall be `CopyConstructible`. If `value_type` has a move constructor, that constructor shall not throw any exceptions.

In 23.5.6 [unord.set] and 23.5.7 [unord.multiset] substitute "`Key`" for "`Value`".

> [ The above substitution is normative as it ties into the requirements table. ]

### 705. type-trait `decay` incompletely specified

Section: 20.10.7.6 [meta.trans.other] Status: CD1 Submitter: Thorsten Ottosen Opened: 2007-07-08 Last modified: 2015-04-08

View other active issues in [meta.trans.other].

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Discussion:

The current working draft has a type-trait `decay` in 20.10.7.6 [meta.trans.other].

Its use is to turn C++03 pass-by-value parameters into efficient C++0x pass-by-rvalue-reference parameters. However, the current definition introduces an incompatible change where the cv-qualification of the parameter type is retained. The deduced type should loose such cv-qualification, as pass-by-value does.

Proposed resolution:

In 20.10.7.6 [meta.trans.other] change the last sentence:

> Otherwise the member typedef `type` equals `remove_cv<U>::type`.

In 20.4.2.4 [tuple.creation]/1 change:

> where each `Vi` in `VTypes` is `X&` if, for the corresponding type `Ti` in `Types`, `remove_cv<remove_reference<Ti>::type>::type` equals `reference_wrapper<X>`, otherwise `Vi` is `decay<Ti>::type`. Let `Ui` be `decay<Ti>::type` for each `Ti` in `Types`. Then each `Vi` in `VTypes` is `X&` if `Ui` equals `reference_wrapper<X>`, otherwise `Vi` is `Ui`.

### 706. `make_pair()` should behave as `make_tuple()` wrt. `reference_wrapper()`

Section: 20.3 [pairs] Status: CD1 Submitter: Thorsten Ottosen Opened: 2007-07-08 Last modified: 2015-04-08

View all other issues in [pairs].

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Discussion:

The current draft has `make_pair()` in 20.3 [pairs]/16 and `make_tuple()` in 20.4.2.4 [tuple.creation]. `make_tuple()` detects the presence of `reference_wrapper<X>` arguments and "unwraps" the reference in such cases. `make_pair()` would OTOH create a `reference_wrapper<X>` member. I suggest that the two functions are made to behave similar in this respect to minimize confusion.

Proposed resolution:

In 20.2 [utility] change the synopsis for make_pair() to read

>     template <class T1, class T2>
>       pair<typename decay<T1>::type V1, typename decay<T2>::type V2> make_pair(T1&&, T2&&);

In 20.3 [pairs]/16 change the declaration to match the above synopsis. Then change the 20.3 [pairs]/17 to:

> Returns: `pair<typename decay<T1>::type V1,typename decay<T2>::type V2>(forward<T1>(x),forward<T2>(y))` where `V1` and `V2` are determined as follows: Let `Ui` be `decay<Ti>::type` for each `Ti`. Then each `Vi` is `X&` if `Ui` equals `reference_wrapper<X>`, otherwise `Vi` is `Ui`.

### 709. `char_traits::not_eof` has wrong signature

Section: 21.2.3 [char.traits.specializations] Status: CD1 Submitter: Bo Persson Opened: 2007-08-13 Last modified: 2015-04-08

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Discussion:

The changes made for `constexpr` in 21.2.3 [char.traits.specializations] have not only changed the `not_eof` function from pass by const reference to pass by value, it has also changed the parameter type from `int_type` to `char_type`.

This doesn't work for type `char`, and is inconsistent with the requirements in Table 56, Traits requirements, 21.2.1 [char.traits.require].

Pete adds:

> For what it's worth, that may not have been an intentional change. N2349, which detailed the changes for adding constant expressions to the library, has strikeout bars through the `const` and the `&` that surround the `char_type` argument, but none through `char_type` itself. So the intention may have been just to change to pass by value, with text incorrectly copied from the standard.

Proposed resolution:

Change the signature in 21.2.3.1 [char.traits.specializations.char], 21.2.3.2 [char.traits.specializations.char16_t], 21.2.3.3 [char.traits.specializations.char32_t], and 21.2.3.4 [char.traits.specializations.wchar.t] to

>     static constexpr int_type not_eof(char_type int_type c);

[ Bellevue: ]

> Resolution: NAD editorial - up to Pete's judgment

[ Post Sophia Antipolis ]

> Moved from Pending NAD Editorial to Review. The proposed wording appears to be correct but non-editorial.

### 710. Missing postconditions

Section: 20.8.2.2 [util.smartptr.shared] Status: CD1 Submitter: Peter Dimov Opened: 2007-08-24 Last modified: 2015-04-08

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Discussion:

A discussion on comp.std.c++ has identified a contradiction in the `shared_ptr` specification. The `shared_ptr` move constructor and the cast functions are missing postconditions for the `get()` accessor.

[ Bellevue: ]

> Move to "ready", adopting the first (Peter's) proposed resolution.

> Note to the project editor: there is an editorial issue here. The wording for the postconditions of the casts is slightly awkward, and the editor should consider rewording "If w is the return value...", e. g. as "For a return value w...".

Proposed resolution:

Add to 20.8.2.2.1 [util.smartptr.shared.const]:

>     shared_ptr(shared_ptr&& r);
>     template<class Y> shared_ptr(shared_ptr<Y>&& r);

> > Postconditions: `*this` shall contain the old value of `r`. `r` shall be empty. `r.get() == 0`.

Add to 20.8.2.2.9 [util.smartptr.shared.cast]:

>     template<class T, class U> shared_ptr<T> static_pointer_cast(shared_ptr<U> const& r);

> > Postconditions: If `w` is the return value, `w.get() == static_cast<T*>(r.get()) && w.use_count() == r.use_count()`.

>     template<class T, class U> shared_ptr<T> dynamic_pointer_cast(shared_ptr<U> const& r);

> > Postconditions: If `w` is the return value, `w.get() == dynamic_cast<T*>(r.get())`.

>     template<class T, class U> shared_ptr<T> const_pointer_cast(shared_ptr<U> const& r);

> > Postconditions: If `w` is the return value, `w.get() == const_cast<T*>(r.get()) && w.use_count() == r.use_count()`.

Alberto Ganesh Barbati has written an alternative proposal where he suggests (among other things) that the casts be respecified in terms of the aliasing constructor as follows:

Change 20.8.2.2.9 [util.smartptr.shared.cast]:

> -2- Returns: If `r` is empty, an `empty shared_ptr<T>;` otherwise, a `shared_ptr<T>` object that stores `static_cast<T*>(r.get())` and shares ownership with `r`. `shared_ptr<T>(r, static_cast<T*>(r.get())`.

> -6- Returns:

> - When `dynamic_cast<T*>(r.get())` returns a nonzero value, a `shared_ptr<T>` object that stores a copy of it and shares ownership with `r`;
> - Otherwise, an empty `shared_ptr<T>` object.
> - If `p = dynamic_cast<T*>(r.get())` is a non-null pointer, `shared_ptr<T>(r, p);`
> - Otherwise, `shared_ptr<T>()`.

> -10- Returns: If `r` is empty, an `empty shared_ptr<T>;` otherwise, a `shared_ptr<T>` object that stores `const_cast<T*>(r.get())` and shares ownership with `r`. `shared_ptr<T>(r, const_cast<T*>(r.get())`.

This takes care of the missing postconditions for the casts by bringing in the aliasing constructor postcondition "by reference".

### 711. Contradiction in empty `shared_ptr`

Section: 20.8.2.2.5 [util.smartptr.shared.obs] Status: C++11 Submitter: Peter Dimov Opened: 2007-08-24 Last modified: 2015-04-08

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Discussion:

A discussion on comp.std.c++ has identified a contradiction in the `shared_ptr` specification. The note:

> [ Note: this constructor allows creation of an empty shared_ptr instance with a non-NULL stored pointer. -end note ]

after the aliasing constructor

>     template<class Y> shared_ptr(shared_ptr<Y> const& r, T *p);

reflects the intent of N2351 to, well, allow the creation of an empty `shared_ptr` with a non-NULL stored pointer.

This is contradicted by the second sentence in the Returns clause of 20.8.2.2.5 [util.smartptr.shared.obs]:

>     T* get() const;

> > Returns: the stored pointer. Returns a null pointer if `*this` is empty.

[ Bellevue: ]

> Adopt option 1 and move to review, not ready.

> There was a lot of confusion about what an empty `shared_ptr` is (the term isn't defined anywhere), and whether we have a good mental model for how one behaves. We think it might be possible to deduce what the definition should be, but the words just aren't there. We need to open an issue on the use of this undefined term. (The resolution of that issue might affect the resolution of issue 711.)

> The LWG is getting more uncomfortable with the aliasing proposal (N2351) now that we realize some of its implications, and we need to keep an eye on it, but there isn't support for removing this feature at this time.

[ Sophia Antipolis: ]

> We heard from Peter Dimov, who explained his reason for preferring solution 1.

> Because it doesn't seem to add anything. It simply makes the behavior for p = 0 undefined. For programmers who don't create empty pointers with p = 0, there is no difference. Those who do insist on creating them presumably have a good reason, and it costs nothing for us to define the behavior in this case.

> The aliasing constructor is sharp enough as it is, so "protecting" users doesn't make much sense in this particular case.

> > Do you have a use case for r being empty and r being non-null?

> I have received a few requests for it from "performance-conscious" people (you should be familiar with this mindset) who don't like the overhead of allocating and maintaining a control block when a null deleter is used to approximate a raw pointer. It is obviously an "at your own risk", low-level feature; essentially a raw pointer behind a shared_ptr facade.

> We could not agree upon a resolution to the issue; some of us thought that Peter's description above is supporting an undesirable behavior.

[ 2009-07 Frankfurt: ]

> We favor option 1, move to Ready.

> [ Howard: Option 2 commented out for clarity, and can be brought back. ]

Proposed resolution:

In keeping the N2351 spirit and obviously my preference, change 20.8.2.2.5 [util.smartptr.shared.obs]:

>     T* get() const;

> > Returns: the stored pointer. Returns a null pointer if `*this` is empty.

### 712. `seed_seq::size` no longer useful

Section: 26.5.7.1 [rand.util.seedseq] Status: CD1 Submitter: Marc Paterno Opened: 2007-08-25 Last modified: 2015-04-08

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Discussion:

One of the motivations for incorporating `seed_seq::size()` was to simplify the wording in other parts of 26.5 [rand]. As a side effect of resolving related issues, all such references to `seed_seq::size()` will have been excised. More importantly, the present specification is contradictory, as "The number of 32-bit units the object can deliver" is not the same as "the result of `v.size()`."

See N2391 and N2423 for some further discussion.

Proposed resolution:

Adopt the proposed resolution in N2423.

[ Kona (2007): The LWG adopted the proposed resolution of N2423 for this issue. The LWG voted to accelerate this issue to Ready status to be voted into the WP at Kona. ]

### 713. `sort()` complexity is too lax

Section: 25.4.1.1 [sort] Status: CD1 Submitter: Matt Austern Opened: 2007-08-30 Last modified: 2015-04-08

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Discussion:

The complexity of `sort()` is specified as "Approximately `N log(N)` (where `N == last - first` ) comparisons on the average", with no worst case complicity specified. The intention was to allow a median-of-three quicksort implementation, which is usually `O(N log N)` but can be quadratic for pathological inputs. However, there is no longer any reason to allow implementers the freedom to have a worst-cast-quadratic sort algorithm. Implementers who want to use quicksort can use a variant like David Musser's "Introsort" (Software Practice and Experience 27:983-993, 1997), which is guaranteed to be `O(N log N)` in the worst case without incurring additional overhead in the average case. Most C++ library implementers already do this, and there is no reason not to guarantee it in the standard.

Proposed resolution:

In 25.4.1.1 [sort], change the complexity to "O(N log N)", and remove footnote 266:

> Complexity: Approximately O(N log(N)) (where N == last - first ) comparisons on the average.266)

> 266) If the worst case behavior is important `stable_sort()` (25.3.1.2) or `partial_sort()` (25.3.1.3) should be used.

### 714. `search_n` complexity is too lax

Section: 25.2.13 [alg.search] Status: CD1 Submitter: Matt Austern Opened: 2007-08-30 Last modified: 2015-04-08

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Discussion:

The complexity for `search_n` (25.2.13 [alg.search] par 7) is specified as "At most (last - first ) * count applications of the corresponding predicate if count is positive, or 0 otherwise." This is unnecessarily pessimistic. Regardless of the value of count, there is no reason to examine any element in the range more than once.

Proposed resolution:

Change the complexity to "At most (last - first) applications of the corresponding predicate".

>     template<class ForwardIterator, class Size, class T>
>       ForwardIterator
>         search_n(ForwardIterator first , ForwardIterator last , Size count ,
>                  const T& value );

>     template<class ForwardIterator, class Size, class T,
>              class BinaryPredicate>
>       ForwardIterator
>         search_n(ForwardIterator first , ForwardIterator last , Size count ,
>                  const T& value , BinaryPredicate pred );

> > Complexity: At most `(last - first ) * count` applications of the corresponding predicate if `count` is positive, or 0 otherwise.

### 715. `minmax_element` complexity is too lax

Section: 25.4.7 [alg.min.max] Status: CD1 Submitter: Matt Austern Opened: 2007-08-30 Last modified: 2015-04-08

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Discussion:

The complexity for `minmax_element` (25.4.7 [alg.min.max] par 16) says "At most `max(2 * (last - first ) - 2, 0)` applications of the corresponding comparisons", i.e. the worst case complexity is no better than calling `min_element` and `max_element` separately. This is gratuitously inefficient. There is a well known technique that does better: see section 9.1 of CLRS (Introduction to Algorithms, by Cormen, Leiserson, Rivest, and Stein).

Proposed resolution:

Change 25.4.7 [alg.min.max] to:

>     template<class ForwardIterator>
>       pair<ForwardIterator, ForwardIterator>
>         minmax_element(ForwardIterator first , ForwardIterator last);
>     template<class ForwardIterator, class Compare>
>       pair<ForwardIterator, ForwardIterator>
>         minmax_element(ForwardIterator first , ForwardIterator last , Compare comp);

> > Returns: `make_pair(m, M)`, where `m` is `min_element(first, last)` or `min_element(first, last, comp)` the first iterator in `[first, last)` such that no iterator in the range refers to a smaller element, and where `M` is `max_element(first, last)` or `max_element(first, last, comp)` the last iterator in `[first, last)` such that no iterator in the range refers to a larger element.

> > Complexity: At most `max(2 * (last - first ) - 2, 0)` `max(⌊(3/2) (N-1)⌋, 0)` applications of the corresponding comparisons predicate, where `N` is `distance(first, last)`.

### 716. Production in [re.grammar] not actually modified

Section: 28.13 [re.grammar] Status: C++11 Submitter: Stephan T. Lavavej Opened: 2007-08-31 Last modified: 2015-04-08

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Discussion:

TR1 7.13 [tr.re.grammar]/3 and C++0x WP 28.13 [re.grammar]/3 say:

> The following productions within the ECMAScript grammar are modified as follows:

> >     CharacterClass ::
> >     [ [lookahead ∉ {^}] ClassRanges ]
> >     [ ^ ClassRanges ]

This definition for `CharacterClass` appears to be exactly identical to that in ECMA-262.

Was an actual modification intended here and accidentally omitted, or was this production accidentally included?

[ Batavia (2009-05): ]

> We agree that what is specified is identical to what ECMA-262 specifies. Pete would like to take a bit of time to assess whether we had intended, but failed, to make a change. It would also be useful to hear from John Maddock on the issue.

> Move to Open.

[ 2009-07 Frankfurt: ]

> Move to Ready.

Proposed resolution:

Remove this mention of the CharacterClass production.

>     CharacterClass ::
>     [ [lookahead ∉ {^}] ClassRanges ]
>     [ ^ ClassRanges ]

### 719. `std::is_literal` type traits should be provided

Section: 20.10 [meta] Status: Resolved Submitter: Daniel Krügler Opened: 2007-08-25 Last modified: 2015-04-08

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Duplicate of: 750

Discussion:

Since the inclusion of `constexpr` in the standard draft N2369 we have a new type category "literal", which is defined in 3.9 [basic.types]/p.11:

> -11- A type is a literal type if it is:

> - a scalar type; or

> - a class type (clause 9) with

>   - a trivial copy constructor,
>   - a trivial destructor,
>   - at least one constexpr constructor other than the copy constructor,
>   - no virtual base classes, and
>   - all non-static data members and base classes of literal types; or
> - an array of literal type.

I strongly suggest that the standard provides a type traits for literal types in 20.10.4.3 [meta.unary.prop] for several reasons:

1. To keep the traits in sync with existing types.
2. I see many reasons for programmers to use this trait in template code to provide optimized template definitions for these types, see below.
3. A user-provided definition of this trait is practically impossible to write portably.

The special problem of reason (c) is that I don't see currently a way to portably test the condition for literal class types:

> - at least one constexpr constructor other than the copy constructor,

[ Alisdair is considering preparing a paper listing a number of missing type traits, and feels that it might be useful to handle them all together rather than piecemeal. This would affect issue 719 and 750. These two issues should move to OPEN pending AM paper on type traits. ]

[ 2009-07 Frankfurt: ]

> Beman, Daniel, and Alisdair will work on a paper proposing new type traits.

[ Addressed in N2947. ]

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Solved by N2984.

Proposed resolution:

In 20.10.2 [meta.type.synop] in the group "type properties", just below the line

>     template <class T> struct is_pod;

add a new one:

>     template <class T> struct is_literal;

In 20.10.4.3 [meta.unary.prop], table Type Property Predicates, just below the line for the `is_pod` property add a new line:

Template | Condition | Preconditions
--- | --- | ---
`template <class T> struct is_literal;` | `T` is a literal type (3.9) | `T` shall be a complete type, an array of unknown bound, or (possibly cv-qualified) `void`.

### 720. Omissions in constexpr usages

Section: 23.3.2 [array], 20.6 [template.bitset] Status: CD1 Submitter: Daniel Krügler Opened: 2007-08-25 Last modified: 2015-04-08

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Discussion:

1. The member function `bool array<T,N>::empty() const` should be a `constexpr` because this is easily to proof and to implement following it's operational semantics defined by Table 87 (Container requirements) which says: `a.size() == 0`.
2. The member function `bool bitset<N>::test() const` must be a `constexpr` (otherwise it would violate the specification of `constexpr bitset<N>::operator[](size_t) const`, because it's return clause delegates to `test()`).
3. I wonder how the constructor `bitset<N>::bitset(unsigned long)` can be declared as a `constexpr`. Current implementations usually have no such `bitset` c'tor which would fulfill the requirements of a `constexpr` c'tor because they have a non-empty c'tor body that typically contains for-loops or `memcpy` to compute the initialisation. What have I overlooked here?

[ Sophia Antipolis: ]

> We handle this as two parts

> 1. The proposed resolution is correct; move to ready.
> 2. The issue points out a real problem, but the issue is larger than just this solution. We believe a paper is needed, applying the full new features of C++ (including extensible literals) to update `std::bitset`. We note that we do not consider this new work, and that is should be handled by the Library Working Group.

> In order to have a consistent working paper, Alisdair and Daniel produced a new wording for the resolution.

Proposed resolution:

1. In the class template definition of 23.3.2 [array]/p. 3 change

   >     constexpr bool empty() const;

2. In the class template definition of 20.6 [template.bitset]/p. 1 change

   >     constexpr bool test(size_t pos ) const;

   and in 20.6.2 [bitset.members] change

   >     constexpr bool test(size_t pos ) const;

### 722. Missing [c.math] functions `nanf` and `nanl`

Section: 26.8 [c.math] Status: CD1 Submitter: Daniel Krügler Opened: 2007-08-27 Last modified: 2015-04-08

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Discussion:

In the listing of 26.8 [c.math], table 108: Header `<cmath>` synopsis I miss the following C99 functions (from 7.12.11.2):

>     float nanf(const char *tagp);
>     long double nanl(const char *tagp);

(Note: These functions cannot be overloaded and they are also not listed anywhere else)

Proposed resolution:

In 26.8 [c.math], table 108, section "Functions", add `nanf` and `nanl` just after the existing entry `nan`.

### 723. `basic_regex` should be moveable

Section: 28.8 [re.regex] Status: C++11 Submitter: Daniel Krügler Opened: 2007-08-29 Last modified: 2015-04-08

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Discussion:

Addresses UK 316

According to the current state of the standard draft, the class template `basic_regex`, as described in 28.8 [re.regex]/3, is neither `MoveConstructible` nor `MoveAssignable`. IMO it should be, because typical regex state machines tend to have a rather large data quantum and I have seen several use cases, where a factory function returns regex values, which would take advantage of moveabilities.

[ Sophia Antipolis: ]

> Needs wording for the semantics, the idea is agreed upon.

[ Post Summit Daniel updated wording to reflect new "swap rules". ]

[ 2009-07 Frankfurt: ]

> Move to Ready.

Proposed resolution:

In the class definition of `basic_regex`, just below 28.8 [re.regex]/3, perform the following changes:

1. Just after `basic_regex(const basic_regex&);` insert:

   >     basic_regex(basic_regex&&);

2. Just after `basic_regex& operator=(const basic_regex&);` insert:

   >     basic_regex& operator=(basic_regex&&);

3. Just after `basic_regex& assign(const basic_regex& that);` insert:

   >     basic_regex& assign(basic_regex&& that);

4. In 28.8.2 [re.regex.construct], just after p.11 add the following new member definition:

   >     basic_regex(basic_regex&& e);

   > > Effects: Move-constructs a `basic_regex` instance from `e`.

   > > Postconditions: `flags()` and `mark_count()` return `e.flags()` and `e.mark_count()`, respectively, that `e` had before construction, leaving `e` in a valid state with an unspecified value.

   > > Throws: nothing.

5. Also in 28.8.2 [re.regex.construct], just after p.18 add the following new member definition:

   >     basic_regex& operator=(basic_regex&& e);

   > > Effects: Returns the result of `assign(std::move(e))`.

6. In 28.8.3 [re.regex.assign], just after p. 2 add the following new member definition:

   >     basic_regex& assign(basic_regex&& rhs);

   > > Effects: Move-assigns a `basic_regex` instance from `rhs` and returns `*this`.

   > > Postconditions: `flags()` and `mark_count()` return `rhs.flags()` and `rhs.mark_count()`, respectively, that `rhs` had before assignment, leaving `rhs` in a valid state with an unspecified value.

   > > Throws: nothing.

### 724. `DefaultConstructible` is not defined

Section: 17.6.3.1 [utility.arg.requirements] Status: C++11 Submitter: Pablo Halpern Opened: 2007-09-12 Last modified: 2015-04-08

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Discussion:

The `DefaultConstructible` requirement is referenced in several places in the August 2007 working draft N2369, but is not defined anywhere.

[ Bellevue: ]

> Walking into the default/value-initialization mess...

> Why two lines? Because we need both expressions to be valid.

> AJM not sure what the phrase "default constructed" means. This is unfortunate, as the phrase is already used 24 times in the library!

> Example: `const int` would not accept first line, but will accept the second.

> This is an issue that must be solved by concepts, but we might need to solve it independantly first.

> It seems that the requirements are the syntax in the proposed first column is valid, but not clear what semantics we need.

> A table where there is no post-condition seems odd, but appears to sum up our position best.

> At a minimum an object is declared and is destructible.

> Move to open, as no-one happy to produce wording on the fly.

[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]

[ 2009-08-17 Daniel adds "[defaultconstructible]" to table title. 408 depends upon this issue. ]

[ 2009-08-18 Alisdair adds: ]

> Looking at the proposed table in this issue, it really needs two rows:

> > Table 33: `DefaultConstructible` requirements [defaultconstructible]
> > expression | post-condition
> > --- | ---
> > `T t;` | `t` is default-initialized.
> > `T{}` | Object of type `T` is value-initialized.

> Note I am using the new brace-initialization syntax that is unambiguous in all use cases (no most vexing parse.)

[ 2009-10-03 Daniel adds: ]

> The suggested definition `T{}` describing it as value-initialization is wrong, because it belongs to list-initialization which would - as the current rules are - always prefer a initializer-list constructor over a default-constructor. I don't consider this as an appropriate definition of `DefaultConstructible`. My primary suggestion is to ask core, whether the special case `T{}` (which also easily leads to ambiguity situations for more than one initializer-list in a class) would always prefer a default-constructor - if any - before considering an initializer-list constructor or to provide another syntax form to prefer value-initialization over list-initialization. If that fails I would fall back to suggest to use the expression `T()` instead of `T{}` with all it's disadvantages for the meaning of the expression

> >     T t();

[ 2009-10 Santa Cruz: ]

> Leave Open. Core is looking to make Alisdair's proposed resolution correct.

[ 2010-01-24 At Alisdair's request, moved his proposal into the proposed wording section. The old wording is preserved here: ]

> In section 17.6.3.1 [utility.arg.requirements], before table 33, add the following table:

> Table 33: `DefaultConstructible` requirements [defaultconstructible]

> expression | post-condition
> `T t;` `T()` | `T` is default constructed.

[ 2010-02-04: Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Rationale:

[ San Francisco: ]

> We believe concepts will solve this problem (N2774).

[ Rationale is obsolete. ]

Proposed resolution:

In section 17.6.3.1 [utility.arg.requirements], before table 33, add the following table:

> Table 33: `DefaultConstructible` requirements [defaultconstructible]
> expression | post-condition
> --- | ---
> `T t;` | Object `t` is default-initialized.
> `T u{};` | Object `u` is value-initialized.
> `T() T{}` | A temporary object of type `T` is value-initialized.

### 727. `regex_replace()` doesn't accept `basic_string`s with custom traits and allocators

Section: 28.11.4 [re.alg.replace] Status: C++11 Submitter: Stephan T. Lavavej Opened: 2007-09-22 Last modified: 2015-04-08

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Discussion:

`regex_match()` and `regex_search()` take `const basic_string<charT, ST, SA>&`. `regex_replace()` takes `const basic_string<charT>&`. This prevents `regex_replace()` from accepting `basic_string`s with custom traits and allocators.

Overloads of `regex_replace()` taking `basic_string` should be additionally templated on `class ST, class SA` and take `const basic_string<charT, ST, SA>&`. Consistency with `regex_match()` and `regex_search()` would place `class ST, class SA` as the first template arguments; compatibility with existing code using TR1 and giving explicit template arguments to `regex_replace()` would place `class ST, class SA` as the last template arguments.

[ Batavia (2009-05): ]

> Bill comments, "We need to look at the depth of this change."

> Pete remarks that we are here dealing with a convenience function that saves a user from calling the iterato-based overload.

> Move to Open.

[ 2009-07 Frankfurt: ]

> Howard to ask Stephan Lavavej to provide wording.

[ 2009-07-17 Stephan provided wording. ]

[ 2009-07-25 Daniel tweaks both this issue and 726. ]

> One relevant part of the proposed resolution below suggests to add a new overload of the format member function in the `match_results` class template that accepts two character pointers defining the `begin` and `end` of a format range. A more general approach could have proposed a pair of iterators instead, but the used pair of char pointers reflects existing practice. If the committee strongly favors an iterator-based signature, this could be simply changed. I think that the minimum requirement should be a `BidirectionalIterator`, but current implementations take advantage (at least partially) of the `RandomAccessIterator` sub interface of the char pointers.

> Suggested Resolution:

> [Moved into the proposed resloution]

[ 2009-07-30 Stephan agrees with Daniel's wording. Howard places Daniel's wording in the Proposed Resolution. ]

[ 2009-10 Santa Cruz: ]

> Move to Review. Chair is anxious to move this to Ready in Pittsburgh.

[ 2010-01-27 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

1. Change 28.4 [re.syn] as indicated:

   >     // 28.11.4, function template regex_replace:
   >     template <class OutputIterator, class BidirectionalIterator,
   >               class traits, class charT, class ST, class SA>
   >       OutputIterator
   >       regex_replace(OutputIterator out,
   >                     BidirectionalIterator first, BidirectionalIterator last,
   >                     const basic_regex<charT, traits>& e,
   >                     const basic_string<charT, ST, SA>& fmt,
   >                     regex_constants::match_flag_type flags =
   >                       regex_constants::match_default);

   >     template <class OutputIterator, class BidirectionalIterator,
   >               class traits, class charT>
   >       OutputIterator
   >       regex_replace(OutputIterator out,
   >                     BidirectionalIterator first, BidirectionalIterator last,
   >                     const basic_regex<charT, traits>& e,
   >                     const charT* fmt,
   >                     regex_constants::match_flag_type flags =
   >                       regex_constants::match_default);

   >     template <class traits, class charT, class ST, class SA,
   >               class FST, class FSA>
   >       basic_string<charT, ST, SA>
   >       regex_replace(const basic_string<charT, ST, SA>& s,
   >                     const basic_regex<charT, traits>& e,
   >                     const basic_string<charT, FST, FSA>& fmt,
   >                     regex_constants::match_flag_type flags =
   >                       regex_constants::match_default);

   >     template <class traits, class charT, class ST, class SA>
   >       basic_string<charT, ST, SA>
   >       regex_replace(const basic_string<charT, ST, SA>& s,
   >                     const basic_regex<charT, traits>& e,
   >                     const charT* fmt,
   >                     regex_constants::match_flag_type flags =
   >                       regex_constants::match_default);

   >     template <class traits, class charT, class ST, class SA>
   >       basic_string<charT>
   >       regex_replace(const charT* s,
   >                     const basic_regex<charT, traits>& e,
   >                     const basic_string<charT, ST, SA>& fmt,
   >                     regex_constants::match_flag_type flags =
   >                       regex_constants::match_default);

   >     template <class traits, class charT>
   >       basic_string<charT>
   >       regex_replace(const charT* s,
   >                     const basic_regex<charT, traits>& e,
   >                     const charT* fmt,
   >                     regex_constants::match_flag_type flags =
   >                       regex_constants::match_default);

2. Change 28.10 [re.results]/3, class template `match_results` as indicated:

   >     template <class OutputIter>
   >       OutputIter
   >       format(OutputIter out,
   >              const char_type* fmt_first, const char_type* fmt_last,
   >              regex_constants::match_flag_type flags =
   >                regex_constants::format_default) const;

   >     template <class OutputIter, class ST, class SA>
   >       OutputIter
   >       format(OutputIter out,
   >              const string_typebasic_string<char_type, ST, SA>& fmt,
   >              regex_constants::match_flag_type flags =
   >                regex_constants::format_default) const;

   >     template <class ST, class SA>
   >       string_typebasic_string<char_type, ST, SA>
   >       format(const string_typebasic_string<char_type, ST, SA>& fmt,
   >              regex_constants::match_flag_type flags =
   >                regex_constants::format_default) const;

   >     string_type
   >     format(const char_type* fmt,
   >            regex_constants::match_flag_type flags =
   >              regex_constants::format_default) const;

3. Insert at the very beginning of 28.10.5 [re.results.form] the following:

   >     template <class OutputIter>
   >       OutputIter
   >       format(OutputIter out,
   >              const char_type* fmt_first, const char_type* fmt_last,
   >              regex_constants::match_flag_type flags =
   >                regex_constants::format_default) const;

   > > 1 Requires: The type `OutputIter` shall satisfy the requirements for an Output Iterator (24.2.4 [output.iterators]).

   > > 2 Effects: Copies the character sequence `[fmt_first,fmt_last)` to `OutputIter out`. Replaces each format specifier or escape sequence in the copied range with either the character(s) it represents or the sequence of characters within `*this` to which it refers. The bitmasks specified in `flags` determine which format specifiers and escape sequences are recognized.

   > > 3 Returns: `out`.

4. Change 28.10.5 [re.results.form], before p. 1 until p. 3 as indicated:

   >     template <class OutputIter, class ST, class SA>
   >       OutputIter
   >       format(OutputIter out,
   >              const string_typebasic_string<char_type, ST, SA>& fmt,
   >              regex_constants::match_flag_type flags =
   >                regex_constants::format_default) const;

   > > 1 Requires: The type `OutputIter` shall satisfy the requirements for an Output Iterator (24.2.3).

   > > 2 Effects: Copies the character sequence `[fmt.begin(),fmt.end())` to `OutputIter out`. Replaces each format specifier or escape sequence in `fmt` with either the character(s) it represents or the sequence of characters within `*this` to which it refers. The bitmasks specified in `flags` determines what format specifiers and escape sequences are recognized Equivalent to `return format(out, fmt.data(), fmt.data() + fmt.size(), flags)`.

   > > 3 Returns: `out`.

5. Change 28.10.5 [re.results.form], before p. 4 until p. 4 as indicated:

   >     template <class ST, class SA>
   >       string_typebasic_string<char_type, ST, SA>
   >       format(const string_typebasic_string<char_type, ST, SA>& fmt,
   >              regex_constants::match_flag_type flags =
   >                regex_constants::format_default) const;

   > > Effects: Returns a copy of the string `fmt`. Replaces each format specifier or escape sequence in `fmt` with either the character(s) it represents or the sequence of characters within `*this` to which it refers. The bitmasks specified in flags determines what format specifiers and escape sequences are recognized. Constructs an empty string `result` of type `basic_string<char_type, ST, SA>`, and calls `format(back_inserter(result), fmt, flags)`.

   > > Returns: `result`

6. At the end of 28.10.5 [re.results.form] insert as indicated:

   >     string_type
   >       format(const char_type* fmt,
   >              regex_constants::match_flag_type flags =
   >                regex_constants::format_default) const;

   > > Effects: Constructs an empty string `result` of type `string_type`, and calls `format(back_inserter(result), fmt, fmt + char_traits<char_type>::length(fmt), flags)`.

   > > Returns: `result`

7. Change 28.11.4 [re.alg.replace] before p. 1 as indicated:

   >     template <class OutputIterator, class BidirectionalIterator,
   >               class traits, class charT, class ST, class SA>
   >       OutputIterator
   >       regex_replace(OutputIterator out,
   >                     BidirectionalIterator first, BidirectionalIterator last,
   >                     const basic_regex<charT, traits>& e,
   >                     const basic_string<charT, ST, SA>& fmt,
   >                     regex_constants::match_flag_type flags =
   >                       regex_constants::match_default);

   >     template <class OutputIterator, class BidirectionalIterator,
   >               class traits, class charT>
   >       OutputIterator
   >       regex_replace(OutputIterator out,
   >                     BidirectionalIterator first, BidirectionalIterator last,
   >                     const basic_regex<charT, traits>& e,
   >                     const charT* fmt,
   >                     regex_constants::match_flag_type flags =
   >                       regex_constants::match_default);

   > > Effects: [..]. If any matches are found then, for each such match, if `!(flags & regex_constants::format_no_copy)` calls `std::copy(m.prefix().first, m.prefix().second, out)`, and then calls `m.format(out, fmt, flags)` for the first form of the function and `m.format(out, fmt, fmt + char_traits<charT>::length(fmt), flags)` for the second form. [..].

8. Change 28.11.4 [re.alg.replace] before p. 3 as indicated:

   >     template <class traits, class charT, class ST, class SA,
   >               class FST, class FSA>
   >       basic_string<charT, ST, SA>
   >       regex_replace(const basic_string<charT, ST, SA>& s,
   >                     const basic_regex<charT, traits>& e,
   >                     const basic_string<charT, FST, FSA>& fmt,
   >                     regex_constants::match_flag_type flags =
   >                       regex_constants::match_default);

   >     template <class traits, class charT, class ST, class SA>
   >       basic_string<charT, ST, SA>
   >       regex_replace(const basic_string<charT, ST, SA>& s,
   >                     const basic_regex<charT, traits>& e,
   >                     const charT* fmt,
   >                     regex_constants::match_flag_type flags =
   >                       regex_constants::match_default);

   > > Effects: Constructs an empty string `result` of type `basic_string<charT, ST, SA>`, calls `regex_replace(back_inserter(result), s.begin(), s.end(), e, fmt, flags)`, and then returns `result`.

9. At the end of 28.11.4 [re.alg.replace] add the following new prototype description:

   >     template <class traits, class charT, class ST, class SA>
   >       basic_string<charT>
   >       regex_replace(const charT* s,
   >                     const basic_regex<charT, traits>& e,
   >                     const basic_string<charT, ST, SA>& fmt,
   >                     regex_constants::match_flag_type flags =
   >                       regex_constants::match_default);

   >     template <class traits, class charT>
   >       basic_string<charT>
   >       regex_replace(const charT* s,
   >                     const basic_regex<charT, traits>& e,
   >                     const charT* fmt,
   >                     regex_constants::match_flag_type flags =
   >                       regex_constants::match_default);

   > > Effects: Constructs an empty string `result` of type `basic_string<charT>`, calls `regex_replace(back_inserter(result), s, s + char_traits<charT>::length(s), e, fmt, flags)`, and then returns `result`.

### 728. Problem in [rand.eng.mers]/6

Section: 26.5.3.2 [rand.eng.mers] Status: CD1 Submitter: Stephan Tolksdorf Opened: 2007-09-21 Last modified: 2015-04-08

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Discussion:

The `mersenne_twister_engine` is required to use a seeding method that is given as an algorithm parameterized over the number of bits `W`. I doubt whether the given generalization of an algorithm that was originally developed only for unsigned 32-bit integers is appropriate for other bit widths. For instance, `W` could be theoretically 16 and `UIntType` a 16-bit integer, in which case the given multiplier would not fit into the `UIntType`. Moreover, T. Nishimura and M. Matsumoto have chosen a dif ferent multiplier for their 64 bit Mersenne Twister [reference].

I see two possible resolutions:

1. Restrict the parameter `W` of the `mersenne_twister_template` to values of 32 or 64 and use the multiplier from [the above reference] for the 64-bit case (my preference)
2. Interpret the state array for any `W` as a 32-bit array of appropriate length (and a specified byte order) and always employ the 32-bit algorithm for seeding

See N2424 for further discussion.

[ Bellevue: ]

> Stephan Tolksdorf has additional comments on N2424. He comments: "there is a typo in the required behaviour for mt19937_64: It should be the 10000th (not 100000th) invocation whose value is given, and the value should be 9981545732273789042 (not 14002232017267485025)." These values need checking.

> Take the proposed recommendation in N2424 and move to REVIEW.

Proposed resolution:

See N2424 for the proposed resolution.

[ Stephan Tolksdorf adds pre-Bellevue: ]

> I support the proposed resolution in N2424, but there is a typo in the required behaviour for `mt19937_64`: It should be the 10000th (not 100000th) invocation whose value is given, and the value should be 9981545732273789042 (not 14002232017267485025). The change to para. 8 proposed by Charles Karney should also be included in the proposed wording.

[ Sophia Antipolis: ]

> Note the main part of the issue is resolved by N2424.

### 732. Defect in [rand.dist.samp.genpdf]

Section: X [rand.dist.samp.genpdf] Status: Resolved Submitter: Stephan Tolksdorf Opened: 2007-09-21 Last modified: 2015-04-08

View all other issues in [rand.dist.samp.genpdf].

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Duplicate of: 795

Discussion:

X [rand.dist.samp.genpdf] describes the interface for a distribution template that is meant to simulate random numbers from any general distribution given only the density and the support of the distribution. I'm not aware of any general purpose algorithm that would be capable of correctly and efficiently implementing the described functionality. From what I know, this is essentially an unsolved research problem. Existing algorithms either require more knowledge about the distribution and the problem domain or work only under very limited circumstances. Even the state of the art special purpose library UNU.RAN does not solve the problem in full generality, and in any case, testing and customer support for such a library feature would be a nightmare.

Possible resolution: For these reasons, I propose to delete section X [rand.dist.samp.genpdf].

[ Bellevue: ]

> Disagreement persists.

> Objection to this issue is that this function takes a general functor. The general approach would be to normalize this function, integrate it, and take the inverse of the integral, which is not possible in general. An example function is sin(1+n*x) — for any spatial frequency that the implementor chooses, there is a value of n that renders that choice arbitrarily erroneous.

> Correction: The formula above should instead read 1+sin(n*x).

> Objector proposes the following possible compromise positions:

> - rand.dist.samp.genpdf takes an number of points so that implementor need not guess.
> - replace rand.disk.samp.genpdf with an extension to either or both of the discrete functions to take arguments that take a functor and number of points in place of the list of probabilities. Reference issues 793 and 794.

Proposed resolution:

See N2813 for the proposed resolution.

Rationale:

Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".

### 734. Unnecessary restriction in [rand.dist.norm.chisq]

Section: 26.5.8.5.3 [rand.dist.norm.chisq] Status: CD1 Submitter: Stephan Tolksdorf Opened: 2007-09-21 Last modified: 2015-04-08

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Discussion:

`chi_squared_distribution`, `fisher_f_distribution` and `student_t_distribution` have parameters for the "degrees of freedom" `n` and `m` that are specified as integers. For the following two reasons this is an unnecessary restriction: First, in many applications such as Bayesian inference or Monte Carlo simulations it is more convenient to treat the respective param- eters as continuous variables. Second, the standard non-naive algorithms (i.e. O(1) algorithms) for simulating from these distributions work with floating-point parameters anyway (all three distributions could be easily implemented using the Gamma distribution, for instance).

Similar arguments could in principle be made for the parameters `t` and `k` of the discrete `binomial_distribution` and `negative_binomial_distribution`, though in both cases continuous parameters are less frequently used in practice and in case of the `binomial_distribution` the implementation would be significantly complicated by a non-discrete parameter (in most implementations one would need an approximation of the log-gamma function instead of just the log-factorial function).

Possible resolution: For these reasons, I propose to change the type of the respective parameters to double.

[ Bellevue: ]

> In N2424. Not wildly enthusiastic, not really felt necessary. Less frequently used in practice. Not terribly bad either. Move to OPEN.

[ Sophia Antipolis: ]

> Marc Paterno: The generalizations were explicitly left out when designing the facility. It's harder to test.

> Marc Paterno: Ask implementers whether floating-point is a significant burden.

> Alisdair: It's neater to do it now, do ask Bill Plauger.

> Disposition: move to review with the option for "NAD" if it's not straightforward to implement; unanimous consent.

Proposed resolution:

See N2424 for the proposed resolution.

[ Stephan Tolksdorf adds pre-Bellevue: ]

> In 26.5.8.5.3 [rand.dist.norm.chisq]:

> > Delete ", where `n` is a positive integer" in the first paragraph.

> > Replace both occurrences of "`explicit chi_squared_distribution(int n = 1);`" with "`explicit chi_squared_distribution(RealType n = 1);`".

> > Replace both occurrences of "`int n() const;`" with "`RealType n() const;`".

> In 26.5.8.5.5 [rand.dist.norm.f]:

> > Delete ", where `m` and `n` are positive integers" in the first paragraph.

> > Replace both occurrences of

> > >     explicit fisher_f_distribution(int m = 1, int n = 1);

> > with

> > >     explicit fisher_f_distribution(RealType m = 1, RealType n = 1);

> > Replace both occurrences of "`int m() const;" with "RealType m() const;`".

> > Replace both occurrences of "`int n() const;" with "RealType n() const;`".

> In 26.5.8.5.6 [rand.dist.norm.t]:

> > Delete ", where `n` is a positive integer" in the first paragraph.

> > Replace both occurrences of "`explicit student_t_distribution(int n = 1);`" with "`explicit student_t_distribution(RealType n = 1);`".

> > Replace both occurrences of "`int n() const;`" with "`RealType n() const;`".

### 740. Please remove `*_ptr<T[N]>`

Section: 20.8.1 [unique.ptr] Status: CD1 Submitter: Herb Sutter Opened: 2007-10-04 Last modified: 2015-04-08

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Discussion:

Please don't provide `*_ptr<T[N]>`. It doesn't enable any useful bounds-checking (e.g., you could imagine that doing `op++` on a `shared_ptr<T[N]>` yields a `shared_ptr<T[N-1]>`, but that promising path immediately falters on `op--` which can't reliably dereference because we don't know the lower bound). Also, most buffers you'd want to point to don't have a compile-time known size.

To enable any bounds-checking would require run-time information, with the usual triplet: base (lower bound), current offset, and max offset (upper bound). And I can sympathize with the point of view that you wouldn't want to require this on `*_ptr` itself. But please let's not follow the `<T[N]>` path, especially not with additional functions to query the bounds etc., because this sets wrong user expectations by embarking on a path that doesn't go all the way to bounds checking as it seems to imply.

If bounds checking is desired, consider a `checked_*_ptr` instead (e.g., `checked_shared_ptr`). And make the interfaces otherwise identical so that user code could easily `#define/typedef` between prepending `checked_` on debug builds and not doing so on release builds (for example).

Note that some may object that `checked_*_ptr` may seem to make the smart pointer more like `vector`, and we don't want two ways to spell `vector`. I don't agree, but if that were true that would be another reason to remove `*_ptr<T[N]>` which equally makes the smart pointer more like `std::array.` :-)

[ Bellevue: ]

> Suggestion that fixed-size array instantiations are going to fail at compile time anyway (if we remove specialization) due to pointer decay, at least that appears to be result from available compilers.

> So concerns about about requiring static_assert seem unfounded.

> After a little more experimentation with compiler, it appears that fixed size arrays would only work at all if we supply these explicit specialization. So removing them appears less breaking than originally thought.

> straw poll unanimous move to Ready.

Proposed resolution:

Change the synopsis under 20.8.1 [unique.ptr] p2:

>     ...
>     template<class T> struct default_delete;
>     template<class T> struct default_delete<T[]>;
>     template<class T, size_t N> struct default_delete<T[N]>;

>     template<class T, class D = default_delete<T>> class unique_ptr;
>     template<class T, class D> class unique_ptr<T[], D>;
>     template<class T, class D, size_t N> class unique_ptr<T[N], D>;
>     ...

Remove the entire section [unique.ptr.dltr.dflt2] `default_delete<T[N]>`.

Remove the entire section [unique.ptr.compiletime] `unique_ptr` for array objects with a compile time length and its subsections: [unique.ptr.compiletime.dtor], [unique.ptr.compiletime.observers], [unique.ptr.compiletime.modifiers].

### 742. Enabling `swap` for proxy iterators

Section: 17.6.3.1 [utility.arg.requirements] Status: Resolved Submitter: Howard Hinnant Opened: 2007-10-10 Last modified: 2015-04-08

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Discussion:

This issue was split from 672. 672 now just deals with changing the requirements of `T` in the `Swappable` requirement from `CopyConstructible` and `CopyAssignable` to `MoveConstructible` and `MoveAssignable`.

This issue seeks to widen the `Swappable` requirement to support proxy iterators. Here is example code:

>     namespace Mine {

>     template <class T>
>     struct proxy {...};

>     template <class T>
>     struct proxied_iterator
>     {
>        typedef T value_type;
>        typedef proxy<T> reference;
>        reference operator*() const;
>        ...
>     };

>     struct A
>     {
>        // heavy type, has an optimized swap, maybe isn't even copyable or movable, just swappable
>        void swap(A&);
>        ...
>     };

>     void swap(A&, A&);
>     void swap(proxy<A>, A&);
>     void swap(A&, proxy<A>);
>     void swap(proxy<A>, proxy<A>);

>     }  // Mine

>     ...

>     Mine::proxied_iterator<Mine::A> i(...)
>     Mine::A a;
>     swap(*i1, a);

The key point to note in the above code is that in the call to `swap`, `*i1` and `a` are different types (currently types can only be `Swappable` with the same type). A secondary point is that to support proxies, one must be able to pass rvalues to `swap`. But note that I am not stating that the general purpose `std::swap` should accept rvalues! Only that overloaded `swap`s, as in the example above, be allowed to take rvalues.

That is, no standard library code needs to change. We simply need to have a more flexible definition of `Swappable`.

[ Bellevue: ]

> While we believe Concepts work will define a swappable concept, we should still resolve this issue if possible to give guidance to the Concepts work.

> Would an ambiguous swap function in two namespaces found by ADL break this wording? Suggest that the phrase "valid expression" means such a pair of types would still not be swappable.

> Motivation is proxy-iterators, but facility is considerably more general. Are we happy going so far?

> We think this wording is probably correct and probably an improvement on what's there in the WP. On the other hand, what's already there in the WP is awfully complicated. Why do we need the two bullet points? They're too implementation-centric. They don't add anything to the semantics of what swap() means, which is there in the post-condition. What's wrong with saying that types are swappable if you can call swap() and it satisfies the semantics of swapping?

[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]

[ 2009-10 Santa Cruz: ]

> Leave as Open. Dave to provide wording.

[ 2009-11-08 Howard adds: ]

> Updated wording to sync with N3000. Also this issue is very closely related to 594.

[ 2010 Pittsburgh: ]

> Moved to NAD Editorial Resolved. Rationale added.

Rationale:

Solved by N3048.

Proposed resolution:

Change 17.6.3.1 [utility.arg.requirements]:

> -1- The template definitions in the C++ Standard Library refer to various named requirements whose details are set out in tables 31-38. In these tables, `T` and `V` are is a types to be supplied by a C++ program instantiating a template; `a`, `b`, and `c` are values of type `const T`; `s` and `t` are modifiable lvalues of type `T`; `u` is a value of type (possibly `const`) `T`; and `rv` is a non-`const` rvalue of type `T`; `w` is a value of type `T`; and `v` is a value of type `V`.

> Table 37: `Swappable` requirements [swappable]
> expression | Return type | Post-condition
> --- | --- | ---
> `swap(sw,tv)` | `void` | `t` `w` has the value originally held by `u` `v`, and `u` `v` has the value originally held by `t` `w`

> The `Swappable` requirement is met by satisfying one or more of the following conditions:
> - `T` is `Swappable` if `T` and `V` are the same type and `T` satisfies the `MoveConstructible` requirements (Table 33) and the `MoveAssignable` requirements (Table 35);
> - `T` is `Swappable` with `V` if a namespace scope function named `swap` exists in the same namespace as the definition of `T` or `V`, such that the expression `swap(sw,t v)` is valid and has the semantics described in this table.
> - `T` is `Swappable` if `T` is an array type whose element type is `Swappable`.

Rationale:

[ post San Francisco: ]

> Solved by N2758.

### 743. rvalue `swap` for `shared_ptr`

Section: 20.8.2.2.8 [util.smartptr.shared.spec] Status: CD1 Submitter: Howard Hinnant Opened: 2007-10-10 Last modified: 2015-04-08

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Discussion:

When the LWG looked at 674 in Kona the following note was made:

> We may need to open an issue to deal with the question of whether `shared_ptr` needs an rvalue `swap`.

This issue was opened in response to that note.

I believe allowing rvalue `shared_ptr`s to `swap` is both appropriate, and consistent with how other library components are currently specified.

[ Bellevue: ]

> Concern that the three signatures for swap is needlessly complicated, but this issue merely brings shared_ptr into equal complexity with the rest of the library. Will open a new issue for concern about triplicate signatures.

> Adopt issue as written.

Proposed resolution:

Change the synopsis in 20.8.2.2 [util.smartptr.shared]:

>     void swap(shared_ptr&& r);
>     ...
>     template<class T> void swap(shared_ptr<T>& a, shared_ptr<T>& b);
>     template<class T> void swap(shared_ptr<T>&& a, shared_ptr<T>& b);
>     template<class T> void swap(shared_ptr<T>& a, shared_ptr<T>&& b);

Change 20.8.2.2.4 [util.smartptr.shared.mod]:

>     void swap(shared_ptr&& r);

Change 20.8.2.2.8 [util.smartptr.shared.spec]:

>     template<class T> void swap(shared_ptr<T>& a, shared_ptr<T>& b);
>     template<class T> void swap(shared_ptr<T>&& a, shared_ptr<T>& b);
>     template<class T> void swap(shared_ptr<T>& a, shared_ptr<T>&& b);

### 744. What is the lifetime of an exception pointed to by an `exception_ptr`?

Section: 18.8.5 [propagation] Status: CD1 Submitter: Alisdair Meredith Opened: 2007-10-10 Last modified: 2015-04-08

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Discussion:

Without some lifetime guarantee, it is hard to know how this type can be used. Very specifically, I don't see how the current wording would guarantee and `exception_ptr` caught at the end of one thread could be safely stored and rethrown in another thread - the original motivation for this API.

(Peter Dimov agreed it should be clearer, maybe a non-normative note to explain?)

[ Bellevue: ]

> Agree the issue is real.

> Intent is lifetime is similar to a shared_ptr (and we might even want to consider explicitly saying that it is a shared_ptr< unspecified type >).

> We expect that most implementations will use shared_ptr, and the standard should be clear that the exception_ptr type is intended to be something whose semantics are smart-pointer-like so that the user does not need to worry about lifetime management. We still need someone to draught those words - suggest emailing Peter Dimov.

> Move to Open.

Proposed resolution:

Change 18.8.5 [propagation]/7:

> -7- Returns: An `exception_ptr` object that refers to the currently handled exception or a copy of the currently handled exception, or a null `exception_ptr` object if no exception is being handled. The referenced object remains valid at least as long as there is an `exception_ptr` that refers to it. If the function needs to allocate memory and the attempt fails, it returns an `exception_ptr` object that refers to an instance of `bad_alloc`. It is unspecified whether the return values of two successive calls to `current_exception` refer to the same exception object. [Note: that is, it is unspecified whether `current_exception` creates a new copy each time it is called. --end note]

### 746. `current_exception` may fail with `bad_alloc`

Section: 18.8.5 [propagation] Status: CD1 Submitter: Alisdair Meredith Opened: 2007-10-10 Last modified: 2015-04-08

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Discussion:

I understand that the attempt to copy an exception may run out of memory, but I believe this is the only part of the standard that mandates failure with specifically `bad_alloc`, as opposed to allowing an implementation-defined type derived from `bad_alloc`. For instance, the Core language for a failed new expression is:

> Any other allocation function that fails to allocate storage shall indicate failure only by throwing an exception of a type that would match a handler (15.3) of type `std::bad_alloc` (18.5.2.1).

I think we should allow similar freedom here (or add a blanket compatible-exception freedom paragraph in 17)

I prefer the clause 17 approach myself, and maybe clean up any outstanding wording that could also rely on it.

Although filed against a specific case, this issue is a problem throughout the library.

[ Bellevue: ]

> Is issue bigger than library?

> No - Core are already very clear about their wording, which is inspiration for the issue.

> While not sold on the original 18.7.5 use case, the generalised 17.4.4.8 wording is the real issue.

> Accept the broad view and move to ready

Proposed resolution:

Add the following exemption clause to 17.6.5.12 [res.on.exception.handling]:

> A function may throw a type not listed in its Throws clause so long as it is derived from a class named in the Throws clause, and would be caught by an exception handler for the base type.

### 749. Currently `has_nothrow_copy_constructor<T>::value` is true if `T` has 'a' nothrow copy constructor.

Section: 20.10.4.3 [meta.unary.prop] Status: CD1 Submitter: Alisdair Meredith Opened: 2007-10-10 Last modified: 2015-04-08

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Discussion:

Unfortunately a class can have multiple copy constructors, and I believe to be useful this trait should only return true is ALL copy constructors are no-throw.

For instance:

>     struct awkward {
>      awkward( const awkward & ) throw() {}
>      awkward( awkward & ) { throw "oops"; } };

Proposed resolution:

Change 20.10.4.3 [meta.unary.prop]:

>     has_trivial_copy_constructor

> > `T` is a trivial type (3.9) or a reference type or a class type with a trivial copy constructor where all copy constructors are trivial (12.8).

>     has_trivial_assign

> > `T` is neither `const` nor a reference type, and `T` is a trivial type (3.9) or a class type with a trivial copy assignment operator where all copy assignment operators are trivial (12.8).

>     has_nothrow_copy_constructor

> > `has_trivial_copy_constructor<T>::value` is `true` or `T` is a class type with a where all copy constructors that is are known not to throw any exceptions or `T` is an array of such a class type

>     has_nothrow_assign

> > `T` is neither `const` nor a reference type, and `has_trivial_assign<T>::value` is `true` or `T` is a class type with a where all copy assignment operators take ing an lvalue of type `T` that is known not to throw any exceptions or `T` is an array of such a class type.

### 752. Allocator complexity requirement

Section: 17.6.3.5 [allocator.requirements] Status: C++11 Submitter: Hans Boehm Opened: 2007-10-11 Last modified: 2015-04-08

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Discussion:

Did LWG recently discuss 17.6.3.5 [allocator.requirements]-2, which states that "All the operations on the allocators are expected to be amortized constant time."?

As I think I pointed out earlier, this is currently fiction for `allocate()` if it has to obtain memory from the OS, and it's unclear to me how to interpret this for `construct()` and `destroy()` if they deal with large objects. Would it be controversial to officially let these take time linear in the size of the object, as they already do in real life?

`Allocate()` more blatantly takes time proportional to the size of the object if you mix in GC. But it's not really a new problem, and I think we'd be confusing things by leaving the bogus requirements there. The current requirement on `allocate()` is generally not important anyway, since it takes O(size) to construct objects in the resulting space. There are real performance issues here, but they're all concerned with the constants, not the asymptotic complexity.

Proposed resolution:

Change 17.6.3.5 [allocator.requirements]/2:

> -2- Table 39 describes the requirements on types manipulated through allocators. All the operations on the allocators are expected to be amortized constant time. Table 40 describes the requirements on allocator types.

### 753. Move constructor in draft

Section: 17.6.3.1 [utility.arg.requirements] Status: C++11 Submitter: Yechezkel Mett Opened: 2007-10-14 Last modified: 2015-04-08

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Discussion:

The draft standard n2369 uses the term move constructor in a few places, but doesn't seem to define it.

`MoveConstructible` requirements are defined in Table 33 in 17.6.3.1 [utility.arg.requirements] as follows:

> `MoveConstructible` requirements
> expression | post-condition
> --- | ---
> `T t = rv` | `t` is equivalent to the value of `rv` before the construction
> [Note: There is no requirement on the value of `rv` after the construction. -- end note]

(where `rv` is a non-const rvalue of type `T`).

So I assume the move constructor is the constructor that would be used in filling the above requirement.

For `vector::reserve`, `vector::resize` and the `vector` modifiers given in 23.3.6.5 [vector.modifiers] we have

> Requires: If `value_type` has a move constructor, that constructor shall not throw any exceptions.

Firstly "If `value_type` has a move constructor" is superfluous; every type which can be put into a `vector` has a move constructor (a copy constructor is also a move constructor). Secondly it means that for any `value_type` which has a throwing copy constructor and no other move constructor these functions cannot be used -- which I think will come as a shock to people who have been using such types in `vector` until now!

I can see two ways to correct this. The simpler, which is presumably what was intended, is to say "If `value_type` has a move constructor and no copy constructor, the move constructor shall not throw any exceptions" or "If `value_type` has a move constructor which changes the value of its parameter,".

The other alternative is add to `MoveConstructible` the requirement that the expression does not throw. This would mean that not every type that satisfies the `CopyConstructible` requirements also satisfies the `MoveConstructible` requirements. It would mean changing requirements in various places in the draft to allow either `MoveConstructible` or `CopyConstructible`, but I think the result would be clearer and possibly more concise too.

Proposed resolution:

Add new defintions to 17.3 [definitions]:

> move constructor

> a constructor which accepts only rvalue arguments of that type, and modifies the rvalue as a side effect during the construction.

> move assignment operator

> an assignment operator which accepts only rvalue arguments of that type, and modifies the rvalue as a side effect during the assignment.

> move assignment

> use of the move assignment operator.

[ Howard adds post-Bellevue: ]

> Unfortunately I believe the wording recommended by the LWG in Bellevue is incorrect. `reserve` et. al. will use a move constructor if one is available, else it will use a copy constructor. A type may have both. If the move constructor is used, it must not throw. If the copy constructor is used, it can throw. The sentence in the proposed wording is correct without the recommended insertion. The Bellevue LWG recommended moving this issue to Ready. I am unfortunately pulling it back to Open. But I'm drafting wording to atone for this egregious action. :-)

### 755. `std::vector` and `std:string` lack explicit shrink-to-fit operations

Section: 23.3.6.3 [vector.capacity], 21.4.4 [string.capacity] Status: CD1 Submitter: Beman Dawes Opened: 2007-10-31 Last modified: 2015-04-08

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Discussion:

A `std::vector` can be shrunk-to-fit via the swap idiom:

>     vector<int> v;
>     ...
>     v.swap(vector<int>(v));  // shrink to fit

> > or:

>     vector<int>(v).swap(v);  // shrink to fit

> > or:

>     swap(v, vector<int>(v));  // shrink to fit

A non-binding request for shrink-to-fit can be made to a `std::string` via:

>     string s;
>     ...
>     s.reserve(0);

Neither of these is at all obvious to beginners, and even some experienced C++ programmers are not aware that shrink-to-fit is trivially available.

Lack of explicit functions to perform these commonly requested operations makes vector and string less usable for non-experts. Because the idioms are somewhat obscure, code readability is impaired. It is also unfortunate that two similar vector-like containers use different syntax for the same operation.

The proposed resolution addresses these concerns. The proposed function takes no arguments to keep the solution simple and focused.

Proposed resolution:

To Class template basic_string 21.4 [basic.string] synopsis, Class template vector 23.3.6 [vector] synopsis, and Class vector<bool> 23.3.7 [vector.bool] synopsis, add:

>     void shrink_to_fit();

To basic_string capacity 21.4.4 [string.capacity] and vector capacity 23.3.6.3 [vector.capacity], add:

>     void shrink_to_fit();

> > Remarks: `shrink_to_fit` is a non-binding request to reduce `capacity()` to `size()`. [Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note]

[ 850 has been added to deal with this issue with respect to `deque`. ]

### 756. Container adaptors push

Section: 23.6 [container.adaptors] Status: Resolved Submitter: Paolo Carlini Opened: 2007-10-31 Last modified: 2015-04-08

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Discussion:

After n2369 we have a single `push_back` overload in the sequence containers, of the "emplace" type. At variance with that, still in n2461, we have two separate overloads, the C++03 one + one taking an rvalue reference in the container adaptors. Therefore, simply from a consistency point of view, I was wondering whether the container adaptors should be aligned with the specifications of the sequence container themselves: thus have a single `push` along the lines:

>     template<typename... _Args>
>     void
>     push(_Args&&... __args)
>       { c.push_back(std::forward<_Args>(__args)...); }

[ Related to 767 ]

Proposed resolution:

Change 23.6.3.1 [queue.defn]:

>     void push(const value_type& x) { c.push_back(x); }
>     void push(value_type&& x) { c.push_back(std::move(x)); }
>     template<class... Args> void push(Args&&... args) { c.push_back(std::forward<Args>(args)...); }

Change 23.6.4 [priority.queue]:

>     void push(const value_type& x) { c.push_back(x); }
>     void push(value_type&& x) { c.push_back(std::move(x)); }
>     template<class... Args> void push(Args&&... args) { c.push_back(std::forward<Args>(args)...); }

Change 23.6.4.3 [priqueue.members]:

>     void push(const value_type& x);

> > Effects:

> > >     c.push_back(x);
> > >     push_heap(c.begin(), c.end(), comp);

>     template<class... Args> void push(value_type Args&&... x args);

> > Effects:

> > >     c.push_back(std::moveforward<Args>(x args)...);
> > >     push_heap(c.begin(), c.end(), comp);

Change 23.6.5.2 [stack.defn]:

>     void push(const value_type& x) { c.push_back(x); }
>     void push(value_type&& x) { c.push_back(std::move(x)); }
>     template<class... Args> void push(Args&&... args) { c.push_back(std::forward<Args>(args)...); }

Rationale:

Addressed by N2680 Proposed Wording for Placement Insert (Revision 1).

### 758. `shared_ptr` and `nullptr`

Section: 20.8.2.2 [util.smartptr.shared] Status: C++11 Submitter: Joe Gottman Opened: 2007-10-31 Last modified: 2015-04-08

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Discussion:

Consider the following program:

>     int main() {
>        shared_ptr<int> p(nullptr);
>        return 0;
>     }

This program will fail to compile because `shared_ptr` uses the following template constructor to construct itself from pointers:

>     template <class Y> shared_ptr(Y *);

According to N2431, the conversion from `nullptr_t` to `Y *` is not deducible, so the above constructor will not be found. There are similar problems with the constructors that take a pointer and a `deleter` or a pointer, a `deleter` and an allocator, as well as the corresponding forms of `reset()`. Note that N2435 will solve this problem for constructing from just `nullptr`, but not for constructors that use `deleters` or allocators or for `reset()`.

In the case of the functions that take deleters, there is the additional question of what argument should be passed to the deleter when it is eventually called. There are two reasonable possibilities: `nullptr` or `static_cast<T *>(0)`, where `T` is the template argument of the `shared_ptr`. It is not immediately clear which of these is better. If `D::operator()` is a template function similar to `shared_ptr`'s constructor, then `d(static_cast<T*>(0))` will compile and `d(nullptr)` will not. On the other hand, if `D::operator()()` takes a parameter that is a pointer to some type other that `T` (for instance `U*` where `U` derives from `T`) then `d(nullptr)` will compile and `d(static_cast<T *>(0))` may not.

[ Bellevue: ]

> The general idea is right, we need to be able to pass a nullptr to a shared_ptr, but there are a few borderline editorial issues here. (For example, the single-argument nullptr_t constructor in the class synopsis isn't marked explicit, but it is marked explicit in the proposed wording for 20.6.6.2.1. There is a missing empty parenthesis in the form that takes a nullptr_t, a deleter, and an allocator.)

> More seriously: this issue says that a shared_ptr constructed from a nullptr is empty. Since "empty" is undefined, it's hard to know whether that's right. This issue is pending on handling that term better.

> Peter suggests definition of empty should be "does not own anything"

> Is there an editorial issue that post-conditions should refer to get() = nullptr, rather than get() = 0?

> No strong feeling towards accept or NAD, but prefer to make a decision than leave it open.

> Seems there are no technical merits between NAD and Ready, comes down to "Do we intentially want to allow/disallow null pointers with these functions". Staw Poll - support null pointers 5 - No null pointers 0

> Move to Ready, modulo editorial comments

[ post Bellevue Peter adds: ]

> The following wording changes are less intrusive:

> In 20.8.2.2.1 [util.smartptr.shared.const], add:

> >     shared_ptr(nullptr_t);

> after:

> >     shared_ptr();

> (Absence of explicit intentional.)

> `px.reset( nullptr )` seems a somewhat contrived way to write `px.reset()`, so I'm not convinced of its utility.

> It's similarly not clear to me whether the deleter constructors need to be extended to take `nullptr`, but if they need to:

> Add

> >     template<class D> shared_ptr(nullptr_t p, D d);
> >     template<class D, class A> shared_ptr(nullptr_t p, D d, A a);

> after

> >     template<class Y, class D> shared_ptr(Y* p, D d);
> >     template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);

> Note that this changes the semantics of the new constructors such that they consistently call `d(p)` instead of `d((T*)0)` when `p` is `nullptr`.

> The ability to be able to pass `0/NULL` to a function that takes a `shared_ptr` has repeatedly been requested by users, but the other additions that the proposed resolution makes are not supported by real world demand or motivating examples.

> It might be useful to split the obvious and non-controversial `nullptr_t` constructor into a separate issue. Waiting for "empty" to be clarified is unnecessary; this is effectively an alias for the default constructor.

[ Sophia Antipolis: ]

> We want to remove the reset functions from the proposed resolution.

> The remaining proposed resolution text (addressing the constructors) are wanted.

> Disposition: move to review. The review should check the wording in the then-current working draft.

Proposed resolution:

In 20.8.2.2 [util.smartptr.shared] p4, add to the definition/synopsis of `shared_ptr`:

>     template<class D> shared_ptr(nullptr_t p, D d);
>     template<class D, class A> shared_ptr(nullptr_t p, D d, A a);

after

>     template<class Y, class D> shared_ptr(Y* p, D d);
>     template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);

In 20.8.2.2.1 [util.smartptr.shared.const] add:

>     template<class D> shared_ptr(nullptr_t p, D d);
>     template<class D, class A> shared_ptr(nullptr_t p, D d, A a);

after

>     template<class Y, class D> shared_ptr(Y* p, D d);
>     template<class Y, class D, class A> shared_ptr(Y* p, D d, A a);

(reusing the following paragraphs 20.8.2.2.1 [util.smartptr.shared.const]/9-13 that speak of p.)

In 20.8.2.2.1 [util.smartptr.shared.const]/10, change

> Effects: Constructs a `shared_ptr` object that owns the pointer object `p` and the deleter `d`. The second constructor shall use a copy of `a` to allocate memory for internal use.

Rationale:

[ San Francisco: ]

> "pointer" is changed to "object" to handle the fact that `nullptr_t` isn't a pointer.

### 759. A reference is not an object

Section: 23.2 [container.requirements] Status: CD1 Submitter: Jens Maurer Opened: 2007-11-06 Last modified: 2015-04-08

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Discussion:

23.2 [container.requirements] says:

> -12- Objects passed to member functions of a container as rvalue references shall not be elements of that container. No diagnostic required.

A reference is not an object, but this sentence appears to claim so.

What is probably meant here:

> An object bound to an rvalue reference parameter of a member function of a container shall not be an element of that container; no diagnostic required.

Proposed resolution:

Change 23.2 [container.requirements]:

> -12- Objects passed to member functions of a container as rvalue references shall not be elements An object bound to an rvalue reference parameter of a member function of a container shall not be an element of that container.; N no diagnostic required.

### 761. `unordered_map` needs an `at()` member function

Section: 23.5.4.3 [unord.map.elem] Status: CD1 Submitter: Joe Gottman Opened: 2007-11-15 Last modified: 2015-04-08

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Discussion:

The new member function `at()` was recently added to `std::map()`. It acts like `operator[]()`, except it throws an exception when the input key is not found. It is useful when the `map` is `const`, the `value_type` of the key doesn't have a default constructor, it is an error if the key is not found, or the user wants to avoid accidentally adding an element to the map. For exactly these same reasons, `at()` would be equally useful in `std::unordered_map`.

Proposed resolution:

Add the following functions to the definition of `unordered_map` under "lookup" (23.5.4 [unord.map]):

>     mapped_type& at(const key_type& k);
>     const mapped_type &at(const key_type &k) const;

Add the following definitions to 23.5.4.3 [unord.map.elem]:

>     mapped_type& at(const key_type& k);
>     const mapped_type &at(const key_type &k) const;

> > Returns: A reference to `x.second`, where `x` is the (unique) element whose key is equivalent to `k`.

> > Throws: An exception object of type `out_of_range` if no such element is present.

[ Bellevue: Editorial note: the "(unique)" differs from map. ]

### 762. `std::unique_ptr` requires complete type?

Section: 20.8.1 [unique.ptr] Status: CD1 Submitter: Daniel Krügler Opened: 2007-11-30 Last modified: 2015-04-08

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Discussion:

In contrast to the proposed `std::shared_ptr`, `std::unique_ptr` does currently not support incomplete types, because it gives no explicit grant - thus instantiating `unique_ptr` with an incomplete pointee type `T` automatically belongs to undefined behaviour according to 17.6.4.8 [res.on.functions]/2, last bullet. This is an unnecessary restriction and prevents many well-established patterns - like the bridge pattern - for `std::unique_ptr`.

[ Bellevue: ]

> Move to open. The LWG is comfortable with the intent of allowing incomplete types and making `unique_ptr` more like `shared_ptr`, but we are not comfortable with the wording. The specification for `unique_ptr` should be more like that of `shared_ptr`. We need to know, for individual member functions, which ones require their types to be complete. The `shared_ptr` specification is careful to say that for each function, and we need the same level of care here. We also aren't comfortable with the "part of the operational semantic" language; it's not used elsewhere in the standard, and it's not clear what it means. We need a volunteer to produce new wording.

Proposed resolution:

The proposed changes in the following revision refers to the current state of N2521 including the assumption that [unique.ptr.compiletime] will be removed according to the current state of 740.

The specialization `unique_ptr<T[]>` has some more restrictive constraints on type-completeness on `T` than `unique_ptr<T>`. The following proposed wordings try to cope with that. If the committee sees less usefulness on relaxed constraints on `unique_ptr<T[]>`, the alternative would be to stop this relaxation e.g. by adding one further bullet to 20.8.1.3 [unique.ptr.runtime]/1: "`T` shall be a complete type, if used as template argument of `unique_ptr<T[], D>`

This issue has some overlap with 673, but it seems not to cause any problems with this one, because 673 adds only optional requirements on `D` that do not conflict with the here discussed ones, provided that `D::pointer`'s operations (including default construction, copy construction/assignment, and pointer conversion) are specified not to throw, otherwise this would have impact on the current specification of `unique_ptr`.

1. In 20.8.1 [unique.ptr]/2 add as the last sentence to the existing para:

   > The `unique_ptr` provides a semantics of strict ownership. A `unique_ptr` owns the object it holds a pointer to. A `unique_ptr` is not `CopyConstructible`, nor `CopyAssignable`, however it is `MoveConstructible` and `MoveAssignable`. The template parameter `T` of `unique_ptr` may be an incomplete type. [ Note: The uses of `unique_ptr` include providing exception safety for dynamically allcoated memory, passing ownership of dynamically allocated memory to a function, and returning dynamically allocated memory from a function. -- end note ]

2. 20.8.1.2.1 [unique.ptr.single.ctor]/1: No changes necessary.

   > [ N.B.: We only need the requirement that `D` is `DefaultConstructible`. The current wording says just this. ]

3. In 20.8.1.2.1 [unique.ptr.single.ctor]/5 change the requires clause to say:

   > Requires: The expression `D()(p)` shall be well formed. The default constructor of `D` shall not throw an exception. `D` must not be a reference type. `D` shall be default constructible, and that construction shall not throw an exception.

   > [ N.B.: There is no need that the expression `D()(p)` is well-formed at this point. I assume that the current wording is based on the corresponding `shared_ptr` wording. In case of `shared_ptr` this requirement is necessary, because the corresponding c'tor *can* fail and must invoke delete `p/d(p)` in this case. `Unique_ptr` is simpler in this regard. The *only* functions that must insist on well-formedness and well-definedness of the expression `get_deleter()(get())` are (1) the destructor and (2) `reset`. The reasoning for the wording change to explicitly require `DefaultConstructible` of `D` is to guarantee that invocation of `D`'s default c'tor is both well-formed and well-defined. Note also that we do *not* need the requirement that `T` must be complete, also in contrast to `shared_ptr`. `Shared_ptr` needs this, because it's c'tor is a template c'tor which potentially requires `Convertible<Y*, X*>`, which again requires Completeness of `Y`, if `!SameType<X, Y>` ]

4. Merge 20.8.1.2.1 [unique.ptr.single.ctor]/12+13 thereby removing the sentence of 12, but transferring the "requires" to 13:

   > Requires: If `D` is not an lvalue-reference type then[..]

   > [ N.B.: For the same reasons as for (3), there is no need that `d(p)` is well-formed/well-defined at this point. The current wording guarantees all what we need, namely that the initialization of both the `T*` pointer and the `D` deleter are well-formed and well-defined. ]

5. 20.8.1.2.1 [unique.ptr.single.ctor]/17: No changes necessary.

6. 20.8.1.2.1 [unique.ptr.single.ctor]/21:

   > Requires: If `D` is not a reference type, construction of the deleter `D` from an rvalue of type `E` shall be well formed and shall not throw an exception. If `D` is a reference type, then `E` shall be the same type as `D` (diagnostic required). `U*` shall be implicitly convertible to `T*`. [Note: These requirements imply that `T` and `U` be complete types. -- end note]

   [ N.B.: The current wording of 21 already implicitly guarantees that `U` is completely defined, because it requires that `Convertible<U*, T*>` is true. If the committee wishes this explicit requirement can be added, e.g. "`U` shall be a complete type." ]

7. 20.8.1.2.2 [unique.ptr.single.dtor]: Just before p1 add a new paragraph:

   > Requires: The expression `get_deleter()(get())` shall be well-formed, shall have well-defined behavior, and shall not throw exceptions. [Note: The use of `default_delete` requires `T` to be a complete type. -- end note]

   > [ N.B.: This requirement ensures that the whole responsibility on type-completeness of `T` is delegated to this expression. ]

8. 20.8.1.2.3 [unique.ptr.single.asgn]/1: No changes necessary, except the current editorial issue, that "must shall" has to be changed to "shall", but this change is not a special part of this resolution.

   [ N.B. The current wording is sufficient, because we can delegate all further requirements on the requirements of the effects clause ]

9. 20.8.1.2.3 [unique.ptr.single.asgn]/6:

   > Requires: Assignment of the deleter `D` from an rvalue `D` shall not throw an exception. `U*` shall be implicitly convertible to `T*`. [Note: These requirements imply that `T` and `U` be complete types. -- end note]

   [ N.B.: The current wording of p. 6 already implicitly guarantees that `U` is completely defined, because it requires that `Convertible<U*, T*>` is true, see (6)+(8). ]

10. 20.8.1.2.3 [unique.ptr.single.asgn]/11: No changes necessary.

    [ N.B.: Delegation to requirements of effects clause is sufficient. ]

11. 20.8.1.2.4 [unique.ptr.single.observers]/1+4+7+9+11:

    >     T* operator->() const;

    > > Note: Use typically requires `T` shall be complete. — end note]

12. 20.8.1.2.5 [unique.ptr.single.modifiers]/1: No changes necessary.

13. 20.8.1.2.5 [unique.ptr.single.modifiers]/4: Just before p. 4 add a new paragraph:

    > Requires: The expression `get_deleter()(get())` shall be well-formed, shall have well-defined behavior, and shall not throw exceptions.

14. 20.8.1.2.5 [unique.ptr.single.modifiers]/7: No changes necessary.

15. 20.8.1.3 [unique.ptr.runtime]: Add one additional bullet on paragraph 1:

    > A specialization for array types is provided with a slightly altered interface.

    > - ...
    > - `T` shall be a complete type.

[ post Bellevue: Daniel provided revised wording. ]

### 765. More on iterator validity

Section: X [iterator.concepts] Status: C++11 Submitter: Martin Sebor Opened: 2007-12-14 Last modified: 2015-04-08

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Discussion:

Issue 278 defines the meaning of the term "invalid iterator" as one that may be singular.

Consider the following code:

       std::deque<int> x, y;
       std::deque<int>::iterator i = x.end(), j = y.end();
       x.swap(y);

Given that `swap()` is required not to invalidate iterators and using the definition above, what should be the expected result of comparing `i` and `j` to `x.end()` and `y.end()`, respectively, after the `swap()`?

I.e., is the expression below required to evaluate to `true`?

       i == y.end() && j == x.end()

(There are at least two implementations where the expression returns `false`.)

More generally, is the definition introduced in issue 278 meant to make any guarantees about whether iterators actually point to the same elements or be associated with the same containers after a non-invalidating operation as they did before?

Here's a motivating example intended to demonstrate the importance of the question:

       Container x, y ({ 1, 2});   // pseudocode to initialize y with { 1, 2 }
       Container::iterator i = y.begin() + 1;
       Container::iterator j = y.end();
       std::swap(x, y);
       std::find(i, j, 3);

`swap()` guarantees that `i` and `j` continue to be valid. Unless the spec says that even though they are valid they may no longer denote a valid range the code above must be well-defined. Expert opinions on this differ as does the behavior of popular implementations for some standard `Containers`.

[ San Francisco: ]

> Pablo: add a note to the last bullet of paragraph 11 of 23.1.1 clarifying that the `end()` iterator doesn't refer to an element and that it can therefore be invalidated.

> Proposed wording:

> > [Note: The `end()` iterator does not refer to any element and can therefore be invalidated. -- end note]

> Howard will add this proposed wording to the issue and then move it to Review.

[ Post Summit: ]

> Lawrence: suggestion: "Note: The `end()` iterator does not refer to any element"

> Walter: "Note: The `end()` iterator can nevertheless be invalidated, because it does not refer to any element."

> Nick: "The `end()` iterator does not refer to any element. It is therefore subject to being invalidated."

> Consensus: go with Nick

> With that update, Recommend Tentatively Ready.

Proposed resolution:

Add to 23.2.1 [container.requirements.general], p11:

> Unless otherwise specified (see 23.1.4.1, 23.1.5.1, 23.2.2.3, and 23.2.6.4) all container types defined in this Clause meet the following additional requirements:

> - ...
> - no `swap()` function invalidates any references, pointers, or iterators referring to the elements of the containers being swapped. [Note: The `end()` iterator does not refer to any element. It is therefore subject to being invalidated. — end note]

### 766. Inconsistent exception guarantees between ordered and unordered associative containers

Section: 23.2 [container.requirements], 23.2.5.1 [unord.req.except] Status: CD1 Submitter: Ion Gaztañaga Opened: 2007-12-22 Last modified: 2015-04-08

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Discussion:

23.2 [container.requirements]p10 states:

> Unless otherwise specified (see 23.2.2.3 and 23.2.5.4) all container types defined in this clause meet the following additional requirements:

> - [...]
> - no `erase()`, `pop_back()` or `pop_front()` function throws an exception.

23.3.3.4 [deque.modifiers] and 23.3.6.5 [vector.modifiers] offer additional guarantees for `deque`/`vector insert()` and `erase()` members. However, 23.2 [container.requirements] p10 does not mention 23.2.5.1 [unord.req.except] that specifies exception safety guarantees for unordered containers. In addition, 23.2.5.1 [unord.req.except] p1 offers the following guaratee for `erase()`:

> No `erase()` function throws an exception unless that exception is thrown by the container's Hash or Pred object (if any).

Summary:

According to 23.2 [container.requirements] p10 no `erase()` function should throw an exception unless otherwise specified. Although does not explicitly mention 23.2.5.1 [unord.req.except], this section offers additional guarantees for unordered containers, allowing `erase()` to throw if predicate or hash function throws.

In contrast, associative containers have no exception safety guarantees section so no `erase()` function should throw, including `erase(k)` that needs to use the predicate function to perform its work. This means that the predicate of an associative container is not allowed to throw.

So:

1. `erase(k)` for associative containers is not allowed to throw. On the other hand, `erase(k)` for unordered associative containers is allowed to throw.
2. `erase(q)` for associative containers is not allowed to throw. On the other hand, `erase(q)` for unordered associative containers is allowed to throw if it uses the hash or predicate.
3. To fulfill 1), predicates of associative containers are not allowed to throw. Predicates of unordered associative containers are allowed to throw.
4. 2) breaks a widely used programming pattern (flyweight pattern) for unordered containers, where objects are registered in a global map in their constructors and unregistered in their destructors. If `erase(q)` is allowed to throw, the destructor of the object would need to rethrow the exception or swallow it, leaving the object registered.

Proposed resolution:

Create a new sub-section of 23.2.4 [associative.reqmts] (perhaps [associative.req.except]) titled "Exception safety guarantees".

> 1 For associative containers, no `clear()` function throws an exception. `erase(k)` does not throw an exception unless that exception is thrown by the container's Pred object (if any).

> 2 For associative containers, if an exception is thrown by any operation from within an `insert()` function inserting a single element, the `insert()` function has no effect.

> 3 For associative containers, no `swap` function throws an exception unless that exception is thrown by the copy constructor or copy assignment operator of the container's Pred object (if any).

Change 23.2.5.1 [unord.req.except]p1:

> For unordered associative containers, no `clear()` function throws an exception. No `erase(k)` function does not throws an exception unless that exception is thrown by the container's Hash or Pred object (if any).

Change 23.2 [container.requirements] p10 to add references to new sections:

> Unless otherwise specified (see [deque.modifiers], and [vector.modifiers], [associative.req.except], [unord.req.except]) all container types defined in this clause meet the following additional requirements:

Change 23.2 [container.requirements] p10 referring to `swap`:

> - no `swap()` function throws an exception unless that exception is thrown by the copy constructor or assignment operator of the container's Compare object (if any; see [associative.reqmts]).

### 767. Forwarding and backward compatibility

Section: 23 [containers] Status: Resolved Submitter: Sylvain Pion Opened: 2007-12-28 Last modified: 2015-04-08

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Discussion:

Playing with g++'s C++0X mode, I noticed that the following code, which used to compile:

>     #include <vector>

>     int main()
>     {
>         std::vector<char *> v;
>         v.push_back(0);
>     }

now fails with the following error message:

> .../include/c++/4.3.0/ext/new_allocator.h: In member function 'void __gnu_cxx::new_allocator<_Tp>::construct(_Tp*, _Args&& ...) [with _Args = int, _Tp = char*]': .../include/c++/4.3.0/bits/stl_vector.h:707: instantiated from 'void std::vector<_Tp, _Alloc>::push_back(_Args&& ...) [with _Args = int, _Tp = char*, _Alloc = std::allocator<char*>]' test.cpp:6: instantiated from here .../include/c++/4.3.0/ext/new_allocator.h:114: error: invalid conversion from 'int' to 'char*'

As far as I know, g++ follows the current draft here.

Does the committee really intend to break compatibility for such cases?

[ Sylvain adds: ]

> I just noticed that `std::pair` has the same issue. The following now fails with GCC's -std=c++0x mode:

> >     #include <utility>

> >     int main()
> >     {
> >        std::pair<char *, char *> p (0,0);
> >     }

> I have not made any general audit for such problems elsewhere.

[ Related to 756 ]

[ Bellevue: ]

> Motivation is to handle the old-style int-zero-valued `NULL` pointers. Problem: this solution requires concepts in some cases, which some users will be slow to adopt. Some discussion of alternatives involving prohibiting variadic forms and additional library-implementation complexity.

> Discussion of "perfect world" solutions, the only such solution put forward being to retroactively prohibit use of the integer zero for a `NULL` pointer. This approach was deemed unacceptable given the large bodies of pre-existing code that do use integer zero for a `NULL` pointer.

> Another approach is to change the member names. Yet another approach is to forbid the extension in absence of concepts.

> Resolution: These issues (756, 767, 760, 763) will be subsumed into a paper to be produced by Alan Talbot in time for review at the 2008 meeting in France. Once this paper is produced, these issues will be moved to NAD Resolved.

Proposed resolution:

Add the following rows to Table 90 "Optional sequence container operations", 23.2.3 [sequence.reqmts]:

> expression | return type | assertion/note pre-/post-condition | container
> --- | --- | --- | ---
> `a.push_front(t)` | `void` | `a.insert(a.begin(), t)` Requires: `T` shall be `CopyConstructible`. | `list, deque`
> `a.push_front(rv)` | `void` | `a.insert(a.begin(), rv)` Requires: `T` shall be `MoveConstructible`. | `list, deque`
> `a.push_back(t)` | `void` | `a.insert(a.end(), t)` Requires: `T` shall be `CopyConstructible`. | `list, deque, vector, basic_string`
> `a.push_back(rv)` | `void` | `a.insert(a.end(), rv)` Requires: `T` shall be `MoveConstructible`. | `list, deque, vector, basic_string`

Change the synopsis in 23.3.3 [deque]:

>     void push_front(const T& x);
>     void push_front(T&& x);
>     void push_back(const T& x);
>     void push_back(T&& x);
>     template <class... Args> requires Constructible<T, Args&&...> void push_front(Args&&... args);
>     template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);

Change 23.3.3.4 [deque.modifiers]:

>     void push_front(const T& x);
>     void push_front(T&& x);
>     void push_back(const T& x);
>     void push_back(T&& x);
>     template <class... Args> requires Constructible<T, Args&&...> void push_front(Args&&... args);
>     template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);

Change the synopsis in 23.3.5 [list]:

>     void push_front(const T& x);
>     void push_front(T&& x);
>     void push_back(const T& x);
>     void push_back(T&& x);
>     template <class... Args> requires Constructible<T, Args&&...> void push_front(Args&&... args);
>     template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);

Change 23.3.5.4 [list.modifiers]:

>     void push_front(const T& x);
>     void push_front(T&& x);
>     void push_back(const T& x);
>     void push_back(T&& x);
>     template <class... Args> requires Constructible<T, Args&&...> void push_front(Args&&... args);
>     template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);

Change the synopsis in 23.3.6 [vector]:

>     void push_back(const T& x);
>     void push_back(T&& x);
>     template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);

Change 23.3.6.5 [vector.modifiers]:

>     void push_back(const T& x);
>     void push_back(T&& x);
>     template <class... Args> requires Constructible<T, Args&&...> void push_back(Args&&... args);

Rationale:

Addressed by N2680 Proposed Wording for Placement Insert (Revision 1).

If there is still an issue with pair, Howard should submit another issue.

### 768. Typos in [atomics]?

Section: 29.5 [atomics.types.generic] Status: CD1 Submitter: Alberto Ganesh Barbati Opened: 2007-12-28 Last modified: 2015-04-08

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Discussion:

in the latest publicly available draft, paper N2641, in section 29.5 [atomics.types.generic], the following specialization of the template `atomic<>` is provided for pointers:

>     template <class T> struct atomic<T*> : atomic_address {
>       T* fetch_add(ptrdiff_t, memory_order = memory_order_seq_cst) volatile;
>       T* fetch_sub(ptrdiff_t, memory_order = memory_order_seq_cst) volatile;

>       atomic() = default;
>       constexpr explicit atomic(T);
>       atomic(const atomic&) = delete;
>       atomic& operator=(const atomic&) = delete;

>       T* operator=(T*) volatile;
>       T* operator++(int) volatile;
>       T* operator--(int) volatile;
>       T* operator++() volatile;
>       T* operator--() volatile;
>       T* operator+=(ptrdiff_t) volatile;
>       T* operator-=(ptrdiff_t) volatile;
>     };

First of all, there is a typo in the non-default constructor which should take a `T*` rather than a `T`.

As you can see, the specialization redefine and therefore hide a few methods from the base class `atomic_address`, namely `fetch_add`, `fetch_sub`, `operator=`, `operator+=` and `operator-=`. That's good, but... what happened to the other methods, in particular these ones:

>     void store(T*, memory_order = memory_order_seq_cst) volatile;
>     T* load( memory_order = memory_order_seq_cst ) volatile;
>     T* swap( T*, memory_order = memory_order_seq_cst ) volatile;
>     bool compare_swap( T*&, T*, memory_order, memory_order ) volatile;
>     bool compare_swap( T*&, T*, memory_order = memory_order_seq_cst ) volatile;

By reading paper N2427 "C++ Atomic Types and Operations", I see that the definition of the specialization `atomic<T*>` matches the one in the draft, but in the example implementation the methods `load()`, `swap()` and `compare_swap()` are indeed present.

Strangely, the example implementation does not redefine the method `store()`. It's true that a `T*` is always convertible to `void*`, but not hiding the `void*` signature from the base class makes the class error-prone to say the least: it lets you assign pointers of any type to a `T*`, without any hint from the compiler.

Is there a true intent to remove them from the specialization or are they just missing from the definition because of a mistake?

[ Bellevue: ]

> The proposed revisions are accepted.

> Further discussion: why is the ctor labeled "constexpr"? Lawrence said this permits the object to be statically initialized, and that's important because otherwise there would be a race condition on initialization.

Proposed resolution:

Change the synopsis in 29.5 [atomics.types.generic]:

>     template <class T> struct atomic<T*> : atomic_address {
>       void store(T*, memory_order = memory_order_seq_cst) volatile;
>       T* load( memory_order = memory_order_seq_cst ) volatile;
>       T* swap( T*, memory_order = memory_order_seq_cst ) volatile;
>       bool compare_swap( T*&, T*, memory_order, memory_order ) volatile;
>       bool compare_swap( T*&, T*, memory_order = memory_order_seq_cst ) volatile;

>       T* fetch_add(ptrdiff_t, memory_order = memory_order_seq_cst) volatile;
>       T* fetch_sub(ptrdiff_t, memory_order = memory_order_seq_cst) volatile;

>       atomic() = default;
>       constexpr explicit atomic(T*);
>       atomic(const atomic&) = delete;
>       atomic& operator=(const atomic&) = delete;

>       T* operator=(T*) volatile;
>       T* operator++(int) volatile;
>       T* operator--(int) volatile;
>       T* operator++() volatile;
>       T* operator--() volatile;
>       T* operator+=(ptrdiff_t) volatile;
>       T* operator-=(ptrdiff_t) volatile;
>     };

### 769. std::function should use nullptr_t instead of "unspecified-null-pointer-type"

Section: 20.9.12.2 [func.wrap.func] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-10 Last modified: 2015-04-08

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Discussion:

N2461 already replaced in 20.9.12.2 [func.wrap.func] it's originally proposed (implicit) conversion operator to "unspecified-bool-type" by the new explicit bool conversion, but the inverse conversion should also use the new `std::nullptr_t` type instead of "unspecified-null-pointer- type".

Proposed resolution:

In 20.9 [function.objects], header `<functional>` synopsis replace:

>     template<class R, class... ArgTypes>
>       bool operator==(const function<R(ArgTypes...)>&, unspecified-null-pointer-type nullptr_t);
>     template<class R, class... ArgTypes>
>       bool operator==(unspecified-null-pointer-type nullptr_t , const function<R(ArgTypes...)>&);
>     template<class R, class... ArgTypes>
>       bool operator!=(const function<R(ArgTypes...)>&, unspecified-null-pointer-type nullptr_t);
>     template<class R, class... ArgTypes>
>       bool operator!=(unspecified-null-pointer-type nullptr_t , const function<R(ArgTypes...)>&);

In the class function synopsis of 20.9.12.2 [func.wrap.func] replace

>     function(unspecified-null-pointer-type nullptr_t);
>     ...
>     function& operator=(unspecified-null-pointer-type nullptr_t);

In 20.9.12.2 [func.wrap.func], "Null pointer comparisons" replace:

>     template <class R, class... ArgTypes>
>       bool operator==(const function<R(ArgTypes...)>&, unspecified-null-pointer-type nullptr_t);
>     template <class R, class... ArgTypes>
>       bool operator==(unspecified-null-pointer-type nullptr_t , const function<R(ArgTypes...)>&);
>     template <class R, class... ArgTypes>
>       bool operator!=(const function<R(ArgTypes...)>&, unspecified-null-pointer-type nullptr_t);
>     template <class R, class... ArgTypes>
>       bool operator!=(unspecified-null-pointer-type nullptr_t , const function<R(ArgTypes...)>&);

In 20.9.12.2.1 [func.wrap.func.con], replace

>     function(unspecified-null-pointer-type nullptr_t);
>     ...
>     function& operator=(unspecified-null-pointer-type nullptr_t);

In 20.9.12.2.6 [func.wrap.func.nullptr], replace

>     template <class R, class... ArgTypes>
>       bool operator==(const function<R(ArgTypes...)>& f, unspecified-null-pointer-type nullptr_t);
>     template <class R, class... ArgTypes>
>       bool operator==(unspecified-null-pointer-type nullptr_t , const function<R(ArgTypes...)>& f);

and replace

>     template <class R, class... ArgTypes>
>       bool operator!=(const function<R(ArgTypes...)>& f, unspecified-null-pointer-type nullptr_t);
>     template <class R, class... ArgTypes>
>       bool operator!=(unspecified-null-pointer-type nullptr_t , const function<R(ArgTypes...)>& f);

### 770. std::function should use rvalue swap

Section: 20.9.12 [func.wrap] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-10 Last modified: 2015-04-08

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Discussion:

It is expected that typical implementations of `std::function` will use dynamic memory allocations at least under given conditions, so it seems appropriate to change the current lvalue swappabilty of this class to rvalue swappability.

Proposed resolution:

In 20.9 [function.objects], header `<functional>` synopsis, just below of

>     template<class R, class... ArgTypes>
>       void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&);
>     template<class R, class... ArgTypes>
>       void swap(function<R(ArgTypes...)>&&, function<R(ArgTypes...)>&);
>     template<class R, class... ArgTypes>
>       void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&&);

In 20.9.12.2 [func.wrap.func] class `function` definition, change

>     void swap(function&&);

In 20.9.12.2 [func.wrap.func], just below of

>     template <class R, class... ArgTypes>
>       void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&);
>     template <class R, class... ArgTypes>
>       void swap(function<R(ArgTypes...)>&&, function<R(ArgTypes...)>&);
>     template <class R, class... ArgTypes>
>       void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&&);

In 20.9.12.2.2 [func.wrap.func.mod] change

>     void swap(function&& other);

In 20.9.12.2.7 [func.wrap.func.alg] add the two overloads

>     template<class R, class... ArgTypes>
>       void swap(function<R(ArgTypes...)>&& f1, function<R(ArgTypes...)>& f2);
>     template<class R, class... ArgTypes>
>       void swap(function<R(ArgTypes...)>& f1, function<R(ArgTypes...)>&& f2);

### 771. Impossible throws clause in [string.conversions]

Section: 21.5 [string.conversions] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-13 Last modified: 2015-04-08

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Discussion:

The new `to_string` and `to_wstring` functions described in 21.5 [string.conversions] have throws clauses (paragraphs 8 and 16) which say:

> Throws: nothing

Since all overloads return either a `std::string` or a `std::wstring` by value this throws clause is impossible to realize in general, since the `basic_string` constructors can fail due to out-of-memory conditions. Either these throws clauses should be removed or should be more detailled like:

> Throws: Nothing if the string construction throws nothing

Further there is an editorial issue in p. 14: All three `to_wstring` overloads return a `string`, which should be `wstring` instead (The header `<string>` synopsis of 21.3 [string.classes] is correct in this regard).

Proposed resolution:

In 21.5 [string.conversions], remove the paragraphs 8 and 16.

>     string to_string(long long val);
>     string to_string(unsigned long long val);
>     string to_string(long double val);

> > Throws: nothing

>     wstring to_wstring(long long val);
>     wstring to_wstring(unsigned long long val);
>     wstring to_wstring(long double val);

> > Throws: nothing

### 772. Impossible return clause in [string.conversions]

Section: 21.5 [string.conversions] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-13 Last modified: 2015-04-08

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Discussion:

The return clause 21.5 [string.conversions] paragraph 15 of the new `to_wstring` overloads says:

> Returns: each function returns a `wstring` object holding the character representation of the value of its argument that would be generated by calling `wsprintf(buf, fmt, val)` with a format specifier of `L"%lld"`, `L"%ulld"`, or `L"%f"`, respectively.

Problem is: There does not exist any `wsprintf` function in C99 (I checked the 2nd edition of ISO 9899, and the first and the second corrigenda from 2001-09-01 and 2004-11-15). What probably meant here is the function `swprintf` from `<wchar.h>/<cwchar>`, but this has the non-equivalent declaration:

>     int swprintf(wchar_t * restrict s, size_t n,
>     const wchar_t * restrict format, ...);

therefore the paragraph needs to mention the `size_t` parameter `n`.

Proposed resolution:

Change the current wording of 21.5 [string.conversions] p. 15 to:

> Returns: e Each function returns a `wstring` object holding the character representation of the value of its argument that would be generated by calling `wsswprintf(buf, bufsz, fmt, val)` with a format specifier `fmt` of `L"%lld"`, `L"%ulld"`, or `L"%f"`, respectively, where `buf` designates an internal character buffer of sufficient size `bufsz`.

[Hint to the editor: The resolution also adds to mention the name of the format specifier "fmt"]

I also would like to remark that the current wording of it's equivalent paragraph 7 should also mention the meaning of `buf` and `fmt`.

Change the current wording of 21.5 [string.conversions] p. 7 to:

> Returns: e Each function returns a string object holding the character representation of the value of its argument that would be generated by calling `sprintf(buf, fmt, val)` with a format specifier `fmt` of `"%lld"`, `"%ulld"`, or `"%f"`, respectively, where `buf` designates an internal character buffer of sufficient size.

### 774. Member `swap` undefined for most containers

Section: 23 [containers] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-01-14 Last modified: 2015-04-08

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Discussion:

It appears most containers declare but do not define a member-swap function.

This is unfortunate, as all overload the `swap` algorithm to call the member-swap function! (required for `swappable` guarantees [Table 37] and Container Requirements [Table 87])

Note in particular that Table 87 gives semantics of `a.swap(b)` as `swap(a,b)`, yet for all containers we define `swap(a,b)` to call `a.swap(b)` - a circular definition.

A quick survey of clause 23 shows that the following containers provide a definition for member-swap:

>     array
>     queue
>     stack
>     vector

Whereas the following declare it, but do not define the semantics:

>     deque
>     list
>     map
>     multimap
>     multiset
>     priority_queue
>     set
>     unordered_map
>     unordered_multi_map
>     unordered_multi_set
>     unordered_set

Suggested resolution:

> Provide a definition for each of the affected containers...

[ Bellevue: ]

> Move to Open and ask Alisdair to provide wording.

[ 2009-07 Frankfurt: ]

> Daniel to provide wording. N2590 is no longer applicable.

[ 2009-07-28 Daniel provided wording. ]

> 1. It assumes that the proposed resolution for 883 is applied, which breaks the circularity of definition between member `swap` and free `swap`.
> 2. It uses the notation of the pre-concept allocator trait `allocator_propagation_map`, which might be renamed after the next refactoring phase of generalized allocators.
> 3. It requires that compare objects, key equal functions and hash functions in containers are swapped via unqualified free `swap` according to 594.

[ 2009-09-30 Daniel adds: ]

> The outcome of this issue should be considered with the outcome of 1198 both in style and in content (e.g. bullet 9 suggests to define the semantic of `void priority_queue::swap(priority_queue&)` in terms of the member `swap` of the container).

[ 2009-10 Santa Cruz: ]

> Looked at, but took no action on as it overlaps too much with N2982. Waiting for a new draft WP.

[ 2009-10 Santa Cruz: ]

> Leave as open. Pablo to provide wording.

[ 2009-10-26 Pablo updated wording. Here is the wording he replaced: ]

> 1. Add a new Throws clause just after X [allocator.propagation.map]/5:

>    >     static void swap(Alloc& a, Alloc& b);

>    > > Effects: [..]

>    > > Throws: Nothing.

>    [ This exception requirement is added, such that it's combination with the general container requirements of N2723 [container.requirements.general]/9 make it unambiguously clear that the following descriptions of "swaps the allocators" have the following meaning: (a) This swap is done by calling `allocator_propagation_map<allocator_type>::swap` and (b) This allocator swap does never propagate an exception ]

> 2. Change 23.2.4.1 [associative.reqmts.except]/3 as indicated:

>    > For associative containers, no `swap` function throws an exception unless that exception is thrown by the copy constructor or copy assignment operator `swap` of the container's `Pred` objects (if any).

> 3. Change 23.2.5.1 [unord.req.except]/3 as indicated:

>    > For unordered associative containers, no `swap` function throws an exception unless that exception is thrown by the copy constructor or copy assignment operator `swap` of the container's `Hash` or `Pred` objects, respectively (if any).

> 4. Insert a new paragraph just after 23.3 [sequences]/1:

>    > In addition to being available via inclusion of the `<algorithm>` header, the `swap` function templates in 25.3.3 [alg.swap] are also available when the header `<queue>` is included.

>    [ There is a new issue in process that will suggest a minimum header for `swap` and `move`. If this one is provided, this text can be removed and the header dependency should be added to `<queue>` ]

> 5. Add one further clause at the end of 23.3.2.3 [array.special]:

>    [This part is added, because otherwise `array::swap` would otherwise contradict the general contract of 23.2.1 [container.requirements.general] p. 10 b. 5]

>    > Throws: Nothing, unless one of the element-wise `swap` calls throws an exception.

> 6. 1. In 23.3.3 [deque], class template `deque` synopsis change as indicated:

>       >     void swap(deque<T,Alloc>&);

>    2. At the end of 23.3.3.4 [deque.modifiers] add as indicated:

>       >     void swap(deque& x);

>       > > Effects: Exchanges the contents and swaps the allocators of `*this` with that of `x`.

>       > > Complexity: Constant time.

> 7. 1. In 23.3.4 [forwardlist], class template `forward_list` synopsis change as indicated:

>       >     void swap(forward_list<T,Allocator>&);

>    2. At the end of 23.3.4.5 [forwardlist.modifiers] add as indicated:

>       >     void swap(forward_list& x);

>       > > Effects: Exchanges the contents and swaps the allocators of `*this` with that of `x`.

>       > > Complexity: Constant time.

> 8. 1. In 23.3.5 [list], class template `list` synopsis change as indicated:

>       >     void swap(list<T,Allocator>&);

>    2. At the end of 23.3.5.4 [list.modifiers] add as indicated:

>       >     void swap(list& x);

>       > > Effects: Exchanges the contents and swaps the allocators of `*this` with that of `x`.

>       > > Complexity: Constant time.

> 9. At the end of 23.6.4.3 [priqueue.members] add a new prototype description:

>    >     void swap(priority_queue& q);

>    > > Requires: `Compare` shall satisfy the `Swappable` requirements ( [swappable]).

>    > > [ This requirement is added to ensure that even a user defined `swap` which is found by ADL for `Compare` satisfies the `Swappable` requirements ]

>    > > Effects: `this->c.swap(q.c); swap(this->comp, q.comp);`

>    > > Throws: What and if `c.swap(q.c)` and `swap(comp, q.comp)` throws.

>    [ This part is added, because otherwise `priority_queue::swap` would otherwise contradict the general contract of 23.2.1 [container.requirements.general] p. 10 b. 5 ]

> 10. 1. In 23.3.6 [vector], class template `vector` synopsis change as indicated:

>        >     void swap(vector<T,Allocator>&);

>     2. Change 23.3.6.3 [vector.capacity] p. 8 as indicated:

>        >     void swap(vector<T,Allocator>& x);

>        > > Effects: Exchanges the contents and `capacity()` and swaps the allocators of `*this` with that of `x`.

> 11. Insert a new paragraph just before 23.4 [associative]/1:

>     > In addition to being available via inclusion of the `<algorithm>` header, the `swap` function templates in 25.3.3 [alg.swap] are also available when any of the headers `<map>` or `<set>` are included.

> 12. 1. In 23.4.4 [map], class template `map` synopsis change as indicated:

>        >     void swap(map<Key,T,Compare,Allocator>&);

>     2. At the end of 23.4.4.4 [map.modifiers] add as indicated:

>        >     void swap(map& x);

>        > > Requires: Compare shall satisfy the `Swappable` requirements ( [swappable]).

>        > > [ This requirement is added to ensure that even a user defined `swap` which is found by ADL for `Compare` satisfies the `Swappable` requirements ]

>        > > Effects: Exchanges the contents and swaps the allocators of `*this` with that of `x`, followed by an unqualified `swap` of the comparison objects of `*this` and `x`.

>        > > Complexity: Constant time

> 13. 1. In 23.4.5 [multimap], class template `multimap` synopsis change as indicated:

>        >     void swap(multimap<Key,T,Compare,Allocator>&);

>     2. At the end of 23.4.5.3 [multimap.modifiers] add as indicated:

>        >     void swap(multimap& x);

>        > > Requires: Compare shall satisfy the `Swappable` requirements ( [swappable]).

>        > > Effects: Exchanges the contents and swaps the allocators of `*this` with that of `x`, followed by an unqualified `swap` of the comparison objects of `*this` and `x`.

>        > > Complexity: Constant time

> 14. 1. In 23.4.6 [set], class template `set` synopsis change as indicated:

>        >     void swap(set<Key,Compare,Allocator>&);

>     2. After section 23.4.6.2 [set.cons] add a new section `set` modifiers [set.modifiers] and add the following paragraphs:

>        >     void swap(set& x);

>        > > Requires: Compare shall satisfy the `Swappable` requirements ( [swappable]).

>        > > Effects: Exchanges the contents and swaps the allocators of `*this` with that of `x`, followed by an unqualified `swap` of the comparison objects of `*this` and `x`.

>        > > Complexity: Constant time

> 15. 1. In 23.4.7 [multiset], class template `multiset` synosis, change as indicated:

>        >     void swap(multiset<Key,Compare,Allocator>&);

>     2. After section 23.4.7.2 [multiset.cons] add a new section `multiset` modifiers [multiset.modifiers] and add the following paragraphs:

>        >     void swap(multiset& x);

>        > > Requires: Compare shall satisfy the `Swappable` requirements ( [swappable]).

>        > > Effects: Exchanges the contents and swaps the allocators of `*this` with that of `x`, followed by an unqualified `swap` of the comparison objects of `*this` and `x`.

>        > > Complexity: Constant time

> 16. Insert a new paragraph just before 23.5 [unord] p. 1:

>     > In addition to being available via inclusion of the `<algorithm>` header, the `swap` function templates in 25.3.3 [alg.swap] are also available when any of the headers `<unordered_map>` or `<unordered_set>` are included.

> 17. After section 23.5.4.3 [unord.map.elem] add a new section unordered_map modifiers 23.5.4.4 [unord.map.modifiers] and add the following paragraphs:

>     >     void swap(unordered_map& x);

>     > > Requires: `Hash` and `Pred` shall satisfy the `Swappable` requirements ( [swappable]).

>     > > [ This requirement is added to ensure that even a user defined `swap` which is found by ADL for `Hash` and `Pred` satisfies the `Swappable` requirements ]

>     > > Effects: Exchanges the contents and hash policy and swaps the allocators of `*this` with that of `x`, followed by an unqualified `swap` of the `Pred` objects and an unqualified `swap` of the `Hash` objects of `*this` and `x`.

>     > > Complexity: Constant time

> 18. After section 23.5.5.2 [unord.multimap.cnstr] add a new section unordered_multimap modifiers 23.5.5.3 [unord.multimap.modifiers] and add the following paragraphs:

>     >     void swap(unordered_multimap& x);

>     > > Requires: `Hash` and `Pred` shall satisfy the `Swappable` requirements ( [swappable]).

>     > > Effects: Exchanges the contents and hash policy and swaps the allocators of `*this` with that of `x`, followed by an unqualified `swap` of the `Pred` objects and an unqualified `swap` of the `Hash` objects of `*this` and `x`

>     > > Complexity: Constant time

> 19. After section 23.5.6.2 [unord.set.cnstr] add a new section unordered_set modifiers [unord.set.modifiers] and add the following paragraphs:

>     >     void swap(unordered_set& x);

>     > > Requires: `Hash` and `Pred` shall satisfy the `Swappable` requirements ( [swappable]).

>     > > Effects: Exchanges the contents and hash policy and swaps the allocators of `*this` with that of `x`, followed by an unqualified `swap` of the `Pred` objects and an unqualified `swap` of the `Hash` objects of `*this` and `x`

>     > > Complexity: Constant time

> 20. After section 23.5.7.2 [unord.multiset.cnstr] add a new section unordered_multiset modifiers [unord.multiset.modifiers] and add the following paragraphs:

>     >     void swap(unordered_multiset& x);

>     > > Requires: `Hash` and `Pred` shall satisfy the `Swappable` requirements ( [swappable]).

>     > > Effects: Exchanges the contents and hash policy and swaps the allocators of `*this` with that of `x`, followed by an unqualified `swap` of the `Pred` objects and an unqualified `swap` of the `Hash` objects of `*this` and `x`

>     > > Complexity: Constant time

[ 2009-10-30 Pablo and Daniel updated wording. ]

[ 2010 Pittsburgh: Ready for Pittsburgh. ]

Proposed resolution:

[ This resolution is based on the September 2009 WP, N2960, except that it assumes that N2982 and issues 883 and 1232 have already been applied. Note in particular that Table 91 in N2960 is refered to as Table 90 because N2982 removed the old Table 90. This resolution also addresses issue 431. ]

In 23.2.1 [container.requirements.general], replace the a.swap(b) row in table 90, "container requirements" (was table 91 before the application of N2982 to the WP):

> `a.swap(b)` | `void` |  | `swap(a,b)Exchange the contents of `a` and `b`.` | (Note A)
> `swap(a,b)` | `void` | `` | `a.swap(b)` | (Note A)

Modify the notes immediately following Table 90 in 23.2.1 [container.requirements.general] as follows (The wording below is after the application of N2982 to N2960. The editor might also want to combine Notes A and B into one.):

> Notes: the algorithms swap(), equal() and lexicographical_compare() are defined in Clause 25. Those entries marked "(Note A)" or "(Note B)" should have linear complexity for array and constant complexity for all other standard containers.

In 23.2.1 [container.requirements.general], before paragraph 8, add:

> The expression `a.swap(b)`, for containers `a` and `b` of a standard container type other than `array`, exchanges the values of `a` and `b` without invoking any move, copy, or swap operations on the individual container elements. Any `Compare`, `Pred`, or `Hash` function objects belonging to `a` and `b` shall be `swappable` and are exchanged by unqualified calls to non-member `swap`. If `allocator_traits<allocator_type>::propagate_on_container_swap::value == true`, then the allocators of `a` and `b` are also exchanged using an unqualified call to non-member `swap`. Otherwise, the behavior is undefined unless `a.get_allocator() == b.get_allocator()`. Each iterator refering to an element in one container before the swap shall refer to the same element in the other container after the swap. It is unspecified whether an iterator with value `a.end()` before the swap will have value `b.end()` after the swap. In addition to being available via inclusion of the `<utility>` header, the `swap` function template in 25.3.3 [alg.swap] is also available within the definition of every standard container's `swap` function.

[ Note to the editor: Paragraph 2 starts with a sentence fragment, clearly from an editing or source-control error. ]

Modify 23.2.4.1 [associative.reqmts.except] as follows:

> 23.2.4.1 Exception safety guarantees 23.2.4.1 [associative.reqmts.except]

> For associative containers, no `clear()` function throws an exception. `erase(k)` does not throw an exception unless that exception is thrown by the container's `PredCompare` object (if any).

> For associative containers, if an exception is thrown by any operation from within an `insert()` function inserting a single element, the `insert()` function has no effect.

> For associative containers, no `swap` function throws an exception unless that exception is thrown by the copy constructor or copy assignment operator swap of the container's `PredCompare` object (if any).

Modify 23.2.5.1 [unord.req.except], paragraph 3 as follows:

> For unordered associative containers, no `swap` function throws an exception unless that exception is thrown by the copy constructor or copy assignment operator swap of the container's `Hash` or `Pred` object (if any).

Modify section 23.3.2.3 [array.special]:

> array specialized algorithms 23.3.2.3 [array.special]

> `template <class T, size_t N> void swap(array<T,N>& x,array<T,N>& y);`

> > Effects: `swap_ranges(x.begin(), x.end(), y.begin() );x.swap(y);`

Add a new section after 23.3.2.6 [array.fill] (Note to the editor: array::fill make use of a concept requirement that must be removed or changed to text.):

> array::swap [array.swap]

> `void swap(array& y);`

> > Effects: `swap_ranges(this->begin(), this->end(), y.begin() );`

> > Throws: Nothing unless one of the element-wise swap calls throws an exception.

> > [Note: Unlike other containers' `swap` functions, `array::swap` takes linear, not constant, time, may exit via an exception, and does not cause iterators to become associated with the other container. — end note]

Insert a new paragraph just after 23.6 [container.adaptors]/1:

> For container adaptors, no `swap` function throws an exception unless that exception is thrown by the swap of the adaptor's `Container` or `Compare` object (if any).

### 775. Tuple indexing should be unsigned?

Section: 20.4.2.5 [tuple.helper] Status: CD1 Submitter: Alisdair Meredith Opened: 2008-01-16 Last modified: 2015-04-08

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Discussion:

The tuple element access API identifies the element in the sequence using signed integers, and then goes on to enforce the requirement that I be >= 0. There is a much easier way to do this - declare I as `unsigned`.

In fact the proposal is to use `std::size_t`, matching the type used in the `tuple_size` API.

A second suggestion is that it is hard to imagine an API that deduces and index at compile time and returns a reference throwing an exception. Add a specific Throws: Nothing paragraph to each element access API.

In addition to `tuple`, update the API applies to `pair` and `array`, and should be updated accordingly.

A third observation is that the return type of the `get` functions for `std::pair` is pseudo-code, but it is not clearly marked as such. There is actually no need for pseudo-code as the return type can be specified precisely with a call to `tuple_element`. This is already done for `std::tuple`, and `std::array` does not have a problem as all elements are of type `T`.

Proposed resolution:

Update header <utility> synopsis in 20.2 [utility]

    // 20.2.3, tuple-like access to pair:
    template <class T> class tuple_size;
    template <intsize_t I, class T> class tuple_element;

    template <class T1, class T2> struct tuple_size<std::pair<T1, T2> >;
    template <class T1, class T2> struct tuple_element<0, std::pair<T1, T2> >;
    template <class T1, class T2> struct tuple_element<1, std::pair<T1, T2> >;

    template<intsize_t I, class T1, class T2>
      Ptypename tuple_element<I, std::pair<T1, T2> >::type & get(std::pair<T1, T2>&);
    template<intsize_t I, class T1, class T2>
      const Ptypename tuple_element<I, std::pair<T1, T2> >::type & get(const std::pair<T1, T2>&);

Update 20.3 [pairs] Pairs

    template<intsize_t I, class T1, class T2>
      Ptypename tuple_element<I, std::pair<T1, T2> >::type & get(pair<T1, T2>&);
    template<intsize_t I, class T1, class T2>
      const Ptypename tuple_element<I, std::pair<T1, T2> >::type & get(const pair<T1, T2>&);

24 Return type: If `I == 0` then `P` is `T1`, if `I == 1` then `P` is `T2`, and otherwise the program is ill-formed.

25 Returns: If `I == 0` returns `p.first`, otherwise if `I == 1` returns `p.second`, and otherwise the program is ill-formed.

Throws: Nothing.

Update header <tuple> synopsis in 20.4 [tuple] with a APIs as below:

    template <intsize_t I, class T> class tuple_element; // undefined
    template <intsize_t I, class... Types> class tuple_element<I, tuple<Types...> >;

    // 20.3.1.4, element access:
    template <intsize_t I, class... Types>
      typename tuple_element<I, tuple<Types...> >::type& get(tuple<Types...>&);
    template <intsize_t I, class ... types>
      typename tuple_element<I, tuple<Types...> >::type const& get(const tuple<Types...>&);

Update 20.4.2.5 [tuple.helper] Tuple helper classes

    template <intsize_t I, class... Types>
    class tuple_element<I, tuple<Types...> > {
    public:
      typedef TI type;
    };

1 Requires: `0 <= I and I < sizeof...(Types)`. The program is ill-formed if `I` is out of bounds.

2 Type: `TI` is the type of the `I`th element of `Types`, where indexing is zero-based.

Update 20.4.2.6 [tuple.elem] Element access

    template <intsize_t I, class... types >
    typename tuple_element<I, tuple<Types...> >::type& get(tuple<Types...>& t);

1 Requires: `0 <= I and I < sizeof...(Types)`. The program is ill-formed if `I` is out of bounds.

2 Returns: A reference to the `I`th element of `t`, where indexing is zero-based. Throws: Nothing.

    template <intsize_t I, class... types>
    typename tuple_element<I, tuple<Types...> >::type const& get(const tuple<Types...>& t);

3 Requires: `0 <= I and I < sizeof...(Types)`. The program is ill-formed if `I` is out of bounds.

4 Returns: A const reference to the `I`th element of `t`, where indexing is zero-based.

Throws: Nothing.

Update header <array> synopsis in 20.2 [utility]

    template <class T> class tuple_size; // forward declaration
    template <intsize_t I, class T> class tuple_element; // forward declaration
    template <class T, size_t N>
      struct tuple_size<array<T, N> >;
    template <intsize_t I, class T, size_t N>
      struct tuple_element<I, array<T, N> >;
    template <intsize_t I, class T, size_t N>
      T& get(array<T, N>&);
    template <intsize_t I, class T, size_t N>
      const T& get(const array<T, N>&);

Update 23.3.2.9 [array.tuple] Tuple interface to class template array

    tuple_element<size_t I, array<T, N> >::type

3 Requires: `0 <= I < N.` The program is ill-formed if `I` is out of bounds.

4 Value: The type `T`.

    template <intsize_t I, class T, size_t N> T& get(array<T, N>& a);

5 Requires: `0 <= I < N`. The program is ill-formed if `I` is out of bounds.

Returns: A reference to the `I`th element of `a`, where indexing is zero-based.

Throws: Nothing.

    template <intsize_t I, class T, size_t N> const T& get(const array<T, N>& a);

6 Requires: `0 <= I < N`. The program is ill-formed if `I` is out of bounds.

7 Returns: A const reference to the `I`th element of `a`, where indexing is zero-based.

Throws: Nothing.

[ Bellevue: Note also that the phrase "The program is ill-formed if I is out of bounds" in the requires clauses are probably unnecessary, and could be removed at the editor's discretion. Also std:: qualification for pair is also unnecessary. ]

### 776. Undescribed `assign` function of `std::array`

Section: 23.3.2 [array] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-20 Last modified: 2015-04-08

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Discussion:

The class template array synopsis in 23.3.2 [array] p. 3 declares a member function

>     void assign(const T& u);

which's semantic is no-where described. Since this signature is not part of the container requirements, such a semantic cannot be derived by those.

I found only one reference to this function in the issue list, 588 where the question is raised:

> what's the effect of calling `assign(T&)` on a zero-sized array?

which does not answer the basic question of this issue.

If this function shall be part of the `std::array`, it's probable semantic should correspond to that of `boost::array`, but of course such wording must be added.

Proposed resolution:

Just after the section 23.3.2.5 [array.data] add the following new section:

23.2.1.5 array::fill [array.fill]

>     void fill(const T& u);

> 1: Effects: `fill_n(begin(), N, u)`

[N.B: I wonder, why class `array` does not have a "modifiers" section. If it had, then `assign` would naturally belong to it]

Change the synopsis in 23.3.2 [array]/3:

>     template <class T, size_t N>
>     struct array {
>       ...
>       void assign fill(const T& u);
>       ...

[ Bellevue: ]

> Suggest substituting "fill" instead of "assign".

> Set state to Review given substitution of "fill" for "assign".

### 777. Atomics Library Issue

Section: 29.6 [atomics.types.operations] Status: CD1 Submitter: Lawrence Crowl Opened: 2008-01-21 Last modified: 2015-04-08

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Discussion:

The load functions are defined as

>     C atomic_load(volatile A* object);
>     C atomic_load_explicit(volatile A* object, memory_order);
>     C A::load(memory_order order = memory_order_seq_cst) volatile;

which prevents their use in `const` contexts.

[ post Bellevue Peter adds: ]

> Issue 777 suggests making `atomic_load` operate on `const` objects. There is a subtle point here. Atomic loads do not generally write to the object, except potentially for the `memory_order_seq_cst` constraint. Depending on the architecture, a dummy write with the same value may be required to be issued by the atomic load to maintain sequential consistency. This, in turn, may make the following code:

> >     const atomic_int x{};

> >     int main()
> >     {
> >       x.load();
> >     }

> dump core under a straightforward implementation that puts const objects in a read-only section.

> There are ways to sidestep the problem, but it needs to be considered.

> The tradeoff is between making the data member of the atomic types mutable and requiring the user to explicitly mark atomic members as mutable, as is already the case with mutexes.

Proposed resolution:

Add the `const` qualifier to `*object` and `*this`.

>     C atomic_load(const volatile A* object);
>     C atomic_load_explicit(const volatile A* object, memory_order);
>     C A::load(memory_order order = memory_order_seq_cst) const volatile;

### 778. std::bitset does not have any constructor taking a string literal

Section: 20.6.1 [bitset.cons] Status: CD1 Submitter: Thorsten Ottosen Opened: 2008-01-24 Last modified: 2015-04-08

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Duplicate of: 116

Discussion:

A small issue with `std::bitset`: it does not have any constructor taking a string literal, which is clumsy and looks like an oversigt when we tried to enable uniform use of `string` and `const char*` in the library.

Suggestion: Add

>     explicit bitset( const char* str );

to std::bitset.

Proposed resolution:

Add to synopsis in 20.6 [template.bitset]

>     explicit bitset( const char* str );

Add to synopsis in 20.6.1 [bitset.cons]

>     explicit bitset( const char* str );

> Effects: Constructs a `bitset` as if `bitset(string(str))`.

### 779. Resolution of #283 incomplete

Section: 25.3.8 [alg.remove] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-25 Last modified: 2015-04-08

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Discussion:

The resolution of 283 did not resolve similar necessary changes for algorithm `remove_copy[_if]`, which seems to be an oversight.

Proposed resolution:

In 25.3.8 [alg.remove] p.6, replace the N2461 requires clause with:

> Requires: Type `T` is `EqualityComparable` (31). The ranges `[first,last)` and `[result,result + (last - first))` shall not overlap. The expression `*result = *first` shall be valid.

### 780. `std::merge()` specification incorrect/insufficient

Section: 25.4.4 [alg.merge] Status: C++11 Submitter: Daniel Krügler Opened: 2008-01-25 Last modified: 2015-04-08

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Discussion:

Though issue 283 has fixed many open issues, it seems that some are still open:

Both 25.3.4 [lib.alg.merge] in 14882:2003 and 25.4.4 [alg.merge] in N2461 have no Requires element and the Effects element contains some requirements, which is probably editorial. Worse is that:

- no assignment requirements are specified (neither implicit nor explicit).
- the effects clause just speaks of "merges", which is badly worded near to a circular definition.
- p. 2 mentions a range `[first, last)`, which is not defined by the function arguments or otherwise.
- p. 2 says "according to the ordering defined by comp" which is both incomplete (because this excludes the first variant with <) and redundant (because the following subordinate clause mentions comp again)

[ Post Summit Alisdair adds: ]

> Suggest:

> > (where `last` is equal to `next(result, distance(first1, last1) + distance(first2, last2))`, such that resulting range will be sorted in non-decreasing order; that is, for every iterator `i` in `[result,last)` other than `result`, the condition `*i < *prev(i)` or, respectively, `comp(*i, *prev(i))` will be `false`.

> Note that this might still not be technically accurate in the case of `InputIterators`, depending on other resolutions working their way through the system (1011).

[ Post Summit Daniel adds: ]

> If we want to use `prev` and `next` here (Note: `merge` is sufficiently satisfied with `InputIterator`) we should instead add more to 25 [algorithms] p. 6, but I can currently not propose any good wording for this.

[ Batavia (2009-05): ]

> Pete points out the existing wording in [algorithms] p. 4 that permits the use of + in algorithm specifications.

> Alisdair points out that that wording may not apply to input iterators.

> Move to Review.

[ 2009-07 Frankfurt: ]

> Move to Ready.

[ 2009-08-23 Daniel reopens: ]

> The proposed wording must be rephrased, because the part

> > for every iterator `i` in `[result,last)` other than `result`, the condition `*i < *(i - 1)` or, respectively, `comp(*i, *(i - 1))` will be `false`"

> isn't meaningful, because the range `[result,last)` is that of a pure `OutputIterator`, which is not readable in general.

> [Howard: Proposed wording updated by Daniel, status moved from Ready to Review.]

[ 2009-10 Santa Cruz: ]

> Matt has some different words to propose. Those words have been moved into the proposed wording section, and the original proposed wording now appears here:

> > In 25.4.4 [alg.merge] replace p.1+ 2:

> > > Effects: Merges Copies all the elements of the two sorted ranges `[first1,last1)` and `[first2,last2)` into the range `[result,result + (last1 - first1) + (last2 - first2))` , such that resulting range will be sorted in non-decreasing order; that is for every pair of iterators `i` and `j` of either input ranges, where `*i` was copied to the output range before `*j` was copied to the output range, the condition `*j < *i` or, respectively, `comp(*j, *i)` will be `false`.

> > > Requires:The resulting range shall not overlap with either of the original ranges. The list will be sorted in non-decreasing order according to the ordering defined by `comp`; that is, for every iterator `i` in `[first,last)` other than `first`, the condition `*i < *(i - 1)` or `comp(*i, *(i - 1))` will be `false`.

[ 2010-02-10 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Change 25.4.4 [alg.merge] 1 and 2:

> 1 Effects: Merges two sorted ranges `[first1,last1)` and `[first2,last2)` into the range `[result, result + (last1 - first1) + (last2 - first2))`.

> Effects: Copies all the elements of the two ranges `[first1,last1)` and `[first2,last2)` into the range `[result, result_last)`, where `result_last` is `result + (last1 - first1) + (last2 - first2)`, such that the resulting range satisfies `is_sorted(result, result_last)` or `is_sorted(result, result_last, comp)`, respectively.

> 2 Requires: The ranges `[first1,last1)` and `[first2,last2)` shall be sorted with respect to `operator<` or `comp`. The resulting range shall not overlap with either of the original ranges. The list will be sorted in non-decreasing order according to the ordering defined by `comp`; that is, for every iterator `i` in `[first,last)` other than `first`, the condition `*i < *(i - 1)` or `comp(*i, *(i - 1))` will be `false`.

Change 25.4.4 [alg.merge] p. 6+7 as indicated [This ensures harmonization between `inplace_merge` and `merge`]

> 6 Effects: Merges two sorted consecutive ranges `[first,middle)` and `[middle,last)`, putting the result of the merge into the range `[first,last)`. The resulting range will be in non-decreasing order; that is, for every iterator `i` in `[first,last)` other than `first`, the condition `*i < *(i - 1)` or, respectively, `comp(*i, *(i - 1))` will be false.

> 7 Requires: The ranges `[first,middle)` and `[middle,last)` shall be sorted with respect to `operator<` or `comp`. The type of `*first` shall satisfy the `Swappable` requirements (37), the `MoveConstructible` requirements (Table 33), and the the `MoveAssignable` requirements (Table 35).

### 781. `std::complex` should add missing C99 functions

Section: 26.4.7 [complex.value.ops] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-26 Last modified: 2015-04-08

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Discussion:

A comparision of the N2461 header `<complex>` synopsis ([complex.syn]) with the C99 standard (ISO 9899, 2nd edition and the two corrigenda) show some complex functions that are missing in C++. These are:

1. 7.3.9.4: (required elements of the C99 library)
   The `cproj` functions
2. 7.26.1: (optional elements of the C99 library)

       cerf    cerfc    cexp2
       cexpm1  clog10   clog1p
       clog2   clgamma  ctgamma

I propose that at least the required `cproj` overloads are provided as equivalent C++ functions. This addition is easy to do in one sentence (delegation to C99 function).

Please note also that the current entry `polar` in 26.4.9 [cmplx.over] p. 1 should be removed from the mentioned overload list. It does not make sense to require that a function already expecting scalar arguments should cast these arguments into corresponding `complex<T>` arguments, which are not accepted by this function.

Proposed resolution:

In 26.4.1 [complex.syn] add just between the declaration of `conj` and `fabs`:

>     template<class T> complex<T> conj(const complex<T>&);
>     template<class T> complex<T> proj(const complex<T>&);
>     template<class T> complex<T> fabs(const complex<T>&);

In 26.4.7 [complex.value.ops] just after p.6 (return clause of `conj`) add:

>     template<class T> complex<T> proj(const complex<T>& x);

> > Effects: Behaves the same as C99 function `cproj`, defined in subclause 7.3.9.4."

In 26.4.9 [cmplx.over] p. 1, add one further entry `proj` to the overload list.

> The following function templates shall have additional overloads:

> >     arg           norm
> >     conj          polar proj
> >     imag          real

### 782. Extended `seed_seq` constructor is useless

Section: 26.5.7.1 [rand.util.seedseq] Status: CD1 Submitter: Daniel Krügler Opened: 2008-01-27 Last modified: 2015-04-08

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Discussion:

Part of the resolution of n2423, issue 8 was the proposal to extend the `seed_seq` constructor accepting an input range as follows (which is now part of N2461):

>     template<class InputIterator,
>     size_t u = numeric_limits<iterator_traits<InputIterator>::value_type>::digits>
>     seed_seq(InputIterator begin, InputIterator end);

First, the expression `iterator_traits<InputIterator>::value_type` is invalid due to missing `typename` keyword, which is easy to fix.

Second (and worse), while the language now supports default template arguments of function templates, this customization point via the second `size_t` template parameter is of no advantage, because `u` can never be deduced, and worse - because it is a constructor function template - it can also never be explicitly provided (14.8.1 [temp.arg.explicit]/7).

The question arises, which advantages result from a compile-time knowledge of `u` versus a run time knowledge? If run time knowledge suffices, this parameter should be provided as normal function default argument [Resolution marked (A)], if compile-time knowledge is important, this could be done via a tagging template or more user-friendly via a standardized helper generator function (`make_seed_seq`), which allows this [Resolution marked (B)].

[ Bellevue: ]

> Fermilab does not have a strong opinion. Would prefer to go with solution A. Bill agrees that solution A is a lot simpler and does the job.

> Proposed Resolution: Accept Solution A.

Issue 803 claims to make this issue moot.

Proposed resolution:

1. In 26.5.7.1 [rand.util.seedseq]/2, class `seed_seq` synopsis replace:

   >     class seed_seq
   >     {
   >     public:
   >        ...
   >        template<class InputIterator,
   >           size_t u = numeric_limits<iterator_traits<InputIterator>::value_type>::digits>
   >               seed_seq(InputIterator begin, InputIterator end,
   >               size_t u = numeric_limits<typename iterator_traits<InputIterator>::value_type>::digits);
   >        ...
   >     };

   and do a similar replacement in the member description between p.3 and p.4.

2. In 26.5.7.1 [rand.util.seedseq]/2, class `seed_seq` synopsis and in the member description between p.3 and p.4 replace:

   >     template<class InputIterator,
   >       size_t u = numeric_limits<iterator_traits<InputIterator>::value_type>::digits>
   >           seed_seq(InputIterator begin, InputIterator end);
   >     template<class InputIterator, size_t u>
   >     seed_seq(InputIterator begin, InputIterator end, implementation-defined s);

   In 26.5.2 [rand.synopsis], header `<random>` synopsis, immediately after the class `seed_seq` declaration and in 26.5.7.1 [rand.util.seedseq]/2, immediately after the class `seed_seq` definition add:

   >     template<size_t u, class InputIterator>
   >       seed_seq make_seed_seq(InputIterator begin, InputIterator end);

   In 26.5.7.1 [rand.util.seedseq], just before p.5 insert two paragraphs:

   > The first constructor behaves as if it would provide an integral constant expression `u` of type `size_t` of value `numeric_limits<typename iterator_traits<InputIterator>::value_type>::digits`.

   > The second constructor uses an implementation-defined mechanism to provide an integral constant expression `u` of type `size_t` and is called by the function `make_seed_seq`.

   In 26.5.7.1 [rand.util.seedseq], just after the last paragraph add:

   >     template<size_t u, class InputIterator>
   >        seed_seq make_seed_seq(InputIterator begin, InputIterator end);

   > > where `u` is used to construct an object `s` of implementation-defined type.

   > > Returns: `seed_seq(begin, end, s)`;

### 783. `thread::id` reuse

Section: 30.3.1.1 [thread.thread.id] Status: CD1 Submitter: Hans Boehm Opened: 2008-02-01 Last modified: 2015-04-08

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Discussion:

The current working paper (N2497, integrated just before Bellevue) is not completely clear whether a given `thread::id` value may be reused once a thread has exited and has been joined or detached. Posix allows thread ids (`pthread_t` values) to be reused in this case. Although it is not completely clear whether this originally was the right decision, it is clearly the established practice, and we believe it was always the intent of the C++ threads API to follow Posix and allow this. Howard Hinnant's example implementation implicitly relies on allowing reuse of ids, since it uses Posix thread ids directly.

It is important to be clear on this point, since it the reuse of thread ids often requires extra care in client code, which would not be necessary if thread ids were unique across all time. For example, a hash table indexed by thread id may have to be careful not to associate data values from an old thread with a new one that happens to reuse the id. Simply removing the old entry after joining a thread may not be sufficient, if it creates a visible window between the join and removal during which a new thread with the same id could have been created and added to the table.

[ post Bellevue Peter adds: ]

> There is a real issue with `thread::id` reuse, but I urge the LWG to reconsider fixing this by disallowing reuse, rather than explicitly allowing it. Dealing with thread id reuse is an incredibly painful exercise that would just force the world to reimplement a non-conflicting `thread::id` over and over.

> In addition, it would be nice if a `thread::id` could be manipulated atomically in a lock-free manner, as motivated by the recursive lock example:

> http://www.decadent.org.uk/pipermail/cpp-threads/2006-August/001091.html

Proposed resolution:

Add a sentence to 30.3.1.1 [thread.thread.id]/p1:

> An object of type `thread::id` provides a unique identifier for each thread of execution and a single distinct value for all thread objects that do not represent a thread of execution ([thread.threads.class]). Each thread of execution has a `thread::id` that is not equal to the `thread::id` of other threads of execution and that is not equal to the `thread::id` of `std::thread` objects that do not represent threads of execution. The library may reuse the value of a `thread::id` of a terminated thread that can no longer be joined.

### 786. Thread library timed waits, UTC and monotonic clocks

Section: 20.12 [time] Status: Resolved Submitter: Christopher Kohlhoff, Jeff Garland Opened: 2008-02-03 Last modified: 2015-04-08

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Discussion:

The draft C++0x thread library requires that the time points of type `system_time` and returned by `get_system_time()` represent Coordinated Universal Time (UTC) (section [datetime.system]). This can lead to surprising behavior when a library user performs a duration-based wait, such as `condition_variable::timed_wait()`. A complete explanation of the problem may be found in the Rationale for the Monotonic Clock section in POSIX, but in summary:

- Operations such as `condition_variable::timed_wait()` (and its POSIX equivalent, `pthread_cond_timedwait()`) are specified using absolute times to address the problem of spurious wakeups.
- The typical use of the timed wait operations is to perform a relative wait. This may be achieved by first calculating an absolute time as the sum of the current time and the desired duration. In fact, the C++0x thread library includes duration-based overloads of `condition_variable::timed_wait()` that behave as if by calling the corresponding absolute time overload with a time point value of `get_system_time() + rel_time`.
- A UTC clock may be affected by changes to the system time, such as synchronization with an external source, leap seconds, or manual changes to the clock.
- Should the clock change during a timed wait operation, the actual duration of the wait will not be the expected length. For example, a user may intend a timed wait of one second duration but, due to an adjustment of the system clock backwards by a minute, the wait instead takes 61 seconds.

POSIX solves the problem by introducing a new monotonic clock, which is unaffected by changes to the system time. When a condition variable is initialized, the user may specify whether the monotonic clock is to be used. (It is worth noting that on POSIX systems it is not possible to use `condition_variable::native_handle()` to access this facility, since the desired clock type must be specified during construction of the condition variable object.)

In the context of the C++0x thread library, there are added dimensions to the problem due to the need to support platforms other than POSIX:

- Some environments (such as embedded systems) do not have a UTC clock, but do have a monotonic clock.
- Some environments do not have a monotonic clock, but do have a UTC clock.
- The Microsoft Windows API's synchronization functions use relative timeouts based on an implied monotonic clock. A program that switches from the Windows API to the C++0x thread library will now find itself susceptible to clock changes.

One possible minimal solution:

- Strike normative references to UTC and an epoch based on 1970-01-01.
- Make the semantics of `system_time` and `get_system_time()` implementation-defined (i.e standard library implementors may choose the appropriate underlying clock based on the capabilities of the target platform).
- Add a non-normative note encouraging use of a monotonic clock.
- Remove `system_time::seconds_since_epoch()`.
- Change the constructor `explicit system_time(time_t secs, nanoseconds ns = 0)` to `explicit system_time(nanoseconds ns)`.

Proposed resolution:

Rationale:

Addressed by N2661: A Foundation to Sleep On.

### 787. complexity of `binary_search`

Section: 25.4.3.4 [binary.search] Status: CD1 Submitter: Daniel Krügler Opened: 2007-09-08 Last modified: 2015-04-08

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Discussion:

In 25.4.3.4 [binary.search] p. 3 the complexity of `binary_search` is described as

> At most `log(last - first) + 2` comparisons.

This should be precised and brought in line with the nomenclature used for `lower_bound`, `upper_bound`, and `equal_range`.

All existing libraries I'm aware of, delegate to `lower_bound` (+ one further comparison). Since issue 384 has now WP status, the resolution of #787 should be brought in-line with 384 by changing the `+ 2` to `+ O(1)`.

[ Sophia Antipolis: ]

> Alisdair prefers to apply an upper bound instead of O(1), but that would require fixing for `lower_bound`, `upper_bound` etc. as well. If he really cares about it, he'll send an issue to Howard.

Proposed resolution:

Change 25.4.3.4 [binary.search]/3

> Complexity: At most `log2(last - first) + 2 O(1)` comparisons.

### 788. Ambiguity in [istream.iterator]

Section: 24.6.1 [istream.iterator] Status: C++11 Submitter: Martin Sebor Opened: 2008-02-06 Last modified: 2015-04-08

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Discussion:

Addresses UK 287

> It is not clear what the initial state of an `istream_iterator` should be. Is _value_ initialized by reading the stream, or default/value initialized? If it is initialized by reading the stream, what happens if the initialization is deferred until first dereference, when ideally the iterator value should have been that of an end-of-stream iterator which is not safely dereferencable?

> Recommendation: Specify _value_ is initialized by reading the stream, or the iterator takes on the end-of-stream value if the stream is empty.

The description of how an istream_iterator object becomes an end-of-stream iterator is a) ambiguous and b) out of date WRT issue 468:

> `istream_iterator` reads (using `operator>>`) successive elements from the input stream for which it was constructed. After it is constructed, and every time `++` is used, the iterator reads and stores a value of `T`. If the end of stream is reached (`operator void*()` on the stream returns `false`), the iterator becomes equal to the end-of-stream iterator value. The constructor with no arguments `istream_iterator()` always constructs an end of stream input iterator object, which is the only legitimate iterator to be used for the end condition. The result of `operator*` on an end of stream is not defined. For any other iterator value a `const T&` is returned. The result of `operator->` on an end of stream is not defined. For any other iterator value a `const T*` is returned. It is impossible to store things into istream iterators. The main peculiarity of the istream iterators is the fact that `++` operators are not equality preserving, that is, `i == j` does not guarantee at all that `++i == ++j`. Every time `++` is used a new value is read.

`istream::operator void*()` returns null if `istream::fail()` is `true`, otherwise non-null. `istream::fail()` returns `true` if `failbit` or `badbit` is set in `rdstate()`. Reaching the end of stream doesn't necessarily imply that `failbit` or `badbit` is set (e.g., after extracting an `int` from `stringstream("123")` the stream object will have reached the end of stream but `fail()` is `false` and `operator void*()` will return a non-null value).

Also I would prefer to be explicit about calling `fail()` here (there is no `operator void*()` anymore.)

[ Summit: ]

> Moved from Ready to Open for the purposes of using this issue to address NB UK 287. Martin to handle.

[ 2009-07 Frankfurt: ]

> This improves the wording.

> Move to Ready.

Proposed resolution:

Change 24.6.1 [istream.iterator]/1:

> `istream_iterator` reads (using `operator>>`) successive elements from the input stream for which it was constructed. After it is constructed, and every time `++` is used, the iterator reads and stores a value of `T`. If the end of stream is reached the iterator fails to read and store a value of `T` (`operator void*() fail()` on the stream returns `false true`), the iterator becomes equal to the end-of-stream iterator value. The constructor with no arguments `istream_iterator()` always constructs an end of stream input iterator object, which is the only legitimate iterator to be used for the end condition. The result of `operator*` on an end of stream is not defined. For any other iterator value a `const T&` is returned. The result of `operator->` on an end of stream is not defined. For any other iterator value a `const T*` is returned. It is impossible to store things into istream iterators. The main peculiarity of the istream iterators is the fact that `++` operators are not equality preserving, that is, `i == j` does not guarantee at all that `++i == ++j`. Every time `++` is used a new value is read.

### 789. `xor_combine_engine(result_type)` should be explicit

Section: X [rand.adapt.xor] Status: CD1 Submitter: P.J. Plauger Opened: 2008-02-09 Last modified: 2015-04-08

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Discussion:

`xor_combine_engine(result_type)` should be `explicit`. (Obvious oversight.)

[ Bellevue: ]

> Non-controversial. Bill is right, but Fermilab believes that this is easy to use badly and hard to use right, and so it should be removed entirely. Got into TR1 by well defined route, do we have permission to remove stuff? Should probably check with Jens, as it is believed he is the originator. Broad consensus that this is not a robust engine adapter.

Proposed resolution:

Remove xor_combine_engine from synopsis of 26.5.2 [rand.synopsis].

Remove X [rand.adapt.xor] `xor_combine_engine`.

### 792. `piecewise_constant_distribution` is undefined for a range with just one endpoint

Section: 26.5.8.6.2 [rand.dist.samp.pconst] Status: CD1 Submitter: P.J. Plauger Opened: 2008-02-09 Last modified: 2015-04-08

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Discussion:

`piecewise_constant_distribution` is undefined for a range with just one endpoint. (Probably should be the same as an empty range.)

Proposed resolution:

Change 26.5.8.6.2 [rand.dist.samp.pconst] paragraph 3b:

> b) If `firstB == lastB` or the sequence `w` has the length zero,

### 793. `discrete_distribution` missing constructor

Section: 26.5.8.6.1 [rand.dist.samp.discrete] Status: Resolved Submitter: P.J. Plauger Opened: 2008-02-09 Last modified: 2015-04-08

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Discussion:

`discrete_distribution` should have a constructor like:

>     template<class _Fn>
>       discrete_distribution(result_type _Count, double _Low, double _High,
>                             _Fn& _Func);

(Makes it easier to fill a histogram with function values over a range.)

[ Bellevue: ]

> How do you specify the function so that it does not return negative values? If you do it is a bad construction. This requirement is already there. Where in each bin does one evaluate the function? In the middle. Need to revisit tomorrow.

[ Sophia Antipolis: ]

> Bill is not requesting this.

> Marc Paterno: `_Fn` cannot return negative values at the points where the function is sampled. It is sampled in the middle of each bin. `_Fn` cannot return 0 everywhere it is sampled.

> Jens: lambda expressions are rvalues

> Add a library issue to provide an `initializer_list<double>` constructor for `discrete_distribution`.

> Marc Paterno: dislikes reference for `_Fn` parameter. Make it pass-by-value (to use lambda), use `std::ref` to wrap giant-state function objects.

> Daniel: See `random_shuffle`, pass-by-rvalue-reference.

> Daniel to draft wording.

[ Pre San Francisco, Daniel provided wording: ]

> The here proposed changes of the WP refer to the current state of N2691. During the Sophia Antipolis meeting two different proposals came up regarding the functor argument type, either by value or by rvalue-reference. For consistence with existing conventions (state-free algorithms and the `general_pdf_distribution` c'tor signature) the author decided to propose a function argument that is provided by value. If severe concerns exists that stateful functions would be of dominant relevance, it should be possible to replace the two occurrences of `Func` by `Func&&` in this proposal as part of an editorial process.

Proposed resolution:

Non-concept version of the proposed resolution

1. In 26.5.8.6.1 [rand.dist.samp.discrete]/1, class `discrete_distribution`, just before the member declaration

   >     explicit discrete_distribution(const param_type& parm);

   insert:

   >     template<typename Func>
   >     discrete_distribution(result_type nf, double xmin, double xmax, Func fw);

2. Between p.4 and p.5 insert a series of new paragraphs as part of the new member description::

   >     template<typename Func>
   >     discrete_distribution(result_type nf, double xmin, double xmax, Func fw);

   > Complexity: Exactly nf invocations of fw.

   > Requires:

   > 1. fw shall be callable with one argument of type double, and shall return values of a type convertible to double;
   > 2. If nf > 0, the relation `xmin` < `xmax` shall hold, and for all sample values `xk`, fw(`xk`) shall return a weight value `wk` that is non-negative, non-NaN, and non-infinity;
   > 3. The following relations shall hold: nf ≥ 0, and 0 < S = `w0`+. . .+`wn-1`.

   > Effects:

   > 1. If nf == 0, sets n = 1 and lets the sequence w have length n = 1 and consist of the single value `w0` = 1.

   > 2. Otherwise, sets n = nf, deltax = (`xmax` - `xmin`)/n and `xcent` = `xmin` + 0.5 * deltax.

   >    For each k = 0, . . . ,n-1, calculates:

   >    `xk` = `xcent` + k * deltax `wk` = fw(`xk`)

   > 3. Constructs a discrete_distribution object with probabilities:

   >    > `pk` = `wk`/S for k = 0, . . . , n-1.

Concept version of the proposed resolution

1. In 26.5.8.6.1 [rand.dist.samp.discrete]/1, class `discrete_distribution`, just before the member declaration

   >     explicit discrete_distribution(const param_type& parm);

   insert:

   >     template<Callable<auto, double> Func>
   >      requires Convertible<Func::result_type, double>
   >     discrete_distribution(result_type nf, double xmin, double xmax, Func fw);

2. Between p.4 and p.5 insert a series of new paragraphs as part of the new member description::

   >     template<Callable<auto, double> Func>
   >      requires Convertible<Func::result_type, double>
   >     discrete_distribution(result_type nf, double xmin, double xmax, Func fw);

   > Complexity: Exactly nf invocations of fw.

   > Requires:

   > 1. If nf > 0, the relation `xmin` < `xmax` shall hold, and for all sample values `xk`, fw(`xk`) shall return a weight value `wk` that is non-negative, non-NaN, and non-infinity;
   > 2. The following relations shall hold: nf ≥ 0, and 0 < S = `w0`+. . .+`wn-1`.

   > Effects:

   > 1. If nf == 0, sets n = 1 and lets the sequence w have length n = 1 and consist of the single value `w0` = 1.

   > 2. Otherwise, sets n = nf, deltax = (`xmax` - `xmin`)/n and `xcent` = `xmin` + 0.5 * deltax.

   >    For each k = 0, . . . ,n-1, calculates:

   >    `xk` = `xcent` + k * deltax `wk` = fw(`xk`)

   > 3. Constructs a discrete_distribution object with probabilities:

   >    > `pk` = `wk`/S for k = 0, . . . , n-1.

Rationale:

Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".

### 794. `piecewise_constant_distribution` missing constructor

Section: 26.5.8.6.2 [rand.dist.samp.pconst] Status: Resolved Submitter: P.J. Plauger Opened: 2008-02-09 Last modified: 2015-04-08

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Discussion:

`piecewise_constant_distribution` should have a constructor like:

>     template<class _Fn>
>        piecewise_constant_distribution(size_t _Count,
>                 _Ty _Low, _Ty _High, _Fn& _Func);

(Makes it easier to fill a histogram with function values over a range. The two (reference 793) make a sensible replacement for `general_pdf_distribution`.)

[ Sophia Antipolis: ]

> Marc: uses variable width of bins and weight for each bin. This is not giving enough flexibility to control both variables.

> Add a library issue to provide an constructor taking an `initializer_list<double>` and `_Fn` for `piecewise_constant_distribution`.

> Daniel to draft wording.

[ Pre San Francisco, Daniel provided wording. ]

> The here proposed changes of the WP refer to the current state of N2691. For reasons explained in 793, the author decided to propose a function argument that is provided by value. The issue proposes a c'tor signature, that does not take advantage of the full flexibility of `piecewise_constant_distribution`, because it restricts on a constant bin width, but the use-case seems to be popular enough to justify it's introduction.

Proposed resolution:

Non-concept version of the proposed resolution

1. In 26.5.8.6.2 [rand.dist.samp.pconst]/1, class `piecewise_constant_distribution`, just before the member declaration

   >     explicit piecewise_constant_distribution(const param_type& parm);

   insert:

   >     template<typename Func>
   >     piecewise_constant_distribution(size_t nf, RealType xmin, RealType xmax, Func fw);

2. Between p.4 and p.5 insert a new sequence of paragraphs nominated below as [p5_1], [p5_2], [p5_3], and [p5_4] as part of the new member description:

   >     template<typename Func>
   >     piecewise_constant_distribution(size_t nf, RealType xmin, RealType xmax, Func fw);

   > > [p5_1] Complexity: Exactly `nf` invocations of `fw`.

   > > [p5_2] Requires:

   > > 1. `fw` shall be callable with one argument of type `RealType`, and shall return values of a type convertible to double;
   > > 2. For all sample values `xk` defined below, fw(`xk`) shall return a weight value `wk` that is non-negative, non-NaN, and non-infinity;
   > > 3. The following relations shall hold: `xmin` < `xmax`, and 0 < S = `w0`+. . .+`wn-1`.

   > > [p5_3] Effects:

   > > 1. If nf == 0,

   > >    1. sets deltax = `xmax` - `xmin`, and
   > >    2. lets the sequence `w` have length `n = 1` and consist of the single value `w0` = 1, and
   > >    3. lets the sequence `b` have length `n+1` with `b0` = `xmin` and `b1` = `xmax`

   > > 2. Otherwise,

   > >    1. sets `n = nf`, `deltax =` (`xmax` - `xmin`)/n, `xcent` = `xmin` + 0.5 * deltax, and

   > >    2. lets the sequences `w` and `b` have length `n` and `n+1`, resp. and

   > >       for each k = 0, . . . ,n-1, calculates:

   > >       `dxk` = k * deltax `bk` = `xmin` + `dxk` `xk` = `xcent` + `dxk` `wk` = fw(`xk`),

   > >       and

   > >    3. sets `bn` = `xmax`

   > > 3. Constructs a `piecewise_constant_distribution` object with the above computed sequence `b` as the interval boundaries and with the probability densities:

   > >    > `ρk` = `wk`/(S * deltax) for k = 0, . . . , n-1.

   > > [p5_4] [Note: In this context, the subintervals [`bk`, `bk+1`) are commonly known as the bins of a histogram. -- end note]

Concept version of the proposed resolution

1. In 26.5.8.6.2 [rand.dist.samp.pconst]/1, class `piecewise_constant_distribution`, just before the member declaration

   >     explicit piecewise_constant_distribution(const param_type& parm);

   insert:

   >     template<Callable<auto, RealType> Func>
   >      requires Convertible<Func::result_type, double>
   >     piecewise_constant_distribution(size_t nf, RealType xmin, RealType xmax, Func fw);

2. Between p.4 and p.5 insert a new sequence of paragraphs nominated below as [p5_1], [p5_2], [p5_3], and [p5_4] as part of the new member description:

   >     template<Callable<auto, RealType> Func>
   >      requires Convertible<Func::result_type, double>
   >     piecewise_constant_distribution(size_t nf, RealType xmin, RealType xmax, Func fw);

   > > [p5_1] Complexity: Exactly `nf` invocations of `fw`.

   > > [p5_2] Requires:

   > > 1. For all sample values `xk` defined below, fw(`xk`) shall return a weight value `wk` that is non-negative, non-NaN, and non-infinity;
   > > 2. The following relations shall hold: `xmin` < `xmax`, and 0 < S = `w0`+. . .+`wn-1`.

   > > [p5_3] Effects:

   > > 1. If nf == 0,

   > >    1. sets deltax = `xmax` - `xmin`, and
   > >    2. lets the sequence `w` have length `n = 1` and consist of the single value `w0` = 1, and
   > >    3. lets the sequence `b` have length `n+1` with `b0` = `xmin` and `b1` = `xmax`

   > > 2. Otherwise,

   > >    1. sets `n = nf`, `deltax =` (`xmax` - `xmin`)/n, `xcent` = `xmin` + 0.5 * deltax, and

   > >    2. lets the sequences `w` and `b` have length `n` and `n+1`, resp. and

   > >       for each k = 0, . . . ,n-1, calculates: `dxk` = k * deltax `bk` = `xmin` + `dxk` `xk` = `xcent` + `dxk` `wk` = fw(`xk`),

   > >       and

   > >    3. sets `bn` = `xmax`

   > > 3. Constructs a `piecewise_constant_distribution` object with the above computed sequence `b` as the interval boundaries and with the probability densities:

   > >    > `ρk` = `wk`/(S * deltax) for k = 0, . . . , n-1.

   > > [p5_4] [Note: In this context, the subintervals [`bk`, `bk+1`) are commonly known as the bins of a histogram. -- end note]

Rationale:

Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".

### 798. Refactoring of binders lead to interface breakage

Section: X [depr.lib.binders] Status: CD1 Submitter: Daniel Krügler Opened: 2008-02-14 Last modified: 2015-04-08

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Discussion:

N2521 and its earlier predecessors have moved the old binders from [lib.binders] to X [depr.lib.binders] thereby introducing some renaming of the template parameter names (`Operation -> Fn`). During this renaming process the protected data member `op` was also renamed to `fn`, which seems as an unnecessary interface breakage to me - even if this user access point is probably rarely used.

Proposed resolution:

Change [depr.lib.binder.1st]:

>     template <class Fn>
>     class binder1st
>       : public unary_function<typename Fn::second_argument_type,
>                               typename Fn::result_type> {
>     protected:
>       Fn fn op;
>       typename Fn::first_argument_type value;
>     public:
>       binder1st(const Fn& x,
>                 const typename Fn::first_argument_type& y);
>       typename Fn::result_type
>         operator()(const typename Fn::second_argument_type& x) const;
>       typename Fn::result_type
>         operator()(typename Fn::second_argument_type& x) const;
>     };

> > -1- The constructor initializes `fn` `op` with `x` and `value` with `y`.

> > -2- `operator()` returns `fnop(value,x)`.

Change [depr.lib.binder.2nd]:

>     template <class Fn>
>     class binder2nd
>       : public unary_function<typename Fn::first_argument_type,
>                               typename Fn::result_type> {
>     protected:
>       Fn fn op;
>       typename Fn::second_argument_type value;
>     public:
>       binder2nd(const Fn& x,
>                 const typename Fn::second_argument_type& y);
>       typename Fn::result_type
>         operator()(const typename Fn::first_argument_type& x) const;
>       typename Fn::result_type
>         operator()(typename Fn::first_argument_type& x) const;
>     };

> > -1- The constructor initializes `fn` `op` with `x` and `value` with `y`.

> > -2- `operator()` returns `fnop(value,x)`.

### 800. Issues in 26.4.7.1 [rand.util.seedseq](6)

Section: 26.5.7.1 [rand.util.seedseq] Status: Resolved Submitter: Stephan Tolksdorf Opened: 2008-02-18 Last modified: 2015-04-08

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Discussion:

The for-loop in the algorithm specification has `n` iterations, where `n` is defined to be `end - begin`, i.e. the number of supplied w-bit quantities. Previous versions of this algorithm and the general logic behind it suggest that this is an oversight and that in the context of the for-loop `n` should be the number of full 32-bit quantities in `b` (rounded upwards). If `w` is 64, the current algorithm throws away half of all bits in `b`. If `w` is 16, the current algorithm sets half of all elements in `v` to 0.

There are two more minor issues:

- Strictly speaking `end - begin` is not defined since `InputIterator` is not required to be a random access iterator.
- Currently all integral types are allowed as input to the `seed_seq` constructor, including `bool`. IMHO allowing `bool`s unnecessarily complicates the implementation without any real benefit to the user. I'd suggest to exclude `bool`s as input.

[ Bellevue: ]

> Move to Open: Bill will try to propose a resolution by the next meeting.

[ post Bellevue: Bill provided wording. ]

This issue is made moot if 803 is accepted.

Proposed resolution:

Replace 26.5.7.1 [rand.util.seedseq] paragraph 6 with:

> Effects: Constructs a `seed_seq` object by effectively concatenating the low-order `u` bits of each of the elements of the supplied sequence `[begin, end)` in ascending order of significance to make a (possibly very large) unsigned binary number `b` having a total of `n` bits, and then carrying out the following algorithm:

> > for( v.clear(); n > 0; n -= 32 ) v.push_back(b mod 232), b /= 232;

Rationale:

Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".

### 801. `tuple` and `pair` trivial members

Section: 20.4 [tuple] Status: Resolved Submitter: Lawrence Crowl Opened: 2008-02-18 Last modified: 2015-04-08

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Discussion:

Classes with trivial special member functions are inherently more efficient than classes without such functions. This efficiency is particularly pronounced on modern ABIs that can pass small classes in registers. Examples include value classes such as complex numbers and floating-point intervals. Perhaps more important, though, are classes that are simple collections, like `pair` and `tuple`. When the parameter types of these classes are trivial, the `pair`s and `tuple`s themselves can be trivial, leading to substantial performance wins.

The current working draft make specification of trivial functions (where possible) much easer through `default`ed and `delete`d functions. As long as the semantics of defaulted and deleted functions match the intended semantics, specification of defaulted and deleted functions will yield more efficient programs.

There are at least two cases where specification of an explicitly defaulted function may be desirable.

First, the `std::pair` template has a non-trivial default constructor, which prevents static initialization of the pair even when the types are statically initializable. Changing the definition to

>     pair() = default;

would enable such initialization. Unfortunately, the change is not semantically neutral in that the current definition effectively forces value initialization whereas the change would not value initialize in some contexts.

** Does the committee confirm that forced value initialization was the intent? If not, does the committee wish to change the behavior of `std::pair` in C++0x?

Second, the same default constructor issue applies to `std::tuple`. Furthermore, the `tuple` copy constructor is current non-trivial, which effectively prevents passing it in registers. To enable passing `tuples` in registers, the copy constructor should be make explicitly `default`ed. The new declarations are:

>     tuple() = default;
>     tuple(const tuple&) = default;

This changes is not implementation neutral. In particular, it prevents implementations based on pointers to the parameter types. It does however, permit implementations using the parameter types as bases.

** How does the committee wish to trade implementation efficiency versus implementation flexibility?

[ Bellevue: ]

> General agreement; the first half of the issue is NAD.

> Before voting on the second half, it was agreed that a "Strongly Favor" vote meant support for trivial tuples (assuming usual requirements met), even at the expense of other desired qualities. A "Weakly Favor" vote meant support only if not at the expense of other desired qualities.

> Concensus: Go forward, but not at expense of other desired qualities.

> It was agreed to Alisdair should fold this work in with his other pair/tuple action items, above, and that issue 801 should be "open", but tabled until Alisdair's proposals are disposed of.

[ 2009-05-27 Daniel adds: ]

> This is partly solved by 1117.

[ 2009-07 Frankfurt: ]

> Wait for dust to settle from fixing exception safety problem with rvalue refs.

[ 2009-07-20 Alisdair adds: ]

> Basically, this issue is what should we do with the default constructor for pairs and tuples of trivial types. The motivation of the issue was to force static initialization rather than dynamic initialization, and was rejected in the case of pair as it would change the meaning of existing programs. The advice was "do the best we can" for tuple without changing existing meaning.

> Frankfurt seems to simply wait and see the resolution on no-throw move constructors, which (I believe) is only tangentially related to this issue, but as good as any to defer until Santa Cruz.

> Looking again now, I think constant (static) initialization for pair can be salvaged by making the default construct constexpr. I have a clarification from Core that this is intended to work, even if the constructor is not trivial/constexpr, so long as no temporaries are implied in the process (even if elided).

[ 2009-10 Santa Cruz: ]

> Leave as open. Alisdair to provide wording.

[ 2010 Pittsburgh: ]

> We believe this may be NAD Editorial since both pair and tuple now have constexpr default constructors, but we're not sure.

[ 2010 Rapperswil: ]

> Daniel believes his pair/tuple paper will resolve this issue. `constexpr` will allow static initialization, and he is already changing the move and copy constructors to be defaulted.

[ 2010-10-24 Daniel adds: ]

> The proposed resolution of n3140 should resolve this issue.

Proposed resolution:

See n3140.

### 803. Simplification of `seed_seq::seq_seq`

Section: 26.5.7.1 [rand.util.seedseq] Status: Resolved Submitter: Charles Karney Opened: 2008-02-22 Last modified: 2015-04-08

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Discussion:

`seed_seq(InputIterator begin, InputIterator end);` constructs a `seed_seq` object repacking the bits of supplied sequence `[begin, end)` into a 32-bit vector.

This repacking triggers several problems:

1. Distinctness of the output of `seed_seq::generate` required the introduction of the initial "`if (w < 32) v.push_back(n);`" (Otherwise the unsigned short vectors [1, 0] and [1] generate the same sequence.)
2. Portability demanded the introduction of the template parameter `u`. (Otherwise some sequences could not be obtained on computers where no integer types are exactly 32-bits wide.)
3. The description and algorithm have become unduly complicated.

I propose simplifying this `seed_seq` constructor to be "32-bit only". Despite it's being simpler, there is NO loss of functionality (see below).

Here's how the description would read

> 26.5.7.1 [rand.util.seedseq] Class `seed_seq`

> >     template<class InputIterator>
> >       seed_seq(InputIterator begin, InputIterator end);

> > > 5 Requires: NO CHANGE

> > > 6 Effects: Constructs a `seed_seq` object by

> > > > for (InputIterator s = begin; s != end; ++s) v.push_back((*s) mod 232);

Discussion:

The chief virtues here are simplicity, portability, and generality.

- Simplicity — compare the above specification with the n2461 proposal.
- Portability — with `iterator_traits<InputIterator>::value_type = uint_least32_t` the user is guaranteed to get the same behavior across platforms.
- Generality — any behavior that the n2461 proposal can achieve can be obtained with this simpler proposal (albeit with a shuffling of bits in the input sequence).

Arguments (and counter-arguments) against making this change (and retaining the n2461 behavior) are:

- The user can pass an array of `unsigned char` and `seed_seq` will nicely repack it.

  Response: So what? Consider the seed string "ABC". The n2461 proposal results in

  >     v = { 0x3, 0x434241 };

  while the simplified proposal yields

  >     v = { 0x41, 0x42, 0x43 };

  The results produced by `seed_seq::generate` with the two inputs are different but nevertheless equivalently "mixed up" and this remains true even if the seed string is long.

- With long strings (e.g., with bit-length comparable to the number of bits in the state), `v` is longer (by a factor of 4) with the simplified proposal and `seed_seq::generate` will be slower.

  Response: It's unlikely that the efficiency of `seed_seq::generate` will be a big issue. If it is, the user is free to repack the seed vector before constructing `seed_seq`.

- A user can pass an array of 64-bit integers and all the bits will be used.

  Response: Indeed. However, there are many instances in the n2461 where integers are silently coerced to a narrower width and this should just be a case of the user needing to read the documentation. The user can of course get equivalent behavior by repacking his seed into 32-bit pieces. Furthermore, the unportability of the n2461 proposal with

  >     unsigned long s[] = {1, 2, 3, 4};
  >     seed_seq q(s, s+4);

  which typically results in `v = {1, 2, 3, 4}` on 32-bit machines and in `v = {1, 0, 2, 0, 3, 0, 4, 0}` on 64-bit machines is a major pitfall for unsuspecting users.

Note: this proposal renders moot issues 782 and 800.

[ Bellevue: ]

> Walter needs to ask Fermilab for guidance. Defer till tomorrow. Bill likes the proposed resolution.

[ Sophia Antipolis: ]

> Marc Paterno wants portable behavior between 32bit and 64bit machines; we've gone to significant trouble to support portability of engines and their values.

> Jens: the new algorithm looks perfectly portable

> Marc Paterno to review off-line.

> Modify the proposed resolution to read "Constructs a seed_seq object by the following algorithm ..."

> Disposition: move to review; unanimous consent.

> (moots 782 and 800)

Proposed resolution:

Change 26.5.7.1 [rand.util.seedseq]:

>     template<class InputIterator,
>       size_t u = numeric_limits<iterator_traits<InputIterator>::value_type>::digits>
>       seed_seq(InputIterator begin, InputIterator end);

> > -5- Requires: `InputIterator` shall satisfy the requirements of an input iterator (24.1.1) such that `iterator_traits<InputIterator>::value_type` shall denote an integral type.

> > -6- Constructs a `seed_seq` object by the following algorithm rearranging some or all of the bits of the supplied sequence `[begin,end)` of w-bit quantities into 32-bit units, as if by the following:

> > First extract the rightmost `u` bits from each of the `n = end - begin` elements of the supplied sequence and concatenate all the extracted bits to initialize a single (possibly very large) unsigned binary number, `b = ∑n-1i=0 (begin[i] mod 2u) · 2w·i` (in which the bits of each `begin[i]` are treated as denoting an unsigned quantity). Then carry out the following algorithm:

> > > v.clear(); if ($w$ < 32) v.push_back($n$); for( ; $n$ > 0; --$n$) v.push_back(b mod 232), b /= 232;

> > > for (InputIterator s = begin; s != end; ++s) v.push_back((*s) mod 232);

Rationale:

Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".

### 804. Some problems with classes `error_code`/`error_condition`

Section: 19.5 [syserr] Status: CD1 Submitter: Daniel Krügler Opened: 2008-02-24 Last modified: 2015-04-08

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Discussion:

1. 19.5.2.1 [syserr.errcode.overview]/1, class `error_code` and 19.5.3.1 [syserr.errcondition.overview]/, class `error_condition` synopses declare an expository data member `cat_`:

   >     const error_category& cat_; // exposition only

   which is used to define the semantics of several members. The decision to use a member of reference type lead to several problems:

   1. The classes are not `(Copy)Assignable`, which is probably not the intent.
   2. The post conditions of all modifiers from 19.5.2.3 [syserr.errcode.modifiers] and 19.5.3.3 [syserr.errcondition.modifiers], resp., cannot be fulfilled.

   The simple fix would be to replace the reference by a pointer member.

2. I would like to give the editorial remark that in both classes the constrained `operator=` overload (template with `ErrorCodeEnum` argument) makes in invalid usage of `std::enable_if`: By using the default value for the second `enable_if` parameter the return type would be defined to be `void&` even in otherwise valid circumstances - this return type must be explicitly provided (In `error_condition` the first declaration uses an explicit value, but of wrong type).
3. The member function `message` throws clauses ( 19.5.1.2 [syserr.errcat.virtuals]/10, 19.5.2.4 [syserr.errcode.observers]/8, and 19.5.3.4 [syserr.errcondition.observers]/6) guarantee "throws nothing", although they return a `std::string` by value, which might throw in out-of-memory conditions (see related issue 771).

[ Sophia Antipolis: ]

> Part A: NAD (editorial), cleared by the resolution of issue 832.

> Part B: Technically correct, save for typo. Rendered moot by the concept proposal (N2620) NAD (editorial).

> Part C: We agree; this is consistent with the resolution of issue 721.

> Howard: please ping Beman, asking him to clear away parts A and B from the wording in the proposed resolution, so it is clear to the editor what needs to be applied to the working paper.

> Beman provided updated wording. Since issue 832 is not going forward, the provided wording includes resolution of part A.

Proposed resolution:

Resolution of part A:

> Change 19.5.2.1 [syserr.errcode.overview] Class error_code overview synopsis as indicated:

> >     private:
> >       int val_;                    // exposition only
> >       const error_category&* cat_; // exposition only

> Change 19.5.2.2 [syserr.errcode.constructors] Class error_code constructors as indicated:

> >     error_code();

> > > Effects: Constructs an object of type `error_code`.

> > > Postconditions: `val_ == 0` and `cat_ == &system_category`.

> > > Throws: Nothing.

> >     error_code(int val, const error_category& cat);

> > > Effects: Constructs an object of type `error_code`.

> > > Postconditions: `val_ == val` and `cat_ == &cat`.

> > > Throws: Nothing.

> Change 19.5.2.3 [syserr.errcode.modifiers] Class error_code modifiers as indicated:

> >     void assign(int val, const error_category& cat);

> > > Postconditions: `val_ == val` and `cat_ == &cat`.

> > > Throws: Nothing.

> Change 19.5.2.4 [syserr.errcode.observers] Class error_code observers as indicated:

> >     const error_category& category() const;

> > > Returns: `*cat_`.

> > > Throws: Nothing.

> Change 19.5.3.1 [syserr.errcondition.overview] Class error_condition overview synopsis as indicated:

> >     private:
> >       int val_;                    // exposition only
> >       const error_category&* cat_; // exposition only

> Change 19.5.3.2 [syserr.errcondition.constructors] Class error_condition constructors as indicated:

> [ (If the proposed resolution of issue 805 has already been applied, the name `posix_category` will have been changed to `generic_category`. That has no effect on this resolution.) ]

> >     error_condition();

> > > Effects: Constructs an object of type `error_condition`.

> > > Postconditions: `val_ == 0` and `cat_ == &posix_category`.

> > > Throws: Nothing.

> >     error_condition(int val, const error_category& cat);

> > > Effects: Constructs an object of type `error_condition`.

> > > Postconditions: `val_ == val` and `cat_ == &cat`.

> > > Throws: Nothing.

> Change 19.5.3.3 [syserr.errcondition.modifiers] Class error_condition modifiers as indicated:

> >     void assign(int val, const error_category& cat);

> > > Postconditions: `val_ == val` and `cat_ == &cat`.

> > > Throws: Nothing.

> Change 19.5.3.4 [syserr.errcondition.observers] Class error_condition observers as indicated:

> >     const error_category& category() const;

> > > Returns: `*cat_`.

> > > Throws: Nothing.

Resolution of part C:

> In 19.5.1.2 [syserr.errcat.virtuals], remove the throws clause p. 10.

> >     virtual string message(int ev) const = 0;

> > > Returns: A string that describes the error condition denoted by `ev`.

> > > Throws: Nothing.

> In 19.5.2.4 [syserr.errcode.observers], remove the throws clause p. 8.

> >     string message() const;

> > > Returns: `category().message(value())`.

> > > Throws: Nothing.

> In 19.5.3.4 [syserr.errcondition.observers], remove the throws clause p. 6.

> >     string message() const;

> > > Returns: `category().message(value())`.

> > > Throws: Nothing.

### 805. `posix_error::posix_errno` concerns

Section: 19.5 [syserr] Status: CD1 Submitter: Jens Maurer Opened: 2008-02-24 Last modified: 2015-04-08

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Discussion:

19.5 [syserr]

>     namespace posix_error {
>       enum posix_errno {
>         address_family_not_supported, // EAFNOSUPPORT
>         ...

should rather use the new scoped-enum facility (7.2 [dcl.enum]), which would avoid the necessity for a new `posix_error` namespace, if I understand correctly.

[ Further discussion: ]

> See N2347, Strongly Typed Enums, since renamed Scoped Enums.

> Alberto Ganesh Barbati also raised this issue in private email, and also proposed the scoped-enum solution.

> Nick Stoughton asked in Bellevue that `posix_error` and `posix_errno` not be used as names. The LWG agreed.

> The wording for the Proposed resolution was provided by Beman Dawes.

Proposed resolution:

Change System error support 19.5 [syserr] as indicated:

>     namespace posix_error {
>       enum posix_errno class errc {
>         address_family_not_supported, // EAFNOSUPPORT
>         ...
>         wrong_protocol_type, // EPROTOTYPE
>       };
>     } // namespace posix_error

>     template <> struct is_error_condition_enum<posix_error::posix_errno errc>
>       : public true_type {}

>     namespace posix_error {
>       error_code make_error_code(posix_errno errc e);
>       error_condition make_error_condition(posix_errno errc e);
>     } // namespace posix_error

Change System error support 19.5 [syserr] :

> The `is_error_code_enum` and `is_error_condition_enum` templates may be specialized for user-defined types to indicate that such a type is eligible for class `error_code` and class `error_condition` automatic conversions, respectively.

Change System error support 19.5 [syserr] and its subsections:

> - remove all occurrences of `posix_error::`
> - change all instances of `posix_errno` to `errc`
> - change all instances of `posix_category` to `generic_category`
> - change all instances of `get_posix_category` to `get_generic_category`

Change Error category objects 19.5.1.5 [syserr.errcat.objects], paragraph 2:

> Remarks: The object's `default_error_condition` and equivalent virtual functions shall behave as specified for the class `error_category`. The object's name virtual function shall return a pointer to the string "POSIX" "generic".

Change 19.5.2.5 [syserr.errcode.nonmembers] Class `error_code` non-member functions as indicated:

>     error_code make_error_code(posix_errno errc e);

> > Returns: `error_code(static_cast<int>(e), posixgeneric_category)`.

Change 19.5.3.5 [syserr.errcondition.nonmembers] Class `error_condition` non-member functions as indicated:

>     error_condition make_error_condition(posix_errno errc e);

> > Returns: `error_condition(static_cast<int>(e), posixgeneric_category)`.

Rationale:

Names Considered
`portable` | Too non-specific. Did not wish to reserve such a common word in namespace std. Not quite the right meaning, either.
`portable_error` | Too long. Explicit qualification is always required for scoped enums, so a short name is desirable. Not quite the right meaning, either. May be misleading because `*_error` in the std lib is usually an exception class name.
`std_error` | Fairly short, yet explicit. But in fully qualified names like `std::std_error::not_enough_memory`, the std_ would be unfortunate. Not quite the right meaning, either. May be misleading because `*_error` in the std lib is usually an exception class name.
`generic` | Short enough. The category could be `generic_category`. Fully qualified names like `std::generic::not_enough_memory` read well. Reserving in namespace std seems dicey.
`generic_error` | Longish. The category could be `generic_category`. Fully qualified names like `std::generic_error::not_enough_memory` read well. Misleading because `*_error` in the std lib is usually an exception class name.
`generic_err` | A bit less longish. The category could be `generic_category`. Fully qualified names like `std::generic_err::not_enough_memory` read well.
`gen_err` | Shorter still. The category could be `generic_category`. Fully qualified names like `std::gen_err::not_enough_memory` read well.
`generr` | Shorter still. The category could be `generic_category`. Fully qualified names like `std::generr::not_enough_memory` read well.
`error` | Shorter still. The category could be `generic_category`. Fully qualified names like `std::error::not_enough_memory` read well. Do we want to use this general a name?

`err`

Shorter still. The category could be `generic_category`. Fully qualified names like `std::err::not_enough_memory` read well. Although alone it looks odd as a name, given the existing `errno` and `namespace std` names, it seems fairly intuitive. Problem: `err` is used throughout the standard library as an argument name and in examples as a variable name; it seems too confusing to add yet another use of the name.

`errc` | Short enough. The "c" stands for "constant". The category could be `generic_category`. Fully qualified names like `std::errc::not_enough_memory` read well. Although alone it looks odd as a name, given the existing `errno` and `namespace std` names, it seems fairly intuitive. There are no uses of `errc` in the current C++ standard.

### 806. `unique_ptr::reset` effects incorrect, too permissive

Section: 20.8.1.2.5 [unique.ptr.single.modifiers] Status: CD1 Submitter: Peter Dimov Opened: 2008-03-13 Last modified: 2015-04-08

View all other issues in [unique.ptr.single.modifiers].

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Discussion:

`void unique_ptr::reset(T* p = 0)` is currently specified as:

> Effects: If `p == get()` there are no effects. Otherwise `get_deleter()(get())`.

There are two problems with this. One, if `get() == 0` and `p != 0`, the deleter is called with a `NULL` pointer, and this is probably not what's intended (the destructor avoids calling the deleter with 0.)

Two, the special check for `get() == p` is generally not needed and such a situation usually indicates an error in the client code, which is being masked. As a data point, `boost::shared_ptr` was changed to assert on such self-resets in 2001 and there were no complaints.

One might think that self-resets are necessary for `operator=` to work; it's specified to perform

>     reset( u.release() );

and the self-assignment

>     p = move(p);

might appear to result in a self-reset. But it doesn't; the `release()` is performed first, zeroing the stored pointer. In other words, `p.reset( q.release() )` works even when `p` and `q` are the same `unique_ptr`, and there is no need to special-case `p.reset( q.get() )` to work in a similar scenario, as it definitely doesn't when `p` and `q` are separate.

Proposed resolution:

Change 20.8.1.2.5 [unique.ptr.single.modifiers]:

>     void reset(T* p = 0);

> > -4- Effects: If `p == get() == 0` there are no effects. Otherwise `get_deleter()(get())`.

Change 20.8.1.3.4 [unique.ptr.runtime.modifiers]:

>     void reset(T* p = 0);

> > ...

> > -2- Effects: If `p == get() == 0` there are no effects. Otherwise `get_deleter()(get())`.

### 807. `tuple` construction should not fail unless its element's construction fails

Section: 20.4.2.1 [tuple.cnstr] Status: CD1 Submitter: Howard Hinnant Opened: 2008-03-13 Last modified: 2015-04-08

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Discussion:

527 Added a throws clause to `bind` constructors. I believe the same throws clause should be added to `tuple` except it ought to take into account move constructors as well.

Proposed resolution:

Add to 20.4.2.1 [tuple.cnstr]:

> For each `tuple` constructor and assignment operator, an exception is thrown only if the construction or assignment of one of the types in `Types` throws an exception.

### 808. [forward] incorrect redundant specification

Section: 20.2.4 [forward] Status: CD1 Submitter: Jens Maurer Opened: 2008-03-13 Last modified: 2015-04-08

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Discussion:

p4 (forward) says:

> Return type: If `T` is an lvalue-reference type, an lvalue; otherwise, an rvalue.

First of all, lvalue-ness and rvalue-ness are properties of an expression, not of a type (see 3.10 [basic.lval]). Thus, the phrasing "Return type" is wrong. Second, the phrase says exactly what the core language wording says for folding references in 14.3.1 [temp.arg.type]/p4 and for function return values in 5.2.2 [expr.call]/p10. (If we feel the wording should be retained, it should at most be a note with cross-references to those sections.)

The prose after the example talks about "forwarding as an `int&` (an lvalue)" etc. In my opinion, this is a category error: "`int&`" is a type, "lvalue" is a property of an expression, orthogonal to its type. (Btw, expressions cannot have reference type, ever.)

Similar with move:

> Return type: an rvalue.

is just wrong and also redundant.

Proposed resolution:

Change 20.2.4 [forward] as indicated:

>     template <class T> T&& forward(typename identity<T>::type&& t);

> > ...

> > Return type: If `T` is an lvalue-reference type, an lvalue; otherwise, an rvalue.

> > ...

> > -7- In the first call to `factory`, `A1` is deduced as `int`, so 2 is forwarded to `A`'s constructor as an `int&&` (an rvalue). In the second call to factory, `A1` is deduced as `int&`, so `i` is forwarded to `A`'s constructor as an `int&` (an lvalue). In both cases, `A2` is deduced as double, so 1.414 is forwarded to `A`'s constructor as `double&&` (an rvalue).

>     template <class T> typename remove_reference<T>::type&& move(T&& t);

> > ...

> > Return type: an rvalue.

### 809. `std::swap` should be overloaded for array types

Section: 25.3.3 [alg.swap] Status: CD1 Submitter: Niels Dekker Opened: 2008-02-28 Last modified: 2015-04-08

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Discussion:

For the sake of generic programming, the header `<algorithm>` should provide an overload of `std::swap` for array types:

    template<class T, size_t N> void swap(T (&a)[N], T (&b)[N]);

It became apparent to me that this overload is missing, when I considered how to write a swap function for a generic wrapper class template. (Actually I was thinking of Boost's value_initialized.) Please look at the following template, `W`, and suppose that is intended to be a very generic wrapper:

    template<class T> class W {
    public:
      T data;
    };

Clearly `W<T>` is CopyConstructible and CopyAssignable, and therefore Swappable, whenever `T` is CopyConstructible and CopyAssignable. Moreover, `W<T>` is also Swappable when `T` is an array type whose element type is CopyConstructible and CopyAssignable. Still it is recommended to add a custom swap function template to such a class template, for the sake of efficiency and exception safety. (E.g., Scott Meyers, Effective C++, Third Edition, item 25: Consider support for a non-throwing swap.) This function template is typically written as follows:

    template<class T> void swap(W<T>& x, W<T>& y) {
      using std::swap;
      swap(x.data, y.data);
    }

Unfortunately, this will introduce an undesirable inconsistency, when `T` is an array. For instance, `W<std::string[8]>` is Swappable, but the current Standard does not allow calling the custom swap function that was especially written for `W`!

    W<std::string[8]> w1, w2;  // Two objects of a Swappable type.
    std::swap(w1, w2);  // Well-defined, but inefficient.
    using std::swap;
    swap(w1, w2);  // Ill-formed, just because ADL finds W's swap function!!!

`W`'s `swap` function would try to call `std::swap` for an array, `std::string[8]`, which is not supported by the Standard Library. This issue is easily solved by providing an overload of `std::swap` for array types. This swap function should be implemented in terms of swapping the elements of the arrays, so that it would be non-throwing for arrays whose element types have a non-throwing swap.

Note that such an overload of `std::swap` should also support multi-dimensional arrays. Fortunately that isn't really an issue, because it would do so automatically, by means of recursion.

For your information, there was a discussion on this issue at comp.lang.c++.moderated: [Standard Library] Shouldn't std::swap be overloaded for C-style arrays?

Proposed resolution:

Add an extra condition to the definition of Swappable requirements [swappable] in 17.6.3.1 [utility.arg.requirements]:

> - `T` is `Swappable` if `T` is an array type whose element type is `Swappable`.

Add the following to 25.3.3 [alg.swap]:

>     template<class T, size_t N> void swap(T (&a)[N], T (&b)[N]);

> > Requires: Type `T` shall be `Swappable`.

> > Effects: `swap_ranges(a, a + N, b);`

### 810. Missing traits dependencies in operational semantics of extended manipulators

Section: 27.7.5 [ext.manip] Status: C++11 Submitter: Daniel Krügler Opened: 2008-03-01 Last modified: 2015-04-08

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Discussion:

The recent draft (as well as the original proposal n2072) uses an operational semantic for `get_money` ([ext.manip]/4) and `put_money` ([ext.manip]/6), which uses

>     istreambuf_iterator<charT>

and

>     ostreambuf_iterator<charT>

resp, instead of the iterator instances, with explicitly provided traits argument (The operational semantic defined by `f` is also traits dependent). This is an obvious oversight because both `*stream_buf` c'tors expect a `basic_streambuf<charT,traits>` as argument.

The same problem occurs within the `get_time` and `put_time` semantic where additional to the problem we have an editorial issue in `get_time` (`streambuf_iterator` instead of `istreambuf_iterator`).

[ Batavia (2009-05): ]

> This appears to be an issue of presentation.

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

In 27.7.5 [ext.manip]/4 within function `f` replace the first line

>     template <class charT, class traits, class moneyT>
>     void f(basic_ios<charT, traits>& str, moneyT& mon, bool intl) {
>        typedef istreambuf_iterator<charT, traits> Iter;
>        ...

In 27.7.5 [ext.manip]/5 remove the first template `charT` parameter:

>     template <class charT, class moneyT> unspecified put_money(const moneyT& mon, bool intl = false);

In 27.7.5 [ext.manip]/6 within function `f` replace the first line

>     template <class charT, class traits, class moneyT>
>     void f(basic_ios<charT, traits>& str, const moneyT& mon, bool intl) {
>       typedef ostreambuf_iterator<charT, traits> Iter;
>       ...

In 27.7.5 [ext.manip]/8 within function `f` replace the first line

>     template <class charT, class traits>
>     void f(basic_ios<charT, traits>& str, struct tm *tmb, const charT *fmt) {
>       typedef istreambuf_iterator<charT, traits> Iter;
>       ...

In 27.7.5 [ext.manip]/10 within function `f` replace the first line

>     template <class charT, class traits>
>     void f(basic_ios<charT, traits>& str, const struct tm *tmb, const charT *fmt) {
>       typedef ostreambuf_iterator<charT, traits> Iter;
>       ...

In 27.7 [iostream.format], Header `<iomanip>` synopsis change:

>     template <class charT, class moneyT> T8 put_money(const moneyT& mon, bool intl = false);

### 811. `pair` of pointers no longer works with literal 0

Section: 20.3 [pairs] Status: C++11 Submitter: Doug Gregor Opened: 2008-03-14 Last modified: 2015-04-08

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Discussion:

>     #include <utility>

>     int main()
>     {
>        std::pair<char *, char *> p (0,0);
>     }

I just got a bug report about that, because it's valid C++03, but not C++0x. The important realization, for me, is that the emplace proposal---which made `push_back` variadic, causing the `push_back(0)` issue---didn't cause this break in backward compatibility. The break actually happened when we added this pair constructor as part of adding rvalue references into the language, long before variadic templates or emplace came along:

>     template<class U, class V> pair(U&& x, V&& y);

Now, concepts will address this issue by constraining that `pair` constructor to only `U`'s and `V`'s that can properly construct "first" and "second", e.g. (from N2322):

>     template<class U , class V >
>     requires Constructible<T1, U&&> && Constructible<T2, V&&>
>     pair(U&& x , V&& y );

[ San Francisco: ]

> Suggested to resolve using pass-by-value for that case.

> Side question: Should pair interoperate with tuples? Can construct a tuple of a pair, but not a pair from a two-element tuple.

> Related to 885.

[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]

[ 2009-10 Santa Cruz: ]

> Leave as open. Howard to provide wording.

[ 2010-02-06 Howard provided wording. ]

[ 2010-02-09 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

Rationale:

[ San Francisco: ]

> Solved by N2770.

[ The rationale is obsolete. ]

Proposed resolution:

Add a paragraph to 20.3 [pairs]:

>     template<class U, class V> pair(U&& x, V&& y);

> > 6 Effects: The constructor initializes `first` with `std::forward<U>(x)` and second with `std::forward<V>(y)`.

> > Remarks: `U` shall be implicitly convertible to `first_type` and `V` shall be implicitly convertible to `second_type`, else this constructor shall not participate in overload resolution.

### 813. "empty" undefined for `shared_ptr`

Section: 20.8.2.2 [util.smartptr.shared] Status: CD1 Submitter: Matt Austern Opened: 2008-02-26 Last modified: 2015-04-08

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Discussion:

Several places in 20.8.2.2 [util.smartptr.shared] refer to an "empty" `shared_ptr`. However, that term is nowhere defined. The closest thing we have to a definition is that the default constructor creates an empty `shared_ptr` and that a copy of a default-constructed `shared_ptr` is empty. Are any other `shared_ptr`s empty? For example, is `shared_ptr((T*) 0)` empty? What are the properties of an empty `shared_ptr`? We should either clarify this term or stop using it.

One reason it's not good enough to leave this term up to the reader's intuition is that, in light of N2351 and issue 711, most readers' intuitive understanding is likely to be wrong. Intuitively one might expect that an empty `shared_ptr` is one that doesn't store a pointer, but, whatever the definition is, that isn't it.

[ Peter adds: ]

> Or, what is an "empty" `shared_ptr`?

> - Are any other `shared_ptrs` empty?

>   Yes. Whether a given `shared_ptr` instance is empty or not is (*) completely specified by the last mutating operation on that instance. Give me an example and I'll tell you whether the `shared_ptr` is empty or not.

>   > (*) If it isn't, this is a legitimate defect.

> - For example, is `shared_ptr((T*) 0)` empty?

>   No. If it were empty, it would have a `use_count()` of 0, whereas it is specified to have an `use_count()` of 1.

> - What are the properties of an empty `shared_ptr`?

>   The properties of an empty `shared_ptr` can be derived from the specification. One example is that its destructor is a no-op. Another is that its `use_count()` returns 0. I can enumerate the full list if you really like.

> - We should either clarify this term or stop using it.

>   I don't agree with the imperative tone

>   A clarification would be either a no-op - if it doesn't contradict the existing wording - or a big mistake if it does.

>   I agree that a clarification that is formally a no-op may add value.

> - However, that term is nowhere defined.

>   Terms can be useful without a definition. Consider the following simplistic example. We have a type `X` with the following operations defined:

>   >     X x;
>   >     X x2(x);
>   >     X f(X x);
>   >     X g(X x1, X x2);

>   A default-constructed value is green.
>   A copy has the same color as the original.
>   `f(x)` returns a red value if the argument is green, a green value otherwise.
>   `g(x1,x2)` returns a green value if the arguments are of the same color, a red value otherwise.

>   Given these definitions, you can determine the color of every instance of type `X`, even if you have absolutely no idea what green and red mean.

>   Green and red are "nowhere defined" and completely defined at the same time.

> Alisdair's wording is fine.

Proposed resolution:

Append the following sentance to 20.8.2.2 [util.smartptr.shared]

> The `shared_ptr` class template stores a pointer, usually obtained via `new`. `shared_ptr` implements semantics of shared ownership; the last remaining owner of the pointer is responsible for destroying the object, or otherwise releasing the resources associated with the stored pointer. A `shared_ptr` object that does not own a pointer is said to be empty.

### 814. `vector<bool>::swap(reference, reference)` not defined

Section: 23.3.7 [vector.bool] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-03-17 Last modified: 2015-04-08

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Discussion:

`vector<bool>::swap(reference, reference)` has no definition.

[ San Francisco: ]

> Move to Open. Alisdair to provide a resolution.

[ Post Summit Daniel provided wording. ]

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Just after 23.3.7 [vector.bool]/5 add the following prototype and description:

> static void swap(reference x, reference y);

> > -6- Effects: Exchanges the contents of `x` and `y` as-if by:

> > >     bool b = x;
> > >     x = y;
> > >     y = b;

### 815. `std::function` and `reference_closure` do not use perfect forwarding

Section: 20.9.12.2.4 [func.wrap.func.inv] Status: Resolved Submitter: Alisdair Meredith Opened: 2008-03-16 Last modified: 2015-04-08

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Discussion:

`std::function` and `reference_closure` should use "perfect forwarding" as described in the rvalue core proposal.

[ Sophia Antipolis: ]

> According to Doug Gregor, as far as `std::function` is concerned, perfect forwarding can not be obtained because of type erasure. Not everyone agreed with this diagnosis of forwarding.

[ 2009-05-01 Howard adds: ]

> Sebastian Gesemann brought to my attention that the `CopyConstructible` requirement on `function`'s `ArgTypes...` is an unnecessary restriction.

> >     template<Returnable R, CopyConstructible... ArgTypes>
> >     class function<R(ArgTypes...)>
> >     ...

> On further investigation, this complaint seemed to be the same issue as this one. I believe the reason `CopyConstructible` was put on `ArgTypes` in the first place was because of the nature of the invoke member:

> >     template<class R, class ...ArgTypes>
> >     R
> >     function<R(ArgTypes...)>::operator()(ArgTypes... arg) const
> >     {
> >         if (f_ == 0)
> >             throw bad_function_call();
> >         return (*f_)(arg...);
> >     }

> However now with rvalue-refs, "by value" no longer implies `CopyConstructible` (as Sebastian correctly points out). If rvalue arguments are supplied, `MoveConstructible` is sufficient. Furthermore, the constraint need not be applied in `function` if I understand correctly. Rather the client must apply the proper constraints at the call site. Therefore, at the very least, I recommend that `CopyConstructible` be removed from the template class `function`.

> Furthermore we need to mandate that the invoker is coded as:

> >     template<class R, class ...ArgTypes>
> >     R
> >     function<R(ArgTypes...)>::operator()(ArgTypes... arg) const
> >     {
> >         if (f_ == 0)
> >             throw bad_function_call();
> >         return (*f_)(std::forward<ArgTypes>(arg)...);
> >     }

> Note that `ArgTypes&&` (the "perfect forwarding signature") is not appropriate here as this is not a deduced context for `ArgTypes`. Instead the client's arguments must implicitly convert to the non-deduced `ArgType` type. Catching these arguments by value makes sense to enable decay.

> Next `forward` is used to move the `ArgTypes` as efficiently as possible, and also with minimum requirements (not `CopyConstructible`) to the type-erased functor. For object types, this will be a `move`. For reference type `ArgTypes`, this will be a copy. The end result must be that the following is a valid program:

> >     #include <functional>
> >     #include <memory>
> >     #include <cassert>

> >     std::unique_ptr<int>
> >     f(std::unique_ptr<int> p, int& i)
> >     {
> >         ++i;
> >         return std::move(p);
> >     }

> >     int main()
> >     {
> >         int i = 2;
> >         std::function<std::unique_ptr<int>(std::unique_ptr<int>,
> >                                            int&> g(f);
> >         std::unique_ptr<int> p = g(std::unique_ptr<int>(new int(1)), i);
> >         assert(*p == 1);
> >         assert(i == 3);
> >     }

> [ Tested in pre-concepts rvalue-ref-enabled compiler. ]

> In the example above, the first `ArgType` is `unique_ptr<int>` and the second `ArgType` is `int&`. Both must work!

[ 2009-05-27 Daniel adds: ]

> in the 2009-05-01 comment of above mentioned issue Howard

> 1. Recommends to replace the `CopyConstructible` requirement by a `MoveConstructible` requirement
> 2. Says: "Furthermore, the constraint need not be applied in `function` if I understand correctly. Rather the client must apply the proper constraints at the call site"

> I'm fine with (a), but I think comment (b) is incorrect, at least in the sense I read these sentences. Let's look at Howard's example code:

> >     function<R(ArgTypes...)>::operator()(ArgTypes... arg) const
> >     {
> >        if (f_ == 0)
> >            throw bad_function_call();
> >        return (*f_)(std::forward<ArgTypes>(arg)...);
> >     }

> In the constrained scope of this `operator()` overload the expression "`(*f_)(std::forward<ArgTypes>(arg)...)`" must be valid. How can it do so, if `ArgTypes` aren't at least `MoveConstructible`?

[ 2009-07 Frankfurt: ]

> Leave this open and wait until concepts are removed from the Working Draft so that we know how to write the proposed resolution in terms of diffs to otherwise stable text.

[ 2009-10 Santa Cruz: ]

> Leave as open. Howard to provide wording. Howard welcomes any help.

[ 2009-12-12 Jonathan Wakely adds: ]

> 20.9.12.2 [func.wrap.func] says

> > 2 A function object `f` of type `F` is Callable for argument types `T1, T2, ..., TN` in `ArgTypes` and a return type `R`, if, given lvalues `t1, t2, ..., tN` of types `T1, T2, ..., TN`, respectively, `INVOKE (f, t1, t2, ..., tN)` is well formed (20.7.2) and, if `R` is not `void`, convertible to `R`.

> N.B. lvalues, which means you can't use `function<R(T&&)>` or `function<R(unique_ptr<T>)>`

> I recently implemented rvalue arguments in GCC's `std::function`, all that was needed was to use `std::forward<ArgTypes>` in a few places. The example in issue 815 works.

> I think 815 could be resolved by removing the requirement that the target function be callable with lvalues. Saying `ArgTypes` need to be `CopyConstructible` is wrong, and IMHO saying `MoveConstructible` is unnecessary, since the by-value signature implies that already, but if it is needed it should only be on `operator()`, not the whole class (you could in theory instantiate `std::function<R(noncopyable)>` as long as you don't invoke the call operator.)

> I think defining invocation in terms of `INVOKE` already implies perfect forwarding, so we don't need to say explicitly that `std::forward` should be used (N.B. the types that are forwarded are those in `ArgTypes`, which can differ from the actual parameter types of the target function. The actual parameter types have gone via type erasure, but that's not a problem - IMHO forwarding the arguments as `ArgTypes` is the right thing to do anyway.)

> Is it sufficient to simply replace "lvalues" with "values"? or do we need to say something like "lvalues when `Ti` is an lvalue-reference and rvalues otherwise"? I prefer the former, so I propose the following resolution for 815:

> Edit 20.9.12.2 [func.wrap.func] paragraph 2:

> > 2 A function object `f` of type `F` is Callable for argument types `T1, T2, ..., TN` in `ArgTypes` and a return type `R`, if, given lvalues `t1, t2, ..., tN` of types `T1, T2, ..., TN`, respectively, `INVOKE (f, t1, t2, ..., tN)` is well formed (20.7.2) and, if `R` is not `void`, convertible to `R`.

[ 2009-12-12 Daniel adds: ]

> I don't like the reduction to "values" and prefer the alternative solution suggested using "lvalues when Ti is an lvalue-reference and rvalues otherwise". The reason why I dislike the shorter version is based on different usages of "values" as part of defining the semantics of requirement tables via expressions. E.g. 17.6.3.1 [utility.arg.requirements]/1 says "`a`, `b`, and `c` are values of type `const T;`" or similar in 23.2.1 [container.requirements.general]/4 or /14 etc. My current reading of all these parts is that both rvalues and lvalues are required to be supported, but this interpretation would violate the intention of the suggested fix of #815, if I correctly understand Jonathan's rationale.

[ 2009-12-12 Howard adds: ]

> > "lvalues when Ti is an lvalue-reference and rvalues otherwise"

> doesn't quite work here because the `Ti` aren't deduced. They are specified by the `function` type. `Ti` might be `const int&` (an lvalue reference) and a valid `ti` might be `2` (a non-const rvalue). I've taken another stab at the wording using "expressions" and "bindable to".

[ 2010-02-09 Wording updated by Jonathan, Ganesh and Daniel. ]

[ 2010-02-09 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010-02-10 Daniel opens to improve wording. ]

[ 2010-02-11 This issue is now addressed by 870. ]

[ 2010-02-12 Moved to Tentatively NAD Editorial after 5 positive votes on c++std-lib. Rationale added below. ]

Rationale:

Addressed by 870.

Proposed resolution:

Edit 20.9.12.2 [func.wrap.func] paragraph 2:

> 2 A function object `f` of type `F` is Callable for argument types `T1, T2, ..., TN` in `ArgTypes` and a return type `R`, if, given lvalues `t1, t2, ..., tN` of types `T1, T2, ..., TN`, respectively, the expression `INVOKE(f, declval<ArgTypes>()..., Rt1, t2, ..., tN)`, considered as an unevaluated operand (5 [expr]), is well formed (20.7.2) and, if `R` is not `void`, convertible to `R`.

### 816. Should `bind()`'s returned functor have a nofail copy ctor when `bind()` is nofail?

Section: 20.9.10.3 [func.bind.bind] Status: Resolved Submitter: Stephan T. Lavavej Opened: 2008-02-08 Last modified: 2015-04-08

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Discussion:

Library Issue 527 notes that `bind(f, t1, ..., tN)` should be nofail when `f, t1, ..., tN` have nofail copy ctors.

However, no guarantees are provided for the copy ctor of the functor returned by `bind()`. (It's guaranteed to have a copy ctor, which can throw implementation-defined exceptions: `bind()` returns a forwarding call wrapper, TR1 3.6.3/2. A forwarding call wrapper is a call wrapper, TR1 3.3/4. Every call wrapper shall be CopyConstructible, TR1 3.3/4. Everything without an exception-specification may throw implementation-defined exceptions unless otherwise specified, C++03 17.4.4.8/3.)

Should the nofail guarantee requested by Library Issue 527 be extended to cover both calling `bind()` and copying the returned functor?

[ Howard adds: ]

> `tuple` construction should probably have a similar guarantee.

[ San Francisco: ]

> Howard to provide wording.

[ Post Summit, Anthony provided wording. ]

[ Batavia (2009-05): ]

> Part of all of this issue appears to be rendered moot by the proposed resolution to issue 817 (q.v.). We recommend the issues be considered simultaneously (or possibly even merged) to ensure there is no overlap. Move to Open, and likewise for issue 817.

[ 2009-07 Frankfurt: ]

> Related to 817 (see below). Leave Open.

[ 2009-10 Santa Cruz: ]

> Move to Ready. Decoupling from issue 817.

[ 2010-02-11 Moved from Ready to Tentatively NAD Editorial, rationale added below. ]

Rationale:

This issue is solved as proposed by 817.

Proposed resolution:

Add a new sentence to the end of paragraphs 2 and 4 of 20.9.10.3 [func.bind.bind]:

> -2- Returns: A forwarding call wrapper `g` with a weak result type (20.6.2). The effect of `g(u1, u2, ..., uM)` shall be `INVOKE(f, v1, v2, ..., vN, Callable<F cv,V1, V2, ..., VN>::result_type)`, where cv represents the cv-qualifiers of `g` and the values and types of the bound arguments `v1, v2, ..., vN` are determined as specified below. The copy constructor and move constructor of the forwarding call wrapper shall throw an exception if and only if the corresponding constructor of `F` or any of the types in `BoundArgs...` throw an exception.

> ...

> -5- Returns: A forwarding call wrapper `g` with a nested type `result_type` defined as a synonym for `R`. The effect of `g(u1, u2, ..., uM)` shall be `INVOKE(f, v1, v2, ..., vN, R)`, where the values and types of the bound arguments `v1, v2, ..., vN` are determined as specified below. The copy constructor and move constructor of the forwarding call wrapper shall throw an exception if and only if the corresponding constructor of `F` or any of the types in `BoundArgs...` throw an exception.

### 817. `bind` needs to be moved

Section: 20.9.10.3 [func.bind.bind] Status: C++11 Submitter: Howard Hinnant Opened: 2008-03-17 Last modified: 2015-04-08

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Discussion:

Addresses US 72, JP 38 and DE 21

The functor returned by `bind()` should have a move constructor that requires only move construction of its contained functor and bound arguments. That way move-only functors can be passed to objects such as `thread`.

This issue is related to issue 816.

US 72:

> `bind` should support move-only functors and bound arguments.

JP 38:

> add the move requirement for bind's return type.

> For example, assume following `th1` and `th2`,

> >     void f(vector<int> v) { }

> >     vector<int> v{ ... };
> >     thread th1([v]{ f(v); });
> >     thread th2(bind(f, v));

> When function object are set to thread, `v` is moved to `th1`'s lambda expression in a Move Constructor of lambda expression because `th1`'s lambda expression has a Move Constructor. But `bind` of `th2`'s return type doesn't have the requirement of Move, so it may not moved but copied.

> Add the requirement of move to get rid of this useless copy.

> And also, add the `MoveConstructible` as well as `CopyConstructible`.

DE 21

> The specification for bind claims twice that "the values and types for the bound arguments v1, v2, ..., vN are determined as specified below". No such specification appears to exist.

[ San Francisco: ]

> Howard to provide wording.

[ Post Summit Alisdair and Howard provided wording. ]

> Several issues are being combined in this resolution. They are all touching the same words so this is an attempt to keep one issue from stepping on another, and a place to see the complete solution in one place.

> 1. `bind` needs to be "moved".
> 2. 20.9.10.3 [func.bind.bind]/p3, p6 and p7 were accidently removed from N2798.
> 3. Issue 929 argues for a way to pass by && for efficiency but retain the decaying behavior of pass by value for the `thread` constructor. That same solution is applicable here.

[ Batavia (2009-05): ]

> We were going to recommend moving this issue to Tentatively Ready until we noticed potential overlap with issue 816 (q.v.).

> Move to Open, and recommend both issues be considered together (and possibly merged).

[ 2009-07 Frankfurt: ]

> The proposed resolution uses concepts. Leave Open.

[ 2009-10 Santa Cruz: ]

> Leave as Open. Howard to provide deconceptified wording.

[ 2009-11-07 Howard updates wording. ]

[ 2009-11-15 Further updates by Peter, Chris and Daniel. ]

[ Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

Proposed resolution:

Change 20.9 [function.objects] p2:

>     template<class Fn, class... Types BoundArgs>
>       unspecified bind(Fn&&, Types BoundArgs&&...);
>     template<class R, class Fn, class... Types BoundArgs>
>       unspecified bind(Fn&&, Types BoundArgs&&...);

Change 20.9.2 [func.require]:

> 4 Every call wrapper (20.9.1 [func.def]) shall be `CopyMoveConstructible`. A simple call wrapper is a call wrapper that is `CopyConstructible` and `CopyAssignable` and whose copy constructor, move constructor and assignment operator do not throw exceptions. A forwarding call wrapper is a call wrapper that can be called with an argument list. [Note: in a typical implementation forwarding call wrappers have an overloaded function call operator of the form

> >     template<class... ArgTypesUnBoundsArgs>
> >     R operator()(ArgTypesUnBoundsArgs&&... unbound_args) cv-qual;

> — end note]

Change 20.9.10.3 [func.bind.bind]:

> Within this clause:

> - Let `FD` be a synonym for the type `decay<F>::type`.
> - Let `fd` be an lvalue of type `FD` constructed from `std::forward<F>(f)`.
> - Let `Ti` be a synonym for the ith type in the template parameter pack `BoundArgs`.
> - Let `TiD` be a synonym for the type `decay<Ti>::type`.
> - Let `ti` be the ith argument in the function parameter pack `bound_args`.
> - Let `tid` be an lvalue of type `TiD` constructed from `std::forward<Ti>(ti)`.
> - Let `Uj` be the jth deduced type of the `UnBoundArgs&&...` parameter of the `operator()` of the forwarding call wrapper.
> - Let `uj` be the jth argument associated with `Uj`.

>     template<class F, class... BoundArgs>
>       unspecified bind(F&& f, BoundArgs&&... bound_args);

> > -1- Requires: `is_constructible<FD, F>::value` shall be `true`. For each `Ti` in `BoundArgs`, `is_constructible<TiD, Ti>::value` shall be `true`. `F` and each `Ti` in `BoundArgs` shall be CopyConstructible. `INVOKE(fd, w1, w2, ..., wN)` (20.9.2 [func.require]) shall be a valid expression for some values w1, w2, ..., wN, where `N == sizeof...(bound_args)`.

> > -2- Returns: A forwarding call wrapper `g` with a weak result type (20.9.2 [func.require]). The effect of `g(u1, u2, ..., uM)` shall be `INVOKE(fd, v1, v2, ..., vN, result_of<FD cv (V1, V2, ..., VN)>::type)`, where cv represents the cv-qualifiers of `g` and the values and types of the bound arguments `v1, v2, ..., vN` are determined as specified below. The copy constructor and move constructor of the forwarding call wrapper shall throw an exception if and only if the corresponding constructor of `FD` or of any of the types `TiD` throws an exception.

> > -3- Throws: Nothing unless the copy construction or of `Ffd` or of one of the values `tid` types in the `BoundArgs...` pack expansion throws an exception.

> > Remarks: The unspecified return type shall satisfy the requirements of `MoveConstructible`. If all of `FD` and `TiD` satisfy the requirements of `CopyConstructible` then the unspecified return type shall satisfy the requirements of `CopyConstructible`. [Note: This implies that all of `FD` and `TiD` shall be `MoveConstructible` — end note]

>     template<class R, class F, class... BoundArgs>
>       unspecified bind(F&& f, BoundArgs&&... bound_args);

> > -4- Requires: `is_constructible<FD, F>::value` shall be `true`. For each `Ti` in `BoundArgs`, `is_constructible<TiD, Ti>::value` shall be `true`. `F` and each `Ti` in `BoundArgs` shall be CopyConstructible. `INVOKE(fd, w1, w2, ..., wN)` shall be a valid expression for some values w1, w2, ..., wN, where `N == sizeof...(bound_args)`.

> > -5- Returns: A forwarding call wrapper `g` with a nested type `result_type` defined as a synonym for `R`. The effect of `g(u1, u2, ..., uM)` shall be `INVOKE(fd, v1, v2, ..., vN, R)`, where the values and types of the bound arguments `v1, v2, ..., vN` are determined as specified below. The copy constructor and move constructor of the forwarding call wrapper shall throw an exception if and only if the corresponding constructor of `FD` or of any of the types `TiD` throws an exception.

> > -6- Throws: Nothing unless the copy construction or of `Ffd` or of one of the values `tid` types in the `BoundArgs...` pack expansion throws an exception.

> > Remarks: The unspecified return type shall satisfy the requirements of `MoveConstructible`. If all of `FD` and `TiD` satisfy the requirements of `CopyConstructible` then the unspecified return type shall satisfy the requirements of `CopyConstructible`. [Note: This implies that all of `FD` and `TiD` shall be `MoveConstructible` — end note]

> -7- The values of the bound arguments `v1, v2, ..., vN` and their corresponding types `V1, V2, ..., VN` depend on the types `TiD` derived from of the corresponding argument `ti` in `bound_args` of type `Ti` in `BoundArgs` in the call to `bind` and the cv-qualifiers cv of the call wrapper `g` as follows:

> - if `ti TiD` is of type `reference_wrapper<T>` the argument is `tid.get()` and its type `Vi` is `T&`;
> - if the value of `std::is_bind_expression<TiD>::value` is `true` the argument is `tid(std::forward<Uj>(uj)... u1, u2, ..., uM)` and its type `Vi` is `result_of<TiD cv (Uj... U1&, U2&, ..., UM&)>::type`;
> - if the value `j` of `std::is_placeholder<TiD>::value` is not zero the argument is `std::forward<Uj>(uj)` and its type `Vi` is `Uj&&`;
> - otherwise the value is `tid` and its type `Vi` is `TiD cv &`.

### 818. wording for memory ordering

Section: 29.3 [atomics.order] Status: CD1 Submitter: Jens Maurer Opened: 2008-03-22 Last modified: 2015-04-08

View other active issues in [atomics.order].

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Discussion:

29.3 [atomics.order] p1 says in the table that

> Element | Meaning
> --- | ---
> `memory_order_acq_rel` | the operation has both acquire and release semantics

To my naked eye, that seems to imply that even an atomic read has both acquire and release semantics.

Then, p1 says in the table:

> Element | Meaning
> --- | ---
> `memory_order_seq_cst` | the operation has both acquire and release semantics, and, in addition, has sequentially-consistent operation ordering

So that seems to be "the same thing" as `memory_order_acq_rel`, with additional constraints.

I'm then reading p2, where it says:

> The `memory_order_seq_cst` operations that load a value are acquire operations on the affected locations. The `memory_order_seq_cst` operations that store a value are release operations on the affected locations.

That seems to imply that atomic reads only have acquire semantics. If that is intended, does this also apply to `memory_order_acq_rel` and the individual load/store operations as well?

Also, the table in p1 contains phrases with "thus" that seem to indicate consequences of normative wording in 1.10 [intro.multithread]. That shouldn't be in normative text, for the fear of redundant or inconsistent specification with the other normative text.

Double-check 29.6 [atomics.types.operations] that each operation clearly says whether it's a load or a store operation, or both. (It could be clearer, IMO. Solution not in current proposed resolution.)

29.3 [atomics.order] p2: What's a "consistent execution"? It's not defined in 1.10 [intro.multithread], it's just used in notes there.

And why does 29.6 [atomics.types.operations] p9 for "load" say:

> Requires: The order argument shall not be `memory_order_acquire` nor `memory_order_acq_rel`.

(Since this is exactly the same restriction as for "store", it seems to be a typo.)

And then: 29.6 [atomics.types.operations] p12:

> These operations are read-modify-write operations in the sense of the "synchronizes with" definition (1.10 [intro.multithread]), so both such an operation and the evaluation that produced the input value synchronize with any evaluation that reads the updated value.

This is redundant with 1.10 [intro.multithread], see above for the reasoning.

[ San Francisco: ]

> Boehm: "I don't think that this changes anything terribly substantive, but it improves the text."

> Note that "Rephrase the table in as [sic] follows..." should read "Replace the table in [atomics.order] with the following...."

> The proposed resolution needs more work. Crowl volunteered to address all of the atomics issues in one paper.

> This issue is addressed in N2783.

Proposed resolution:

edit 29.3 [atomics.order], paragraph 1 as follows.

> The enumeration `memory_order` specifies the detailed regular (non-atomic) memory synchronization order as defined in Clause 1.7 section 1.10 and may provide for operation ordering. Its enumerated values and their meanings are as follows:

> > For `memory_order_relaxed`,
> >   no operation orders memory.
> > For `memory_order_release`, `memory_order_acq_rel`, and `memory_order_seq_cst`,
> >   a store operation performs a release operation on the affected memory location.
> > For `memory_order_consume`,
> >   a load operation performs a consume operation on the affected memory location.
> > For `memory_order_acquire`, `memory_order_acq_rel`, and `memory_order_seq_cst`,
> >   a load operation performs an acquire operation on the affected memory location.

remove table 136 in 29.3 [atomics.order].

> Table 136 — memory_order effects
> Element | Meaning
> --- | ---
> `memory_order_relaxed` | the operation does not order memory
> `memory_order_release` | the operation performs a release operation on the affected memory location, thus making regular memory writes visible to other threads through the atomic variable to which it is applied
> `memory_order_acquire` | the operation performs an acquire operation on the affected memory location, thus making regular memory writes in other threads released through the atomic variable to which it is applied visible to the current thread
> `memory_order_consume` | the operation performs a consume operation on the affected memory location, thus making regular memory writes in other threads released through the atomic variable to which it is applied visible to the regular memory reads that are dependencies of this consume operation.
> `memory_order_acq_rel` | the operation has both acquire and release semantics
> `memory_order_seq_cst` | the operation has both acquire and release semantics, and, in addition, has sequentially-consistent operation ordering

edit 29.3 [atomics.order], paragraph 2 as follows.

> The `memory_order_seq_cst` operations that load a value are acquire operations on the affected locations. The `memory_order_seq_cst` operations that store a value are release operations on the affected locations. In addition, in a consistent execution, there There must be is a single total order S on all `memory_order_seq_cst` operations, consistent with the happens before order and modification orders for all affected locations, such that each `memory_order_seq_cst` operation observes either the last preceding modification according to this order S, or the result of an operation that is not `memory_order_seq_cst`. [Note: Although it is not explicitly required that S include locks, it can always be extended to an order that does include lock and unlock operations, since the ordering between those is already included in the happens before ordering. —end note]

### 819. rethrow_if_nested

Section: 18.8.6 [except.nested] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-03-25 Last modified: 2015-04-08

View other active issues in [except.nested].

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Discussion:

Looking at the wording I submitted for `rethrow_if_nested`, I don't think I got it quite right.

The current wording says:

>     template <class E> void rethrow_if_nested(const E& e);

> > Effects: Calls `e.rethrow_nested()` only if `e` is publicly derived from `nested_exception`.

This is trying to be a bit subtle, by requiring `e` (not `E`) to be publicly derived from `nested_exception` the idea is that a `dynamic_cast` would be required to be sure. Unfortunately, if `e` is dynamically but not statically derived from `nested_exception`, `e.rethrow_nested()` is ill-formed.

[ San Francisco: ]

> Alisdair was volunteered to provide wording.

[ 2009-10 Santa Cruz: ]

> Leave as Open. Alisdair to provide wording.

[ 2009-11-09 Alisdair provided wording. ]

[ 2010-03-10 Dietmar updated wording. ]

[ 2010 Pittsburgh: ]

> Moved to Ready for Pittsburgh.

Proposed resolution:

Change 18.8.6 [except.nested], p8:

>     template <class E> void rethrow_if_nested(const E& e);

> > -8- Effects: Calls `e.rethrow_nested()` o Only if the dynamic type of `e` is publicly and unambiguously derived from `nested_exception` this calls `dynamic_cast<const nested_exception&>(e).rethrow_nested()`.

### 820. `current_exception()`'s interaction with throwing copy ctors

Section: 18.8.5 [propagation] Status: CD1 Submitter: Stephan T. Lavavej Opened: 2008-03-26 Last modified: 2015-04-08

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Discussion:

As of N2521, the Working Paper appears to be silent about what `current_exception()` should do if it tries to copy the currently handled exception and its copy constructor throws. 18.8.5 [propagation]/7 says "If the function needs to allocate memory and the attempt fails, it returns an `exception_ptr` object that refers to an instance of `bad_alloc`.", but doesn't say anything about what should happen if memory allocation succeeds but the actual copying fails.

I see three alternatives: (1) return an `exception_ptr` object that refers to an instance of some fixed exception type, (2) return an `exception_ptr` object that refers to an instance of the copy ctor's thrown exception (but if that has a throwing copy ctor, an infinite loop can occur), or (3) call `terminate()`.

I believe that `terminate()` is the most reasonable course of action, but before we go implement that, I wanted to raise this issue.

[ Peter's summary: ]

> The current practice is to not have throwing copy constructors in exception classes, because this can lead to `terminate()` as described in 15.5.1 [except.terminate]. Thus calling `terminate()` in this situation seems consistent and does not introduce any new problems.

> However, the resolution of core issue 475 may relax this requirement:

> > The CWG agreed with the position that `std::uncaught_exception()` should return `false` during the copy to the exception object and that `std::terminate()` should not be called if that constructor exits with an exception.

> Since throwing copy constructors will no longer call `terminate()`, option (3) doesn't seem reasonable as it is deemed too drastic a response in a recoverable situation.

> Option (2) cannot be adopted by itself, because a potential infinite recursion will need to be terminated by one of the other options.

Proposed resolution:

Add the following paragraph after 18.8.5 [propagation]/7:

> Returns (continued): If the attempt to copy the current exception object throws an exception, the function returns an `exception_ptr` that refers to the thrown exception or, if this is not possible, to an instance of `bad_exception`.

> [Note: The copy constructor of the thrown exception may also fail, so the implementation is allowed to substitute a `bad_exception` to avoid infinite recursion. -- end note.]

Rationale:

[ San Francisco: ]

> Pete: there may be an implied assumption in the proposed wording that current_exception() copies the existing exception object; the implementation may not actually do that.

> Pete will make the required editorial tweaks to rectify this.

### 821. Minor cleanup : `unique_ptr`

Section: 20.8.1.3.4 [unique.ptr.runtime.modifiers] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-03-30 Last modified: 2015-04-08

View all other issues in [unique.ptr.runtime.modifiers].

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Discussion:

Reading resolution of LWG issue 673 I noticed the following:

>     void reset(T* pointer p = 0 pointer());

> -1- Requires: Does not accept pointer types which are convertible to `T*` `pointer` (diagnostic required). [Note: One implementation technique is to create a private templated overload. -- end note]

This could be cleaned up by mandating the overload as a public deleted function. In addition, we should probably overload `reset` on `nullptr_t` to be a stronger match than the deleted overload. Words...

Proposed resolution:

Add to class template definition in 20.8.1.3 [unique.ptr.runtime]

>     // modifiers
>     pointer release();
>     void reset(pointer p = pointer());
>     void reset( nullptr_t );
>     template< typename U > void reset( U ) = delete;
>     void swap(unique_ptr&& u);

Update 20.8.1.3.4 [unique.ptr.runtime.modifiers]

>     void reset(pointer p = pointer());
>     void reset(nullptr_t);

> -1- Requires: Does not accept pointer types which are convertible to `pointer` (diagnostic required). [Note: One implementation technique is to create a private templated overload. -- end note]

> Effects: If `get() == nullptr` there are no effects. Otherwise `get_deleter()(get())`.

> ...

[ Note this wording incorporates resolutions for 806 (New) and 673 (Ready). ]

### 823. `identity<void>` seems broken

Section: 20.2.4 [forward] Status: Resolved Submitter: Walter Brown Opened: 2008-04-09 Last modified: 2015-04-08

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Discussion:

N2588 seems to have added an `operator()` member function to the `identity<>` helper in 20.2.4 [forward]. I believe this change makes it no longer possible to instantiate `identity<void>`, as it would require forming a reference-to-`void` type as this `operator()`'s parameter type.

Suggested resolution: Specialize `identity<void>` so as not to require the member function's presence.

[ Sophia Antipolis: ]

> Jens: suggests to add a requires clause to avoid specializing on `void`.

> Alisdair: also consider cv-qualified `void`.

> Alberto provided proposed wording.

[ 2009-07-30 Daniel reopens: ]

> This issue became closed, because the `ReferentType` requirement fixed the problem - this is no longer the case. In retrospective it seems to be that the root of current issues around `std::identity` (823, 700, 939) is that it was standardized as something very different (an unconditional type mapper) than traditional usage indicated (a function object that should derive from `std::unary_function)`, as the SGI definition does. This issue could be solved, if `std::identity` is removed (one proposal of 939), but until this has been decided, this issue should remain open. An alternative for removing it, would be, to do the following:

> 1. Let `identity` stay as a real function object, which would now properly derive from `unary_function`:

>    >     template <class T> struct identity : unary_function<T, T> {
>    >       const T& operator()(const T&) const;
>    >     };

> 2. Invent (if needed) a generic type wrapper (corresponding to concept `IdentityOf`), e.g. `identity_of`, and move it's prototype description back to 20.2.4 [forward]:

>    >     template <class T> struct identity_of {
>    >       typedef T type;
>    >     };

>    and adapt the `std::forward` signature to use `identity_of` instead of `identity`.

[ 2009-10 Santa Cruz: ]

> Mark as NAD Editorial Resolved, fixed by 939.

Proposed resolution:

Change definition of `identity` in 20.2.4 [forward], paragraph 2, to:

>     template <class T>  struct identity {
>         typedef T type;

>         requires ReferentType<T>
>           const T& operator()(const T& x) const;
>       };

...

>       requires ReferentType<T>
>         const T& operator()(const T& x) const;

Rationale:

The point here is to able to write `T&` given `T` and `ReferentType` is precisely the concept that guarantees so, according to N2677 (Foundational concepts). Because of this, it seems preferable than an explicit check for `cv void` using `SameType/remove_cv` as it was suggested in Sophia. In particular, Daniel remarked that there may be types other than `cv void` which aren't referent types (`int[]`, perhaps?).

### 824. rvalue ref issue with `basic_string` inserter

Section: 21.4.8.9 [string.io] Status: CD1 Submitter: Alisdair Meredith Opened: 2008-04-10 Last modified: 2015-04-08

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Discussion:

In the current working paper, the `<string>` header synopsis at the end of 21.3 [string.classes] lists a single `operator<<` overload for `basic_string`.

>     template<class charT, class traits, class Allocator>
>      basic_ostream<charT, traits>&
>        operator<<(basic_ostream<charT, traits>&& os,
>                   const basic_string<charT,traits,Allocator>& str);

The definition in 21.4.8.9 [string.io] lists two:

>     template<class charT, class traits, class Allocator>
>      basic_ostream<charT, traits>&
>        operator<<(basic_ostream<charT, traits>& os,
>                   const basic_string<charT,traits,Allocator>& str);

>     template<class charT, class traits, class Allocator>
>      basic_ostream<charT, traits>&
>        operator<<(basic_ostream<charT, traits>&& os,
>                   const basic_string<charT,traits,Allocator>& str);

I believe the synopsis in 21.3 [string.classes] is correct, and the first of the two signatures in 21.4.8.9 [string.io] should be deleted.

Proposed resolution:

Delete the first of the two signatures in 21.4.8.9 [string.io]:

>     template<class charT, class traits, class Allocator>
>      basic_ostream<charT, traits>&
>        operator<<(basic_ostream<charT, traits>& os,
>                   const basic_string<charT,traits,Allocator>& str);

>     template<class charT, class traits, class Allocator>
>      basic_ostream<charT, traits>&
>        operator<<(basic_ostream<charT, traits>&& os,
>                   const basic_string<charT,traits,Allocator>& str);

### 825. Missing rvalues reference stream insert/extract operators?

Section: 19.5.2.1 [syserr.errcode.overview], 20.8.2.2.11 [util.smartptr.shared.io], 22.4.8 [facets.examples], 20.6.4 [bitset.operators], 26.4.6 [complex.ops], 27.6 [stream.buffers], 28.9 [re.submatch] Status: Resolved Submitter: Alisdair Meredith Opened: 2008-04-10 Last modified: 2015-04-08

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Discussion:

Addresses UK 220

Should the following use rvalues references to stream in insert/extract operators?

- 19.5.2.1 [syserr.errcode.overview]
- 20.8.2.2.11 [util.smartptr.shared.io]
- 22.4.8 [facets.examples]
- 20.6.4 [bitset.operators]
- 26.4.6 [complex.ops]
- Doubled signatures in 27.6 [stream.buffers] for character inserters (ref 27.7.3.6.4 [ostream.inserters.character]) + definition 27.7.3.6.4 [ostream.inserters.character]
- 28.9 [re.submatch]

[ Sophia Antipolis ]

> Agree with the idea in the issue, Alisdair to provide wording.

[ Daniel adds 2009-02-14: ]

> The proposal given in the paper N2831 apparently resolves this issue.

[ Batavia (2009-05): ]

> The cited paper is an earlier version of N2844, which changed the rvalue reference binding rules. That paper includes generic templates `operator<<` and `operator>>` that adapt rvalue streams.

> We therefore agree with Daniel's observation. Move to NAD Editorial Resolved.

Proposed resolution:

### 827. `constexpr shared_ptr::shared_ptr()?`

Section: 20.8.2.2.1 [util.smartptr.shared.const] Status: Resolved Submitter: Peter Dimov Opened: 2008-04-11 Last modified: 2015-04-08

View all other issues in [util.smartptr.shared.const].

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Discussion:

Would anyone object to making the default constructor of `shared_ptr` (and `weak_ptr` and `enable_shared_from_this`) `constexpr`? This would enable static initialization for `shared_ptr` variables, eliminating another unfair advantage of raw pointers.

[ San Francisco: ]

> It's not clear to us that you can initialize a pointer with the literal 0 in a constant expression. We need to ask CWG to make sure this works. Bjarne has been appointed to do this.

> Core got back to us and assured as that `nullptr` would do the job nicely here.

[ 2009-05-01 Alisdair adds: ]

> I don't believe that `constexpr` will buy anything in this case. `shared_ptr/weak_ptr/enable_shared_from_this` cannot be literal types as they have a non-trivial copy constructor. As they do not produce literal types, then the `constexpr` default constructor will not guarantee constant initialization, and so not buy the hoped for optimization.

> I recommend referring this back to Core to see if we can get static initialization for types with `constexpr` constructors, even if they are not literal types. Otherwise this should be closed as NAD.

[ 2009-05-26 Daniel adds: ]

> If Alisdair's 2009-05-01 comment is correct, wouldn't that also make `constexpr mutex()` useless, because this class has a non-trivial destructor? (828)

[ 2009-07-21 Alisdair adds: ]

> The feedback from core is that this and similar uses of `constexpr` constructors to force static initialization should be supported. If there are any problems with this in the working draught, we should file core issues.

> Recommend we declare the default constructor `constexpr` as the issue suggests (proposed wording added).

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Solved by N2994.

Proposed resolution:

Change 20.8.2.2 [util.smartptr.shared] and 20.8.2.2.1 [util.smartptr.shared.const]:

>     constexpr shared_ptr();

Change 20.8.2.3 [util.smartptr.weak] and 20.8.2.3.1 [util.smartptr.weak.const]:

>     constexpr weak_ptr();

Change 20.8.2.5 [util.smartptr.enab] (2 places):

>     constexpr enable_shared_from_this();

### 828. Static initialization for `std::mutex`?

Section: 30.4.1.2.1 [thread.mutex.class] Status: Resolved Submitter: Peter Dimov Opened: 2008-04-18 Last modified: 2015-04-08

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Discussion:

[Note: I'm assuming here that 3.6.2 [basic.start.init]/1 will be fixed.]

Currently `std::mutex` doesn't support static initialization. This is a regression with respect to `pthread_mutex_t`, which does. I believe that we should strive to eliminate such regressions in expressive power where possible, both to ease migration and to not provide incentives to (or force) people to forego the C++ primitives in favor of pthreads.

[ Sophia Antipolis: ]

> We believe this is implementable on POSIX, because the initializer-list feature and the constexpr feature make this work. Double-check core language about static initialization for this case. Ask core for a core issue about order of destruction of statically-initialized objects wrt. dynamically-initialized objects (should come afterwards). Check non-POSIX systems for implementability.

> If ubiquitous implementability cannot be assured, plan B is to introduce another constructor, make this constexpr, which is conditionally-supported. To avoid ambiguities, this new constructor needs to have an additional parameter.

[ Post Summit: ]

> Jens: constant initialization seems to be ok core-language wise

> Consensus: Defer to threading experts, in particular a Microsoft platform expert.

> Lawrence to send e-mail to Herb Sutter, Jonathan Caves, Anthony Wiliams, Paul McKenney, Martin Tasker, Hans Boehm, Bill Plauger, Pete Becker, Peter Dimov to alert them of this issue.

> Lawrence: What about header file shared with C? The initialization syntax is different in C and C++.

> Recommend Keep in Review

[ Batavia (2009-05): ]

> Keep in Review status pending feedback from members of the Concurrency subgroup.

[ See related comments from Alisdair and Daniel in 827. ]

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Addressed by N2994.

Proposed resolution:

Change 30.4.1.2.1 [thread.mutex.class]:

>     class mutex {
>     public:
>       constexpr mutex();
>       ...

### 829. current_exception wording unclear about exception type

Section: 18.8.5 [propagation] Status: CD1 Submitter: Beman Dawes Opened: 2008-04-20 Last modified: 2015-04-08

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Discussion:

Consider this code:

>     exception_ptr xp;

>     try {do_something(); }

>     catch (const runtime_error& ) {xp = current_exception();}

>     ...

>     rethrow_exception(xp);

Say `do_something()` throws an exception object of type `range_error`. What is the type of the exception object thrown by `rethrow_exception(xp)` above? It must have a type of `range_error`; if it were of type `runtime_error` it still isn't possible to propagate an exception and the exception_ptr/current_exception/rethrow_exception machinery serves no useful purpose.

Unfortunately, the current wording does not explicitly say that. Different people read the current wording and come to different conclusions. While it may be possible to deduce the correct type from the current wording, it would be much clearer to come right out and explicitly say what the type of the referred to exception is.

[ Peter adds: ]

> I don't like the proposed resolution of 829. The normative text is unambiguous that the `exception_ptr` refers to the currently handled exception. This term has a standard meaning, see 15.3 [except.handle]/8; this is the exception that `throw;` would rethrow, see 15.1 [except.throw]/7.

> A better way to address this is to simply add the non-normative example in question as a clarification. The term currently handled exception should be italicized and cross-referenced. A [Note: the currently handled exception is the exception that a throw expression without an operand (15.1 [except.throw]/7) would rethrow. --end note] is also an option.

Proposed resolution:

After 18.8.5 [propagation] , paragraph 7, add the indicated text:

>     exception_ptr current_exception();

> > Returns: `exception_ptr` object that refers to the currently handled exception (15.3 [except.handle]) or a copy of the currently handled exception, or a null `exception_ptr` object if no exception is being handled. If the function needs to allocate memory and the attempt fails, it returns an `exception_ptr` object that refers to an instance of `bad_alloc`. It is unspecified whether the return values of two successive calls to `current_exception` refer to the same exception object. [Note: that is, it is unspecified whether `current_exception` creates a new copy each time it is called. -- end note]

> > Throws: nothing.

### 834. `unique_ptr::pointer` requirements underspecified

Section: 20.8.1.2 [unique.ptr.single] Status: Resolved Submitter: Daniel Krügler Opened: 2008-05-14 Last modified: 2015-04-08

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Discussion:

Issue 673 (including recent updates by 821) proposes a useful extension point for `unique_ptr` by granting support for an optional `deleter_type::pointer` to act as pointer-like replacement for `element_type*` (In the following: `pointer`).

Unfortunately no requirements are specified for the type `pointer` which has impact on at least two key features of `unique_ptr`:

1. Operational fail-safety.
2. (Well-)Definedness of expressions.

The `unique_ptr` specification makes great efforts to require that essentially all operations cannot throw and therefore adds proper wording to the affected operations of the deleter as well. If user-provided `pointer`-emulating types ("smart pointers") will be allowed, either all throw-nothing clauses have to be replaced by weaker "An exception is thrown only if `pointer`'s {op} throws an exception"-clauses or it has to be said explicitly that all used operations of `pointer` are required not to throw. I understand the main focus of `unique_ptr` to be as near as possible to the advantages of native pointers which cannot fail and thus strongly favor the second choice. Also, the alternative position would make it much harder to write safe and simple template code for `unique_ptr`. Additionally, I assume that a general statement need to be given that all of the expressions of `pointer` used to define semantics are required to be well-formed and well-defined (also as back-end for 762).

[ Sophia Antipolis: ]

> Howard: We maybe need a core concept `PointerLike`, but we don't need the arithmetic (see `shared_ptr` vs. `vector<T>::iterator`.

> Howard will go through and enumerate the individual requirements wrt. `pointer` for each member function.

[ 2009-07 Frankfurt: ]

> Move to Ready.

[ 2009-10-15 Alisdair pulls from Ready: ]

> I hate to pull an issue out of Ready status, but I don't think 834 is fully baked yet.

> For reference the proposed resolution is to add the following words:

> > `unique_ptr<T, D>::pointer`'s operations shall be well-formed, shall have well defined behavior, and shall not throw exceptions.

> This leaves me with a big question : which operations?

> Are all pointer operations required to be nothrow, including operations that have nothing to do with interactions with `unique_ptr`? This was much simpler with concepts where we could point to operations within a certain concept, and so nail down the interactions.

[ 2009-10-15 Daniel adds: ]

> I volunteer to prepare a more fine-grained solution, but I would like to ask for feedback that helps me doing so. If this question is asked early in the meeting I might be able to fix it within the week, but I cannot promise that now.

[ 2009-10 Santa Cruz: ]

> Leave in open. Daniel to provide wording as already suggested.

[ 2009-12-22 Daniel provided wording and rationale. ]

[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]

Rationale:

The here proposed resolution has considerable overlap with the requirements that are used in the allocator requirements.

This might be a convincing argument to isolate the common subset into one requirement. The reason I did not do that is basically because we might find out that they are either over-constraining or under-constraining at this late point of specification. Note also that as a result of the idea of a general requirement set I added the requirement:

> A default-initialized object may have a singular value

even though this does not play a relevant role for `unique_ptr`.

One further characteristics of the resolution is that availability of relational operators of `unique_ptr<T, D>::pointer` is not part of the basic requirements, which is in sync with the allocator requirements on pointer-like (this means that `unique_ptr` can hold a `void_pointer` or `const_void_pointer`).

Solved by N3073.

Proposed resolution:

1. Change 20.8.1.2 [unique.ptr.single] p. 1 as indicated: [The intent is to replace the coupling between `T*` and the deleter's `operator()` by a coupling between `unique_ptr<T, D>::pointer` and this `operator()`]

   > 1 - The default type for the template parameter `D` is `default_delete`. A client-supplied template argument `D` shall be a function pointer or functor for which, given a value `d` of type `D` and a pointer value `ptr` of type `T* unique_ptr<T, D>::pointer`, the expression `d(ptr)` is valid and has the effect of deallocating the pointer as appropriate for that deleter. `D` may also be an lvalue-reference to a deleter.

2. Change 20.8.1.2 [unique.ptr.single] p. 3 as indicated:

   > 3 - If the type `remove_reference<D>::type::pointer` exists, then `unique_ptr<T, D>::pointer` shall be a synonym for `remove_reference<D>::type::pointer`. Otherwise `unique_ptr<T, D>::pointer` shall be a synonym for `T*`. The type `unique_ptr<T, D>::pointer` shall be satisfy the requirements of `EqualityComparable`, `DefaultConstructible`, `CopyConstructible` (Table 34) and, `CopyAssignable` (Table 36), `Swappable`, and `Destructible` (17.6.3.1 [utility.arg.requirements]). A default-initialized object may have a singular value. A value-initialized object produces the null value of the type. The null value shall be equivalent only to itself. An object of this type can be copy-initialized with a value of type `nullptr_t`, compared for equality with a value of type `nullptr_t`, and assigned a value of type `nullptr_t`. The effect shall be as if a value-initialized object had been used in place of the null pointer constant. An object `p` of this type can be contextually converted to `bool`. The effect shall be as if `p != nullptr` had been evaluated in place of `p`. No operation on this type which is part of the above mentioned requirements shall exit via an exception.

   > [Note: Given an allocator type `X` (17.6.3.5 [allocator.requirements]), the types `X::pointer`, `X::const_pointer`, `X::void_pointer`, and `X::const_void_pointer` may be used as `unique_ptr<T, D>::pointer` — end note]

   > In addition to being available via inclusion of the `<utility>` header, the `swap` function template in 20.2.2 [utility.swap] is also available within the definition of `unique_ptr`'s `swap` function.

3. Change 20.8.1.2.1 [unique.ptr.single.ctor] p. 2+3 as indicated: [The first change ensures that we explicitly say, how the stored pointer is initialized. This is important for a `constexpr` function, because this may make a difference for user-defined pointer-like types]

   >     constexpr unique_ptr();

   > > ...

   > > 2 - Effects: Constructs a `unique_ptr` which owns nothing, value-initializing the stored pointer.

   > > 3 - Postconditions: `get() == 0 nullptr`.

4. Change 20.8.1.2.1 [unique.ptr.single.ctor] p. 6+7 as indicated: [This is a step-by-fix to ensure consistency to the changes of N2976]

   >     unique_ptr(pointer p);

   > > ...

   > > 6 - Effects: Constructs a `unique_ptr` which owns `p`, initializing the stored pointer with `p`.

   > > 7 - Postconditions: `get() == p`. `get_deleter()` returns a reference to a default constructed value-initialized deleter `D`.

5. Insert a new effects clause in 20.8.1.2.1 [unique.ptr.single.ctor] just before p. 14: [The intent is to fix the current lack of specification in which way the stored pointer is initialized]

   >     unique_ptr(pointer p, implementation-defined see below d1);
   >     unique_ptr(pointer p, implementation-defined see below d2);

   > > ...

   > > Effects: Constructs a `unique_ptr` which owns `p`, initializing the stored pointer with `p` and the initializing the deleter as described above.

   > > 14 - Postconditions: `get() == p`. `get_deleter()` returns a reference to the internally stored deleter. If `D` is a reference type then `get_deleter()` returns a reference to the lvalue `d`.

6. Change 20.8.1.2.1 [unique.ptr.single.ctor] p. 18+22 as indicated: [The intent is to clarify that the moved-from source must contain a null pointer, there is no other choice left]

   >     unique_ptr(unique_ptr&& u);

   > > [..]

   > > 18 - Postconditions: `get() == value u.get()` had before the construction and `u.get() == nullptr`. `get_deleter()` returns a reference to the internally stored deleter which was constructed from `u.get_deleter()`. If `D` is a reference type then `get_deleter()` and `u.get_deleter()` both reference the same lvalue deleter.

   >     template <class U, class E> unique_ptr(unique_ptr<U, E>&& u);

   > > [..]

   > > 22 - Postconditions: `get() == value u.get()` had before the construction, modulo any required offset adjustments resulting from the cast from `unique_ptr<U, E>::pointer` to `pointer` and `u.get() == nullptr`. `get_deleter()` returns a reference to the internally stored deleter which was constructed from `u.get_deleter()`.

7. Change 20.8.1.2.1 [unique.ptr.single.ctor] p. 20 as indicated: [With the possibility of user-defined pointer-like types the implication does only exist, if those are built-in pointers. Note that this change should also be applied with the acceptance of 950]

   >     template <class U, class E> unique_ptr(unique_ptr<U, E>&& u);

   > > 20 - Requires: If `D` is not a reference type, construction of the deleter `D` from an rvalue of type `E` shall be well formed and shall not throw an exception. If `D` is a reference type, then `E` shall be the same type as `D` (diagnostic required). `unique_ptr<U, E>::pointer` shall be implicitly convertible to `pointer`. [Note: These requirements imply that `T` and `U` are complete types. — end note]

8. Change 20.8.1.2.2 [unique.ptr.single.dtor] p. 2 as indicated:

   >     ~unique_ptr();

   > > ...

   > > 2 - Effects: If `get() == 0 nullptr` there are no effects. Otherwise `get_deleter()(get())`.

9. Change 20.8.1.2.3 [unique.ptr.single.asgn] p. 3+8 as indicated: [The intent is to clarify that the moved-from source must contain a null pointer, there is no other choice left]

   >     unique_ptr& operator=(unique_ptr&& u);

   > > [..]

   > > 3 - Postconditions: This `unique_ptr` now owns the pointer which `u` owned, and `u` no longer owns it, `u.get() == nullptr`. [Note: If `D` is a reference type, then the referenced lvalue deleters are move assigned. — end note]

   >     template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u);

   > > [..]

   > > 8 - Postconditions: This `unique_ptr` now owns the pointer which `u` owned, and `u` no longer owns it, `u.get() == nullptr`.

10. Change 20.8.1.2.3 [unique.ptr.single.asgn] p. 6 as indicated: [With the possibility of user-defined pointer-like types the implication does only exist, if those are built-in pointers. Note that this change should also be applied with the acceptance of 950]

    >     template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u);

    > > [..]

    > > 6 - Requires: Assignment of the deleter `D` from an rvalue `D` shall not throw an exception. `unique_ptr<U, E>::pointer` shall be implicitly convertible to `pointer`. [Note: These requirements imply that `T` and `U` are complete types. — end note]

11. Change 20.8.1.2.3 [unique.ptr.single.asgn] before p. 11 and p. 12 as indicated: [The first change is a simple typo fix]

    >     unique_ptr& operator=(nullptr_t});

    > > 11 - Effects: `reset()`.

    > > 12 - Postcondition: `get() == 0 nullptr`

12. Change 20.8.1.2.4 [unique.ptr.single.observers] p. 1+4+12 as indicated:

    >     typename add_lvalue_reference<T>::type operator*() const;

    > > 1 - Requires: `get() != 0 nullptr`. The variable definition `add_lvalue_reference<T>::type t = *get()` shall be well formed, shall have well-defined behavior, and shall not exit via an exception.

    > > [..]

    >     pointer operator->() const;

    > > 4 - Requires: `get() != 0 nullptr`.

    > > [..]

    >     explicit operator bool() const;

    > > 12 - Returns: `get() != 0nullptr`.

13. Change 20.8.1.2.5 [unique.ptr.single.modifiers] p. 1 as indicated:

    >     pointer release();

    > > 1 - Postcondition: `get() == 0 nullptr`.

14. Change 20.8.1.2.5 [unique.ptr.single.modifiers] p. 9 as indicated: [The intent is to ensure that potentially user-defined swaps are used. A side-step fix and harmonization with the specification of the the deleter is realized. Please note the additional requirement in bullet 2 of this proposed resolution regarding the availability of the generic `swap` templates within the member `swap` function.]

    >     void swap(unique_ptr& u);

    > > 8 - Requires: The deleter `D` shall be `Swappable` and shall not throw an exception under `swap`.

    > > 9 - Effects: The stored pointers of `*this` and `u` are exchanged by an unqualified call to non-member `swap`. The stored deleters are `swap`'d (unqualified) exchanged by an unqualified call to non-member `swap`.

15. Change 20.8.1.3.3 [unique.ptr.runtime.observers] p. 1 as indicated:

    >     T& operator[](size_t i) const;

    > > Requires: `i <` the size of the array to which the stored pointer points. The variable definition `T& t = get()[i]` shall be well formed, shall have well-defined behavior, and shall not exit via an exception.

16. Change 20.8.1.3.4 [unique.ptr.runtime.modifiers] p. 1 as indicated:

    >     void reset(pointer p = pointer());
    >     void reset(nullptr_t p);

    > > 1 - Effects: If `get() == 0 nullptr` there are no effects. Otherwise `get_deleter()(get())`.

17. Change 20.8.1.5 [unique.ptr.special] as indicated: [We don't add the relational operators to the basic requirement set, therefore we need special handling here]

    >     template <class T1, class D1, class T2, class D2>
    >       bool operator==(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

    > > Requires: The variable definition `bool b = x.get() == y.get();` shall be well formed, shall have well-defined behavior, and shall not exit via an exception.

    > > 2 - Returns: `x.get() == y.get()`.

    > > Throws: nothing.

    >     template <class T1, class D1, class T2, class D2>
    >       bool operator!=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

    > > Requires: The variable definition `bool b = x.get() != y.get();` shall be well formed, shall have well-defined behavior, and shall not exit via an exception.

    > > 3 - Returns: `x.get() != y.get()`.

    > > Throws: nothing.

    >     template <class T1, class D1, class T2, class D2>
    >       bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

    > > Requires: The variable definition `bool b = x.get() < y.get()`; shall be well formed, shall have well-defined behavior, and shall not exit via an exception.

    > > 4 - Returns: `x.get() < y.get()`.

    > > Throws: nothing.

    >     template <class T1, class D1, class T2, class D2>
    >       bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

    > > Requires: The variable definition `bool b = x.get() <= y.get();` shall be well formed, shall have well-defined behavior, and shall not exit via an exception.

    > > 5 - Returns: `x.get() <= y.get()`.

    > > Throws: nothing.

    >     template <class T1, class D1, class T2, class D2>
    >       bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

    > > Requires: The variable definition `bool b = x.get() > y.get();` shall be well formed, shall have well-defined behavior, and shall not exit via an exception.

    > > 6 - Returns: `x.get() > y.get()`.

    > > Throws: nothing.

    >     template <class T1, class D1, class T2, class D2>
    >       bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

    > > Requires: The variable definition `bool b = x.get() >= y.get();` shall be well formed, shall have well-defined behavior, and shall not exit via an exception.

    > > 7 - Returns: `x.get() >= y.get()`.

    > > Throws: nothing.

### 835. Tying two streams together (correction to DR 581)

Section: 27.5.5.3 [basic.ios.members] Status: C++11 Submitter: Martin Sebor Opened: 2008-05-17 Last modified: 2015-04-08

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Discussion:

The fix for issue 581, now integrated into the working paper, overlooks a couple of minor problems.

First, being an unformatted function once again, `flush()` is required to create a sentry object whose constructor must, among other things, flush the tied stream. When two streams are tied together, either directly or through another intermediate stream object, flushing one will also cause a call to `flush()` on the other tied stream(s) and vice versa, ad infinitum. The program below demonstrates the problem.

Second, as Bo Persson notes in his comp.lang.c++.moderated post, for streams with the `unitbuf` flag set such as `std::stderr`, the destructor of the sentry object will again call `flush()`. This seems to create an infinite recursion for `std::cerr << std::flush;`

>     #include <iostream>

>     int main ()
>     {
>        std::cout.tie (&std::cerr);
>        std::cerr.tie (&std::cout);
>        std::cout << "cout\n";
>        std::cerr << "cerr\n";
>     }

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Review.

[ 2009-05-26 Daniel adds: ]

> I think that the most recently suggested change in 27.7.3.4 [ostream::sentry] need some further word-smithing. As written, it would make the behavior undefined, if under conditions when `pubsync()` should be called, but when in this scenario `os.rdbuf()` returns 0.

> This case is explicitly handled in `flush()` and needs to be taken care of. My suggested fix is:

> > If `((os.flags() & ios_base::unitbuf) && !uncaught_exception()` `&& os.rdbuf() != 0`) is true, calls `os.flush()` `os.rdbuf()->pubsync()`.

> Two secondary questions are:

> 1. Should `pubsync()` be invoked in any case or shouldn't a base requirement for this trial be that `os.good() == true` as required in the original `flush()` case?
> 2. Since `uncaught_exception()` is explicitly tested, shouldn't a return value of -1 of `pubsync()` produce `setstate(badbit)` (which may throw `ios_base::failure`)?

[ 2009-07 Frankfurt: ]

> Daniel volunteered to modify the proposed resolution to address his two questions.

> Move back to Open.

[ 2009-07-26 Daniel provided wording. Moved to Review. ]

[ 2009-10-13 Daniel adds: ]

> This proposed wording is written to match the outcome of 397.

[ 2009 Santa Cruz: ]

> Move to Open. Martin to propose updated wording that will also resolve issue 397 consistently.

[ 2010-02-15 Martin provided wording. ]

[ 2010 Pittsburgh: ]

> Moved to Ready for Pittsburgh.

Proposed resolution:

1. Just before 27.5.5.3 [basic.ios.members] p. 2 insert a new paragraph:

   > Requires: If `(tiestr != 0)` is `true`, `tiestr` must not be reachable by traversing the linked list of tied stream objects starting from `tiestr->tie()`.

2. Change 27.7.3.4 [ostream::sentry] p. 4 as indicated:

   > If `((os.flags() & ios_base::unitbuf) && !uncaught_exception() && os.good())` is `true`, calls `os.flush()` `os.rdbuf()->pubsync()`. If that function returns -1 sets `badbit` in `os.rdstate()` without propagating an exception.

3. Add after 27.7.3.4 [ostream::sentry] p17, the following paragraph:

   > Throws: Nothing.

### 836. Effects of `money_base::space` and `money_base::none` on `money_get`

Section: 22.4.6.1.2 [locale.money.get.virtuals] Status: C++11 Submitter: Martin Sebor Opened: 2008-05-17 Last modified: 2015-04-08

View all other issues in [locale.money.get.virtuals].

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Duplicate of: 670

Discussion:

In paragraph 2, 22.4.6.1.2 [locale.money.get.virtuals] specifies the following:

> Where `space` or `none` appears in the format pattern, except at the end, optional white space (as recognized by `ct.is`) is consumed after any required space.

This requirement can be (and has been) interpreted two mutually exclusive ways by different readers. One possible interpretation is that:

> 1. where `money_base::space` appears in the format, at least one space is required, and
> 2. where `money_base::none` appears in the format, space is allowed but not required.

The other is that:

> where either `money_base::space` or `money_base::none` appears in the format, white space is optional.

[ San Francisco: ]

> Martin will revise the proposed resolution.

[ 2009-07 Frankfurt: ]

> There is a noun missing from the proposed resolution. It's not clear that the last sentence would be helpful, even if the word were not missing:

> > In either case, any required MISSINGWORD followed by all optional whitespace (as recognized by `ct.is()`) is consumed.

> Strike this sentence and move to Review.

> [ Howard: done. ]

[ 2009-10 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

I propose to change the text to make it clear that the first interpretation is intended, that is, to make following change to 22.4.6.1.2 [locale.money.get.virtuals], p. 2:

> When `money_base::space` or `money_base::none` appears as the last element in the format pattern, except at the end, optional white space (as recognized by `ct.is`) is consumed after any required space. no white space is consumed. Otherwise, where `money_base::space` appears in any of the initial elements of the format pattern, at least one white space character is required. Where `money_base::none` appears in any of the initial elements of the format pattern, white space is allowed but not required. If `(str.flags() & str.showbase)` is `false`, ...

### 838. Can an end-of-stream iterator become a non-end-of-stream one?

Section: 24.6.1 [istream.iterator] Status: C++11 Submitter: Martin Sebor Opened: 2008-05-17 Last modified: 2015-04-08

View all other issues in [istream.iterator].

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Discussion:

From message c++std-lib-20003...

The description of `istream_iterator` in 24.6.1 [istream.iterator], p. 1 specifies that objects of the class become the end-of-stream (EOS) iterators under the following condition (see also issue 788 another problem with this paragraph):

> If the end of stream is reached (`operator void*()` on the stream returns `false`), the iterator becomes equal to the end-of-stream iterator value.

One possible implementation approach that has been used in practice is for the iterator to set its `in_stream` pointer to 0 when it reaches the end of the stream, just like the default ctor does on initialization. The problem with this approach is that the Effects clause for `operator++()` says the iterator unconditionally extracts the next value from the stream by evaluating `*in_stream >> value`, without checking for `(in_stream == 0)`.

Conformance to the requirement outlined in the Effects clause can easily be verified in programs by setting `eofbit` or `failbit` in `exceptions()` of the associated stream and attempting to iterate past the end of the stream: each past-the-end access should trigger an exception. This suggests that some other, more elaborate technique might be intended.

Another approach, one that allows `operator++()` to attempt to extract the value even for EOS iterators (just as long as `in_stream` is non-0) is for the iterator to maintain a flag indicating whether it has reached the end of the stream. This technique would satisfy the presumed requirement implied by the Effects clause mentioned above, but it isn't supported by the exposition-only members of the class (no such flag is shown). This approach is also found in existing practice.

The inconsistency between existing implementations raises the question of whether the intent of the specification is that a non-EOS iterator that has reached the EOS become a non-EOS one again after the stream's `eofbit` flag has been cleared? That is, are the assertions in the program below expected to pass?

>        sstream strm ("1 ");
>        istream_iterator eos;
>        istream_iterator it (strm);
>        int i;
>        i = *it++
>        assert (it == eos);
>        strm.clear ();
>        strm << "2 3 ";
>        assert (it != eos);
>        i = *++it;
>        assert (3 == i);

Or is it intended that once an iterator becomes EOS it stays EOS until the end of its lifetime?

[ San Francisco: ]

> We like the direction of the proposed resolution. We're not sure about the wording, and we need more time to reflect on it,

> Move to Open. Detlef to rewrite the proposed resolution in such a way that no reference is made to exposition only members of `istream_iterator`.

[ 2009-07 Frankfurt: ]

> Move to Ready.

Proposed resolution:

The discussion of this issue on the reflector suggests that the intent of the standard is for an `istreambuf_iterator` that has reached the EOS to remain in the EOS state until the end of its lifetime. Implementations that permit EOS iterators to return to a non-EOS state may only do so as an extension, and only as a result of calling `istream_iterator` member functions on EOS iterators whose behavior is in this case undefined.

To this end we propose to change 24.6.1 [istream.iterator], p1, as follows:

> The result of operator-> on an end-of-stream is not defined. For any other iterator value a `const T*` is returned. Invoking `operator++()` on an end-of-stream iterator is undefined. It is impossible to store things into istream iterators...

Add pre/postconditions to the member function descriptions of `istream_iterator` like so:

>     istream_iterator();

> Effects: Constructs the end-of-stream iterator.
> Postcondition: `in_stream == 0`.

>     istream_iterator(istream_type &s);

> Effects: Initializes `in_stream` with &s. value may be initialized during construction or the first time it is referenced.
> Postcondition: `in_stream == &s`.

>     istream_iterator(const istream_iterator &x);

> Effects: Constructs a copy of `x`.
> Postcondition: `in_stream == x.in_stream`.

>     istream_iterator& operator++();

> Requires: `in_stream != 0`.
> Effects: `*in_stream >> value`.

>     istream_iterator& operator++(int);

> Requires: `in_stream != 0`.
> Effects:

> >     istream_iterator tmp (*this);
> >     *in_stream >> value;
> >     return tmp;

### 842. `ConstructibleAsElement` and bit containers

Section: 23.2 [container.requirements], 23.3.7 [vector.bool], 20.6 [template.bitset] Status: CD1 Submitter: Howard Hinnant Opened: 2008-06-03 Last modified: 2015-04-08

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Discussion:

23.2 [container.requirements] p. 3 says:

> Objects stored in these components shall be constructed using `construct_element` (20.6.9). For each operation that inserts an element of type `T` into a container (`insert`, `push_back`, `push_front`, `emplace`, etc.) with arguments `args... T` shall be `ConstructibleAsElement`, as described in table 88. [Note: If the component is instantiated with a scoped allocator of type `A` (i.e., an allocator for which `is_scoped_allocator<A>::value` is `true`), then `construct_element` may pass an inner allocator argument to `T`'s constructor. -- end note]

However `vector<bool, A>` (23.3.7 [vector.bool]) and `bitset<N>` (20.6 [template.bitset]) store bits, not `bool`s, and `bitset<N>` does not even have an allocator. But these containers are governed by this clause. Clearly this is not implementable.

Proposed resolution:

Change 23.2 [container.requirements] p. 3:

> Objects stored in these components shall be constructed using `construct_element` (20.6.9), unless otherwise specified. For each operation that inserts an element of type `T` into a container (`insert`, `push_back`, `push_front`, `emplace`, etc.) with arguments `args... T` shall be `ConstructibleAsElement`, as described in table 88. [Note: If the component is instantiated with a scoped allocator of type `A` (i.e., an allocator for which `is_scoped_allocator<A>::value` is `true`), then `construct_element` may pass an inner allocator argument to `T`'s constructor. -- end note]

Change 23.3.7 [vector.bool]/p2:

> Unless described below, all operations have the same requirements and semantics as the primary `vector` template, except that operations dealing with the `bool` value type map to bit values in the container storage, and `construct_element` (23.2 [container.requirements]) is not used to construct these values.

Move 20.6 [template.bitset] to clause 20.

### 843. Reference Closure

Section: X [func.referenceclosure.cons] Status: CD1 Submitter: Lawrence Crowl Opened: 2008-06-02 Last modified: 2015-04-08

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Discussion:

The `std::reference_closure` type has a deleted copy assignment operator under the theory that references cannot be assigned, and hence the assignment of its reference member must necessarily be ill-formed.

However, other types, notably `std::reference_wrapper` and `std::function` provide for the "copying of references", and thus the current definition of `std::reference_closure` seems unnecessarily restrictive. In particular, it should be possible to write generic functions using both `std::function` and `std::reference_closure`, but this generality is much harder when one such type does not support assignment.

The definition of `reference_closure` does not necessarily imply direct implementation via reference types. Indeed, the `reference_closure` is best implemented via a frame pointer, for which there is no standard type.

The semantics of assignment are effectively obtained by use of the default destructor and default copy assignment operator via

>     x.~reference_closure(); new (x) reference_closure(y);

So the copy assignment operator generates no significant real burden to the implementation.

Proposed resolution:

In [func.referenceclosure] Class template reference_closure, replace the `=delete` in the copy assignment operator in the synopsis with `=default`.

>     template<class R , class... ArgTypes >
>       class reference_closure<R (ArgTypes...)> {
>       public:
>          ...
>          reference_closure& operator=(const reference_closure&) = delete default;
>          ...

In X [func.referenceclosure.cons] Construct, copy, destroy, add the member function description

>     reference_closure& operator=(const reference_closure& f)

> > Postcondition: `*this` is a copy of `f`.

> > Returns: `*this`.

### 844. `complex pow` return type is ambiguous

Section: 26.4.9 [cmplx.over] Status: CD1 Submitter: Howard Hinnant Opened: 2008-06-03 Last modified: 2015-04-08

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Discussion:

The current working draft is in an inconsistent state.

26.4.8 [complex.transcendentals] says that:

> `pow(complex<float>(), int())` returns a `complex<float>`.

26.4.9 [cmplx.over] says that:

> `pow(complex<float>(), int())` returns a `complex<double>`.

[ Sophia Antipolis: ]

> Since `int` promotes to `double`, and C99 doesn't have an `int`-based overload for `pow`, the C99 result is `complex<double>`, see also C99 7.22, see also library issue 550.

> Special note: ask P.J. Plauger.

> > Looks fine.

Proposed resolution:

Strike this `pow` overload in 26.4.1 [complex.syn] and in 26.4.8 [complex.transcendentals]:

>     template<class T> complex<T> pow(const complex<T>& x, int y);

### 845. atomics cannot support aggregate initialization

Section: 29.5 [atomics.types.generic] Status: CD1 Submitter: Alisdair Meredith Opened: 2008-06-03 Last modified: 2015-04-08

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Discussion:

The atomic classes (and class templates) are required to support aggregate initialization (X [atomics.types.integral] p. 2 / X [atomics.types.address] p. 1) yet also have user declared constructors, so cannot be aggregates.

This problem might be solved with the introduction of the proposed initialization syntax at Antipolis, but the wording above should be altered. Either strike the sentence as redundant with new syntax, or refer to 'brace initialization'.

[ Jens adds: ]

> Note that

> >     atomic_itype a1 = { 5 };

> would be aggregate-initialization syntax (now coming under the disguise of brace initialization), but would be ill-formed, because the corresponding constructor for atomic_itype is explicit. This works, though:

> >     atomic_itype a2 { 6 };

[ San Francisco: ]

> The preferred approach to resolving this is to remove the explicit specifiers from the atomic integral type constructors.

> Lawrence will provide wording.

> This issue is addressed in N2783.

Proposed resolution:

within the synopsis in X [atomics.types.integral] edit as follows.

>     ....
>     typedef struct atomic_bool {
>     ....
>       constexpr explicit atomic_bool(bool);
>     ....
>     typedef struct atomic_itype {
>     ....
>       constexpr explicit atomic_itype(integral);
>     ....

edit X [atomics.types.integral] paragraph 2 as follows.

> The atomic integral types shall have standard layout. They shall each have a trivial default constructor, a constexpr explicit value constructor, a deleted copy constructor, a deleted copy assignment operator, and a trivial destructor. They shall each support aggregate initialization syntax.

within the synopsis of X [atomics.types.address] edit as follows.

>     ....
>     typedef struct atomic_address {
>     ....
>       constexpr explicit atomic_address(void*);
>     ....

edit X [atomics.types.address] paragraph 1 as follows.

> The type `atomic_address` shall have standard layout. It shall have a trivial default constructor, a constexpr explicit value constructor, a deleted copy constructor, a deleted copy assignment operator, and a trivial destructor. It shall support aggregate initialization syntax.

within the synopsis of 29.5 [atomics.types.generic] edit as follows.

>     ....
>     template <class T> struct atomic {
>     ....
>       constexpr explicit atomic(T);
>     ....
>     template <> struct atomic<integral> : atomic_itype {
>     ....
>       constexpr explicit atomic(integral);
>     ....
>     template <> struct atomic<T*> : atomic_address {
>     ....
>       constexpr explicit atomic(T*);
>     ....

edit 29.5 [atomics.types.generic] paragraph 2 as follows.

> Specializations of the `atomic` template shall have a deleted copy constructor, a deleted copy assignment operator, and a constexpr explicit value constructor.

### 846. No definition for constructor

Section: 29.6 [atomics.types.operations] Status: CD1 Submitter: Alisdair Meredith Opened: 2008-06-03 Last modified: 2015-04-08

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Discussion:

The atomic classes and class templates (X [atomics.types.integral] / X [atomics.types.address]) have a constexpr constructor taking a value of the appropriate type for that atomic. However, neither clause provides semantics or a definition for this constructor. I'm not sure if the initialization is implied by use of constexpr keyword (which restricts the form of a constructor) but even if that is the case, I think it is worth spelling out explicitly as the inference would be far too subtle in that case.

[ San Francisco: ]

> Lawrence will provide wording.

> This issue is addressed in N2783.

Proposed resolution:

before the description of ...`is_lock_free`, that is before 29.6 [atomics.types.operations] paragraph 4, add the following description.

>     constexpr A::A(C desired);

> Effects:
>   Initializes the object with the value `desired`. [Note: Construction is not atomic. —end note]

### 847. string exception safety guarantees

Section: 21.4.1 [string.require] Status: C++11 Submitter: Hervé Brönnimann Opened: 2008-06-05 Last modified: 2015-04-08

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Discussion:

In March, on comp.lang.c++.moderated, I asked what were the string exception safety guarantees are, because I cannot see *any* in the working paper, and any implementation I know offers the strong exception safety guarantee (string unchanged if a member throws exception). The closest the current draft comes to offering any guarantees is 21.4 [basic.string], para 3:

> The class template `basic_string` conforms to the requirements for a Sequence Container (23.1.1), for a Reversible Container (23.1), and for an Allocator-aware container (91). The iterators supported by `basic_string` are random access iterators (24.1.5).

However, the chapter 23 only says, on the topic of exceptions: 23.2 [container.requirements], para 10:

> Unless otherwise specified (see 23.2.2.3 and 23.2.6.4) all container types defined in this clause meet the following additional requirements:

> - if an exception is thrown by...

I take it as saying that this paragraph has *no* implication on `std::basic_string`, as `basic_string` isn't defined in Clause 23 and this paragraph does not define a *requirement* of Sequence nor Reversible Container, just of the models defined in Clause 23. In addition, LWG Issue 718 proposes to remove 23.2 [container.requirements], para 3.

Finally, the fact that no operation on Traits should throw exceptions has no bearing, except to suggest (since the only other throws should be allocation, `out_of_range`, or `length_error`) that the strong exception guarantee can be achieved.

The reaction in that group by Niels Dekker, Martin Sebor, and Bo Persson, was all that this would be worth an LWG issue.

A related issue is that `erase()` does not throw. This should be stated somewhere (and again, I don't think that the 23.2 [container.requirements], para 1 applies here).

[ San Francisco: ]

> Implementors will study this to confirm that it is actually possible.

[ Daniel adds 2009-02-14: ]

> The proposed resolution of paper N2815 interacts with this issue (the paper does not refer to this issue).

[ 2009-07 Frankfurt: ]

> Move to Ready.

Proposed resolution:

Add a blanket statement in 21.4.1 [string.require]:

> - if any member function or operator of `basic_string<charT, traits, Allocator>` throws, that function or operator has no effect.

> - no `erase()` or `pop_back()` function throws.

As far as I can tell, this is achieved by any implementation. If I made a mistake and it is not possible to offer this guarantee, then either state all the functions for which this is possible (certainly at least `operator+=`, `append`, `assign`, and `insert`), or add paragraphs to Effects clauses wherever appropriate.

### 848. Missing `std::hash` specializations for `std::bitset/std::vector<bool>`

Section: 20.9.13 [unord.hash] Status: CD1 Submitter: Thorsten Ottosen Opened: 2008-06-05 Last modified: 2015-04-08

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Discussion:

In the current working draft, `std::hash<T>` is specialized for builtin types and a few other types. Bitsets seems like one that is missing from the list, not because it cannot not be done by the user, but because it is hard or impossible to write an efficient implementation that works on 32bit/64bit chunks at a time. For example, `std::bitset` is too much encapsulated in this respect.

Proposed resolution:

Add the following to the synopsis in 20.9 [function.objects]/2:

>     template<class Allocator> struct hash<std::vector<bool,Allocator>>;
>     template<size_t N> struct hash<std::bitset<N>>;

Modify the last sentence of 20.9.13 [unord.hash]/1 to end with:

> ... and `std::string`, `std::u16string`, `std::u32string`, `std::wstring`, `std::error_code`, `std::thread::id`, `std::bitset`, `and std::vector<bool>`.

### 850. Should `shrink_to_fit` apply to `std::deque`?

Section: 23.3.3.3 [deque.capacity] Status: CD1 Submitter: Niels Dekker Opened: 2008-06-05 Last modified: 2015-04-08

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Discussion:

Issue 755 added a `shrink_to_fit` function to `std::vector` and `std::string`. It did not yet deal with `std::deque`, because of the fundamental difference between `std::deque` and the other two container types. The need for `std::deque` may seem less evident, because one might think that for this container, the overhead is a small map, and some number of blocks that's bounded by a small constant.

The container overhead can in fact be arbitrarily large (i.e. is not necessarily O(N) where N is the number of elements currently held by the `deque`). As Bill Plauger noted in a reflector message, unless the map of block pointers is shrunk, it must hold at least maxN⁄B pointers where maxN is the maximum of N over the lifetime of the `deque` since its creation. This is independent of how the map is implemented (`vector`-like circular buffer and all), and maxN bears no relation to N, the number of elements it currently holds.

Hervé Brönnimann reports a situation where a `deque` of requests grew very large due to some temporary backup (the front request hanging), and the map of the `deque` grew quite large before getting back to normal. Just to put some color on it, assuming a `deque` with 1K pointer elements in steady regime, that held, at some point in its lifetime, maxN=10M pointers, with one block holding 128 elements, the spine must be at least (maxN ⁄ 128), in that case 100K. In that case, shrink-to-fit would allow to reuse about 100K which would otherwise never be reclaimed in the lifetime of the `deque`.

An added bonus would be that it allows implementations to hang on to empty blocks at the end (but does not care if they do or not). A `shrink_to_fit` would take care of both shrinks, and guarantee that at most O(B) space is used in addition to the storage to hold the N elements and the N⁄B block pointers.

Proposed resolution:

To class template `deque` 23.3.3 [deque] synopsis, add:

>     void shrink_to_fit();

To deque capacity 23.3.3.3 [deque.capacity], add:

>     void shrink_to_fit();

> > Remarks: `shrink_to_fit` is a non-binding request to reduce memory use. [Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note]

### 852. unordered containers `begin(n)` mistakenly `const`

Section: 23.5 [unord] Status: CD1 Submitter: Robert Klarer Opened: 2008-06-12 Last modified: 2015-04-08

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Discussion:

In 3 of the four unordered containers the local `begin` member is mistakenly declared `const`:

>     local_iterator begin(size_type n) const;

Proposed resolution:

Change the synopsis in 23.5.4 [unord.map], 23.5.5 [unord.multimap], and 23.5.7 [unord.multiset]:

>     local_iterator begin(size_type n) const;

### 853. `to_string` needs updating with `zero` and `one`

Section: 20.6 [template.bitset] Status: C++11 Submitter: Howard Hinnant Opened: 2008-06-18 Last modified: 2015-04-08

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Discussion:

Issue 396 adds defaulted arguments to the `to_string` member, but neglects to update the three newer `to_string` overloads.

[ post San Francisco: ]

> Daniel found problems with the wording and provided fixes. Moved from Ready to Review.

[ Post Summit: ]

> Alisdair: suggest to not repeat the default arguments in B, C, D (definition of to_string members)

> Walter: This is not really a definition.

> Consensus: Add note to the editor: Please apply editor's judgement whether default arguments should be repeated for B, C, D changes.

> Recommend Tentatively Ready.

[ 2009-05-09: See alternative solution in issue 1113. ]

Proposed resolution:

1. replace in 20.6 [template.bitset]/1 (class `bitset`)

   >     template <class charT, class traits>
   >       basic_string<charT, traits, allocator<charT> >
   >       to_string(charT zero = charT('0'), charT one = charT('1')) const;
   >     template <class charT>
   >       basic_string<charT, char_traits<charT>, allocator<charT> >
   >       to_string(charT zero = charT('0'), charT one = charT('1')) const;
   >     basic_string<char, char_traits<char>, allocator<char> >
   >       to_string(char zero = '0', char one = '1') const;

2. replace in 20.6.2 [bitset.members]/37

   >     template <class charT, class traits>
   >       basic_string<charT, traits, allocator<charT> >
   >       to_string(charT zero = charT('0'), charT one = charT('1')) const;

   > > 37 Returns: `to_string<charT, traits, allocator<charT> >(zero, one)`.

3. replace in 20.6.2 [bitset.members]/38

   >     template <class charT>
   >       basic_string<charT, char_traits<charT>, allocator<charT> >
   >       to_string(charT zero = charT('0'), charT one = charT('1')) const;

   > > 38 Returns: `to_string<charT, char_traits<charT>, allocator<charT> >(zero, one)`.

4. replace in 20.6.2 [bitset.members]/39

   >     basic_string<char, char_traits<char>, allocator<char> >
   >       to_string(char zero = '0', char one = '1') const;

   > > 39 Returns: `to_string<char, char_traits<char>, allocator<char> >(zero, one)`.

### 854. `default_delete` converting constructor underspecified

Section: 20.8.1.1.2 [unique.ptr.dltr.dflt] Status: C++11 Submitter: Howard Hinnant Opened: 2008-06-18 Last modified: 2015-04-08

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Discussion:

No relationship between `U` and `T` in the converting constructor for `default_delete` template.

Requirements: `U*` is convertible to `T*` and `has_virtual_destructor<T>`; the latter should also become a concept.

Rules out cross-casting.

The requirements for `unique_ptr` conversions should be the same as those on the deleter.

[ Howard adds 2008-11-26: ]

> I believe we need to be careful to not outlaw the following use case, and I believe the current proposed wording (`requires Convertible<U*, T*> && HasVirtualDestructor<T>`) does so:

> >     #include <memory>

> >     int main()
> >     {
> >         std::unique_ptr<int> p1(new int(1));
> >         std::unique_ptr<const int> p2(move(p1));
> >         int i = *p2;
> >     //    *p2 = i;  // should not compile
> >     }

> I've removed "`&& HasVirtualDestructor<T>`" from the `requires` clause in the proposed wording.

[ Post Summit: ]

> Alisdair: This issue has to stay in review pending a paper constraining `unique_ptr`.

> Consensus: We agree with the resolution, but `unique_ptr` needs to be constrained, too.

> Recommend Keep in Review.

[ Batavia (2009-05): ]

> Keep in Review status for the reasons cited.

[ 2009-07 Frankfurt: ]

> The proposed resolution uses concepts. Howard needs to rewrite the proposed resolution.

> Move back to Open.

[ 2009-07-26 Howard provided rewritten proposed wording and moved to Review. ]

[ 2009-10 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

Add after 20.8.1.1.2 [unique.ptr.dltr.dflt], p1:

>     template <class U> default_delete(const default_delete<U>& other);

> > -1- Effects: ...

> > Remarks: This constructor shall participate in overload resolution if and only if `U*` is implicitly convertible to `T*`.

### 856. Removal of `aligned_union`

Section: 20.10.7.6 [meta.trans.other] Status: CD1 Submitter: Jens Maurer Opened: 2008-06-12 Last modified: 2015-04-08

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Discussion:

With the arrival of extended unions (N2544), there is no known use of `aligned_union` that couldn't be handled by the "extended unions" core-language facility.

Proposed resolution:

Remove the following signature from 20.10.2 [meta.type.synop]:

>     template <std::size_t Len, class... Types> struct aligned_union;

Remove the second row from table 51 in 20.10.7.6 [meta.trans.other], starting with:

>     template <std::size_t Len,
>     class... Types>
>     struct aligned_union;

### 857. `condition_variable::time_wait` return `bool` error prone

Section: 30.5.1 [thread.condition.condvar] Status: C++11 Submitter: Beman Dawes Opened: 2008-06-13 Last modified: 2015-04-08

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Discussion:

The meaning of the `bool` returned by `condition_variable::timed_wait` is so obscure that even the class' designer can't deduce it correctly. Several people have independently stumbled on this issue.

It might be simpler to change the return type to a scoped enum:

>     enum class timeout { not_reached, reached };

That's the same cost as returning a `bool`, but not subject to mistakes. Your example below would be:

>     if (cv.wait_until(lk, time_limit) == timeout::reached )
>       throw time_out();

[ Beman to supply exact wording. ]

[ San Francisco: ]

> There is concern that the enumeration names are just as confusing, if not more so, as the bool. You might have awoken because of a signal or a spurious wakeup, for example.

> Group feels that this is a defect that needs fixing.

> Group prefers returning an enum over a void return.

> Howard to provide wording.

[ 2009-06-14 Beman provided wording. ]

[ 2009-07 Frankfurt: ]

> Move to Ready.

Proposed resolution:

Change Condition variables 30.5 [thread.condition], Header condition_variable synopsis, as indicated:

>     namespace std {
>       class condition_variable;
>       class condition_variable_any;

>       enum class cv_status { no_timeout, timeout };
>     }

Change class `condition_variable` 30.5.1 [thread.condition.condvar] as indicated:

>     class condition_variable {
>     public:
>       ...
>       template <class Clock, class Duration>
>         bool cv_status wait_until(unique_lock<mutex>& lock,
>                         const chrono::time_point<Clock, Duration>& abs_time);
>       template <class Clock, class Duration, class Predicate>
>         bool wait_until(unique_lock<mutex>& lock,
>                         const chrono::time_point<Clock, Duration>& abs_time,
>                         Predicate pred);

>       template <class Rep, class Period>
>         bool cv_status wait_for(unique_lock<mutex>& lock,
>                       const chrono::duration<Rep, Period>& rel_time);
>       template <class Rep, class Period, class Predicate>
>         bool wait_for(unique_lock<mutex>& lock,
>                       const chrono::duration<Rep, Period>& rel_time,
>                       Predicate pred);
>       ...
>     };

>     ...

>     template <class Clock, class Duration>
>       bool cv_status wait_until(unique_lock<mutex>& lock,
>                       const chrono::time_point<Clock, Duration>& abs_time);

> > -15- Precondition: `lock` is locked by the calling thread, and either

> > - no other thread is waiting on this `condition_variable` object or
> > - `lock.mutex()` returns the same value for each of the `lock` arguments supplied by all concurrently waiting threads (via `wait`, `wait_for` or `wait_until`.).

> > -16- Effects:

> > - Atomically calls `lock.unlock()` and blocks on `*this`.
> > - When unblocked, calls `lock.lock()` (possibly blocking on the lock) and returns.
> > - The function will unblock when signaled by a call to `notify_one()`, a call to `notify_all()`, by the current time exceeding `abs_time` if `Clock::now() >= abs_time`, or spuriously.
> > - If the function exits via an exception, `lock.unlock()` shall be called prior to exiting the function scope.

> > -17- Postcondition: `lock` is locked by the calling thread.

> > -18- Returns: `Clock::now() < abs_time` `cv_status::timeout` if the function unblocked because `abs_time` was reached, otherwise `cv_status::no_timeout`.

> > -19- Throws: `std::system_error` when the effects or postcondition cannot be achieved.

> > -20- Error conditions:

> > - `operation_not_permitted` — if the thread does not own the lock.
> > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

>     template <class Rep, class Period>
>       bool cv_status wait_for(unique_lock<mutex>& lock,
>                     const chrono::duration<Rep, Period>& rel_time);

> > -21- Effects Returns:

> > >     wait_until(lock, chrono::monotonic_clock::now() + rel_time)

> > -22- Returns: `false` if the call is returning because the time duration specified by `rel_time` has elapsed, otherwise `true`.

> > [ This part of the wording may conflict with 859 in detail, but does not do so in spirit. If both issues are accepted, there is a logical merge. ]

>     template <class Clock, class Duration, class Predicate>
>       bool wait_until(unique_lock<mutex>& lock,
>                       const chrono::time_point<Clock, Duration>& abs_time,
>                       Predicate pred);

> > -23- Effects:

> > >     while (!pred())
> > >       if (!wait_until(lock, abs_time) == cv_status::timeout)
> > >         return pred();
> > >     return true;

> > -24- Returns: `pred()`.

> > -25- [Note: The returned value indicates whether the predicate evaluates to `true` regardless of whether the timeout was triggered. — end note].

Change Class condition_variable_any 30.5.2 [thread.condition.condvarany] as indicated:

>     class condition_variable_any {
>     public:
>       ...
>       template <class Lock, class Clock, class Duration>
>         bool cv_status wait_until(Lock& lock,
>                         const chrono::time_point<Clock, Duration>& abs_time);
>       template <class Lock, class Clock, class Duration, class Predicate>
>         bool wait_until(Lock& lock,
>                         const chrono::time_point<Clock, Duration>& abs_time,
>                         Predicate pred);

>       template <class Lock, class Rep, class Period>
>         bool cv_status wait_for(Lock& lock,
>                       const chrono::duration<Rep, Period>& rel_time);
>       template <class Lock, class Rep, class Period, class Predicate>
>         bool wait_for(Lock& lock,
>                       const chrono::duration<Rep, Period>& rel_time,
>                       Predicate pred);
>       ...
>     };

>     ...

>     template <class Lock, class Clock, class Duration>
>       bool cv_status wait_until(Lock& lock,
>                       const chrono::time_point<Clock, Duration>& abs_time);

> > -13- Effects:

> > - Atomically calls `lock.unlock()` and blocks on `*this`.
> > - When unblocked, calls `lock.lock()` (possibly blocking on the lock) and returns.
> > - The function will unblock when signaled by a call to `notify_one()`, a call to `notify_all()`, by the current time exceeding `abs_time` if `Clock::now() >= abs_time`, or spuriously.
> > - If the function exits via an exception, `lock.unlock()` shall be called prior to exiting the function scope.

> > -14- Postcondition: `lock` is locked by the calling thread.

> > -15- Returns: `Clock::now() < abs_time` `cv_status::timeout` if the function unblocked because `abs_time` was reached, otherwise `cv_status::no_timeout`.

> > -16- Throws: `std::system_error` when the effects or postcondition cannot be achieved.

> > -17- Error conditions:

> > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

>     template <class Lock, class Rep, class Period>
>       bool cv_status wait_for(Lock& lock,
>                     const chrono::duration<Rep, Period>& rel_time);

> > -18- Effects Returns:

> > >     wait_until(lock, chrono::monotonic_clock::now() + rel_time)

> > -19- Returns: `false` if the call is returning because the time duration specified by `rel_time` has elapsed, otherwise `true`.

> > [ This part of the wording may conflict with 859 in detail, but does not do so in spirit. If both issues are accepted, there is a logical merge. ]

>     template <class Lock, class Clock, class Duration, class Predicate>
>       bool wait_until(Lock& lock,
>                       const chrono::time_point<Clock, Duration>& rel_time abs_time,
>                       Predicate pred);

> > -20- Effects:

> > >     while (!pred())
> > >       if (!wait_until(lock, abs_time) == cv_status::timeout)
> > >         return pred();
> > >     return true;

> > -21- Returns: `pred()`.

> > -22- [Note: The returned value indicates whether the predicate evaluates to `true` regardless of whether the timeout was triggered. — end note].

### 858. Wording for Minimal Support for Garbage Collection

Section: 20.7.4 [util.dynamic.safety] Status: CD1 Submitter: Pete Becker Opened: 2008-06-21 Last modified: 2015-04-08

View all other issues in [util.dynamic.safety].

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Discussion:

The first sentence of the Effects clause for `undeclare_reachable` seems to be missing some words. I can't parse

> ... for all non-null `p` referencing the argument is no longer declared reachable...

I take it the intent is that `undeclare_reachable` should be called only when there has been a corresponding call to `declare_reachable`. In particular, although the wording seems to allow it, I assume that code shouldn't call `declare_reachable` once then call `undeclare_reachable` twice.

I don't know what "shall be live" in the Requires clause means.

In the final Note for `undeclare_reachable`, what does "cannot be deallocated" mean? Is this different from "will not be able to collect"?

For the wording on nesting of `declare_reachable` and `undeclare_reachable`, the words for locking and unlocking recursive mutexes probably are a good model.

[ San Francisco: ]

> Nick: what does "shall be live" mean?

> Hans: I can provide an appropriate cross-reference to the Project Editor to clarify the intent.

Proposed resolution:

In 20.7.4 [util.dynamic.safety] (N2670, Minimal Support for Garbage Collection)

Add at the beginning, before paragraph 39

> A complete object is declared reachable while the number of calls to `declare_reachable` with an argument referencing the object exceeds the number of `undeclare_reachable` calls with pointers to the same complete object.

Change paragraph 42 (Requires clause for `undeclare_reachable`)

> If `p` is not null, `declare_reachable(p)` was previously called the complete object referenced by `p` shall have been previously declared reachable, and shall be live (3.8 [basic.life]) from the time of the call until the last `undeclare_reachable(p)` call on the object.

Change the first sentence in paragraph 44 (Effects clause for `undeclare_reachable`):

> Effects: Once the number of calls to `undeclare_reachable(p)` equals the number of calls to `declare_reachable(p)` for all non-null `p` referencing the argument is no longer declared reachable. When this happens, pointers to the object referenced by p may not be subsequently dereferenced. After a call to `undeclare_reachable(p)`, if `p` is not null and the object `q` referenced by `p` is no longer declared reachable, then dereferencing any pointer to `q` that is not safely derived results in undefined behavior. ...

Change the final note:

[Note: It is expected that calls to `declare_reachable(p)` will consume a small amount of memory, in addition to that occupied by the referenced object, until the matching call to `undeclare_reachable(p)` is encountered. In addition, the referenced object cannot be deallocated during this period, and garbage collecting implementations will not be able to collect the object while it is declared reachable. Long running programs should arrange that calls for short-lived objects are matched. --end note]

### 859. Monotonic Clock is Conditionally Supported?

Section: 30.5 [thread.condition] Status: C++11 Submitter: Pete Becker Opened: 2008-06-23 Last modified: 2015-04-08

View all other issues in [thread.condition].

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Discussion:

Related to 958, 959.

N2661 says that there is a class named `monotonic_clock`. It also says that this name may be a synonym for `system_clock`, and that it's conditionally supported. So the actual requirement is that it can be monotonic or not, and you can tell by looking at `is_monotonic`, or it might not exist at all (since it's conditionally supported). Okay, maybe too much flexibility, but so be it.

A problem comes up in the threading specification, where several variants of `wait_for` explicitly use `monotonic_clock::now()`. What is the meaning of an effects clause that says

>     wait_until(lock, chrono::monotonic_clock::now() + rel_time)

when `monotonic_clock` is not required to exist?

[ San Francisco: ]

> Nick: maybe instead of saying that `chrono::monotonic_clock` is conditionally supported, we could say that it's always there, but not necessarily supported..

> Beman: I'd prefer a typedef that identifies the best clock to use for `wait_for` locks.

> Tom: combine the two concepts; create a duration clock type, but keep the is_monotonic test.

> Howard: if we create a `duration_clock` type, is it a typedef or an entirely true type?

> There was broad preference for a typedef.

> Move to Open. Howard to provide wording to add a typedef for duration_clock and to replace all uses of monotonic_clock in function calls and signatures with duration_clock.

[ Howard notes post-San Francisco: ]

> After further thought I do not believe that creating a `duration_clock typedef` is the best way to proceed. An implementation may not need to use a `time_point` to implement the `wait_for` functions.

> For example, on POSIX systems `sleep_for` can be implemented in terms of `nanosleep` which takes only a duration in terms of nanoseconds. The current working paper does not describe `sleep_for` in terms of `sleep_until`. And paragraph 2 of 30.2.4 [thread.req.timing] has the words strongly encouraging implementations to use monotonic clocks for `sleep_for`:

> > 2 The member functions whose names end in `_for` take an argument that specifies a relative time. Implementations should use a monotonic clock to measure time for these functions.

> I believe the approach taken in describing the effects of `sleep_for` and `try_lock_for` is also appropriate for `wait_for`. I.e. these are not described in terms of their `_until` variants.

[ 2009-07 Frankfurt: ]

> Beman will send some suggested wording changes to Howard.

> Move to Ready.

[ 2009-07-21 Beman added the requested wording changes to 962. ]

Proposed resolution:

Change 30.5.1 [thread.condition.condvar], p21-22:

>     template <class Rep, class Period>
>       bool wait_for(unique_lock<mutex>& lock,
>                     const chrono::duration<Rep, Period>& rel_time);

> > Precondition: `lock` is locked by the calling thread, and either

> > - no other thread is waiting on this `condition_variable` object or
> > - `lock.mutex()` returns the same value for each of the `lock` arguments supplied by all concurrently waiting threads (via `wait`, `wait_for` or `wait_until`).

> > 21 Effects:

> > >     wait_until(lock, chrono::monotonic_clock::now() + rel_time)

> > - Atomically calls `lock.unlock()` and blocks on `*this`.
> > - When unblocked, calls `lock.lock()` (possibly blocking on the lock) and returns.
> > - The function will unblock when signaled by a call to `notify_one()`, a call to `notify_all()`, by the elapsed time `rel_time` passing (30.2.4 [thread.req.timing]), or spuriously.
> > - If the function exits via an exception, `lock.unlock()` shall be called prior to exiting the function scope.

> > Postcondition: `lock` is locked by the calling thread.

> > 22 Returns: `false` if the call is returning because the time duration specified by `rel_time` has elapsed, otherwise `true`.

> > [ This part of the wording may conflict with 857 in detail, but does not do so in spirit. If both issues are accepted, there is a logical merge. ]

> > Throws: `std::system_error` when the effects or postcondition cannot be achieved.

> > Error conditions:

> > - `operation_not_permitted` -- if the thread does not own the lock.
> > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

Change 30.5.1 [thread.condition.condvar], p26-p29:

>     template <class Rep, class Period, class Predicate>
>       bool wait_for(unique_lock<mutex>& lock,
>                     const chrono::duration<Rep, Period>& rel_time,
>                     Predicate pred);

> > Precondition: `lock` is locked by the calling thread, and either

> > - no other thread is waiting on this `condition_variable` object or
> > - `lock.mutex()` returns the same value for each of the `lock` arguments supplied by all concurrently waiting threads (via `wait`, `wait_for` or `wait_until`).

> > 26 Effects:

> > >     wait_until(lock, chrono::monotonic_clock::now() + rel_time, std::move(pred))

> > > - Executes a loop: Within the loop the function first evaluates `pred()` and exits the loop if the result of `pred()` is `true`.
> > > - Atomically calls `lock.unlock()` and blocks on `*this`.
> > > - When unblocked, calls `lock.lock()` (possibly blocking on the lock).
> > > - The function will unblock when signaled by a call to `notify_one()`, a call to `notify_all()`, by the elapsed time `rel_time` passing (30.1.4 [thread.req.timing]), or spuriously.
> > > - If the function exits via an exception, `lock.unlock()` shall be called prior to exiting the function scope.
> > > - The loop terminates when `pred()` returns `true` or when the time duration specified by `rel_time` has elapsed.

> > 27 [Note: There is no blocking if `pred()` is initially `true`, even if the timeout has already expired. -- end note]

> > Postcondition: `lock` is locked by the calling thread.

> > 28 Returns: `pred()`

> > 29 [Note: The returned value indicates whether the predicate evaluates to `true` regardless of whether the timeout was triggered. -- end note]

> > Throws: `std::system_error` when the effects or postcondition cannot be achieved.

> > Error conditions:

> > - `operation_not_permitted` -- if the thread does not own the lock.
> > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

Change 30.5.2 [thread.condition.condvarany], p18-19:

>     template <class Lock, class Rep, class Period>
>       bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);

> > 18 Effects:

> > >     wait_until(lock, chrono::monotonic_clock::now() + rel_time)

> > - Atomically calls `lock.unlock()` and blocks on `*this`.
> > - When unblocked, calls `lock.lock()` (possibly blocking on the lock) and returns.
> > - The function will unblock when signaled by a call to `notify_one()`, a call to `notify_all()`, by the elapsed time `rel_time` passing (30.2.4 [thread.req.timing]), or spuriously.
> > - If the function exits via an exception, `lock.unlock()` shall be called prior to exiting the function scope.

> > Postcondition: `lock` is locked by the calling thread.

> > 19 Returns: `false` if the call is returning because the time duration specified by `rel_time` has elapsed, otherwise `true`.

> > Throws: `std::system_error` when the returned value, effects, or postcondition cannot be achieved.

> > Error conditions:

> > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

Change 30.5.2 [thread.condition.condvarany], p23-p26:

>     template <class Lock, class Rep, class Period, class Predicate>
>       bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);

> > Precondition: `lock` is locked by the calling thread, and either

> > - no other thread is waiting on this `condition_variable` object or
> > - `lock.mutex()` returns the same value for each of the `lock` arguments supplied by all concurrently waiting threads (via `wait`, `wait_for` or `wait_until`).

> > 23 Effects:

> > >     wait_until(lock, chrono::monotonic_clock::now() + rel_time, std::move(pred))

> > > - Executes a loop: Within the loop the function first evaluates `pred()` and exits the loop if the result of `pred()` is `true`.
> > > - Atomically calls `lock.unlock()` and blocks on `*this`.
> > > - When unblocked, calls `lock.lock()` (possibly blocking on the lock).
> > > - The function will unblock when signaled by a call to `notify_one()`, a call to `notify_all()`, by the elapsed time `rel_time` passing (30.1.4 [thread.req.timing]), or spuriously.
> > > - If the function exits via an exception, `lock.unlock()` shall be called prior to exiting the function scope.
> > > - The loop terminates when `pred()` returns `true` or when the time duration specified by `rel_time` has elapsed.

> > 24 [Note: There is no blocking if `pred()` is initially `true`, even if the timeout has already expired. -- end note]

> > Postcondition: `lock` is locked by the calling thread.

> > 25 Returns: `pred()`

> > 26 [Note: The returned value indicates whether the predicate evaluates to `true` regardless of whether the timeout was triggered. -- end note]

> > Throws: `std::system_error` when the effects or postcondition cannot be achieved.

> > Error conditions:

> > - `operation_not_permitted` -- if the thread does not own the lock.
> > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

### 860. Floating-Point State

Section: 26 [numerics] Status: C++11 Submitter: Lawrence Crowl Opened: 2008-06-23 Last modified: 2015-04-08

View all other issues in [numerics].

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Discussion:

There are a number of functions that affect the floating point state. These function need to be thread-safe, but I'm unsure of the right approach in the standard, as we inherit them from C.

[ San Francisco: ]

> Nick: I think we already say that these functions do not introduce data races; see 17.6.5.6/20

> Pete: there's more to it than not introducing data races; are these states maintained per thread?

> Howard: 21.5/14 says that strtok and strerror are not required to avoid data races, and 20.9/2 says the same about asctime, gmtime, ctime, and gmtime.

> Nick: POSIX has a list of not-safe functions. All other functions are implicitly thread safe.

> Lawrence is to form a group between meetings to attack this issue. Nick and Tom volunteered to work with Lawrence.

> Move to Open.

[ Post Summit: ]

> Hans: Sane oses seem ok. Sensible thing is implementable and makes sense.

> Nick: Default wording seems to cover this? Hole in POSIX, these functions need to be added to list of thread-unsafe functions.

> Lawrence: Not sufficient, not "thread-safe" per our definition, but think of state as a thread-local variable. Need something like "these functions only affect state in the current thread."

> Hans: Suggest the following wording: "The floating point environment is maintained per-thread."

> Walter: Any other examples of state being thread safe that are not already covered elsewhere?

> Have thread unsafe functions paper which needs to be updated. Should just fold in 26.3 [cfenv] functions.

> Recommend Open. Lawrence instead suggests leaving it open until we have suitable wording that may or may not include the thread local commentary.

[ 2009-09-23 Hans provided wording. ]

> If I understand the history correctly, Nick, as the Posix liaison, should probably get a veto on this, since I think it came from Posix (?) via WG14 and should probably really be addressed there (?). But I think we are basically in agreement that there is no other sane way to do this, and hence we don't have to worry too much about stepping on toes. As far as I can tell, this same issue also exists in the latest Posix standard (?).

[ 2009-10 Santa Cruz: ]

> Moved to Ready.

Proposed resolution:

Add at the end of 26.3.1 [cfenv.syn]:

> 2 The header defines all functions, types, and macros the same as C99 7.6.

> A separate floating point environment shall be maintained for each thread. Each function accesses the environment corresponding to its calling thread.

### 861. Incomplete specification of `EqualityComparable` for `std::forward_list`

Section: 23.2 [container.requirements] Status: C++11 Submitter: Daniel Krügler Opened: 2008-06-24 Last modified: 2015-04-08

View other active issues in [container.requirements].

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Discussion:

Table 89, Container requirements, defines `operator==` in terms of the container member function `size()` and the algorithm `std::equal`:

> `==` is an equivalence relation. `a.size() == b.size() && equal(a.begin(), a.end(), b.begin()`

The new container `forward_list` does not provide a `size` member function by design but does provide `operator==` and `operator!=` without specifying it's semantic.

Other parts of the (sequence) container requirements do also depend on `size()`, e.g. `empty()` or `clear()`, but this issue explicitly attempts to solve the missing `EqualityComparable` specification, because of the special design choices of `forward_list`.

I propose to apply one of the following resolutions, which are described as:

1. Provide a definition, which is optimal for this special container without previous size test. This choice prevents two `O(N)` calls of `std::distance()` with the corresponding container ranges and instead uses a special `equals` implementation which takes two container ranges instead of 1 1/2.
2. The simple fix where the usual test is adapted such that `size()` is replaced by `distance` with corresponding performance disadvantages.

Both proposal choices are discussed, the preferred choice of the author is to apply (A).

[ San Francisco: ]

> There's an Option C: change the requirements table to use distance().

> LWG found Option C acceptable.

> Martin will draft the wording for Option C.

[ post San Francisco: ]

> Martin provided wording for Option C.

[ 2009-07 Frankfurt ]

> Other operational semantics (see, for example, Tables 82 and 83) are written in terms of a container's size() member. Daniel to update proposed resolution C.

> [ Howard: Commented out options A and B. ]

[ 2009-07-26 Daniel updated proposed resolution C. ]

[ 2009-10 Santa Cruz: ]

> Mark NAD Editorial. Addressed by N2986.

[ 2009-10 Santa Cruz: ]

> Reopened. N2986 was rejected in full committee on procedural grounds.

[ 2010-01-30 Howard updated Table numbers. ]

[ 2010 Pittsburgh: ]

> Moved to Ready for Pittsburgh.

Proposed resolution:

Option (C):

> 1. In 23.2.1 [container.requirements.general] change Table 90 -- Container requirements as indicated:

>    1. Change the text in the Assertion/note column in the row for "`X u`;" as follows:

>       > post: `u.size() == 0empty() == true`

>    2. Change the text in the Assertion/note column in the row for "`X();`" as follows:

>       > `X().size() == 0empty() == true`

>    3. Change the text in the Operational Semantics column in the row for "`a == b`" as follows:

>       > `==` is an equivalence relation. `a.size()distance(a.begin(), a.end()) == b.size()distance(b.begin(), b.end()) && equal(a.begin(), a.end(), b.begin())`

>    4. Add text in the Ass./Note/pre-/post-condition column in the row for "`a == b`" as follows:

>       > Requires: `T` is `EqualityComparable`

>    5. Change the text in the Operational Semantics column in the row for "`a.size()`" as follows:

>       > `a.end() - a.begin()distance(a.begin(), a.end())`

>    6. Change the text in the Operational Semantics column in the row for "`a.max_size()`" as follows:

>       > `size()distance(begin(), end())` of the largest possible container

>    7. Change the text in the Operational Semantics column in the row for "`a.empty()`" as follows:

>       > `a.size() == 0a.begin() == a.end()`

> 2. In 23.2.1 [container.requirements.general] change Table 93 — Allocator-aware container requirements as indicated:

>    1. Change the text in the Assertion/note column in the row for "`X() / X u;`" as follows:

>       > Requires: `A` is `DefaultConstructible` post: `u.size() == 0u.empty() == true`, `get_allocator() == A()`

>    2. Change the text in the Assertion/note column in the row for "`X(m) / X u(m);`" as follows:

>       > post: `u.size() == 0u.empty() == true, get_allocator() == m`

> 3. In 23.2.3 [sequence.reqmts] change Table 94 — Sequence container requirements as indicated:

>    1. Change the text in the Assertion/note column in the row for "`X(n, t) / X a(n, t)`" as follows:

>       > post: `size()distance(begin(), end()) == n [..]`

>    2. Change the text in the Assertion/note column in the row for "`X(i, j) / X a(i, j)`" as follows:

>       > [..] post: `size() ==` distance between `i` and `j` `distance(begin(), end()) == distance(i, j)` [..]

>    3. Change the text in the Assertion/note column in the row for "`a.clear()`" as follows:

>       > `a.erase(a.begin(), a.end())` post: `size() == 0a.empty() == true`

> 4. In 23.2.4 [associative.reqmts] change Table 96 — Associative container requirements as indicated:

>    [ Not every occurrence of `size()` was replaced, because all current associative containers have a `size`. The following changes ensure consistency regarding the semantics of "`erase`" for all tables and adds some missing objects ]

>    1. Change the text in the Complexity column in the row for `X(i,j,c)/Xa(i,j,c);` as follows:

>       > `N` log `N` in general (`N` `== distance(i, j)`is the distance from `i` to `j`); ...

>    2. Change the text in the Complexity column in the row for "`a.insert(i, j)`" as follows:

>       > `N log(a.size() + N)` (`N` is the distance from `i` to `j`) where `N == distance(i, j)`

>    3. Change the text in the Complexity column in the row for "`a.erase(k)`" as follows:

>       > `log(a.size()) + a.count(k)`

>    4. Change the text in the Complexity column in the row for "`a.erase(q1, q2)`" as follows:

>       > `log(a.size()) + N` where `N` is the distance from `q1` to `q2` `== distance(q1, q2)`.

>    5. Change the text in the Assertion/note column in the row for "`a.clear()`" as follows:

>       > `a.erase(a.begin(),a.end())` post: `size() == 0a.empty() == true`

>    6. Change the text in the Complexity column in the row for "`a.clear()`" as follows:

>       > linear in `a.size()`

>    7. Change the text in the Complexity column in the row for "`a.count(k)`" as follows:

>       > `log(a.size()) + a.count(k)`

> 5. In 23.2.5 [unord.req] change Table 98 — Unordered associative container requirements as indicated:

>    [ The same rational as for Table 96 applies here ]

>    1. Change the text in the Assertion/note column in the row for "`a.clear()`" as follows:

>       > [..] Post: `a.size() == 0empty() == true`

### 865. More algorithms that throw away information

Section: 25.3.6 [alg.fill], 25.3.7 [alg.generate] Status: C++11 Submitter: Daniel Krügler Opened: 2008-07-13 Last modified: 2015-04-08

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Discussion:

In regard to library defect 488 I found some more algorithms which unnecessarily throw away information. These are typically algorithms, which sequentially write into an `OutputIterator`, but do not return the final value of this output iterator. These cases are:

1.     template<class OutputIterator, class Size, class T>
       void fill_n(OutputIterator first, Size n, const T& value);

2.     template<class OutputIterator, class Size, class Generator>
       void generate_n(OutputIterator first, Size n, Generator gen);

In both cases the minimum requirements on the iterator are `OutputIterator`, which means according to the requirements of 24.2.4 [output.iterators] p. 2 that only single-pass iterations are guaranteed. So, if users of `fill_n` and `generate_n` have only an `OutputIterator` available, they have no chance to continue pushing further values into it, which seems to be a severe limitation to me.

[ Post Summit Daniel "conceptualized" the wording. ]

[ Batavia (2009-05): ]

> Alisdair likes the idea, but has concerns about the specific wording about the returns clauses.

> Alan notes this is a feature request.

> Bill notes we have made similar changes to other algorithms.

> Move to Open.

[ 2009-07 Frankfurt ]

> We have a consensus for moving forward on this issue, but Daniel needs to deconceptify it.

[ 2009-07-25 Daniel provided non-concepts wording. ]

[ 2009-10 Santa Cruz: ]

> Moved to Ready.

Proposed resolution:

1. Replace the current declaration of `fill_n` in 25 [algorithms]/2, header `<algorithm>` synopsis and in 25.3.6 [alg.fill] by

   >     template<class OutputIterator, class Size, class T>
   >       voidOutputIterator fill_n(OutputIterator first, Size n, const T& value);

   Just after the effects clause add a new returns clause saying:

   > Returns: For `fill_n` and positive `n`, returns `first + n`. Otherwise returns `first` for `fill_n`.

2. Replace the current declaration of `generate_n` in 25 [algorithms]/2, header `<algorithm>` synopsis and in 25.3.7 [alg.generate] by

   >     template<class OutputIterator, class Size, class Generator>
   >       voidOutputIterator generate_n(OutputIterator first, Size n, Generator gen);

   Just after the effects clause add a new returns clause saying:

   > For `generate_n` and positive `n`, returns `first + n`. Otherwise returns `first` for `generate_n`.

### 866. Qualification of placement new-expressions

Section: 20.7.12 [specialized.algorithms], 20.8.2.2.6 [util.smartptr.shared.create] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2008-07-14 Last modified: 2015-04-08

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Discussion:

LWG issue 402 replaced "`new`" with "`::new`" in the placement new-expression in 20.7.9.1 [allocator.members]. I believe the rationale given in 402 applies also to the following other contexts:

- in 20.7.12 [specialized.algorithms], all four algorithms `unitialized_copy`, `unitialized_copy_n`, `unitialized_fill` and `unitialized_fill_n` use the unqualified placement new-expression in some variation of the form:

  >     new  (static_cast<void*>(&*result)) typename  iterator_traits<ForwardIterator>::value_type(*first);

- in 20.8.2.2.6 [util.smartptr.shared.create] there is a reference to the unqualified placement new-expression:

  >     new  (pv)  T(std::forward<Args>(args)...),

I suggest to add qualification in all those places. As far as I know, these are all the remaining places in the whole library that explicitly use a placement new-expression. Should other uses come out, they should be qualified as well.

As an aside, a qualified placement new-expression does not need additional requirements to be compiled in a constrained context. By adding qualification, the `HasPlacementNew` concept introduced recently in N2677 (Foundational Concepts) would no longer be needed by library and should therefore be removed.

[ San Francisco: ]

> Detlef: If we move this to Ready, it's likely that we'll forget about the side comment about the `HasPlacementNew` concept.

[ post San Francisco: ]

> Daniel: `HasPlacementNew` has been removed from N2774 (Foundational Concepts).

Proposed resolution:

Replace "`new`" with "`::new`" in:

- 20.7.12.2 [uninitialized.copy], paragraphs 1 and 3
- 20.7.12.3 [uninitialized.fill] paragraph 1
- 20.7.12.4 [uninitialized.fill.n] paragraph 1
- 20.8.2.2.6 [util.smartptr.shared.create] once in paragraph 1 and twice in paragraph 2.

### 868. Default construction and value-initialization

Section: 23 [containers] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2008-07-22 Last modified: 2015-04-08

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Discussion:

The term "default constructed" is often used in wording that predates the introduction of the concept of value-initialization. In a few such places the concept of value-initialization is more correct than the current wording (for example when the type involved can be a built-in) so a replacement is in order. Two of such places are already covered by issue 867. This issue deliberately addresses the hopefully non-controversial changes in the attempt of being approved more quickly. A few other occurrences (for example in `std::tuple`, `std::reverse_iterator` and `std::move_iterator`) are left to separate issues. For `std::reverse_iterator`, see also issue 408. This issue is related with issue 724.

[ San Francisco: ]

> The list provided in the proposed resolution is not complete. James Dennett will review the library and provide a complete list and will double-check the vocabulary.

> This issue relates to Issue 886 tuple construction

[ 2009-07 Frankfurt ]

> The proposed resolution is incomplete.

> Move to Tentatively NAD Future. Howard will contact Ganesh for wording. If wording is forthcoming, Howard will move it back to Review.

[ 2009-07-18 Ganesh updated the proposed wording. ]

> Howard: Moved back to Review. Note that 17.6.3.1 [utility.arg.requirements] refers to a section that is not in the current working paper, but does refer to a section that we expect to reappear after the de-concepts merge. This was a point of confusion we did not recognize when we reviewed this issue in Frankfurt.

> Howard: Ganesh also includes a survey of places in the WP surveyed for changes of this nature and purposefully not treated:

> > Places where changes are not being proposed

> > In the following paragraphs, we are not proposing changes because it's not clear whether we actually prefer value-initialization over default-initialization (now partially covered by 1012):

> > - 20.8.1.2.1 [unique.ptr.single.ctor] para 3 e 7

> > - 24.5.1.3.1 [reverse.iter.cons] para 1

> > - 24.5.3.3.1 [move.iter.op.const] para 1

> > In the following paragraphs, the expression "default constructed" need not be changed, because the relevant type does not depend on a template parameter and has a user-provided constructor:

> > - [func.referenceclosure.invoke] para 12, type: reference_closure

> > - 30.3.1.2 [thread.thread.constr] para 30, type: thread

> > - 30.3.1.5 [thread.thread.member] para 52, type: thread_id

> > - 30.3.2 [thread.thread.this], para 1, type: thread_id

[ 2009-08-18 Daniel adds: ]

> I have no objections against the currently suggested changes, but I also cross-checked with the list regarding intentionally excluded changes, and from this I miss the discussion of

> 1. 21.4.1 [string.require] p. 2:

>    > "[..] The `Allocator` object used shall be a copy of the `Allocator>` object passed to the `basic_string` object's constructor or, if the constructor does not take an `Allocator` argument, a copy of a default-constructed `Allocator` object."

> 2. N2723, 26.5.1.4 [rand.req.eng], Table 109, expression "`T()`":

>    > Pre-/post-condition: "Creates an engine with the same initial state as all other default-constructed engines of type `X`."

>    as well as in 26.5.5 [rand.predef]/1-9 (N2914), 26.5.7.1 [rand.util.seedseq]/3, 27.7.2.1.1 [istream.cons]/3, 27.7.3.2 [ostream.cons]/9 (N2914), 28.13 [re.grammar]/2, 30.3.1.4 [thread.thread.assign]/1 (N2914),

>    [ Candidates for the "the expression "default constructed" need not be changed" list ]

>    I'm fine, if these would be added to the intentionally exclusion list, but mentioning them makes it easier for other potential reviewers to decide on the relevance or not-relevance of them for this issue.

> 3. I suggest to remove the reference of [func.referenceclosure.invoke] in the "it's not clear" list, because this component does no longer exist.

> 4. I also suggest to add a short comment that all paragraphs in the resolution whether they refer to N2723 or to N2914 numbering, because e.g. "Change 23.3.3.2 [deque.cons] para 5" is an N2723 coordinate, while "Change 23.3.3.3 [deque.capacity] para 1" is an N2914 coordinate. Even better would be to use one default document for the numbering (probably N2914) and mention special cases (e.g. "Change 17.6.3.1 [utility.arg.requirements] para 2" as referring to N2723 numbering).

[ 2009-08-18 Alisdair adds: ]

> I strongly believe the term "default constructed" should not appear in the library clauses unless we very clearly define a meaning for it, and I am not sure what that would be.

> In those cases where we do not want to replace "default constructed" with "vale initialized" we should be using "default initialized". If we have a term that could mean either, we reduce portability of programs.

> I have not done an exhaustive review to clarify if that is a vendor freedom we have reason to support (e.g. value-init in debug, default-init in release) so I may yet be convinced that LWG has reason to define this new term of art, but generally C++ initialization is confusing enough without supporting further ill-defined terms.

[ 2009-10 Santa Cruz: ]

> Move to Ready.

[ 2010 Pittsburgh: ]

> Moved to review in order to enable conflict resolution with 704.

[ 2010-03-26 Daniel harmonized the wording with the upcoming FCD. ]

[ 2010 Rapperswil: ]

> Move to Ready.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 17.6.3.1 [utility.arg.requirements] para 2:

> 2 In general, a default constructor is not required. Certain container class member function signatures specify the default constructor `T()` as a default argument. `T()` shall be a well-defined expression (8.5) if one of those signatures is called using the default argument (8.3.6).

Change 23.3.3.2 [deque.cons] para 3:

> 3 Effects: Constructs a deque with `n` default constructed value-initialized elements.

Change 23.3.3.3 [deque.capacity] para 1:

> 1 Effects: If `sz < size()`, equivalent to `erase(begin() + sz, end());`. If `size() < sz`, appends `sz - size()` default constructed value-initialized elements to the sequence.

Change 23.3.4.2 [forwardlist.cons] para 3:

> 3 Effects: Constructs a `forward_list` object with `n` default constructed value-initialized elements.

Change 23.3.4.5 [forwardlist.modifiers] para 22:

> 22 Effects: [...] For the first signature the inserted elements are default constructed value-initialized, and for the second signature they are copies of `c`.

Change 23.3.5.2 [list.cons] para 3:

> 3 Effects: Constructs a `list` with `n` default constructed value-initialized elements.

Change 23.3.5.3 [list.capacity] para 1:

> 1 Effects: If `sz < size()`, equivalent to `list<T>::iterator it = begin(); advance(it, sz); erase(it, end());`. If `size() < sz`, `appends sz - size()` default constructed value-initialized elements to the sequence.

Change 23.3.6.2 [vector.cons] para 3:

> 3 Effects: Constructs a `vector` with `n` default constructed value-initialized elements.

Change 23.3.6.3 [vector.capacity] para 9:

> 9 Effects: If `sz < size()`, equivalent to `erase(begin() + sz, end());`. If `size() < sz`, appends `sz - size()` default constructed value-initialized elements to the sequence.

### 869. Bucket (local) iterators and iterating past end

Section: 23.2.5 [unord.req] Status: C++11 Submitter: Sohail Somani Opened: 2008-07-22 Last modified: 2015-04-08

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Discussion:

Is there any language in the current draft specifying the behaviour of the following snippet?

>     unordered_set<int> s;
>     unordered_set<int>::local_iterator it = s.end(0);

>     // Iterate past end - the unspecified part
>     it++;

I don't think there is anything about `s.end(n)` being considered an iterator for the past-the-end value though (I think) it should be.

[ San Francisco: ]

> We believe that this is not a substantive change, but the proposed change to the wording is clearer than what we have now.

[ Post Summit: ]

> Recommend Tentatively Ready.

Proposed resolution:

Change Table 97 "Unordered associative container requirements" in 23.2.5 [unord.req]:

> Table 97: Unordered associative container requirements
> expression | return type | assertion/note pre/post-condition | complexity
> --- | --- | --- | ---
> `b.begin(n)` | `local_iterator` `const_local_iterator` for const `b`. | Pre: n shall be in the range [0,b.bucket_count()). Note: [b.begin(n), b.end(n)) is a valid range containing all of the elements in the nth bucket. `b.begin(n)` returns an iterator referring to the first element in the bucket. If the bucket is empty, then `b.begin(n) == b.end(n)`. | Constant
> `b.end(n)` | `local_iterator` `const_local_iterator` for const `b`. | Pre: n shall be in the range `[0, b.bucket_count())`. `b.end(n)` returns an iterator which is the past-the-end value for the bucket. | Constant

### 870. Do unordered containers not support function pointers for predicate/hasher?

Section: 23.2.5 [unord.req] Status: C++11 Submitter: Daniel Krügler Opened: 2008-08-17 Last modified: 2015-04-08

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Discussion:

Good ol' associative containers allow both function pointers and function objects as feasible comparators, as described in 23.2.4 [associative.reqmts]/2:

> Each associative container is parameterized on `Key` and an ordering relation `Compare` that induces a strict weak ordering (25.3) on elements of Key. [..]. The object of type `Compare` is called the comparison object of a container. This comparison object may be a pointer to function or an object of a type with an appropriate function call operator.[..]

The corresponding wording for unordered containers is not so clear, but I read it to disallow function pointers for the hasher and I miss a clear statement for the equality predicate, see 23.2.5 [unord.req]/3+4+5:

> Each unordered associative container is parameterized by `Key`, by a function object `Hash` that acts as a hash function for values of type `Key`, and by a binary predicate `Pred` that induces an equivalence relation on values of type `Key`.[..]

> A hash function is a function object that takes a single argument of type `Key` and returns a value of type `std::size_t`.

> Two values `k1` and `k2` of type `Key` are considered equal if the container's equality function object returns `true` when passed those values.[..]

and table 97 says in the column "assertion...post-condition" for the expression `X::hasher`:

> `Hash` shall be a unary function object type such that the expression `hf(k)` has type `std::size_t`.

Note that 20.9 [function.objects]/1 defines as "Function objects are objects with an `operator()` defined.[..]"

Does this restriction exist by design or is it an oversight? If an oversight, I suggest that to apply the following

[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]

[ 2009-10 Santa Cruz: ]

> Ask Daniel to provide proposed wording that: makes it explicit that function pointers are function objects at the beginning of 20.9 [function.objects]; fixes the "requirements" for typedefs in 20.9.4 [refwrap] to instead state that the function objects defined in that clause have these typedefs, but not that these typedefs are requirements on function objects; remove the wording that explicitly calls out that associative container comparators may be function pointers.

[ 2009-12-19 Daniel updates wording and rationale. ]

[ 2010-02-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Rationale:

The below provided wording also affects some part of the library which is involved with callable types (20.9.1 [func.def]/3). Reason for this is that callable objects do have a lot in common with function objects. A simple formula seems to be:

> callable objects = function objects + pointers to member

The latter group is excluded from function objects because of the expression-based usage of function objects in the algorithm clause, which is incompatible with the notation to dereference pointers to member without a concept map available in the language.

This analysis showed some currently existing normative definition differences between the above subset of callable objects and function objects which seem to be unintended: Backed by the Santa Cruz outcome function objects should include both function pointers and "object[s] with an operator() defined". This clearly excludes class types with a conversion function to a function pointer or all similar conversion function situations described in 13.3 [over.match]/2 b. 2. In contrast to this, the wording for callable types seems to be less constrained (20.9.1 [func.def]/3):

> A callable type is a [..] class type whose objects can appear immediately to the left of a function call operator.

The rationale given in N1673 and a recent private communication with Peter Dimov revealed that the intention of this wording was to cover the above mentioned class types with conversion functions as well. To me the current wording of callable types can be read either way and I suggest to make the intention more explicit by replacing

> [..] class type whose objects can appear immediately to the left of a function call operator

by

> [..] class type whose objects can appear as the leftmost subexpression of a function call expression 5.2.2 [expr.call].

and to use the same definition for the class type part of function objects, because there is no reason to exclude class types with a conversion function to e.g. pointer to function from being used in algorithms.

Now this last term "function objects" itself brings us to a third unsatisfactory state: The term is used both for objects (e.g. "Function objects are objects[..]" in 20.9 [function.objects]/1) and for types (e.g. "Each unordered associative container is parameterized [..] by a function object Hash that acts as a hash function [..]" in 23.2.5 [unord.req]/3). This impreciseness should be fixed and I suggest to introduce the term function object type as the counter part to callable type. This word seems to be a quite natural choice, because the library already uses it here and there (e.g. "Hash shall be a unary function object type such that the expression `hf(k)` has type `std::size_t`." in Table 98, "`X::hasher`" or "Requires: `T` shall be a function object type [..]" in 20.9.12.2.5 [func.wrap.func.targ]/3).

Finally I would like to add that part of the issue 870 discussion related to the requirements for typedefs in 20.9.4 [refwrap] during the Santa Cruz meeting is now handled by the new issue 1290.

Obsolete rationale:

> [ San Francisco: ]

> > This is fixed by N2776.

Proposed resolution:

1. Change 20.9 [function.objects]/1 as indicated:

   > 1 Function objects are objects with an `operator()` defined. An object type (3.9 [basic.types]) that can be the type of the postfix-expression in a function call (5.2.2 [expr.call], 13.3.1.1 [over.match.call]) is called a function object type*. A function object is an object of a function object type. In the places where one would expect to pass a pointer to a function to an algorithmic template (Clause 25 [algorithms]), the interface is specified to accept an object with an `operator()` defined a function object. This not only makes algorithmic templates work with pointers to functions, but also enables them to work with arbitrary function objects.

   > > * Such a type is either a function pointer or a class type which often has a member `operator()`, but in some cases it can omit that member and provide a conversion to a pointer to function.

2. Change 20.9.1 [func.def]/3 as indicated: [The intent is to make the commonality of callable types and function object types more explicit and to get rid of wording redundancies]

   > 3 A callable type is a pointer to function, a pointer to member function, a pointer to member data, or a class type whose objects can appear immediately to the left of a function call operator function object type (20.9 [function.objects]).

3. Change [bind]/1 as indicated:

   > 1 The function template `bind` returns an object that binds a function callable object passed as an argument to additional arguments.

4. Change 20.9.10 [func.bind]/1 as indicated:

   > 1 This subclause describes a uniform mechanism for binding arguments of function callable objects.

5. Change 20.9.12 [func.wrap]/1 as indicated:

   > 1 This subclause describes a polymorphic wrapper class that encapsulates arbitrary function callable objects.

6. Change 20.9.12.2 [func.wrap.func]/2 as indicated [The reason for this change is that 20.9.12.2 [func.wrap.func]/1 clearly says that all callable types may be wrapped by `std::function` and current implementations indeed do provide support for pointer to members as well. One further suggested improvement is to set the below definition of Callable in italics]:

   > 2 A function callable object `f` of type `F` is Callable Callable for argument types `T1, T2, ..., TN` in `ArgTypes` and a return type `R`, if, given lvalues `t1, t2, ..., tN` of types `T1, T2, ..., TN`, respectively, the expression `INVOKE(f, declval<ArgTypes>()..., Rt1, t2, ..., tN)`, considered as an unevaluated operand (5 [expr]), is well formed (20.7.2) and, if `R` is not `void`, convertible to `R`.

7. Change 20.9.12.2.1 [func.wrap.func.con]/7 as indicated:

   >     function(const function& f);
   >     template <class A> function(allocator_arg_t, const A& a, const function& f);

   > > ...

   > > 7 Throws: shall not throw exceptions if `f`'s target is a function pointer or a function callable object passed via `reference_wrapper`. Otherwise, may throw `bad_alloc` or any exception thrown by the copy constructor of the stored function callable object. [Note: Implementations are encouraged to avoid the use of dynamically allocated memory for small function callable objects, e.g., where `f`'s target is an object holding only a pointer or reference to an object and a member function pointer. — end note]

8. Change 20.9.12.2.1 [func.wrap.func.con]/11 as indicated:

   >     template<class F> function(F f);
   >     template <class F, class A> function(allocator_arg_t, const A& a, F f);

   > > ...

   > > 11 [..] [Note: implementations are encouraged to avoid the use of dynamically allocated memory for small function callable objects, for example, where `f`'s target is an object holding only a pointer or reference to an object and a member function pointer. — end note]

9. Change 20.9.12.2.4 [func.wrap.func.inv]/3 as indicated:

   >     R operator()(ArgTypes... args) const

   > > ...

   > > 3 Throws: `bad_function_call` if `!*this`; otherwise, any exception thrown by the wrapped function callable object.

10. Change 20.9.12.2.5 [func.wrap.func.targ]/3 as indicated:

    >     template<typename T>       T* target();
    >     template<typename T> const T* target() const;

    > > ...

    > > 3 Requires: `T` shall be a function object type that is Callable (20.9.12.2 [func.wrap.func]) for parameter types `ArgTypes` and return type `R`.

11. Change 23.2.4 [associative.reqmts]/2 as indicated: [The suggested removal seems harmless, because 25.4 [alg.sorting]1 already clarifies that `Compare` is a function object type. Nevertheless it is recommended, because the explicit naming of "pointer to function" is misleading]

    > 2 Each associative container is parameterized on `Key` and an ordering relation `Compare` that induces a strict weak ordering (25.4 [alg.sorting]) on elements of `Key`. In addition, `map` and `multimap` associate an arbitrary type `T` with the `Key`. The object of type `Compare` is called the comparison object of a container. This comparison object may be a pointer to function or an object of a type with an appropriate function call operator.

12. Change 23.2.5 [unord.req]/3 as indicated:

    > 3 Each unordered associative container is parameterized by `Key`, by a function object type `Hash` that acts as a hash function for values of type `Key`, and by a binary predicate `Pred` that induces an equivalence relation on values of type Key. [..]

13. Change 25.1 [algorithms.general]/7 as indicated: [The intent is to bring this part in sync with 20.9 [function.objects]]

    > 7 The `Predicate` parameter is used whenever an algorithm expects a function object (20.9 [function.objects]) that when applied to the result of dereferencing the corresponding iterator returns a value testable as `true`. In other words, if an algorithm takes `Predicate pred` as its argument and `first` as its iterator argument, it should work correctly in the construct `if (pred(*first)){...}`. The function object `pred` shall not apply any nonconstant function through the dereferenced iterator. This function object may be a pointer to function, or an object of a type with an appropriate function call operator.

14. Change 20.8.1.2 [unique.ptr.single]/1 as indicated:

    > 1 The default type for the template parameter `D` is `default_delete`. A client-supplied template argument `D` shall be a function pointer or functor object type for which, given a value `d` of type `D` and a pointer `ptr` of type `T*`, the expression `d(ptr)` is valid and has the effect of deallocating the pointer as appropriate for that deleter. `D` may also be an lvalue-reference to a deleter.

### 871. Iota's requirements on `T` are too strong

Section: 26.7.6 [numeric.iota] Status: C++11 Submitter: Daniel Krügler Opened: 2008-08-20 Last modified: 2015-04-08

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Discussion:

According to the recent WP N2691, 26.7.6 [numeric.iota]/1, the requires clause of `std::iota` says:

> `T` shall meet the requirements of `CopyConstructible` and `Assignable` types, and shall be convertible to `ForwardIterator`'s value type.[..]

Neither `CopyConstructible` nor `Assignable` is needed, instead `MoveConstructible` seems to be the correct choice. I guess the current wording resulted as an artifact from comparing it with similar numerical algorithms like `accumulate`.

Note: If this function will be conceptualized, the here proposed `MoveConstructible` requirement can be removed, because this is an implied requirement of function arguments, see N2710/[temp.req.impl]/3, last bullet.

[ post San Francisco: ]

> Issue pulled by author prior to review.

[ 2009-07-30 Daniel reopened: ]

> with the absence of concepts, this issue (closed) is valid again and I suggest to reopen it. I also revised by proposed resolution based on N2723 wording:

[ 2009-10 Santa Cruz: ]

> Change 'convertible' to 'assignable', Move To Ready.

Proposed resolution:

Change the first sentence of 26.7.6 [numeric.iota]/1:

> Requires: `T` shall meet the requirements of `CopyConstructible` and `Assignable` types, and shall be assignable to `ForwardIterator`'s value type. [..]

### 872. `move_iterator::operator[]` has wrong return type

Section: 24.5.3.3.12 [move.iter.op.index] Status: C++11 Submitter: Doug Gregor Opened: 2008-08-21 Last modified: 2015-04-08

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Discussion:

`move_iterator`'s `operator[]` is declared as:

>     reference operator[](difference_type n) const;

This has the same problem that `reverse_iterator`'s `operator[]` used to have: if the underlying iterator's `operator[]` returns a proxy, the implicit conversion to `value_type&&` could end up referencing a temporary that has already been destroyed. This is essentially the same issue that we dealt with for `reverse_iterator` in DR 386.

[ 2009-07-28 Reopened by Alisdair. No longer solved by concepts. ]

[ 2009-08-15 Howard adds: ]

> I recommend closing this as a duplicate of 1051 which addresses this issue for both `move_iterator` and `reverse_iterator`.

[ 2009-10 Santa Cruz: ]

> Move to Ready. Note that if 1051 is reopened, it may yield a better resolution, but 1051 is currently marked NAD.

Proposed resolution:

In 24.5.3.1 [move.iterator] and 24.5.3.3.12 [move.iter.op.index], change the declaration of `move_iterator`'s `operator[]` to:

>     reference unspecified operator[](difference_type n) const;

Rationale:

[ San Francisco: ]

> NAD Editorial, see N2777.

### 874. Missing `initializer_list` constructor for `discrete_distribution`

Section: 26.5.8.6.1 [rand.dist.samp.discrete] Status: Resolved Submitter: Daniel Krügler Opened: 2008-08-22 Last modified: 2015-04-08

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Discussion:

During the Sophia Antipolis meeting it was decided to separate from 793 a subrequest that adds initializer list support to `discrete_distribution`, specifically, the issue proposed to add a c'tor taking a `initializer_list<double>`.

Proposed resolution:

1. In 26.5.8.6.1 [rand.dist.samp.discrete] p. 1, class `discrete_distribution`, just before the member declaration

   >     explicit discrete_distribution(const param_type& parm);

   insert

   >     discrete_distribution(initializer_list<double> wl);

2. Between p.4 and p.5 of the same section insert a new paragraph as part of the new member description:

   >     discrete_distribution(initializer_list<double> wl);

   > > Effects: Same as `discrete_distribution(wl.begin(), wl.end())`.

Rationale:

Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".

### 875. Missing `initializer_list` constructor for `piecewise_constant_distribution`

Section: 26.5.8.6.2 [rand.dist.samp.pconst] Status: Resolved Submitter: Daniel Krügler Opened: 2008-08-22 Last modified: 2015-04-08

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Discussion:

During the Sophia Antipolis meeting it was decided to separate from 794 a subrequest that adds initializer list support to `piecewise_constant_distribution`, specifically, the issue proposed to add a c'tor taking a `initializer_list<double>` and a `Callable` to evaluate weight values. For consistency with the remainder of this class and the remainder of the `initializer_list`-aware library the author decided to change the list argument type to the template parameter `RealType` instead. For the reasoning to use `Func` instead of `Func&&` as c'tor function argument see issue 793.

Proposed resolution:

Non-concept version of the proposed resolution

1. In 26.5.8.6.2 [rand.dist.samp.pconst]/1, class `piecewise_constant_distribution`, just before the member declaration

   >     explicit piecewise_constant_distribution(const param_type& parm);

   insert

   >     template<typename Func>
   >     piecewise_constant_distribution(initializer_list<RealType> bl, Func fw);

2. Between p.4 and p.5 of the same section insert a series of new paragraphs nominated below as [p5_1], [p5_2], and [p5_3] as part of the new member description:

   >     template<typename Func>
   >     piecewise_constant_distribution(initializer_list<RealType> bl, Func fw);

   > > [p5_1] Complexity: Exactly `nf = max(bl.size(), 1) - 1` invocations of `fw`.

   > > [p5_2] Requires:

   > > 1. `fw` shall be callable with one argument of type `RealType`, and shall return values of a type convertible to `double`;
   > > 2. The relation `0 < S = w0+. . .+wn-1` shall hold. For all sampled values `xk` defined below, `fw(xk)` shall return a weight value `wk` that is non-negative, non-NaN, and non-infinity;
   > > 3. If `nf > 0` let `bk = *(bl.begin() + k), k = 0, . . . , bl.size()-1` and the following relations shall hold for `k = 0, . . . , nf-1: bk < bk+1`.

   > > [p5_3] Effects:

   > > 1. If `nf == 0`,

   > >    1. lets the sequence `w` have length `n = 1` and consist of the single value `w0 = 1`, and
   > >    2. lets the sequence `b` have length `n+1` with `b0 = 0` and `b1 = 1`.

   > > 2. Otherwise,

   > >    1. sets `n = nf`, and `[bl.begin(), bl.end())` shall form the sequence `b` of length `n+1`, and

   > >    2. lets the sequences `w` have length `n` and for each `k = 0, . . . ,n-1`, calculates:

   > >       > xk = 0.5*(bk+1 + bk) wk = fw(xk)

   > > 3. Constructs a `piecewise_constant_distribution` object with the above computed sequence `b` as the interval boundaries and with the probability densities:

   > >    > ρk = wk/(S * (bk+1 - bk)) for k = 0, . . . , n-1.

Concept version of the proposed resolution

1. In 26.5.8.6.2 [rand.dist.samp.pconst]/1, class `piecewise_constant_distribution`, just before the member declaration

   >     explicit piecewise_constant_distribution(const param_type& parm);

   insert

   >     template<Callable<auto, RealType> Func>
   >      requires Convertible<Func::result_type, double>
   >     piecewise_constant_distribution(initializer_list<RealType> bl, Func fw);

2. Between p.4 and p.5 of the same section insert a series of new paragraphs nominated below as [p5_1], [p5_2], and [p5_3] as part of the new member description:

   >     template<Callable<auto, RealType> Func>
   >      requires Convertible<Func::result_type, double>
   >     piecewise_constant_distribution(initializer_list<RealType> bl, Func fw);

   > > [p5_1] Complexity: Exactly `nf = max(bl.size(), 1) - 1` invocations of `fw`.

   > > [p5_2] Requires:

   > > 1. The relation `0 < S = w0+. . .+wn-1` shall hold. For all sampled values `xk` defined below, `fw(xk)` shall return a weight value `wk` that is non-negative, non-NaN, and non-infinity;
   > > 2. If `nf > 0` let `bk = *(bl.begin() + k), k = 0, . . . , bl.size()-1` and the following relations shall hold for `k = 0, . . . , nf-1: bk < bk+1`.

   > > [p5_3] Effects:

   > > 1. If `nf == 0`,

   > >    1. lets the sequence `w` have length `n = 1` and consist of the single value `w0 = 1`, and
   > >    2. lets the sequence `b` have length `n+1` with `b0 = 0` and `b1 = 1`.

   > > 2. Otherwise,

   > >    1. sets `n = nf`, and `[bl.begin(), bl.end())` shall form the sequence `b` of length `n+1`, and

   > >    2. lets the sequences `w` have length `n` and for each `k = 0, . . . ,n-1`, calculates:

   > >       > xk = 0.5*(bk+1 + bk) wk = fw(xk)

   > > 3. Constructs a `piecewise_constant_distribution` object with the above computed sequence `b` as the interval boundaries and with the probability densities:

   > >    > ρk = wk/(S * (bk+1 - bk)) for k = 0, . . . , n-1.

Rationale:

Addressed by N2836 "Wording Tweaks for Concept-enabled Random Number Generation in C++0X".

### 876. `basic_string` access operations should give stronger guarantees

Section: 21.4 [basic.string] Status: C++11 Submitter: Daniel Krügler Opened: 2008-08-22 Last modified: 2015-04-08

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Discussion:

During the Sophia Antipolis meeting it was decided to split-off some parts of the n2647 ("Concurrency modifications for `basic_string`") proposal into a separate issue, because these weren't actually concurrency-related. The here proposed changes refer to the recent update document n2668 and attempt to take advantage of the stricter structural requirements.

Indeed there exists some leeway for more guarantees that would be very useful for programmers, especially if interaction with transactionary or exception-unaware C API code is important. This would also allow compilers to take advantage of more performance optimizations, because more functions can have throw() specifications. This proposal uses the form of "Throws: Nothing" clauses to reach the same effect, because there already exists a different issue in progress to clean-up the current existing "schizophrenia" of the standard in this regard.

Due to earlier support for copy-on-write, we find the following unnecessary limitations for C++0x:

1. Missing no-throw guarantees: `data()` and `c_str()` simply return a pointer to their guts, which is a non-failure operation. This should be spelled out. It is also noteworthy to mention that the same guarantees should also be given by the size query functions, because the combination of pointer to content and the length is typically needed during interaction with low-level API.
2. Missing complexity guarantees: `data()` and `c_str()` simply return a pointer to their guts, which is guaranteed O(1). This should be spelled out.
3. Missing reading access to the terminating character: Only the const overload of `operator[]` allows reading access to the terminator char. For more intuitive usage of strings, reading access to this position should be extended to the non-const case. In contrast to C++03 this reading access should now be homogeneously an lvalue access.

The proposed resolution is split into a main part (A) and a secondary part (B) (earlier called "Adjunct Adjunct Proposal"). (B) extends (A) by also making access to index position size() of the at() overloads a no-throw operation. This was separated, because this part is theoretically observable in specifically designed test programs.

[ San Francisco: ]

> We oppose part 1 of the issue but hope to address `size()` in issue 877.

> We do not support part B. 4 of the issue because of the breaking API change.

> We support part A. 2 of the issue.

> On support part A. 3 of the issue:

> > Pete's broader comment: now that we know that `basic_string` will be a block of contiguous memory, we should just rewrite its specification with that in mind. The expression of the specification will be simpler and probably more correct as a result.

[ 2009-07 Frankfurt ]

> Move proposed resolution A to Ready.

> [ Howard: Commented out part B. ]

Proposed resolution:

1. 1. In 21.4.4 [string.capacity], just after p. 1 add a new paragraph:

      > Throws: Nothing.

   2. In 21.4.5 [string.access] replace p. 1 by the following 4 paragraghs:

      > Requires: `pos ≤ size()`.

      > Returns: If `pos < size()`, returns `*(begin() + pos)`. Otherwise, returns a reference to a `charT()` that shall not be modified.

      > Throws: Nothing.

      > Complexity: Constant time.

   3. In 21.4.7.1 [string.accessors] replace the now common returns clause of `c_str()` and `data()` by the following three paragraphs:

      > Returns: A pointer `p` such that `p+i == &operator[](i)` for each `i` in `[0, size()]`.

      > Throws: Nothing.

      > Complexity: Constant time.

### 878. `forward_list` preconditions

Section: 23.3.4 [forwardlist] Status: C++11 Submitter: Martin Sebor Opened: 2008-08-23 Last modified: 2015-04-08

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Discussion:

`forward_list` member functions that take a `forward_list::iterator` (denoted `position` in the function signatures) argument have the following precondition:

> Requires: `position` is dereferenceable or equal to `before_begin()`.

I believe what's actually intended is this:

> Requires: `position` is in the range [`before_begin()`, `end()`).

That is, when it's dereferenceable, `position` must point into `*this`, not just any `forward_list` object.

[ San Francisco: ]

> Robert suggested alternate proposed wording which had large support.

[ Post Summit: ]

> Walter: "position is before_begin() or a dereferenceable": add "is" after the "or"

> With that minor update, Recommend Tentatively Ready.

Proposed resolution:

Change the Requires clauses [forwardlist] , p21, p24, p26, p29, and, 23.3.4.6 [forwardlist.ops], p39, p43, p47 as follows:

> Requires: `position` is `before_begin()` or is a dereferenceable iterator in the range `[begin(), end())` or equal to `before_begin()`. ...

### 880. Missing atomic exchange parameter

Section: 29 [atomics] Status: Resolved Submitter: Lawrence Crowl Opened: 2008-08-24 Last modified: 2015-04-08

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Duplicate of: 942

Discussion:

The `atomic_exchange` and `atomic_exchange_explicit` functions seem to be inconsistently missing parameters.

[ Post Summit: ]

> Lawrence: Need to write up a list for Pete with details.

> Detlef: Should not be New, we already talked about in Concurrency group.

> Recommend Open.

[ 2009-07 Frankfurt ]

> Lawrence will handle all issues relating to atomics in a single paper.

> LWG will defer discussion on atomics until that paper appears.

> Move to Open.

[ 2009-08-17 Handled by N2925. ]

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Solved by N2992.

Proposed resolution:

Add the appropriate parameters. For example,

>     bool atomic_exchange(volatile atomic_bool*, bool);
>     bool atomic_exchange_explicit(volatile atomic_bool*, bool, memory_order);

### 881. `shared_ptr` conversion issue

Section: 20.8.2.2.1 [util.smartptr.shared.const] Status: C++11 Submitter: Peter Dimov Opened: 2008-08-30 Last modified: 2015-04-08

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Discussion:

We've changed `shared_ptr<Y>` to not convert to `shared_ptr<T>` when `Y*` doesn't convert to `T*` by resolving issue 687. This only fixed the converting copy constructor though. N2351 later added move support, and the converting move constructor is not constrained.

[ San Francisco: ]

> We might be able to move this to NAD, Editorial once shared_ptr is conceptualized, but we want to revisit this issue to make sure.

[ 2009-07 Frankfurt ]

> Moved to Ready.

> This issue now represents the favored format for specifying constrained templates.

Proposed resolution:

We need to change the Requires clause of the move constructor:

>     shared_ptr(shared_ptr&& r);
>     template<class Y> shared_ptr(shared_ptr<Y>&& r);

> > Requires Remarks: For the second constructor `Y*` shall be convertible to `T*`. The second constructor shall not participate in overload resolution unless `Y*` is convertible to `T*`.

in order to actually make the example in 687 compile (it now resolves to the move constructor).

### 882. `duration` non-member arithmetic requirements

Section: 20.12.5.5 [time.duration.nonmember] Status: CD1 Submitter: Howard Hinnant Opened: 2008-09-08 Last modified: 2015-04-08

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Discussion:

N2661 specified the following requirements for the non-member `duration` arithmetic:

>     template <class Rep1, class Period, class Rep2>
>       duration<typename common_type<Rep1, Rep2>::type, Period>
>       operator*(const duration<Rep1, Period>& d, const Rep2& s);

> > Requires: Let `CR` represent the `common_type` of `Rep1` and `Rep2`. Both `Rep1` and `Rep2` shall be implicitly convertible to CR, diagnostic required.

>     template <class Rep1, class Period, class Rep2>
>       duration<typename common_type<Rep1, Rep2>::type, Period>
>       operator*(const Rep1& s, const duration<Rep2, Period>& d);

> > Requires: Let `CR` represent the `common_type` of `Rep1` and `Rep2`. Both `Rep1` and `Rep2` shall be implicitly convertible to `CR`, diagnostic required.

>     template <class Rep1, class Period, class Rep2>
>       duration<typename common_type<Rep1, Rep2>::type, Period>
>       operator/(const duration<Rep1, Period>& d, const Rep2& s);

> > Requires: Let `CR` represent the `common_type` of `Rep1` and `Rep2`. Both `Rep1` and `Rep2` shall be implicitly convertible to `CR`, and `Rep2` shall not be an instantiation of `duration`, diagnostic required.

During transcription into the working paper, the requirements clauses on these three functions was changed to:

> Requires: `CR(Rep1, Rep2)` shall exist. Diagnostic required.

This is a non editorial change with respect to N2661 as user written representations which are used in `duration` need not be implicitly convertible to or from arithmetic types in order to interoperate with `duration`s based on arithmetic types. An explicit conversion will do fine for most expressions as long as there exists a `common_type` specialization relating the user written representation and the arithmetic type. For example:

>     class saturate
>     {
>     public:
>       explicit saturate(long long i);
>       ...
>     };

>     namespace std {

>     template <>
>     struct common_type<saturate, long long>
>     {
>         typedef saturate type;
>     };

>     template <>
>     struct common_type<long long, saturate>
>     {
>         typedef saturate type;
>     };

>     }  // std

>     millisecond ms(3);  // integral-based duration
>     duration<saturate, milli> my_ms = ms;  // ok, even with explicit conversions
>     my_ms = my_ms + ms;                    // ok, even with explicit conversions

However, when dealing with multiplication of a duration and its representation, implicit convertibility is required between the rhs and the lhs's representation for the member `*=` operator:

>     template <class Rep, class Period = ratio<1>>
>     class duration {
>     public:
>        ...
>        duration& operator*=(const rep& rhs);
>        ...
>     };
>     ...
>     ms *= 2;               // ok, 2 is implicitly convertible to long long
>     my_ms *= saturate(2);  // ok, rhs is lhs's representation
>     my_ms *= 2;            // error, 2 is not implicitly convertible to saturate

The last line does not (and should not) compile. And we want non-member multiplication to have the same behavior as member arithmetic:

>     my_ms = my_ms * saturate(2);  // ok, rhs is lhs's representation
>     my_ms = my_ms * 2;            // should be error, 2 is not implicitly convertible to saturate

The requirements clauses of N2661 make the last line an error as expected. However the latest working draft at this time (N2723) allows the last line to compile.

All that being said, there does appear to be an error in these requirements clauses as specified by N2661.

> Requires: ... Both `Rep1` and `Rep2` shall be implicitly convertible to CR, diagnostic required.

It is not necessary for both `Rep`s to be implicitly convertible to the `CR`. It is only necessary for the rhs `Rep` to be implicitly convertible to the `CR`. The `Rep` within the `duration` should be allowed to only be explicitly convertible to the `CR`. The explicit-conversion-requirement is covered under 20.12.5.7 [time.duration.cast].

Proposed resolution:

Change the requirements clauses under 20.12.5.5 [time.duration.nonmember]:

>     template <class Rep1, class Period, class Rep2>
>       duration<typename common_type<Rep1, Rep2>::type, Period>
>       operator*(const duration<Rep1, Period>& d, const Rep2& s);

> > Requires: `CR(Rep1, Rep2)` shall exist. `Rep2` shall be implicitly convertible to `CR(Rep1, Rep2)`. Diagnostic required.

>     template <class Rep1, class Period, class Rep2>
>       duration<typename common_type<Rep1, Rep2>::type, Period>
>       operator*(const Rep1& s, const duration<Rep2, Period>& d);

> > Requires d behavior: `CR(Rep1, Rep2)` shall exist. `Rep1` shall be implicitly convertible to `CR(Rep1, Rep2)`. Diagnostic required.

>     template <class Rep1, class Period, class Rep2>
>       duration<typename common_type<Rep1, Rep2>::type, Period>
>       operator/(const duration<Rep1, Period>& d, const Rep2& s);

> > Requires: `CR(Rep1, Rep2)` shall exist `Rep2` shall be implicitly convertible to `CR(Rep1, Rep2)` and `Rep2` shall not be an instantiation of `duration`. Diagnostic required.

### 883. swap circular definition

Section: 23 [containers] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-09-10 Last modified: 2015-04-08

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Discussion:

Note in particular that Table 90 "Container Requirements" gives semantics of `a.swap(b)` as `swap(a,b)`, yet for all containers we define `swap(a,b)` to call `a.swap(b)` - a circular definition.

[ San Francisco: ]

> Robert to propose a resolution along the lines of "Postcondition: "a = b, b = a" This will be a little tricky for the hash containers, since they don't have `operator==`.

[ Post Summit Anthony Williams provided proposed wording. ]

[ 2009-07 Frankfurt ]

> Moved to Ready with minor edits (which have been made).

Proposed resolution:

In table 80 in section 23.2.1 [container.requirements.general], replace the postcondition of `a.swap(b)` with the following:

> Table 80 -- Container requirements
> Expression | Return type | Operational semantics | Assertion/note pre-/post-conidtion | Complexity
> --- | --- | --- | --- | ---
> ... | ... | ... | ... | ...
> `a.swap(b);` | `void` |  | `swap(a,b)` Exchange the contents of `a` and `b`. | (Note A)

Remove the reference to swap from the paragraph following the table.

> Notes: the algorithms `swap()`, `equal()` and `lexicographical_compare()` are defined in Clause 25. ...

### 884. `shared_ptr` swap

Section: 20.8.2.2.4 [util.smartptr.shared.mod] Status: Resolved Submitter: Jonathan Wakely Opened: 2008-09-15 Last modified: 2015-04-08

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Discussion:

>     #include <memory>
>     #include <cassert>

>     struct A { };
>     struct B : A { };

>     int main()
>     {
>         std::shared_ptr<A> pa(new A);
>         std::shared_ptr<B> pb(new B);
>         std::swap<A>(pa, pb);  // N.B. no argument deduction
>         assert( pa.get() == pb.get() );
>         return 0;
>     }

Is this behaviour correct (I believe it is) and if so, is it unavoidable, or not worth worrying about?

This calls the lvalue/rvalue swap overload for `shared_ptr`:

>     template<class T> void swap( shared_ptr<T> & a, shared_ptr<T> && b );

silently converting the second argument from `shared_ptr<B>` to `shared_ptr<A>` and binding the rvalue ref to the produced temporary.

This is not, in my opinion, a `shared_ptr` problem; it is a general issue with the rvalue swap overloads. Do we want to prevent this code from compiling? If so, how?

Perhaps we should limit rvalue args to swap to those types that would benefit from the "swap trick". Or, since we now have `shrink_to_fit()`, just eliminate the rvalue swap overloads altogether. The original motivation was:

>     vector<A> v = ...;
>     ...
>     swap(v, vector<A>(v));

N1690.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to NAD Editorial Resolved.

Proposed resolution:

Recommend NAD Editorial Resolved, fixed by N2844.

### 885. `pair` assignment

Section: 20.3 [pairs] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-09-15 Last modified: 2015-04-08

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Discussion:

>     20.2.3 pairs
>     Missing assignemnt operator:
>     template<class U , class V>
>       requires CopyAssignable<T1, U> && CopyAssignable<T2, V>
>         pair& operator=(pair<U , V> const & p );

Well, that's interesting. This assignment operator isn't in the current working paper, either. Perhaps we deemed it acceptable to build a temporary of type `pair` from `pair<U, V>`, then move-assign from that temporary?

It sounds more like an issue waiting to be opened, unless you want to plug it now. As written we risk moving from lvalues.

[ San Francisco: ]

> Would be NAD if better ctors fixed it.

> Related to 811.

[ post San Francisco: ]

> Possibly NAD Editorial, solved by N2770.

[ 2009-05-25 Alisdair adds: ]

> Issue 885 was something I reported while reviewing the library concepts documents ahead of San Francisco. The missing operator was added as part of the paper adopted at that meeting (N2770) and I can confirm this operator is present in the current working paper. I recommend NAD.

[ 2009-07 Frankfurt ]

> We agree with the intent, but we need to wait for the dust to settle on concepts.

[ 2010-03-11 Stefanus provided wording. ]

[ 2010 Pittsburgh: Moved to Ready for Pittsburgh. ]

Proposed resolution:

Add the following declaration 20.3.2 [pairs.pair], before the declaration of `pair& operator=(pair&& p);`:

>     template<class U, class V> pair& operator=(const pair<U, V>& p);

Add the following description to 20.3.2 [pairs.pair] after paragraph 11 (before the description of `pair& operator=(pair&& p);)`:

>     template<class U, class V> pair& operator=(const pair<U, V>& p);

> > Requires: `T1` shall satisfy the requirements of `CopyAssignable` from `U`. `T2` shall satisfy the requirements of `CopyAssignable` from `V`.

> > Effects: Assigns `p.first` to `first` and `p.second` to `second`.

> > Returns: `*this`.

### 886. `tuple` construction

Section: 20.4.2.1 [tuple.cnstr] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-09-15 Last modified: 2015-04-08

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Discussion:

20.4.2.1 [tuple.cnstr]:

> Effects: Default initializes each element.

Could be clarified to state each "non-trivial" element. Otherwise we have a conflict with Core deinfition of default initialization - trivial types do not get initialized (rather than initialization having no effect)

I'm going to punt on this one, because it's not an issue that's related to concepts. I suggest bringing it to Howard's attention on the reflector.

[ San Francisco: ]

> Text in draft doesn't mean anything, changing to "non-trivial" makes it meaningful.

> We prefer "value initializes". Present implementations use value-initialization. Users who don't want value initialization have alternatives.

> Request resolution text from Alisdair.

> This issue relates to Issue 868 default construction and value-initialization.

[ 2009-05-04 Alisdair provided wording and adds: ]

> Note: This IS a change of semantic from TR1, although one the room agreed with during the discussion. To preserve TR1 semantics, this would have been worded:

> >     requires DefaultConstructible<Types>... tuple();

> > > -2- Effects: Default-initializes each non-trivial element.

[ 2009-07 Frankfurt ]

> Move to Ready.

Proposed resolution:

Change p2 in Construction 20.4.2.1 [tuple.cnstr]:

>     requires DefaultConstructible<Types>... tuple();

> > -2- Effects: Default Value-initializes each element.

### 888. `this_thread::yield` too strong

Section: 30.3.2 [thread.thread.this] Status: C++11 Submitter: Lawrence Crowl Opened: 2008-09-15 Last modified: 2015-04-08

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Discussion:

I never thought I'd say this, but `this_thread::yield` seems to be too strong in specification. The issue is that some systems distinguish between yielding to another thread in the same process and yielding to another process. Given that the C++ standard only talks about a single program, one can infer that the specification allows yielding only to another thread within the same program. Posix has no facility for that behavior. Can you please file an issue to weaken the wording. Perhaps "Offers the operating system the opportunity to reschedule."

[ Post Summit: ]

> Recommend move to Tentatively Ready.

Proposed resolution:

Change 30.3.2 [thread.thread.this]/3:

>     void this_thread::yield();

> > Effects: Offers the operating system implementation the opportunity to reschedule. another thread.

### 889. `thread::id` comparisons

Section: 30.3.1.1 [thread.thread.id] Status: Resolved Submitter: Lawrence Crowl Opened: 2008-09-15 Last modified: 2015-04-08

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Discussion:

Addresses UK 324

The `thread::id` type supports the full set of comparison operators. This is substantially more than is required for the associative containers that justified them. Please place an issue against the threads library.

[ San Francisco: ]

> Would depend on proposed extension to POSIX, or non-standard extension. What about hash? POSIX discussing op. POSIX not known to be considering support needed for hash, op.

> Group expresses support for putting ids in both unordered and ordered containers.

[ post San Francisco: ]

> Howard: It turns out the current working paper N2723 already has `hash<thread::id>` (20.9 [function.objects], 20.9.13 [unord.hash]). We simply overlooked it in the meeting. It is a good thing we voted in favor of it (again). :-)

> Recommend NAD.

[ Post Summit: ]

> Recommend to close as NAD. For POSIX, see if we need to add a function to convert `pthread_t` to integer.

[ Post Summit, Alisdair adds: ]

> The recommendation for LWG-889/UK-324 is NAD, already specified.

> It is not clear to me that the specification is complete.

> In particular, the synopsis of `<functional>` in 20.9 [function.objects] does not mention `hash< thread::id>` nor `hash< error_code >`, although their existence is implied by 20.9.13 [unord.hash], p1.

> I am fairly uncomfortable putting the declaration for the `thread_id` specialization into `<functional>` as `id` is a nested class inside `std::thread`, so it implies that `<functional>` would require the definition of the `thread` class template in order to forward declared `thread::id` and form this specialization.

> It seems better to me that the dependency goes the other way around (`<thread>` will more typically make use of `<functional>` than vice-versa) and the `hash<thread::id>` specialization be declared in the `<thread>` header.

> I think `hash<error_code>` could go into either `<system_error>` or `<functional>` and have no immediate preference either way. However, it should clearly appear in the synopsis of one of these two.

> Recommend moving 889 back to open, and tying in a reference to UK-324.

[ Batavia (2009-05): ]

> Howard observes that `thread::id` need not be a nested class; it could be a `typedef` for a more visible type.

[ 2009-05-24 Alisdair adds: ]

> I do not believe this is correct. `thread::id` is explicitly documents as a nested class, rather than as an unspecified typedef analogous to an iterator. If the intent is that this is not implemented as a nested class (under the as-if freedoms) then this is a novel form of standardese.

[ 2009-07 Frankfurt ]

> Decided we want to move hash specialization for thread_id to the thread header. Alisdair to provide wording.

[ 2009-07-28 Alisdair provided wording, moved to Review. ]

[ 2009-10 Santa Cruz: ]

> Add a strike for `hash<thread::id>`. Move to Ready

[ 2009-11-13 The proposed wording of 1182 is a superset of the wording in this issue. ]

[ 2010-02-09 Moved from Ready to Open: ]

> Issue 1182 is not quite a superset of this issue and it is controversial whether or not the note:

> > hash template specialization allows `thread::id` objects to be used as keys in unordered containers.

> should be added to the WP.

[ 2010-02-09 Objections to moving this to NAD Editorial Resolved, addressed by 1182 have been removed. Set to Tentatively NAD Editorial Resolved. ]

Rationale:

Solved by 1182.

Proposed resolution:

Remove the following prototype from the synopsis in 20.9 [function.objects]:

>     template <> struct hash<std::thread::id>;

Add to 30.3 [thread.threads], p1 Header `<thread>` synopsis:

>     template <class T> struct hash;
>     template <> struct hash<thread::id>;

Add template specialization below class definition in 30.3.1.1 [thread.thread.id]

>     template <>
>     struct hash<thread::id> : public unary_function<thread::id, size_t> {
>        size_t operator()(thread::id val) const;
>     };

Extend note in p2 30.3.1.1 [thread.thread.id] with second sentence:

> [Note: Relational operators allow `thread::id` objects to be used as keys in associative containers. `hash` template specialization allows `thread::id` objects to be used as keys in unordered containers. — end note]

Add new paragraph to end of 30.3.1.1 [thread.thread.id]

>     template <> struct hash<thread::id>;

> > An explicit specialization of the class template hash (20.9.13 [unord.hash]) shall be provided for the type `thread::id`.

### 890. Improving `<system_error>` initialization

Section: 19.5.1 [syserr.errcat] Status: C++11 Submitter: Beman Dawes Opened: 2008-09-14 Last modified: 2015-04-08

View all other issues in [syserr.errcat].

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Discussion:

The `static const error_category` objects `generic_category` and `system_category` in header `<system_error>` are currently declared:

>     const error_category& get_generic_category();
>     const error_category& get_system_category();

>     static const error_category& generic_category = get_generic_category();
>     static const error_category& system_category = get_system_category();

This formulation has several problems:

- Implementation details are exposed, since initialization is specified in the interface. This over-constrains implementations without offsetting user benefits. The form of initialization specified may be less than maximally efficient on some platforms.
- Use of the objects is more expensive in terms of number of machine level instructions. See Implementation experience below.
- Depending on the compiler, some cost may be incurred by each translation unit that includes the header, even if the objects are not used. This is a common scenario in user code, since the header is included by other standard library headers. It should be mentioned that at least one compilers is able to optimize this cost away, however.

IO streams uses a somewhat different formulation for iostream_category, but still suffer much the same problems.

The original plan was to eliminate these problems by applying the C++0x `constexpr` feature. See LWG issue 832. However, that approach turned out to be unimplementable, since it would require a `constexpr` object of a class with virtual functions, and that is not allowed by the core language.

The proposed resolution was developed as an alternative. It mitigates the above problems by removing initialization from the visible interface, allowing implementations flexibility.

Implementation experience:

Prototype implementations of the current WP interface and proposed resolution interface were tested with recent Codegear, GCC, Intel, and Microsoft compilers on Windows. The code generated by the Microsoft compiler was studied at length; the WP and proposal versions generated very similar code. For both versions the compiler did make use of static initialization; apparently the compiler applied an implicit `constexpr` where useful, even in cases where `constexpr` would not be permitted by the language!

Acknowledgements:

Martin Sebor, Chris Kohlhoff, and John Lakos provided useful ideas and comments on initialization issues.

[ San Francisco: ]

> Martin: prefers not to create more file-scope static objects, and would like to see `get_*` functions instead.

[Pre-Summit:]

> Beman: The proposed resolution has been reworked to remove the file-scope static objects, per Martin's suggestions. The `get_` prefix has been eliminated from the function names as no longer necessary and to conform with standard library naming practice.

[ Post Summit: ]

> Agreement that this is wise and essential, text provided works and has been implemented. Seems to be widespread consensus. Move to Tentative Ready.

Proposed resolution:

Change 17.6.5.14 [value.error.codes] Value of error codes as indicated:

> Certain functions in the C++ standard library report errors via a `std::error_code` (19.4.2.2) object. That object's `category()` member shall return a reference to `std::system_category```()`` for errors originating from the operating system, or a reference to an implementation-defined error_category object for errors originating elsewhere. The implementation shall define the possible values of value() for each of these error categories. [Example: For operating systems that are based on POSIX, implementations are encouraged to define the `std::system_category```()`` values as identical to the POSIX `errno` values, with additional values as defined by the operating system's documentation. Implementations for operating systems that are not based on POSIX are encouraged to define values identical to the operating system's values. For errors that do not originate from the operating system, the implementation may provide enums for the associated values --end example]

Change 19.5.1.1 [syserr.errcat.overview] Class `error_category` overview `error_category` synopsis as indicated:

>     const error_category& get_generic_category();
>     const error_category& get_system_category();

>     static storage-class-specifier const error_category& generic_category = get_generic_category();
>     static storage-class-specifier const error_category& system_category = get_system_category();

Change 19.5.1.5 [syserr.errcat.objects] Error category objects as indicated:

>     const error_category& get_generic_category();

> > Returns: A reference to an object of a type derived from class `error_category`.

> > Remarks: The object's `default_error_condition` and `equivalent` virtual functions shall behave as specified for the class `error_category`. The object's `name` virtual function shall return a pointer to the string `"GENERIC"`.

>     const error_category& get_system_category();

> > Returns: A reference to an object of a type derived from class `error_category`.

> > Remarks: The object's `equivalent` virtual functions shall behave as specified for class `error_category`. The object's `name` virtual function shall return a pointer to the string `"system"`. The object's `default_error_condition` virtual function shall behave as follows:

> > > If the argument `ev` corresponds to a POSIX `errno` value `posv`, the function shall return `error_condition(posv, generic_category())`. Otherwise, the function shall return `error_condition(ev, system_category())`. What constitutes correspondence for any given operating system is unspecified. [Note: The number of potential system error codes is large and unbounded, and some may not correspond to any POSIX `errno` value. Thus implementations are given latitude in determining correspondence. — end note]

Change 19.5.2.2 [syserr.errcode.constructors] Class `error_code` constructors as indicated:

>     error_code();

> > Effects: Constructs an object of type error_code.

> > Postconditions: `val_ == 0` and `cat_ == &system_category``()`.

Change 19.5.2.3 [syserr.errcode.modifiers] Class `error_code` modifiers as indicated:

>     void clear();

> > Postconditions: `value() == 0` and `category() == system_category``()`.

Change 19.5.2.5 [syserr.errcode.nonmembers] Class `error_code` non-member functions as indicated:

>     error_code make_error_code(errc e);

> > Returns: `error_code(static_cast<int>(e), generic_category``()``)`.

Change 19.5.3.2 [syserr.errcondition.constructors] Class `error_condition` constructors as indicated:

>     error_condition();

> > Effects: Constructs an object of type `error_condition`.

> > Postconditions: `val_ == 0` and `cat_ == &generic_category``()`.

Change 19.5.3.3 [syserr.errcondition.modifiers] Class `error_condition` modifiers as indicated:

>     void clear();

> > Postconditions: `value() == 0` and `category() == generic_category``()`.

Change 19.5.3.5 [syserr.errcondition.nonmembers] Class `error_condition` non-member functions as indicated:

>     error_condition make_error_condition(errc e);

> > Returns: `error_condition(static_cast<int>(e), generic_category())`.

Change 27.5 [iostreams.base] Iostreams base classes, Header `<ios>` synopsis as indicated:

>     concept_map ErrorCodeEnum<io_errc> { };
>     error_code make_error_code(io_errc e);
>     error_condition make_error_condition(io_errc e);
>     storage-class-specifier const error_category& iostream_category();

Change 27.5.3.1.1 [ios::failure] Class ios_base::failure, paragraph 2 as indicated:

> When throwing `ios_base::failure` exceptions, implementations should provide values of `ec` that identify the specific reason for the failure. [ Note: Errors arising from the operating system would typically be reported as `system_category``()` errors with an error value of the error number reported by the operating system. Errors arising from within the stream library would typically be reported as `error_code(io_errc::stream, iostream_category())`. — end note ]

Change 27.5.6.5 [error.reporting] Error reporting as indicated:

>     error_code make_error_code(io_errc e);

> > Returns: `error_code(static_cast<int>(e), iostream_category`()`)`.

>     error_condition make_error_condition(io_errc e);

> > Returns: `error_condition(static_cast<int>(e), iostream_category`()`)`.

>     storage-class-specifier const error_category& iostream_category();

> > The implementation shall initialize iostream_category. Its storage-class-specifier may be static or extern. It is unspecified whether initialization is static or dynamic (3.6.2). If initialization is dynamic, it shall occur before completion of the dynamic initialization of the first translation unit dynamically initialized that includes header <system_error>.

> > Returns: A reference to an object of a type derived from class `error_category`.

> > Remarks: The object's default_error_condition and equivalent virtual functions shall behave as specified for the class error_category. The object's name virtual function shall return a pointer to the string "iostream".

### 891. `std::thread`, `std::call_once` issue

Section: 30.3.1.2 [thread.thread.constr], 30.4.4.2 [thread.once.callonce] Status: C++11 Submitter: Peter Dimov Opened: 2008-09-15 Last modified: 2015-04-08

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Discussion:

I notice that the vararg overloads of `std::thread` and `std::call_once` (N2723 30.3.1.2 [thread.thread.constr] and 30.4.4.2 [thread.once.callonce]) are no longer specified in terms of `std::bind`; instead, some of the `std::bind` wording has been inlined into the specification.

There are two problems with this.

First, the specification (and implementation) in terms of `std::bind` allows, for example:

>     std::thread th( f, 1, std::bind( g ) );

which executes `f( 1, g() )` in a thread. This can be useful. The "inlined" formulation changes it to execute `f( 1, bind(g) )` in a thread.

Second, assuming that we don't want the above, the specification has copied the wording

> `INVOKE(func, w1, w2, ..., wN)` (20.6.2) shall be a valid expression for some values `w1, w2, ..., wN`

but this is not needed since we know that our argument list is args; it should simply be

> `INVOKE(func, args...)` (20.6.2) shall be a valid expression

[ Summit: ]

> Move to open.

[ Post Summit Anthony provided proposed wording. ]

[ 2009-07 Frankfurt ]

> Leave Open. Await decision for thread variadic constructor.

[ 2009-10 Santa Cruz: ]

> See proposed wording for 929 for this, for the formulation on how to solve this. 929 modifies the thread constructor to have "pass by value" behavior with pass by reference efficiency through the use of the `decay` trait. This same formula would be useful for `call_once`.

[ 2010-02-11 Anthony updates wording. ]

[ 2010-02-12 Moved to Tentatively Ready after 5 postive votes on c++std-lib. ]

Proposed resolution:

Modify 30.4.4.2 [thread.once.callonce] p1-p2 with the following:

>     template<class Callable, class ...Args>
>       void call_once(once_flag& flag, Callable&& func, Args&&... args);

> > Given a function as follows:

> > >     template<typename T> typename decay<T>::type decay_copy(T&& v)
> > >        { return std::forward<T>(v); }

> > 1 Requires: The template parameters `Callable` and each `Ti` in `Args` shall be `CopyConstructible` if an lvalue and otherwise satisfy the `MoveConstructible` requirements. `INVOKE(decay_copy(std::forward<Callable>(func), w1, w2, ..., wN decay_copy(std::forward<Args>(args))...)` (20.9.2 [func.require]) shall be a valid expression for some values `w1, w2, ..., wN`, where `N == sizeof...(Args)`.

> > 2 Effects: Calls to `call_once` on the same `once_flag` object are serialized. If there has been a prior effective call to `call_once` on the same `once_flag` object, the call to `call_once` returns without invoking `func`. If there has been no prior effective call to `call_once` on the same `once_flag` object, the argument `func` (or a copy thereof) is called as if by invoking `func(args)` `INVOKE(decay_copy(std::forward<Callable>(func)), decay_copy(std::forward<Args>(args))...)` is executed. The call to `call_once` is effective if and only if `func(args)` `INVOKE(decay_copy(std::forward<Callable>(func)), decay_copy(std::forward<Args>(args))...)` returns without throwing an exception. If an exception is thrown it is propagated to the caller.

### 893. `std::mutex` issue

Section: 30.4.1.2.1 [thread.mutex.class] Status: C++11 Submitter: Peter Dimov Opened: 2008-09-15 Last modified: 2015-04-08

View all other issues in [thread.mutex.class].

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Duplicate of: 905

Discussion:

30.4.1.2.1 [thread.mutex.class]/27 (in N2723) says that the behavior is undefined if:

- a thread that owns a `mutex` object calls `lock()` or `try_lock()` on that object

I don't believe that this is right. Calling `lock()` or `try_lock()` on a locked `mutex` is well defined in the general case. `try_lock()` is required to fail and return `false`. `lock()` is required to either throw an exception (and is allowed to do so if it detects deadlock) or to block until the `mutex` is free. These general requirements apply regardless of the current owner of the `mutex`; they should apply even if it's owned by the current thread.

Making double `lock()` undefined behavior probably can be justified (even though I'd still disagree with the justification), but `try_lock()` on a locked `mutex` must fail.

[ Summit: ]

> Move to open. Proposed resolution:

> - In 30.4.1 [thread.mutex.requirements] paragraph 12, change the error condition for `resource_deadlock_would_occur` to: "if the implementation detects that a deadlock would occur"
> - Strike 30.4.1.2.1 [thread.mutex.class] paragraph 3 bullet 2 "a thread that owns a mutex object calls `lock()` or `try_lock()` on that object, or"

[ 2009-07 Frankfurt ]

> Move to Review. Alisdair to provide note.

[ 2009-07-31 Alisdair provided note. ]

[ 2009-10 Santa Cruz: ]

> Moved to Ready.

[ 2009-11-18 Peter Opens: ]

> I don't believe that the proposed note:

> > [Note: a program may deadlock if the thread that owns a `mutex` object calls `lock()` or `try_lock()` on that object. If the program can detect the deadlock, a `resource_deadlock_would_occur` error condition may be observed. — end note]

> is entirely correct. "or `try_lock()`" should be removed, because `try_lock` is non-blocking and doesn't deadlock; it just returns `false` when it fails to lock the mutex.

> [ Howard: I've set to Open and updated the wording per Peter's suggestion. ]

[ 2009-11-18 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

In 30.4.1 [thread.mutex.requirements] paragraph 12 change:

> - ...
> - `resource_deadlock_would_occur` -- if the current thread already owns the mutex and is able to detect it implementation detects that a deadlock would occur.
> - ...

Strike 30.4.1.2.1 [thread.mutex.class] paragraph 3 bullet 2:

> -3- The behavior of a program is undefined if:

> - ...
> - a thread that owns a `mutex` object calls `lock()` or `try_lock()` on that object, or
> - ...

Add the following note after p3 30.4.1.2.1 [thread.mutex.class]

> [Note: a program may deadlock if the thread that owns a `mutex` object calls `lock()` on that object. If the implementation can detect the deadlock, a `resource_deadlock_would_occur` error condition may be observed. — end note]

### 894. longjmp and destructors

Section: 18.10 [support.runtime] Status: C++11 Submitter: Lawrence Crowl, Alisdair Meredith Opened: 2008-09-17 Last modified: 2015-04-08

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Discussion:

The interaction between `longjmp` and exceptions seems unnecessarily restrictive and not in keeping with existing practice.

Proposed resolution:

Edit paragraph 4 of 18.10 [support.runtime] as follows:

> The function signature `longjmp(jmp_buf jbuf, int val)` has more restricted behavior in this International Standard. A `setjmp/longjmp` call pair has undefined behavior if replacing the `setjmp` and `longjmp` by `catch` and `throw` would destroy invoke any non-trivial destructors for any automatic objects.

### 896. Library thread safety issue

Section: 20.8.2.2 [util.smartptr.shared] Status: C++11 Submitter: Hans Boehm Opened: 2008-09-16 Last modified: 2015-04-08

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Discussion:

It is unclear whether `shared_ptr` is thread-safe in the sense that multiple threads may simultaneously copy a `shared_ptr`. However this is a critical piece of information for the client, and it has significant impact on usability for many applications. (Detlef Vollman thinks it is currently clear that it is not thread-safe. Hans Boehm thinks it currently requires thread safety, since the `use_count` is not an explicit field, and constructors and assignment take a const reference to an existing `shared_ptr`.)

Pro thread-safety:

Many multi-threaded usages are impossible. A thread-safe version can be used to destroy an object when the last thread drops it, something that is often required, and for which we have no other easy mechanism.

Against thread-safety:

The thread-safe version is well-known to be far more expensive, even if used by a single thread. Many applications, including all single-threaded ones, do not care.

[ San Francisco: ]

> Beman: this is a complicated issue, and would like to move this to Open and await comment from Peter Dimov; we need very careful and complete rationale for any decision we make; let's go slow

> Detlef: I think that `shared_ptr` should not be thread-safe.

> Hans: When you create a thread with a lambda, it in some cases makes it very difficult for the lambda to reference anything in the heap. It's currently ambiguous as to whether you can use a `shared_ptr` to get at an object.

> Leave in Open. Detlef will submit an alternative proposed resolution that makes `shared_ptr` explicitly unsafe.

> A third option is to support both threadsafe and non-safe share_ptrs, and to let the programmer decide which behavior they want.

> Beman: Peter, do you support the PR?

> Peter:

> > Yes, I support the proposed resolution, and I certainly oppose any attempts to `make shared_ptr` thread-unsafe.

> > I'd mildly prefer if

> > > [Note: This is true in spite of that fact that such functions often modify `use_count()` --end note]

> > is changed to

> > > [Note: This is true in spite of that fact that such functions often cause a change in `use_count()` --end note]

> > (or something along these lines) to emphasise that `use_count()` is not, conceptually, a variable, but a return value.

[ 2009-07 Frankfurt ]

> Vote: Do we want one thread-safe shared pointer or two? If two, one would allow concurrent construction and destruction of shared pointers, and one would not be thread-safe. If one, then it would be thread-safe.

> No concensus on that vote.

> Hans to improve wording in consultation with Pete. Leave Open.

[ 2009-10 Santa Cruz: ]

> Move to Ready. Ask Editor to clear up wording a little when integrating to make it clear that the portion after the first comma only applies for the presence of data races.

[ 2009-10-24 Hans adds: ]

> I think we need to pull 896 back from ready, unfortunately. My wording doesn't say the right thing.

> I suspect we really want to say something along the lines of:

> > For purposes of determining the presence of a data race, member functions access and modify only the `shared_ptr` and `weak_ptr` objects themselves and not objects they refer to. Changes in `use_count()` do not reflect modifications that can introduce data races.

> But I think this needs further discussion by experts to make sure this is right.

> Detlef and I agree continue to disagree on the resolution, but I think we agree that it would be good to try to expedite this so that it can be in CD2, since it's likely to generate NB comments no matter what we do. And lack of clarity of intent is probably the worst option. I think it would be good to look at this between meetings.

[ 2010-01-20 Howard: ]

> I've moved Hans' suggested wording above into the proposed resolution section and preserved the previous wording here:

> > Make it explicitly thread-safe, in this weak sense, as I believe was intended:

> > Insert in 20.8.2.2 [util.smartptr.shared], before p5:

> > > For purposes of determining the presence of a data race, member functions do not modify `const shared_ptr` and const `weak_ptr` arguments, nor any objects they refer to. [Note: This is true in spite of that fact that such functions often cause a change in `use_count()` --end note]

> > On looking at the text, I'm not sure we need a similar disclaimer anywhere else, since nothing else has the problem with the modified `use_count()`. I think Howard arrived at a similar conclusion.

[ 2010 Pittsburgh: Moved to Ready for Pittsburgh ]

Proposed resolution:

Insert a new paragraph at the end of 20.8.2.2 [util.smartptr.shared]:

> For purposes of determining the presence of a data race, member functions access and modify only the `shared_ptr` and `weak_ptr` objects themselves and not objects they refer to. Changes in `use_count()` do not reflect modifications that can introduce data races.

### 897. Forward_list issues... Part 2

Section: 23.3.4.5 [forwardlist.modifiers] Status: Resolved Submitter: Howard Hinnant Opened: 2008-09-22 Last modified: 2015-04-08

View all other issues in [forwardlist.modifiers].

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Discussion:

This issue was split off from 892 at the request of the LWG.

[ San Francisco: ]

> This issue is more complicated than it looks.

> paragraph 47: replace each `(first, last) with (first, last]`

> add a statement after paragraph 48 that complexity is O(1)

> remove the complexity statement from the first overload of splice_after

> We may have the same problems with other modifiers, like erase_after. Should it require that all iterators in the range (position, last] be dereferenceable?

There are actually 3 issues here:

1. What value should `erase_after` return? With `list`, code often looks like:

   >     for (auto i = l.begin(); i != l.end();)
   >     {
   >         // inspect *i and decide if you want to erase it
   >         // ...
   >         if (I want to erase *i)
   >             i = l.erase(i);
   >         else
   >             ++i;
   >     }

   I.e. the iterator returned from `erase` is useful for setting up the logic for operating on the next element. For `forward_list` this might look something like:

   >     auto i = fl.before_begin();
   >     auto ip1 = i;
   >     for (++ip1; ip1 != fl.end(); ++ip1)
   >     {
   >         // inspect *(i+1) and decide if you want to erase it
   >         // ...
   >         if (I want to erase *(i+1))
   >             i = fl.erase_after(i);
   >         else
   >             ++i;
   >         ip1 = i;
   >     }

   In the above example code, it is convenient if `erase_after` returns the element prior to the erased element (range) instead of the element after the erase element (range).

   Existing practice:

   - SGI slist returns an iterator referencing the element after the erased range.
   - CodeWarrior slist returns an iterator referencing the element before the erased range.

   There is not a strong technical argument for either solution over the other.

2. With all other containers, operations always work on the range `[first, last)` and/or prior to the given `position`.

   With `forward_list`, operations sometimes work on the range `(first, last]` and/or after the given `position`.

   This is simply due to the fact that in order to operate on `*first` (with `forward_list`) one needs access to `*(first-1)`. And that's not practical with `forward_list`. So the operating range needs to start with `(first`, not `[first` (as the current working paper says).

   Additionally, if one is interested in splicing the range `(first, last)`, then (with `forward_list`), one needs practical (constant time) access to `*(last-1)` so that one can set the next field in this node to the proper value. As this is not possible with `forward_list`, one must specify the last element of interest instead of one past the last element of interest. The syntax for doing this is to pass `(first, last]` instead of `(first, last)`.

   With `erase_after` we have a choice of either erasing the range `(first, last]` or `(first, last)`. Choosing the latter enables:

   >     x.erase_after(pos, x.end());

   With the former, the above statement is inconvenient or expensive due to the lack of constant time access to `x.end()-1`. However we could introduce:

   >     iterator erase_to_end(const_iterator position);

   to compensate.

   The advantage of the former (`(first, last]`) for `erase_after` is a consistency with `splice_after` which uses `(first, last]` as the specified range. But this either requires the addition of `erase_to_end` or giving up such functionality.

3. As stated in the discussion of 892, and reienforced by point 2 above, a `splice_after` should work on the source range `(first, last]` if the operation is to be Ο(1). When splicing an entire list `x` the algorithm needs `(x.before_begin(), x.end()-1]`. Unfortunately `x.end()-1` is not available in constant time unless we specify that it must be. In order to make `x.end()-1` available in constant time, the implementation would have to dedicate a pointer to it. I believe the design of N2543 intended a nominal overhead of `foward_list` of 1 pointer. Thus splicing one entire `forward_list` into another can not be Ο(1).

[ Batavia (2009-05): ]

> We agree with the proposed resolution.

> Move to Review.

[ 2009-07 Frankfurt ]

> We may need a new issue to correct splice_after, because it may no longer be correct to accept an rvalues as an argument. Merge may be affected, too. This might be issue 1133. (Howard: confirmed)

> Move this to Ready, but the Requires clause of the second form of splice_after should say "(first, last)," not "(first, last]" (there are three occurrences). There was considerable discussion on this. (Howard: fixed)

> Alan suggested removing the "foward_last<T. Alloc>&& x" parameter from the second form of splice_after, because it is redundant. PJP wanted to keep it, because it allows him to check for bad ranges (i.e. "Granny knots").

> We prefer to keep `x`.

> Beman. Whenever we deviate from the customary half-open range in the specification, we should add a non-normative comment to the standard explaining the deviation. This clarifies the intention and spares the committee much confusion in the future.

> Alan to write a non-normative comment to explain the use of fully-closed ranges.

> Move to Ready, with the changes described above. (Howard: awaiting note from Alan)

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved, addressed by N2988.

Proposed resolution:

Wording below assumes issue 878 is accepted, but this issue is independent of that issue.

Change 23.3.4.5 [forwardlist.modifiers]:

>     iterator erase_after(const_iterator position);

> > Requires: The iterator following `position` is dereferenceable.

> > Effects: Erases the element pointed to by the iterator following `position`.

> > Returns: An iterator pointing to the element following the one that was erased, or `end()` if no such element exists An iterator equal to `position`.

>     iterator erase_after(const_iterator position, const_iterator last);

> > Requires: All iterators in the range `[(position,last)` are dereferenceable.

> > Effects: Erases the elements in the range `[(position,last)`.

> > Returns: An iterator equal to `position` `last`

Change 23.3.4.6 [forwardlist.ops]:

>     void splice_after(const_iterator position, forward_list<T,Allocator>&& x);

> > Requires: `position` is `before_begin()` or a dereferenceable iterator in the range `[begin(), end))`. `&x != this`.

> > Effects: Inserts the contents of `x` after `position`, and `x` becomes empty. Pointers and references to the moved elements of `x` now refer to those same elements but as members of `*this`. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into `*this`, not into `x`.

> > Throws: Nothing.

> > Complexity: Ο(1) Ο(`distance(x.begin(), x.end())`)

> ...

>     void splice_after(const_iterator position, forward_list<T,Allocator>&& x,
>                       const_iterator first, const_iterator last);

> > Requires: `position` is `before_begin()` or a dereferenceable iterator in the range `[begin(), end))`. `(first,last)` is a valid range in `x`, and all iterators in the range `(first,last)` are dereferenceable. `position` is not an iterator in the range `(first,last)`.

> > Effects: Inserts elements in the range `(first,last)` after `position` and removes the elements from `x`. Pointers and references to the moved elements of `x` now refer to those same elements but as members of `*this`. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into `*this`, not into `x`.

> > Complexity: Ο(1).

### 898. Small contradiction in n2723 to forward to committee

Section: 23.3.4.6 [forwardlist.ops] Status: C++11 Submitter: Arch Robison Opened: 2008-09-08 Last modified: 2015-04-08

View all other issues in [forwardlist.ops].

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Discussion:

I ran across a small contradiction in working draft n2723.

> 23.3.4 [forwardlist]p2: A `forward_list` satisfies all of the requirements of a container (table 90), except that the `size()` member function is not provided.

> 23.3.4.6 [forwardlist.ops]p57: Complexity: At most `size() + x.size() - 1` comparisons.

Presumably 23.3.4.6 [forwardlist.ops]p57 needs to be rephrased to not use `size()`, or note that it is used there only for sake of notational convenience.

[ 2009-03-29 Beman provided proposed wording. ]

[ Batavia (2009-05): ]

> We agree with the proposed resolution.

> Move to Tentatively Ready.

Proposed resolution:

Change 23.3.4.6 [forwardlist.ops], forward_list operations, paragraph 19, merge complexity as indicated:

> Complexity: At most `size() + x.size() distance(begin(), end()) + distance(x.begin(), x.end()) - 1` comparisons.

### 899. Adjusting `shared_ptr` for `nullptr_t`

Section: 20.8.2.2.2 [util.smartptr.shared.dest] Status: C++11 Submitter: Peter Dimov Opened: 2008-09-18 Last modified: 2015-04-08

View all other issues in [util.smartptr.shared.dest].

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Discussion:

James Dennett, message c++std-lib-22442:

> The wording below addresses one case of this, but opening an issue to address the need to sanity check uses of the term "pointer" in 20.8.2.2 [util.smartptr.shared] would be a good thing.

There's one more reference, in `~shared_ptr;` we can apply your suggested change to it, too. That is:

Change 20.8.2.2.2 [util.smartptr.shared.dest]/1 second bullet from:

> Otherwise, if *this owns a pointer p and a deleter d, d(p) is called.

to:

> Otherwise, if *this owns an object p and a deleter d, d(p) is called.

[ Post Summit: ]

> Recommend Review.

[ Batavia (2009-05): ]

> Peter Dimov notes the analogous change has already been made to "the new `nullptr_t` taking constructors in 20.8.2.2.1 [util.smartptr.shared.const] p9-13."

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change 20.8.2.2.2 [util.smartptr.shared.dest] p1 second bullet:

> - ...
> - Otherwise, if `*this` owns a pointer an object `p` and a deleter `d`, `d(p)` is called.

### 900. Stream move-assignment

Section: 27.9.1.8 [ifstream.assign] Status: C++11 Submitter: Niels Dekker Opened: 2008-09-20 Last modified: 2015-04-08

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Discussion:

It appears that we have an issue similar to issue 675 regarding the move-assignment of stream types. For example, when assigning to an `std::ifstream`, `ifstream1`, it seems preferable to close the file originally held by `ifstream1`:

>     ifstream1 = std::move(ifstream2);

The current Draft (N2723) specifies that the move-assignment of stream types like `ifstream` has the same effect as a swap:

> Assign and swap 27.9.1.8 [ifstream.assign]

>     basic_ifstream& operator=(basic_ifstream&& rhs);

> > Effects: `swap(rhs)`.

[ Batavia (2009-05): ]

> Howard agrees with the analysis and the direction proposed.

> Move to Open pending specific wording to be supplied by Howard.

[ 2009-07 Frankfurt: ]

> Howard is going to write wording.

[ 2009-07-26 Howard provided wording. ]

[ 2009-09-13 Niels adds: ]

> Note: The proposed change of 27.9.1.3 [filebuf.assign] p1 depends on the resolution of LWG 1204, which allows implementations to assume that `*this` and `rhs` refer to different objects.

[ 2009 Santa Cruz: ]

> Leave as Open. Too closely related to 911 to move on at this time.

[ 2010 Pittsburgh: ]

> Moved to Ready for Pittsburgh.

Proposed resolution:

Change 27.8.2.2 [stringbuf.assign]/1:

>     basic_stringbuf& operator=(basic_stringbuf&& rhs);

> > -1- Effects: `swap(rhs)`. After the move assignment `*this` reflects the same observable state it would have if it had been move constructed from `rhs` (27.8.2.1 [stringbuf.cons]).

Change 27.8.3.2 [istringstream.assign]/1:

>     basic_istringstream& operator=(basic_istringstream&& rhs);

> > -1- Effects: `swap(rhs)`. Move assigns the base and members of `*this` with the respective base and members of `rhs`.

Change 27.8.4.2 [ostringstream.assign]/1:

>     basic_ostringstream& operator=(basic_ostringstream&& rhs);

> > -1- Effects: `swap(rhs)`. Move assigns the base and members of `*this` with the respective base and members of `rhs`.

Change 27.8.5.2 [stringstream.assign]/1:

>     basic_stringstream& operator=(basic_stringstream&& rhs);

> > -1- Effects: `swap(rhs)`. Move assigns the base and members of `*this` with the respective base and members of `rhs`.

Change 27.9.1.3 [filebuf.assign]/1:

>     basic_filebuf& operator=(basic_filebuf&& rhs);

> > -1- Effects: `swap(rhs)`. Begins by calling `this->close()`. After the move assignment `*this` reflects the same observable state it would have if it had been move constructed from `rhs` (27.9.1.2 [filebuf.cons]).

Change 27.9.1.8 [ifstream.assign]/1:

>     basic_ifstream& operator=(basic_ifstream&& rhs);

> > -1- Effects: `swap(rhs)`. Move assigns the base and members of `*this` with the respective base and members of `rhs`.

Change 27.9.1.12 [ofstream.assign]/1:

>     basic_ofstream& operator=(basic_ofstream&& rhs);

> > -1- Effects: `swap(rhs)`. Move assigns the base and members of `*this` with the respective base and members of `rhs`.

Change 27.9.1.16 [fstream.assign]/1:

>     basic_fstream& operator=(basic_fstream&& rhs);

> > -1- Effects: `swap(rhs)`. Move assigns the base and members of `*this` with the respective base and members of `rhs`.

### 904. `result_of` argument types

Section: X [func.ret] Status: C++11 Submitter: Jonathan Wakely Opened: 2008-09-10 Last modified: 2015-04-08

View all other issues in [func.ret].

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Discussion:

The WP and TR1 have the same text regarding the argument types of a `result_of` expression:

> The values `ti` are lvalues when the corresponding type `Ti` is a reference type, and rvalues otherwise.

I read this to mean that this compiles:

>     typedef int (*func)(int&);
>     result_of<func(int&&)>::type i = 0;

even though this doesn't:

>     int f(int&);
>     f( std::move(0) );

Should the text be updated to say "when `Ti` is an lvalue-reference type" or am I missing something?

I later came up with this self-contained example which won't compile, but I think it should:

>     struct X {
>       void operator()(int&);
>       int operator()(int&&);
>     } x;

>     std::result_of< X(int&&) >::type i = x(std::move(0));

[ Post Summit: ]

> Recommend Tentatively Ready.

Proposed resolution:

Change X [func.ret], p1:

> ... The values `ti` are lvalues when the corresponding type `Ti` is an lvalue-reference type, and rvalues otherwise.

### 907. Bitset's immutable element retrieval is inconsistently defined

Section: 20.6.2 [bitset.members] Status: C++11 Submitter: Daniel Krügler Opened: 2008-09-26 Last modified: 2015-04-08

View all other issues in [bitset.members].

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Discussion:

The current standard 14882::2003(E) as well as the current draft N2723 have in common a contradiction of the operational semantics of member function `test` 20.6.2 [bitset.members] p.56-58 and the immutable member `operator[]` overload 20.6.2 [bitset.members] p.64-66 (all references are defined in terms of N2723):

1.     bool test(size_t pos) const;

   > Requires: `pos` is valid

   > Throws: `out_of_range` if `pos` does not correspond to a valid bit position.

   > Returns: `true` if the bit at position `pos` in `*this` has the value one.

2.     constexpr bool operator[](size_t pos) const;

   > Requires: `pos` shall be valid.

   > Throws: nothing.

   > Returns: `test(pos)`.

Three interpretations:

1. The `operator[]` overload is indeed allowed to throw an exception (via `test()`, if `pos` corresponds to an invalid bit position) which does not leave the call frame. In this case this function cannot be a `constexpr` function, because `test()` is not, due to 5.20 [expr.const]/2, last bullet.
2. The intend was not to throw an exception in `test` in case of an invalid bit position. There is only little evidence for this interpretation.
3. The intend was that `operator[]` should not throw any exception, but that `test` has the contract to do so, if the provided bit position is invalid.

The problem became worse, because issue 720 recently voted into WP argued that member `test` logically must be a `constexpr` function, because it was used to define the semantics of another `constexpr` function (the `operator[]` overload).

Three alternatives are proposed, corresponding to the three bullets (A), (B), and (C), the author suggests to follow proposal (C).

Proposed alternatives:

1. Remove the `constexpr` specifier in front of `operator[]` overload and undo that of member `test` (assuming 720 is accepted) in both the class declaration 20.6 [template.bitset]/1 and in the member description before 20.6.2 [bitset.members]/56 and before /64 to read:

   >     constexpr bool test(size_t pos) const;
   >     ..
   >     constexpr bool operator[](size_t pos) const;

   Change the throws clause of p. 65 to read:

   > Throws: nothing `out_of_range` if `pos` does not correspond to a valid bit position.

2. Replace the throws clause p. 57 to read:

   > Throws: `out_of_range` if `pos` does not correspond to a valid bit position nothing.

3. Undo the addition of the `constexpr` specifier to the `test` member function in both class declaration 20.6 [template.bitset]/1 and in the member description before 20.6.2 [bitset.members]/56, assuming that 720 was applied.

   >     constexpr bool test(size_t pos) const;

   Change the returns clause p. 66 to read:

   > Returns: `test(pos)` `true` if the bit at position `pos` in `*this` has the value one, otherwise `false`.

[ Post Summit: ]

> Lawrence: proposed resolutions A, B, C are mutually exclusive.

> Recommend Review with option C.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

1. […]
2. […]

3. Undo the addition of the `constexpr` specifier to the `test` member function in both class declaration 20.6 [template.bitset] p.1 and in the member description before 20.6.2 [bitset.members] p.56, assuming that 720 was applied.

   >     constexpr bool test(size_t pos) const;

   Change the returns clause p. 66 to read:

   > Returns: `test(pos)` `true` if the bit at position `pos` in `*this` has the value one, otherwise `false`.

### 908. Deleted assignment operators for atomic types must be volatile

Section: X [atomics.types] Status: Resolved Submitter: Anthony Williams Opened: 2008-09-26 Last modified: 2015-04-08

View all issues with Resolved status.

Discussion:

Addresses US 90

The deleted copy-assignment operators for the atomic types are not marked as volatile in N2723, whereas the assignment operators from the associated non-atomic types are. e.g.

>     atomic_bool& operator=(atomic_bool const&) = delete;
>     atomic_bool& operator=(bool) volatile;

This leads to ambiguity when assigning a non-atomic value to a non-volatile instance of an atomic type:

>     atomic_bool b;
>     b=false;

Both assignment operators require a standard conversions: the copy-assignment operator can use the implicit `atomic_bool(bool)` conversion constructor to convert `false` to an instance of `atomic_bool`, or `b` can undergo a qualification conversion in order to use the assignment from a plain `bool`.

This is only a problem once issue 845 is applied.

[ Summit: ]

> Move to open. Assign to Lawrence. Related to US 90 comment.

[ 2009-08-17 Handled by N2925. ]

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Addressed by N2992.

Proposed resolution:

Add volatile qualification to the deleted copy-assignment operator of all the atomic types:

>     atomic_bool& operator=(atomic_bool const&) volatile = delete;
>     atomic_itype& operator=(atomic_itype const&) volatile = delete;

etc.

This will mean that the deleted copy-assignment operator will require two conversions in the above example, and thus be a worse match than the assignment from plain `bool`.

### 909. `regex_token_iterator` should use `initializer_list`

Section: 28.12.2 [re.tokiter] Status: C++11 Submitter: Daniel Krügler Opened: 2008-09-26 Last modified: 2015-04-08

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Discussion:

Addresses UK 319

Construction of a `regex_token_iterator` (28.12.2 [re.tokiter]/6+) usually requires the provision of a sequence of integer values, which can currently be done via a `std::vector<int>` or a C array of `int`. Since the introduction of `initializer_list` in the standard it seems much more reasonable to provide a corresponding constructor that accepts an `initializer_list<int>` instead. This could be done as a pure addition or one could even consider replacement. The author suggests the replacement strategy (A), but provides an alternative additive proposal (B) as a fall-back, because of the handiness of this range type:

[ Batavia (2009-05): ]

> We strongly recommend alternative B of the proposed resolution in order that existing code not be broken. With that understanding, move to Tentatively Ready.

Original proposed wording:

1. 1. In 28.12.2 [re.tokiter]/6 and the list 28.12.2.1 [re.tokiter.cnstr]/10-11 change the constructor declaration:

      >     template <std::size_t N>
      >     regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
      >                          const regex_type& re,
      >                          const int (&submatches)[N] initializer_list<int> submatches,
      >                          regex_constants::match_flag_type m =
      >                            regex_constants::match_default);

   2. In 28.12.2.1 [re.tokiter.cnstr]/12 change the last sentence

      > The third constructor initializes the member `subs` to hold a copy of the sequence of integer values pointed to by the iterator range `[&submatches.begin(), &submatches.end() + N)`.

2. 1. In 28.12.2 [re.tokiter]/6 and the list 28.12.2.1 [re.tokiter.cnstr]/10-11 insert the following constructor declaration between the already existing ones accepting a `std::vector` and a C array of `int`, resp.:

      >     regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
      >                          const regex_type& re,
      >                          initializer_list<int> submatches,
      >                          regex_constants::match_flag_type m =
      >                            regex_constants::match_default);

   2. In 28.12.2.1 [re.tokiter.cnstr]/12 change the last sentence

      > The third and fourth constructor initializes the member `subs` to hold a copy of the sequence of integer values pointed to by the iterator range `[&submatches,&submatches + N)` and `[submatches.begin(),submatches.end())`, respectively.

Proposed resolution:

1. […]

2. 1. In 28.12.2 [re.tokiter]/6 and the list 28.12.2.1 [re.tokiter.cnstr]/10-11 insert the following constructor declaration between the already existing ones accepting a `std::vector` and a C array of `int`, resp.:

      >     regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
      >                          const regex_type& re,
      >                          initializer_list<int> submatches,
      >                          regex_constants::match_flag_type m =
      >                            regex_constants::match_default);

   2. In 28.12.2.1 [re.tokiter.cnstr]/12 change the last sentence

      > The third and fourth constructor initializes the member `subs` to hold a copy of the sequence of integer values pointed to by the iterator range `[&submatches,&submatches + N)` and `[submatches.begin(),submatches.end())`, respectively.

### 911. I/O streams and `move/swap` semantic

Section: 27.7.2 [input.streams], 27.7.3 [output.streams] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2008-09-29 Last modified: 2015-04-08

View all issues with C++11 status.

Discussion:

Class template `basic_istream`, `basic_ostream` and `basic_iostream` implements public move constructors, move assignment operators and `swap` method and free functions. This might induce both the user and the compiler to think that those types are `MoveConstructible`, `MoveAssignable` and `Swappable`. However, those class templates fail to fulfill the user expectations. For example:

>     std::ostream os(std::ofstream("file.txt"));
>     assert(os.rdbuf() == 0); // buffer object is not moved to os, file.txt has been closed

>     std::vector<std::ostream> v;
>     v.push_back(std::ofstream("file.txt"));
>     v.reserve(100); // causes reallocation
>     assert(v[0].rdbuf() == 0); // file.txt has been closed!

>     std::ostream&& os1 = std::ofstream("file1.txt");
>     os1 = std::ofstream("file2.txt");
>     os1 << "hello, world"; // still writes to file1.txt, not to file2.txt!

>     std::ostream&& os1 = std::ofstream("file1.txt");
>     std::ostream&& os2 = std::ofstream("file2.txt");
>     std::swap(os1, os2);
>     os1 << "hello, world"; // writes to file1.txt, not to file2.txt!

This is because the move constructor, the move assignment operator and `swap` are all implemented through calls to `std::basic_ios` member functions `move()` and `swap()` that do not move nor swap the controlled stream buffers. That can't happen because the stream buffers may have different types.

Notice that for `basic_streambuf`, the member function `swap()` is protected. I believe that is correct and all of `basic_istream`, `basic_ostream`, `basic_iostream` should do the same as the move ctor, move assignment operator and swap member function are needed by the derived `fstream`s and `stringstream`s template. The free swap functions for `basic_(i|o|io)stream` templates should be removed for the same reason.

[ Batavia (2009-05): ]

> We note that the rvalue swap functions have already been removed.

> Bill is unsure about making the affected functions protected; he believes they may need to be public.

> We are also unsure about removing the lvalue swap functions as proposed.

> Move to Open.

[ 2009-07 Frankfurt: ]

> It's not clear that the use case is compelling.

> Howard: This needs to be implemented and tested.

[ 2009-07-26 Howard adds: ]

> I started out thinking I would recommend NAD for this one. I've turned around to agree with the proposed resolution (which I've updated to the current draft). I did not fully understand Ganesh's rationale, and attempt to describe my improved understanding below.

> The move constructor, move assignment operator, and swap function are different for `basic_istream`, `basic_ostream` and `basic_iostream` than other classes. A timely conversation with Daniel reminded me of this long forgotten fact. These members are sufficiently different that they would be extremely confusing to use in general, but they are very much needed for derived clients.

> - The move constructor moves everything but the `rdbuf` pointer.
> - The move assignment operator moves everything but the `rdbuf` pointer.
> - The swap function swaps everything but the `rdbuf` pointer.

> The reason for this behavior is that for the std-derived classes (stringstreams, filestreams), the `rdbuf` pointer points back into the class itself (self referencing). It can't be swapped or moved. But this fact isn't born out at the `stream` level. Rather it is born out at the `fstream`/`sstream` level. And the lower levels just need to deal with that fact by not messing around with the `rdbuf` pointer which is stored down at the lower levels.

> In a nutshell, it is very confusing for all of those who are not so intimately related with streams that they've implemented them. And it is even fairly confusing for some of those who have (including myself). I do not think it is safe to swap or move `istreams` or `ostreams` because this will (by necessary design) separate stream state from streambuffer state. Derived classes (such as `fstream` and `stringstream` must be used to keep the stream state and stream buffer consistently packaged as one unit during a move or swap.

> I've implemented this proposal and am living with it day to day.

[ 2009 Santa Cruz: ]

> Leave Open. Pablo expected to propose alternative wording which would rename move construction, move assignment and swap, and may or may not make them protected. This will impact issue 900.

[ 2010 Pittsburgh: ]

> Moved to Ready for Pittsburgh.

Proposed resolution:

27.7.2.1 [istream]: make the following member functions protected:

>     basic_istream(basic_istream&&  rhs);
>     basic_istream&  operator=(basic_istream&&  rhs);
>     void  swap(basic_istream&  rhs);

Ditto: remove the swap free function signature

>     // swap:
>     template <class charT, class traits>
>       void swap(basic_istream<charT, traits>& x, basic_istream<charT, traits>& y);

27.7.2.1.2 [istream.assign]: remove paragraph 4

>     template <class charT, class traits>
>       void swap(basic_istream<charT, traits>& x, basic_istream<charT, traits>& y);

> > Effects: `x.swap(y)`.

27.7.2.5 [iostreamclass]: make the following member function protected:

>     basic_iostream(basic_iostream&&  rhs);
>     basic_iostream&  operator=(basic_iostream&&  rhs);
>     void  swap(basic_iostream&  rhs);

Ditto: remove the swap free function signature

>     template <class charT, class traits>
>       void swap(basic_iostream<charT, traits>& x, basic_iostream<charT, traits>& y);

27.7.2.5.3 [iostream.assign]: remove paragraph 3

>     template <class charT, class traits>
>       void swap(basic_iostream<charT, traits>& x, basic_iostream<charT, traits>& y);

> > Effects: `x.swap(y)`.

27.7.3.1 [ostream]: make the following member function protected:

>     basic_ostream(basic_ostream&&  rhs);
>     basic_ostream&  operator=(basic_ostream&&  rhs);
>     void  swap(basic_ostream&  rhs);

Ditto: remove the swap free function signature

>     // swap:
>     template <class charT, class traits>
>       void swap(basic_ostream<charT, traits>& x, basic_ostream<charT, traits>& y);

27.7.3.3 [ostream.assign]: remove paragraph 4

>     template <class charT, class traits>
>       void swap(basic_ostream<charT, traits>& x, basic_ostream<charT, traits>& y);

> > Effects: `x.swap(y)`.

### 920. Ref-qualification support in the library

Section: 20.9.11 [func.memfn] Status: C++11 Submitter: Bronek Kozicki Opened: 2008-10-06 Last modified: 2015-04-08

View all other issues in [func.memfn].

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Duplicate of: 1230

Discussion:

Daniel Krügler wrote:

> Shouldn't above list be completed for &- and &&-qualified member functions This would cause to add:

> >     template<Returnable R, class T, CopyConstructible... Args>
> >     unspecified mem_fn(R (T::* pm)(Args...) &);
> >     template<Returnable R, class T, CopyConstructible... Args>
> >     unspecified mem_fn(R (T::* pm)(Args...) const &);
> >     template<Returnable R, class T, CopyConstructible... Args>
> >     unspecified mem_fn(R (T::* pm)(Args...) volatile &);
> >     template<Returnable R, class T, CopyConstructible... Args>
> >     unspecified mem_fn(R (T::* pm)(Args...) const volatile &);
> >     template<Returnable R, class T, CopyConstructible... Args>
> >     unspecified mem_fn(R (T::* pm)(Args...) &&);
> >     template<Returnable R, class T, CopyConstructible... Args>
> >     unspecified mem_fn(R (T::* pm)(Args...) const &&);
> >     template<Returnable R, class T, CopyConstructible... Args>
> >     unspecified mem_fn(R (T::* pm)(Args...) volatile &&);
> >     template<Returnable R, class T, CopyConstructible... Args>
> >     unspecified mem_fn(R (T::* pm)(Args...) const volatile &&);

yes, absolutely. Thanks for spotting this. Without this change `mem_fn` cannot be initialized from pointer to ref-qualified member function. I believe semantics of such function pointer is well defined.

[ Post Summit Daniel provided wording. ]

[ Batavia (2009-05): ]

> We need to think about whether we really want to go down the proposed path of combinatorial explosion. Perhaps a Note would suffice.

> We would really like to have an implementation before proceeding.

> Move to Open, and recommend this be deferred until after the next Committee Draft has been issued.

[ 2009-10-10 Daniel updated wording to post-concepts. ]

> 1230 has a similar proposed resolution

[ 2009-10 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

1. Change 20.9 [function.objects]/2, header `<functional>` synopsis as follows:

   >     // 20.7.14, member function adaptors:
   >     template<class R, class T> unspecified mem_fn(R T::*);

   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...));
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) const);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) volatile);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) const volatile);

   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) &);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) const &);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) volatile &);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) const volatile &);

   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) &&);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) const &&);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) volatile &&);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::*)(Args...) const volatile &&);

2. Change the prototype list of 20.9.11 [func.memfn] as follows [NB: The following text, most notably p.2 and p.3 which discuss influence of the cv-qualification on the definition of the base class's first template parameter remains unchanged. ]:

   >     template<class R, class T> unspecified mem_fn(R T::* pm);

   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...));
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) volatile);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const volatile);

   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) &);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const &);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) volatile &);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const volatile &);

   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) &&);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const &&);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) volatile &&);
   >     template<class R, class T, class ...Args> unspecified mem_fn(R (T::* pm)(Args...) const volatile &&);

3. Remove 20.9.11 [func.memfn]/5:

   > Remarks: Implementations may implement `mem_fn` as a set of overloaded function templates.

### 921. Rational Arithmetic should use template aliases

Section: 20.11.3 [ratio.ratio] Status: C++11 Submitter: Pablo Halpern Opened: 2008-10-07 Last modified: 2015-04-08

View all other issues in [ratio.ratio].

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Discussion:

The compile-time functions that operate on `ratio<N,D>` require the cumbersome and error-prone "evaluation" of a `type` member using a meta-programming style that predates the invention of template aliases. Thus, multiplying three ratios `a`, `b`, and `c` requires the expression:

>     ratio_multiply<a, ratio_multiply<b, c>::type>::type

The simpler expression:

>     ratio_multiply<a, ratio_multiply<b, c>>

Could be used by if template aliases were employed in the definitions.

[ Post Summit: ]

> Jens: not a complete proposed resolution: "would need to make similar change"

> Consensus: We agree with the direction of the issue.

> Recommend Open.

[ 2009-05-11 Daniel adds: ]

> Personally I'm not in favor for the addition of:

> >     typedef ratio type;

> For a reader of the standard it's usage or purpose is unclear. I haven't seen similar examples of attempts to satisfy non-feature complete compilers.

[ 2009-05-11 Pablo adds: ]

> The addition of type to the `ratio` template allows the previous style (i.e., in the prototype implementations) to remain valid and permits the use of transitional library implementations for C++03 compilers. I do not feel strongly about its inclusion, however, and leave it up to the reviewers to decide.

[ Batavia (2009-05): ]

> Bill asks for additional discussion in the issue that spells out more details of the implementation. Howard points us to issue 948 which has at least most of the requested details. Tom is strongly in favor of overflow-checking at compile time. Pete points out that there is no change of functionality implied. We agree with the proposed resolution, but recommend moving the issue to Review to allow time to improve the discussion if needed.

[ 2009-07-21 Alisdair adds: ]

> See 1121 for a potentially incompatible proposal.

[ 2009-10 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

1. In 20.11 [ratio] p.3 change as indicated:

   >     // ratio arithmetic
   >     template <class R1, class R2> structusing ratio_add = see below;
   >     template <class R1, class R2> structusing ratio_subtract = see below;
   >     template <class R1, class R2> structusing ratio_multiply = see below;
   >     template <class R1, class R2> structusing ratio_divide = see below;

2. In 20.11.3 [ratio.ratio], change as indicated:

   >     namespace std {
   >       template <intmax_t N, intmax_t D = 1>
   >       class ratio {
   >       public:
   >         typedef ratio type;
   >         static const intmax_t num;
   >         static const intmax_t den;
   >       };
   >     }

3. In 20.11.4 [ratio.arithmetic] change as indicated:

   >     template <class R1, class R2> structusing ratio_add = see below{
   >       typedef see below type;
   >     };

   > > 1 The nested typedef type `ratio_add<R1, R2>` shall be a synonym for `ratio<T1, T2>` where `T1` has the value `R1::num * R2::den + R2::num * R1::den` and `T2` has the value `R1::den * R2::den`.

   >     template <class R1, class R2> structusing ratio_subtract = see below{
   >       typedef see below type;
   >     };

   > > 2 The nested typedef type `ratio_subtract<R1, R2>` shall be a synonym for `ratio<T1, T2>` where `T1` has the value `R1::num * R2::den - R2::num * R1::den` and `T2` has the value `R1::den * R2::den`.

   >     template <class R1, class R2> structusing ratio_multiply = see below{
   >       typedef see below type;
   >     };

   > > 3 The nested typedef type `ratio_multiply<R1, R2>` shall be a synonym for `ratio<T1, T2>` where `T1` has the value `R1::num * R2::num` and `T2` has the value `R1::den * R2::den`.

   >     template <class R1, class R2> structusing ratio_divide = see below{
   >       typedef see below type;
   >     };

   > > 4 The nested typedef type `ratio_divide<R1, R2>` shall be a synonym for `ratio<T1, T2>` where `T1` has the value `R1::num * R2::den` and `T2` has the value `R1::den * R2::num`.

4. In 20.12.5.1 [time.duration.cons] p.4 change as indicated:

   > Requires: `treat_as_floating_point<rep>::value` shall be true or `ratio_divide<Period2, period>::type::den` shall be 1.[..]

5. In 20.12.5.7 [time.duration.cast] p.2 change as indicated:

   > Returns: Let CF be `ratio_divide<Period, typename ToDuration::period>::type`, and [..]

### 922. [func.bind.place] Number of placeholders

Section: B [implimits] Status: C++11 Submitter: Sohail Somani Opened: 2008-10-11 Last modified: 2015-04-08

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Discussion:

Addresses DE 24

With respect to the section 20.9.10.4 [func.bind.place]:

TR1 dropped some suggested implementation quantities for the number of placeholders. The purpose of this defect is to put these back for C++0x.

[ Post Summit: ]

> see DE 24

> Recommend applying the proposed resolution from DE 24, with that Tentatively Ready.

Original proposed resolution:

Add 20.9.10.4 [func.bind.place] p.2:

> While the exact number of placeholders (`_M`) is implementation defined, this number shall be at least 10.

Proposed resolution:

Add to B [implimits]:

- Number of placeholders (20.9.10.4 [func.bind.place]) [10].

### 923. atomics with floating-point

Section: 29 [atomics] Status: Resolved Submitter: Herb Sutter Opened: 2008-10-17 Last modified: 2015-04-08

View all other issues in [atomics].

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Discussion:

Right now, C++0x doesn't have `atomic<float>`. We're thinking of adding the words to support it for TR2 (note: that would be slightly post-C++0x). If we need it, we could probably add the words.

Proposed resolutions: Using `atomic<FP>::compare_exchange` (weak or strong) should be either:

1. ill-formed, or
2. well-defined.

I propose Option 1 for C++0x for expediency. If someone wants to argue for Option 2, they need to say what exactly they want `compare_exchange` to mean in this case (IIRC, C++0x doesn't even assume IEEE 754).

[ Summit: ]

> Move to open. Blocked until concepts for atomics are addressed.

[ Post Summit Anthony adds: ]

> Recommend NAD. C++0x does have `std::atomic<float>`, and both `compare_exchange_weak` and `compare_exchange_strong` are well-defined in this case. Maybe change the note in 29.6 [atomics.types.operations] paragraph 20 to:

> > [Note: The effect of the compare-and-exchange operations is

> > >     if (!memcmp(object,expected,sizeof(*object)))
> > >         *object = desired;
> > >     else
> > >         *expected = *object;

> > This may result in failed comparisons for values that compare equal if the underlying type has padding bits or alternate representations of the same value. -- end note]

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Addressed by N2992.

Proposed resolution:

Change the note in 29.6 [atomics.types.operations] paragraph 20 to:

> [Note: The effect of the compare-and-exchange operations is

> >     if (*object == *expected !memcmp(object,expected,sizeof(*object)))
> >         *object = desired;
> >     else
> >         *expected = *object;

> This may result in failed comparisons for values that compare equal if the underlying type has padding bits or alternate representations of the same value. -- end note]

### 924. structs with internal padding

Section: 29 [atomics] Status: Resolved Submitter: Herb Sutter Opened: 2008-10-17 Last modified: 2015-04-08

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Discussion:

Right now, the `compare_exchange_weak` loop should rapidly converge on the padding contents. But `compare_exchange_strong` will require a bit more compiler work to ignore padding for comparison purposes.

Note that this isn't a problem for structs with no padding, and we do already have one portable way to ensure that there is no padding that covers the key use cases: Have elements be the same type. I suspect that the greatest need is for a structure of two pointers, which has no padding problem. I suspect the second need is a structure of a pointer and some form of an integer. If that integer is `intptr_t`, there will be no padding.

Related but separable issue: For unused bitfields, or other unused fields for that matter, we should probably say it's the programmer's responsibility to set them to zero or otherwise ensure they'll be ignored by `memcmp`.

Proposed resolution: Using `atomic<struct-with-padding>::compare_exchange_strong` should be either:

1. ill-formed, or
2. well-defined.

I propose Option 1 for C++0x for expediency, though I'm not sure how to say it. I would be happy with Option 2, which I believe would mean that `compare_exchange_strong` would be implemented to avoid comparing padding bytes, or something equivalent such as always zeroing out padding when loading/storing/comparing. (Either implementation might require compiler support.)

[ Summit: ]

> Move to open. Blocked until concepts for atomics are addressed.

[ Post Summit Anthony adds: ]

> The resolution of LWG 923 should resolve this issue as well.

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Addressed by N2992.

Proposed resolution:

### 925. `shared_ptr`'s explicit conversion from `unique_ptr`

Section: 20.8.2.2.1 [util.smartptr.shared.const] Status: C++11 Submitter: Rodolfo Lima Opened: 2008-10-12 Last modified: 2015-04-08

View all other issues in [util.smartptr.shared.const].

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Discussion:

The current working draft (N2798), section 20.8.2.2.1 [util.smartptr.shared.const] declares `shared_ptr`'s constructor that takes a rvalue reference to `unique_ptr` and `auto_ptr` as being explicit, affecting several valid smart pointer use cases that would take advantage of this conversion being implicit, for example:

>     class A;
>     std::unique_ptr<A> create();
>     void process(std::shared_ptr<A> obj);

>     int main()
>     {
>        process(create());                  // use case #1
>        std::unique_ptr<A> uobj = create();
>        process(std::move(uobj));           // use case #2
>        return 0;
>     }

If `unique_ptr` to `shared_ptr` conversions are explicit, the above lines should be written:

>     process(std::shared_ptr<A>(create()));        // use case #1
>     process(std::shared_ptr<A>(std::move(uobj))); // use case #2

The extra cast required doesn't seems to give any benefits to the user, nor protects him of any unintended conversions, this being the raison d'etre of explicit constructors.

It seems that this constructor was made explicit to mimic the conversion from `auto_ptr` in pre-rvalue reference days, which accepts both lvalue and rvalue references. Although this decision was valid back then, C++0x allows the user to express in a clear and non verbose manner when he wants move semantics to be employed, be it implicitly (use case 1) or explicitly (use case 2).

[ Batavia (2009-05): ]

> Howard and Alisdair like the motivating use cases and the proposed resolution.

> Move to Tentatively Ready.

Proposed resolution:

In both 20.8.2.2 [util.smartptr.shared] paragraph 1 and 20.8.2.2.1 [util.smartptr.shared.const] change:

>     template <class Y> explicit shared_ptr(auto_ptr<Y> &&r);
>     template <class Y, class D> explicit shared_ptr(unique_ptr<Y, D> &&r);

### 929. Thread constructor

Section: 30.3.1.2 [thread.thread.constr] Status: C++11 Submitter: Anthony Williams Opened: 2008-10-23 Last modified: 2015-04-08

View all other issues in [thread.thread.constr].

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Discussion:

Addresses UK 323

The `thread` constructor for starting a new thread with a function and arguments is overly constrained by the signature requiring rvalue references for `func` and `args` and the `CopyConstructible` requirements for the elements of `args`. The use of an rvalue reference for the function restricts the potential use of a plain function name, since the type of the bound parameter will be deduced to be a function reference and decay to pointer-to-function will not happen. This therefore complicates the implementation in order to handle a simple case. Furthermore, the use of rvalue references for args prevents the array to pointer decay. Since arrays are not `CopyConstructible` or even `MoveConstructible`, this essentially prevents the passing of arrays as parameters. In particular it prevents the passing of string literals. Consequently a simple case such as

>     void f(const char*);
>     std::thread t(f,"hello");

is ill-formed since the type of the string literal is `const char[6]`.

By changing the signature to take all parameters by value we can eliminate the `CopyConstructible` requirement and permit the use of arrays, as the parameter passing semantics will cause the necessary array-to-pointer decay. They will also cause the function name to decay to a pointer to function and allow the implementation to handle functions and function objects identically.

The new signature of the `thread` constructor for a function and arguments is thus:

>     template<typename F,typename... Args>
>     thread(F,Args... args);

Since the parameter pack `Args` can be empty, the single-parameter constructor that takes just a function by value is now redundant.

[ Howard adds: ]

> I agree with everything Anthony says in this issue. However I believe we can optimize in such a way as to get the pass-by-value behavior with the pass-by-rvalue-ref performance. The performance difference is that the latter removes a `move` when passing in an lvalue.

> This circumstance is very analogous to `make_pair` (20.3 [pairs]) where we started with passing by const reference, changed to pass by value to get pointer decay, and then changed to pass by rvalue reference, but modified with `decay<T>` to retain the pass-by-value behavior. If we were to apply the same solution here it would look like:

> >     template <class F> explicit thread(F f);
> >     template <class F, class ...Args> thread(F&& f, Args&&... args);

> > > -4- Requires: `F` and each `Ti` in `Args` shall be `CopyConstructible` if an lvalue and otherwise `MoveConstructible`. `INVOKE(f, w1, w2, ..., wN)` (20.9.2 [func.require]) shall be a valid expression for some values `w1, w2, ... , wN,` where `N == sizeof...(Args)`.

> > > -5- Effects: Constructs an object of type `thread` and executes `INVOKE(f, t1, t2, ..., tN)` in a new thread of execution, where `t1, t2, ..., tN` are the values in `args...`. Constructs the following objects in memory which is accessible to a new thread of execution as if:

> > > >     typename decay<F>::type g(std::forward<F>(f));
> > > >     tuple<typename decay<Args>::type...> w(std::forward<Args>(args)...);

> > > The new thread of execution executes `INVOKE(g, wi...)` where the `wi...` refers to the elements stored in the `tuple w`. Any return value from `g` is ignored. If `f` terminates with an uncaught exception, `std::terminate()` shall be called. If the evaluation of `INVOKE(g, wi...)` terminates with an uncaught exception, `std::terminate()` shall be called [Note: `std::terminate()` could be called before entering `g`. -- end note]. Any exception thrown before the evaluation of `INVOKE` has started shall be catchable in the calling thread.

> Text referring to when `terminate()` is called was contributed by Ganesh.

[ Batavia (2009-05): ]

> We agree with the proposed resolution, but would like the final sentence to be reworded since "catchable" is not a term of art (and is used nowhere else).

[ 2009-07 Frankfurt: ]

> This is linked to N2901.

> Howard to open a separate issue to remove (1176).

> In Frankfurt there is no consensus for removing the variadic constructor.

[ 2009-10 Santa Cruz: ]

> We want to move forward with this issue. If we later take it out via 1176 then that's ok too. Needs small group to improve wording.

[ 2009-10 Santa Cruz: ]

> Stefanus provided revised wording. Moved to Review Here is the original wording:

> > Modify the class definition of `std::thread` in 30.3.1 [thread.thread.class] to remove the following signature:

> > >     template<class F> explicit thread(F f);
> > >     template<class F, class ... Args> explicit thread(F&& f, Args&& ... args);

> > Modify 30.3.1.2 [thread.thread.constr] to replace the constructors prior to paragraph 4 with the single constructor as above. Replace paragraph 4 - 6 with the following:

> > > -4- Requires: `F` and each `Ti` in `Args` shall be `CopyConstructible` if an lvalue and otherwise `MoveConstructible`. `INVOKE(f, w1, w2, ..., wN)` (20.9.2 [func.require]) shall be a valid expression for some values `w1, w2, ... , wN,` where `N == sizeof...(Args)`.

> > > -5- Effects: Constructs an object of type `thread` and executes `INVOKE(f, t1, t2, ..., tN)` in a new thread of execution, where `t1, t2, ..., tN` are the values in `args...`. Constructs the following objects:

> > > >     typename decay<F>::type g(std::forward<F>(f));
> > > >     tuple<typename decay<Args>::type...> w(std::forward<Args>(args)...);

> > > and executes `INVOKE(g, wi...)` in a new thread of execution. These objects shall be destroyed when the new thread of execution completes. Any return value from `g` is ignored. If `f` terminates with an uncaught exception, `std::terminate()` shall be called. If the evaluation of `INVOKE(g, wi...)` terminates with an uncaught exception, `std::terminate()` shall be called [Note: `std::terminate()` could be called before entering `g`. -- end note]. Any exception thrown before the evaluation of `INVOKE` has started shall be catchable in the calling thread.

> > > -6- Synchronization: The invocation of the constructor happens before the invocation of `f` `g`.

[ 2010-01-19 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Modify the class definition of `std::thread` in 30.3.1 [thread.thread.class] to remove the following signature:

>     template<class F> explicit thread(F f);
>     template<class F, class ... Args> explicit thread(F&& f, Args&& ... args);

Modify 30.3.1.2 [thread.thread.constr] to replace the constructors prior to paragraph 4 with the single constructor as above. Replace paragraph 4 - 6 with the following:

> Given a function as follows:

> >     template<typename T> typename decay<T>::type decay_copy(T&& v)
> >         { return std::forward<T>(v); }

> -4- Requires: `F` and each `Ti` in `Args` shall be `CopyConstructible` if an lvalue and otherwise satisfy the `MoveConstructible` requirements. `INVOKE(f, w1, w2, ..., wN)` (20.9.2 [func.require]) shall be a valid expression for some values `w1, w2, ... , wN,` where `N == sizeof...(Args)`. `INVOKE(decay_copy(std::forward<F>(f)), decay_copy(std::forward<Args>(args))...)` (20.9.2 [func.require]) shall be a valid expression.

> -5- Effects: Constructs an object of type `thread` and executes `INVOKE(f, t1, t2, ..., tN)` in a new thread of execution, where `t1, t2, ..., tN` are the values in `args...`. Any return value from `f` is ignored. If `f` terminates with an uncaught exception, `std::terminate()` shall be called. The new thread of execution executes `INVOKE(decay_copy(std::forward<F>(f)), decay_copy(std::forward<Args>(args))...)` with the calls to `decay_copy()` being evaluated in the constructing thread. Any return value from this invocation is ignored. [Note: this implies any exceptions not thrown from the invocation of the copy of `f` will be thrown in the constructing thread, not the new thread. — end note]. If the invocation of `INVOKE(decay_copy(std::forward<F>(f)), decay_copy(std::forward<Args>(args))...)` terminates with an uncaught exception, `std::terminate` shall be called.

> -6- Synchronization: The invocation of the constructor happens before the invocation of the copy of `f`.

### 931. type trait `extent<T, I>`

Section: 20.10.4.3 [meta.unary.prop] Status: C++11 Submitter: Yechezkel Mett Opened: 2008-11-04 Last modified: 2015-04-08

View other active issues in [meta.unary.prop].

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Discussion:

The draft (N2798) says in 20.10.4.3 [meta.unary.prop] Table 44:

> Table 44 -- Type property queries
> Template | Value
> --- | ---
> `template <class T, unsigned I = 0> struct extent;` | If `T` is not an array type (8.3.4), or if it has rank less than `I`, or if `I` is 0 and `T` has type "array of unknown bound of `U`", then 0; otherwise, the size of the `I`'th dimension of `T`

Firstly it isn't clear from the wording if `I` is 0-based or 1-based ("the `I`'th dimension" sort of implies 1-based). From the following example it is clear that the intent is 0-based, in which case it should say "or if it has rank less than or equal to `I`".

Sanity check:

The example says `assert((extent<int[2], 1>::value) == 0);`

Here the rank is 1 and `I` is 1, but the desired result is 0.

[ Post Summit: ]

> Do not use "size" or "value", use "bound". Also, move the cross-reference to 8.3.4 to just after "bound".

> Recommend Tentatively Ready.

Proposed resolution:

In Table 44 of 20.10.4.3 [meta.unary.prop], third row, column "Value", change the cell content:

> Table 44 -- Type property queries
> Template | Value
> --- | ---
> `template <class T, unsigned I = 0> struct extent;` | If `T` is not an array type (8.3.4), or if it has rank less than or equal to `I`, or if `I` is 0 and `T` has type "array of unknown bound of `U`", then 0; otherwise, the size bound (8.3.4) of the `I`'th dimension of `T`, where indexing of `I` is zero-based.

[ Wording supplied by Daniel. ]

### 932. `unique_ptr(pointer p)` for pointer deleter types

Section: 20.8.1.2.1 [unique.ptr.single.ctor] Status: Resolved Submitter: Howard Hinnant Opened: 2008-11-26 Last modified: 2015-04-08

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Discussion:

Addresses US 79

20.8.1.2.1 [unique.ptr.single.ctor]/5 no longer requires for `D` not to be a pointer type. I believe this restriction was accidently removed when we relaxed the completeness reuqirements on `T`. The restriction needs to be put back in. Otherwise we have a run time failure that could have been caught at compile time:

>     {
>     unique_ptr<int, void(*)(void*)> p1(malloc(sizeof(int)));  // should not compile
>     }  // p1.~unique_ptr() dereferences a null function pointer
>     unique_ptr<int, void(*)(void*)> p2(malloc(sizeof(int)), free);  // ok

[ Post Summit: ]

> Recommend Tentatively Ready.

[ 2009-07 Frankfurt ]

> Moved from Tentatively Ready to Open only because the wording needs to be improved for `enable_if` type constraining, possibly following Robert's formula.

[ 2009-07 Frankfurt: ]

> We need to consider whether some requirements in the Requires paragraphs of [unique.ptr] should instead be Remarks.

> Leave Open. Howard to provide wording, and possibly demonstrate how this can be implemented using enable_if.

[ 2009-07-27 Howard adds: ]

> The two constructors to which this issue applies are not easily constrained with `enable_if` as they are not templated:

> >     unique_ptr();
> >     explicit unique_ptr(pointer p);

> To "SFINAE" these constructors away would take heroic effort such as specializing the entire `unique_ptr` class template on pointer deleter types. There is insufficient motivation for such heroics. Here is the expected and reasonable implementation for these constructors:

> >     unique_ptr()
> >         : ptr_(pointer())
> >         {
> >             static_assert(!is_pointer<deleter_type>::value,
> >                 "unique_ptr constructed with null function pointer deleter");
> >         }
> >     explicit unique_ptr(pointer p)
> >         : ptr_(p)
> >         {
> >             static_assert(!is_pointer<deleter_type>::value,
> >                 "unique_ptr constructed with null function pointer deleter");
> >         }

> I.e. just use `static_assert` to verify that the constructor is not instantiated with a function pointer for a deleter. The compiler will automatically take care of issuing a diagnostic if the deleter is a reference type (uninitialized reference error).

> In keeping with our discussions in Frankfurt, I'm moving this requirement on the implementation from the Requires paragraph to a Remarks paragraph.

[ 2009-08-17 Daniel adds: ]

> It is insufficient to require a diagnostic. This doesn't imply an ill-formed program as of 1.3 [defns.diagnostic] (a typical alternative would be a compiler warning), but exactly that seems to be the intend. I suggest to use the following remark instead:

> > Remarks: The program shall be ill-formed if this constructor is instantiated when `D` is a pointer type or reference type.

> Via the general standard rules of 1.4 [intro.compliance] the "diagnostic required" is implied.

[ 2009-10 Santa Cruz: ]

> Moved to Ready.

[ 2010-03-14 Howard adds: ]

> We moved N3073 to the formal motions page in Pittsburgh which should obsolete this issue. I've moved this issue to NAD Editorial, solved by N3073.

Rationale:

Solved by N3073.

Proposed resolution:

Change the description of the default constructor in 20.8.1.2.1 [unique.ptr.single.ctor]:

>     unique_ptr();

> > -1- Requires: `D` shall be default constructible, and that construction shall not throw an exception. `D` shall not be a reference type or pointer type (diagnostic required).

> > ...

> > Remarks: The program shall be ill-formed if this constructor is instantiated when `D` is a pointer type or reference type.

Add after 20.8.1.2.1 [unique.ptr.single.ctor]/8:

>     unique_ptr(pointer p);

> > ...

> > Remarks: The program shall be ill-formed if this constructor is instantiated when `D` is a pointer type or reference type.

### 934. `duration` is missing `operator%`

Section: 20.12.5 [time.duration] Status: C++11 Submitter: Terry Golubiewski Opened: 2008-11-30 Last modified: 2015-04-08

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Discussion:

Addresses US 81

`duration` is missing `operator%`. This operator is convenient for computing where in a time frame a given `duration` lies. A motivating example is converting a `duration` into a "broken-down" time duration such as hours::minutes::seconds:

>     class ClockTime
>     {
>         typedef std::chrono::hours hours;
>         typedef std::chrono::minutes minutes;
>         typedef std::chrono::seconds seconds;
>     public:
>         hours hours_;
>         minutes minutes_;
>         seconds seconds_;

>         template <class Rep, class Period>
>           explicit ClockTime(const std::chrono::duration<Rep, Period>& d)
>             : hours_  (std::chrono::duration_cast<hours>  (d)),
>               minutes_(std::chrono::duration_cast<minutes>(d % hours(1))),
>               seconds_(std::chrono::duration_cast<seconds>(d % minutes(1)))
>               {}
>     };

[ Summit: ]

> Agree except that there is a typo in the proposed resolution. The member operators should be `operator%=`.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

[ 2009-07 Frankfurt ]

> Moved from Tentatively Ready to Open only because the wording needs to be improved for enable_if type constraining, possibly following Robert's formula.

[ 2009-07 Frankfurt: ]

> Howard to open a separate issue (1177) to handle the removal of member functions from overload sets, provide wording, and possibly demonstrate how this can be implemented using enable_if (see 947).

> Move to Ready.

Proposed resolution:

Add to the synopsis in 20.12 [time]:

>     template <class Rep1, class Period, class Rep2>
>       duration<typename common_type<Rep1, Rep2>::type, Period>
>       operator%(const duration<Rep1, Period>& d, const Rep2& s);
>     template <class Rep1, class Period1, class Rep2, class Period2>
>       typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type
>       operator%(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);

Add to the synopsis of `duration` in 20.12.5 [time.duration]:

>     template <class Rep, class Period = ratio<1>>
>     class duration {
>     public:
>       ...
>       duration& operator%=(const rep& rhs);
>       duration& operator%=(const duration& d);
>       ...
>     };

Add to 20.12.5.3 [time.duration.arithmetic]:

>     duration& operator%=(const rep& rhs);

> > Effects: `rep_ %= rhs`.

> > Returns: `*this`.

>     duration& operator%=(const duration& d);

> > Effects: `rep_ %= d.count()`.

> > Returns: `*this`.

Add to 20.12.5.5 [time.duration.nonmember]:

>     template <class Rep1, class Period, class Rep2>
>       duration<typename common_type<Rep1, Rep2>::type, Period>
>       operator%(const duration<Rep1, Period>& d, const Rep2& s);

> > Requires: `Rep2` shall be implicitly convertible to `CR(Rep1, Rep2)` and `Rep2` shall not be an instantiation of `duration`. Diagnostic required.

> > Returns: `duration<CR, Period>(d) %= s`.

>     template <class Rep1, class Period1, class Rep2, class Period2>
>       typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type
>       operator%(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);

> > Returns: `common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type(lhs) %= rhs`.

### 938. `default_delete<T[]>::operator()` should only accept `T*`

Section: 20.8.1.1.3 [unique.ptr.dltr.dflt1] Status: C++11 Submitter: Howard Hinnant Opened: 2008-12-07 Last modified: 2015-04-08

View all issues with C++11 status.

Discussion:

Consider:

>     derived* p = new derived[3];
>     std::default_delete<base[]> d;
>     d(p);  // should fail

Currently the marked line is a run time failure. We can make it a compile time failure by "poisoning" `op(U*)`.

[ Post Summit: ]

> Recommend Review.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Add to 20.8.1.1.3 [unique.ptr.dltr.dflt1]:

>     namespace std {
>       template <class T> struct default_delete<T[]> {
>         void operator()(T*) const;
>       template <class U> void operator()(U*) const = delete;
>     };
>     }

### 939. Problem with `std::identity` and reference-to-temporaries

Section: 20.2.4 [forward] Status: C++11 Submitter: Alisdair Meredith Opened: 2008-12-11 Last modified: 2015-04-08

View all other issues in [forward].

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Discussion:

`std::identity` takes an argument of type `T const &` and returns a result of `T const &`.

Unfortunately, this signature will accept a value of type other than `T` that is convertible-to-`T`, and then return a reference to the dead temporary. The constraint in the concepts version simply protects against returning reference-to-`void`.

Solutions:

> i/ Return-by-value, potentially slicing bases and rejecting non-copyable types

> ii/ Provide an additional overload:

> >     template< typename T >
> >     template operator( U & ) = delete;

> This seems closer on intent, but moves beyond the original motivation for the operator, which is compatibility with existing (non-standard) implementations.

> iii/ Remove the `operator()` overload. This restores the original definition of the `identity`, although now effectively a type_trait rather than part of the perfect forwarding protocol.

> iv/ Remove `std::identity` completely; its original reason to exist is replaced with the `IdentityOf` concept.

My own preference is somewhere between (ii) and (iii) - although I stumbled over the issue with a specific application hoping for resolution (i)!

[ Batavia (2009-05): ]

> We dislike options i and iii, and option ii seems like overkill. If we remove it (option iv), implementers can still provide it under a different name.

> Move to Open pending wording (from Alisdair) for option iv.

[ 2009-05-23 Alisdair provided wording for option iv. ]

[ 2009-07-20 Alisdair adds: ]

> I'm not sure why this issue was not discussed at Frankfurt (or I missed the discussion) but the rationale is now fundamentally flawed. With the removal of concepts, `std::identity` again becomes an important library type so we cannot simply remove it.

> At that point, we need to pick one of the other suggested resolutions, but have no guidance at the moment.

[ 2009-07-20 Howard adds: ]

> I believe the rationale for not addressing this issue in Frankfurt was that it did not address a national body comment.

> I also believe that removal of `identity` is still a practical option as my latest reformulation of `forward`, which is due to comments suggested at Summit, no longer uses `identity`. :-)

> >     template <class T, class U,
> >         class = typename enable_if
> >                 <
> >                     !is_lvalue_reference<T>::value ||
> >                      is_lvalue_reference<T>::value &&
> >                      is_lvalue_reference<U>::value
> >                 >::type,
> >         class = typename enable_if
> >                 <
> >                     is_same<typename remove_all<T>::type,
> >                             typename remove_all<U>::type>::value
> >                 >::type>
> >     inline
> >     T&&
> >     forward(U&& t)
> >     {
> >         return static_cast<T&&>(t);

> >     }

> > [ The above code assumes acceptance of 1120 for the definition of `remove_all`. This is just to make the syntax a little more palatable. Without this trait the above is still very implementable. ]

> Paper with rationale is on the way ... really, I promise this time! ;-)

[ 2009-07-30 Daniel adds: See 823 for an alternative resolution. ]

[ 2009-10 Santa Cruz: ]

> Move to Ready. Howard will update proposed wording to reflect current draft.

Proposed resolution:

Strike from 20.2 [utility]:

>     template <class T> struct identity;

Remove from 20.2.4 [forward]:

>     template <class T> struct identity {
>       typedef T type;

>       const T& operator()(const T& x) const;
>     };

>     const T& operator()(const T& x) const;

> > -2- Returns: `x`

### 940. `std::distance`

Section: 24.4.4 [iterator.operations] Status: Resolved Submitter: Thomas Opened: 2008-12-14 Last modified: 2015-04-08

View all other issues in [iterator.operations].

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Discussion:

Addresses UK 270

Regarding the `std::distance` - function, 24.4.4 [iterator.operations] p.4 says:

> Returns the number of increments or decrements needed to get from first to last.

This sentence is completely silent about the sign of the return value. 24.4.4 [iterator.operations] p.1 gives more information about the underlying operations, but again no inferences about the sign can be made. Strictly speaking, that is taking that sentence literally, I think this sentence even implies a positive return value in all cases, as the number of increments or decrements is clearly a ratio scale variable, with a natural zero bound.

Practically speaking, my implementations did what common sense and knowledge based on pointer arithmetic forecasts, namely a positive sign for increments (that is, going from `first` to `last` by `operator++`), and a negative sign for decrements (going from `first` to `last` by `operator--`).

Here are my two questions:

First, is that paragraph supposed to be interpreted in the way what I called 'common sense', that is negative sign for decrements ? I am fairly sure that's the supposed behavior, but a double-check here in this group can't hurt.

Second, is the present wording (2003 standard version - no idea about the draft for the upcoming standard) worth an edit to make it a bit more sensible, to mention the sign of the return value explicitly ?

[ Daniel adds: ]

> My first thought was that resolution 204 would already cover the issue report, but it seems that current normative wording is in contradiction to that resolution:

> Referring to N2798, 24.4.4 [iterator.operations]/ p.4 says:

> > Effects: Returns the number of increments or decrements needed to get from `first` to `last`.

> IMO the part " or decrements" is in contradiction to p. 5 which says

> > Requires: `last` shall be reachable from `first`.

> because "reachable" is defined in X [iterator.concepts]/7 as

> > An iterator `j` is called reachable from an iterator `i` if and only if there is a finite sequence of applications of the expression `++i` that makes `i == j`.[..]

> Here is wording that would be consistent with this definition of "reachable":

> Change 24.4.4 [iterator.operations] p4 as follows:

> > Effects: Returns the number of increments or decrements needed to get from `first` to `last`.

Thomas adds more discussion and an alternative view point here.

[ Summit: ]

> The proposed wording below was verbally agreed to. Howard provided.

[ Batavia (2009-05): ]

> Pete reports that a recent similar change has been made for the `advance()` function.

> We agree with the proposed resolution. Move to Tentatively Ready.

[ 2009-07 Frankfurt ]

> Moved from Tentatively Ready to Open only because the wording needs to be tweaked for concepts removal.

[ 2009-07 Frankfurt: ]

> Leave Open pending arrival of a post-Concepts WD.

[ 2009-10-14 Daniel provided de-conceptified wording. ]

[ 2009-10 Santa Cruz: ]

> Move to Ready, replacing the Effects clause in the proposed wording with "If InputIterator meets the requirements of random access iterator then returns (last - first), otherwise returns the number of increments needed to get from first to list.".

[ 2010 Pittsburgh: ]

> Moved to NAD Editorial Resolved. Rationale added below.

Rationale:

Solved by N3066.

Proposed resolution:

1. Change 24.2.7 [random.access.iterators], Table 105 as indicated [This change is not essential but it simplifies the specification] for the row with expression "`b - a`" and the column Operational semantics:

   >     (a < b) ? distance(a,b)
   >     : -distance(b,a)

2. Change 24.4.4 [iterator.operations]/4+5 as indicated:

   >     template<class InputIterator>
   >       typename iterator_traits<InputIterator>::difference_type
   >         distance(InputIterator first, InputIterator last);

   > > 4 Effects: If `InputIterator` meets the requirements of random access iterator then returns `(last - first)`, otherwise R returns the number of increments or decrements needed to get from `first` to `last`.

   > > 5 Requires: If `InputIterator` meets the requirements of random access iterator then `last` shall be reachable from `first` or `first` shall be reachable from `last`, otherwise `last` shall be reachable from `first`.

### 943. `ssize_t` undefined

Section: X [atomics.types.address] Status: C++11 Submitter: Holger Grund Opened: 2008-12-19 Last modified: 2015-04-08

View all other issues in [atomics.types.address].

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Discussion:

There is a row in "Table 122 - Atomics for standard typedef types" in X [atomics.types.integral] with `atomic_ssize_t` and `ssize_t`. Unless, I'm missing something `ssize_t` is not defined by the standard.

[ Summit: ]

> Move to review. Proposed resolution: Remove the typedef. Note: `ssize_t` is a POSIX type.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Remove the row containing `ssize_t` from Table 119 "Atomics for standard typedef types" in X [atomics.types.address].

### 944. `atomic<bool>` derive from `atomic_bool`?

Section: 29.5 [atomics.types.generic] Status: Resolved Submitter: Holger Grund Opened: 2008-12-19 Last modified: 2015-04-08

View other active issues in [atomics.types.generic].

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Discussion:

I think it's fairly obvious that `atomic<bool>` is supposed to be derived from `atomic_bool` (and otherwise follow the `atomic<integral>` interface), though I think the current wording doesn't support this. I raised this point along with `atomic<floating-point>` privately with Herb and I seem to recall it came up in the resulting discussion on this list. However, I don't see anything on the current libs issue list mentioning this problem.

29.5 [atomics.types.generic]/3 reads

> There are full specializations over the integral types on the atomic class template. For each integral type integral in the second column of table 121 or table 122, the specialization `atomic<integral>` shall be publicly derived from the corresponding atomic integral type in the first column of the table. These specializations shall have trivial default constructors and trivial destructors.

Table 121 does not include (`atomic_bool`, `bool`), so that this should probably be mentioned explicitly in the quoted paragraph.

[ Summit: ]

> Move to open. Lawrence will draft a proposed resolution. Also, ask Howard to fix the title.

[ Post Summit Anthony provided proposed wording. ]

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Addressed by N2992.

Proposed resolution:

Replace paragraph 3 in 29.5 [atomics.types.generic] with

> -3- There are full specializations over the integral types on the `atomic` class template. For each integral type `integral` in the second column of table 121 or table 122, the specialization `atomic<integral>` shall be publicly derived from the corresponding atomic integral type in the first column of the table. In addition, the specialization `atomic<bool>` shall be publicly derived from `atomic_bool`. These specializations shall have trivial default constructors and trivial destructors.

### 947. `duration` arithmetic: contradictory requirements

Section: 20.12.5.5 [time.duration.nonmember] Status: Resolved Submitter: Pete Becker Opened: 2008-12-20 Last modified: 2015-04-08

View all other issues in [time.duration.nonmember].

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Discussion:

In 20.12.5.5 [time.duration.nonmember], paragraph 8 says that calling `dur / rep` when `rep` is an instantiation of `duration` requires a diagnostic. That's followed by an `operator/` that takes two durations. So `dur1 / dur2` is legal under the second version, but requires a diagnostic under the first.

[ Howard adds: ]

> Please see the thread starting with c++std-lib-22980 for more information.

[ Batavia (2009-05): ]

> Move to Open, pending proposed wording (and preferably an implementation).

[ 2009-07-27 Howard adds: ]

> I've addressed this issue under the proposed wording for 1177 which cleans up several places under 20.12.5 [time.duration] which used the phrase "diagnostic required".

> For clarity's sake, here is an example implementation of the constrained `operator/`:

> >     template <class _Duration, class _Rep, bool = __is_duration<_Rep>::value>
> >     struct __duration_divide_result
> >     {
> >     };

> >     template <class _Duration, class _Rep2,
> >         bool = is_convertible<_Rep2,
> >                               typename common_type<typename _Duration::rep, _Rep2>::type>::value>
> >     struct __duration_divide_imp
> >     {
> >     };

> >     template <class _Rep1, class _Period, class _Rep2>
> >     struct __duration_divide_imp<duration<_Rep1, _Period>, _Rep2, true>
> >     {
> >         typedef duration<typename common_type<_Rep1, _Rep2>::type, _Period> type;
> >     };

> >     template <class _Rep1, class _Period, class _Rep2>
> >     struct __duration_divide_result<duration<_Rep1, _Period>, _Rep2, false>
> >         : __duration_divide_imp<duration<_Rep1, _Period>, _Rep2>
> >     {
> >     };

> >     template <class _Rep1, class _Period, class _Rep2>
> >     inline
> >     typename __duration_divide_result<duration<_Rep1, _Period>, _Rep2>::type
> >     operator/(const duration<_Rep1, _Period>& __d, const _Rep2& __s)
> >     {
> >         typedef typename common_type<_Rep1, _Rep2>::type _Cr;
> >         duration<_Cr, _Period> __r = __d;
> >         __r /= static_cast<_Cr>(__s);
> >         return __r;
> >     }

> `__duration_divide_result` is basically a custom-built `enable_if` that will contain `type` only if `Rep2` is not a `duration` and if `Rep2` is implicitly convertible to `common_type<typename Duration::rep, Rep2>::type`. `__is_duration` is simply a private trait that answers `false`, but is specialized for `duration` to answer `true`.

> The constrained `operator%` works identically.

[ 2009-10 Santa Cruz: ]

> Mark NAD Editorial Resolved, fixed by 1177.

Proposed resolution:

### 948. `ratio` arithmetic tweak

Section: 20.11.4 [ratio.arithmetic] Status: C++11 Submitter: Howard Hinnant Opened: 2008-12-26 Last modified: 2015-04-08

View all other issues in [ratio.arithmetic].

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Discussion:

N2800, 20.11.4 [ratio.arithmetic] lacks a paragraph from the proposal N2661:

> ratio arithmetic [ratio.arithmetic]

> ... If the implementation is unable to form the indicated `ratio` due to overflow, a diagnostic shall be issued.

The lack of a diagnostic on compile-time overflow is a significant lack of functionality. This paragraph could be put back into the WP simply editorially. However in forming this issue I realized that we can do better than that. This paragraph should also allow alternative formulations which go to extra lengths to avoid overflow when possible. I.e. we should not mandate overflow when the implementation can avoid it.

For example:

>     template <class R1, class R2> struct ratio_multiply {
>       typedef see below} type;

> > The nested typedef type shall be a synonym for `ratio<T1, T2>` where `T1` has the value `R1::num * R2::num` and `T2` has the value `R1::den * R2::den`.

Consider the case where `intmax_t` is a 64 bit 2's complement signed integer, and we have:

>     typedef std::ratio<0x7FFFFFFFFFFFFFFF, 0x7FFFFFFFFFFFFFF0> R1;
>     typedef std::ratio<8, 7> R2;
>     typedef std::ratio_multiply<R1, R2>::type RT;

According to the present formulation the implementaiton will multiply `0x7FFFFFFFFFFFFFFF * 8` which will result in an overflow and subsequently require a diagnostic.

However if the implementation is first allowed to divde `0x7FFFFFFFFFFFFFFF` by `7` obtaining `0x1249249249249249 / 1` and divide `8` by `0x7FFFFFFFFFFFFFF0` obtaining `1 / 0x0FFFFFFFFFFFFFFE`, then the exact result can then be computed without overflow:

>     [0x7FFFFFFFFFFFFFFF/0x7FFFFFFFFFFFFFF0] * [8/7] = [0x1249249249249249/0x0FFFFFFFFFFFFFFE]

Example implmentation which accomplishes this:

>     template <class R1, class R2>
>     struct ratio_multiply
>     {
>     private:
>         typedef ratio<R1::num, R2::den> _R3;
>         typedef ratio<R2::num, R1::den> _R4;
>     public:
>         typedef ratio<__ll_mul<_R3::num, _R4::num>::value,
>                       __ll_mul<_R3::den, _R4::den>::value> type;
>     };

[ Post Summit: ]

> Recommend Tentatively Ready.

Proposed resolution:

Add a paragraph prior to p1 in 20.11.4 [ratio.arithmetic]:

> Implementations may use other algorithms to compute the indicated ratios to avoid overflow. If overflow occurs, a diagnostic shall be issued.

### 949. `owner_less`

Section: 20.8.2.4 [util.smartptr.ownerless] Status: C++11 Submitter: Thomas Plum Opened: 2008-12-30 Last modified: 2015-04-08

View all issues with C++11 status.

Discussion:

20.8.2.4 [util.smartptr.ownerless] (class template `owner_less`) says that `operator()(x,y)` shall return `x.before(y)`.

However, `shared_ptr` and `weak_ptr` have an `owner_before()` but not a `before()`, and there's no base class to provide a missing `before()`.

Being that the class is named `owner_less` , I'm guessing that "`before()`" should be "`owner_before()`", right?

[ Herve adds: ]

> Agreed with the typo, it should be "shall return `x.owner_before(y)`".

[ Post Summit: ]

> Recommend Tentatively Ready.

Proposed resolution:

Change 20.8.2.4 [util.smartptr.ownerless] p2:

> -2- `operator()(x,y)` shall return `x.owner_before(y)`. [Note: ...

### 950. `unique_ptr` converting ctor shouldn't accept array form

Section: 20.8.1.2.1 [unique.ptr.single.ctor] Status: Resolved Submitter: Howard Hinnant Opened: 2009-01-07 Last modified: 2015-04-08

View all other issues in [unique.ptr.single.ctor].

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Discussion:

`unique_ptr`'s of array type should not convert to `unique_ptr`'s which do not have an array type.

>     struct Deleter
>     {
>        void operator()(void*) {}
>     };

>     int main()
>     {
>        unique_ptr<int[], Deleter> s;
>        unique_ptr<int, Deleter> s2(std::move(s));  // should not compile
>     }

[ Post Summit: ]

> Walter: Does the "diagnostic required" apply to both arms of the "and"?

> Tom Plum: suggest to break into several sentences

> Walter: suggest "comma" before the "and" in both places

> Recommend Review.

[ Batavia (2009-05): ]

> The post-Summit comments have been applied to the proposed resolution. We now agree with the proposed resolution. Move to Tentatively Ready.

[ 2009-07 Frankfurt ]

> Moved from Tentatively Ready to Open only because the wording needs to be improved for `enable_if` type constraining, possibly following Robert's formula.

[ 2009-08-01 Howard updates wording and sets to Review. ]

[ 2009-10 Santa Cruz: ]

> Move to Ready.

[ 2010-02-27 Pete Opens: ]

> The proposed replacement text doesn't make sense.

> > If `D` is a reference type, then `E` shall be the same type as `D`, else this constructor shall not participate in overload resolution.

> This imposes two requirements. 1. If `D` is a reference type, `E` has to be `D`. 2. If `D` is not a reference type, the constructor shall not participate in overload resolution. If the latter apples, the language in the preceding paragraph that this constructor shall not throw an exception if `D` is not a reference type is superfluous. I suspect that's not the intention, but I can't parse this text any other way.

> > `U` shall not be an array type, else this constructor shall not participate in overload resolution.

> I don't know what this means.

[ 2010-02-27 Peter adds: ]

> I think that the intent is (proposed text):

> > Remarks: this constructor shall only participate in overload resolution if:

> > - `unique_ptr<U, E>::pointer` is implicitly convertible to `pointer`,
> > - `U` is not an array type, and
> > - if `D` is a reference type, `E` is the same type as `D`.

[ 2010-02-28 Howard adds: ]

> I like Peter's proposal. Here is a tweak of it made after looking at my implementation. I believe this fixes a further defect not addressed by the current proposed wording:

> > Remarks: this constructor shall only participate in overload resolution if:

> > - `unique_ptr<U, E>::pointer` is implicitly convertible to `pointer`, and
> > - `U` is not an array type, and
> > - if `D` is a reference type, `E` is the same type as `D`, else `E` shall be implicitly convertible to `D`.

[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]

Rationale:

Solved by N3073.

Proposed resolution:

Change 20.8.1.2.1 [unique.ptr.single.ctor]:

>     template <class U, class E> unique_ptr(unique_ptr<U, E>&& u);

> > -20- Requires: If `D` is not a reference type, construction of the deleter `D` from an rvalue of type `E` shall be well formed and shall not throw an exception. If `D` is a reference type, then `E` shall be the same type as `D` (diagnostic required). `unique_ptr<U, E>::pointer` shall be implicitly convertible to `pointer`. [Note: These requirements imply that `T` and `U` are complete types. — end note]

> > Remarks: If `D` is a reference type, then `E` shall be the same type as `D`, else this constructor shall not participate in overload resolution. `unique_ptr<U, E>::pointer` shall be implicitly convertible to `pointer`, else this constructor shall not participate in overload resolution. `U` shall not be an array type, else this constructor shall not participate in overload resolution. [Note: These requirements imply that `T` and `U` are complete types. — end note]

Change 20.8.1.2.3 [unique.ptr.single.asgn]:

>     template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u);

> > -6- Requires: Assignment of the deleter `D` from an rvalue `D` shall not throw an exception. `unique_ptr<U, E>::pointer` shall be implicitly convertible to `pointer`. [Note: These requirements imply that `T` and `U` are complete types. — end note]

> > Remarks: `unique_ptr<U, E>::pointer` shall be implicitly convertible to `pointer`, else this operator shall not participate in overload resolution. `U` shall not be an array type, else this operator shall not participate in overload resolution. [Note: These requirements imply that `T` and `U` are complete types. — end note]

### 951. Various threading bugs #1

Section: 20.12.4.1 [time.traits.is_fp] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2015-04-08

View all issues with C++11 status.

Discussion:

Related to 953.

20.12.4.1 [time.traits.is_fp] says that the type `Rep` "is assumed to be ... a class emulating an integral type." What are the requirements for such a type?

[ 2009-05-10 Howard adds: ]

> `IntegralLike`.

[ Batavia (2009-05): ]

> As with issue 953, we recommend this issue be addressed in the context of providing concepts for the entire `thread` header.

> We look forward to proposed wording.

> Move to Open.

[ 2009-08-01 Howard adds: ]

> I have surveyed all clauses of 20.12.4.2 [time.traits.duration_values], 20.12.4.3 [time.traits.specializations] and 20.12.5 [time.duration]. I can not find any clause which involves the use of a `duration::rep` type where the requirements on the `rep` type are not clearly spelled out. These requirements were carefully crafted to allow any arithmetic type, or any user-defined type emulating an arithmetic type.

> Indeed, `treat_as_floating_point` becomes completely superfluous if `duration::rep` can never be a class type.

> There will be some `Rep` types which will not meet the requirements of every `duration` operation. This is no different than the fact that `vector<T>` can easily be used for types `T` which are not `DefaultConstructible`, even though some members of `vector<T>` require `T` to be `DefaultConstructible`. This is why the requirements on `Rep` are specified for each operation individually.

> In 20.12.4.1 [time.traits.is_fp] p1:

> >     template <class Rep> struct treat_as_floating_point
> >       : is_floating_point<Rep> { };

> > > The `duration` template uses the `treat_as_floating_point` trait to help determine if a `duration` object can be converted to another `duration` with a different tick period. If `treat_as_floating_point<Rep>::value` is `true`, then `Rep` is a floating-point type and implicit conversions are allowed among `duration`s. Otherwise, the implicit convertibility depends on the tick periods of the `duration`s. If `Rep` is a class type which emulates a floating-point type, the author of `Rep` can specialize `treat_as_floating_point` so that `duration` will treat this `Rep` as if it were a floating-point type. Otherwise `Rep` is assumed to be an integral type or a class emulating an integral type.

> The phrases "a class type which emulates a floating-point type" and "a class emulating an integral type" are clarifying phrases which refer to the summation of all the requirements on the `Rep` type specified in detail elsewhere (and should not be repeated here).

> This specification has been implemented, now multiple times, and the experience has been favorable. The current specification clearly specifies the requirements at each point of use (though I'd be happy to fix any place I may have missed, but none has been pointed out).

> I am amenable to improved wording of this paragraph (and any others), but do not have any suggestions for improved wording at this time. I am strongly opposed to changes which would significantly alter the semantics of the specification under 20.12 [time] without firmly grounded and documented rationale, example implementation, testing, and user experience which relates a positive experience.

> I recommend NAD unless someone wants to produce some clarifying wording.

[ 2009-10 Santa Cruz: ]

> Stefanus to provide wording to turn this into a note.

[ 2010-02-11 Stefanus provided wording. ]

[ 2010 Rapperswil: ]

> Move to Ready.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 20.12.4.1 [time.traits.is_fp]/1:

> 1 The `duration` template uses the `treat_as_floating_point` trait to help determine if a `duration` object can be converted to another `duration` with a different tick period. If `treat_as_floating_point<Rep>::value` is `true`, then `Rep` is a floating-point type and implicit conversions are allowed among `duration`s. Otherwise, the implicit convertibility depends on the tick periods of the `duration`s. If `Rep` is a class type which emulates a floating-point type, the author of `Rep` can specialize `treat_as_floating_point` so that duration will treat this `Rep` as if it were a floating-point type. Otherwise `Rep` is assumed to be an integral type or a class emulating an integral type. [Note: The intention of this trait is to indicate whether a given class behaves like a floating point type, and thus allows division of one value by another with acceptable loss of precision. If `treat_as_floating_point<Rep>::value` is `false`, `Rep` will be treated as if it behaved like an integral type for the purpose of these conversions. — end note]

### 953. Various threading bugs #3

Section: 20.12.3 [time.clock.req] Status: Resolved Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2015-04-08

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Discussion:

Related to 951.

20.12.3 [time.clock.req] says that a clock's `rep` member is "an arithmetic type or a class emulating an arithmetic type." What are the requirements for such a type?

[ 2009-05-10 Howard adds: ]

> This wording was aimed directly at the `ArithmeticLike` concept.

[ Batavia (2009-05): ]

> We recommend this issue be addressed in the context of providing concepts for the entire `thread` header.

> May resolve for now by specifying arithmetic types, and in future change to `ArithmeticLike`. However, Alisdair believes this is not feasible.

> Bill disagrees.

> We look forward to proposed wording. Move to Open.

[ 2009-08-01 Howard adds: ]

> See commented dated 2009-08-01 in 951.

[ 2009-10 Santa Cruz: ]

> Stefanus to provide wording to turn this into a note.

[ 2010-02-11 Stephanus provided wording for 951 which addresses this issue as well. ]

[ 2010 Rapperswil: ]

> Move to NAD Editorial Resolved, resolved by 951.

Proposed resolution:

### 954. Various threading bugs #4

Section: 20.12.3 [time.clock.req] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2015-04-08

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Discussion:

Table 55 — Clock Requirements (in 20.12.3 [time.clock.req])

1. the requirements for `C1::time_point` require `C1` and `C2` to "refer to the same epoch", but "epoch" is not defined.
2. "Different clocks may share a `time_point` definition if it is valid to compare their `time_point`s by comparing their respective `duration`s." What does "valid" mean here? And, since `C1::rep` is "THE representation type of the native `duration` and `time_point`" (emphasis added), there doesn't seem to be much room for some other representation.
3. `C1::is_monotonic` has type "`const bool`". The "`const`" should be removed.
4. `C1::period` has type `ratio`. `ratio` isn't a type, it's a template. What is the required type?

[ 2009-05-10 Howard adds: ]

1. "epoch" is purposefully not defined beyond the common English definition. The C standard also chose not to define epoch, though POSIX did. I believe it is a strength of the C standard that epoch is not defined. When it is known that two `time_point`s refer to the same epoch, then a definition of the epoch is not needed to compare the two `time_point`s, or subtract them.

   A `time_point` and a `Clock` implicitly refer to an (unspecified) epoch. The `time_point` represents an offset (`duration`) from an epoch.

2. The sentence:

   > Different clocks may share a `time_point` definition if it is valid to compare their `time_point`s by comparing their respective `duration`s.

   is redundant and could be removed. I believe the sentence which follows the above:

   > `C1` and `C2` shall refer to the same epoch.

   is sufficient. If two clocks share the same epoch, then by definition, comparing their `time_point`s is valid.

3. `is_monotonic` is meant to never change (be `const`). It is also desired that this value be usable in compile-time computation and branching.

4. This should probably instead be worded:

   > An instantiation of `ratio`.

[ Batavia (2009-05): ]

> Re (a): It is not clear to us whether "epoch" is a term of art.

> Re (b), (c), and (d): We agree with Howard's comments, and would consider adding to (c) a `static constexpr` requirement.

> Move to Open pending proposed wording.

[ 2009-05-25 Daniel adds: ]

> In regards to (d) I suggest to say "a specialization of ratio" instead of "An instantiation of ratio". This seems to be the better matching standard core language term for this kind of entity.

[ 2009-05-25 Ganesh adds: ]

> Regarding (a), I found this paper on the ISO website using the term "epoch" consistently with the current wording:

> http://standards.iso.org/ittf/PubliclyAvailableStandards/C030811e_FILES/MAIN_C030811e/text/ISOIEC_18026E_TEMPORAL_CS.HTM

> which is part of ISO/IEC 18026 "Information technology -- Spatial Reference Model (SRM)".

[ 2009-08-01 Howard: Moved to Reivew as the wording requested in Batavia has been provided. ]

[ 2009-10 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

1. Change 20.12.3 [time.clock.req] p1:

   > -1- A clock is a bundle consisting of a native `duration`, a native `time_point`, and a function `now()` to get the current `time_point`. The origin of the clock's `time_point` is referred to as the clock's epoch as defined in section 6.3 of ISO/IEC 18026. A clock shall meet the requirements in Table 45.

2. Remove the sentence from the `time_point` row of the table "Clock Requirements":

   Clock requirements
   `C1::time_point` | `chrono::time_point<C1>` or `chrono::time_point<C2, C1::duration>` | The native `time_point` type of the clock. Different clocks may share a `time_point` definition if it is valid to compare their `time_point`s by comparing their respective `duration`s. `C1` and `C2` shall refer to the same epoch.

3. Change the row starting with `C1::period` of the table "Clock Requirements":

   Clock requirements
   `C1::period` | a specialization of `ratio` | The tick period of the clock in seconds.

### 956. Various threading bugs #6

Section: 20.12.3 [time.clock.req] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2015-04-08

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Discussion:

20.12.3 [time.clock.req] uses the word "native" in several places, but doesn't define it. What is a "native `duration`"?

[ 2009-05-10 Howard adds: ]

> The standard uses "native" in several places without defining it (e.g. 2.13.3 [lex.ccon]). It is meant to mean "that which is defined by the facility", or something along those lines. In this case it refers to the nested `time_point` and `duration` types of the clock. Better wording is welcome.

[ Batavia (2009-05): ]

> Move to Open pending proposed wording from Pete.

[ 2009-10-23 Pete provides wording: ]

[ 2009-11-18 Daniel adds: ]

> I see that 30.4.1.3 [thread.timedmutex.requirements]/3 says:

> > Precondition: If the tick `period` of `rel_time` is not exactly convertible to the native tick `period`, the `duration` shall be rounded up to the nearest native tick `period`.

> I would prefer to see that adapted as well. Following the same style as the proposed resolution I come up with

> > Precondition: If the tick `period` of `rel_time` is not exactly convertible to the native tick `period` of the execution environment, the `duration` shall be rounded up to the nearest native tick `period` of the execution environment.

[ 2010-03-28 Daniel synced wording with N3092 ]

[ Post-Rapperswil, Howard provides wording: ]

> Moved to Tentatively Ready with revised wording from Howard Hinnant after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 20.12.3 [time.clock.req]:

> 1 A clock is a bundle consisting of a native `duration`, a native `time_point`, and a function `now()` to get the current `time_point`. The origin of the clock's `time_point` is referred to as the clock's epoch. A clock shall meet the requirements in Table 56.

> 2 ...

> Table 56 — Clock requirements
> Expression | Return type | Operational semantics
> --- | --- | ---
> `C1::rep` | An arithmetic type or a class emulating an arithmetic type | The representation type of the native `C1::duration`. and `time_point`.
> `C1::period` | ... | ...
> `C1::duration` | `chrono::duration<C1::rep, C1::period>` | The native `duration` type of the clock.
> `C1::time_point` | `chrono::time_point<C1>` or `chrono::time_point<C2, C1::duration>` | The native `time_point` type of the clock. `C1` and `C2` shall refer to the same epoch.
> ...

### 957. Various threading bugs #7

Section: 20.12.7.1 [time.clock.system] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2015-04-08

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Discussion:

20.12.7.1 [time.clock.system]: `to_time_t` is overspecified. It requires truncation, but should allow rounding. For example, suppose a system has a clock that gives times in milliseconds, but `time()` rounds those times to the nearest second. Then `system_clock` can't use any resolution finer than one second, because if it did, truncating times between half a second and a full second would produce the wrong `time_t` value.

[ Post Summit Anthony Williams provided proposed wording. ]

[ Batavia (2009-05): ]

> Move to Review pending input from Howard. and other stakeholders.

[ 2009-05-23 Howard adds: ]

> I am in favor of the wording provided by Anthony.

[ 2009-10 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

In 20.12.7.1 [time.clock.system] replace paragraphs 3 and 4 with:

>     time_t to_time_t(const time_point& t);

> > -3- Returns: A `time_t` object that represents the same point in time as `t` when both values are truncated restricted to the coarser of the precisions of `time_t` and `time_point`. It is implementation defined whether values are rounded or truncated to the required precision.

>     time_point from_time_t(time_t t);

> > -4- Returns: A `time_point` object that represents the same point in time as `t` when both values are truncated restricted to the coarser of the precisions of `time_t` and `time_point`. It is implementation defined whether values are rounded or truncated to the required precision.

### 958. Various threading bugs #8

Section: 30.5.1 [thread.condition.condvar] Status: Resolved Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2015-04-08

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Discussion:

30.5.1 [thread.condition.condvar]: the specification for `wait_for` with no predicate has an effects clause that says it calls `wait_until`, and a returns clause that sets out in words how to determine the return value. Is this description of the return value subtly different from the description of the value returned by `wait_until`? Or should the effects clause and the returns clause be merged?

[ Summit: ]

> Move to open. Associate with LWG 859 and any other monotonic-clock related issues.

[ 2009-08-01 Howard adds: ]

> I believe that 859 (currently Ready) addresses this issue, and that this issue should be marked NAD, solved by 859 (assuming it moves to WP).

[ 2009-10 Santa Cruz: ]

> Mark as NAD Editorial Resolved, addressed by resolution of Issue 859.

Proposed resolution:

### 960. Various threading bugs #10

Section: 30.4.1 [thread.mutex.requirements] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2015-04-08

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Discussion:

30.4.1 [thread.mutex.requirements]: paragraph 4 is entitled "Error conditions", but according to 17.5.1.4 [structure.specifications], "Error conditions:" specifies "the error conditions for error codes reported by the function." It's not clear what this should mean when there is no function in sight.

[ Summit: ]

> Move to open.

[ Beman provided proposed wording. ]

[ 2009-10 Santa Cruz: ]

> Move to Ready. Fix the proposed wording with "functions of type Mutex" -> "functions of Mutex type"

Proposed resolution:

Change 30.4.1 [thread.mutex.requirements] Mutex requirements, paragraph 4 as indicated:

> -4- Error conditions: The error conditions for error codes, if any, reported by member functions of Mutex type shall be:

> - `not_enough_memory` — if there is not enough memory to construct the mutex object.
> - `resource_unavailable_try_again` — if any native handle type manipulated is not available.
> - `operation_not_permitted` — if the thread does not have the necessary permission to change the state of the mutex object.
> - `device_or_resource_busy` — if any native handle type manipulated is already locked.
> - `invalid_argument` — if any native handle type manipulated as part of mutex construction is incorrect.

### 962. Various threading bugs #12

Section: 30.4.2.2.2 [thread.lock.unique.locking] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2015-04-08

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Discussion:

30.4.2.2.2 [thread.lock.unique.locking]: `unique_lock::lock` is required to throw an object of type `std::system_error` "when the postcondition cannot be achieved." The postcondition is `owns == true`, and this is trivial to achieve. Presumably, the requirement is intended to mean something more than that.

[ Summit: ]

> Move to open.

[ Beman has volunteered to provide proposed wording. ]

[ 2009-07-21 Beman added wording to address 30.2.2 [thread.req.exception] in response to the Frankfurt notes in 859. ]

[ 2009-09-25 Beman: minor update to wording. ]

[ 2009-10 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

Change Exceptions 30.2.2 [thread.req.exception] as indicated:

> Some functions described in this Clause are specified to throw exceptions of type `system_error` (19.5.5). Such exceptions shall be thrown if any of the Error conditions are detected or a call to an operating system or other underlying API results in an error that prevents the library function from satisfying its postconditions or from returning a meaningful value meeting its specifications. Failure to allocate storage shall be reported as described in 17.6.5.12 [res.on.exception.handling].

Change thread assignment 30.3.1.5 [thread.thread.member], join(), paragraph 8 as indicated:

> Throws: `std::system_error` when the postconditions cannot be achieved an exception is required (30.2.2 [thread.req.exception]).

Change thread assignment 30.3.1.5 [thread.thread.member], detach(), paragraph 13 as indicated:

> Throws: `std::system_error` when the effects or postconditions cannot be achieved an exception is required (30.2.2 [thread.req.exception]).

Change Mutex requirements 30.4.1 [thread.mutex.requirements], paragraph 11, as indicated:

> Throws: `std::system_error` when the effects or postcondition cannot be achieved an exception is required (30.2.2 [thread.req.exception]).

Change unique_lock locking 30.4.2.2.2 [thread.lock.unique.locking], paragraph 3, as indicated:

> Throws: `std::system_error` when the postcondition cannot be achieved an exception is required (30.2.2 [thread.req.exception]).

Change unique_lock locking 30.4.2.2.2 [thread.lock.unique.locking], paragraph 8, as indicated:

> Throws: `std::system_error` when the postcondition cannot be achieved an exception is required (30.2.2 [thread.req.exception]).

Change unique_lock locking 30.4.2.2.2 [thread.lock.unique.locking], paragraph 13, as indicated:

> Throws: `std::system_error` when the postcondition cannot be achieved an exception is required (30.2.2 [thread.req.exception]).

Change unique_lock locking 30.4.2.2.2 [thread.lock.unique.locking], paragraph 18, as indicated:

> Throws: `std::system_error` when the postcondition cannot be achieved an exception is required (30.2.2 [thread.req.exception]).

Change unique_lock locking 30.4.2.2.2 [thread.lock.unique.locking], paragraph 22, as indicated:

> Throws: `std::system_error` when the postcondition cannot be achieved an exception is required (30.2.2 [thread.req.exception]).

Change Function call_once 30.4.4.2 [thread.once.callonce], paragraph 4, as indicated

> Throws: `std::system_error` when the effects cannot be achieved an exception is required (30.2.2 [thread.req.exception]), or any exception thrown by `func`.

Change Class condition_variable 30.5.1 [thread.condition.condvar], paragraph 12, as indicated:

> Throws: `std::system_error` when the effects or postcondition cannot be achieved an exception is required (30.2.2 [thread.req.exception]).

Change Class condition_variable 30.5.1 [thread.condition.condvar], paragraph 19, as indicated:

> Throws: `std::system_error` when the effects or postcondition cannot be achieved an exception is required (30.2.2 [thread.req.exception]).

Change Class condition_variable_any 30.5.2 [thread.condition.condvarany], paragraph 10, as indicated:

> Throws: `std::system_error` when the effects or postcondition cannot be achieved an exception is required (30.2.2 [thread.req.exception]).

Change Class condition_variable_any 30.5.2 [thread.condition.condvarany], paragraph 16, as indicated:

> Throws: `std::system_error` when the returned value, effects, or postcondition cannot be achieved an exception is required (30.2.2 [thread.req.exception]).

Assuming issue 859, Monotonic Clock is Conditionally Supported?, has been applied to the working paper, change Change 30.5.1 [thread.condition.condvar] as indicated:

>     template <class Rep, class Period>
>     bool wait_for(unique_lock<mutex>& lock,
>                   const chrono::duration<Rep, Period>& rel_time);

>     ...

> Throws: `std::system_error` when the effects or postcondition cannot be achieved an exception is required ([thread.req.exception]).

Assuming issue 859, Monotonic Clock is Conditionally Supported?, has been applied to the working paper, change Change 30.5.1 [thread.condition.condvar] as indicated:

>     template <class Rep, class Period, class Predicate>
>       bool wait_for(unique_lock<mutex>& lock,
>                     const chrono::duration<Rep, Period>& rel_time,
>                     Predicate pred);

>     ...

> Throws: `std::system_error` when the effects or postcondition cannot be achieved an exception is required (30.2.2 [thread.req.exception]).

Assuming issue 859, Monotonic Clock is Conditionally Supported?, has been applied to the working paper, change 30.5.2 [thread.condition.condvarany] as indicated:

>     template <class Lock, class Rep, class Period>
>       bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);

>     ...

> Throws: `std::system_error` when the returned value, effects or postcondition cannot be achieved an exception is required (30.2.2 [thread.req.exception]).

Assuming issue 859, Monotonic Clock is Conditionally Supported?, has been applied to the working paper, change 30.5.2 [thread.condition.condvarany] as indicated:

>     template <class Lock, class Rep, class Period, class Predicate>
>       bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);

>     ...

> Throws: `std::system_error` when the returned value, effects or postcondition cannot be achieved an exception is required (30.2.2 [thread.req.exception]).

### 963. Various threading bugs #13

Section: 30.3.1.5 [thread.thread.member] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2015-04-08

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Discussion:

30.3.1.5 [thread.thread.member]: `thread::detach` is required to throw an exception if the thread is "not a detachable thread". "Detachable" is never defined.

[ Howard adds: ]

> Due to a mistake on my part, 3 proposed resolutions appeared at approximately the same time. They are all three noted below in the discussion.

[ Summit, proposed resolution: ]

> In 30.3.1.5 [thread.thread.member] change:

> >     void detach();

> > > ...

> > > -14- Error conditions:

> > > - `no_such_process` -- if the thread is not a valid thread.
> > > - `invalid_argument` -- if the thread is not a detachable joinable thread.

[ Post Summit, Jonathan Wakely adds: ]

> A `thread` is detachable if it is joinable. As we've defined joinable, we can just use that.

> This corresponds to the pthreads specification, where pthread_detach fails if the thread is not joinable:

> > EINVAL: The implementation has detected that the value specified by thread does not refer to a joinable thread.

> Jonathan recommends this proposed wording:

> > In 30.3.1.5 [thread.thread.member] change:

> > >     void detach();

> > > > ...

> > > > -14- Error conditions:

> > > > - ...
> > > > - `invalid_argument` -- not a detachable joinable thread.

[ Post Summit, Anthony Williams adds: ]

> This is covered by the precondition that `joinable()` be `true`.

> Anthony recommends this proposed wording:

> > In 30.3.1.5 [thread.thread.member] change:

> > >     void detach();

> > > > ...

> > > > -14- Error conditions:

> > > > - ...
> > > > - `invalid_argument` -- not a detachable thread.

[ 2009-10 Santa Cruz: ]

> Mark as Ready with proposed resolution from Summit.

Proposed resolution:

In 30.3.1.5 [thread.thread.member] change:

>     void detach();

> > ...

> > -14- Error conditions:

> > - `no_such_process` -- if the thread is not a valid thread.
> > - `invalid_argument` -- if the thread is not a detachable joinable thread.

### 964. Various threading bugs #14

Section: 30.5.2 [thread.condition.condvarany] Status: Resolved Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2015-04-08

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Discussion:

The requirements for the constructor for `condition_variable` has several error conditions, but the requirements for the constructor for `condition_variable_any` has none. Is this difference intentional?

[ Summit: ]

> Move to open, pass to Howard. If this is intentional, a note may be helpful. If the error conditions are to be copied from `condition_variable`, this depends on LWG 965.

[ Post Summit Howard adds: ]

> The original intention (N2447) was to let the OS return whatever errors it was going to return, and for those to be translated into exceptions, for both `condition_variable` and `condition_variable_any`. I have not received any complaints about specific error conditions from vendors on non-POSIX platforms, but such complaints would not surprise me if they surfaced.

[ 2009-10 Santa Cruz: ]

> Leave open. Benjamin to provide wording.

[ 2010 Pittsburgh: ]

> We don't have throw clauses for condition variables.

> This issue may be dependent on LWG 1268.

> Leave open. Detlef will coordinate with Benjamin.

> Consider merging LWG 964, 966, and 1268 into a single paper.

Proposed resolution:

Resolved 2011-03 Madrid meeting by paper N3278

### 965. Various threading bugs #15

Section: 30.5.1 [thread.condition.condvar] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2015-04-08

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Discussion:

30.5.1 [thread.condition.condvar]: the constructor for `condition_variable` throws an exception with error code `device_or_resource_busy` "if attempting to initialize a previously-initialized but as of yet undestroyed `condition_variable`." How can this occur?

[ Summit: ]

> Move to review. Proposed resolution: strike the `device_or_resource_busy` error condition from the constructor of `condition_variable`.

> - This is a POSIX error that cannot occur in this interface because the C++ interface does not separate declaration from initialization.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change 30.5.1 [thread.condition.condvar] p3:

> - ...
> - `device_or_resource_busy` -- if attempting to initialize a previously-initialized but as of yet undestroyed `condition_variable`.

### 966. Various threading bugs #16

Section: 30.5.1 [thread.condition.condvar] Status: Resolved Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2015-04-08

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Discussion:

30.5.1 [thread.condition.condvar]: `condition_variable::wait` and `condition_variable::wait_until` both have a postcondition that `lock` is locked by the calling thread, and a throws clause that requires throwing an exception if this postcondition cannot be achieved. How can the implementation detect that this `lock` can never be obtained?

[ Summit: ]

> Move to open. Requires wording. Agreed this is an issue, and the specification should not require detecting deadlocks.

[ 2009-08-01 Howard provides wording. ]

> The proposed wording is inspired by the POSIX spec which says:

> > [EINVAL]
> >   The value specified by cond or mutex is invalid.
> > [EPERM]
> >   The mutex was not owned by the current thread at the time of the call.

> I do not believe [EINVAL] is possible without memory corruption (which we don't specify). [EPERM] is possible if this thread doesn't own the mutex, which is listed as a precondition. "May" is used instead of "Shall" because not all OS's are POSIX.

[ 2009-10 Santa Cruz: ]

> Leave open, Detlef to provide improved wording.

[ 2009-10-23 Detlef Provided wording. ]

> Detlef's wording put in Proposed resolution. Original wording here:

> > Change 30.5.1 [thread.condition.condvar] p12, p19 and 30.5.2 [thread.condition.condvarany] p10, p16:

> > > Throws: May throw `std::system_error` if a precondition is not met. when the effects or postcondition cannot be achieved.

[ 2009-10 Santa Cruz: ]

> Leave open, Detlef to provide improved wording.

[ 2009-11-18 Anthony adds: ]

> `condition_variable::wait` takes a `unique_lock<mutex>`. We know whether or not a `unique_lock` owns a lock, through use of its `owns_lock()` member.

> I would like to propose the following resolution:

> > Modify the first sentence of 30.5.1 [thread.condition.condvar] p9:

> > >     void wait(unique_lock<mutex>& lock);

> > > > 9 Precondition: `lock` is locked by the calling thread `lock.owns_lock()` is `true`, and either

> > > > ...

> > Replace 30.5.1 [thread.condition.condvar] p11-13 with:

> > >     void wait(unique_lock<mutex>& lock);

> > > > ...

> > > > 11 Postcondition: `lock` is locked by the calling thread `lock.owns_lock()` is `true`.

> > > > 12 Throws: `std::system_error` when the effects or postcondition cannot be achieved if the implementation detects that the preconditions are not met or the effects cannot be achieved. Any exception thrown by `lock.lock()` or `lock.unlock()`.

> > > > 13 Error Conditions: The error conditions are implementation defined.

> > > > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

[ 2010 Pittsburgh: ]

> There are heavy conflicts with adopted papers.

> This issue is dependent on LWG 1268.

> Leave open pending outstanding edits to the working draft. Detlef will provide wording.

> Possibly related to 964.

[2011-03-24 Madrid]

Rationale:

This has been resolved since filing, with the introduction of `system_error` to the thread specification.

Proposed resolution:

Replace 30.5.1 [thread.condition.condvar] p12, p19 and 30.5.2 [thread.condition.condvarany] p10, p16:

> Throws: `std::system_error` when the effects or postcondition cannot be achieved.

> Error conditions:

> - equivalent error condition from `lock.lock()` or `lock.unlock()`.

> Throws: It is implementation-defined whether a `std::system_error` with implementation-defined error condition is thrown if the precondition is not met.

### 967. Various threading bugs #17

Section: 30.3.1.2 [thread.thread.constr] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2015-04-08

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Discussion:

the error handling for the constructor for `condition_variable` distinguishes lack of memory from lack of other resources, but the error handling for the thread constructor does not. Is this difference intentional?

[ Beman has volunteered to provide proposed wording. ]

[ 2009-09-25 Beman provided proposed wording. ]

> The proposed resolution assumes 962 has been accepted and its proposed resolution applied to the working paper.

[ 2009-10 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

Change Mutex requirements 30.4.1 [thread.mutex.requirements], paragraph 4, as indicated:

> Error conditions:

> > - `not_enough_memory` — if there is not enough memory to construct the mutex object.
> > - `resource_unavailable_try_again` — if any native handle type manipulated is not available.
> > - `operation_not_permitted` — if the thread does not have the necessary permission to change the state of the mutex object.
> > - `device_or_resource_busy` — if any native handle type manipulated is already locked.
> > - `invalid_argument` — if any native handle type manipulated as part of mutex construction is incorrect.

Change Class `condition_variable` 30.5.1 [thread.condition.condvar], default constructor, as indicated:

> `condition_variable();`

> > Effects: Constructs an object of type `condition_variable`.

> > Throws: `std::system_error` when an exception is required (30.2.2 [thread.req.exception]).

> > Error conditions:

> > > - `not_enough_memory` — if a memory limitation prevents initialization.
> > > - `resource_unavailable_try_again` — if some non-memory resource limitation prevents initialization.
> > > - `device_or_resource_busy` — if attempting to initialize a previously-initialized but as of yet undestroyed `condition_variable`.

### 968. Various threading bugs #18

Section: 30.4.1 [thread.mutex.requirements] Status: C++11 Submitter: Pete Becker Opened: 2009-01-07 Last modified: 2015-04-08

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Discussion:

30.4.1 [thread.mutex.requirements]: several functions are required to throw exceptions "if the thread does not have the necessary permission ...". "The necessary permission" is not defined.

[ Summit: ]

> Move to open.

[ Beman has volunteered to provide proposed wording. ]

[ 2009-10 Santa Cruz: ]

> Moved to Ready with minor word-smithing in the example.

Proposed resolution:

Change Exceptions 30.2.2 [thread.req.exception] as indicated:

> Some functions described in this Clause are specified to throw exceptions of type `system_error` (19.5.5). Such exceptions shall be thrown if any of the Error conditions are detected or a call to an operating system or other underlying API results in an error that prevents the library function from meeting its specifications. [Note: See 17.6.5.12 [res.on.exception.handling] for exceptions thrown to report storage allocation failures. —end note]

> [Example:

> > Consider a function in this clause that is specified to throw exceptions of type `system_error` and specifies Error conditions that include `operation_not_permitted` for a thread that does not have the privilege to perform the operation. Assume that, during the execution of this function, an `errno` of `EPERM` is reported by a POSIX API call used by the implementation. Since POSIX specifies an `errno` of `EPERM` when "the caller does not have the privilege to perform the operation", the implementation maps `EPERM` to an `error_condition` of `operation_not_permitted` (19.5 [syserr]) and an exception of type `system_error` is thrown.

> —end example]

> Editorial note: For the sake of exposition, the existing text above is shown with the changes proposed in issues 962 and 967. The proposed additional example is independent of whether or not the 962 and 967 proposed resolutions are accepted.

Change Mutex requirements 30.4.1 [thread.mutex.requirements], paragraph 4, as indicated:

> — `operation_not_permitted` — if the thread does not have the necessary permission to change the state of the mutex object privilege to perform the operation.

Change Mutex requirements 30.4.1 [thread.mutex.requirements], paragraph 12, as indicated:

> — `operation_not_permitted` — if the thread does not have the necessary permission to change the state of the mutex privilege to perform the operation.

### 970. `addressof` overload unneeded

Section: 20.7.12.1 [specialized.addressof] Status: C++11 Submitter: Howard Hinnant Opened: 2009-01-16 Last modified: 2015-04-08

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Discussion:

20.7.12.1 [specialized.addressof] specifies:

>     template <ObjectType T> T* addressof(T& r);
>     template <ObjectType T> T* addressof(T&& r);

The two signatures are ambiguous when the argument is an lvalue. The second signature seems not useful: what does it mean to take the address of an rvalue?

[ Post Summit: ]

> Recommend Review.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

[ 2009-11-18 Moved from Pending WP to WP. Confirmed in N3000. ]

Proposed resolution:

Change 20.7.12.1 [specialized.addressof]:

>     template <ObjectType T> T* addressof(T& r);
>     template <ObjectType T> T* addressof(T&& r);

### 974. `duration<double>` should not implicitly convert to `duration<int>`

Section: 20.12.5.1 [time.duration.cons] Status: C++11 Submitter: Howard Hinnant Opened: 2009-01-21 Last modified: 2015-04-08

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Discussion:

The following code should not compile because it involves implicit truncation errors (against the design philosophy of the `duration` library).

>     duration<double> d(3.5);
>     duration<int> i = d;  // implicit truncation, should not compile

This intent was codified in the example implementation which drove this proposal but I failed to accurately translate the code into the specification in this regard.

[ Batavia (2009-05): ]

> We agree with the proposed resolution.

> Move to Tentatively Ready.

[ 2009-07 Frankfurt ]

> Moved from Tentatively Ready to Open only because the wording needs to be improved for `enable_if` type constraining, possibly following Robert's formula.

[ 2009-08-01 Howard adds: ]

> Addressed by 1177.

[ 2009-10 Santa Cruz: ]

> Not completely addressed by 1177. Move to Ready.

Proposed resolution:

Change 20.12.5.1 [time.duration.cons], p4:

>     template <class Rep2, class Period2>
>       duration(const duration<Rep2, Period2>& d);

> > -4- Requires: `treat_as_floating_point<rep>::value` shall be `true` or both `ratio_divide<Period2, period>::type::den` shall be 1 and `treat_as_floating_point<Rep2>::value` shall be `false`. Diagnostic required. [Note: This requirement prevents implicit truncation error when converting between integral-based `duration` types. Such a construction could easily lead to confusion about the value of the `duration`. — end note]

### 975. `is_convertible` cannot be instantiated for non-convertible types

Section: 20.10.6 [meta.rel] Status: C++11 Submitter: Daniel Krügler Opened: 2009-01-25 Last modified: 2015-04-08

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Discussion:

Addresses UK 206

Related to 1114.

The current specification of `std::is_convertible` (reference is draft N2798) is basically defined by 20.10.6 [meta.rel] p.4:

> In order to instantiate the template `is_convertible<From, To>`, the following code shall be well formed:

> >     template <class T>
> >       typename add_rvalue_reference<T>::type create();

> >     To test() {
> >       return create<From>();
> >     }

> [Note: This requirement gives well defined results for reference types, void types, array types, and function types. — end note]

The first sentence can be interpreted, that e.g. the expression

>     std::is_convertible<double, int*>::value

is ill-formed because `std::is_convertible<double, int*>` could not be instantiated, or in more general terms: The wording requires that `std::is_convertible<X, Y>` cannot be instantiated for otherwise valid argument types `X` and `Y` if `X` is not convertible to `Y`.

This semantic is both unpractical and in contradiction to what the last type traits paper N2255 proposed:

> If the following `test` function is well formed code `b` is `true`, else it is `false`.

> >     template <class T>
> >       typename add_rvalue_reference<T>::type create();

> >     To test() {
> >       return create<From>();
> >     }

> [Note: This definition gives well defined results for `reference` types, `void` types, array types, and function types. — end note]

[ Post Summit: ]

> Jens: Checking that code is well-formed and then returning `true`/`false` sounds like speculative compilation. John Spicer would really dislike this. Please find another wording suggesting speculative compilation.

> Recommend Open.

[ Post Summit, Howard adds: ]

> John finds the following wording clearer:

> > Template | Condition | Comments
> > --- | --- | ---
> > `template <class From, class To> struct is_convertible;` | see below | `From` and `To` shall be complete types, arrays of unknown bound, or (possibly cv-qualified) `void` types.

> > Given the following function prototype:

> > >     template <class T>
> > >       typename add_rvalue_reference<T>::type create();

> > `is_convertible<From, To>::value` shall be `true` if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function, else `is_convertible<From, To>::value` shall be `false`.

> > >     To test() {
> > >       return create<From>();
> > >     }

Original proposed wording:

In 20.10.6 [meta.rel]/4 change:

> In order to instantiate the template `is_convertible<From, To>`, the following code shall be well formed If the following code is well formed `is_convertible<From, To>::value` is `true`, otherwise `false`:[..]

Revision 2

> In 20.10.6 [meta.rel] change:

> > Template | Condition | Comments
> > --- | --- | ---
> > ... | ... | ...
> > `template <class From, class To> struct is_convertible;` | The code set out below shall be well formed. see below | `From` and `To` shall be complete types, arrays of unknown bound, or (possibly cv-qualified) `void` types.

> > -4- In order to instantiate the template `is_convertible<From, To>`, the following code shall be well formed: Given the following function prototype:

> > >     template <class T>
> > >       typename add_rvalue_reference<T>::type create();

> > `is_convertible<From, To>::value` inherits either directly or indirectly from `true_type` if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function, else `is_convertible<From, To>::value` inherits either directly or indirectly from `false_type`.

> > >     To test() {
> > >       return create<From>();
> > >     }

> > [Note: This requirement gives well defined results for reference types, void types, array types, and function types. -- end note]

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

In 20.10.6 [meta.rel] change:

> Template | Condition | Comments
> --- | --- | ---
> ... | ... | ...
> `template <class From, class To> struct is_convertible;` | The code set out below shall be well formed. see below | `From` and `To` shall be complete types, arrays of unknown bound, or (possibly cv-qualified) `void` types.

> -4- In order to instantiate the template `is_convertible<From, To>`, the following code shall be well formed: Given the following function prototype:

> >     template <class T>
> >       typename add_rvalue_reference<T>::type create();

> the predicate condition for a template specialization `is_convertible<From, To>` shall be satisfied, if and only if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function.

> >     To test() {
> >       return create<From>();
> >     }

> [Note: This requirement gives well defined results for reference types, void types, array types, and function types. — end note]

### 976. Class template `std::stack` should be movable

Section: 23.6.5.2 [stack.defn] Status: Resolved Submitter: Daniel Krügler Opened: 2009-02-01 Last modified: 2015-04-08

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Discussion:

The synopsis given in 23.6.5.2 [stack.defn] does not show up

>     requires MoveConstructible<Cont> stack(stack&&);
>     requires MoveAssignable<Cont> stack& operator=(stack&&);

although the other container adaptors do provide corresponding members.

[ Batavia (2009-05): ]

> We agree with the proposed resolution.

> Move to Tentatively Ready.

[ 2009-07 Frankfurt ]

> Moved from Tentatively Ready to Open only because the wording needs to be tweaked for concepts removal.

[ 2009-08-18 Daniel updates the wording and Howard sets to Review. ]

[ 2009-08-23 Howard adds: ]

> 1194 also adds these move members using an editorially different style.

[ 2009-10 Santa Cruz: ]

> Mark NAD Editorial Resolved, addressed by issue 1194.

Proposed resolution:

In the class stack synopsis of 23.6.5.2 [stack.defn] insert:

>     template <class T, class Container = deque<T> >
>     class stack {
>       [..]
>       explicit stack(const Container&);
>       explicit stack(Container&& = Container());
>       stack(stack&& s) : c(std::move(s.c)) {}
>       stack& operator=(stack&& s) { c = std::move(s.c); return *this; }
>       [..]
>     };

### 978. Hashing smart pointers

Section: 20.9.13 [unord.hash] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-02-02 Last modified: 2015-04-08

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Discussion:

Addresses UK 208

I don't see an open issue on supporting `std::hash` for smart pointers (`unique_ptr` and `shared_ptr` at least).

It seems reasonable to at least expect support for the smart pointers, especially as they support comparison for use in ordered associative containers.

[ Batavia (2009-05): ]

> Howard points out that the client can always supply a custom hash function.

> Alisdair replies that the smart pointer classes are highly likely to be frequently used as hash keys.

> Bill would prefer to be conservative.

> Alisdair mentions that this issue may also be viewed as a subissue or duplicate of issue 1025.

> Move to Open, and recommend the issue be deferred until after the next Committee Draft is issued.

[ 2009-05-31 Peter adds: ]

> > Howard points out that the client can always supply a custom hash function.

> Not entirely true. The client cannot supply the function that hashes the address of the control block (the equivalent of the old `operator<`, now proudly carrying the awkward name of '`owner_before`'). Only the implementation can do that, not necessarily via specializing `hash<>`, of course.

> This hash function makes sense in certain situations for `shared_ptr` (when one needs to switch from `set/map` using ownership ordering to `unordered_set/map`) and is the only hash function that makes sense for `weak_ptr`.

[ 2009-07-28 Alisdair provides wording. ]

[ 2009-10 Santa Cruz: ]

> Move to Ready.

[ 2009-11-16 Moved from Ready to Open: ]

> Pete writes:

> > As far as I can see, "...suitable for using this type as key in unordered associative containers..." doesn't define any semantics. It's advice to the reader, and if it's present at all it should be in a note. But we have far too much of this sort of editorial commentary as it is.

> > And in the resolution of 978 it's clearly wrong: it says that if there is no hash specialization available for `D::pointer`, the implementation may provide `hash<unique_ptr<T,D>>` if the result is not suitable for use in unordered containers.

> Howard writes:

> > Is this a request to pull 978 from Ready?

> Barry writes:

> > I read this as more than a request. The PE says it's wrong, so it can't be Ready.

[ 2010-01-31 Alisdair: related to 1245 and 1182. ]

[ 2010-02-08 Beman updates wording. ]

[ 2010-02-09 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Add the following declarations to the synopsis of `<memory>` in 20.7 [memory]

>     // [util.smartptr.hash] hash support
>     template <class T> struct hash;
>     template <class T, class D> struct hash<unique_ptr<T,D>>;
>     template <class T> struct hash<shared_ptr<T>>;

Add a new subclause under 20.8.2 [util.smartptr] called hash support

> ### hash support [util.smartptr.hash]

>     template <class T, class D> struct hash<unique_ptr<T,D>>;

> > Specialization meeting the requirements of class template `hash` (20.9.13 [unord.hash]). For an object `p` of type `UP`, where `UP` is a type `unique_ptr<T,D>`, `hash<UP>()(p)` shall evaluate to the same value as `hash<typename UP::pointer>()(p.get())`. The specialization `hash<typename UP::pointer>` is required to be well-formed.

>     template <class T> struct hash<shared_ptr<T>>;

> > Specialization meeting the requirements of class template `hash` (20.9.13 [unord.hash]). For an object `p` of type `shared_ptr<T>`, `hash<shared_ptr<T>>()(p)` shall evaluate to the same value as `hash<T*>()(p.get())`.

### 981. Unordered container requirements should add `initializer_list` support

Section: 23.2.5 [unord.req] Status: C++11 Submitter: Daniel Krügler Opened: 2009-02-08 Last modified: 2015-04-08

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Discussion:

Refering to N2800 all container requirements tables (including those for associative containers) provide useful member function overloads accepting `std::initializer_list` as argument, the only exception is Table 87. There seems to be no reason for not providing them, because 23.5 [unord] is already `initializer_list`-aware. For the sake of library interface consistency and user-expectations corresponding overloads should be added to the table requirements of unordered containers as well.

[ Batavia (2009-05): ]

> We agree with the proposed resolution.

> Move to Tentatively Ready.

Proposed resolution:

In 23.2.5 [unord.req]/9 insert:

> ... `[q1, q2)` is a valid range in `a`, `il` designates an object of type `initializer_list<value_type>`, `t` is a value of type `X::value_type`, ...

In 23.2.5 [unord.req], Table 87 insert:

> Table 87 - Unordered associative container requirements (in addition to container)
> Expression | Return type | Assertion/note pre-/post-condition | Complexity
> --- | --- | --- | ---
> `X(i, j) X a(i, j)` | `X` | ... | ...
> `X(il)` | `X` | Same as `X(il.begin(), il.end())`. | Same as `X(il.begin(), il.end())`.
> ... | ... | ... | ...
> `a = b` | `X` | ... | ...
> `a = il` | `X&` | `a = X(il); return *this;` | Same as `a = X(il)`.
> ... | ... | ... | ...
> `a.insert(i, j)` | `void` | ... | ...
> `a.insert(il)` | `void` | Same as `a.insert(il.begin(), il.end())`. | Same as `a.insert(il.begin(), il.end())`.

### 982. Wrong complexity for initializer_list assignment in Table 85

Section: 23.2.4 [associative.reqmts] Status: C++11 Submitter: Daniel Krügler Opened: 2009-02-08 Last modified: 2015-04-08

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Discussion:

According to N2800, the associative container requirements table 85 says that assigning an `initializer_list` to such a container is of constant complexity, which is obviously wrong.

[ Batavia (2009-05): ]

> We agree with the proposed resolution.

> Move to Tentatively Ready.

Proposed resolution:

In 23.2.4 [associative.reqmts], Table 85 change:

> Table 85 - Associative container requirements (in addition to container)
> Expression | Return type | Assertion/note pre-/post-condition | Complexity
> --- | --- | --- | ---
> `a = il` | `X&` | `a = X(il); return *this;` | constant Same as `a = X(il)`.

### 983. `unique_ptr` reference deleters should not be moved from

Section: 20.8.1.2 [unique.ptr.single] Status: Resolved Submitter: Howard Hinnant Opened: 2009-02-10 Last modified: 2015-04-08

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Discussion:

Dave brought to my attention that when a `unique_ptr` has a non-const reference type deleter, move constructing from it, even when the `unique_ptr` containing the reference is an rvalue, could have surprising results:

>     D d(some-state);
>     unique_ptr<A, D&> p(new A, d);
>     unique_ptr<A, D> p2 = std::move(p);
>     // has d's state changed here?

I agree with him. It is the `unique_ptr` that is the rvalue, not the deleter. When the deleter is a reference type, the `unique_ptr` should respect the "lvalueness" of the deleter.

Thanks Dave.

[ Batavia (2009-05): ]

> Seems correct, but complicated enough that we recommend moving to Review.

[ 2009-10 Santa Cruz: ]

> Move to Ready.

[ 2010-03-14 Howard adds: ]

> We moved N3073 to the formal motions page in Pittsburgh which should obsolete this issue. I've moved this issue to NAD Editorial, solved by N3073.

Rationale:

Solved by N3073.

Proposed resolution:

Change 20.8.1.2.1 [unique.ptr.single.ctor], p20-21

>     template <class U, class E> unique_ptr(unique_ptr<U, E>&& u);

> > -20- Requires: If `D E` is not a reference type, construction of the deleter `D` from an rvalue of type `E` shall be well formed and shall not throw an exception. Otherwise `E` is a reference type and construction of the deleter `D` from an lvalue of type `E` shall be well formed and shall not throw an exception. If `D` is a reference type, then `E` shall be the same type as `D` (diagnostic required). `unique_ptr<U, E>::pointer` shall be implicitly convertible to `pointer`. [`Note:` These requirements imply that `T` and `U` are complete types. — end note]

> > -21- Effects: Constructs a `unique_ptr` which owns the pointer which `u` owns (if any). If the deleter `E` is not a reference type, it this deleter is move constructed from `u`'s deleter, otherwise the reference this deleter is copy constructed from `u`.'s deleter. After the construction, `u` no longer owns a pointer. [Note: The deleter constructor can be implemented with `std::forward<DE>`. — end note]

Change 20.8.1.2.3 [unique.ptr.single.asgn], p1-3

>     unique_ptr& operator=(unique_ptr&& u);

> > -1- Requires: If the deleter `D` is not a reference type, A assignment of the deleter `D` from an rvalue `D` shall not throw an exception. Otherwise the deleter `D` is a reference type, and assignment of the deleter `D` from an lvalue `D` shall not throw an exception.

> > -2- Effects: reset(u.release()) followed by an move assignment from `u`'s deleter to this deleter `std::forward<D>(u.get_deleter())`.

> > -3- Postconditions: This `unique_ptr` now owns the pointer which `u` owned, and `u` no longer owns it. [Note: If `D` is a reference type, then the referenced lvalue deleters are move assigned. — end note]

Change 20.8.1.2.3 [unique.ptr.single.asgn], p6-7

>     template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u);

> > Requires: If the deleter `E` is not a reference type, A assignment of the deleter `D` from an rvalue `DE` shall not throw an exception. Otherwise the deleter `E` is a reference type, and assignment of the deleter `D` from an lvalue `E` shall not throw an exception. `unique_ptr<U, E>::pointer` shall be implicitly convertible to `pointer`. [Note: These requirements imply that `T` and `U>` are complete types. — end note]

> > Effects: `reset(u.release())` followed by an move assignment from `u`'s deleter to this deleter `std::forward<E>(u.get_deleter())`. If either `D` or `E` is a reference type, then the referenced lvalue deleter participates in the move assignment.

### 984. Does `<cinttypes>` have macro guards?

Section: 27.9.2 [c.files] Status: C++11 Submitter: Howard Hinnant Opened: 2009-02-12 Last modified: 2015-04-08

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Discussion:

The C standard says about `<inttypes.h>`:

> C++ implementations should define these macros only when `__STDC_FORMAT_MACROS`is defined before `<inttypes.h>` is included.

The C standard has a similar note about `<stdint.h>`. For `<cstdint>` we adopted a "thanks but no thanks" policy and documented that fact in 18.4.1 [cstdint.syn]:

> ... [Note: The macros defined by `<stdint>` are provided unconditionally. In particular, the symbols `__STDC_LIMIT_MACROS` and `__STDC_CONSTANT_MACROS` (mentioned in C99 footnotes 219, 220, and 222) play no role in C++. — end note]

I recommend we put a similar note in 27.9.2 [c.files] regarding `<cinttypes>`.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Add to 27.9.2 [c.files]:

> Table 112 describes header `<cinttypes>`. [Note: The macros defined by `<cintypes>` are provided unconditionally. In particular, the symbol `__STDC_FORMAT_MACROS` (mentioned in C99 footnote 182) plays no role in C++. — end note]

### 985. Allowing throwing move

Section: 23.2.1 [container.requirements.general] Status: Resolved Submitter: Rani Sharoni Opened: 2009-02-12 Last modified: 2015-04-08

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Discussion:

Introduction

This proposal is meant to resolve potential regression of the N2800 draft, see next section, and to relax the requirements for containers of types with throwing move constructors.

The basic problem is that some containers operations, like `push_back`, have a strong exception safety guarantee (i.e. no side effects upon exception) that are not achievable when throwing move constructors are used since there is no way to guarantee revert after partial move. For such operations the implementation can at most provide the basic guarantee (i.e. valid but unpredictable) as it does with multi copying operations (e.g. range insert).

For example, `vector<T>::push_back()` (where `T` has a move constructor) might resize the `vector` and move the objects to the new underlying buffer. If move constructor throws it might not be possible to recover the throwing object or to move the old objects back to the original buffer.

The current draft is explicit by disallowing throwing move for some operations (e.g. `vector<>::reserve`) and not clear about other operations mentioned in 23.2.1 [container.requirements.general]/10 (e.g. single element `insert`): it guarantees strong exception safety without explicitly disallowing a throwing move constructor.

Regression

This section only refers to cases in which the contained object is by itself a standard container.

Move constructors of standard containers are allowed to throw and therefore existing operations are broken, compared with C++03, due to move optimization. (In fact existing implementations like Dinkumware are actually throwing).

For example, `vector< list<int> >::reserve` yields undefined behavior since `list<int>`'s move constructor is allowed to throw. On the other hand, the same operation has strong exception safety guarantee in C++03.

There are few options to solve this regression:

1. Disallow throwing move and throwing default constructor
2. Disallow throwing move but disallowing usage after move
3. Special casing
4. Disallow throwing move and making it optional

Option 1 is suggested by proposal N2815 but it might not be applicable for existing implementations for which containers default constructors are throwing.

Option 2 limits the usage significantly and it's error prone by allowing zombie objects that are nothing but destructible (e.g. no `clear()` is allowed after move). It also potentially complicates the implementation by introducing special state.

Option 3 is possible, for example, using default construction and `swap` instead of move for standard containers case. The implementation is also free to provide special hidden operation for non throwing move without forcing the user the cope with the limitation of option-2 when using the public move.

Option 4 impact the efficiency in all use cases due to rare throwing move.

The proposed wording will imply option 1 or 3 though option 2 is also achievable using more wording. I personally oppose to option 2 that has impact on usability.

Relaxation for user types

Disallowing throwing move constructors in general seems very restrictive since, for example, common implementation of move will be default construction + `swap` so move will throw if the default constructor will throw. This is currently the case with the Dinkumware implementation of node based containers (e.g. `std::list`) though this section doesn't refer to standard types.

For throwing move constructors it seem that the implementation should have no problems to provide the basic guarantee instead of the strong one. It's better to allow throwing move constructors with basic guarantee than to disallow it silently (compile and run), via undefined behavior.

There might still be cases in which the relaxation will break existing generic code that assumes the strong guarantee but it's broken either way given a throwing move constructor since this is not a preserving optimization.

[ Batavia (2009-05): ]

> Bjarne comments (referring to his draft paper): "I believe that my suggestion simply solves that. Thus, we don't need a throwing move."

> Move to Open and recommend it be deferred until after the next Committee Draft is issued.

[ 2009-10 Santa Cruz: ]

> Should wait to get direction from Dave/Rani (N2983).

[ 2010-03-28 Daniel updated wording to sync with N3092. ]

> The suggested change of 23.3.3.4 [deque.modifiers]/2 should be removed, because the current wording does say more general things:

> > 2 Remarks: If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of `T` there are no effects. If an exception is thrown by the move constructor of a non-CopyConstructible `T`, the effects are unspecified.

> The suggested change of 23.3.6.3 [vector.capacity]/2 should be removed, because the current wording does say more general things:

> > 2 Effects: A directive that informs a `vector` of a planned change in size, so that it can manage the storage allocation accordingly. After `reserve()`, `capacity()` is greater or equal to the argument of `reserve` if reallocation happens; and equal to the previous value of `capacity()` otherwise. Reallocation happens at this point if and only if the current capacity is less than the argument of `reserve()`. If an exception is thrown other than by the move constructor of a non-`CopyConstructible` type, there are no effects.

[2011-03-15: Daniel updates wording to sync with N3242 and comments]

> The issue has nearly been resolved by previous changes to the working paper, in particular all suggested changes for `deque` and `vector` are no longer necessary. The still remaining parts involve the unordered associative containers.

[2011-03-24 Madrid meeting]

It looks like this issue has been resolved already by `noexcept` paper N3050

Rationale:

Resolved by N3050

Proposed resolution:

23.2.1 [container.requirements.general] paragraph 10 add footnote:

> -10- Unless otherwise specified (see 23.2.4.1 [associative.reqmts.except], 23.2.5.1 [unord.req.except], 23.3.3.4 [deque.modifiers], and 23.3.6.5 [vector.modifiers]) all container types defined in this Clause meet the following additional requirements:

> - …

> [Note: for compatibility with C++ 2003, when "no effect" is required, standard containers should not use the `value_type`'s throwing move constructor when the contained object is by itself a standard container. — end note]

23.2.5.1 [unord.req.except] change paragraph 2+4 to say:

> -2- For unordered associative containers, if an exception is thrown by any operation other than the container's hash function from within an `insert()` function inserting a single element, the `insert()` function has no effect unless the exception is thrown by the contained object move constructor.

> […]

> -4- For unordered associative containers, if an exception is thrown from within a `rehash()` function other than by the container's hash function or comparison function, the `rehash()` function has no effect unless the exception is thrown by the contained object move constructor.

Keep 23.3.3.4 [deque.modifiers] paragraph 2 unchanged [Drafting note: The originally proposed wording did suggest to add a last sentence as follows:

> If an exception is thrown by `push_back()` or `emplace_back()` function, that function has no effects unless the exception is thrown by the move constructor of `T`.

— end drafting note ]

> -2- Remarks: If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of `T` there are no effects. If an exception is thrown by the move constructor of a non-CopyInsertable `T`, the effects are unspecified.

Keep 23.3.6.3 [vector.capacity] paragraph 2 unchanged [Drafting note: The originally proposed wording did suggest to change the last sentence as follows:

> If an exception is thrown, there are no effects unless the exception is thrown by the contained object move constructor.

— end drafting note ]

> -2- Effects: A directive that informs a `vector` of a planned change in size, so that it can manage the storage allocation accordingly. After `reserve()`, `capacity()` is greater or equal to the argument of `reserve` if reallocation happens; and equal to the previous value of `capacity()` otherwise. Reallocation happens at this point if and only if the current capacity is less than the argument of `reserve()`. If an exception is thrown other than by the move constructor of a non-`CopyInsertable` type, there are no effects.

Keep 23.3.6.3 [vector.capacity] paragraph 12 unchanged [Drafting note: The originally proposed wording did suggest to change the old paragraph as follows:

> -12- Requires: If `value_type` has a move constructor, that constructor shall not throw any exceptions. If an exception is thrown, there are no effects unless the exception is thrown by the contained object move constructor.

— end drafting note ]

> -12- Requires: If an exception is thrown other than by the move constructor of a non-`CopyInsertable` `T` there are no effects.

Keep 23.3.6.5 [vector.modifiers] paragraph 1 unchanged [Drafting note: The originally proposed wording did suggest to change the old paragraph as follows:

> -1- Requires: If `value_type` has a move constructor, that constructor shall not throw any exceptions. Remarks: If an exception is thrown by `push_back()` or `emplace_back()` function, that function has no effect unless the exception is thrown by the move constructor of `T`.

— end drafting note ]

> -1- Remarks: Causes reallocation if the new size is greater than the old capacity. If no reallocation happens, all the iterators and references before the insertion point remain valid. If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of `T` or by any `InputIterator` operation there are no effects. If an exception is thrown by the move constructor of a non-`CopyInsertable` `T`, the effects are unspecified.

### 986. Generic `try_lock` contradiction

Section: 30.4.3 [thread.lock.algorithm] Status: C++11 Submitter: Chris Fairles Opened: 2009-02-14 Last modified: 2015-04-08

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Discussion:

In 30.4.3 [thread.lock.algorithm], the generic `try_lock` effects (p2) say that a failed `try_lock` is when it either returns `false` or throws an exception. In the event a call to `try_lock` does fail, by either returning `false` or throwing an exception, it states that `unlock` shall be called for all prior arguments. Then the returns clause (p3) goes on to state in a note that after returning, either all locks are locked or none will be. So what happens if multiple locks fail on `try_lock`?

Example:

>     #include <mutex>

>     int main() {
>      std::mutex m0, m1, m2;
>      std::unique_lock<std::mutex> l0(m0, std::defer_lock);
>      std::unique_lock<std::mutex> l1(m1); //throws on try_lock
>      std::unique_lock<std::mutex> l2(m2); //throws on try_lock

>      int result = std::try_lock(l0, l1, l2);

>      assert( !l0.owns_lock() );
>      assert( l1.owns_lock() ); //??
>      assert( l2.owns_lock() ); //??
>     }

The first lock's `try_lock` succeeded but, being a prior argument to a lock whose `try_lock` failed, it gets unlocked as per the effects clause of 30.4.3 [thread.lock.algorithm]. However, 2 locks remain locked in this case but the return clause states that either all arguments shall be locked or none will be. This seems to be a contradiction unless the intent is for implementations to make an effort to unlock not only prior arguments, but the one that failed and those that come after as well. Shouldn't the note only apply to the arguments that were successfully locked?

Further discussion and possible resolutions in c++std-lib-23049.

[ Summit: ]

> Move to review. Agree with proposed resolution.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change 30.4.3 [thread.lock.algorithm], p2:

> -2- Effects: Calls `try_lock()` for each argument in order beginning with the first until all arguments have been processed or a call to `try_lock()` fails, either by returning `false` or by throwing an exception. If a call to `try_lock()` fails, `unlock()` shall be called for all prior arguments and there shall be no further calls to `try_lock()`.

Delete the note from 30.4.3 [thread.lock.algorithm], p3

> -3- Returns: -1 if all calls to `try_lock()` returned `true`, otherwise a 0-based index value that indicates the argument for which `try_lock()` returned `false`. [Note: On return, either all arguments will be locked or none will be locked. -- end note]

### 987. `reference_wrapper` and function types

Section: 20.9.4 [refwrap] Status: C++11 Submitter: Howard Hinnant Opened: 2009-02-18 Last modified: 2015-04-08

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Discussion:

The synopsis in 20.9.4 [refwrap] says:

>     template <ObjectType T> class reference_wrapper
>     ...

And then paragraph 3 says:

> The template instantiation `reference_wrapper<T>` shall be derived from `std::unary_function<T1, R>` only if the type `T` is any of the following:

> - a function type or a pointer to function type taking one argument of type `T1` and returning `R`

But function types are not `ObjectType`s.

Paragraph 4 contains the same contradiction.

[ Post Summit: ]

> Jens: restricted reference to ObjectType

> Recommend Review.

[ Post Summit, Peter adds: ]

> In https://svn.boost.org/trac/boost/ticket/1846 however Eric Niebler makes the very reasonable point that `reference_wrapper<F>`, where `F` is a function type, represents a reference to a function, a legitimate entity. So `boost::ref` was changed to allow it.

> http://svn.boost.org/svn/boost/trunk/libs/bind/test/ref_fn_test.cpp

> Therefore, I believe an alternative proposed resolution for issue 987 could simply allow `reference_wrapper` to be used with function types.

[ Post Summit, Howard adds: ]

> I agree with Peter (and Eric). I got this one wrong on my first try. Here is code that demonstrates how easy (and useful) it is to instantiate `reference_wrapper` with a function type:

> >     #include <functional>

> >     template <class F>
> >     void test(F f);

> >     void f() {}

> >     int main()
> >     {
> >         test(std::ref(f));
> >     }

> Output (link time error shows type of `reference_wrapper` instantiated with function type):

> >     Undefined symbols:
> >       "void test<std::reference_wrapper<void ()()> >(std::reference_wrapper<void ()()>)",...

> I've taken the liberty of changing the proposed wording to allow function types and set to Open. I'll also freely admit that I'm not positive `ReferentType` is the correct concept.

[ Batavia (2009-05): ]

> Howard observed that `FunctionType`, a concept not (yet?) in the Working Paper, is likely the correct constraint to be applied. However, the proposed resolution provides an adequate approximation.

> Move to Review.

[ 2009-05-23 Alisdair adds: ]

> By constraining to `PointeeType` we rule out the ability for `T` to be a reference, and call in reference-collapsing. I'm not sure if this is correct and intended, but would like to be sure the case was considered.

> Is dis-allowing reference types and the implied reference collapsing the intended result?

[ 2009-07 Frankfurt ]

> Moved from Review to Open only because the wording needs to be tweaked for concepts removal.

[ 2009-10-14 Daniel provided de-conceptified wording. ]

[ 2009-10 post-Santa Cruz: ]

> Move to Tentatively Ready.

Proposed resolution:

Change 20.9.4 [refwrap]/1 as indicated:

> `reference_wrapper<T>` is a `CopyConstructible` and `CopyAssignable` wrapper around a reference to an object or function of type `T`.

### 990. `monotonic_clock::is_monotonic` must be `true`

Section: X [time.clock.monotonic] Status: C++11 Submitter: Howard Hinnant Opened: 2009-03-09 Last modified: 2015-04-08

View all other issues in [time.clock.monotonic].

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Discussion:

There is some confusion over what the value of `monotonic_clock::is_monotonic` when `monotonic_clock` is a synonym for `system_clock`. The intent is that if `monotonic_clock` exists, then `monotonic_clock::is_monotonic` is `true`.

[ Batavia (2009-05): ]

> We agree with the proposed resolution.

> Move to Tentatively Ready.

Proposed resolution:

Change X [time.clock.monotonic], p1:

> -1- Objects of class `monotonic_clock` represent clocks for which values of `time_point` never decrease as physical time advances. `monotonic_clock` may be a synonym for `system_clock` if and only if `system_clock::is_monotonic` is `true`.

### 991. Provide allocator for `wstring_convert`

Section: 22.3.3.2.2 [conversions.string] Status: C++11 Submitter: P.J. Plauger Opened: 2009-03-03 Last modified: 2015-04-08

View other active issues in [conversions.string].

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Discussion:

Addresses JP-50 [CD1]

Add custom allocator parameter to `wstring_convert`, since we cannot allocate memory for strings from a custom allocator.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change 22.3.3.2.2 [conversions.string]:

>     template<class Codecvt, class Elem = wchar_t,
>              class Wide_alloc = std::allocator<Elem>,
>              class Byte_alloc = std::allocator<char> > class wstring_convert {
>       public:
>         typedef std::basic_string<char, char_traits<char>, Byte_alloc> byte_string;
>         typedef std::basic_string<Elem, char_traits<Elem>, Wide_alloc> wide_string;
>          ...

Change 22.3.3.2.2 [conversions.string], p3:

> -3- The class template describes an ob ject that controls conversions between wide string ob jects of class `std::basic_string<Elem, char_traits<Elem>, Wide_alloc>` and byte string objects of class `std::basic_string<char, char_traits<char>, Byte_alloc>` (also known as `std::string`).

### 993. `_Exit` needs better specification

Section: 18.5 [support.start.term] Status: C++11 Submitter: P.J. Plauger Opened: 2009-03-03 Last modified: 2015-04-08

View all other issues in [support.start.term].

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Discussion:

Addresses UK-188 [CD1]

The function `_Exit` does not appear to be defined in this standard. Should it be added to the table of functions included-by-reference to the C standard?

[ 2009-05-09 Alisdair fixed some minor issues in the wording. ]

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Add to 18.5 [support.start.term] Table 20 (Header `<cstdlib>` synopsis) Functions:

>     _Exit

Add before the description of `abort(void)`:

>     void _Exit [[noreturn]] (int status)

> > The function `_Exit(int status)` has additional behavior in this International Standard:

> > - The program is terminated without executing destructors for objects of automatic, thread, or static storage duration and without calling the functions passed to `atexit()` (3.6.3 [basic.start.term]).

### 994. `quick_exit` should terminate well-defined

Section: 18.6.2.3 [new.handler] Status: C++11 Submitter: P.J. Plauger Opened: 2009-03-03 Last modified: 2015-04-08

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Discussion:

Addresses UK-193 [CD1]

`quick_exit` has been added as a new valid way to terminate a program in a well defined way.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change 18.6.2.3 [new.handler], p2:

> -2- Required behavior: ...

> - ...
> - call either `abort()` or `exit();` terminate execution of the program without returning to the caller

### 997. "Effects: Equivalent to" is underspecified

Section: 17.5.1.4 [structure.specifications] Status: C++11 Submitter: Thomas Plum Opened: 2009-03-03 Last modified: 2015-04-08

View all other issues in [structure.specifications].

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Discussion:

Addresses UK-163 [CD1]

Many functions are defined as "Effects: Equivalent to a...", which seems to also define the preconditions, effects, etc. But this is not made clear.

After studying the occurrences of "Effects: Equivalent to", I agree with the diagnosis but disagree with the solution. In 21.4.2 [string.cons] we find

> 14 Effects: If `InputIterator` is an integral type, equivalent to `basic_string(static_cast<size_type>(begin), static_cast<value_type>(end), a)`

> 15 Otherwise constructs a string from the values in the range `[begin, end)`, as indicated in the Sequence Requirements table (see 23.1.3).

This would be devishly difficult to re-write with an explicit "Equivalent to:" clause. Instead, I propose the following, which will result in much less editorial re-work.

[ 2009-05-09 Alisdair adds: ]

> This issue is related to 492.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Add a new paragraph after 17.5.1.4 [structure.specifications], p3:

> -3- Descriptions of function semantics contain the following elements (as appropriate):154

> - Requires: the preconditions for calling the function
> - Effects: the actions performed by the function
> - Postconditions: the observable results established by the function
> - Returns: a description of the value(s) returned by the function
> - Throws: any exceptions thrown by the function, and the conditions that would cause the exception
> - Complexity: the time and/or space complexity of the function
> - Remarks: additional semantic constraints on the function
> - Error conditions: the error conditions for error codes reported by the function.
> - Notes: non-normative comments about the function

> Whenever the Effects element specifies that the semantics of some function `F` are Equivalent to some code-sequence, then the various elements are interpreted as follows. If `F`'s semantics specifies a Requires element, then that requirement is logically imposed prior to the equivalent-to semantics. Then, the semantics of the code-sequence are determined by the Requires, Effects, Postconditions, Returns, Throws, Complexity, Remarks, Error Conditions and Notes specified for the (one or more) function invocations contained in the code-sequence. The value returned from `F` is specified by `F`'s Returns element, or if `F` has no Returns element, a non-`void` return from `F` is specified by the Returns elements in code-sequence. If `F`'s semantics contains a Throws (or Postconditions, or Complexity) element, then that supersedes any occurrences of that element in the code-sequence.

### 998. Smart pointer referencing its owner

Section: 20.8.1.2.5 [unique.ptr.single.modifiers] Status: C++11 Submitter: Pavel Minaev Opened: 2009-02-26 Last modified: 2015-04-08

View all other issues in [unique.ptr.single.modifiers].

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Discussion:

Consider the following (simplified) implementation of `std::auto_ptr<T>::reset()`:

>     void reset(T* newptr = 0) {
>        if (this->ptr && this->ptr != newptr) {
>          delete this->ptr;
>        }
>        this->ptr = newptr;
>     }

Now consider the following code which uses the above implementation:

>     struct foo {
>        std::auto_ptr<foo> ap;
>        foo() : ap(this) {}
>        void reset() { ap.reset(); }
>     };
>     int main() {
>        (new foo)->reset();
>     }

With the above implementation of auto_ptr, this results in U.B. at the point of auto_ptr::reset(). If this isn't obvious yet, let me explain how this goes step by step:

1. `foo::reset()` entered
2. `auto_ptr::reset()` entered
3. `auto_ptr::reset()` tries to delete `foo`
4. `foo::~foo()` entered, tries to destruct its members
5. `auto_ptr::~auto_ptr()` executed - `auto_ptr` is no longer a valid object!
6. `foo::~foo()` left
7. `auto_ptr::reset()` sets its "ptr" field to 0 <- U.B.! `auto_ptr` is not a valid object here already!

[ Thanks to Peter Dimov who recognized the connection to `unique_ptr` and brought this to the attention of the LWG, and helped with the solution. ]

[ Howard adds: ]

> To fix this behavior `reset` must be specified such that deleting the pointer is the last action to be taken within `reset`.

[ Alisdair adds: ]

> The example providing the rationale for LWG 998 is poor, as it relies on broken semantics of having two object believing they are unique owners of a single resource. It should not be surprising that UB results from such code, and I feel no need to go out of our way to support such behaviour.

> If an example is presented that does not imply multiple ownership of a unique resource, I would be much more ready to accept the proposed resolution.

[ Batavia (2009-05): ]

> Howard summarizes:

> > This issue has to do with circular ownership, and affects `auto_ptr`, too (but we don't really care about that). It is intended to spell out the order in which operations must be performed so as to avoid the possibility of undefined behavior in the self-referential case.

> Howard points to message c++std-lib-23175 for another example, requested by Alisdair.

> We agree with the issue and with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change 20.8.1.2.5 [unique.ptr.single.modifiers], p5 (Effects clause for `reset`), and p6:

> -5- Effects: If `get() == nullptr` there are no effects. Otherwise `get_deleter()(get())`. Assigns `p` to the stored `pointer`, and then if the old value of the `pointer` is not equal to `nullptr`, calls `get_deleter()(`the old value of the `pointer)`. [Note: The order of these operations is significant because the call to `get_deleter()` may destroy `*this`. -- end note]

> -6- Postconditions: `get() == p`. [Note: The postcondition does not hold if the call to `get_deleter()` destroys `*this` since `this->get()` is no longer a valid expression. -- end note]

### 999. Taking the address of a function

Section: 20.7.12 [specialized.algorithms] Status: C++11 Submitter: Peter Dimov Opened: 2009-03-09 Last modified: 2015-04-08

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Discussion:

The same fix (reference 987) may be applied to `addressof`, which is also constrained to `ObjectType`. (That was why `boost::ref` didn't work with functions - it tried to apply `boost::addressof` and the `reinterpret_cast<char&>` implementation of `addressof` failed.)

[ Batavia (2009-05): ]

> We agree.

> Move to Tentatively Ready.

[ 2009-07 Frankfurt ]

> Moved from Tentatively Ready to Open only because the wording needs to be tweaked for concepts removal.

[ 2009-10-10 Daniel updates wording to concept-free. ]

[ 2009-10 post-Santa Cruz: ]

> Move to Tentatively Ready.

Proposed resolution:

[ The resolution assumes that `addressof` is reintroduced as described in n2946 ]

In 20.7.12 [specialized.algorithms] change as described:

>     template <class T> T* addressof(T& r);

> > Returns: The actual address of the object or function referenced by `r`, even in the presence of an overloaded `operator&`.

### 1004. Clarify "throws an exception"

Section: 17.6.4.8 [res.on.functions] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2015-04-08

View all other issues in [res.on.functions].

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Discussion:

Addresses UK 179

According to the 4th bullet there is a problem if "if any replacement function or handler function or destructor operation throws an exception". There should be no problem throwing exceptions so long as they are caught within the function.

[ Batavia (2009-05): ]

> The phrasing "throws an exception" is commonly used elsewhere to mean "throws or propagates an exception." Move to Open pending a possible more general resolution.

[ 2009-07 Frankfurt: ]

> Replace "propagates" in the proposed resolution with the phrase "exits via" and move to Ready.

Proposed resolution:

Change the 4th bullet of 17.6.4.8 [res.on.functions], p2:

> - if any replacement function or handler function or destructor operation throws exits via an exception, unless specifically allowed in the applicable Required behavior: paragraph.

### 1006. `operator delete` in garbage collected implementation

Section: 18.6.1 [new.delete] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2015-04-08

View all other issues in [new.delete].

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Discussion:

Addresses UK 190

It is not entirely clear how the current specification acts in the presence of a garbage collected implementation.

[ Summit: ]

> Agreed.

[ 2009-05-09 Alisdair adds: ]

> Proposed wording is too strict for implementations that do not support garbage collection. Updated wording supplied.

[ Batavia (2009-05): ]

> We recommend advancing this to Tentatively Ready with the understanding that it will not be moved for adoption unless and until the proposed resolution to Core issue #853 is adopted.

Proposed resolution:

(Editorial note: This wording ties into the proposed resolution for Core #853)

Add paragraphs to 18.6.1.1 [new.delete.single]:

>     void operator delete(void* ptr) throw();
>     void operator delete(void* ptr, const std::nothrow_t&) throw();

> [ The second signature deletion above is editorial. ]

> > Requires: If an implementation has strict pointer safety (3.7.4.3 [basic.stc.dynamic.safety]) then `ptr` shall be a safely-derived pointer.

> > -10- ...

>     void operator delete(void* ptr, const std::nothrow_t&) throw();

> > Requires: If an implementation has strict pointer safety (3.7.4.3 [basic.stc.dynamic.safety]) then `ptr` shall be a safely-derived pointer.

> > -15- ...

Add paragraphs to 18.6.1.2 [new.delete.array]:

>     void operator delete[](void* ptr) throw();
>     void operator delete[](void* ptr, const std::nothrow_t&) throw();

> [ The second signature deletion above is editorial. ]

> > Requires: If an implementation has strict pointer safety (3.7.4.3 [basic.stc.dynamic.safety]) then `ptr` shall be a safely-derived pointer.

> > -9- ...

>     void operator delete[](void* ptr, const std::nothrow_t&) throw();

> > Requires: If an implementation has strict pointer safety (3.7.4.3 [basic.stc.dynamic.safety]) then `ptr` shall be a safely-derived pointer.

> > -13- ...

Add paragraphs to 18.6.1.3 [new.delete.placement]:

>     void operator delete(void* ptr, void*) throw();

> > Requires: If an implementation has strict pointer safety (3.7.4.3 [basic.stc.dynamic.safety]) then `ptr` shall be a safely-derived pointer.

> > -7- ...

>     void operator delete[](void* ptr, void*) throw();

> > Requires: If an implementation has strict pointer safety (3.7.4.3 [basic.stc.dynamic.safety]) then `ptr` shall be a safely-derived pointer.

> > -9- ...

### 1011. `next/prev` wrong iterator type

Section: 24.4.4 [iterator.operations] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2015-04-08

View all other issues in [iterator.operations].

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Discussion:

Addresses UK 271

`next/prev` return an incremented iterator without changing the value of the original iterator. However, even this may invalidate an `InputIterator`. A `ForwardIterator` is required to guarantee the 'multipass' property.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

[ 2009-07 Frankfurt ]

> Moved from Tentatively Ready to Open only because the wording needs to be tweaked for concepts removal.

[ 2009-10-14 Daniel provided de-conceptified wording. ]

[ 2009-10 Santa Cruz: ]

> Moved to Ready.

Proposed resolution:

1. Change header `<iterator>` synopsis 24.3 [iterator.synopsis] as indicated:

   >     // 24.4.4, iterator operations:
   >     ...
   >     template <class InputForwardIterator>
   >       InputForwardIterator
   >       next(InputForwardIterator x, typename std::iterator_traits<InputForwardIterator>::difference_type n = 1);

2. Change 24.4.4 [iterator.operations] before p.6 as indicated:

   >     template <class InputForwardIterator>
   >       InputForwardIterator
   >       next(InputForwardIterator x, typename std::iterator_traits<InputForwardIterator>::difference_type n = 1);

### 1012. `reverse_iterator` default ctor should value initialize

Section: 24.5.1.3.1 [reverse.iter.cons] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2015-04-08

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Discussion:

Addresses UK 277

The default constructor default-initializes current, rather than value-initializes. This means that when Iterator corresponds to a trivial type, the current member is left un-initialized, even when the user explictly requests value intialization! At this point, it is not safe to perform any operations on the reverse_iterator other than assign it a new value or destroy it. Note that this does correspond to the basic definition of a singular iterator.

[ Summit: ]

> Agree with option i.

Related issue: 408

[ Batavia (2009-05): ]

> We believe this should be revisited in conjunction with issue 408, which nearly duplicates this issue. Move to Open.

[ 2009-07 post-Frankfurt: ]

> Change "constructed" to "initialized" in two places in the proposed resolution.

> Move to Tentatively Ready.

[ 2009 Santa Cruz: ]

> Moved to Ready for this meeting.

Proposed resolution:

Change [reverse.iter.con]:

>     reverse_iterator();

> > -1- Effects: Default Value initializes `current`. Iterator operations applied to the resulting iterator have defined behavior if and only if the corresponding operations are defined on a default constructed value initialized iterator of type `Iterator`.

Change 24.5.3.3.1 [move.iter.op.const]:

>     move_iterator();

> > -1- Effects: Constructs a `move_iterator`, default value initializing `current`. Iterator operations applied to the resulting iterator have defined behavior if and only if the corresponding operations are defined on a value initialized iterator of type `Iterator`.

### 1014. `basic_regex` should be created/assigned from initializer lists

Section: 28.8.2 [re.regex.construct] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2015-04-08

View all other issues in [re.regex.construct].

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Discussion:

Addresses UK 317 and JP 74

UK 317:

> `basic_string` has both a constructor and an assignment operator that accepts an initializer list, `basic_regex` should have the same.

JP 74:

> `basic_regex & operator= (initializer_list<T>);` is not defined.

[ Batavia (2009-05): ]

> UK 317 asks for both assignment and constructor, but the requested constructor is already present in the current Working Paper. We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change 28.8 [re.regex]:

>     template <class charT,
>               class traits = regex_traits<charT> >
>     class basic_regex {
>       ...
>       basic_regex& operator=(const charT* ptr);
>       basic_regex& operator=(initializer_list<charT> il);
>       template <class ST, class SA>
>         basic_regex& operator=(const basic_string<charT, ST, SA>& p);
>       ...
>     };

Add in 28.8.2 [re.regex.construct]:

> > -20- ...

>     basic_regex& operator=(initializer_list<charT> il);

> > -21- Effects: returns `assign(il.begin(), il.end());`

### 1019. Make `integral_constant` objects useable in integral-constant-expressions

Section: 20.10.3 [meta.help] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2015-04-08

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Discussion:

Addresses UK 205 [CD1]

`integral_constant` objects should be usable in integral-constant-expressions. The addition to the language of literal types and the enhanced rules for constant expressions make this possible.

[ Batavia (2009-05): ]

> We agree that the `static` data member ought be declared `constexpr`, but do not see a need for the proposed `operator value_type()`. (A use case would be helpful.) Move to Open.

[ 2009-05-23 Alisdair adds: ]

> The motivating case in my mind is that we can then use `true_type` and `false_type` as integral Boolean expressions, for example inside a `static_assert` declaration. In that sense it is purely a matter of style.

> Note that Boost has applied the non-explicit conversion operator for many years as it has valuable properties for extension into other metaprogramming libraries, such as MPL. If additional rationale is desired I will poll the Boost lists for why this extension was originally applied. I would argue that explicit conversion is more appropriate for 0x though.

[ 2009-07-04 Howard adds: ]

> Here's a use case which demonstrates the syntactic niceness which Alisdair describes:

> >     #define requires(...) class = typename std::enable_if<(__VA_ARGS__)>::type

> >     template <class T, class U,
> >         requires(!is_lvalue_reference<T>() ||
> >                   is_lvalue_reference<T>() && is_lvalue_reference<U>()),
> >         requires(is_same<typename base_type<T>::type,
> >                          typename base_type<U>::type>)>
> >     inline
> >     T&&
> >     forward(U&& t)
> >     {
> >         return static_cast<T&&>(t);
> >     }

[ 2009-07 post-Frankfurt: ]

> Move to Tentatively Ready.

[ 2009 Santa Cruz: ]

> Moved to Ready for this meeting.

Proposed resolution:

Add to the `integral_constant` struct definition in 20.10.3 [meta.help]:

>     template <class T, T v>
>     struct integral_constant {
>       static constexpr T value = v;
>       typedef T value_type;
>       typedef integral_constant<T,v> type;
>       constexpr operator value_type() { return value; }
>     };

### 1021. Allow `nullptr_t` assignments to `unique_ptr`

Section: 20.8.1.2.3 [unique.ptr.single.asgn] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2015-04-08

View all other issues in [unique.ptr.single.asgn].

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Discussion:

Addresses UK 211 [CD1]

The `nullptr_t` type was introduced to resolve the null pointer literal problem. It should be used for the assignment operator, as with the constructor and elsewhere through the library.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change the synopsis in 20.8.1.2 [unique.ptr.single]:

>     unique_ptr& operator=(unspecified-pointer-type nullptr_t);

Change 20.8.1.2.3 [unique.ptr.single.asgn]:

>     unique_ptr& operator=(unspecified-pointer-type nullptr_t);

> > Assigns from the literal 0 or `NULL`. [Note: The unspecified-pointer-type is often implemented as a pointer to a private data member, avoiding many of the implicit conversion pitfalls. — end note]

### 1030. Missing requirements for smart-pointer safety API

Section: 20.8.2.6 [util.smartptr.shared.atomic] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-11 Last modified: 2015-04-08

View all other issues in [util.smartptr.shared.atomic].

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Discussion:

Addresses JP 44 [CD1]

The 1st parameter `p` and 2nd parameter `v` is now `shared_ptr<T>*`.

It should be `shared_ptr<T>&`, or if these are `shared_ptr<T>*` then add the "`p` shall not be a null pointer" at the requires.

[ Summit: ]

> Agree. All of the functions need a requirement that `p` (or `v`) is a pointer to a valid object.

[ 2009-07 post-Frankfurt: ]

> Lawrence explained that these signatures match the regular atomics. The regular atomics must not use references because these signatures are shared with C. The decision to pass shared_ptrs by pointer rather than by reference was deliberate and was motivated by the principle of least surprise.

> Lawrence to write wording that requires that the pointers not be null.

[ 2009-09-20 Lawrence provided wording: ]

> The parameter types for atomic shared pointer access were deliberately chosen to be pointers to match the corresponding parameters of the atomics chapter. Those in turn were deliberately chosen to match C functions, which do not have reference parameters.

> We adopt the second suggestion, to require that such pointers not be null.

[ 2009-10 Santa Cruz: ]

> Moved to Ready.

Proposed resolution:

In section "`shared_ptr` atomic access" 20.8.2.6 [util.smartptr.shared.atomic], add to each function the following clause.

> Requires: `p` shall not be null.

### 1033. `thread::join()` effects?

Section: 30.3.1.5 [thread.thread.member] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2009-03-12 Last modified: 2015-04-08

View all other issues in [thread.thread.member].

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Discussion:

While looking at `thread::join()` I think I spotted a couple of possible defects in the specifications. I could not find a previous issue or NB comment about that, but I might have missed it.

The postconditions clause for `thread::join()` is:

> Postconditions: If `join()` throws an exception, the value returned by `get_id()` is unchanged. Otherwise, `get_id() == id()`.

and the throws clause is:

> Throws: `std::system_error` when the postconditions cannot be achieved.

Now... how could the postconditions not be achieved? It's just a matter of resetting the value of `get_id()` or leave it unchanged! I bet we can always do that. Moreover, it's a chicken-and-egg problem: in order to decide whether to throw or not I depend on the postconditions, but the postconditions are different in the two cases.

I believe the throws clause should be:

> Throws: `std::system_error` when the effects or postconditions cannot be achieved.

as it is in `detach()`, or, even better, as the postcondition is trivially satisfiable and to remove the circular dependency:

> Throws: `std::system_error` if the effects cannot be achieved.

Problem is that... ehm... `join()` has no "Effects" clause. Is that intentional?

[ See the thread starting at c++std-lib-23204 for more discussion. ]

[ Batavia (2009-05): ]

> Pete believes there may be some more general language (in frontmatter) that can address this and related issues such as 962.

> Move to Open.

[ 2009-11-18 Anthony provides wording. ]

[ 2010-02-12 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Edit 30.3.1.5 [thread.thread.member] as indicated:

>     void join();

> > 5 Precondition: `joinable()` is `true`.

> > Effects: Blocks until the thread represented by `*this` has completed.

> > 6 Synchronization: The completion of the thread represented by `*this` happens before (1.10 [intro.multithread]) `join()` returns. [Note: Operations on `*this` are not synchronized. — end note]

> > 7 Postconditions: If `join()` throws an exception, the value returned by `get_id()` is unchanged. Otherwise, The thread represented by `*this` has completed. `get_id() == id()`.

> > 8 ...

### 1034. Clarify generality of Container Requirement tables

Section: 23.2.1 [container.requirements.general] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2015-04-08

View other active issues in [container.requirements.general].

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Discussion:

Addresses UK 222 [CD1]

It is not clear what purpose the Requirement tables serve in the Containers clause. Are they the definition of a library Container? Or simply a conventient shorthand to factor common semantics into a single place, simplifying the description of each subsequent container? This becomes an issue for 'containers' like `array`, which does not meet the default-construct-to-empty requirement, or `forward_list` which does not support the size operation. Are these components no longer containers? Does that mean the remaining requirements don't apply? Or are these contradictions that need fixing, despite being a clear design decision?

Recommend:

Clarify all the tables in 23.2 [container.requirements] are there as a convenience for documentation, rather than a strict set of requirements. Containers should be allowed to relax specific requirements if they call attention to them in their documentation. The introductory text for `array` should be expanded to mention a default constructed `array` is not empty, and `forward_list` introduction should mention it does not provide the required `size` operation as it cannot be implemented efficiently.

[ Summit: ]

> Agree in principle.

[ 2009-07 post-Frankfurt: ]

> We agree in principle, but we have a timetable. This group feels that the issue should be closed as NAD unless a proposed resolution is submitted prior to the March 2010 meeting.

[ 2009-10 Santa Cruz: ]

> Looked at this and still intend to close as NAD in March 2010 unless there is proposed wording that we like.

[ 2010-02-02 Nicolai M. Josuttis updates proposed wording and adds: ]

> I just came across issue #1034 (response to UK 222), which covers the role of container requirements. The reason I found this issue was that I am wondering why `array<>` is specified to be a sequence container. For me, currently, this follows from Sequence containers 23.2.3 [sequence.reqmts] saying:

> > The library provides five basic kinds of sequence containers: `array`, `vector`, `forward_list`, `list`, and `deque`. while later on in Table 94 "Sequence container requirements" are defined.

> IMO, you can hardly argue that this is NAD. We MUST say somewhere that either array is not a sequence container or does not provide all operations of a sequence container (even not all requirements of a container in general).

> Here is the number of requirements `array<>` does not meet (AFAIK):

> general container requirements:

> - a default constructed `array` is not empty
> - `swap` has no constant complexity

> Note also that `swap` not only has linear complexity it also invalidates iterators (or to be more precise, assigns other values to the elements), which is different from the effect swap has for other containers. For this reason, I must say that i tend to propose to remove `swap()` for `arrays`.

> sequence container requirements:

> - There is no constructor and assignment for a range
> - There is no constructor and assignment for `n` copies of `t`
> - There are no `emplace`, `insert`, `erase`, `clear`, `assign` operations

> In fact, out of all sequence container requirements `array<>` only provides the following operations: from sequence requirements (Table 94):

> >     X(il);
> >     a = il;

> and from optional requirements (Table 95):

> >     [], at(), front(), back()

> This is almost nothing!

> Note in addition, that due to the fact that `array` is an aggregate and not a container with `initializer_lists` a construction or assignment with an initializer list is valid for all sequence containers but not valid for array:

> >     vector<int>  v({1,2,3});   // OK
> >     v = {4,5,6};               // OK

> >     array<int,3> a({1,2,3});   // Error
> >     array<int,3> a = {1,2,3};  // OK
> >     a = {4,5,6};               // Error

> BTW, for this reason, I am wondering, why `<array>` includes `<initializer_list>`.

> IMO, we can't really say that `array` is a sequence container. `array` is special. As the solution to this issue seemed to miss some proposed wording where all could live with, let me try to suggest some.

[ 2010-02-12 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010 Pittsburgh: Ok with move to Ready except for "OPEN:" part. ]

Proposed resolution:

In Sequence containers 23.2.3 [sequence.reqmts] modify paragraph 1 as indicated:

> 1 A sequence container organizes a finite set of objects, all of the same type, into a strictly linear arrangement. The library provides five four basic kinds of sequence containers: `array`, `vector`, `forward_list`, `list`, and `deque`. In addition, `array` is provided as a sequence container that only provides limited sequence operations because it has a fixed number of elements. It The library also provides container adaptors that make it easy to construct abstract data types, such as `stack`s or `queue`s, out of the basic sequence container kinds (or out of other kinds of sequence containers that the user might define).

Modify paragraph 2 as follows (just editorial):

> 2 The five basic sequence containers offer the programmer different complexity trade-offs and should be used accordingly. `vector` or `array` is the type of sequence container that should be used by default. `list` or `forward_list` should be used when there are frequent insertions and deletions from the middle of the sequence. `deque` is the data structure of choice when most insertions and deletions take place at the beginning or at the end of the sequence.

In Class template array 23.3.2 [array] modify paragraph 3 as indicated:

> 3 Unless otherwise specified, all `array` operations are as described in 23.2. An array satisfies all of the requirements of a container and of a reversible container (given in two tables in 23.2 [container.requirements]) except that a default constructed `array` is not empty, `swap` does not have constant complexity, and `swap` may throw exceptions. An `array` satisfies some of the requirements of a sequence container (given in 23.2.3 [sequence.reqmts]). Descriptions are provided here only for operations on `array` that are not described in that Clause in one of these tables or for operations where there is additional semantic information.

In array specialized algorithms 23.3.2.3 [array.special] add to the specification of `swap()`:

>     template <class T, size_t N> void swap(array<T,N>& x, array<T,N>& y);

> > 1 Effects: ...

> > Complexity: Linear in `N`.

### 1037. Unclear status of `match_results` as library container

Section: 23.2.3 [sequence.reqmts] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2015-04-08

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Discussion:

Addresses UK 232 [CD1]

`match_results` may follow the requirements but is not listed a general purpose library container.

Remove reference to `match_results` against `a[n]` operation.

[ Summit: ]

> Agree. `operator[]` is defined elsewhere.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

In 23.2.3 [sequence.reqmts] Table 84, remove reference to `match_results` in the row describing the `a[n]` operation.

### 1038. Sequence requirement table needs to reference several new containers

Section: 23.2.3 [sequence.reqmts] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2015-04-08

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Discussion:

Addresses UK 233 [CD1]

Table 84 is missing references to several new container types.

[ Summit: ]

> Agree.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

In 23.2.3 [sequence.reqmts] Table 84, Add reference to listed containers to the following rows:

> Table 84 -- Optional sequence container operations
> Expression | Return type | Operational semantics | Container
> --- | --- | --- | ---
> `a.front()` | ... | ... | `vector, list, deque, basic_string, array, forward_list`
> `a.back()` | ... | ... | `vector, list, deque, basic_string, array`
> `a.emplace_front(args)` | ... | ... | `list, deque, forward_list`
> `a.push_front(t)` | ... | ... | `list, deque, forward_list`
> `a.push_front(rv)` | ... | ... | `list, deque, forward_list`
> `a.pop_front()` | ... | ... | `list, deque, forward_list`
> `a[n]` | ... | ... | `vector, deque, basic_string, array`
> `a.at(n)` | ... | ... | `vector, deque, basic_string, array`

### 1039. Sequence container `back` function should also support `const_iterator`

Section: 23.2.3 [sequence.reqmts] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2015-04-08

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Discussion:

Addresses UK 234 [CD1]

The reference to `iterator` in semantics for `back` should also allow for `const_iterator` when called on a const-qualified container. This would be ugly to specify in the 03 standard, but is quite easy with the addition of `auto` in this new standard.

[ Summit: ]

> Agree.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

In 23.2.3 [sequence.reqmts] Table 84, replace iterator with auto in semantics for back:

> Table 84 — Optional sequence container operations
> Expression | Return type | Operational semantics | Container
> --- | --- | --- | ---
> `a.back()` | `reference; const_reference` for constant `a` | `{ iterator auto tmp = a.end(); --tmp; return *tmp; }` | `vector, list, deque, basic_string`

### 1040. Clarify possible sameness of associative container's `iterator` and `const_iterator`

Section: 23.2.4 [associative.reqmts] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2015-04-08

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Discussion:

Addresses UK 238 [CD1]

Leaving it unspecified whether or not `iterator` and `const_iterator` are the same type is dangerous, as user code may or may not violate the One Definition Rule by providing overloads for both types. It is probably too late to specify a single behaviour, but implementors should document what to expect. Observing that problems can be avoided by users restricting themselves to using `const_iterator`, add a note to that effect.

Suggest Change 'unspecified' to 'implementation defined'.

[ Summit: ]

> Agree with issue. Agree with adding the note but not with changing the normative text. We believe the note provides sufficient guidance.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

In 23.2.4 [associative.reqmts] p6, add:

> -6- `iterator` of an associative container meets the requirements of the `BidirectionalIterator` concept. For associative containers where the value type is the same as the key type, both `iterator` and `const_iterator` are constant iterators. It is unspecified whether or not `iterator` and `const_iterator` are the same type. [Note: `iterator` and `const_iterator` have identical semantics in this case, and `iterator` is convertible to `const_iterator`. Users can avoid violating the One Definition Rule by always using `const_iterator` in their function parameter lists -- end note]

### 1043. Clarify that `compare_exchange` is not a read-modify-write operation

Section: 29.6 [atomics.types.operations] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2015-04-08

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Discussion:

Addresses US 91 [CD1]

It is unclear whether or not a failed `compare_exchange` is a RMW operation (as used in 1.10 [intro.multithread]).

Suggested solution:

Make failing `compare_exchange` operations not be RMW.

[ Anthony Williams adds: ]

> In 29.6 [atomics.types.operations] p18 it says that "These operations are atomic read-modify-write operations" (final sentence). This is overly restrictive on the implementations of `compare_exchange_weak` and `compare_exchange_strong` on platforms without a native CAS instruction.

[ Summit: ]

> Group agrees with the resolution as proposed by Anthony Williams in the attached note.

[ Batavia (2009-05): ]

> We recommend the proposed resolution be reviewed by members of the Concurrency Subgroup.

[ 2009-07 post-Frankfurt: ]

> This is likely to be addressed by Lawrence's upcoming paper. He will adopt the proposed resolution.

[ 2009-08-17 Handled by N2925. ]

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Addressed by N2992.

Proposed resolution:

Change 29.6 [atomics.types.operations] p18:

> -18- Effects: Atomically, compares the value pointed to by `object` or by `this` for equality with that in `expected`, and if true, replaces the value pointed to by `object` or by `this` with desired, and if false, updates the value in `expected` with the value pointed to by `object` or by `this`. Further, if the comparison is true, memory is affected according to the value of `success`, and if the comparison is false, memory is affected according to the value of `failure`. When only one `memory_order` argument is supplied, the value of `success` is `order`, and the value of `failure` is `order` except that a value of `memory_order_acq_rel` shall be replaced by the value `memory_order_acquire` and a value of `memory_order_release` shall be replaced by the value `memory_order_relaxed`. If the comparison is `true`, T these operations are atomic read-modify-write operations (1.10). If the comparison is `false`, these operations are atomic load operations.

### 1044. Empty tag types should be `constexpr` literals

Section: 30.4 [thread.mutex] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2015-04-08

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Discussion:

Addresses UK 325 [CD1]

We believe `constexpr` literal values should be a more natural expression of empty tag types than extern objects as it should improve the compiler's ability to optimize the empty object away completely.

[ Summit: ]

> Move to review. The current specification is a "hack", and the proposed specification is a better "hack".

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change the synopsis in 30.4 [thread.mutex]:

>     struct defer_lock_t {};
>     struct try_to_lock_t {};
>     struct adopt_lock_t {};

>     extern constexpr defer_lock_t defer_lock {};
>     extern constexpr try_to_lock_t try_to_lock {};
>     extern constexpr adopt_lock_t adopt_lock {};

### 1045. Remove unnecessary preconditions from `unique_lock` constructor

Section: 30.4.2.2.1 [thread.lock.unique.cons] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2015-04-08

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Discussion:

Addresses UK 326 [CD1]

The precondition that the mutex is not owned by this thread offers introduces the risk of unnecessary undefined behaviour into the program. The only time it matters whether the current thread owns the mutex is in the lock operation, and that will happen subsequent to construction in this case. The lock operation has the identical pre-condition, so there is nothing gained by asserting that precondition earlier and denying the program the right to get into a valid state before calling lock.

[ Summit: ]

> Agree, move to review.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Strike 30.4.2.2.1 [thread.lock.unique.cons] p7:

>     unique_lock(mutex_type& m, defer_lock_t);

> > -7- Precondition: If `mutex_type` is not a recursive mutex the calling thread does not own the mutex.

### 1046. Provide simple facility to start asynchronous operations

Section: 30.6 [futures] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2015-04-08

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Discussion:

Addresses UK 329 [CD1]

`future`, `promise` and `packaged_task` provide a framework for creating future values, but a simple function to tie all three components together is missing. Note that we only need a simple facility for C++0x. Advanced thread pools are to be left for TR2.

Simple Proposal:

Provide a simple function along the lines of:

>     template< typename F, typename ... Args >
>       requires Callable< F, Args... >
>         future< Callable::result_type > async( F&& f, Args && ... );

Semantics are similar to creating a `thread` object with a `packaged_task` invoking `f` with `forward<Args>(args...)` but details are left unspecified to allow different scheduling and thread spawning implementations.

It is unspecified whether a task submitted to async is run on its own thread or a thread previously used for another async task. If a call to `async` succeeds, it shall be safe to wait for it from any thread.

The state of `thread_local` variables shall be preserved during `async` calls.

No two incomplete async tasks shall see the same value of `this_thread::get_id()`.

[Note: this effectively forces new tasks to be run on a new thread, or a fixed-size pool with no queue. If the library is unable to spawn a new thread or there are no free worker threads then the `async` call should fail. --end note]

[ Summit: ]

> The concurrency subgroup has revisited this issue and decided that it could be considered a defect according to the Kona compromise. A task group was formed lead by Lawrence Crowl and Bjarne Stroustrup to write a paper for Frankfort proposing a simple asynchronous launch facility returning a `future`. It was agreed that the callable must be run on a separate thread from the caller, but not necessarily a brand-new thread. The proposal might or might not allow for an implementation that uses fixed-size or unlimited thread pools.

> Bjarne in c++std-lib-23121: I think that what we agreed was that to avoid deadlock `async()` would almost certainly be specified to launch in a different thread from the thread that executed `async()`, but I don't think it was a specific design constraint.

[ 2009-10 Santa Cruz: ]

> Proposed resolution: see N2996 (Herb's and Lawrence's paper on Async). Move state to NAD editorial Resolved.

Proposed resolution:

### 1047. Ensure that future's `get()` blocks when not ready

Section: 30.6.6 [futures.unique_future] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2015-04-08

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Discussion:

Addresses UK 334 [CD1]

Behaviour of `get()` is undefined if calling `get()` while not `is_ready()`. The intent is that `get()` is a blocking call, and will wait for the future to become ready.

[ Summit: ]

> Agree, move to Review.

[ 2009-04-03 Thomas J. Gritzan adds: ]

> This issue also applies to `shared_future::get()`.

> Suggested wording:

> Add a paragraph to 30.6.7 [futures.shared_future]:

> >     void shared_future<void>::get() const;

> > > Effects: If `is_ready()` would return `false`, block on the asynchronous result associated with `*this`.

[ Batavia (2009-05): ]

> It is not clear to us that this is an issue, because the proposed resolution's Effects clause seems to duplicate information already present in the Synchronization clause. Keep in Review status.

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Addressed by N2997.

Proposed resolution:

Add a paragraph to 30.6.6 [futures.unique_future]:

>     R&& unique_future::get();
>     R& unique_future<R&>::get();
>     void unique_future<void>::get();

> > Note:...

> > Effects: If `is_ready()` would return `false`, block on the asynchronous result associated with `*this`.

> > Synchronization: if `*this` is associated with a `promise` object, the completion of `set_value()` or `set_exception()` to that `promise` happens before (1.10) `get()` returns.

### 1048. Provide empty-state inspection for `std::unique_future`

Section: 30.6.6 [futures.unique_future] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2015-04-08

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Discussion:

Addresses UK 335 [CD1]

`std::unique_future` is `MoveConstructible`, so you can transfer the association with an asynchronous result from one instance to another. However, there is no way to determine whether or not an instance has been moved from, and therefore whether or not it is safe to wait for it.

>     std::promise<int> p;
>     std::unique_future<int> uf(p.get_future());
>     std::unique_future<int> uf2(std::move(uf));
>     uf.wait(); // oops, uf has no result to wait for.

Suggest we add a `waitable()` function to `unique_future` (and `shared_future`) akin to `std::thread::joinable()`, which returns `true` if there is an associated result to wait for (whether or not it is ready).

Then we can say:

>     if(uf.waitable()) uf.wait();

[ Summit: ]

> Create an issue. Requires input from Howard. Probably NAD.

[ Post Summit, Howard throws in his two cents: ]

> Here is a copy/paste of my last prototype of `unique_future` which was several years ago. At that time I was calling `unique_future` `future`:

> >     template <class R>
> >     class future
> >     {
> >     public:
> >         typedef R result_type;
> >     private:
> >         future(const future&);// = delete;
> >         future& operator=(const future&);// = delete;

> >         template <class R1, class F1> friend class prommise;
> >     public:
> >         future();
> >         ~future();

> >         future(future&& f);
> >         future& operator=(future&& f);

> >         void swap(future&& f);

> >         bool joinable() const;
> >         bool is_normal() const;
> >         bool is_exceptional() const;
> >         bool is_ready() const;

> >         R get();

> >         void join();
> >         template <class ElapsedTime>
> >             bool timed_join(const ElapsedTime&);
> >     };

> `shared_future` had a similar interface. I intentionally reused the `thread` interface where possible to lessen the learning curve std::lib clients will be faced with.

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Addressed by N2997.

Proposed resolution:

### 1049. Move assignment of `promise` inverted

Section: 30.6.5 [futures.promise] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2015-04-08

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Discussion:

Addresses UK 339 [CD1]

Move assignment is going in the wrong direction, assigning from `*this` to the passed rvalue, and then returning a reference to an unusable `*this`.

[ Summit: ]

> Agree, move to Review.

[ Batavia (2009-05): ]

> We recommend deferring this issue until after Detlef's paper (on futures) has been issued.

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Addressed by N2997.

Proposed resolution:

Strike 30.6.5 [futures.promise] p6 and change p7:

>     promise& operator=(promise&& rhs);

> > -6- Effects: move assigns its associated state to `rhs`.

> > -7- Postcondition: `*this` has no associated state. associated state of `*this` is the same as the associated state of `rhs` before the call. `rhs` has no associated state.

### 1050. Clarify postconditions for `get_future()`

Section: 30.6.5 [futures.promise] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2015-04-08

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Discussion:

Addresses UK 340 [CD1]

There is an implied postcondition for `get_future()` that the state of the `promise` is transferred into the `future` leaving the `promise` with no associated state. It should be spelled out.

[ Summit: ]

> Agree, move to Review.

[ 2009-04-03 Thomas J. Gritzan adds: ]

> `promise::get_future()` must not invalidate the state of the promise object.

> A promise is used like this:

> >     promise<int> p;
> >     unique_future<int> f = p.get_future();
> >     // post 'p' to a thread that calculates a value
> >     // use 'f' to retrieve the value.

> So `get_future()` must return an object that shares the same associated state with `*this`.

> But still, this function should throw an `future_already_retrieved` error when it is called twice.

> `packaged_task::get_future()` throws `std::bad_function_call` if its `future` was already retrieved. It should throw `future_error(future_already_retrieved)`, too.

> Suggested resolution:

> Replace p12/p13 30.6.5 [futures.promise]:

> > -12- Throws: `future_error` if `*this` has no associated state the `future` has already been retrieved.

> > -13- Error conditions: `future_already_retrieved` if `*this` has no associated state the `future` associated with the associated state has already been retrieved.

> > Postcondition: The returned object and `*this` share the associated state.

> Replace p14 30.6.9 [futures.task]:

> > -14- Throws: `std::bad_function_call future_error` if the future associated with the task has already been retrieved.

> > Error conditions: `future_already_retrieved` if the `future` associated with the task has already been retrieved.

> > Postcondition: The returned object and `*this` share the associated task.

[ Batavia (2009-05): ]

> Keep in Review status pending Detlef's forthcoming paper on futures.

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Addressed by N2997.

Proposed resolution:

Add after p13 30.6.5 [futures.promise]:

>     unique_future<R> get_future();

> > -13- ...

> > Postcondition: `*this` has no associated state.

### 1054. `forward` broken

Section: 20.2.4 [forward] Status: Resolved Submitter: Howard Hinnant Opened: 2009-03-13 Last modified: 2015-04-08

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Discussion:

This is a placeholder issue to track the fact that we (well I) put the standard into an inconsistent state by requesting that we accept N2844 except for the proposed changes to [forward].

There will exist in the post meeting mailing N2835 which in its current state reflects the state of affairs prior to the Summit meeting. I hope to update it in time for the post Summit mailing, but as I write this issue I have not done so yet.

[ Batavia (2009-05): ]

> Move to Open, awaiting the promised paper.

[ 2009-08-02 Howard adds: ]

> My current preferred solution is:

> >     template <class T>
> >     struct __base_type
> >     {
> >        typedef typename remove_cv<typename remove_reference<T>::type>::type type;
> >     };

> >     template <class T, class U,
> >        class = typename enable_if<
> >            !is_lvalue_reference<T>::value ||
> >             is_lvalue_reference<T>::value &&
> >             is_lvalue_reference<U>::value>::type,
> >        class = typename enable_if<
> >             is_same<typename __base_type<T>::type,
> >                     typename __base_type<U>::type>::value>::type>
> >     inline
> >     T&&
> >     forward(U&& t)
> >     {
> >        return static_cast<T&&>(t);
> >     }

> This has been tested by Bill, Jason and myself.

> It allows the following lvalue/rvalue casts:

> 1. Cast an lvalue `t` to an lvalue `T` (identity).
> 2. Cast an lvalue `t` to an rvalue `T`.
> 3. Cast an rvalue `t` to an rvalue `T` (identity).

> It disallows:

> 1. Cast an rvalue `t` to an lvalue `T`.
> 2. Cast one type `t` to another type `T` (such as `int` to `double`).

> "a." is disallowed as it can easily lead to dangling references. "b." is disallowed as this function is meant to only change the lvalue/rvalue characteristic of an expression.

> Jason has expressed concern that "b." is not dangerous and is useful in contexts where you want to "forward" a derived type as a base type. I find this use case neither dangerous, nor compelling. I.e. I could live with or without the "b." constraint. Without it, forward would look like:

> >     template <class T, class U,
> >        class = typename enable_if<
> >            !is_lvalue_reference<T>::value ||
> >             is_lvalue_reference<T>::value &&
> >             is_lvalue_reference<U>::value>::type>
> >     inline
> >     T&&
> >     forward(U&& t)
> >     {
> >        return static_cast<T&&>(t);
> >     }

> Or possibly:

> >     template <class T, class U,
> >        class = typename enable_if<
> >            !is_lvalue_reference<T>::value ||
> >             is_lvalue_reference<T>::value &&
> >             is_lvalue_reference<U>::value>::type,
> >        class = typename enable_if<
> >             is_base_of<typename __base_type<U>::type,
> >                        typename __base_type<T>::type>::value>::type>
> >     inline
> >     T&&
> >     forward(U&& t)
> >     {
> >        return static_cast<T&&>(t);
> >     }

> The "promised paper" is not in the post-Frankfurt mailing only because I'm waiting for the non-concepts draft. But I'm hoping that by adding this information here I can keep people up to date.

[ 2009-08-02 David adds: ]

> `forward` was originally designed to do one thing: perfect forwarding. That is, inside a function template whose actual argument can be a const or non-const lvalue or rvalue, restore the original "rvalue-ness" of the actual argument:

> >     template <class T>
> >     void f(T&& x)
> >     {
> >         // x is an lvalue here.  If the actual argument to f was an
> >         // rvalue, pass static_cast<T&&>(x) to g; otherwise, pass x.
> >         g( forward<T>(x) );
> >     }

> Attempting to engineer `forward` to accomodate uses other than perfect forwarding dilutes its idiomatic meaning. The solution proposed here declares that `forward<T>(x)` means nothing more than `static_cast<T&&>(x)`, with a patchwork of restrictions on what `T` and `x` can be that can't be expressed in simple English.

> I would be happy with either of two approaches, whose code I hope (but can't guarantee) I got right.

> 1. Use a simple definition of `forward` that accomplishes its original purpose without complications to accomodate other uses:

>    >     template <class T, class U>
>    >     T&& forward(U& x)
>    >     {
>    >         return static_cast<T&&>(x);
>    >     }

> 2. Use a definition of `forward` that protects the user from as many potential mistakes as possible, by actively preventing all other uses:

>    >     template <class T, class U>
>    >     boost::enable_if_c<
>    >         // in forward<T>(x), x is a parameter of the caller, thus an lvalue
>    >         is_lvalue_reference<U>::value
>    >         // in caller's deduced T&& argument, T can only be non-ref or lvalue ref
>    >         && !is_rvalue_reference<T>::value
>    >         // Must not cast cv-qualifications or do any type conversions
>    >         && is_same<T&,U&>::value
>    >         , T&&>::type forward(U&& a)
>    >     {
>    >         return static_cast<T&&>(a);
>    >     }

[ 2009-09-27 Howard adds: ]

> A paper, N2951, is available which compares several implementations (including David's) with respect to several use cases (including Jason's) and provides wording for one implementation.

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Solved by N2951.

Proposed resolution:

### 1055. Provide a trait that returns the underlying type of an enumeration type

Section: 20.10.7.6 [meta.trans.other] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2015-04-08

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Discussion:

Addresses UK 98 [CD1]

It would be useful to be able to determine the underlying type of an arbitrary enumeration type. This would allow safe casting to an integral type (especially needed for scoped enums, which do not promote), and would allow use of `numeric_limits`. In general it makes generic programming with enumerations easier.

[ Batavia (2009-05): ]

> Pete observes (and Tom concurs) that the proposed resolution seems to require compiler support for its implementation, as it seems necessary to look at the range of values of the enumerated type. To a first approximation, a library solution could give an answer based on the size of the type. If the user has specialized `numeric_limits` for the enumerated type, then the library might be able to do better, but there is no such requirement. Keep status as Open and solicit input from CWG.

[ 2009-05-23 Alisdair adds: ]

> Just to confirm that the BSI originator of this comment assumed it did indeed imply a compiler intrinsic. Rather than request a Core extension, it seemed in keeping with that the type traits interface provides a library API to unspecified compiler features - where we require several other traits (e.g. `has_trivial_*`) to get the 'right' answer now, unlike in TR1.

[ Addressed in N2947. ]

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Solved by N2984.

Proposed resolution:

Add a new row to the table in 20.10.7.6 [meta.trans.other]:

> Table 41 -- Other transformations
> Template | Condition | Comments
> --- | --- | ---
> `template< class T > struct enum_base;` | `T` shall be an enumeration type (7.2 [dcl.enum]) | The member typedef `type` shall name the underlying type of the enum `T`.

### 1065. Allow inline namespaces within namespace `std` for implementations

Section: 17.6.1.1 [contents] Status: C++11 Submitter: Howard Hinnant Opened: 2009-03-15 Last modified: 2015-04-08

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Discussion:

Addresses UK 168 [CD1]

We should make it clear (either by note or normatively) that namespace `std` may contain inline namespaces, and that entities specified to be defined in std may in fact be defined in one of these inline namespaces. (If we're going to use them for versioning, eg when TR2 comes along, we're going to need that.)

Replace "namespace std or namespaces nested within namespace std" with "namespace std or namespaces nested within namespace std or inline namespaces nested directly or indirectly within namespace std"

[ Summit: ]

> adopt UK words (some have reservations whether it is correct)

[ 2009-05-09 Alisdair improves the wording. ]

[ Batavia (2009-05): ]

> Bill believes there is strictly speaking no need to say that because no portable test can detect the difference. However he agrees that it doesn't hurt to say this.

> Move to Tentatively Ready.

Proposed resolution:

Change 17.6.1.1 [contents] p2:

> All library entities except macros, `operator new` and `operator delete` are defined within the namespace `std` or namespaces nested within namespace `std`. It is unspecified whether names declared in a specific namespace are declared directly in that namespace, or in an inline namespace inside that namespace. [Footnote: This gives implementers freedom to support multiple configurations of the library.]

### 1066. Use `[[noreturn]]` attribute in the library

Section: 18 [language.support] Status: C++11 Submitter: Howard Hinnant Opened: 2009-03-15 Last modified: 2015-04-08

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Discussion:

Addresses UK 189 and JP 27 [CD1]

The addition of the `[[noreturn]]` attribute to the language will be an important aid for static analysis tools.

The following functions should be declared in C++ with the `[[noreturn]]` attribute: `abort` `exit` `quick_exit` `terminate` `unexpected` `rethrow_exception` `throw_with_nested`.

[ Summit: ]

> Agreed.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change 18.5 [support.start.term] p3:

> -2- ...

>     void abort [[noreturn]] (void)

> -3- ...

> -6- ...

>     void exit [[noreturn]] (int status)

> -7- ...

> -11- ...

>     void quick_exit [[noreturn]] (int status)

> -12- ...

Change the `<exception>` synopsis in 18.8 [support.exception]:

>     void unexpected [[noreturn]] ();
>     ...
>     void terminate [[noreturn]] ();
>     ...
>     void rethrow_exception [[noreturn]] (exception_ptr p);
>     ...
>     template <class T> void throw_with_nested [[noreturn]] (T&& t); // [[noreturn]]

Change D.8.4 [unexpected]:

>     void unexpected [[noreturn]] ();

Change 18.8.3.4 [terminate]:

>     void terminate [[noreturn]] ();

Change 18.8.5 [propagation]:

>     void rethrow_exception [[noreturn]] (exception_ptr p);

In the synopsis of 18.8.6 [except.nested] and the definition area change:

>     template <class T> void throw_with_nested [[noreturn]] (T&& t); // [[noreturn]]

### 1070. Ambiguous move overloads in function

Section: 20.9.12.2 [func.wrap.func] Status: C++11 Submitter: Howard Hinnant Opened: 2009-03-19 Last modified: 2015-04-08

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Discussion:

The synopsis in 20.9.12.2 [func.wrap.func] says:

>     template<Returnable R, CopyConstructible... ArgTypes>
>     class function<R(ArgTypes...)>
>     {
>         ...
>         template<class F>
>           requires CopyConstructible<F> && Callable<F, ArgTypes...>
>                 && Convertible<Callable<F, ArgTypes...>::result_type, R>
>           function(F);
>         template<class F>
>           requires CopyConstructible<F> && Callable<F, ArgTypes...>
>                 && Convertible<Callable<F, ArgTypes...>::result_type, R>
>           function(F&&);
>         ...
>         template<class F, Allocator Alloc> function(allocator_arg_t, const Alloc&, F);
>         template<class F, Allocator Alloc> function(allocator_arg_t, const Alloc&, F&&);
>         ...
>         template<class F>
>           requires CopyConstructible<F> && Callable<F, ArgTypes..>
>                 && Convertible<Callable<F, ArgTypes...>::result_type
>           function& operator=(F);
>         template<class F>
>           requires CopyConstructible<F> && Callable<F, ArgTypes...>
>                 && Convertible<Callable<F, ArgTypes...>::result_type, R>
>           function& operator=(F&&);
>         ...
>     };

Each of the 3 pairs above are ambiguous. We need only one of each pair, and we could do it with either one. If we choose the `F&&` version we need to bring `decay` into the definition to get the pass-by-value behavior. In the proposed wording I've gotten lazy and just used the pass-by-value signature.

[ 2009-05-01 Daniel adds: ]

> 1024 modifies the second removed constructor.

[ Batavia (2009-05): ]

> We briefly discussed whether we ought support moveable function objects, but decided that should be a separate issue if someone cares to propose it.

> Move to Tentatively Ready.

Proposed resolution:

Change the synopsis of 20.9.12.2 [func.wrap.func], and remove the associated definitions in 20.9.12.2.1 [func.wrap.func.con]:

>     template<Returnable R, CopyConstructible... ArgTypes>
>     class function<R(ArgTypes...)>
>     {
>         ...
>         template<class F>
>           requires CopyConstructible<F> && Callable<F, ArgTypes...>
>                 && Convertible<Callable<F, ArgTypes...>::result_type, R>
>           function(F);
>         template<class F>
>           requires CopyConstructible<F> && Callable<F, ArgTypes...>
>                 && Convertible<Callable<F, ArgTypes...>::result_type, R>
>           function(F&&);
>         ...
>         template<class F, Allocator Alloc> function(allocator_arg_t, const Alloc&, F);
>         template<class F, Allocator Alloc> function(allocator_arg_t, const Alloc&, F&&);
>         ...
>         template<class F>
>           requires CopyConstructible<F> && Callable<F, ArgTypes..>
>                 && Convertible<Callable<F, ArgTypes...>::result_type
>           function& operator=(F);
>         template<class F>
>           requires CopyConstructible<F> && Callable<F, ArgTypes...>
>                 && Convertible<Callable<F, ArgTypes...>::result_type, R>
>           function& operator=(F&&);
>         ...
>     };

### 1071. `is_bind_expression` should derive from `integral_constant<bool>`

Section: 20.9.10.1 [func.bind.isbind] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-19 Last modified: 2015-04-08

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Discussion:

Class template is_bind_expression 20.9.10.1 [func.bind.isbind]:

>     namespace std {
>       template<class T> struct is_bind_expression {
>         static const bool value = see below;
>       };
>     }

`is_bind_expression` should derive from `std::integral_constant<bool>` like other similar trait types.

[ Daniel adds: ]

> We need the same thing for the trait `is_placeholder` as well.

[ 2009-03-22 Daniel provided wording. ]

[ Batavia (2009-05): ]

> We recommend this be deferred until after the next Committee Draft is issued.

> Move to Open.

[ 2009-05-31 Peter adds: ]

> I am opposed to the proposed resolution and to the premise of the issue in general. The traits's default definitions should NOT derive from `integral_constant`, because this is harmful, as it misleads people into thinking that `is_bind_expression<E>` always derives from `integral_constant`, whereas it may not.

> `is_bind_expression` and `is_placeholder` allow user specializations, and in fact, this is their primary purpose. Such user specializations may not derive from `integral_constant`, and the places where `is_bind_expression` and `is_placeholder` are used intentionally do not require such derivation.

> The long-term approach here is to switch to `BindExpression<E>` and `Placeholder<P>` explicit concepts, of course, but until that happens, I say leave them alone.

[ 2009-10 post-Santa Cruz: ]

> Move to Tentatively Ready. We are comfortable with requiring user specializations to derive from `integral_constant`.

Proposed resolution:

1. In 20.9.10.1 [func.bind.isbind] change as indicated:

   >     namespace std {
   >      template<class T> struct is_bind_expression : integral_constant<bool, see below> { };{
   >        static const bool value = see below;
   >      };
   >     }

2. In 20.9.10.1 [func.bind.isbind]/2 change as indicated:

   >     static const bool value;

   > > -2- `true` if `T` is a type returned from `bind`, `false` otherwise. If `T` is a type returned from `bind`, `is_bind_expression<T>` shall be publicly derived from `integral_constant<bool, true>`, otherwise it shall be publicly derived from `integral_constant<bool, false>`.

3. In 20.9.10.2 [func.bind.isplace] change as indicated:

   >     namespace std {
   >      template<class T> struct is_placeholder : integral_constant<int, see below> { };{
   >        static const int value = see below;
   >      };
   >     }

4. In 20.9.10.2 [func.bind.isplace]/2 change as indicated:

   >     static const int value;

   > > -2- value is `J` if `T` is the type of `std::placeholders::_J`, 0 otherwise. If `T` is the type of `std::placeholders::_J`, `is_placeholder<T>` shall be publicly derived from `integral_constant<int, J>` otherwise it shall be publicly derived from `integral_constant<int, 0>`.

### 1073. Declaration of `allocator_arg` should be `constexpr`

Section: 20.7 [memory] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-19 Last modified: 2015-04-08

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Discussion:

Declaration of `allocator_arg` should be `constexpr` to ensure constant initialization.

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change 20.7 [memory] p2:

>     // 20.8.1, allocator argument tag
>     struct allocator_arg_t { };
>     constexpr allocator_arg_t allocator_arg = allocator_arg_t();

### 1075. Scoped allocators are too complex

Section: 20 [utilities], 23 [containers] Status: Resolved Submitter: Alan Talbot Opened: 2009-03-20 Last modified: 2015-04-08

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Discussion:

Addresses US 65 and US 74.1 [CD1]

US 65:

> Scoped allocators and allocator propagation traits add a small amount of utility at the cost of a great deal of machinery. The machinery is user visible, and it extends to library components that don't have any obvious connection to allocators, including basic concepts and simple components like `pair` and `tuple`.

> Suggested resolution:

> Sketch of proposed resolution: Eliminate scoped allocators, replace allocator propagation traits with a simple uniform rule (e.g. always propagate on copy and move), remove all mention of allocators from components that don't explicitly allocate memory (e.g. pair), and adjust container interfaces to reflect this simplification.

> Components that I propose eliminating include `HasAllocatorType`, `is_scoped_allocator`, `allocator_propagation_map`, `scoped_allocator_adaptor`, and `ConstructibleAsElement`.

US 74.1:

> Scoped allocators represent a poor trade-off for standardization, since (1) scoped-allocator--aware containers can be implemented outside the C++ standard library but used with its algorithms, (2) scoped allocators only benefit a tiny proportion of the C++ community (since few C++ programmers even use today's allocators), and (3) all C++ users, especially the vast majority of the C++ community that won't ever use scoped allocators are forced to cope with the interface complexity introduced by scoped allocators.

> In essence, the larger community will suffer to support a very small subset of the community who can already implement their own data structures outside of the standard library. Therefore, scoped allocators should be removed from the working paper.

> Some evidence of the complexity introduced by scoped allocators:

> > 20.3 [pairs], 20.4 [tuple]: Large increase in the number of pair and tuple constructors.

> > 23 [containers]: Confusing "AllocatableElement" requirements throughout.

> Suggested resolution:

> Remove support for scoped allocators from the working paper. This includes at least the following changes:

> > Remove X [allocator.element.concepts]

> > Remove 20.13 [allocator.adaptor]

> > Remove [construct.element]

> > In Clause 23 [containers]: replace requirements naming the `AllocatableElement` concept with requirements naming `CopyConstructible`, `MoveConstructible`, `DefaultConstructible`, or `Constructible`, as appropriate.

[ Post Summit Alan moved from NAD to Open. ]

[ 2009-05-15 Ganesh adds: ]

> The requirement `AllocatableElement` should not be replaced with `Constructible` on the `emplace_xxx()` functions as suggested. In the one-parameter case the `Constructible` requirement is not satisfied when the constructor is explicit (as per [concept.map.fct], twelfth bullet) but we do want to allow explicit constructors in emplace, as the following example shows:

> >     vector<shared_ptr<int>> v;
> >     v.emplace_back(new int); // should be allowed

> If the issue is accepted and scoped allocators are removed, I suggest to add a new pair of concepts to [concept.construct], namely:

> >     auto concept HasExplicitConstructor<typename T, typename... Args> {
> >      explicit T::T(Args...);
> >     }

> >     auto concept ExplicitConstructible<typename T, typename... Args>
> >      : HasExplicitConstructor<T, Args...>, NothrowDestructible<T>
> >     { }

> We should then use `ExplicitConstructible` as the requirement for all `emplace_xxx()` member functions.

> For coherence and consistency with the similar concepts `Convertible/ExplicitlyConvertible`, we might also consider changing `Constructible` to:

> >     auto concept Constructible<typename T, typename... Args>
> >      : HasConstructor<T, Args...>, ExplicitConstructible<T, Args...>
> >     { }

> Moreover, all emplace-related concepts in [container.concepts] should also use `ExplicitConstructible` instead of `Constructible` in the definitions of their axioms. In fact the concepts in [container.concepts] should be corrected even if the issue is not accepted.

> On the other hand, if the issue is not accepted, the scoped allocator adaptors should be fixed because the following code:

> >     template <typename T> using scoped_allocator = scoped_allocator_adaptor<allocator<T>>;

> >     vector<shared_ptr<int>, scoped_allocator<shared_ptr<int>>> v;
> >     v.emplace_back(new int); // ops! doesn't compile

> doesn't compile, as the member function `construct()` of the scoped allocator requires non-explicit constructors through concept `ConstructibleWithAllocator`. Fixing that is not difficult but probably more work than it's worth and is therefore, IMHO, one more reason in support of the complete removal of scoped allocators.

[ 2009-06-09 Alan adds: ]

> I reopened this issue because I did not think that these National Body comments were adequately addressed by marking them NAD. My understanding is that something can be marked NAD if it is clearly a misunderstanding or trivial, but a substantive issue that has any technical merit requires a disposition that addresses the concerns.

> The notes in the NB comment list (US 65 & US 74.1) say that:

> 1. this issue has not introduced any new arguments not previously discussed,
> 2. the vote (4-9-3) was not a consensus for removing scoped allocators,
> 3. the issue is resolved by N2840.

> My opinion is:

> 1. there are new arguments in both comments regarding concepts (which were not present in the library when the scoped allocator proposal was voted in),
> 2. the vote was clearly not a consensus for removal, but just saying there was a vote does not provide a rationale,
> 3. I do not believe that N2840 addresses these comments (although it does many other things and was voted in with strong approval).

> My motivation to open the issue was to ensure that the NB comments were adequately addressed in a way that would not risk a "no" vote on our FCD. If there are responses to the technical concerns raised, then perhaps they should be recorded. If the members of the NB who authored the comments are satisfied with N2840 and the other disposition remarks in the comment list, then I am sure they will say so. In either case, this issue can be closed very quickly in Frankfurt, and hopefully will have helped make us more confident of approval with little effort. If in fact there is controversy, my thought is that it is better to know now rather than later so there is more time to deal with it.

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Addressed by N2982.

Proposed resolution:

Rationale:

Scoped allocators have been revised significantly.

### 1079. UK-265: `RandomAccessIterator`'s `operator-` has nonsensical effects clause

Section: 24.2.7 [random.access.iterators] Status: C++11 Submitter: Doug Gregor Opened: 2009-03-20 Last modified: 2015-04-08

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Discussion:

Addresses UK 265

UK-265:

This effects clause is nonesense. It looks more like an axiom stating equivalence, and certainly an effects clause cannot change the state of two arguments passed by const reference

[ 2009-09-18 Alisdair adds: ]

> For random access iterators, the definitions of `(b-a)` and `(a<b)` are circular:

> From table Table 104 -- Random access iterator requirements:

> >     b - a :==>  (a < b) ? distance(a,b) : -distance(b,a)

> >     a < b :==>  b - a > 0

[ 2009-10 Santa Cruz: ]

> Moved to Ready.

[ 2010-02-13 Alisdair opens. ]

> Looking again at LWG 1079, the wording in the issue no longer exists, and appears to be entirely an artefact of the concepts wording.

> This issue is currently on our Ready list (not even Tentative!) but I think it has to be pulled as there is no way to apply the resolution.

> Looking at the current paper, I think this issue is now "NAD, solved by the removal of concepts". Unfortunately it is too late to poll again, so we will have to perform that review in Pittsburgh.

[ 2010-02-13 Daniel updates the wording to address the circularity problem. ]

> [ The previous wording is preserved here: ]

> > Modify 24.2.7 [random.access.iterators]p7-9 as follows:

> > >     difference_type operator-(const X& a, const X& b);

> > > > -7- Precondition: there exists a value `n` of `difference_type` such that `a == b + n`.

> > > > -8- Effects: `b == a + (b - a)`

> > > > -9- Returns: `(a < b) ? distance(a,b) : -distance(b,a)``n`

[ 2010 Pittsburgh: ]

> Moved to Ready for Pittsburgh.

Proposed resolution:

Modify Table 105 in 24.2.7 [random.access.iterators]:

> Table 105 — Random access iterator requirements (in addition to bidirectional iterator)
> Expression | Return type | Operational semantics | Assertion/note pre-/post-condition
> --- | --- | --- | ---
> `b - a` | `Distance` | `distance(a,b)` return `n` | pre: there exists a value `n` of `Distance` such that `a + n == b`. `b == a + (b - a)`.

### 1088. `std::promise` should provide non-member `swap` overload

Section: 30.6.5 [futures.promise] Status: Resolved Submitter: Howard Hinnant Opened: 2009-03-22 Last modified: 2015-04-08

View other active issues in [futures.promise].

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Discussion:

Addresses UK 342 [CD1]

`std::promise` is missing a non-member overload of `swap`. This is inconsistent with other types that provide a `swap` member function.

Add a non-member overload `void swap(promise&& x,promise&& y){ x.swap(y); }`

[ Summit: ]

> Create an issue. Move to review, attention: Howard. Detlef will also look into it.

[ Post Summit Daniel provided wording. ]

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Addressed by N2997.

Proposed resolution:

1. In 30.6.5 [futures.promise], before p.1, immediately after class template promise add:

   >     template <class R>
   >     void swap(promise<R>& x, promise<R>& y);

2. Change 30.6.5 [futures.promise]/10 as indicated (to fix a circular definition):

   > -10- Effects: swap(*this, other)Swaps the associated state of `*this` and `other`

   > Throws: Nothing.

3. After the last paragraph in 30.6.5 [futures.promise] add the following prototype description:

   >     template <class R>
   >     void swap(promise<R>& x, promise<R>& y);

   > > Effects: `x.swap(y)`

   > > Throws: Nothing.

### 1089. Unify "Throws: Nothing." specifications

Section: 30 [thread] Status: C++11 Submitter: Howard Hinnant Opened: 2009-03-22 Last modified: 2015-04-08

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Discussion:

Addresses JP 76 [CD1]

A description for "Throws: Nothing." are not unified.

At the part without throw, "Throws: Nothing." should be described.

Add "Throws: Nothing." to the following.

- 30.3.1.6 [thread.thread.static] p1
- 30.4.2.1 [thread.lock.guard] p4
- 30.4.2.2.1 [thread.lock.unique.cons] p6
- 30.5.1 [thread.condition.condvar] p7 and p8
- 30.5.2 [thread.condition.condvarany] p6, p7, p19, p21 and p25

[ Summit: ]

> Pass on to editor.

[ Post Summit: Editor declares this non-editorial. ]

[ 2009-08-01 Howard provided wording: ]

> The definition of "Throws: Nothing." that I added is probably going to be controversial, but I beg you to consider it seriously.

> > In C++ there are three "flow control" options for a function:

> > 1. It can return, either with a value, or with `void`.
> > 2. It can call a function which never returns, such as `std::exit` or `std::terminate`.
> > 3. It can throw an exception.

> > The above list can be abbreviated with:

> > 1. Returns.
> > 2. Ends program.
> > 3. Throws exception.

> > In general a function can have the behavior of any of these 3, or any combination of any of these three, depending upon run time data.

> > 1. R
> > 2. E
> > 3. T
> > 4. RE
> > 5. RT
> > 6. ET
> > 7. RET

> > A function with no throw spec, and no documentation, is in general a RET function. It may return, it may end the program, or it may throw. When we specify a function with an empty throw spec:

> > >     void f() throw();

> > We are saying that `f()` is an RE function: It may return or end the program, but it will not throw.

> > I posit that there are very few places in the library half of the standard where we intend for functions to be able to end the program (call `terminate`). And none of those places where we do say `terminate` could be called, do we currently say "Throws: Nothing.".

> > I believe that if we define "Throws: Nothing." to mean R, we will both clarify many, many places in the standard, and give us a good rationale for choosing between "Throws: Nothing." (R) and `throw()` (RE) in the future. Indeed, this may give us motivation to change several `throw()`s to "Throws: Nothing.".

> I did not add the following changes as JP 76 requested as I believe we want to allow these functions to throw:

> > Add a paragraph under 30.4.2.1 [thread.lock.guard] p4:

> > >     explicit lock_guard(mutex_type& m);

> > > Throws: Nothing.

> > Add a paragraph under 30.4.2.2.1 [thread.lock.unique.cons] p6:

> > >     explicit unique_lock(mutex_type& m);

> > > Throws: Nothing.

> > Add a paragraph under 30.5.2 [thread.condition.condvarany] p19, p21 and p25:

> > >     template <class Lock, class Rep, class Period>
> > >       bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);

> > > Throws: Nothing.

> > >     template <class Lock, class Duration, class Predicate>
> > >       bool wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& rel_time, Predicate pred);

> > > Throws: Nothing.

> > >     template <class Lock, class Rep, class Period, class Predicate>
> > >       bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);

> > > Throws: Nothing.

[ 2009-10 Santa Cruz: ]

> Defer pending further developments with exception restriction annotations.

[ 2010-02-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010-02-24 Pete moved to Open: ]

> A "Throws: Nothing" specification is not the place to say that a function is not allowed to call `exit()`. While I agree with the thrust of the proposed resolution, "doesn't throw exceptions" is a subset of "always returns normally". If it's important to say that most library functions don't call `exit()`, say so.

[ 2010 Pittsburgh: ]

> Move to Ready except for the added paragraph to 17.5.1.4 [structure.specifications].

Proposed resolution:

Add a paragraph under 30.3.1.6 [thread.thread.static] p1:

>     unsigned hardware_concurrency();

> -1- Returns: ...

> Throws: Nothing.

Add a paragraph under 30.5.1 [thread.condition.condvar] p7 and p8:

> [Informational, not to be incluced in the WP: The POSIX spec allows only:

> [EINVAL]
>   The value `cond` does not refer to an initialized condition variable. — end informational]

>     void notify_one();

> -7- Effects: ...

> Throws: Nothing.

>     void notify_all();

> -8- Effects: ...

> Throws: Nothing.

Add a paragraph under 30.5.2 [thread.condition.condvarany] p6 and p7:

>     void notify_one();

> -6- Effects: ...

> Throws: Nothing.

>     void notify_all();

> -7- Effects: ...

> Throws: Nothing.

### 1090. Missing description of `packaged_task` member `swap`, missing non-member `swap`

Section: 30.6.9 [futures.task] Status: Resolved Submitter: Daniel Krügler Opened: 2009-03-22 Last modified: 2015-04-08

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Discussion:

Class template `packaged_task` in 30.6.9 [futures.task] shows a member `swap` declaration, but misses to document it's effects (No prototype provided). Further on this class misses to provide a non-member swap.

[ Batavia (2009-05): ]

> Alisdair notes that paragraph 2 of the proposed resolution has already been applied in the current Working Draft.

> We note a pending `future`-related paper by Detlef; we would like to wait for this paper before proceeding.

> Move to Open.

[ 2009-05-24 Daniel removed part 2 of the proposed resolution. ]

[ 2009-10 post-Santa Cruz: ]

> Move to Tentatively Ready, removing bullet 3 from the proposed resolution but keeping the other two bullets.

[ 2010 Pittsburgh: ]

> Moved to NAD Editorial Resolved. Rationale added below.

Rationale:

Solved by N3058.

Proposed resolution:

1. In 30.6.9 [futures.task], immediately after the definition of class template packaged_task add:

   >     template<class R, class... Argtypes>
   >     void swap(packaged_task<R(ArgTypes...)>&, packaged_task<R(ArgTypes...)>&);

2. At the end of 30.6.9 [futures.task] (after p. 20), add the following prototype description:

   >     template<class R, class... Argtypes>
   >     void swap(packaged_task<R(ArgTypes...)>& x, packaged_task<R(ArgTypes...)>& y);

   > > Effects: `x.swap(y)`

   > > Throws: Nothing.

### 1093. Multiple definitions for `random_shuffle` algorithm

Section: 25.3.12 [alg.random.shuffle] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-22 Last modified: 2015-04-08

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Discussion:

There are a couple of issues with the declaration of the `random_shuffle` algorithm accepting a random number engine.

1. The Iterators must be shuffle iterators, yet this requirement is missing.
2. The `RandomNumberEngine` concept is now provided by the random number library (n2836) and the placeholder should be removed.

[ 2009-05-02 Daniel adds: ]

> this issue completes adding necessary requirement to the third new `random_shuffle` overload. The current suggestion is:

> >     template<RandomAccessIterator Iter, UniformRandomNumberGenerator Rand>
> >     requires ShuffleIterator<Iter>
> >     void random_shuffle(Iter first, Iter last, Rand&& g);

> IMO this is still insufficient and I suggest to add the requirement

> >     Convertible<Rand::result_type, Iter::difference_type>

> to the list (as the two other overloads already have).

> Rationale:

> > Its true that this third overload is somewhat different from the remaining two. Nevertheless we know from `UniformRandomNumberGenerator`, that it's `result_type` is an integral type and that it satisfies `UnsignedIntegralLike<result_type>`.

> > To realize it's designated task, the algorithm has to invoke the `Callable` aspect of `g` and needs to perform some algebra involving it's `min()/max()` limits to compute another index value that at this point is converted into `Iter::difference_type`. This is so, because 24.2.7 [random.access.iterators] uses this type as argument of it's algebraic operators. Alternatively consider the equivalent iterator algorithms in 24.4.4 [iterator.operations] with the same result.

> > This argument leads us to the conclusion that we also need `Convertible<Rand::result_type, Iter::difference_type>` here.

[ Batavia (2009-05): ]

> Alisdair notes that point (ii) has already been addressed.

> We agree with the proposed resolution to point (i) with Daniel's added requirement.

> Move to Review.

[ 2009-06-05 Daniel updated proposed wording as recommended in Batavia. ]

[ 2009-07-28 Alisdair adds: ]

> Revert to Open, with a note there is consensus on direction but the wording needs updating to reflect removal of concepts.

[ 2009-10 post-Santa Cruz: ]

> Leave Open, Walter to work on it.

[ 2010 Pittsburgh: Moved to NAD Editorial Resolved, addressed by N3056. ]

Rationale:

Solved by N3056.

Proposed resolution:

Change in [algorithms.syn] and 25.3.12 [alg.random.shuffle]:

>     concept UniformRandomNumberGenerator<typename Rand> { }
>     template<RandomAccessIterator Iter, UniformRandomNumberGenerator Rand>
>       requires ShuffleIterator<Iter> &&
>       Convertible<Rand::result_type, Iter::difference_type>
>       void random_shuffle(Iter first, Iter last, Rand&& g);

### 1094. Replace "unspecified-bool-type" by "`explicit operator bool() const`" in I/O library

Section: 27.5.5.4 [iostate.flags] Status: C++11 Submitter: P.J. Plauger Opened: 2009-03-24 Last modified: 2015-04-08

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Discussion:

Addresses JP 65 and JP 66 [CD1]

Switch from "unspecified-bool-type" to "explicit operator bool() const".

Replace `operator unspecified-bool-type() const;`" with `explicit operator bool() const;`

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Review.

[ 2009 Santa Cruz: ]

> Moved to Ready.

Proposed resolution:

Change the synopis in 27.5.5 [ios]:

>     explicit operator unspecified-bool-type bool() const;

Change 27.5.5.4 [iostate.flags]:

>     explicit operator unspecified-bool-type bool() const;

> > -1- Returns: `!fail()` If `fail()` then a value that will evaluate false in a boolean context; otherwise a value that will evaluate true in a boolean context. The value type returned shall not be convertible to int.

> > [Note: This conversion can be used in contexts where a bool is expected (e.g., an `if` condition); however, implicit conversions (e.g., to `int`) that can occur with `bool` are not allowed, eliminating some sources of user error. One possible implementation choice for this type is pointer-to-member. -- end note]

### 1095. Shared objects and the library wording unclear

Section: 17.6.4.10 [res.on.objects] Status: C++11 Submitter: Beman Dawes Opened: 2009-03-27 Last modified: 2015-04-08

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Discussion:

N2775, Small library thread-safety revisions, among other changes, removed a note from 17.6.4.10 [res.on.objects] that read:

> [Note: This prohibition against concurrent non-const access means that modifying an object of a standard library type shared between threads without using a locking mechanism may result in a data race. --end note.]

That resulted in wording which is technically correct but can only be understood by reading the lengthy and complex 17.6.5.9 [res.on.data.races] Data race avoidance. This has the effect of making 17.6.4.10 [res.on.objects] unclear, and has already resulted in a query to the LWG reflector. See c++std-lib-23194.

[ Batavia (2009-05): ]

> The proposed wording seems to need a bit of tweaking ("really bad idea" isn't quite up to standardese). We would like feedback as to whether the original Note's removal was intentional.

> Change the phrase "is a really bad idea" to "risks undefined behavior" and move to Review status.

[ 2009-10 Santa Cruz: ]

> Note: Change to read: "Modifying...", Delete 'thus', move to Ready

Proposed resolution:

Change 17.6.4.10 [res.on.objects] as indicated:

> The behavior of a program is undefined if calls to standard library functions from different threads may introduce a data race. The conditions under which this may occur are specified in 17.6.4.7.

> [Note: Modifying an object of a standard library type shared between threads risks undefined behavior unless objects of the type are explicitly specified as being sharable without data races or the user supplies a locking mechanism. --end note]

### 1097. #define __STDCPP_THREADS

Section: 18.2 [support.types] Status: C++11 Submitter: Jens Maurer Opened: 2009-04-03 Last modified: 2015-04-08

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Discussion:

Addresses DE 18

Freestanding implementations do not (necessarily) have support for multiple threads (see 1.10 [intro.multithread]). Applications and libraries may want to optimize for the absence of threads. I therefore propose a preprocessor macro to indicate whether multiple threads can occur.

There is ample prior implementation experience for this feature with various spellings of the macro name. For example, gcc implicitly defines `_REENTRANT` if multi-threading support is selected on the compiler command-line.

While this is submitted as a library issue, it may be more appropriate to add the macro in 16.8 cpp.predefined in the core language.

See also N2693.

[ Batavia (2009-05): ]

> We agree with the issue, and believe it is properly a library issue.

> We prefer that the macro be conditionally defined as part of the `<thread>` header.

> Move to Review.

[ 2009-10 Santa Cruz: ]

> Move to Ready.

[ 2010-02-25 Pete moved to Open: ]

> The proposed resolution adds a feature-test macro named `__STDCPP_THREADS`, described after the following new text:

> > The standard library defines the following macros; no explicit prior inclusion of any header file is necessary.

> The correct term here is "header", not "header file". But that's minor. The real problem is that library entities are always defined in headers. If `__STDCPP_THREADS` is defined without including any header it's part of the language and belongs with the other predefined macros in the Preprocessor clause.

> Oddly enough, the comments from Batavia say "We prefer that the macro be conditionally defined as part of the `<thread>` header." There's no mention of a decision to change this.

[ 2010-02-26 Ganesh updates wording. ]

[ 2010 Pittsburgh: Adopt Ganesh's wording and move to Review. ]

[ 2010-03-08 Pete adds: ]

> Most macros we have begin and end with with double underbars, this one only begins with double underbars.

[ 2010 Pittsburgh: Ganesh's wording adopted and moved to Ready for Pittsburgh. ]

Proposed resolution:

Change 17.6.1.3 [compliance]/3:

> 3 The supplied version of the header `<cstdlib>` shall declare at least the functions `abort()`, `atexit()`, and `exit()` (18.5). The supplied version of the header `<thread>` either shall meet the same requirements as for a hosted implementation or including it shall have no effect. The other headers listed in this table shall meet the same requirements as for a hosted implementation.

Add the following line to table 15:

> Table 15 — C++ headers for freestanding implementations
> Subclause | Header(s)
> --- | ---
> ...
> 30.3 [thread.threads] Threads | `<thread>`

Add to the `<thread>` synopsis in 30.3 [thread.threads]/1 the line:

>     namespace std {

>     #define __STDCPP_THREADS __cplusplus

>       class thread;
>       ...

### 1098. definition of `get_pointer_safety()`

Section: 20.7.4 [util.dynamic.safety] Status: C++11 Submitter: Jens Maurer Opened: 2009-04-03 Last modified: 2015-04-08

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Discussion:

Addresses DE 18

In 20.7.4 [util.dynamic.safety], `get_pointer_safety()` purports to define behavior for non-safely derived pointers (3.7.4.3 [basic.stc.dynamic.safety]). However, the cited core-language section in paragraph 4 specifies undefined behavior for the use of such pointer values. This seems an unfortunate near-contradiction. I suggest to specify the term relaxed pointer safety in the core language section and refer to it from the library description. This issue deals with the library part, the corresponding core issue (c++std-core-13940) deals with the core modifications.

See also N2693.

[ Batavia (2009-05): ]

> We recommend if this issue is to be moved, the issue be moved concurrently with the cited Core issue.

> We agree with the intent of the proposed resolution. We would like input from garbage collection specialists.

> Move to Open.

[ 2009-10 Santa Cruz: ]

> The core issue is 853 and is in Ready status.

Proposed resolution:

In 20.7.4 [util.dynamic.safety] p16, replace the description of `get_pointer_safety()` with:

> `pointer_safety get_pointer_safety();`

> > Returns: an enumeration value indicating the implementation's treatment of pointers that are not safely derived (3.7.4.3). Returns `pointer_safety::relaxed` if pointers that are not safely derived will be treated the same as pointers that are safely derived for the duration of the program. Returns `pointer_safety::preferred` if pointers that are not safely derived will be treated the same as pointers that are safely derived for the duration of the program but allows the implementation to hint that it could be desirable to avoid dereferencing pointers that are not safely derived as described. [Example: `pointer_safety::preferred` might be returned to detect if a leak detector is running to avoid spurious leak reports. -- end note] Returns `pointer_safety::strict` if pointers that are not safely derived might be treated differently than pointers that are safely derived.

> > Returns: Returns `pointer_safety::strict` if the implementation has strict pointer safety (3.7.4.3 [basic.stc.dynamic.safety]). It is implementation-defined whether `get_pointer_safety` returns `pointer_safety::relaxed` or `pointer_safety::preferred` if the implementation has relaxed pointer safety (3.7.4.3 [basic.stc.dynamic.safety]).Footnote

> > Throws: nothing

> > Footnote) `pointer_safety::preferred` might be returned to indicate to the program that a leak detector is running so that the program can avoid spurious leak reports.

### 1100. `auto_ptr` to `unique_ptr` conversion

Section: 20.8.1.2.1 [unique.ptr.single.ctor] Status: Resolved Submitter: Howard Hinnant Opened: 2009-04-25 Last modified: 2015-04-08

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Discussion:

Message c++std-lib-23182 led to a discussion in which several people expressed interest in being able to convert an `auto_ptr` to a `unique_ptr` without the need to call `release`. Below is wording to accomplish this.

[ Batavia (2009-05): ]

> Pete believes it not a good idea to separate parts of a class's definition. Therefore, if we do this, it should be part of `unique-ptr`'s specification.

> Alisdair believes the lvalue overload may be not necessary.

> Marc believes it is more than just sugar, as it does ease the transition to `unique-ptr`.

> We agree with the resolution as presented. Move to Tentatively Ready.

[ 2009-07 Frankfurt ]

> Moved from Tentatively Ready to Open only because the wording needs to be tweaked for concepts removal.

[ 2009-08-01 Howard deconceptifies wording: ]

> I also moved the change from X [depr.auto.ptr] to 20.8.1.2 [unique.ptr.single] per the Editor's request in Batavia (as long as I was making changes anyway). Set back to Review.

[ 2009-10 Santa Cruz: ]

> Move to Ready.

[ 2010-03-14 Howard adds: ]

> We moved N3073 to the formal motions page in Pittsburgh which should obsolete this issue. I've moved this issue to NAD Editorial, solved by N3073.

Rationale:

Solved by N3073.

Proposed resolution:

Add to 20.8.1.2 [unique.ptr.single]:

>     template <class T, class D>
>     class unique_ptr
>     {
>     public:
>         template <class U>
>           unique_ptr(auto_ptr<U>& u);
>         template <class U>
>           unique_ptr(auto_ptr<U>&& u);
>     };

Add to 20.8.1.2.1 [unique.ptr.single.ctor]:

>     template <class U>
>       unique_ptr(auto_ptr<U>& u);
>     template <class U>
>       unique_ptr(auto_ptr<U>&& u);

> > Effects: Constructs a `unique_ptr` with `u.release()`.

> > Postconditions: `get() ==` the value `u.get()` had before the construciton, modulo any required offset adjustments resulting from the cast from `U*` to `T*`. `u.get() == nullptr`.

> > Throws: nothing.

> > Remarks: `U*` shall be implicitly convertible to `T*` and `D` shall be the same type as `default_delete<T>`, else these constructors shall not participate in overload resolution.

### 1103. `system_error` constructor postcondition overly strict

Section: 19.5.6.2 [syserr.syserr.members] Status: C++11 Submitter: Howard Hinnant Opened: 2009-04-25 Last modified: 2015-04-08

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Discussion:

19.5.6.2 [syserr.syserr.members] says:

>     system_error(error_code ec, const string& what_arg);

> > Effects: Constructs an object of class `system_error`.

> > Postconditions: `code() == ec` and `strcmp(runtime_error::what(), what_arg.c_str()) == 0`.

However the intent is for:

>     std::system_error se(std::errc::not_a_directory, "In FooBar");
>     ...
>     se.what();  // returns something along the lines of:
>                 //   "In FooBar: Not a directory"

The way the constructor postconditions are set up now, to achieve both conformance, and the desired intent in the `what()` string, the `system_error` constructor must store "In FooBar" in the base class, and then form the desired output each time `what()` is called. Or alternatively, store "In FooBar" in the base class, and store the desired `what()` string in the derived `system_error`, and override `what()` to return the string in the derived part.

Both of the above implementations seem suboptimal to me. In one I'm computing a new string every time `what()` is called. And since `what()` can't propagate exceptions, the client may get a different string on different calls.

The second solution requires storing two strings instead of one.

What I would like to be able to do is form the desired `what()` string once in the `system_error` constructor, and store that in the base class. Now I'm:

1. Computing the desired `what()` only once.
2. The base class `what()` definition is sufficient and nothrow.
3. I'm not storing multiple strings.

This is smaller code, smaller data, and faster.

`ios_base::failure` has the same issue.

[ Comments about this change received favorable comments from the `system_error` designers. ]

[ Batavia (2009-05): ]

> We agree with the proposed resolution.

> Move to Tentatively Ready.

Proposed resolution:

In 19.5.6.2 [syserr.syserr.members], change the following constructor postconditions:

>     system_error(error_code ec, const string& what_arg);

> > -2- Postconditions: `code() == ec` and `strcmp(runtime_error::what(), what_arg.c_str()) == 0 string(what()).find(what_arg) != string::npos`.

>     system_error(error_code ec, const char* what_arg);

> > -4- Postconditions: `code() == ec` and `strcmp(runtime_error::what(), what_arg) == 0 string(what()).find(what_arg) != string::npos`.

>     system_error(error_code ec);

> > -6- Postconditions: `code() == ec` and `strcmp(runtime_error::what(), ""`.

>     system_error(int ev, const error_category& ecat, const string& what_arg);

> > -8- Postconditions: `code() == error_code(ev, ecat)` and `strcmp(runtime_error::what(), what_arg.c_str()) == 0 string(what()).find(what_arg) != string::npos`.

>     system_error(int ev, const error_category& ecat, const char* what_arg);

> > -10- Postconditions: `code() == error_code(ev, ecat)` and `strcmp(runtime_error::what(), what_arg) == 0 string(what()).find(what_arg) != string::npos`.

>     system_error(int ev, const error_category& ecat);

> > -12- Postconditions: `code() == error_code(ev, ecat)` and `strcmp(runtime_error::what(), "") == 0`.

In 19.5.6.2 [syserr.syserr.members], change the description of `what()`:

>     const char *what() const throw();

> > -14- Returns: An NTBS incorporating `runtime_error::what()` and `code().message()` the arguments supplied in the constructor.

> > [Note: One possible implementation would be: The return NTBS might take the form: `what_arg + ": " + code().message()`

> > >     if (msg.empty()) {
> > >       try {
> > >         string tmp = runtime_error::what();
> > >         if (code()) {
> > >           if (!tmp.empty())
> > >             tmp += ": ";
> > >           tmp += code().message();
> > >         }
> > >         swap(msg, tmp);
> > >       } catch(...) {
> > >         return runtime_error::what();
> > >       }
> > >     return msg.c_str();

> > — end note]

In 27.5.3.1.1 [ios::failure], change the synopsis:

>     namespace std {
>       class ios_base::failure : public system_error {
>       public:
>         explicit failure(const string& msg, const error_code& ec = io_errc::stream);
>         explicit failure(const char* msg, const error_code& ec = io_errc::stream);
>         virtual const char* what() const throw();
>       };
>     }

In 27.5.3.1.1 [ios::failure], change the description of the constructors:

>     explicit failure(const string& msg, , const error_code& ec = io_errc::stream);

> > -3- Effects: Constructs an object of class `failure` by constructing the base class with `msg` and `ec`.

> > -4- Postcondition: `code() == ec` and `strcmp(what(), msg.c_str()) == 0`

>     explicit failure(const char* msg, const error_code& ec = io_errc::stream);

> > -5- Effects: Constructs an object of class `failure` by constructing the base class with `msg` and `ec`.

> > -6- Postcondition: `code() == ec and strcmp(what(), msg) == 0`

In 27.5.3.1.1 [ios::failure], remove `what` (the base class definition need not be repeated here).

>     const char* what() const;

> > -7- Returns: The message `msg` with which the exception was created.

### 1104. `basic_ios::move` should accept lvalues

Section: 27.5.5.3 [basic.ios.members] Status: C++11 Submitter: Howard Hinnant Opened: 2009-04-25 Last modified: 2015-04-08

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Discussion:

With the rvalue reference changes in N2844 `basic_ios::move` no longer has the most convenient signature:

>     void move(basic_ios&& rhs);

This signature should be changed to accept lvalues. It does not need to be overloaded to accept rvalues. This is a special case that only derived clients will see. The generic `move` still needs to accept rvalues.

[ Batavia (2009-05): ]

> Tom prefers, on general principles, to provide both overloads. Alisdair agrees.

> Howard points out that there is no backward compatibility issue as this is new to C++0X.

> We agree that both overloads should be provided, and Howard will provide the additional wording. Move to Open.

[ 2009-05-23 Howard adds: ]

> Added overload, moved to Review.

[ 2009 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

Add a signature to the existing prototype in the synopsis of 27.5.5 [ios] and in 27.5.5.3 [basic.ios.members]:

>     void move(basic_ios& rhs);
>     void move(basic_ios&& rhs);

### 1106. Multiple exceptions from connected `shared_future::get()`?

Section: 30.6.7 [futures.shared_future] Status: Resolved Submitter: Thomas J. Gritzan Opened: 2009-04-03 Last modified: 2015-04-08

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Discussion:

It is not clear, if multiple threads are waiting in a `shared_future::get()` call, if each will rethrow the stored exception.

Paragraph 9 reads:

> Throws: the stored exception, if an exception was stored and not retrieved before.

The "not retrieved before" suggests that only one exception is thrown, but one exception for each call to `get()` is needed, and multiple calls to `get()` even on the same `shared_future` object seem to be allowed.

I suggest removing "and not retrieved before" from the Throws paragraph. I recommend adding a note that explains that multiple calls on `get()` are allowed, and each call would result in an exception if an exception was stored.

[ Batavia (2009-05): ]

> We note there is a pending paper by Detlef on such `future`-related issues; we would like to wait for his paper before proceeding.

> Alisdair suggests we may want language to clarify that this `get()` function can be called from several threads with no need for explicit locking.

> Move to Open.

[ 2010-01-23 Moved to Tentatively NAD Editorial after 5 positive votes on c++std-lib. ]

Rationale:

Resolved by paper N2997.

Proposed resolution:

Change 30.6.7 [futures.shared_future]:

>     const R& shared_future::get() const;
>     R& shared_future<R&>::get() const;
>     void shared_future<void>::get() const;

> > ...

> > -9- Throws: the stored exception, if an exception was stored and not retrieved before. [Note: Multiple calls on `get()` are allowed, and each call would result in an exception if an exception was stored. — end note]

### 1108. thread.req.exception overly constrains implementations

Section: 30.2.2 [thread.req.exception] Status: C++11 Submitter: Christopher Kohlhoff Opened: 2009-04-25 Last modified: 2015-04-08

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Discussion:

The current formulation of 30.2.2 [thread.req.exception]/2 reads:

> The `error_category` of the `error_code` reported by such an exception's `code()` member function is as specified in the error condition Clause.

This constraint on the code's associated `error_categor` means an implementation must perform a mapping from the system-generated error to a `generic_category()` error code. The problems with this include:

- The mapping is always performed, even if the resultant value is never used.

- The original error produced by the operating system is lost.

The latter was one of Peter Dimov's main objections (in a private email discussion) to the original `error_code`-only design, and led to the creation of `error_condition` in the first place. Specifically, `error_code` and `error_condition` are intended to perform the following roles:

- `error_code` holds the original error produced by the operating system.
- `error_condition` and the generic category provide a set of well known error constants that error codes may be tested against.

Any mapping determining correspondence of the returned error code to the conditions listed in the error condition clause falls under the "latitude" granted to implementors in 19.5.1.5 [syserr.errcat.objects]. (Although obviously their latitude is restricted a little by the need to match the right error condition when returning an error code from a library function.)

It is important that this `error_code/error_condition` usage is done correctly for the thread library since it is likely to set the pattern for future TR libraries that interact with the operating system.

[ Batavia (2009-05): ]

> Move to Open, and recommend the issue be deferred until after the next Committee Draft is issued.

[ 2009-10 post-Santa Cruz: ]

> Move to Tentatively Ready.

Proposed resolution:

Change 30.2.2 [thread.req.exception] p.2:

> -2- The `error_category` (19.5.1.1) of the `error_code` reported by such an exception's `code()` member function is as specified in the error condition Clause. The `error_code` reported by such an exception's `code()` member function shall compare equal to one of the conditions specified in the function's error condition Clause. [Example: When the thread constructor fails:

> >     ec.category() == implementation-defined // probably system_category
> >     ec == errc::resource_unavailable_try_again // holds true

> — end example]

### 1110. Is `for_each` overconstrained?

Section: 25.2.4 [alg.foreach] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-04-29 Last modified: 2015-04-08

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Discussion:

Quoting working paper for reference (25.2.4 [alg.foreach]):

>     template<InputIterator Iter, Callable<auto, Iter::reference> Function>
>       requires CopyConstructible<Function>
>       Function for_each(Iter first, Iter last, Function f);

> > 1 Effects: Applies f to the result of dereferencing every iterator in the range [first,last), starting from first and proceeding to last - 1.

> > 2 Returns: f.

> > 3 Complexity: Applies f exactly last - first times.

P2 implies the passed object `f` should be invoked at each stage, rather than some copy of `f`. This is important if the return value is to usefully accumulate changes. So the requirements are an object of type `Function` can be passed-by-value, invoked multiple times, and then return by value. In this case, `MoveConstructible` is sufficient. This would open support for move-only functors, which might become important in concurrent code as you can assume there are no other references (copies) of a move-only type and so freely use them concurrently without additional locks.

[ See further discussion starting with c++std-lib-23686. ]

[ Batavia (2009-05): ]

> Pete suggests we may want to look at this in a broader context involving other algorithms. We should also consider the implications of parallelism.

> Move to Open, and recommend the issue be deferred until after the next Committee Draft is issued.

[ 2009-10-14 Daniel de-conceptified the proposed resolution. ]

> The note in 25.1 [algorithms.general]/9 already says the right thing:

> > Unless otherwise specified, algorithms that take function objects as arguments are permitted to copy those function objects freely.

> So we only need to ensure that the wording for `for_each` is sufficiently clear, which is the intend of the following rewording.

[ 2009-10-15 Daniel proposes: ]

> - Add a new Requires clause just after the prototype declaration (25.2.4 [alg.foreach]):

>   > Requires: `Function` shall be `MoveConstructible` ( [moveconstructible]), `CopyConstructible` is not required.

> - Change 25.2.4 [alg.foreach]/2 as indicated:

>   > Returns: std::move(f).

[ 2009-10 post-Santa Cruz: ]

> Move to Tentatively Ready, using Daniel's wording without the portion saying "CopyConstructible is not required".

[ 2009-10-27 Daniel adds: ]

> I see that during the Santa Cruz meeting the originally proposed addition

> > , `CopyConstructible` is not required.

> was removed. I don't think that this removal was a good idea. The combination of 25.1 [algorithms.general] p.9

> > [Note: Unless otherwise specified, algorithms that take function objects as arguments are permitted to copy those function objects freely.[..]

> with the fact that `CopyConstructible` is a refinement `MoveConstructible` makes it necessary that such an explicit statement is given. Even the existence of the usage of `std::move` in the Returns clause doesn't help much, because this would still be well-formed for a `CopyConstructible` without move constructor. Let me add that the originally proposed addition reflects current practice in the standard, e.g. 25.3.9 [alg.unique] p.5 usages a similar terminology.

> For similar wording need in case for auto_ptr see 973.

> [ Howard: Moved from Tentatively Ready to Open. ]

[ 2009-11-20 Howard restores "not `CopyConstructible`" to the spec. ]

[ 2009-11-22 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

- Add a new Requires clause just after the prototype declaration (25.2.4 [alg.foreach]):

  > Requires: `Function` shall meet the requirements of `MoveConstructible` ( [moveconstructible]). `Function` need not meet the requirements of `CopyConstructible` ( [copyconstructible]).

- Change 25.2.4 [alg.foreach]/2 as indicated:

  > Returns: std::move(f).

### 1113. `bitset::to_string` could be simplified

Section: 20.6 [template.bitset] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-05-09 Last modified: 2015-04-08

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Discussion:

In 853 our resolution is changing the signature by adding two defaulting arguments to 3 calls. In principle, this means that ABI breakage is not an issue, while API is preserved.

With that observation, it would be very nice to use the new ability to supply default template parameters to function templates to collapse all 3 signatures into 1. In that spirit, this issue offers an alternative resolution than that of 853.

[ Batavia (2009-05): ]

> Move to Open, and look at the issue again after 853 has been accepted. We further recommend this be deferred until after the next Committee Draft.

[ 2009-10 post-Santa Cruz: ]

> Move to Tentatively Ready.

Proposed resolution:

1. In 20.6 [template.bitset]/1 (class bitset) ammend:

   >     template <class charT = char,
   >                 class traits = char_traits<charT>,
   >                 class Allocator = allocator<charT>>
   >       basic_string<charT, traits, Allocator>
   >       to_string(charT zero = charT('0'), charT one = charT('1')) const;
   >     template <class charT, class traits>
   >       basic_string<charT, traits, allocator<charT> > to_string() const;
   >     template <class charT>
   >       basic_string<charT, char_traits<charT>, allocator<charT> > to_string() const;
   >     basic_string<char, char_traits<char>, allocator<char> > to_string() const;

2. In 20.6.2 [bitset.members] prior to p35 ammend:

   >     template <class charT = char,
   >                 class traits = char_traits<charT>,
   >                 class Allocator = allocator<charT>>
   >       basic_string<charT, traits, Allocator>
   >       to_string(charT zero = charT('0'), charT one = charT('1')) const;

3. Strike 20.6.2 [bitset.members] paragraphs 37 -> 39 (including signature above 37)

### 1114. Type traits underspecified

Section: 20.10 [meta] Status: C++11 Submitter: Daniel Krügler Opened: 2009-05-12 Last modified: 2015-04-08

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Discussion:

Related to 975 and 1023.

The current wording in 20.10.1 [meta.rqmts] is still unclear concerning it's requirements on the type traits classes regarding ambiguities. Specifically it's unclear

- if a predicate trait (20.10.4 [meta.unary], 20.10.6 [meta.rel]) could derive from both `true_type`/`false_type`.
- if any of the type traits (20.10.1 [meta.rqmts], 20.10.4 [meta.unary], 20.10.6 [meta.rel]) could ambiguously derive from the same specified result type.
- if any of the type traits (20.10.1 [meta.rqmts], 20.10.4 [meta.unary], 20.10.6 [meta.rel]) could derive from other `integral_constant` types making the contained names ambiguous
- if any of the type traits (20.10.1 [meta.rqmts], 20.10.4 [meta.unary], 20.10.6 [meta.rel]) could have other base classes that contain members hiding the name of the result type members or make the contained member names ambiguous.

[ Batavia (2009-05): ]

> Alisdair would prefer to factor some of the repeated text, but modulo a corner case or two, he believes the proposed wording is otherwise substantially correct.

> Move to Open.

[ 2009-10 post-Santa Cruz: ]

> Move to Tentatively Ready.

Proposed resolution:

[ The usage of the notion of a BaseCharacteristic below might be useful in other places - e.g. to define the base class relation in 20.9.4 [refwrap], 20.9.11 [func.memfn], or 20.9.12.2 [func.wrap.func]. In this case it's definition should probably be moved to Clause 17 ]

1. Change 20.10.1 [meta.rqmts] p.1 as indicated:

   > [..] It shall be `DefaultConstructible`, `CopyConstructible`, and publicly and unambiguously derived, directly or indirectly, from its BaseCharacteristic, which is a specialization of the template `integral_constant` (20.6.3), with the arguments to the template `integral_constant` determined by the requirements for the particular property being described. The member names of the BaseCharacteristic shall be unhidden and unambiguously available in the UnaryTypeTrait.

2. Change 20.10.1 [meta.rqmts] p.2 as indicated:

   > [..] It shall be `DefaultConstructible`, `CopyConstructible`, and publicly and unambiguously derived, directly or indirectly, from an instance its BaseCharacteristic, which is a specialization of the template `integral_constant` (20.6.3), with the arguments to the template `integral_constant` determined by the requirements for the particular relationship being described. The member names of the BaseCharacteristic shall be unhidden and unambiguously available in the BinaryTypeTrait.

3. Change 20.10.4 [meta.unary] p.2 as indicated:

   > Each of these templates shall be a UnaryTypeTrait (20.6.1), publicly derived directly or indirectly from `true_type` if the corresponding condition is true, otherwise from `false_type` where its BaseCharacteristic shall be `true_type` if the corresponding condition is true, otherwise `false_type`.

4. Change 20.10.6 [meta.rel] p.2 as indicated:

   > Each of these templates shall be a BinaryTypeTrait (20.6.1), publicly derived directly or indirectly from `true_type` if the corresponding condition is true, otherwise from `false_type` where its BaseCharacteristic shall be `true_type` if the corresponding condition is true, otherwise `false_type`.

### 1116. Literal constructors for tuple

Section: 20.4.2 [tuple.tuple] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-05-23 Last modified: 2015-04-08

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Discussion:

It is not currently possible to construct `tuple` literal values, even if the elements are all literal types. This is because parameters are passed to constructor by reference.

An alternative would be to pass all constructor arguments by value, where it is known that *all* elements are literal types. This can be determined with concepts, although note that the negative constraint really requires factoring out a separate concept, as there is no way to provide an 'any of these fails' constraint inline.

Note that we will have similar issues with `pair` (and `tuple` constructors from `pair`) although I am steering clear of that class while other constructor-related issues settle.

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Solved by N2994.

Proposed resolution:

Ammend the tuple class template declaration in 20.4.2 [tuple.tuple] as follows

> Add the following concept:

> >     auto concept AllLiteral< typename ... Types > {
> >       requires LiteralType<Types>...;
> >     }

> ammend the constructor

> >     template <class... UTypes>
> >       requires AllLiteral<Types...>
> >             && Constructible<Types, UTypes>...
> >       explicit tuple(UTypes...);

> >     template <class... UTypes>
> >       requires !AllLiteral<Types...>
> >             && Constructible<Types, UTypes&&>...
> >       explicit tuple(UTypes&&...);

> ammend the constructor

> >     template <class... UTypes>
> >       requires AllLiteral<Types...>
> >             && Constructible<Types, UTypes>...
> >       tuple(tuple<UTypes...>);

> >     template <class... UTypes>
> >       requires !AllLiteral<Types...>
> >             && Constructible<Types, const UTypes&>...
> >       tuple(const tuple<UTypes...>&);

Update the same signatures in 20.4.2.1 [tuple.cnstr], paras 3 and 5.

### 1117. `tuple` copy constructor

Section: 20.4.2.1 [tuple.cnstr] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-05-23 Last modified: 2015-04-08

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Discussion:

The copy constructor for the `tuple` template is constrained. This seems an unusual strategy, as the copy constructor will be implicitly deleted if the constraints are not met. This is exactly the same effect as requesting an `=default;` constructor. The advantage of the latter is that it retains triviality, and provides support for `tuple`s as literal types if issue 1116 is also accepted.

Actually, it might be worth checking with core if a constrained copy constructor is treated as a constructor template, and as such does not suppress the implicit generation of the copy constructor which would hide the template in this case.

[ 2009-05-27 Daniel adds: ]

> This would solve one half of the suggested changes in 801.

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Solved by N2994.

Proposed resolution:

Change 20.4.2 [tuple.tuple] and 20.4.2.1 [tuple.cnstr] p4:

>     requires CopyConstructible<Types>... tuple(const tuple&) = default;

### 1118. `tuple` query APIs do not support cv-qualification

Section: 20.4.2.5 [tuple.helper] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-05-23 Last modified: 2015-04-08

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Discussion:

The APIs `tuple_size` and `tuple_element` do not support cv-qualified `tuple`s, `pair`s or `array`s.

The most generic solution would be to supply partial specializations once for each cv-type in the `tuple` header. However, requiring this header for cv-qualified `pair`s/`array`s seems unhelpful. The BSI editorial suggestion (UK-198/US-69, N2533) to merge `tuple` into `<utility>` would help with `pair`, but not `array`. That might be resolved by making a dependency between the `<array>` header and `<utility>`, or simply recognising the dependency be fulfilled in a Remark.

[ 2009-05-24 Daniel adds: ]

> All `tuple_size` templates with a base class need to derive publicly, e.g.

> >     template <IdentityOf T> class tuple_size< const T > :
> >        public tuple_size<T> {};

> The same applies to the tuple_element class hierarchies.

> What is actually meant with the comment

> > this solution relies on 'metafunction forwarding' to inherit the nested typename type

> ?

> I ask, because all base classes are currently unconstrained and their instantiation is invalid in the constrained context of the `tuple_element` partial template specializations.

[ 2009-05-24 Alisdair adds: ]

> I think a better solution might be to ask Pete editorially to change all declarations of tupling APIs to use the struct specifier instead of class.

> "metafunction forwarding" refers to the MPL metafunction protocol, where a metafunction result is declared as a nested typedef with the name "type", allowing metafunctions to be chained by means of inheritance. It is a neater syntax than repeatedly declaring a typedef, and inheritance syntax is slightly nicer when it comes to additional typename keywords.

> The constrained template with an unconstrained base is a good observation though.

[ 2009-10 post-Santa Cruz: ]

> Move to Open, Alisdair to provide wording. Once wording is provided, Howard will move to Review.

[ 2010-03-28 Daniel deconceptified wording. ]

[ Post-Rapperswil - Daniel provides wording: ]

The below given P/R reflects the discussion from the Rapperswil meeting that the wording should not constrain implementation freedom to realize the actual issue target. Thus the original code form was replaced by normative words.

While preparing this wording it turned out that several `tuple_size` specializations as that of `pair` and `array` are underspecified, because the underlying type of the member value is not specified except that it is an integral type. For the specializations we could introduce a canonical one - like `size_t` - or we could use the same type as the specialization of the unqualified type uses. The following wording follows the second approach.

The wording refers to N3126.

> Moved to Tentatively Ready after 6 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

1. Change 20.4.1 [tuple.general] p.2, header `<tuple>` synopsis, as indicated:

   >     // 20.4.2.5, tuple helper classes:
   >     template <class T> class tuple_size; // undefined
   >     template <class T> class tuple_size<const T>;
   >     template <class T> class tuple_size<volatile T>;
   >     template <class T> class tuple_size<const volatile T>;

   >     template <class... Types> class tuple_size<tuple<Types...> >;

   >     template <size_t I, class T> class tuple_element; // undefined
   >     template <size_t I, class T> class tuple_element<I, const T>;
   >     template <size_t I, class T> class tuple_element<I, volatile T>;
   >     template <size_t I, class T> class tuple_element<I, const volatile T>;

   >     template <size_t I, class... Types> class tuple_element<I, tuple<Types...> >;

2. Add the end of subclause 20.4.2.5 [tuple.helper] insert the following two paragraphs:

   >     template <class T> class tuple_size<const T>;
   >     template <class T> class tuple_size<volatile T>;
   >     template <class T> class tuple_size<const volatile T>;

   > > Let TS denote `tuple_size<T>` of the cv-unqualified type `T`. Then each of the three templates shall meet the UnaryTypeTrait requirements (20.7.1) with a BaseCharacteristic of `integral_constant<remove_cv<decltype(TS::value)>::type, TS::value>`.

   >     template <size_t I, class T> class tuple_element<I, const T>;
   >     template <size_t I, class T> class tuple_element<I, volatile T>;
   >     template <size_t I, class T> class tuple_element<I, const volatile T>;

   > > Let TE denote `tuple_element<I, T>` of the cv-unqualified type `T`. Then each of the three templates shall meet the TransformationTrait requirements (20.7.1) with a member typedef `type` that shall name the same type as the following type:

   > > - for the first specialization, the type `add_const<TE::type>::type`,
   > > - for the second specialization, the type `add_volatile<TE::type>::type`, and
   > > - for the third specialization, the type `add_cv<TE::type>::type`

### 1122. Ratio values should be `constexpr`

Section: 20.11.3 [ratio.ratio] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-05-25 Last modified: 2015-04-08

View all other issues in [ratio.ratio].

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Discussion:

The values `num` and `den` in the `ratio` template should be declared `constexpr`.

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Solved by N2994.

Proposed resolution:

20.11.3 [ratio.ratio]

>     namespace std {
>       template <intmax_t N, intmax_t D = 1>
>       class ratio {
>       public:
>         static constexpr intmax_t num;
>         static constexpr intmax_t den;
>       };
>     }

### 1123. No requirement that standard streams be flushed

Section: 27.5.3.1.6 [ios::Init] Status: C++11 Submitter: James Kanze Opened: 2009-05-14 Last modified: 2015-04-08

View all other issues in [ios::Init].

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Discussion:

As currently formulated, the standard doesn't require that there is ever a flush of `cout`, etc. (This implies, for example, that the classical hello, world program may have no output.) In the current draft (N2798), there is a requirement that the objects be constructed before `main`, and before the dynamic initialization of any non-local objects defined after the inclusion of `<iostream>` in the same translation unit. The only requirement that I can find concerning flushing, however, is in 27.5.3.1.6 [ios::Init], where the destructor of the last `std::ios_base::Init` object flushes. But there is, as far as I can see, no guarantee that such an object ever exists.

Also, the wording in [iostreams.objects] says that:

> The objects are constructed and the associations are established at some time prior to or during the first time an object of class `ios_base::Init` is constructed, and in any case before the body of main begins execution.

In 27.5.3.1.6 [ios::Init], however, as an effect of the constructor, it says that

> If `init_cnt` is zero, the function stores the value one in `init_cnt`, then constructs and initializes the objects `cin`, `cout`, `cerr`, `clog` `wcin`, `wcout`, `wcerr`, and `wclog`"

which seems to forbid earlier construction.

(Note that with these changes, the exposition only "`static int init_cnt`" in `ios_base::Init` can be dropped.)

Of course, a determined programmer can still inhibit the flush with things like:

>     new std::ios_base::Init ;       //  never deleted

or (in a function):

>     std::ios_base::Init ensureConstruction ;
>     //  ...
>     exit( EXIT_SUCCESS ) ;

Perhaps some words somewhere to the effect that all `std::ios_base::Init` objects should have static lifetime would be in order.

[ 2009 Santa Cruz: ]

> Moved to Ready. Some editorial changes are expected (in addition to the proposed wording) to remove `init_cnt` from `Init`.

Proposed resolution:

Change 27.4 [iostream.objects]/2:

> -2- The objects are constructed and the associations are established at some time prior to or during the first time an object of class `ios_base::Init` is constructed, and in any case before the body of main begins execution.292 The objects are not destroyed during program execution.293 If a translation unit includes `<iostream>` or explicitly constructs an `ios_base::Init` object, these stream objects shall be constructed before dynamic initialization of non-local objects defined later in that translation unit. The results of including `<iostream>` in a translation unit shall be as if `<iostream>` defined an instance of `ios_base::Init` with static lifetime. Similarly, the entire program shall behave as if there were at least one instance of `ios_base::Init` with static lifetime.

Change 27.5.3.1.6 [ios::Init]/3:

>     Init();

> > -3- Effects: Constructs an object of class `Init`. If `init_cnt` is zero, the function stores the value one in `init_cnt`, then constructs and initializes the objects `cin`, `cout`, `cerr`, `clog` (27.4.1), `wcin`, `wcout`, `wcerr`, and `wclog` (27.4.2). In any case, the function then adds one to the value stored in `init_cnt`. Constructs and initializes the objects `cin`, `cout`, `cerr`, `clog`, `wcin`, `wcout`, `wcerr` and `wclog` if they have not already been constructed and initialized.

Change 27.5.3.1.6 [ios::Init]/4:

>     ~Init();

> > -4- Effects: Destroys an object of class `Init`. The function subtracts one from the value stored in `init_cnt` and, if the resulting stored value is one, If there are no other instances of the class still in existance, calls `cout.flush()`, `cerr.flush()`, `clog.flush()`, `wcout.flush()`, `wcerr.flush()`, `wclog.flush()`.

### 1126. `istreambuff_iterator::equal` needs a const & parameter

Section: 24.6.3.5 [istreambuf.iterator::equal] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-05-28 Last modified: 2015-04-08

View all other issues in [istreambuf.iterator::equal].

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Discussion:

The `equal` member function of `istreambuf_iterator` is declared `const`, but takes its argument by non-const reference.

This is not compatible with the `operator==` free function overload, which is defined in terms of calling `equal` yet takes both arguments by reference to const.

[ The proposed wording is consistent with 110 with status TC1. ]

[ 2009-11-02 Howard adds: ]

> Set to Tentatively Ready after 5 positive votes on c++std-lib.

Proposed resolution:

Ammend in both:
24.6.3 [istreambuf.iterator]
24.6.3.5 [istreambuf.iterator::equal]

>     bool equal(const istreambuf_iterator& b) const;

### 1129. `istream(buf)_iterator` should support literal sentinel value

Section: 24.6.1.1 [istream.iterator.cons], 24.6.3 [istreambuf.iterator] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-05-30 Last modified: 2015-04-08

View all other issues in [istream.iterator.cons].

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Discussion:

`istream_iterator` and `istreambuf_iterator` should support literal sentinel values. The default constructor is frequently used to terminate ranges, and could easily be a literal value for `istreambuf_iterator`, and `istream_iterator` when iterating value types. A little more work using a suitably sized/aligned char-array for storage (or an updated component like `boost::optional` proposed for TR2) would allow `istream_iterator` to support `constexpr` default constructor in all cases, although we might leave this tweak as a QoI issue. Note that requiring `constexpr` be supported also allows us to place no-throw guarantees on this constructor too.

[ 2009-06-02 Daniel adds: ]

> I agree with the usefulness of the issue suggestion, but we need to ensure that `istream_iterator` can satisfy be literal if needed. Currently this is not clear, because 24.6.1 [istream.iterator]/3 declares a copy constructor and a destructor and explains their semantic in 24.6.1.1 [istream.iterator.cons]/3+4.

> The prototype semantic specification is ok (although it seems somewhat redundant to me, because the semantic doesn't say anything interesting in both cases), but for support of trivial class types we also need a trivial copy constructor and destructor as of 9 [class]/6. The current non-informative specification of these two special members suggests to remove their explicit declaration in the class and add explicit wording that says that if `T` is trivial a default constructed iterator is also literal, alternatively it would be possible to mark both as defaulted and add explicit (memberwise) wording that guarantees that they are trivial.

> Btw.: I'm quite sure that the `istreambuf_iterator` additions to ensure triviality are not sufficient as suggested, because the library does not yet give general guarantees that a defaulted special member declaration makes this member also trivial. Note that e.g. the atomic types do give a general statement!

> Finally there is a wording issue: There does not exist something like a "literal constructor". The core language uses the term "constexpr constructor" for this.

> Suggestion:

> 1. Change 24.6.1 [istream.iterator]/3 as indicated:

>    >     constexpr istream_iterator();
>    >     istream_iterator(istream_type& s);
>    >     istream_iterator(const istream_iterator<T,charT,traits,Distance>& x) = default;
>    >     ~istream_iterator() = default;

> 2. Change 24.6.1.1 [istream.iterator.cons]/1 as indicated:

>    >     constexpr istream_iterator();

>    > > -1- Effects: Constructs the end-of-stream iterator. If `T` is a literal type, then this constructor shall be a constexpr constructor.

> 3. Change 24.6.1.1 [istream.iterator.cons]/3 as indicated:

>    >     istream_iterator(const istream_iterator<T,charT,traits,Distance>& x) = default;

>    > > -3- Effects: Constructs a copy of `x`. If `T` is a literal type, then this constructor shall be a trivial copy constructor.

> 4. Change 24.6.1.1 [istream.iterator.cons]/4 as indicated:

>    >     ~istream_iterator() = default;

>    > > -4- Effects: The iterator is destroyed. If `T` is a literal type, then this destructor shall be a trivial destructor.

> 5. Change 24.6.3 [istreambuf.iterator] before p. 1 as indicated:

>    >     constexpr istreambuf_iterator() throw();
>    >     istreambuf_iterator(const istreambuf_iterator&)  throw() = default;
>    >     ~istreambuf_iterator()  throw() = default;

> 6. Change 24.6.3 [istreambuf.iterator]/1 as indicated:

>    > [..] The default constructor `istreambuf_iterator()` and the constructor `istreambuf_iterator(0)` both construct an end of stream iterator object suitable for use as an end-of-range. All specializations of `istreambuf_iterator` shall have a trivial copy constructor, a constexpr default constructor and a trivial destructor.

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Addressed by N2994.

Proposed resolution:

24.6.1 [istream.iterator] para 3

>     constexpr istream_iterator();

24.6.1.1 [istream.iterator.cons]

>     constexpr istream_iterator();

> > -1- Effects: Constructs the end-of-stream iterator. If `T` is a literal type, then this constructor shall be a literal constructor.

24.6.3 [istreambuf.iterator]

>     constexpr istreambuf_iterator() throw();

### 1130. `copy_exception` name misleading

Section: 18.8.5 [propagation] Status: C++11 Submitter: Peter Dimov Opened: 2009-05-13 Last modified: 2015-04-08

View all other issues in [propagation].

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Discussion:

The naming of `std::copy_exception` misleads almost everyone (experts included!) to think that it is the function that copies an `exception_ptr`:

>     exception_ptr p1 = current_exception();
>     exception_ptr p2 = copy_exception( p1 );

But this is not at all what it does. The above actually creates an `exception_ptr p2` that contains a copy of `p1`, not of the exception to which `p1` refers!

This is, of course, all my fault; in my defence, I used `copy_exception` because I was unable to think of a better name.

But I believe that, based on what we've seen so far, any other name would be better.

Therefore, I propose `copy_exception` to be renamed to `create_exception`:

>     template<class E> exception_ptr create_exception(E e);

with the following explanatory paragraph after it:

> Creates an `exception_ptr` that refers to a copy of `e`.

[ 2009-05-13 Daniel adds: ]

> What about

> >     make_exception_ptr

> in similarity to `make_pair` and `make_tuple`, `make_error_code` and `make_error_condition`, or `make_shared`? Or, if a stronger symmetry to `current_exception` is preferred:

> >     make_exception

> We have not a single `create_*` function in the library, it was always `make_*` used.

[ 2009-05-13 Peter adds: ]

> `make_exception_ptr` works for me.

[ 2009-06-02 Thomas J. Gritzan adds: ]

> To avoid surprises and unwanted recursion, how about making a call to `std::make_exception_ptr` with an `exception_ptr` illegal?

> It might work like this:

> >     template<class E>
> >     exception_ptr make_exception_ptr(E e);
> >     template<>
> >     exception_ptr make_exception_ptr<exception_ptr>(exception_ptr e) = delete;

[ 2009 Santa Cruz: ]

> Move to Review for the time being. The subgroup thinks this is a good idea, but doesn't want to break compatibility unnecessarily if someone is already shipping this. Let's talk to Benjamin and PJP tomorrow to make sure neither objects.

[ 2009-11-16 Jonathan Wakely adds: ]

> GCC 4.4 shipped with `copy_exception` but we could certainly keep that symbol in the library (but not the headers) so it can still be found by any apps foolishly relying on the experimental C++0x mode being ABI stable.

[ 2009-11-16 Peter adopts wording supplied by Daniel. ]

[ 2009-11-16 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

1. Change 18.8 [support.exception]/1, header `<exception>` synopsis as indicated:

   >     exception_ptr current_exception();
   >     void rethrow_exception [[noreturn]] (exception_ptr p);
   >     template<class E> exception_ptr copy_exceptionmake_exception_ptr(E e);

2. Change 18.8.5 [propagation]:

   >     template<class E> exception_ptr copy_exceptionmake_exception_ptr(E e);

   > > -11- Effects: Creates an `exception_ptr` that refers to a copy of `e`, as if

   > > >     try {
   > > >       throw e;
   > > >     } catch(...) {
   > > >       return current_exception();
   > > >     }

   > > ...

3. Change 30.6.5 [futures.promise]/7 as indicated:

   > Effects: if the associated state of `*this` is not ready, stores an exception object of type `future_error` with an error code of `broken_promise` as if by `this->set_exception(copy_exceptionmake_exception_ptr( future_error(future_errc::broken_promise))`. Destroys ...

### 1131. C++0x does not need `alignment_of`

Section: 20.10.4.3 [meta.unary.prop] Status: C++11 Submitter: Niels Dekker Opened: 2009-06-01 Last modified: 2015-04-08

View other active issues in [meta.unary.prop].

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Discussion:

The `alignment_of` template is no longer necessary, now that the core language will provide `alignof`. Scott Meyers raised this issue at comp.std.c++, C++0x: alignof vs. alignment_of, May 21, 2009. In a reply, Daniel Krügler pointed out that `alignof` was added to the working paper after `alignment_of`. So it appears that `alignment_of` is only part of the current Working Draft (N2857) because it is in TR1.

Having both `alignof` and `alignment_of` would cause unwanted confusion. In general, I think TR1 functionality should not be brought into C++0x if it is entirely redundant with other C++0x language or library features.

[ 2009-11-16 Chris adds: ]

> I would like to suggest the following new wording for this issue, based on recent discussions. Basically this doesn't delete `alignment_of`, it just makes it clear that it is just a wrapper for `alignof`. This deletes the first part of the proposed resolution, changes the second part by leaving in `alignof(T)` but changing the precondition and leaves the 3rd part unchanged.

> Suggested Resolution:

> > Change the first row of Table 44 ("Type property queries"), from Type properties 20.10.4.3 [meta.unary.prop]:

> > > Table 44 — Type property queries
> > > `template <class T> struct alignment_of;` | `alignof(T)`. Precondition: `T` shall be a complete type, a reference type, or an array of unknown bound, but shall not be a function type or (possibly cv-qualified) `void`. `alignof(T)` shall be defined

> > Change text in Table 51 ("Other transformations"), from Other transformations 20.10.7.6 [meta.trans.other], as follows:

> > > Table 51 — Other transformations
> > > …`aligned_storage;` | `Len` shall not be zero. `Align` shall be equal to `alignment_of<T>::value alignof(T)` for some type `T` or to default-alignment. | …

[ 2010-01-30 Alisdair proposes that Chris' wording be moved into the proposed wording section and tweaks it on the way. ]

> Original proposed wording saved here:

> > Remove from Header <type_traits> synopsis 20.10.2 [meta.type.synop]:

> > >     template <class T> struct alignment_of;

> > Remove the first row of Table 44 ("Type property queries"), from Type properties 20.10.4.3 [meta.unary.prop]:

> > > Table 44 — Type property queries
> > > `template <class T> struct alignment_of;` | `alignof(T)`. Precondition: `T` shall be a complete type, a reference type, or an array of unknown bound, but shall not be a function type or (possibly cv-qualified) `void`.

> > Change text in Table 51 ("Other transformations"), from Other transformations 20.10.7.6 [meta.trans.other], as follows:

> > > Table 51 — Other transformations
> > > …`aligned_storage;` | `Len` shall not be zero. Align shall be equal to `alignment_of<T>::value alignof(T)` for some type `T` or to default-alignment. | …

[ 2010-01-30 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Change the first row of Table 43 ("Type property queries"), from Type properties 20.10.4.3 [meta.unary.prop]:

> Table 43 — Type property queries
> `template <class T> struct alignment_of;` | `alignof(T)`. Precondition: `T` shall be a complete type, a reference type, or an array of unknown bound, but shall not be a function type or (possibly cv-qualified) `void`. `alignof(T)` is a valid expression (5.3.6 [expr.alignof])

Change text in Table 51 ("Other transformations"), from Other transformations 20.10.7.6 [meta.trans.other], as follows:

> Table 51 — Other transformations
> …`aligned_storage;` | `Len` shall not be zero. `Align` shall be equal to `alignment_of<T>::value alignof(T)` for some type `T` or to default-alignment. | …

### 1133. Does N2844 break current specification of list::splice?

Section: 23.3.4.6 [forwardlist.ops], 23.3.5.5 [list.ops] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-05-09 Last modified: 2015-04-08

View all other issues in [forwardlist.ops].

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Discussion:

IIUC, N2844 means that lvalues will no longer bind to rvalue references. Therefore, the current specification of `list::splice` (list operations 23.3.5.5 [list.ops]) will be a breaking change of behaviour for existing programs. That is because we changed the signature to swallow via an rvalue reference rather than the lvalue reference used in 03.

Retaining this form would be safer, requiring an explicit move when splicing from lvalues. However, this will break existing programs. We have the same problem with `forward_list`, although without the risk of breaking programs so here it might be viewed as a positive feature.

The problem signatures:

>     void splice_after(const_iterator position, forward_list<T,Alloc>&& x);
>     void splice_after(const_iterator position, forward_list<T,Alloc>&& x,
>                       const_iterator i);
>     void splice_after(const_iterator position, forward_list<T,Alloc>&& x,
>                       const_iterator first, const_iterator last);

>     void splice(const_iterator position, list<T,Alloc>&& x);
>     void splice(const_iterator position, list<T,Alloc>&& x,
>                 const_iterator i);
>     void splice(const_iterator position, list<T,Alloc>&& x,
>                 const_iterator first, const_iterator last);

Possible resolutions:

Option A. Add an additional (non-const) lvalue-reference overload in each case

Option B. Change rvalue reference back to (non-const) lvalue-reference overload in each case

Option C. Add an additional (non-const) lvalue-reference overload in just the `std::list` cases

I think (B) would be very unfortunate, I really like the `forward_list` behaviour in (C) but feel (A) is needed for consistency.

My actual preference would be NAD, ship with this as a breaking change as it is a more explicit interface. I don't think that will fly though!

See the thread starting with c++std-lib-23725 for more discussion.

[ 2009-10-27 Christopher Jefferson provides proposed wording for Option C. ]

[ 2009-12-08 Jonathan Wakely adds: ]

> As Bill Plauger pointed out, `list::merge` needs similar treatment.

> [ Wording updated. ]

[ 2009-12-13 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

In 23.3.5 [list]

Add lvalue overloads before rvalue ones:

>     void splice(const_iterator position, list<T,Allocator>& x);
>     void splice(const_iterator position, list<T,Allocator>&& x);
>     void splice(const_iterator position, list<T,Allocator>& x, const_iterator i);
>     void splice(const_iterator position, list<T,Allocator>&& x, const_iterator i);
>     void splice(const_iterator position, list<T,Allocator>& x,
>                 const_iterator first, const_iterator last);
>     void splice(const_iterator position, list<T,Allocator>&& x,
>                 const_iterator first, const_iterator last);
>     void merge(list<T,Allocator>& x);
>     template <class Compare> void merge(list<T,Allocator>& x, Compare comp);
>     void merge(list<T,Allocator>&& x);
>     template <class Compare> void merge(list<T,Allocator>&& x, Compare comp);

In 23.3.5.5 [list.ops], similarly add lvalue overload before each rvalue one:

(After paragraph 2)

>     void splice(const_iterator position, list<T,Allocator>& x);
>     void splice(const_iterator position, list<T,Allocator>&& x);

(After paragraph 6)

>     void splice(const_iterator position, list<T,Allocator>& x, const_iterator i);
>     void splice(const_iterator position, list<T,Allocator>&& x, const_iterator i);

(After paragraph 10)

>     void splice(const_iterator position, list<T,Allocator>& x,
>                 const_iterator first, const_iterator last);
>     void splice(const_iterator position, list<T,Allocator>&& x,
>                 const_iterator first, const_iterator last);

In 23.3.5.5 [list.ops], after paragraph 21

>     void merge(list<T,Allocator>& x);
>     template <class Compare> void merge(list<T,Allocator>& x, Compare comp);
>     void merge(list<T,Allocator>&& x);
>     template <class Compare> void merge(list<T,Allocator>&& x, Compare comp);

### 1134. Redundant specification of `<stdint.h>`, `<fenv.h>`, `<tgmath.h>`, and maybe `<complex.h>`

Section: X [stdinth], X [fenv], 26.8 [c.math], X [cmplxh] Status: C++11 Submitter: Robert Klarer Opened: 2009-05-26 Last modified: 2015-04-08

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Discussion:

This is probably editorial.

The following items should be removed from the draft, because they're redundant with Annex D, and they arguably make some *.h headers non-deprecated:

X [stdinth] (regarding `<stdint.h>`)

X [fenv] (regarding `<fenv.h>`

Line 3 of 26.8 [c.math] (regarding `<tgmath.h>`)

X [cmplxh] (regarding `<complex.h>`, though the note in this subclause is not redundant)

[ 2009-06-10 Ganesh adds: ]

> While searching for `stdint` in the CD, I found that `<stdint.h>` is also mentioned in 3.9.1 [basic.fundamental] p.5. I guess it should refer to `<cstdint>` instead.

[ 2009 Santa Cruz: ]

> Real issue. Maybe just editorial, maybe not. Move to Ready.

Proposed resolution:

Remove the section X [stdinth].

Remove the section X [fenv].

Remove 26.8 [c.math], p3:

> -3- The header `<tgmath.h>` effectively includes the headers `<complex.h>` and `<math.h>`.

Remove the section X [cmplxh].

### 1135. `exception_ptr` should support contextual conversion to `bool`

Section: 18.8.5 [propagation] Status: Resolved Submitter: Daniel Krügler Opened: 2007-06-06 Last modified: 2015-04-08

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Discussion:

As of N2857 18.8.5 [propagation] p.5, the implementation-defined type `exception_ptr` does provide the following ways to check whether it is a null value:

>     void f(std::exception_ptr p) {
>       p == nullptr;
>       p == 0;
>       p == exception_ptr();
>     }

This is rather cumbersome way of checking for the null value and I suggest to require support for evaluation in a boolean context like so:

>     void g(std::exception_ptr p) {
>       if (p) {}
>       !p;
>     }

[ 2009 Santa Cruz: ]

> Move to Ready. Note to editor: considering putting in a cross-reference to 4 [conv], paragraph 3, which defines the phrase "contextually converted to `bool`".

[ 2010-03-14 Howard adds: ]

> We moved N3073 to the formal motions page in Pittsburgh which should obsolete this issue. I've moved this issue to NAD Editorial, solved by N3073.

Rationale:

Solved by N3073.

Proposed resolution:

In section 18.8.5 [propagation] insert a new paragraph between p.5 and p.6:

> An object `e` of type `exception_ptr` can be contextually converted to `bool`. The effect shall be as if `e != exception_ptr()` had been evaluated in place of `e`. There shall be no implicit conversion to arithmetic type, to enumeration type or to pointer type.

### 1136. Incomplete specification of `nested_exception::rethrow_nested()`

Section: 18.8.6 [except.nested] Status: C++11 Submitter: Daniel Krügler Opened: 2007-06-06 Last modified: 2015-04-08

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Discussion:

It was recently mentioned in a newsgroup article http://groups.google.de/group/comp.std.c++/msg/f82022aff68edf3d that the specification of the member function `rethrow_nested()` of the class `nested_exception` is incomplete, specifically it remains unclear what happens, if member `nested_ptr()` returns a null value. In 18.8.6 [except.nested] we find only the following paragraph related to that:

>     void rethrow_nested() const; // [[noreturn]]

> > -4- Throws: the stored exception captured by this `nested_exception` object.

This is a problem, because it is possible to create an object of `nested_exception` with exactly such a state, e.g.

>     #include <exception>
>     #include <iostream>

>     int main() try {
>       std::nested_exception e; // OK, calls current_exception() and stores it's null value
>       e.rethrow_nested(); // ?
>       std::cout << "A" << std::endl;
>     }
>     catch(...) {
>       std::cout << "B" << std::endl;
>     }

I suggest to follow the proposal of the reporter, namely to invoke `terminate()` if `nested_ptr()` return a null value of `exception_ptr` instead of relying on the fallback position of undefined behavior. This would be consistent to the behavior of a `throw;` statement when no exception is being handled.

[ 2009 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

Change around 18.8.6 [except.nested] p.4 as indicated:

> -4- Throws: the stored exception captured by this `nested_exception` object, if `nested_ptr() != nullptr`

> - Remarks: If `nested_ptr() == nullptr`, `terminate()` shall be called.

### 1137. Return type of `conj` and `proj`

Section: 26.4.9 [cmplx.over] Status: C++11 Submitter: Marc Steinbach Opened: 2009-06-11 Last modified: 2015-04-08

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Discussion:

In clause 1, the Working Draft (N2857) specifies overloads of the functions

>     arg, conj, imag, norm, proj, real

for non-complex arithmetic types (`float`, `double`, `long double`, and integers). The only requirement (clause 2) specifies effective type promotion of arguments.

I strongly suggest to add the following requirement on the return types:

> All the specified overloads must return real (i.e., non-complex) values, specifically, the nested `value_type` of effectively promoted arguments.

(This has no effect on `arg`, `imag`, `norm`, `real`: they are real-valued anyway.)

Rationale:

Mathematically, `conj()` and `proj()`, like the transcendental functions, are complex-valued in general but map the (extended) real line to itself. In fact, both functions act as identity on the reals. A typical user will expect `conj()` and `proj()` to preserve this essential mathematical property in the same way as `exp()`, `sin()`, etc. A typical use of `conj()`, e.g., is the generic scalar product of n-vectors:

>     template<typename T>
>     inline T
>     scalar_product(size_t n, T const* x, T const* y) {
>       T result = 0;
>       for (size_t i = 0; i < n; ++i)
>         result += x[i] * std::conj(y[i]);
>       return result;
>     }

This will work equally well for real and complex floating-point types `T` if `conj()` returns `T`. It will not work with real types if `conj()` returns complex values.

Instead, the implementation of `scalar_product` becomes either less efficient and less useful (if a complex result is always returned), or unnecessarily complicated (if overloaded versions with proper return types are defined). In the second case, the real-argument overload of `conj()` cannot be used. In fact, it must be avoided.

Overloaded `conj()` and `proj()` are principally needed in generic programming. All such use cases will benefit from the proposed return type requirement, in a similar way as the `scalar_product` example. The requirement will not harm use cases where a complex return value is expected, because of implicit conversion to complex. Without the proposed return type guarantee, I find overloaded versions of `conj()` and `proj()` not only useless but actually troublesome.

[ 2009-11-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Insert a new paragraph after 26.4.9 [cmplx.over]/2:

> All of the specified overloads shall have a return type which is the nested `value_type` of the effectively cast arguments.

### 1138. Unusual return value for `operator+`

Section: 21.4.8.1 [string::op+] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-06-12 Last modified: 2015-04-08

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Discussion:

Many of the `basic_string operator+` overloads return an rvalue-reference. Is that really intended?

I'm considering it might be a mild performance tweak to avoid making un-necessary copies of a cheaply movable type, but it opens risk to dangling references in code like:

>     auto && s = string{"x"} + string{y};

and I'm not sure about:

>     auto s = string{"x"} + string{y};

[ 2009-10-11 Howard updated Returns: clause for each of these. ]

[ 2009-11-05 Howard adds: ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

Proposed resolution:

Strike the `&&` from the return type in the following function signatures:

> 21.3 [string.classes] p2 Header Synopsis

> >     template<class charT, class traits, class Allocator>
> >       basic_string<charT,traits,Allocator>&&
> >         operator+(basic_string<charT,traits,Allocator>&& lhs,
> >                   const basic_string<charT,traits,Allocator>& rhs);

> >     template<class charT, class traits, class Allocator>
> >       basic_string<charT,traits,Allocator>&&
> >         operator+(const basic_string<charT,traits,Allocator>& lhs,
> >                   basic_string<charT,traits,Allocator>&& rhs);

> >     template<class charT, class traits, class Allocator>
> >       basic_string<charT,traits,Allocator>&&
> >         operator+(basic_string<charT,traits,Allocator>&& lhs,
> >                   basic_string<charT,traits,Allocator>&& rhs);

> >     template<class charT, class traits, class Allocator>
> >       basic_string<charT,traits,Allocator>&&
> >         operator+(const charT* lhs,
> >                   basic_string<charT,traits,Allocator>&& rhs);

> >     template<class charT, class traits, class Allocator>
> >       basic_string<charT,traits,Allocator>&&
> >         operator+(charT lhs, basic_string<charT,traits,Allocator>&& rhs);

> >     template<class charT, class traits, class Allocator>
> >       basic_string<charT,traits,Allocator>&&
> >         operator+(basic_string<charT,traits,Allocator>&& lhs,
> >                   const charT* rhs);

> >     template<class charT, class traits, class Allocator>
> >       basic_string<charT,traits,Allocator>&&
> >         operator+(basic_string<charT,traits,Allocator>&& lhs, charT rhs);

> 21.4.8.1 [string::op+]

> >     template<class charT, class traits, class Allocator>
> >       basic_string<charT,traits,Allocator>&&
> >         operator+(basic_string<charT,traits,Allocator>&& lhs,
> >                   const basic_string<charT,traits,Allocator>& rhs);

> > > Returns: `std::move(lhs.append(rhs))`

> >     template<class charT, class traits, class Allocator>
> >       basic_string<charT,traits,Allocator>&&
> >         operator+(const basic_string<charT,traits,Allocator>& lhs,
> >                   basic_string<charT,traits,Allocator>&& rhs);

> > > Returns: `std::move(rhs.insert(0, lhs))`

> >     template<class charT, class traits, class Allocator>
> >       basic_string<charT,traits,Allocator>&&
> >         operator+(basic_string<charT,traits,Allocator>&& lhs,
> >                   basic_string<charT,traits,Allocator>&& rhs);

> > > Returns: `std::move(lhs.append(rhs))` [Note: Or equivalently `std::move(rhs.insert(0, lhs))` — end note]

> >     template<class charT, class traits, class Allocator>
> >       basic_string<charT,traits,Allocator>&&
> >         operator+(const charT* lhs,
> >                   basic_string<charT,traits,Allocator>&& rhs);

> > > Returns: `std::move(rhs.insert(0, lhs))`.

> >     template<class charT, class traits, class Allocator>
> >       basic_string<charT,traits,Allocator>&&
> >         operator+(charT lhs, basic_string<charT,traits,Allocator>&& rhs);

> > > Returns: `std::move(rhs.insert(0, 1, lhs))`.

> >     template<class charT, class traits, class Allocator>
> >       basic_string<charT,traits,Allocator>&&
> >         operator+(basic_string<charT,traits,Allocator>&& lhs,
> >                   const charT* rhs);

> > > Returns: `std::move(lhs.append(rhs))`.

> >     template<class charT, class traits, class Allocator>
> >       basic_string<charT,traits,Allocator>&&
> >         operator+(basic_string<charT,traits,Allocator>&& lhs, charT rhs);

> > > Returns: `std::move(lhs.append(1, rhs))`.

### 1143. Atomic operations library not concept enabled

Section: 29 [atomics] Status: Resolved Submitter: LWG Opened: 2009-06-15 Last modified: 2015-04-08

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Discussion:

Addresses US 87, UK 311

The atomics chapter is not concept enabled.

Needs to also consider issues 923 and 924.

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Addressed by N2992.

Proposed resolution:

### 1144. "thread safe" is undefined

Section: 18.5 [support.start.term] Status: C++11 Submitter: LWG Opened: 2009-06-16 Last modified: 2015-04-08

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Discussion:

Addresses UK 187

The term "thread safe" is not defined nor used in this context anywhere else in the standard.

Suggested action:

Clarify the meaning of "thread safe".

[ 2009 Santa Cruz: ]

> The "thread safe" language has already been change in the WP. It was changed to "happen before", but the current WP text is still a little incomplete: "happen before" is binary, but the current WP text only mentions one thing.

> Move to Ready.

Proposed resolution:

For the following functions in 18.5 [support.start.term].

>     extern "C" int at_quick_exit(void (*f)(void));
>     extern "C++" int at_quick_exit(void (*f)(void));

Edit paragraph 10 as follows. The intent is to provide the other half of the happens before relation; to note indeterminate ordering; and to clean up some formatting.

> Effects: The `at_quick_exit()` functions register the function pointed to by `f` to be called without arguments when `quick_exit` is called. It is unspecified whether a call to `at_quick_exit()` that does not happen-before happen before (1.10) all calls to `quick_exit` will succeed. [Note: the `at_quick_exit()` functions shall not introduce a data race (17.6.4.7). exitnote —end note] [Note: The order of registration may be indeterminate if `at_quick_exit` was called from more than one thread. —end note] [Note: The `at_quick_exit` registrations are distinct from the `atexit` registrations, and applications may need to call both registration functions with the same argument. —end note]

For the following function.

>     void quick_exit [[noreturn]] (int status)

Edit paragraph 13 as follows. The intent is to note that thread-local variables may be different.

> Effects: Functions registered by calls to `at_quick_exit` are called in the reverse order of their registration, except that a function shall be called after any previously registered functions that had already been called at the time it was registered. Objects shall not be destroyed as a result of calling `quick_exit`. If control leaves a registered function called by `quick_exit` because the function does not provide a handler for a thrown exception, `terminate()` shall be called. [Note: Functions registered by one thread may be called by any thread, and hence should not rely on the identity of thread-storage-duration objects. —end note] After calling registered functions, `quick_exit` shall call `_Exit(status)`. [Note: The standard file buffers are not flushed. See: ISO C 7.20.4.4. —end note]

### 1145. Inappropriate headers for atomics

Section: 29 [atomics] Status: Resolved Submitter: LWG Opened: 2009-06-16 Last modified: 2015-04-08

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Discussion:

Addresses UK 312

The contents of the `<stdatomic.h>` header are not listed anywhere, and `<cstdatomic>` is listed as a C99 header in chapter 17. If we intend to use these for compatibility with a future C standard, we should not use them now.

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Solved by N2992.

Proposed resolution:

Remove `<cstdatomic>` from the C99 headers in table 14. Add a new header `<atomic>` to the headers in table 13. Update chapter 29 to remove reference to `<stdatomic.h>` and replace the use of `<cstdatomic>` with `<atomic>`.

[ If and when WG14 adds atomic operations to C we can add corresponding headers to table 14 with a TR. ]

### 1146. "lockfree" does not say enough

Section: 29.4 [atomics.lockfree] Status: Resolved Submitter: Jeffrey Yasskin Opened: 2009-06-16 Last modified: 2015-04-08

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Discussion:

Addresses US 88

The "lockfree" facilities do not tell the programmer enough.

There are 2 problems here. First, at least on x86, it's less important to me whether some integral types are lock free than what is the largest type I can pass to atomic and have it be lock-free. For example, if `long long`s are not lock-free, `ATOMIC_INTEGRAL_LOCK_FREE` is probably 1, but I'd still be interested in knowing whether longs are always lock-free. Or if long longs at any address are lock-free, I'd expect `ATOMIC_INTEGRAL_LOCK_FREE` to be 2, but I may actually care whether I have access to the `cmpxchg16b` instruction. None of the support here helps with that question. (There are really 2 related questions here: what alignment requirements are there for lock-free access; and what processor is the program actually running on, as opposed to what it was compiled for?)

Second, having `atomic_is_lock_free` only apply to individual objects is pretty useless (except, as Lawrence Crowl points out, for throwing an exception when an object is unexpectedly not lock-free). I'm likely to want to use its result to decide what algorithm to use, and that algorithm is probably going to allocate new memory containing atomic objects and then try to act on them. If I can't predict the lock-freedom of the new object by checking the lock-freedom of an existing object, I may discover after starting the algorithm that I can't continue.

[ 2009-06-16 Jeffrey Yasskin adds: ]

> To solve the first problem, I think 2 macros would help: `MAX_POSSIBLE_LOCK_FREE_SIZE` and `MAX_GUARANTEED_LOCK_FREE_SIZE`, which expand to the maximum value of `sizeof(T)` for which atomic may (or will, respectively) use lock-free operations. Lawrence points out that this "relies heavily on implementations using word-size compare-swap on sub-word-size types, which in turn requires address modulation." He expects that to be the end state anyway, so it doesn't bother him much.

> To solve the second, I think one could specify that equally aligned objects of the same type will return the same value from `atomic_is_lock_free()`. I don't know how to specify "equal alignment". Lawrence suggests an additional function, `atomic_is_always_lock_free()`.

[ 2009-10-22 Benjamin Kosnik: ]

> In the evolution discussion of N2925, "More Collected Issues with Atomics," there is an action item with respect to LWG 1146, US 88

> This is stated in the paper as:

> Relatedly, Mike Sperts will create an issue to propose adding a traits mechanism to check the compile-time properties through a template mechanism rather than macros

> Here is my attempt to do this. I don't believe that a separate trait is necessary for this, and that instead `atomic_integral::is_lock_free` can be re-purposed with minimal work as follows.

> [ Howard: Put Benjamin's wording in the proposed wording section. ]

[ 2009-10-22 Alberto Ganesh Barbati: ]

> Just a thought... wouldn't it be better to use a scoped enum instead of plain integers? For example:

> >     enum class is_lock_free
> >     {
> >         never = 0, sometimes = 1, always = 2;
> >     };

> if compatibility with C is deemed important, we could use an unscoped enum with suitably chosen names. It would still be more descriptive than 0, 1 and 2.

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Solved by N2992.

Proposed resolution:

Header `<cstdatomic>` synopsis [atomics.synopsis]

Edit as follows:

>     namespace std {
>     ...
>     // 29.4, lock-free property
>     #define ATOMIC_INTEGRAL_LOCK_FREE unspecified
>     #define ATOMIC_CHAR_LOCK_FREE unspecified
>     #define ATOMIC_CHAR16_T_LOCK_FREE unspecified
>     #define ATOMIC_CHAR32_T_LOCK_FREE unspecified
>     #define ATOMIC_WCHAR_T_LOCK_FREE unspecified
>     #define ATOMIC_SHORT_LOCK_FREE unspecified
>     #define ATOMIC_INT_LOCK_FREE unspecified
>     #define ATOMIC_LONG_LOCK_FREE unspecified
>     #define ATOMIC_LLONG_LOCK_FREE unspecified
>     #define ATOMIC_ADDRESS_LOCK_FREE unspecified

Lock-free Property 29.4 [atomics.lockfree]

Edit the synopsis as follows.

>     namespace std {
>        #define ATOMIC_INTEGRAL_LOCK_FREE unspecified
>        #define ATOMIC_CHAR_LOCK_FREE unspecified
>        #define ATOMIC_CHAR16_T_LOCK_FREE unspecified
>        #define ATOMIC_CHAR32_T_LOCK_FREE unspecified
>        #define ATOMIC_WCHAR_T_LOCK_FREE unspecified
>        #define ATOMIC_SHORT_LOCK_FREE unspecified
>        #define ATOMIC_INT_LOCK_FREE unspecified
>        #define ATOMIC_LONG_LOCK_FREE unspecified
>        #define ATOMIC_LLONG_LOCK_FREE unspecified
>        #define ATOMIC_ADDRESS_LOCK_FREE unspecified
>     }

Edit paragraph 1 as follows.

> The ATOMIC_...._LOCK_FREE macros ATOMIC_INTEGRAL_LOCK_FREE and ATOMIC_ADDRESS_LOCK_FREE indicate the general lock-free property of integral and address atomic the corresponding atomic integral types, with the signed and unsigned variants grouped together. The properties also apply to the corresponding specializations of the atomic template. A value of 0 indicates that the types are never lock-free. A value of 1 indicates that the types are sometimes lock-free. A value of 2 indicates that the types are always lock-free.

Operations on Atomic Types 29.6 [atomics.types.operations]

Edit as follows.

>     void static constexpr bool A::is_lock_free() const volatile;

> > Returns: True if the object's types's operations are lock-free, false otherwise. [Note: In the same way that `<limits>` `std::numeric_limits<short>::max()` is related to `<limits.h>` `__LONG_LONG_MAX__`, `<atomic> std::atomic_short::is_lock_free` is related to `<stdatomic.h>` and `ATOMIC_SHORT_LOCK_FREE` — end note]

### 1147. Non-volatile atomic functions

Section: 29.6 [atomics.types.operations] Status: Resolved Submitter: Jeffrey Yasskin Opened: 2009-06-16 Last modified: 2015-04-08

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Discussion:

Addresses US 90

The C++0X draft declares all of the functions dealing with atomics (section 29.6 [atomics.types.operations]) to take volatile arguments. Yet it also says (29.4-3),

> [ Note: Many operations are volatile-qualified. The "volatile as device register" semantics have not changed in the standard. This qualification means that volatility is preserved when applying these operations to volatile objects. It does not mean that operations on non-volatile objects become volatile. Thus, volatile qualified operations on non-volatile objects may be merged under some conditions. — end note ]

I was thinking about how to implement this in gcc, and I believe that we'll want to overload most of the functions on volatile and non-volatile. Here's why:

To let the compiler take advantage of the permission to merge non-volatile atomic operations and reorder atomics in certain, we'll need to tell the compiler backend about exactly which atomic operation was used. So I expect most of the functions of the form `atomic_<op>_explicit()` (e.g. `atomic_load_explicit`, `atomic_exchange_explicit`, `atomic_fetch_add_explicit`, etc.) to become compiler builtins. A builtin can tell whether its argument was volatile or not, so those functions don't really need extra explicit overloads. However, I don't expect that we'll want to add builtins for every function in chapter 29, since most can be implemented in terms of the `_explicit` free functions:

    class atomic_int {
      __atomic_int_storage value;
     public:
      int fetch_add(int increment, memory_order order = memory_order_seq_cst) volatile {
        // &value has type "volatile __atomic_int_storage*".
        atomic_fetch_add_explicit(&value, increment, order);
      }
      ...
    };

But now this always calls the volatile builtin version of `atomic_fetch_add_explicit()`, even if the `atomic_int` wasn't declared volatile. To preserve volatility and the compiler's permission to optimize, I'd need to write:

    class atomic_int {
      __atomic_int_storage value;
     public:
      int fetch_add(int increment, memory_order order = memory_order_seq_cst) volatile {
        atomic_fetch_add_explicit(&value, increment, order);
      }
      int fetch_add(int increment, memory_order order = memory_order_seq_cst) {
        atomic_fetch_add_explicit(&value, increment, order);
      }
      ...
    };

But this is visibly different from the declarations in the standard because it's now overloaded. (Consider passing `&atomic_int::fetch_add` as a template parameter.)

The implementation may already have permission to add overloads to the member functions:

> 17.6.5.5 [member.functions] An implementation may declare additional non-virtual member function signatures within a class:
> ...

> - by adding a member function signature for a member function name.

but I don't see an equivalent permission to add overloads to the free functions.

[ 2009-06-16 Lawrence adds: ]

> I recommend allowing non-volatile overloads.

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Addressed by N2992.

Proposed resolution:

### 1151. Behavior of the library in the presence of threads is incompletely specified

Section: 17 [library] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2015-04-08

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Discussion:

Addresses US 63

Description

The behavior of the library in the presence of threads is incompletely specified.

For example, if thread 1 assigns to `X`, then writes data to file `f`, which is read by thread 2, and then accesses variable `X`, is thread 2 guaranteed to be able to see the value assigned to `X` by thread 1? In other words, does the write of the data "happen before" the read?

Another example: does simultaneous access using `operator at()` to different characters in the same non-const string really introduce a data race?

Suggestion

Notes

17 SG: should go to threads group; misclassified in document

Concurrency SG: Create an issue. Hans will look into it.

[ 2009 Santa Cruz: ]

> Move to "Open". Hans and the rest of the concurrency working group will study this. We can't make progress without a thorough review and a paper.

[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]

Rationale:

Solved by N3069.

Proposed resolution:

### 1152. Expressions parsed differently than intended

Section: 22.4.2.2.2 [facet.num.put.virtuals] Status: C++11 Submitter: Seungbeom Kim Opened: 2009-06-27 Last modified: 2015-04-08

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Discussion:

In Table 73 — Floating-point conversions, 22.4.2.2.2 [facet.num.put.virtuals], in N2914, we have the following entries:

Table 73 — Floating-point conversions
State | `stdio` equivalent
--- | ---
`floatfield == ios_base::fixed | ios_base::scientific && !uppercase` | `%a`
`floatfield == ios_base::fixed | ios_base::scientific` | `%A`

These expressions are supposed to mean:

>     floatfield == (ios_base::fixed | ios_base::scientific) && !uppercase
>     floatfield == (ios_base::fixed | ios_base::scientific)

but technically parsed as:

>     ((floatfield == ios_base::fixed) | ios_base::scientific) && (!uppercase)
>     ((floatfield == ios_base::fixed) | ios_base::scientific)

and should be corrected with additional parentheses, as shown above.

[ 2009-10-28 Howard: ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

Proposed resolution:

Change Table 83 — Floating-point conversions in 22.4.2.2.2 [facet.num.put.virtuals]:

Table 83 — Floating-point conversions
State | `stdio` equivalent
--- | ---
`floatfield == (ios_base::fixed | ios_base::scientific) && !uppercase` | `%a`
`floatfield == (ios_base::fixed | ios_base::scientific)` | `%A`

### 1157. Local types can now instantiate templates

Section: 17.6.4.2.1 [namespace.std] Status: C++11 Submitter: LWG Opened: 2009-06-28 Last modified: 2015-04-08

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Discussion:

Addresses UK 175

Description

Local types can now be used to instantiate templates, but don't have external linkage.

Suggestion

Remove the reference to external linkage.

Notes

We accept the proposed solution. Martin will draft an issue.

[ 2009-07-28 Alisdair provided wording. ]

[ 2009-10 Santa Cruz: ]

> Moved to Ready.

Proposed resolution:

17.6.4.2.1 [namespace.std]

Strike "of external linkage" in p1 and p2:

> -1- The behavior of a C++ program is undefined if it adds declarations or definitions to namespace `std` or to a namespace within namespace `std` unless otherwise specified. A program may add a concept map for any standard library concept or a template specialization for any standard library template to namespace `std` only if the declaration depends on a user-defined type of external linkage and the specialization meets the standard library requirements for the original template and is not explicitly prohibited.179

> -2- The behavior of a C++ program is undefined if it declares

> - an explicit specialization of any member function of a standard library class template, or
> - an explicit specialization of any member function template of a standard library class or class template, or
> - an explicit or partial specialization of any member class template of a standard library class or class template.

> A program may explicitly instantiate a template defined in the standard library only if the declaration depends on the name of a user-defined type of external linkage and the instantiation meets the standard library requirements for the original template.

### 1158. Encouragement to use monotonic clock

Section: 30.2.4 [thread.req.timing] Status: C++11 Submitter: LWG Opened: 2009-06-28 Last modified: 2015-04-08

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Discussion:

Addresses UK 322, US 96

Description

Not all systems can provide a monotonic clock. How are they expected to treat a _for function?

Suggestion

Add at least a note explaining the intent for systems that do not support a monotonic clock.

Notes

Create an issue, together with UK 96. Note that the specification as is already allows a non-monotonic clock due to the word “should” rather than “shall”. If this wording is kept, a footnote should be added to make the meaning clear.

[ 2009-06-29 Beman provided a proposed resolution. ]

[ 2009-10-31 Howard adds: ]

> Set to Tentatively Ready after 5 positive votes on c++std-lib.

[ 2010-02-24 Pete moved to Open: ]

> LWG 1158's proposed resolution replaces the ISO-specified normative term "should" with "are encouraged but not required to", which presumably means the same thing, but has no ISO normative status. The WD used the latter formulation in quite a few non-normative places, but only three normative ones. I've changed all the normative uses to "should".

[ 2010-03-06 Beman updates wording. ]

[ 2010 Pittsburgh: Moved to Ready. ]

Proposed resolution:

Change Timing specifications 30.2.4 [thread.req.timing] as indicated:

The member functions whose names end in `_for` take an argument that specifies a relative time. Implementations should use a monotonic clock to measure time for these functions. [Note: Implementations are not required to use a monotonic clock because such a clock may be unavailable. — end note]

### 1159. Unclear spec for `resource_deadlock_would_occur`

Section: 30.4.2.2.2 [thread.lock.unique.locking] Status: C++11 Submitter: LWG Opened: 2009-06-28 Last modified: 2015-04-08

View all other issues in [thread.lock.unique.locking].

View all issues with C++11 status.

Duplicate of: 1219

Discussion:

Addresses UK 327, UK 328

UK 327 Description

Not clear what the specification for error condition `resource_deadlock_would_occur` means. It is perfectly possible for this thread to own the mutex without setting owns to true on this specific lock object. It is also possible for lock operations to succeed even if the thread does own the mutex, if the mutex is recursive. Likewise, if the mutex is not recursive and the mutex has been locked externally, it is not always possible to know that this error condition should be raised, depending on the host operating system facilities. It is possible that 'i.e.' was supposed to be 'e.g.' and that suggests that recursive locks are not allowed. That makes sense, as the exposition-only member owns is boolean and not a integer to count recursive locks.

UK 327 Suggestion

Add a precondition `!owns`. Change the 'i.e.' in the error condition to be 'e.g.' to allow for this condition to propogate deadlock detection by the host OS.

UK 327 Notes

Create an issue. Assigned to Lawrence Crowl. Note: not sure what try_lock means for recursive locks when you are the owner. POSIX has language on this, which should ideally be followed. Proposed fix is not quite right, for example, try_lock should have different wording from lock.

UK 328 Description

There is a missing precondition that `owns` is true, or an `if(owns)` test is missing from the effect clause

UK 328 Suggestion

Add a precondition that `owns == true`. Add an error condition to detect a violation, rather than yield undefined behaviour.

UK 328 Notes

Handle in same issue as UK 327. Also uncertain that the proposed resolution is the correct one.

[ 2009-11-11 Alisdair notes that this issue is very closely related to 1219, if not a dup. ]

[ 2010-02-12 Anthony provided wording. ]

[ 2010 Pittsburgh: ]

> Wording updated and moved to Ready for Pittsburgh.

Proposed resolution:

Modify 30.4.2.2.2 [thread.lock.unique.locking] p3 to say:

>     void lock();

> > ...

> > 3 Throws: Any exception thrown by `pm->lock()`. `std::system_error` if an exception is required (30.2.2 [thread.req.exception]). `std::system_error` with an error condition of `operation_not_permitted` if `pm` is `0`. `std::system_error` with an error condition of `resource_deadlock_would_occur` if on entry `owns` is `true`. `std::system_error` when the postcondition cannot be achieved.

Remove 30.4.2.2.2 [thread.lock.unique.locking] p4 (Error condition clause).

Modify 30.4.2.2.2 [thread.lock.unique.locking] p8 to say:

>     bool try_lock();

> > ...

> > 8 Throws: Any exception thrown by `pm->try_lock()`. `std::system_error` if an exception is required (30.2.2 [thread.req.exception]). `std::system_error` with an error condition of `operation_not_permitted` if `pm` is `0`. `std::system_error` with an error condition of `resource_deadlock_would_occur` if on entry `owns` is `true`. `std::system_error` when the postcondition cannot be achieved.

Remove 30.4.2.2.2 [thread.lock.unique.locking] p9 (Error condition clause).

Modify 30.4.2.2.2 [thread.lock.unique.locking] p13 to say:

>     template <class Clock, class Duration>
>       bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time);

> > ...

> > 13 Throws: Any exception thrown by `pm->try_lock_until()`. `std::system_error` if an exception is required (30.2.2 [thread.req.exception]). `std::system_error` with an error condition of `operation_not_permitted` if `pm` is `0`. `std::system_error` with an error condition of `resource_deadlock_would_occur` if on entry `owns` is `true`. `std::system_error` when the postcondition cannot be achieved.

Remove 30.4.2.2.2 [thread.lock.unique.locking] p14 (Error condition clause).

Modify 30.4.2.2.2 [thread.lock.unique.locking] p18 to say:

>     template <class Rep, class Period>
>       bool try_lock_for(const chrono::duration<Rep, Period>& rel_time);

> > ...

> > 18 Throws: Any exception thrown by `pm->try_lock_for()`. `std::system_error` if an exception is required (30.2.2 [thread.req.exception]). `std::system_error` with an error condition of `operation_not_permitted` if `pm` is `0`. `std::system_error` with an error condition of `resource_deadlock_would_occur` if on entry `owns` is `true`. `std::system_error` when the postcondition cannot be achieved.

Remove 30.4.2.2.2 [thread.lock.unique.locking] p19 (Error condition clause).

### 1160. `future_error` public constructor is 'exposition only'

Section: 30.6.3 [futures.future_error] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2015-04-08

View all issues with Resolved status.

Discussion:

Addresses UK 331

Description

Not clear what it means for a public constructor to be 'exposition only'. If the intent is purely to support the library calling this constructor then it can be made private and accessed through friendship. Otherwise it should be documented for public consumption.

Suggestion

Declare the constructor as private with a note about intended friendship, or remove the exposition-only comment and document the semantics.

Notes

Create an issue. Assigned to Detlef. Suggested resolution probably makes sense.

[ 2009-07 Frankfurt ]

> Pending a paper from Anthony Williams / Detlef Vollmann.

[ 2009-10-14 Pending paper: N2967. ]

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Solved by N2997.

Proposed resolution:

### 1161. Unnecessary `unique_future` limitations

Section: 30.6.6 [futures.unique_future] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2015-04-08

View all other issues in [futures.unique_future].

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Discussion:

Addresses UK 336

Description

It is possible to transfer ownership of the asynchronous result from one unique_future instance to another via the move-constructor. However, it is not possible to transfer it back, and nor is it possible to create a default-constructed unique_future instance to use as a later move target. This unduly limits the use of `unique_future` in code. Also, the lack of a move-assignment operator restricts the use of `unique_future` in containers such as `std::vector` - `vector::insert` requires move-assignable for example.

Suggestion

Add a default constructor with the semantics that it creates a `unique_future` with no associated asynchronous result. Add a move-assignment operator which transfers ownership.

Notes

Create an issue. Detlef will look into it.

[ 2009-07 Frankfurt ]

> Pending a paper from Anthony Williams / Detlef Vollmann.

[ 2009-10-14 Pending paper: N2967. ]

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Addressed by N2997.

Proposed resolution:

### 1162. `shared_future` should support an efficient move constructor

Section: 30.6.7 [futures.shared_future] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2015-04-08

View all other issues in [futures.shared_future].

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Discussion:

Addresses UK 337

Description

`shared_future` should support an efficient move constructor that can avoid unnecessary manipulation of a reference count, much like `shared_ptr`

Suggestion

Add a move constructor

Notes

Create an issue. Detlef will look into it.

[ 2009-07 Frankfurt ]

> Pending a paper from Anthony Williams / Detlef Vollmann.

[ 2009-10-14 Pending paper: N2967. ]

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Addressed by N2997.

Proposed resolution:

### 1163. `shared_future` is inconsistent with `shared_ptr`

Section: 30.6.7 [futures.shared_future] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2015-04-08

View all other issues in [futures.shared_future].

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Discussion:

Addresses UK 338

Description

`shared_future` is currently `CopyConstructible`, but not `CopyAssignable`. This is inconsistent with `shared_ptr`, and will surprise users. Users will then write work-arounds to provide this behaviour. We should provide it simply and efficiently as part of shared_future. Note that since the shared_future member functions for accessing the state are all declared const, the original usage of an immutable shared_future value that can be freely copied by multiple threads can be retained by declaring such an instance as "`const shared_future`".

Suggestion

Remove "=delete" from the copy-assignment operator of shared_future. Add a move-constructor `shared_future(shared_future&& rhs)`, and a move-assignment operator `shared_future& operator=(shared_future&& rhs)`. The postcondition for the copy-assignment operator is that `*this` has the same associated state as `rhs`. The postcondition for the move-constructor and move assignment is that `*this` has the same associated as `rhs` had before the constructor/assignment call and that `rhs` has no associated state.

Notes

Create an issue. Detlef will look into it.

[ 2009-07 Frankfurt ]

> Pending a paper from Anthony Williams / Detlef Vollmann.

[ 2009-10-14 Pending paper: N2967. ]

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Adressed by N2997.

Proposed resolution:

### 1165. Unneeded `promise` move constructor

Section: 30.6.5 [futures.promise] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2015-04-08

View other active issues in [futures.promise].

View all other issues in [futures.promise].

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Discussion:

Addresses UK 343

Description

The move constructor of a `std::promise` object does not need to allocate any memory, so the move-construct-with-allocator overload of the constructor is superfluous.

Suggestion

Remove the constructor with the signature `template <class Allocator> promise(allocator_arg_t, const Allocator& a, promise& rhs);`

Notes

Create an issue. Detlef will look into it. Will solicit feedback from Pablo. Note that "rhs" argument should also be an rvalue reference in any case.

[ 2009-07 Frankfurt ]

> Pending a paper from Anthony Williams / Detlef Vollmann.

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Adressed by N2997.

Proposed resolution:

### 1166. Allocator-specific move/copy break model of move-constructor and move-assignment

Section: X [allocator.propagation], X [allocator.propagation.map], 23 [containers] Status: Resolved Submitter: LWG Opened: 2009-06-28 Last modified: 2015-04-08

View all issues with Resolved status.

Discussion:

Addresses US 77

Description

Allocator-specific move and copy behavior for containers (N2525) complicates a little-used and already-complicated portion of the standard library (allocators), and breaks the conceptual model of move-constructor and move-assignment operations on standard containers being efficient operations. The extensions for allocator-specific move and copy behavior should be removed from the working paper.

With the introduction of rvalue references, we are teaching programmers that moving from a standard container (e.g., a `vector<string>`) is an efficient, constant-time operation. The introduction of N2525 removed that guarantee; depending on the behavior of four different traits (20.8.4), the complexity of copy and move operations can be constant or linear time. This level of customization greatly increases the complexity of standard containers, and benefits only a tiny fraction of the C++ community.

Suggestion

Remove 20.8.4.

Remove 20.8.5.

Remove all references to the facilities in 20.8.4 and 20.8.5 from clause 23.

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Addressed by N2982.

Proposed resolution:

### 1170. String char-like types no longer PODs

Section: 21.1 [strings.general] Status: C++11 Submitter: Beman Dawes Opened: 2009-06-22 Last modified: 2015-04-08

View all issues with C++11 status.

Discussion:

Addresses UK 218

Prior to the introduction of constant expressions into the library, `basic_string` elements had to be POD types, and thus had to be both trivially copyable and standard-layout. This ensured that they could be memcpy'ed and would be compatible with other libraries and languages, particularly the C language and its library.

N2349, Constant Expressions in the Standard Library Revision 2, changed the requirement in 21/1 from "POD type" to "literal type". That change had the effect of removing the trivially copyable and standard-layout requirements from `basic_string` elements.

This means that `basic_string` elements no longer are guaranteed to be memcpy'able, and are no longer guaranteed to be standard-layout types:

> 3.9/p2 and 3.9/p3 both make it clear that a "trivially copyable type" is required for memcpy to be guaranteed to work.

> Literal types (3.9p12) may have a non-trivial copy assignment operator, and that violates the trivially copyable requirements given in 9/p 6, bullet item 2.

> Literal types (3.9p12) have no standard-layout requirement, either.

This situation probably arose because the wording for "Constant Expressions in the Standard Library" was in process at the same time the C++ POD deconstruction wording was in process.

Since trivially copyable types meet the C++0x requirements for literal types, and thus work with constant expressions, it seems an easy fix to revert the `basic_string` element wording to its original state.

[ 2009-07-28 Alisdair adds: ]

> When looking for any resolution for this issue, consider the definition of "character container type" in 17.3 [defns.character.container]. This does require the character type to be a POD, and this term is used in a number of places through clause 21 and 28. This suggests the PODness constraint remains, but is much more subtle than before. Meanwhile, I suspect the change from POD type to literal type was intentional with the assumption that trivially copyable types with non-trivial-but-constexpr constructors should serve as well. I don't believe the current wording offers the right guarantees for either of the above designs.

[ 2009-11-04 Howard modifies proposed wording to disallow array types as char-like types. ]

[ 2010-01-23 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Change General 21.1 [strings.general] as indicated:

> This Clause describes components for manipulating sequences of any literal non-array POD (3.9) type. In this Clause such types are called char-like types, and objects of char-like types are called char-like objects or simply characters.

### 1171. duration types should be literal

Section: 20.12.5 [time.duration] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-07-06 Last modified: 2015-04-08

View all other issues in [time.duration].

View all issues with C++11 status.

Discussion:

The `duration` types in 20.12.5 [time.duration] are exactly the sort of type that should be "literal types" in the new standard. Likewise, arithmetic operations on `duration`s should be declared `constexpr`.

[ 2009-09-21 Daniel adds: ]

> An alternative (and possibly preferable solution for potentially heap-allocating `big_int` representation types) would be to ask the core language to allow references to `const` literal types as feasible arguments for `constexpr` functions.

[ 2009-10-30 Alisdair adds: ]

> I suggest this issue moves from New to Open.

> Half of this issue was dealt with in paper n2994 on constexpr constructors.

> The other half (duration arithmetic) is on hold pending Core support for `const &` in `constexpr` functions.

[ 2010-03-15 Alisdair updated wording to be consistent with N3078. ]

[ 2010 Rapperswil: ]

> This issue was the motivation for Core adding the facility for `constexpr` functions to take parameters by `const &`. Move to Tentatively Ready.

[ Adopted at 2010-11 Batavia. ]

Proposed resolution:

Add `constexpr` to declaration of following functions and constructors:

Modify p1 20.12 [time], and the prototype definitions in 20.12.5.5 [time.duration.nonmember], 20.12.5.6 [time.duration.comparisons], and 20.12.5.7 [time.duration.cast]:

> Header `<chrono>` synopsis

>     // duration arithmetic
>     template <class Rep1, class Period1, class Rep2, class Period2>
>        typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type
>        constexpr operator+(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);
>     template <class Rep1, class Period1, class Rep2, class Period2>
>        typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type
>        constexpr operator-(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);
>     template <class Rep1, class Period, class Rep2>
>        duration<typename common_type<Rep1, Rep2>::type, Period>
>        constexpr operator*(const duration<Rep1, Period>& d, const Rep2& s);
>     template <class Rep1, class Period, class Rep2>
>        duration<typename common_type<Rep1, Rep2>::type, Period>
>        constexpr operator*(const Rep1& s, const duration<Rep2, Period>& d);
>     template <class Rep1, class Period, class Rep2>
>        duration<typename common_type<Rep1, Rep2>::type, Period>
>        constexpr operator/(const duration<Rep1, Period>& d, const Rep2& s);
>     template <class Rep1, class Period1, class Rep2, class Period2>
>        typename common_type<Rep1, Rep2>::type
>        constexpr operator/(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);

>     // duration comparisons
>     template <class Rep1, class Period1, class Rep2, class Period2>
>        constexpr bool operator==(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);
>     template <class Rep1, class Period1, class Rep2, class Period2>
>        constexpr bool operator!=(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);
>     template <class Rep1, class Period1, class Rep2, class Period2>
>        constexpr bool operator< (const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);
>     template <class Rep1, class Period1, class Rep2, class Period2>
>        constexpr bool operator<=(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);
>     template <class Rep1, class Period1, class Rep2, class Period2>
>        constexpr bool operator> (const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);
>     template <class Rep1, class Period1, class Rep2, class Period2>
>        constexpr bool operator>=(const  duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);

>     // duration_cast
>     template <class ToDuration, class Rep, class Period>
>        constexpr ToDuration duration_cast(const duration<Rep, Period>& d);

Change 20.12.5 [time.duration]:

>     template <class Rep, class Period = ratio<1>>
>     class duration {
>       ...
>     public:
>       ...
>       constexpr duration(const duration&) = default;
>       ...

>     };

[ Note - this edit already seems assumed by definition of the duration static members `zero/min/max`. They cannot meaningfully be `constexpr` without this change. ]

### 1172. `select_on_container_(copy|move)_construction` over-constrained

Section: X [allocator.concepts.members] Status: Resolved Submitter: Alberto Ganesh Barbati Opened: 2009-07-08 Last modified: 2015-04-08

View all issues with Resolved status.

Discussion:

I believe the two functions `select_on_container_(copy|move)_construction()` are over-constrained. For example, the return value of the "copy" version is (see X [allocator.concepts.members]/21):

> Returns: `x` if the allocator should propagate from the existing container to the new container on copy construction, otherwise `X()`.

Consider the case where a user decides to provide an explicit concept map for Allocator to adapt some legacy allocator class, as he wishes to provide customizations that the `LegacyAllocator` concept map template does not provide. Now, although it's true that the legacy class is required to have a default constructor, the user might have reasons to prefer a different constructor to implement `select_on_container_copy_construction()`. However, the current wording requires the use of the default constructor.

Moreover, it's not said explicitly that `x` is supposed to be the allocator of the existing container. A clarification would do no harm.

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Addressed by N2982.

Proposed resolution:

Replace X [allocator.concepts.members]/21 with:

>     X select_on_container_copy_construction(const X& x);

> -21- Returns: `x` if the allocator should propagate from the existing container to the new container on copy construction, otherwise `X()`. an allocator object to be used by the new container on copy construction. [Note: `x` is the allocator of the existing container that is being copied. The most obvious choices for the return value are `x`, if the allocator should propagate from the existing container, and `X()`. — end note]

Replace X [allocator.concepts.members]/25 with:

>     X select_on_container_move_construction(X&& x);

> -25- Returns: `move(x)` if the allocator should propagate from the existing container to the new container on move construction, otherwise `X()`. an allocator object to be used by the new container on move construction. [Note: `x` is the allocator of the existing container that is being moved. The most obvious choices for the return value are `move(x)`, if the allocator should propagate from the existing container, and `X()`. — end note]

### 1174. Type property predicates

Section: 20.10.4.3 [meta.unary.prop] Status: Resolved Submitter: Jason Merrill Opened: 2009-07-16 Last modified: 2015-04-08

View other active issues in [meta.unary.prop].

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Discussion:

I've been implementing compiler support for `is_standard_layout`, and noticed a few nits about 20.10.4.3 [meta.unary.prop]:

1. There's no trait for "trivially copyable type", which is now the property that lets you do bitwise copying of a type, and therefore seems useful to be able to query. `has_trivial_assign` && `has_trivial_copy_constructor` && `has_trivial_destructor` is similar, but not identical, specifically with respect to const types.
2. `has_trivial_copy_constructor` and `has_trivial_assign` lack the "or an array of such a class type" language that most other traits in that section, including `has_nothrow_copy_constructor` and `has_nothrow_assign`, have; this seems like an oversight.

[ See the thread starting with c++std-lib-24420 for further discussion. ]

[ Addressed in N2947. ]

[ 2009-10 Santa Cruz: ]

> NAD Editorial Resolved. Solved by N2984.

Proposed resolution:

### 1177. Improve "diagnostic required" wording

Section: 20.12.5 [time.duration] Status: C++11 Submitter: Howard Hinnant Opened: 2009-07-18 Last modified: 2015-04-08

View all other issues in [time.duration].

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Discussion:

"diagnostic required" has been used (by me) for code words meaning "use `enable_if` to constrain templated functions. This needs to be improved by referring to the function signature as not participating in the overload set, and moving this wording to a Remarks paragraph.

[ 2009-10 Santa Cruz: ]

> Moved to Ready.

[ 2009-11-19 Pete opens: ]

> Oh, and speaking of 1177, most of the changes result in rather convoluted prose. Instead of saying

> > A shall be B, else C

> it should be

> > C if A is not B

> That is:

> > `Rep2` shall be implicitly convertible to `CR(Rep1, Rep2)`, else this signature shall not participate in overload resolution.

> should be

> > This signature shall not participate in overload resolution if `Rep2` is not implicitly convertible to `CR(Rep1, Rep2)`.

> That is clearer, and eliminates the false requirement that `Rep2` "shall be" convertible.

[ 2009-11-19 Howard adds: ]

> I've updated the wording to match Pete's suggestion and included bullet 16 from 1195.

[ 2009-11-19 Jens adds: ]

> Further wording suggestion using "unless":

> > This signature shall not participate in overload resolution unless `Rep2` is implicitly convertible to `CR(Rep1, Rep2)`.

[ 2009-11-20 Howard adds: ]

> I've updated the wording to match Jens' suggestion.

[ 2009-11-22 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

[ This proposed resolution addresses 947 and 974. ]

1. Change 20.12.5.1 [time.duration.cons] (and reorder the Remarks paragraphs per 17.5.1.4 [structure.specifications]):

   >     template <class Rep2>
   >       explicit duration(const Rep2& r);

   > > Requires: Remarks: This constructor shall not participate in overload resolution unless `Rep2` shall be is implicitly convertible to `rep` and

   > > - `treat_as_floating_point<rep>::value` shall be is `true`, or
   > > - `treat_as_floating_point<Rep2>::value` shall be is `false`.

   > > Diagnostic required [Example:

   > > >     duration<int, milli> d(3); // OK
   > > >     duration<int, milli> d(3.5); // error

   > > — end example]

   > > Effects: Constructs an object of type `duration`.

   > > Postcondition: `count() == static_cast<rep>(r)`.

   >     template <class Rep2, class Period2>
   >       duration(const duration<Rep2, Period2>& d);

   > > Requires: Remarks: This constructor shall not participate in overload resolution unless `treat_as_floating_point<rep>::value` shall be is `true` or `ratio_divide<Period2, period>::type::den` shall be is 1. Diagnostic required. [Note: This requirement prevents implicit truncation error when converting between integral-based duration types. Such a construction could easily lead to confusion about the value of the duration. — end note] [Example:

   > > >     duration<int, milli> ms(3);
   > > >     duration<int, micro> us = ms; // OK
   > > >     duration<int, milli> ms2 = us; // error

   > > — end example]

   > > Effects: Constructs an object of type `duration`, constructing `rep_` from `duration_cast<duration>(d).count()`.

2. Change the following paragraphs in 20.12.5.5 [time.duration.nonmember]:

   >     template <class Rep1, class Period, class Rep2>
   >       duration<typename common_type<Rep1, Rep2>::type, Period>
   >       operator*(const duration<Rep1, Period>& d, const Rep2& s);

   > > Requires Remarks: This operator shall not participate in overload resolution unless `Rep2` shall be is implicitly convertible to `CR(Rep1, Rep2)`. Diagnostic required.

   >     template <class Rep1, class Period, class Rep2>
   >       duration<typename common_type<Rep1, Rep2>::type, Period>
   >       operator*(const Rep1& s, const duration<Rep2, Period>& d);

   > > Requires Remarks: This operator shall not participate in overload resolution unless `Rep1` shall be is implicitly convertible to `CR(Rep1, Rep2)`. Diagnostic required.

   >     template <class Rep1, class Period, class Rep2>
   >       duration<typename common_type<Rep1, Rep2>::type, Period>
   >       operator/(const duration<Rep1, Period>& d, const Rep2& s);

   > > Requires Remarks: This operator shall not participate in overload resolution unless `Rep2` shall be is implicitly convertible to `CR(Rep1, Rep2)` and `Rep2` shall not be is not an instantiation of `duration`. Diagnostic required.

   >     template <class Rep1, class Period, class Rep2>
   >       duration<typename common_type<Rep1, Rep2>::type, Period>
   >       operator%(const duration<Rep1, Period>& d, const Rep2& s);

   > > Requires Remarks: This operator shall not participate in overload resolution unless `Rep2` shall be is implicitly convertible to `CR(Rep1, Rep2)` and `Rep2` shall not be is not an instantiation of `duration`. Diagnostic required.

3. Change the following paragraphs in 20.12.5.7 [time.duration.cast]:

   >     template <class ToDuration, class Rep, class Period>
   >       ToDuration duration_cast(const duration<Rep, Period>& d);

   > > Requires Remarks: This function shall not participate in overload resolution unless `ToDuration` shall be is an instantiation of `duration`. Diagnostic required.

4. Change 20.12.6.1 [time.point.cons]/3 as indicated:

   > Requires: `Duration2` shall be implicitly convertible to `duration`. Diagnostic required.

   > Remarks: This constructor shall not participate in overload resolution unless `Duration2` is implicitly convertible to `duration`.

5. Change the following paragraphs in 20.12.6.7 [time.point.cast]:

   >     template <class ToDuration, class Clock, class Duration>
   >       time_point<Clock, ToDuration> time_point_cast(const time_point<Clock, Duration>& t);

   > > Requires Remarks: This function shall not participate in overload resolution unless `ToDuration` shall be is an instantiation of `duration`. Diagnostic required.

### 1178. Header dependencies

Section: 17.6.5.2 [res.on.headers] Status: C++11 Submitter: Beman Dawes Opened: 2009-07-18 Last modified: 2015-04-08

View all issues with C++11 status.

Discussion:

See Frankfurt notes of 1001.

Proposed resolution:

Change 17.6.5.2 [res.on.headers], Headers, paragraph 1, as indicated:

> A C++ header may include other C++ headers.[footnote] A C++ header shall provide the declarations and definitions that appear in its synopsis (3.2 [basic.def.odr]). A C++ header shown in its synopsis as including other C++ headers shall provide the declarations and definitions that appear in the synopses of those other headers.

> [footnote] C++ headers must include a C++ header that contains any needed definition (3.2).

### 1180. Missing `string_type` member typedef in class `sub_match`

Section: 28.9.1 [re.submatch.members] Status: C++11 Submitter: Daniel Krügler Opened: 2009-07-25 Last modified: 2015-04-08

View all issues with C++11 status.

Discussion:

The definition of class template `sub_match` is strongly dependent on the type `basic_string<value_type>`, both in interface and effects, but does not provide a corresponding typedef `string_type`, as e.g. class `match_results` does, which looks like an oversight to me that should be fixed.

[ 2009-11-15 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

1. In the class template `sub_match` synopsis 28.9 [re.submatch]/1 change as indicated:

   >     template <class BidirectionalIterator>
   >     class sub_match : public std::pair<BidirectionalIterator, BidirectionalIterator> {
   >     public:
   >       typedef typename iterator_traits<BidirectionalIterator>::value_type value_type;
   >       typedef typename iterator_traits<BidirectionalIterator>::difference_type difference_type;
   >       typedef BidirectionalIterator iterator;
   >       typedef basic_string<value_type> string_type;

   >       bool matched;

   >       difference_type length() const;
   >       operator basic_string<value_type>string_type() const;
   >       basic_string<value_type>string_type str() const;
   >       int compare(const sub_match& s) const;
   >       int compare(const basic_string<value_type>string_type& s) const;
   >       int compare(const value_type* s) const;
   >     };

2. In 28.9.1 [re.submatch.members]/2 change as indicated:

   >     operator basic_string<value_type>string_type() const;

   > > Returns: `matched ? basic_string<value_type> string_type(first, second) : basic_string<value_type> string_type()`.

3. In 28.9.1 [re.submatch.members]/3 change as indicated:

   >     basic_string<value_type>string_type str() const;

   > > Returns: `matched ? basic_string<value_type> string_type(first, second) : basic_string<value_type> string_type()`.

4. In 28.9.1 [re.submatch.members]/5 change as indicated:

   >     int compare(const basic_string<value_type>string_type& s) const;

### 1181. Invalid `sub_match` comparison operators

Section: 28.9.2 [re.submatch.op] Status: C++11 Submitter: Daniel Krügler Opened: 2009-07-25 Last modified: 2015-04-08

View all other issues in [re.submatch.op].

View all issues with C++11 status.

Discussion:

Several heterogeneous comparison operators of class template `sub_match` are specified by return clauses that are not valid in general. E.g. 28.9.2 [re.submatch.op]/7:

>     template <class BiIter, class ST, class SA>
>     bool operator==(
>       const basic_string<
>         typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
>       const sub_match<BiIter>& rhs);

> > Returns: `lhs == rhs.str()`.

The returns clause would be ill-formed for all cases where `ST != std::char_traits<iterator_traits<BiIter>::value_type>` or `SA != std::allocator<iterator_traits<BiIter>::value_type>`.

The generic character of the comparison was intended, so there are basically two approaches to fix the problem: The first one would define the semantics of the comparison using the traits class `ST` (The semantic of `basic_string::compare` is defined in terms of the compare function of the corresponding traits class), the second one would define the semantics of the comparison using the traits class

>     std::char_traits<iterator_traits<BiIter>::value_type>

which is essentially identical to

>     std::char_traits<sub_match<BiIter>::value_type>

I suggest to follow the second approach, because this emphasizes the central role of the `sub_match` object as part of the comparison and would also make sure that a `sub_match` comparison using some `basic_string<char_t, ..>` always is equivalent to a corresponding comparison with a string literal because of the existence of further overloads (beginning from 28.9.2 [re.submatch.op]/19). If users really want to take advantage of their own `traits::compare`, they can simply write a corresponding compare function that does so.

[ Post-Rapperswil ]

The following update is a result of the discussion during the Rapperswil meeting, the P/R expresses all comparisons by delegating to sub_match's compare functions. The processing is rather mechanical: Only `==` and `<` where defined by referring to `sub_match`'s compare function, all remaining ones where replaced by the canonical definitions in terms of these two.

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

The wording refers to N3126.

1. Change 28.9.2 [re.submatch.op]/7 as indicated:

   >     template <class BiIter, class ST, class SA>
   >      bool operator==(
   >        const basic_string<
   >          typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
   >        const sub_match<BiIter>& rhs);

   > 7 Returns: `lhs == rhs.str()rhs.compare(lhs.c_str()) == 0`.

2. Change 28.9.2 [re.submatch.op]/8 as indicated:

   >     template <class BiIter, class ST, class SA>
   >      bool operator!=(
   >        const basic_string<
   >          typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
   >        const sub_match<BiIter>& rhs);

   > 8 Returns: `lhs != rhs.str()!(lhs == rhs)`.

3. Change 28.9.2 [re.submatch.op]/9 as indicated:

   >     template <class BiIter, class ST, class SA>
   >      bool operator<(
   >        const basic_string<
   >          typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
   >        const sub_match<BiIter>& rhs);

   > 9 Returns: `lhs < rhs.str()rhs.compare(lhs.c_str()) > 0`.

4. Change 28.9.2 [re.submatch.op]/10 as indicated:

   >     template <class BiIter, class ST, class SA>
   >      bool operator>(
   >        const basic_string<
   >          typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
   >        const sub_match<BiIter>& rhs);

   > 10 Returns: `lhs > rhs.str()rhs < lhs`.

5. Change 28.9.2 [re.submatch.op]/11 as indicated:

   >     template <class BiIter, class ST, class SA>
   >      bool operator>=(
   >        const basic_string<
   >        typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
   >      const sub_match<BiIter>& rhs);

   > 11 Returns: `lhs >= rhs.str()!(lhs < rhs)`.

6. Change 28.9.2 [re.submatch.op]/12 as indicated:

   >     template <class BiIter, class ST, class SA>
   >      bool operator<=(
   >        const basic_string<
   >          typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
   >        const sub_match<BiIter>& rhs);

   > 12 Returns: `lhs <= rhs.str()!(rhs < lhs)`.

7. Change 28.9.2 [re.submatch.op]/13 as indicated:

   >     template <class BiIter, class ST, class SA>
   >      bool operator==(const sub_match<BiIter>& lhs,
   >        const basic_string<
   >          typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);

   > 13 Returns: `lhs.str() == rhslhs.compare(rhs.c_str()) == 0`.

8. Change 28.9.2 [re.submatch.op]/14 as indicated:

   >     template <class BiIter, class ST, class SA>
   >      bool operator!=(const sub_match<BiIter>& lhs,
   >        const basic_string<
   >          typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);

   > 14 Returns: `lhs.str() != rhs!(lhs == rhs)`.

9. Change 28.9.2 [re.submatch.op]/15 as indicated:

   >     template <class BiIter, class ST, class SA>
   >      bool operator<(const sub_match<BiIter>& lhs,
   >        const basic_string<
   >          typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);

   > 15 Returns: `lhs.str() < rhslhs.compare(rhs.c_str()) < 0`.

10. Change 28.9.2 [re.submatch.op]/16 as indicated:

    >     template <class BiIter, class ST, class SA>
    >      bool operator>(const sub_match<BiIter>& lhs,
    >        const basic_string<
    >          typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);

    > 16 Returns: `lhs.str() > rhsrhs < lhs`.

11. Change 28.9.2 [re.submatch.op]/17 as indicated:

    >     template <class BiIter, class ST, class SA>
    >      bool operator>=(const sub_match<BiIter>& lhs,
    >        const basic_string<
    >          typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);

    > 17 Returns: `lhs.str() >= rhs!(lhs < rhs)`.

12. Change 28.9.2 [re.submatch.op]/18 as indicated:

    >     template <class BiIter, class ST, class SA>
    >      bool operator<=(const sub_match<BiIter>& lhs,
    >        const basic_string<
    >          typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);

    > 18 Returns: `lhs.str() <= rhs!(rhs < lhs)`.

13. Change 28.9.2 [re.submatch.op]/19 as indicated:

    >     template <class BiIter>
    >      bool operator==(typename iterator_traits<BiIter>::value_type const* lhs,
    >        const sub_match<BiIter>& rhs);

    > 19 Returns: `lhs == rhs.str()rhs.compare(lhs) == 0`.

14. Change 28.9.2 [re.submatch.op]/20 as indicated:

    >     template <class BiIter>
    >      bool operator!=(typename iterator_traits<BiIter>::value_type const* lhs,
    >        const sub_match<BiIter>& rhs);

    > 20 Returns: `lhs != rhs.str()!(lhs == rhs)`.

15. Change 28.9.2 [re.submatch.op]/21 as indicated:

    >     template <class BiIter>
    >      bool operator<(typename iterator_traits<BiIter>::value_type const* lhs,
    >        const sub_match<BiIter>& rhs);

    > 21 Returns: `lhs < rhs.str()rhs.compare(lhs) > 0`.

16. Change 28.9.2 [re.submatch.op]/22 as indicated:

    >     template <class BiIter>
    >      bool operator>(typename iterator_traits<BiIter>::value_type const* lhs,
    >        const sub_match<BiIter>& rhs);

    > 22 Returns: `lhs > rhs.str()rhs < lhs`.

17. Change 28.9.2 [re.submatch.op]/23 as indicated:

    >     template <class BiIter>
    >      bool operator>=(typename iterator_traits<BiIter>::value_type const* lhs,
    >        const sub_match<BiIter>& rhs);

    > 23 Returns: `lhs >= rhs.str()!(lhs < rhs)`.

18. Change 28.9.2 [re.submatch.op]/24 as indicated:

    >     template <class BiIter>
    >      bool operator<=(typename iterator_traits<BiIter>::value_type const* lhs,
    >        const sub_match<BiIter>& rhs);

    > 24 Returns: `lhs <= rhs.str()!(rhs < lhs)`.

19. Change 28.9.2 [re.submatch.op]/25 as indicated:

    >     template <class BiIter>
    >      bool operator==(const sub_match<BiIter>& lhs,
    >        typename iterator_traits<BiIter>::value_type const* rhs);

    > 25 Returns: `lhs.str() == rhslhs.compare(rhs) == 0`.

20. Change 28.9.2 [re.submatch.op]/26 as indicated:

    >     template <class BiIter>
    >      bool operator!=(const sub_match<BiIter>& lhs,
    >        typename iterator_traits<BiIter>::value_type const* rhs);

    > 26 Returns: `lhs.str() != rhs!(lhs == rhs)`.

21. Change 28.9.2 [re.submatch.op]/27 as indicated:

    >     template <class BiIter>
    >      bool operator<(const sub_match<BiIter>& lhs,
    >        typename iterator_traits<BiIter>::value_type const* rhs);

    > 27 Returns: `lhs.str() < rhslhs.compare(rhs) < 0`.

22. Change 28.9.2 [re.submatch.op]/28 as indicated:

    >     template <class BiIter>
    >      bool operator>(const sub_match<BiIter>& lhs,
    >        typename iterator_traits<BiIter>::value_type const* rhs);

    > 28 Returns: `lhs.str() > rhsrhs < lhs`.

23. Change 28.9.2 [re.submatch.op]/29 as indicated:

    >     template <class BiIter>
    >      bool operator>=(const sub_match<BiIter>& lhs,
    >        typename iterator_traits<BiIter>::value_type const* rhs);

    > 29 Returns: `lhs.str() >= rhs!(lhs < rhs)`.

24. Change 28.9.2 [re.submatch.op]/30 as indicated:

    >     template <class BiIter>
    >      bool operator<=(const sub_match<BiIter>& lhs,
    >        typename iterator_traits<BiIter>::value_type const* rhs);

    > 30 Returns: `lhs.str() <= rhs!(rhs < lhs)`.

25. Change 28.9.2 [re.submatch.op]/31 as indicated:

    >     template <class BiIter>
    >      bool operator==(typename iterator_traits<BiIter>::value_type const& lhs,
    >        const sub_match<BiIter>& rhs);

    > 31 Returns: `basic_string<typename iterator_traits<BiIter>::value_type>(1, lhs) == rhs.str()`.
    > 31 Returns: `rhs.compare(typename sub_match<BiIter>::string_type(1, lhs)) == 0`.

26. Change 28.9.2 [re.submatch.op]/32 as indicated:

    >     template <class BiIter>
    >      bool operator!=(typename iterator_traits<BiIter>::value_type const& lhs,
    >        const sub_match<BiIter>& rhs);

    > 32 Returns: `basic_string<typename iterator_traits<BiIter>::value_type>(1, lhs) != rhs.str()!(lhs == rhs)`.

27. Change 28.9.2 [re.submatch.op]/33 as indicated:

    >     template <class BiIter>
    >      bool operator<(typename iterator_traits<BiIter>::value_type const& lhs,
    >        const sub_match<BiIter>& rhs);

    > 33 Returns: `basic_string<typename iterator_traits<BiIter>::value_type>(1, lhs) < rhs.str()`.
    > 33 Returns: `rhs.compare(typename sub_match<BiIter>::string_type(1, lhs)) > 0`.

28. Change 28.9.2 [re.submatch.op]/34 as indicated:

    >     template <class BiIter>
    >      bool operator>(typename iterator_traits<BiIter>::value_type const& lhs,
    >        const sub_match<BiIter>& rhs);

    > 34 Returns: `basic_string<typename iterator_traits<BiIter>::value_type>(1, lhs) > rhs.str()rhs < lhs`.

29. Change 28.9.2 [re.submatch.op]/35 as indicated:

    >     template <class BiIter>
    >      bool operator>=(typename iterator_traits<BiIter>::value_type const& lhs,
    >        const sub_match<BiIter>& rhs);

    > 35 Returns: `basic_string<typename iterator_traits<BiIter>::value_type>(1, lhs) >= rhs.str()!(lhs < rhs)`.

30. Change 28.9.2 [re.submatch.op]/36 as indicated:

    >     template <class BiIter>
    >      bool operator<=(typename iterator_traits<BiIter>::value_type const& lhs,
    >        const sub_match<BiIter>& rhs);

    > 36 Returns: `basic_string<typename iterator_traits<BiIter>::value_type>(1, lhs) <= rhs.str()!(rhs < lhs)`.

31. Change 28.9.2 [re.submatch.op]/37 as indicated:

    >     template <class BiIter>
    >      bool operator==(const sub_match<BiIter>& lhs,
    >        typename iterator_traits<BiIter>::value_type const& rhs);

    > 37 Returns: `lhs.str() == basic_string<typename iterator_traits<BiIter>::value_type>(1, rhs)`.
    > 37 Returns: `lhs.compare(typename sub_match<BiIter>::string_type(1, rhs)) == 0`.

32. Change 28.9.2 [re.submatch.op]/38 as indicated:

    >     template <class BiIter>
    >      bool operator!=(const sub_match<BiIter>& lhs,
    >        typename iterator_traits<BiIter>::value_type const& rhs);

    > 38 Returns: `lhs.str() != basic_string<typename iterator_traits<BiIter>::value_type>(1, rhs)!(lhs == rhs)`.

33. Change 28.9.2 [re.submatch.op]/39 as indicated:

    >     template <class BiIter>
    >      bool operator<(const sub_match<BiIter>& lhs,
    >        typename iterator_traits<BiIter>::value_type const& rhs);

    > 39 Returns: `lhs.str() < basic_string<typename iterator_traits<BiIter>::value_type>(1, rhs)`.
    > 39 Returns: `lhs.compare(typename sub_match<BiIter>::string_type(1, rhs)) < 0`.

34. Change 28.9.2 [re.submatch.op]/40 as indicated:

    >     template <class BiIter>
    >      bool operator>(const sub_match<BiIter>& lhs,
    >        typename iterator_traits<BiIter>::value_type const& rhs);

    > 40 Returns: `lhs.str() > basic_string<typename iterator_traits<BiIter>::value_type>(1, rhs)rhs < lhs`.

35. Change 28.9.2 [re.submatch.op]/41 as indicated:

    >     template <class BiIter>
    >      bool operator>=(const sub_match<BiIter>& lhs,
    >        typename iterator_traits<BiIter>::value_type const& rhs);

    > 41 Returns: `lhs.str() >= basic_string<typename iterator_traits<BiIter>::value_type>(1, rhs)!(lhs < rhs)`.

36. Change 28.9.2 [re.submatch.op]/42 as indicated:

    >     template <class BiIter>
    >      bool operator<=(const sub_match<BiIter>& lhs,
    >        typename iterator_traits<BiIter>::value_type const& rhs);

    > 42 Returns: `lhs.str() <= basic_string<typename iterator_traits<BiIter>::value_type>(1, rhs)!(rhs < lhs)`.

### 1182. Unfortunate hash dependencies

Section: 20.9.13 [unord.hash] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-07-28 Last modified: 2015-04-08

View other active issues in [unord.hash].

View all other issues in [unord.hash].

View all issues with C++11 status.

Discussion:

Addresses UK 324

The implied library dependencies created by spelling out all the `hash` template specializations in the `<functional>` synopsis are unfortunate. The potential coupling is greatly reduced if the `hash` specialization is declared in the appropriate header for each library type, as it is much simpler to forward declare the primary template and provide a single specialization than it is to implement a `hash` function for a `string` or `vector` without providing a definition for the whole `string/vector` template in order to access the necessary bits.

Note that the proposed resolution purely involves moving the declarations of a few specializations, it specifically does not make any changes to 20.9.13 [unord.hash].

[ 2009-09-15 Daniel adds: ]

I suggest to add to the current existing proposed resolution the following items.

- Add to the very first strike-list of the currently suggested resolution the following lines:

  >     template <> struct hash<std::error_code>;
  >     template <> struct hash<std::thread::id>;

- Add the following declarations to 19.5 [syserr], header `<system_error>` synopsis after // 19.5.4:

  >     // 19.5.x hash support
  >     template <class T> struct hash;
  >     template <> struct hash<error_code>;

- Add a new clause 19.5.X (probably after 19.5.4):

  > 19.5.X Hash support [syserr.hash]

  >     template <> struct hash<error_code>;

  > > An explicit specialization of the class template hash (20.9.13 [unord.hash]) shall be provided for the type `error_code` suitable for using this type as key in unordered associative containers (23.5 [unord]).

- Add the following declarations to 30.3.1.1 [thread.thread.id] just after the declaration of the comparison operators:

  >     template <class T> struct hash;
  >     template <> struct hash<thread::id>;

- Add a new paragraph at the end of 30.3.1.1 [thread.thread.id]:

  >     template <> struct hash<thread::id>;

  > > An explicit specialization of the class template hash (20.9.13 [unord.hash]) shall be provided for the type `thread::id` suitable for using this type as key in unordered associative containers (23.5 [unord]).

- Issue 889 independently suggests moving the specialization `std::hash<std::thread::id>` to header `<thread>`.

[ 2009-11-13 Alisdair adopts Daniel's suggestion and the extended note from 889. ]

[ 2010-01-31 Alisdair: related to 1245 and 978. ]

[ 2010-02-07 Proposed wording updated by Beman, Daniel, Alisdair and Ganesh. ]

[ 2010-02-09 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Strike the following specializations declared in the `<functional>` synopsis p2 20.9 [function.objects]

>     template <> struct hash<std::string>;
>     template <> struct hash<std::u16string>;
>     template <> struct hash<std::u32string>;
>     template <> struct hash<std::wstring>;

>     template <> struct hash<std::error_code>;
>     template <> struct hash<std::thread::id>;
>     template <class Allocator> struct hash<std::vector<bool, Allocator> >;
>     template <std::size_t N> struct hash<std::bitset<N> >;

Add the following at the end of 20.9.13 [unord.hash]:

>     template <> struct hash<bool>;
>     template <> struct hash<char>;
>     template <> struct hash<signed char>;
>     template <> struct hash<unsigned char>;
>     template <> struct hash<char16_t>;
>     template <> struct hash<char32_t>;
>     template <> struct hash<wchar_t>;
>     template <> struct hash<short>;
>     template <> struct hash<unsigned short>;
>     template <> struct hash<int>;
>     template <> struct hash<unsigned int>;
>     template <> struct hash<long>;
>     template <> struct hash<long long>;
>     template <> struct hash<unsigned long>;
>     template <> struct hash<unsigned long long>;
>     template <> struct hash<float>;
>     template <> struct hash<double>;
>     template <> struct hash<long double>;
>     template<class T> struct hash<T*>;

> Specializations meeting the requirements of class template `hash` 20.9.13 [unord.hash].

Add the following declarations to 19.5 [syserr], header `<system_error>` synopsis after // 19.5.4:

>     // [syserr.hash] hash support
>     template <class T> struct hash;
>     template <> struct hash<error_code>;

Add a new clause 19.5.X (probably after 19.5.4):

> 19.5.X Hash support [syserr.hash]

>     template <> struct hash<error_code>;

> Specialization meeting the requirements of class template `hash` 20.9.13 [unord.hash].

Add the following declarations to the synopsis of `<string>` in 21.3 [string.classes]

>     // [basic.string.hash] hash support
>     template <class T> struct hash;
>     template <> struct hash<string>;
>     template <> struct hash<u16string>;
>     template <> struct hash<u32string>;
>     template <> struct hash<wstring>;

Add a new clause 21.4.X

> 21.4.X Hash support [basic.string.hash]>

>     template <> struct hash<string>;
>     template <> struct hash<u16string>;
>     template <> struct hash<u32string>;
>     template <> struct hash<wstring>;

> Specializations meeting the requirements of class template `hash` 20.9.13 [unord.hash].

Add the following declarations to the synopsis of `<vector>` in 23.3 [sequences]

>     // 21.4.x hash support
>     template <class T> struct hash;
>     template <class Allocator> struct hash<vector<bool, Allocator>>;

Add a new paragraph to the end of 23.3.7 [vector.bool]

>     template <class Allocator> struct hash<vector<bool, Allocator>>;

> Specialization meeting the requirements of class template `hash` 20.9.13 [unord.hash].

Add the following declarations to the synopsis of `<bitset>` in 20.6 [template.bitset]

>     // [bitset.hash] hash support
>     template <class T> struct hash;
>     template <size_t N> struct hash<bitset<N> >;

Add a new subclause 20.3.7.X [bitset.hash]

> 20.3.7.X bitset hash support [bitset.hash]

>     template <size_t N> struct hash<bitset<N> >;

> Specialization meeting the requirements of class template `hash` 20.9.13 [unord.hash].

Add the following declarations to 30.3.1.1 [thread.thread.id] synopsis just after the declaration of the comparison operators:

>     template <class T> struct hash;
>     template <> struct hash<thread::id>;

Add a new paragraph at the end of 30.3.1.1 [thread.thread.id]:

>     template <> struct hash<thread::id>;

> Specialization meeting the requirements of class template `hash` 20.9.13 [unord.hash].

Change Header <typeindex> synopsis 20.14.1 [type.index.synopsis] as indicated:

>     namespace std {
>     class type_index;
>       // [type.index.hash] hash support
>       template <class T> struct hash;
>       template<> struct hash<type_index>;  : public unary_function<type_index, size_t> {
>         size_t operator()(type_index index) const;
>       }
>     }

Change Template specialization hash<type_index> [type.index.templ] as indicated:

> 20.11.4 Template specialization hash<type_index> [type.index.templ] Hash support [type.index.hash]

>     size_t operator()(type_index index) const;

> > Returns: `index.hash_code()`

>     template<> struct hash<type_index>;

> Specialization meeting the requirements of class template `hash` [unord.hash]. For an object `index` of type `type_index`, `hash<type_index>()(index)` shall evaluate to the same value as `index.hash_code()`.

### 1183. `basic_ios::set_rdbuf` may break class invariants

Section: 27.5.5.3 [basic.ios.members] Status: C++11 Submitter: Daniel Krügler Opened: 2009-07-28 Last modified: 2015-04-08

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Discussion:

The protected member function `set_rdbuf` had been added during the process of adding move and swap semantics to IO classes. A relevant property of this function is described by it's effects in 27.5.5.3 [basic.ios.members]/19:

> Effects: Associates the `basic_streambuf` object pointed to by sb with this stream without calling `clear()`.

This means that implementors of or those who derive from existing IO classes could cause an internal state where the stream buffer could be 0, but the IO class has the state `good()`. This would break several currently existing implementations which rely on the fact that setting a stream buffer via the currently only ways, i.e. either by calling

>     void init(basic_streambuf<charT,traits>* sb);

or by calling

>     basic_streambuf<charT,traits>* rdbuf(basic_streambuf<charT,traits>* sb);

to set `rdstate()` to `badbit`, if the buffer is 0. This has the effect that many internal functions can simply check `rdstate()` instead of `rdbuf()` for being 0.

I therefore suggest that a requirement is added for callers of `set_rdbuf` to set a non-0 value.

[ 2009-10 Santa Cruz: ]

> Moved to Open. Martin volunteers to provide new wording, where `set_rdbuf()` sets the `badbit` but does not cause an exception to be thrown like a call to `clear()` would.

[ 2009-10-20 Martin provides wording: ]

Change 27.5.5.3 [basic.ios.members] around p. 19 as indicated:

>     void set_rdbuf(basic_streambuf<charT, traits>* sb);

> > Effects: Associates the `basic_streambuf` object pointed to by `sb` with this stream without calling `clear()`. Postconditions: `rdbuf() == sb`.

> > Effects: As if:

> > >     iostate state = rdstate();
> > >     try { rdbuf(sb); }
> > >     catch(ios_base::failure) {
> > >        if (0 == (state & ios_base::badbit))
> > >            unsetf(badbit);
> > >     }

> > Throws: Nothing.

Rationale:

We need to be able to call `set_rdbuf()` on stream objects for which (`rdbuf() == 0`) holds without causing `ios_base::failure` to be thrown. We also don't want `badbit` to be set as a result of setting `rdbuf()` to 0 if it wasn't set before the call. This changed Effects clause maintains the current behavior (as of N2914) without requiring that `sb` be non-null.

[ Post-Rapperswil ]

Several reviewers and the submitter believe that the best solution would be to add a pre-condition that the buffer shall not be a null pointer value.

> Moved to Tentatively Ready with revised wording provided by Daniel after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

1. Add a new pre-condition just before 27.5.4.2 [basic.ios.members]/23 as indicated:

   >     void set_rdbuf(basic_streambuf<charT, traits>* sb);

   > > ?? Requires: `sb != nullptr`.

   > > 23 Effects: Associates the `basic_streambuf` object pointed to by `sb` with this stream without calling `clear()`.

   > > 24 Postconditions: `rdbuf() == sb`.

   > > 25 Throws: Nothing.

Rationale:

We believe that setting a `nullptr` stream buffer can be prevented.

### 1185. Iterator categories and output iterators

Section: 24.2 [iterator.requirements] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-07-31 Last modified: 2015-04-08

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Discussion:

(wording relative to N2723 pending new working paper)

According to p3 24.2 [iterator.requirements], Forward iterators, Bidirectional iterators and Random Access iterators all satisfy the requirements for an Output iterator:

> XXX iterators satisfy all the requirements of the input and output iterators and can be used whenever either kind is specified ...

Meanwhile, p4 goes on to contradict this:

> Besides its category, a forward, bidirectional, or random access iterator can also be mutable or constant...

> ... Constant iterators do not satisfy the requirements for output iterators

The latter seems to be the overriding concern, as the iterator tag hierarchy does not define `forward_iterator_tag` as multiply derived from both `input_iterator_tag` and `output_iterator_tag`.

The work on concepts for iterators showed us that output iterator really is fundamentally a second dimension to the iterator categories, rather than part of the linear input -> forward -> bidirectional -> random-access sequence. It would be good to clear up these words to reflect that, and separately list output iterator requirements in the requires clauses for the appropriate algorithms and operations.

[ 2010 Pittsburgh: Moved to NAD Editorial Resolved. Rationale added below. ]

Rationale:

Solved by N3066.

Proposed resolution:

### 1187. `std::decay`

Section: 20.10.7.6 [meta.trans.other] Status: C++11 Submitter: Jason Merrill Opened: 2009-08-07 Last modified: 2015-04-08

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Discussion:

I notice that `std::decay` is specified to strip the cv-quals from anything but an array or pointer. This seems incorrect for values of class type, since class rvalues can have cv-qualified type (3.10 [basic.lval]/9).

[ 2009-08-09 Howard adds: ]

> See the thread starting with c++std-lib-24568 for further discussion. And here is a convenience link to the original proposal. Also see the closely related issue 705.

[ 2010 Pittsburgh: Moved to Ready. ]

Proposed resolution:

Add a note to `decay` in 20.10.7.6 [meta.trans.other]:

> [Note: This behavior is similar to the lvalue-to-rvalue (4.1), array-to-pointer (4.2), and function-to-pointer (4.3) conversions applied when an lvalue expression is used as an rvalue, but also strips cv-qualifiers from class types in order to more closely model by-value argument passing. — end note]

### 1189. Awkward interface for changing the number of buckets in an unordered associative container

Section: 23.2.5 [unord.req], 23.5 [unord] Status: C++11 Submitter: Matt Austern Opened: 2009-08-10 Last modified: 2015-04-08

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Discussion:

Consider a typical use case: I create an `unordered_map` and then start adding elements to it one at a time. I know that it will eventually need to store a few million elements, so, for performance reasons, I would like to reserve enough capacity that none of the calls to `insert` will trigger a rehash.

Unfortunately, the existing interface makes this awkward. The user naturally sees the problem in terms of the number of elements, but the interface presents it as buckets. If `m` is the map and `n` is the expected number of elements, this operation is written `m.rehash(n / m.max_load_factor())` — not very novice friendly.

[ 2009-09-30 Daniel adds: ]

> I recommend to replace "`resize`" by a different name like "`reserve`", because that would better match the intended use-case. Rational: Any existing resize function has the on-success post-condition that the provided size is equal to `size()`, which is not satisfied for the proposal. Reserve seems to fit the purpose of the actual renaming suggestion.

[ 2009-10-28 Ganesh summarizes alternative resolutions and expresses a strong preference for the second (and opposition to the first): ]

> 1. In the unordered associative container requirements (23.2.5 [unord.req]), remove the row for rehash and replace it with:

>    > Table 87 — Unordered associative container requirements (in addition to container)
>    > Expression | Return type | Assertion/note pre-/post-condition | Complexity
>    > --- | --- | --- | ---
>    > `a.rehashreserve(n)` | `void` | Post: `a.bucket_count > max(a.size(), n) / a.max_load_factor()` and `a.bucket_count() >= n`. | Average case linear in `a.size()`, worst case quadratic.

>    Make the corresponding change in the class synopses in 23.5.4 [unord.map], 23.5.5 [unord.multimap], 23.5.6 [unord.set], and 23.5.7 [unord.multiset].

> 2. In 23.2.5 [unord.req]/9, table 98, append a new row after the last one:

>    > Table 87 — Unordered associative container requirements (in addition to container)
>    > Expression | Return type | Assertion/note pre-/post-condition | Complexity
>    > --- | --- | --- | ---
>    > `a.rehash(n)` | `void` | Post: `a.bucket_count > a.size() / a.max_load_factor()` and `a.bucket_count() >= n`. | Average case linear in `a.size()`, worst case quadratic.
>    > `a.reserve(n)` | `void` | Same as `a.rehash(ceil(n / a.max_load_factor()))` | Average case linear in `a.size()`, worst case quadratic.

>    In 23.5.4 [unord.map]/3 in the definition of class template `unordered_map`, in 23.5.5 [unord.multimap]/3 in the definition of class template `unordered_multimap`, in 23.5.6 [unord.set]/3 in the definition of class template `unordered_set` and in 23.5.7 [unord.multiset]/3 in the definition of class template `unordered_multiset`, add the following line after member function `rehash()`:

>    >     void reserve(size_type n);

[ 2009-10-28 Howard: ]

> Moved to Tentatively Ready after 5 votes in favor of Ganesh's option 2 above. The original proposed wording now appears here:

> > Informally: instead of providing `rehash(n)` provide `resize(n)`, with the semantics "make the container a good size for `n` elements".

> > In the unordered associative container requirements (23.2.5 [unord.req]), remove the row for rehash and replace it with:

> > > Table 87 — Unordered associative container requirements (in addition to container)
> > > Expression | Return type | Assertion/note pre-/post-condition | Complexity
> > > --- | --- | --- | ---
> > > `a.rehashresize(n)` | `void` | Post: `a.bucket_count > max(a.size(), n) / a.max_load_factor()` and `a.bucket_count() >= n`. | Average case linear in `a.size()`, worst case quadratic.

> > Make the corresponding change in the class synopses in 23.5.4 [unord.map], 23.5.5 [unord.multimap], 23.5.6 [unord.set], and 23.5.7 [unord.multiset].

Proposed resolution:

In 23.2.5 [unord.req]/9, table 98, append a new row after the last one:

> Table 87 — Unordered associative container requirements (in addition to container)
> Expression | Return type | Assertion/note pre-/post-condition | Complexity
> --- | --- | --- | ---
> `a.rehash(n)` | `void` | Post: `a.bucket_count > a.size() / a.max_load_factor()` and `a.bucket_count() >= n`. | Average case linear in `a.size()`, worst case quadratic.
> `a.reserve(n)` | `void` | Same as `a.rehash(ceil(n / a.max_load_factor()))` | Average case linear in `a.size()`, worst case quadratic.

In 23.5.4 [unord.map]/3 in the definition of class template `unordered_map`, in 23.5.5 [unord.multimap]/3 in the definition of class template `unordered_multimap`, in 23.5.6 [unord.set]/3 in the definition of class template `unordered_set` and in 23.5.7 [unord.multiset]/3 in the definition of class template `unordered_multiset`, add the following line after member function `rehash()`:

>     void reserve(size_type n);

### 1191. `tuple get` API should respect rvalues

Section: 20.4.2.6 [tuple.elem] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-08-18 Last modified: 2015-04-08

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Discussion:

The `tuple get` API should respect rvalues. This would allow for moving a single element out of a `tuple`-like type.

[ 2009-10-30 Alisdair adds: ]

> The issue of rvalue overloads of get for tuple-like types was briefly discussed in Santa Cruz.

> The feedback was this would be welcome, but we need full wording for the other types (`pair` and `array`) before advancing.

> I suggest the issue moves to Open from New as it has been considered, feedback given, and it has not (yet) been rejected as NAD.

[ 2010 Rapperswil: ]

> Note that wording has been provided, and this issue becomes more important now that we have added a function to support forwarding argument lists as `tuple`s. Move to Tentatively Ready.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Add the following signature to p2 20.4.1 [tuple.general]

>     template <size_t I, class ... Types>
>     typename tuple_element<I, tuple<Types...> >::type&& get(tuple<Types...> &&);

And again to 20.4.2.6 [tuple.elem].

>     template <size_t I, class ... Types>
>     typename tuple_element<I, tuple<Types...> >::type&& get(tuple<Types...>&& t);

> > Effects: Equivalent to `return std::forward<typename tuple_element<I, tuple<Types...> >::type&&>(get<I>(t));`

> > [Note: If a `T` in `Types` is some reference type `X&`, the return type is `X&`, not `X&&`. However, if the element type is non-reference type `T`, the return type is `T&&`. — end note]

Add the following signature to p1 20.2 [utility]

>     template <size_t I, class T1, class T2>
>     typename tuple_element<I, pair<T1,T2> >::type&& get(pair<T1, T2>&&);

And to p5 20.3.4 [pair.astuple]

>     template <size_t I, class T1, class T2>
>     typename tuple_element<I, pair<T1,T2> >::type&& get(pair<T1, T2>&& p);

> > Returns: If `I == 0` returns `std::forward<T1&&>(p.first)`; if `I == 1` returns `std::forward<T2&&>(p.second)`; otherwise the program is ill-formed.

> > Throws: Nothing.

Add the following signature to 23.3 [sequences] `<array>` synopsis

>     template <size_t I, class T, size_t N>
>     T&& get(array<T,N> &&);

And after p8 23.3.2.9 [array.tuple]

>     template <size_t I, class T, size_t N>
>     T&& get(array<T,N> && a);

> > Effects: Equivalent to `return std::move(get<I>(a));`

### 1192. `basic_string` missing definitions for `cbegin` / `cend` / `crbegin` / `crend`

Section: 21.4.3 [string.iterators] Status: C++11 Submitter: Jonathan Wakely Opened: 2009-08-14 Last modified: 2015-04-08

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Discussion:

Unlike the containers in clause 23, `basic_string` has definitions for `begin()` and `end()`, but these have not been updated to include `cbegin`, `cend`, `crbegin` and `crend`.

[ 2009-10-28 Howard: ]

> Moved to Tentatively NAD after 5 positive votes on c++std-lib. Added rationale.

[ 2009-10-28 Alisdair disagrees: ]

> I'm going to have to speak up as the dissenting voice.

> I agree the issue could be handled editorially, and that would be my preference if Pete feels this is appropriate. Failing that, I really think this issue should be accepted and moved to ready. The other begin/end functions all have a semantic definition for this template, and it is confusing if a small few are missing.

> I agree that an alternative would be to strike all the definitions for `begin/end/rbegin/rend` and defer completely to the requirements tables in clause 23. I think that might be confusing without a forward reference though, as those tables are defined in a later clause than the `basic_string` template itself. If someone wants to pursue this I would support it, but recommend it as a separate issue.

> So my preference is strongly to move Ready over NAD, and a stronger preference for NAD Editorial if Pete is happy to make these changes.

[ 2009-10-29 Howard: ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib. Removed rationale to mark it NAD. :-)

Proposed resolution:

Add to 21.4.3 [string.iterators]

>     iterator       begin();
>     const_iterator begin() const;
>     const_iterator cbegin() const;

> ...

>     iterator       end();
>     const_iterator end() const;
>     const_iterator cend() const;

> ...

>     reverse_iterator       rbegin();
>     const_reverse_iterator rbegin() const;
>     const_reverse_iterator crbegin() const;

> ...

>     reverse_iterator       rend();
>     const_reverse_iterator rend() const;
>     const_reverse_iterator crend() const;

### 1193. `default_delete` cannot be instantiated with incomplete types

Section: 20.8.1.1 [unique.ptr.dltr] Status: C++11 Submitter: Daniel Krügler Opened: 2009-08-18 Last modified: 2015-04-08

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Discussion:

According to the general rules of 17.6.4.8 [res.on.functions] p 2 b 5 the effects are undefined, if an incomplete type is used to instantiate a library template. But neither in 20.8.1.1 [unique.ptr.dltr] nor in any other place of the standard such explicit allowance is given. Since this template is intended to be instantiated with incomplete types, this must be fixed.

[ 2009-11-15 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2009-11-17 Alisdair Opens: ]

> LWG 1193 tries to support `unique_ptr` for incomplete types. I believe the proposed wording goes too far:

> > The template parameter `T` of `default_delete` may be an incomplete type.

> Do we really want to support `cv-void`? Suggested ammendment:

> > The template parameter `T` of `default_delete` may be an incomplete type other than `cv-void`.

> We might also consider saying something about arrays of incomplete types.

> Did we lose support for `unique_ptr<function-type>` when the concept-enabled work was shelved? If so, we might want a `default_delete` partial specialization for function types that does nothing. Alternatively, function types should not be supported by default, but there is no reason a user cannot support them via their own deletion policy.

> Function-type support might also lead to conditionally supporting a function-call operator in the general case, and that seems way too inventive at this stage to me, even if we could largely steal wording directly from `reference_wrapper`. `shared_ptr` would have similar problems too.

[ 2010-01-24 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Add two new paragraphs directly to 20.8.1.1 [unique.ptr.dltr] (before 20.8.1.1.2 [unique.ptr.dltr.dflt]) with the following content:

> The class template `default_delete` serves as the default deleter (destruction policy) for the class template `unique_ptr`.

> The template parameter `T` of `default_delete` may be an incomplete type.

### 1194. Unintended `queue` constructor

Section: 23.6 [container.adaptors] Status: C++11 Submitter: Howard Hinnant Opened: 2009-08-20 Last modified: 2015-04-08

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Discussion:

23.6.3.1 [queue.defn] has the following `queue` constructor:

>     template <class Alloc> explicit queue(const Alloc&);

This will be implemented like so:

>     template <class Alloc> explicit queue(const Alloc& a) : c(a) {}

The issue is that `Alloc` can be anything that a container will construct from, for example an `int`. Is this intended to compile?

>     queue<int> q(5);

Before the addition of this constructor, `queue<int>(5)` would not compile. I ask, not because this crashes, but because it is new and appears to be unintended. We do not want to be in a position of accidently introducing this "feature" in C++0X and later attempting to remove it.

I've picked on `queue`. `priority_queue` and `stack` have the same issue. Is it useful to create a `priority_queue` of 5 identical elements?

[ Daniel, Howard and Pablo collaborated on the proposed wording. ]

[ 2009-10 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

[ This resolution includes a semi-editorial clean up, giving definitions to members which in some cases weren't defined since C++98. This resolution also offers editorially different wording for 976, and it also provides wording for 1196. ]

Change container.adaptors, p1:

> The container adaptors each take a `Container` template parameter, and each constructor takes a `Container` reference argument. This container is copied into the `Container` member of each adaptor. If the container takes an allocator, then a compatible allocator may be passed in to the adaptor's constructor. Otherwise, normal copy or move construction is used for the container argument. [Note: it is not necessary for an implementation to distinguish between the one-argument constructor that takes a `Container` and the one- argument constructor that takes an allocator_type. Both forms use their argument to construct an instance of the container. — end note]

Change queue.defn, p1:

>     template <class T, class Container = deque<T> >
>     class queue {
>     public:
>       typedef typename Container::value_type      value_type;
>       typedef typename Container::reference       reference;
>       typedef typename Container::const_reference const_reference;
>       typedef typename Container::size_type       size_type;
>       typedef Container                           container_type;
>     protected:
>       Container c;

>     public:
>       explicit queue(const Container&);
>       explicit queue(Container&& = Container());
>       queue(queue&& q); : c(std::move(q.c)) {}
>       template <class Alloc> explicit queue(const Alloc&);
>       template <class Alloc> queue(const Container&, const Alloc&);
>       template <class Alloc> queue(Container&&, const Alloc&);
>       template <class Alloc> queue(queue&&, const Alloc&);
>       queue& operator=(queue&& q); { c = std::move(q.c); return *this; }

>       bool empty() const          { return c.empty(); }
>       ...
>     };

Add a new section after 23.6.3.1 [queue.defn], [queue.cons]:

> `queue` constructors [queue.cons]

>     explicit queue(const Container& cont);

> > Effects: Initializes `c` with `cont`.

>     explicit queue(Container&& cont = Container());

> > Effects: Initializes `c` with `std::move(cont)`.

>     queue(queue&& q)

> > Effects: Initializes `c` with `std::move(q.c)`.

> For each of the following constructors, if `uses_allocator<container_type, Alloc>::value` is `false`, then the constructor shall not participate in overload resolution.

>     template <class Alloc>
>       explicit queue(const Alloc& a);

> > Effects: Initializes `c` with `a`.

>     template <class Alloc>
>       queue(const container_type& cont, const Alloc& a);

> > Effects: Initializes `c` with `cont` as the first argument and `a` as the second argument.

>     template <class Alloc>
>       queue(container_type&& cont, const Alloc& a);

> > Effects: Initializes `c` with `std::move(cont)` as the first argument and `a` as the second argument.

>     template <class Alloc>
>       queue(queue&& q, const Alloc& a);

> > Effects: Initializes `c` with `std::move(q.c)` as the first argument and `a` as the second argument.

>     queue& operator=(queue&& q);

> > Effects: Assigns `c` with `std::move(q.c)`.

> > Returns: `*this`.

Add to 23.6.4.1 [priqueue.cons]:

>     priority_queue(priority_queue&& q);

> > Effects: Initializes `c` with `std::move(q.c)` and initializes `comp` with `std::move(q.comp)`.

> For each of the following constructors, if `uses_allocator<container_type, Alloc>::value` is `false`, then the constructor shall not participate in overload resolution.

>     template <class Alloc>
>       explicit priority_queue(const Alloc& a);

> > Effects: Initializes `c` with `a` and value-initializes `comp`.

>     template <class Alloc>
>       priority_queue(const Compare& compare, const Alloc& a);

> > Effects: Initializes `c` with `a` and initializes `comp` with `compare`.

>     template <class Alloc>
>       priority_queue(const Compare& compare, const Container& cont, const Alloc& a);

> > Effects: Initializes `c` with `cont` as the first argument and `a` as the second argument, and initializes `comp` with `compare`.

>     template <class Alloc>
>       priority_queue(const Compare& compare, Container&& cont, const Alloc& a);

> > Effects: Initializes `c` with `std::move(cont)` as the first argument and `a` as the second argument, and initializes `comp` with `compare`.

>     template <class Alloc>
>       priority_queue(priority_queue&& q, const Alloc& a);

> > Effects: Initializes `c` with `std::move(q.c)` as the first argument and `a` as the second argument, and initializes `comp` with `std::move(q.comp)`.

>     priority_queue& operator=(priority_queue&& q);

> > Effects: Assigns `c` with `std::move(q.c)` and assigns `comp` with `std::move(q.comp)`.

> > Returns: `*this`.

Change 23.6.5.2 [stack.defn]:

>     template <class T, class Container = deque<T> >
>     class stack {
>     public:
>       typedef typename Container::value_type      value_type;
>       typedef typename Container::reference       reference;
>       typedef typename Container::const_reference const_reference;
>       typedef typename Container::size_type       size_type;
>       typedef Container                           container_type;
>     protected:
>       Container c;

>     public:
>       explicit stack(const Container&);
>       explicit stack(Container&& = Container());
>       stack(stack&& s);
>       template <class Alloc> explicit stack(const Alloc&);
>       template <class Alloc> stack(const Container&, const Alloc&);
>       template <class Alloc> stack(Container&&, const Alloc&);
>       template <class Alloc> stack(stack&&, const Alloc&);
>       stack& operator=(stack&& s);

>       bool empty() const          { return c.empty(); }
>       ...
>     };

Add a new section after 23.6.5.2 [stack.defn], [stack.cons]:

> `stack` constructors [stack.cons]

>     stack(stack&& s);

> > Effects: Initializes `c` with `std::move(s.c)`.

> For each of the following constructors, if `uses_allocator<container_type, Alloc>::value` is `false`, then the constructor shall not participate in overload resolution.

>     template <class Alloc>
>       explicit stack(const Alloc& a);

> > Effects: Initializes `c` with `a`.

>     template <class Alloc>
>       stack(const container_type& cont, const Alloc& a);

> > Effects: Initializes `c` with `cont` as the first argument and `a` as the second argument.

>     template <class Alloc>
>       stack(container_type&& cont, const Alloc& a);

> > Effects: Initializes `c` with `std::move(cont)` as the first argument and `a` as the second argument.

>     template <class Alloc>
>       stack(stack&& s, const Alloc& a);

> > Effects: Initializes `c` with `std::move(s.c)` as the first argument and `a` as the second argument.

>     stack& operator=(stack&& s);

> > Effects: Assigns `c` with `std::move(s.c)`.

> > Returns: `*this`.

### 1195. "Diagnostic required" wording is insufficient to prevent UB

Section: 17 [library] Status: C++11 Submitter: Daniel Krügler Opened: 2009-08-18 Last modified: 2015-04-08

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Discussion:

Several parts of the library use the notion of "Diagnostic required" to indicate that in the corresponding situation an error diagnostic should occur, e.g. 20.8.1.1.2 [unique.ptr.dltr.dflt]/2

>     void operator()(T *ptr) const;

> > Effects: calls `delete` on `ptr`. A diagnostic is required if `T` is an incomplete type.

The problem with this approach is that such a requirement is insufficient to prevent undefined behavior, if this situation occurs. According to 1.3 [defns.diagnostic] a diagnostic message is defined as

> a message belonging to an implementation-defined subset of the implementation's output messages.

which doesn't indicate any relation to an ill-formed program. In fact, "compiler warnings" are a typical expression of such diagnostics. This means that above wording can be interpreted by compiler writers that they satisfy the requirements of the standard if they just produce such a "warning", if the compiler happens to compile code like this:

>     #include <memory>

>     struct Ukn; // defined somewhere else
>     Ukn* create_ukn(); // defined somewhere else

>     int main() {
>      std::default_delete<Ukn>()(create_ukn());
>     }

In this and other examples discussed here it was the authors intent to guarantee that the program is ill-formed with a required diagnostic, therefore such wording should be used instead. According to the general rules outlined in 1.4 [intro.compliance] it should be sufficient to require that these situations produce an ill-formed program and the "diagnostic required" part should be implied. The proposed resolution also suggests to remove several redundant wording of "Diagnostics required" to ensure that the absence of such saying does not cause a misleading interpretation.

[ 2009 Santa Cruz: ]

> Move to NAD.

> It's not clear that there's any important difference between "ill-formed" and "diagnostic required". From 1.4 [intro.compliance], 1.3 [defns.ill.formed], and 1.3 [defns.well.formed] it appears that an ill-formed program is one that is not correctly constructed according to the syntax rules and diagnosable semantic rules, which means that... "a conforming implementation shall issue at least one diagnostic message." The author's intent seems to be that we should be requiring a fatal error instead of a mere warning, but the standard just doesn't have language to express that distinction. The strongest thing we can ever require is a "diagnostic".

> The proposed rewording may be a clearer way of expressing the same thing that the WP already says, but such a rewording is editorial.

[ 2009 Santa Cruz: ]

> Considered again. Group disagrees that the change is technical, but likes it editorially. Moved to NAD Editorial.

[ 2009-11-19: Moved from NAD Editorial to Open. Please see the thread starting with Message c++std-lib-25916. ]

[ 2009-11-20 Daniel updated wording. ]

> The following resolution differs from the previous one by avoiding the unusual and misleading term "shall be ill-formed", which does also not follow the core language style. This resolution has the advantage of a minimum impact on the current wording, but I would like to mention that a more intrusive solution might be preferrable - at least as a long-term solution: Jens Maurer suggested the following approach to get rid of the usage of the term "ill-formed" from the library by introducing a new category to existing elements to the list of 17.5.1.4 [structure.specifications]/3, e.g. "type requirements" or "static constraints" that define conditions that can be checked during compile-time and any violation would make the program ill-formed. As an example, the currently existing phrase 20.4.2.5 [tuple.helper]/1

> > Requires: `I < sizeof...(Types)`. The program is ill-formed if `I` is out of bounds.

> could then be written as

> > Static constraints: `I < sizeof...(Types)`.

[ 2009-11-21 Daniel updated wording. ]

[ 2009-11-22 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

1. Change 20.11 [ratio]/2 as indicated:

   > Throughout this subclause, if the template argument types `R1` and `R2` shall be are not specializations of the `ratio` template, the program is ill-formed. Diagnostic required.

2. Change 20.11.3 [ratio.ratio]/1 as indicated:

   If t The template argument `D` shall not be is zero, and or the absolute values of the template arguments `N` and `D` shall be are not representable by type `intmax_t`, the program is ill-formed. Diagnostic required. [..]

3. Change 20.11.4 [ratio.arithmetic]/1 as indicated:

   > Implementations may use other algorithms to compute these values. If overflow occurs, the program is ill-formed a diagnostic shall be issued.

4. Change 20.11.5 [ratio.comparison]/2 as indicated:

   > [...] Implementations may use other algorithms to compute this relationship to avoid overflow. If overflow occurs, the program is ill-formed a diagnostic is required.

5. Change 20.8.1.1.2 [unique.ptr.dltr.dflt]/2 as indicated:

   > Effects: calls `delete` on `ptr`. A diagnostic is required if `T` is an incomplete type.

   > Remarks: If `T` is an incomplete type, the program is ill-formed.

6. Change 20.8.1.1.3 [unique.ptr.dltr.dflt1]/1 as indicated:

   >     void operator()(T* ptr) const;

   > > Effects: `operator()` calls `delete[]` on `ptr`. A diagnostic is required if `T` is an incomplete type.

   > > Remarks: If `T` is an incomplete type, the program is ill-formed.

7. Change 20.8.1.2.1 [unique.ptr.single.ctor] as indicated: [Note: This editorially improves the currently suggested wording of 932 by replacing

   > "shall be ill-formed" by "is ill-formed"]

   [If N3025 is accepted this bullet is applied identically in that paper as well.]

   > -1- Requires: `D` shall be default constructible, and that construction shall not throw an exception. `D` shall not be a reference type or pointer type (diagnostic required).

   > ...

   > Remarks: If this constructor is instantiated with a pointer type or reference type for the template argument `D`, the program is ill-formed.

8. Change 20.8.1.2.1 [unique.ptr.single.ctor]/8 as indicated: [Note: This editorially improves the currently suggested wording of 932 by replacing

   > "shall be ill-formed" by "is ill-formed"]

   [If N3025 is accepted this bullet is applied identically in that paper as well.]

   >     unique_ptr(pointer p);

   > > ...

   > > Remarks: If this constructor is instantiated with a pointer type or reference type for the template argument `D`, the program is ill-formed.

9. Change 20.8.1.2.1 [unique.ptr.single.ctor]/13 as indicated:

   > [..] If `d` is an rvalue, it will bind to the second constructor of this pair and the program is ill-formed. That constructor shall emit a diagnostic. [Note: The diagnostic could be implemented using a `static_assert` which assures that `D` is not a reference type. — end note] Else `d` is an lvalue and will bind to the first constructor of this pair. [..]

10. 20.8.1.2.1 [unique.ptr.single.ctor]/20: Solved by 950.

11. Change 20.8.1.3 [unique.ptr.runtime]/1 as indicated:

    > A specialization for array types is provided with a slightly altered interface.

    > - Conversions among different types of `unique_ptr<T[], D>` or to or from the non-array forms of `unique_ptr` are disallowed (diagnostic required) produce an ill-formed program.
    > - ...

12. Change 20.12.5 [time.duration]/2-4 as indicated:

    > 2 Requires: `Rep` shall be an arithmetic type or a class emulating an arithmetic type. If a program instantiates `duration` with a `duration` type for the template argument `Rep` a diagnostic is required.

    > 3 Remarks: If `duration` is instantiated with a `duration` type for the template argument `Rep`, the program is ill-formed.

    > 3 4 Requires Remarks: If `Period` shall be is not a specialization of `ratio`, diagnostic required the program is ill-formed.

    > 4 5 Requires Remarks: If `Period::num` shall be is not positive, diagnostic required the program is ill-formed.

13. 20.12.5.1 [time.duration.cons]/1+4: Apply 1177
14. 20.12.5.5 [time.duration.nonmember]/4+6+8+11: Apply 1177
15. 20.12.5.7 [time.duration.cast]/1: Apply 1177

16. Change 20.12.6 [time.point]/2 as indicated:

    > If `Duration` shall be is not an instance of `duration`, the program is ill-formed. Diagnostic required.

17. 20.12.6.1 [time.point.cons]/3: Apply 1177
18. 20.12.6.7 [time.point.cast]/1: Apply 1177

### 1196. move semantics undefined for priority_queue

Section: 23.6.4.1 [priqueue.cons] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-08-19 Last modified: 2015-04-08

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Discussion:

The class template `priority_queue` declares signatures for a move constructor and move assignment operator in its class definition. However, it does not provide a definition (unlike `std::queue`, and proposed resolution for `std::stack`.) Nor does it provide a text clause specifying their behaviour.

[ 2009-08-23 Daniel adds: ]

> 1194 provides wording that solves this issue.

[ 2009-10 Santa Cruz: ]

> Mark NAD Editorial Resolved, solved by issue 1194.

Proposed resolution:

### 1197. Can unordered containers have `bucket_count() == 0`?

Section: 23.2.5 [unord.req] Status: C++11 Submitter: Howard Hinnant Opened: 2009-08-24 Last modified: 2015-04-08

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Discussion:

Table 97 "Unordered associative container requirements" in 23.2.5 [unord.req] says:

> Table 97 — Unordered associative container requirements (in addition to container)
> Expression | Return type | Assertion/note pre-/post-condition | Complexity
> --- | --- | --- | ---
> `b.bucket(k)` | `size_type` | Returns the index of the bucket in which elements with keys equivalent to `k` would be found, if any such element existed. Post: the return value shall be in the range `[0, b.bucket_count())`. | Constant

What should `b.bucket(k)` return if `b.bucket_count() == 0`?

I believe allowing `b.bucket_count() == 0` is important. It is a very reasonable post-condition of the default constructor, or of a moved-from container.

I can think of several reasonable results from `b.bucket(k)` when `b.bucket_count() == 0`:

1. Return 0.
2. Return `numeric_limits<size_type>::max()`.
3. Throw a `domain_error`.
4. Requires: `b.bucket_count() != 0`.

[ 2009-08-26 Daniel adds: ]

> A forth choice would be to add the pre-condition "`b.bucket_count() != 0`" and thus imply undefined behavior if this is violated.

> [ Howard: I like this option too, added to the list. ]

> Further on here my own favorite solution (rationale see below):

> Suggested resolution:

> [Rationale: I suggest to follow choice (1). The main reason is that all associative container functions which take a key argument, are basically free of pre-conditions and non-disrupting, therefore excluding choices (3) and (4). Option (2) seems a bit unexpected to me. It would be more natural, if several similar functions would exist which would also justify the existence of a symbolic constant like npos for this situation. The value 0 is both simple and consistent, it has exactly the same role as a past-the-end iterator value. A typical use-case is:

> >     size_type pos = m.bucket(key);
> >     if (pos != m.bucket_count()) {
> >      ...
> >     } else {
> >      ...
> >     }

> — end Rationale]

> - Change Table 97 in 23.2.5 [unord.req] as follows (Row b.bucket(k), Column "Assertion/..."):

> > Table 97 — Unordered associative container requirements (in addition to container)
> > Expression | Return type | Assertion/note pre-/post-condition | Complexity
> > --- | --- | --- | ---
> > `b.bucket(k)` | `size_type` | Returns the index of the bucket in which elements with keys equivalent to `k` would be found, if any such element existed. Post: if b.bucket_count() != 0, the return value shall be in the range `[0, b.bucket_count())`, otherwise 0. | Constant

[ 2010-01-25 Choice 4 put into proposed resolution section. ]

[ 2010-01-31 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Change Table 97 in 23.2.5 [unord.req] as follows (Row b.bucket(k), Column "Assertion/..."):

> Table 97 — Unordered associative container requirements (in addition to container)
> Expression | Return type | Assertion/note pre-/post-condition | Complexity
> --- | --- | --- | ---
> `b.bucket(k)` | `size_type` | Pre: `b.bucket_count() > 0` Returns the index of the bucket in which elements with keys equivalent to `k` would be found, if any such element existed. Post: the return value shall be in the range `[0, b.bucket_count())`. | Constant

### 1198. Container adaptor swap: member or non-member?

Section: 23.6 [container.adaptors] Status: C++11 Submitter: Pablo Halpern Opened: 2009-08-26 Last modified: 2015-04-08

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Discussion:

Under 23.6 [container.adaptors] of N2914 the member function of `swap` of `queue` and `stack` call:

>     swap(c, q.c);

But under 23.6 [container.adaptors] of N2723 these members are specified to call:

>     c.swap(q.c);

Neither draft specifies the semantics of member `swap` for `priority_queue` though it is declared.

Although the distinction between member `swap` and non-member `swap` is not important when these adaptors are adapting standard containers, it may be important for user-defined containers.

We (Pablo and Howard) feel that it is more likely for a user-defined container to support a namespace scope `swap` than a member `swap`, and therefore these adaptors should use the container's namespace scope `swap`.

[ 2009-09-30 Daniel adds: ]

> The outcome of this issue should be considered with the outcome of 774 both in style and in content (e.g. 774 bullet 9 suggests to define the semantic of `void priority_queue::swap(priority_queue&)` in terms of the member `swap` of the container).

[ 2010-03-28 Daniel update to diff against N3092. ]

[ 2010 Rapperswil: ]

> Preference to move the wording into normative text, rather than inline function definitions in the class synopsis. Move to Tenatively Ready.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 23.6.3.1 [queue.defn]:

>     template <class T, class Container = deque<T> >
>     class queue {
>        ...
>        void swap(queue& q) { using std::swap;
>                               c.swap(c, q.c); }
>        ...
>     };

Change 23.6.4 [priority.queue]:

>     template <class T, class Container = vector<T>,
>               class Compare = less<typename Container::value_type> >
>     class priority_queue {
>         ...
>         void swap(priority_queue& q); { using std::swap;
>                                          swap(c, q.c);
>                                          swap(comp, q.comp); }
>         ...
>     };

Change 23.6.5.2 [stack.defn]:

>     template <class T, class Container = deque<T> >
>     class stack {
>        ...
>        void swap(stack& s) { using std::swap;
>                               c.swap(c, s.c); }
>        ...
>     };

### 1199. Missing extended copy constructor in container adaptors

Section: 23.6 [container.adaptors] Status: C++11 Submitter: Pablo Halpern Opened: 2009-08-26 Last modified: 2015-04-08

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Discussion:

`queue` has a constructor:

>     template <class Alloc>
>       queue(queue&&, const Alloc&);

but it is missing a corresponding constructor:

>     template <class Alloc>
>       queue(const queue&, const Alloc&);

The same is true of `priority_queue`, and `stack`. This "extended copy constructor" is needed for consistency and to ensure that the user of a container adaptor can always specify the allocator for his adaptor.

[ 2010-02-01 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

[ This resolution has been harmonized with the proposed resolution to issue 1194 ]

Change 23.6.3.1 [queue.defn], p1:

>     template <class T, class Container = deque<T> >
>     class queue {
>     public:
>       typedef typename Container::value_type      value_type;
>       typedef typename Container::reference       reference;
>       typedef typename Container::const_reference const_reference;
>       typedef typename Container::size_type       size_type;
>       typedef Container                           container_type;
>     protected:
>       Container c;

>     public:
>       explicit queue(const Container&);
>       explicit queue(Container&& = Container());
>       queue(queue&& q);

>       template <class Alloc> explicit queue(const Alloc&);
>       template <class Alloc> queue(const Container&, const Alloc&);
>       template <class Alloc> queue(Container&&, const Alloc&);
>       template <class Alloc> queue(const queue&, const Alloc&);
>       template <class Alloc> queue(queue&&, const Alloc&);
>       queue& operator=(queue&& q);

>       bool empty() const          { return c.empty(); }
>       ...
>     };

To the new section 23.6.3.2 [queue.cons], introduced in 1194, add:

>     template <class Alloc>
>       queue(const queue& q, const Alloc& a);

> > Effects: Initializes `c` with `q.c` as the first argument and `a` as the second argument.

Change 23.6.4 [priority.queue] as follows (I've an included an editorial change to move the poorly-placed move-assignment operator):

>     template <class T, class Container = vector<T>,
>               class Compare = less<typename Container::value_type> >
>     class priority_queue {
>     public:
>       typedef typename Container::value_type      value_type;
>       typedef typename Container::reference       reference;
>       typedef typename Container::const_reference const_reference;
>       typedef typename Container::size_type       size_type;
>       typedef          Container                  container_type;
>     protected:
>       Container c;
>       Compare comp;

>     public:
>       priority_queue(const Compare& x, const Container&);
>       explicit priority_queue(const Compare& x = Compare(), Container&& = Container());
>       template <class InputIterator>
>         priority_queue(InputIterator first, InputIterator last,
>                        const Compare& x, const Container&);
>       template <class InputIterator>
>         priority_queue(InputIterator first, InputIterator last,
>                        const Compare& x = Compare(), Container&& = Container());
>       priority_queue(priority_queue&&);
>       priority_queue& operator=(priority_queue&&);
>       template <class Alloc> explicit priority_queue(const Alloc&);
>       template <class Alloc> priority_queue(const Compare&, const Alloc&);
>       template <class Alloc> priority_queue(const Compare&,
>                                             const Container&, const Alloc&);
>       template <class Alloc> priority_queue(const Compare&,
>                                             Container&&, const Alloc&);
>       template <class Alloc> priority_queue(const priority_queue&, const Alloc&);
>       template <class Alloc> priority_queue(priority_queue&&, const Alloc&);

>       priority_queue& operator=(priority_queue&&);
>       ...
>     };

Add to 23.6.4.1 [priqueue.cons]:

>     template <class Alloc>
>       priority_queue(const priority_queue& q, const Alloc& a);

> > Effects: Initializes `c` with `q.c` as the first argument and `a` as the second argument, and initializes `comp` with `q.comp`.

Change 23.6.5.2 [stack.defn]:

>     template <class T, class Container = deque<T> >
>     class stack {
>     public:
>       typedef typename Container::value_type      value_type;
>       typedef typename Container::reference       reference;
>       typedef typename Container::const_reference const_reference;
>       typedef typename Container::size_type       size_type;
>       typedef Container                           container_type;
>     protected:
>       Container c;

>     public:
>       explicit stack(const Container&);
>       explicit stack(Container&& = Container());
>       stack(stack&& s);

>       template <class Alloc> explicit stack(const Alloc&);
>       template <class Alloc> stack(const Container&, const Alloc&);
>       template <class Alloc> stack(Container&&, const Alloc&);
>       template <class Alloc> stack(const stack&, const Alloc&);
>       template <class Alloc> stack(stack&&, const Alloc&);
>       stack& operator=(stack&& s);

>       bool empty() const          { return c.empty(); }
>       ...
>     };

To the new section 23.6.5.3 [stack.cons], introduced in 1194, add:

>     template <class Alloc>
>       stack(const stack& s, const Alloc& a);

> > Effects: Initializes `c` with `s.c` as the first argument and `a` as the second argument.

### 1204. Global permission to move

Section: 17.6.4.9 [res.on.arguments] Status: C++11 Submitter: Howard Hinnant Opened: 2009-09-12 Last modified: 2015-04-08

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Discussion:

When a library function binds an rvalue reference parameter to an argument, the library must be able to assume that the bound argument is a temporary, and not a moved-from lvalue. The reason for this is that the library function must be able to modify that argument without concern that such modifications will corrupt the logic of the calling code. For example:

>     template <class T, class A>
>     void
>     vector<T, A>::push_back(value_type&& v)
>     {
>         // This function should move from v, potentially modifying
>         // the object v is bound to.
>     }

If `v` is truly bound to a temporary, then `push_back` has the only reference to this temporary in the entire program. Thus any modifications will be invisible to the rest of the program.

If the client supplies `std::move(x)` to `push_back`, the onus is on the client to ensure that the value of `x` is no longer important to the logic of his program after this statement. I.e. the client is making a statement that `push_back` may treat `x` as a temporary.

> The above statement is the very foundation upon which move semantics is based.

The standard is currently lacking a global statement to this effect. I propose the following addition to 17.6.4.9 [res.on.arguments]:

> Each of the following statements applies to all arguments to functions defined in the C++ standard library, unless explicitly stated otherwise.

> - If an argument to a function has an invalid value (such as a value outside the domain of the function, or a pointer invalid for its intended use), the behavior is undefined.
> - If a function argument is described as being an array, the pointer actually passed to the function shall have a value such that all address computations and accesses to objects (that would be valid if the pointer did point to the first element of such an array) are in fact valid.
> - If a function argument binds to an rvalue reference parameter, the C++ standard library may assume that this parameter is a unique reference to this argument. If the parameter is a generic parameter of the form `T&&`, and an lvalue of type `A` is bound, then the binding is considered to be to an lvalue reference (14.8.2.1 [temp.deduct.call]) and thus not covered by this clause. [Note: If a program casts an lvalue to an rvalue while passing that lvalue to a library function (e.g. `move(x)`), then the program is effectively asking the library to treat that lvalue as a temporary. The library is at liberty to optimize away aliasing checks which might be needed if the argument were an lvalue. — end note]

Such a global statement will eliminate the need for piecemeal statements such as 23.2.1 [container.requirements.general]/13:

> An object bound to an rvalue reference parameter of a member function of a container shall not be an element of that container; no diagnostic required.

Additionally this clarifies that move assignment operators need not perform the traditional `if (this != &rhs)` test commonly found (and needed) in copy assignment operators.

[ 2009-09-13 Niels adds: ]

> Note: This resolution supports the change of 27.9.1.3 [filebuf.assign]/1, proposed by LWG 900.

[ 2009 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

Add a bullet to 17.6.4.9 [res.on.arguments]:

> Each of the following statements applies to all arguments to functions defined in the C++ standard library, unless explicitly stated otherwise.

> - If an argument to a function has an invalid value (such as a value outside the domain of the function, or a pointer invalid for its intended use), the behavior is undefined.
> - If a function argument is described as being an array, the pointer actually passed to the function shall have a value such that all address computations and accesses to objects (that would be valid if the pointer did point to the first element of such an array) are in fact valid.
> - If a function argument binds to an rvalue reference parameter, the C++ standard library may assume that this parameter is a unique reference to this argument. If the parameter is a generic parameter of the form `T&&`, and an lvalue of type `A` is bound, then the binding is considered to be to an lvalue reference (14.8.2.1 [temp.deduct.call]) and thus not covered by this clause. [Note: If a program casts an lvalue to an rvalue while passing that lvalue to a library function (e.g. `move(x)`), then the program is effectively asking the library to treat that lvalue as a temporary. The library is at liberty to optimize away aliasing checks which might be needed if the argument were an lvalue. — end note]

Delete 23.2.1 [container.requirements.general]/13:

> An object bound to an rvalue reference parameter of a member function of a container shall not be an element of that container; no diagnostic required.

### 1205. Some algorithms could more clearly document their handling of empty ranges

Section: 25 [algorithms] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-09-13 Last modified: 2015-04-08

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Discussion:

There are a number of algorithms whose result might depend on the handling of an empty range. In some cases the result is not clear, while in others it would help readers to clearly mention the result rather than require some subtle intuition of the supplied wording.

25.2.1 [alg.all_of]

> Returns: `true` if `pred(*i)` is `true` for every iterator `i` in the range `[first,last)`, ...

What does this mean if the range is empty?

I believe that we intend this to be `true` and suggest a non-normative note to clarify:

Add to p1 25.2.1 [alg.all_of]:

> [Note: Returns `true` if `[first,last)` is empty. — end note]

25.2.3 [alg.none_of]

> Returns: `true` if `pred(*i)` is `false` for every iterator `i` in the range `[first,last)`, ...

What does this mean if the range empty?

I believe that we intend this to be `true` and suggest a non-normative note to clarify:

Add to p1 25.2.3 [alg.none_of]:

> [Note: Returns `true` if `[first,last)` is empty. — end note]

25.2.2 [alg.any_of]

The specification for an empty range is actually fairly clear in this case, but a note wouldn't hurt and would be consistent with proposals for `all_of`/`none_of` algorithms.

Add to p1 25.2.2 [alg.any_of]:

> [Note: Returns `false` if `[first,last)` is empty. — end note]

25.2.6 [alg.find.end]

what does this mean if `[first2,last2)` is empty?

I believe the wording suggests the algorithm should return `last1` in this case, but am not 100% sure. Is this in fact the correct result anyway? Surely an empty range should always match and the naive expected result would be `first1`?

My proposed wording is a note to clarify the current semantic:

Add to p2 25.2.6 [alg.find.end]:

> [Note: Returns `last1` if `[first2,last2)` is empty. — end note]

I would prefer a normative wording treating empty ranges specially, but do not believe we can change semantics at this point in the process, unless existing implementations actually yield this result:

Alternative wording: (NOT a note)

Add to p2 25.2.6 [alg.find.end]:

> Returns `first1` if `[first2,last2)` is empty.

25.2.7 [alg.find.first.of]

The phrasing seems precise when `[first2, last2)` is empty, but a small note to confirm the reader's understanding might still help.

Add to p2 25.2.7 [alg.find.first.of]

> [Note: Returns `last1` if `[first2,last2)` is empty. — end note]

25.2.13 [alg.search]

What is the expected result if `[first2, last2)` is empty?

I believe the wording suggests the algorithm should return `last1` in this case, but am not 100% sure. Is this in fact the correct result anyway? Surely an empty range should always match and the naive expected result would be `first1`?

My proposed wording is a note to clarify the current semantic:

Add to p2 25.2.13 [alg.search]:

> [Note: Returns `last1` if `[first2,last2)` is empty. — end note]

Again, I would prefer a normative wording treating empty ranges specially, but do not believe we can change semantics at this point in the process, unless existing implementations actually yield this result:

Alternative wording: (NOT a note)

Add to p2 25.2.13 [alg.search]:

> Returns `first1` if `[first2,last2)` is empty.

25.3.13 [alg.partitions]

Is an empty range partitioned or not?

Proposed wording:

Add to p1 25.3.13 [alg.partitions]:

> [Note: Returns `true` if `[first,last)` is empty. — end note]

25.4.5.1 [includes]

> Returns: `true` if every element in the range `[first2,last2)` is contained in the range `[first1,last1)`. ...

I really don't know what this means if `[first2,last2)` is empty. I could loosely guess that this implies empty ranges always match, and my proposed wording is to clarify exactly that:

Add to p1 25.4.5.1 [includes]:

> [Note: Returns `true` if `[first2,last2)` is empty. — end note]

25.4.6.2 [pop.heap]

The effects clause is invalid if the range `[first,last)` is empty, unlike all the other heap alogorithms. The should be called out in the requirements.

Proposed wording:

Revise p2 25.4.6.2 [pop.heap]

> Requires: The range `[first,last)` shall be a valid non-empty heap.

[Editorial] Reverse order of 25.4.6.2 [pop.heap] p1 and p2.

25.4.7 [alg.min.max]

`minmax_element` does not clearly specify behaviour for an empty range in the same way that `min_element` and `max_element` do.

Add to p31 25.4.7 [alg.min.max]:

> Returns `make_pair(first, first)` if `first == last`.

25.4.8 [alg.lex.comparison]

The wording here seems quite clear, especially with the sample algorithm implementation. A note is recommended purely for consistency with the rest of these issue resolutions:

Add to p1 25.4.8 [alg.lex.comparison]:

> [Note: An empty sequence is lexicographically less than any other non-empty sequence, but not to another empty sequence. — end note]

[ 2009-11-11 Howard changes Notes to Remarks and changed `search` to return `first1` instead of `last1`. ]

[ 2009-11-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Add to 25.2.1 [alg.all_of]:

> Remarks: Returns `true` if `[first,last)` is empty.

Add to 25.2.2 [alg.any_of]:

> Remarks: Returns `false` if `[first,last)` is empty.

Add to 25.2.3 [alg.none_of]:

> Remarks: Returns `true` if `[first,last)` is empty.

Add to 25.2.6 [alg.find.end]:

> Remarks: Returns `last1` if `[first2,last2)` is empty.

Add to 25.2.7 [alg.find.first.of]

> Remarks: Returns `last1` if `[first2,last2)` is empty.

Add to 25.2.13 [alg.search]:

> Remarks: Returns `first1` if `[first2,last2)` is empty.

Add to 25.3.13 [alg.partitions]:

> Remarks: Returns `true` if `[first,last)` is empty.

Add to 25.4.5.1 [includes]:

> Remarks: Returns `true` if `[first2,last2)` is empty.

Revise p2 25.4.6.2 [pop.heap]

> Requires: The range `[first,last)` shall be a valid non-empty heap.

[Editorial]

> Reverse order of 25.4.6.2 [pop.heap] p1 and p2.

Add to p35 25.4.7 [alg.min.max]:

>     template<class ForwardIterator, class Compare>
>       pair<ForwardIterator, ForwardIterator>
>         minmax_element(ForwardIterator first, ForwardIterator last, Compare comp);

> > Returns: `make_pair(m, M)`, where `m` is the first iterator in `[first,last)` such that no iterator in the range refers to a smaller element, and where `M` is the last iterator in `[first,last)` such that no iterator in the range refers to a larger element. Returns `make_pair(first, first)` if `first == last`.

Add to 25.4.8 [alg.lex.comparison]:

> Remarks: An empty sequence is lexicographically less than any other non-empty sequence, but not less than another empty sequence.

### 1206. Incorrect requires for `move_backward` and `copy_backward`

Section: 25.3.2 [alg.move] Status: C++11 Submitter: Howard Hinnant Opened: 2009-09-13 Last modified: 2015-04-08

View all issues with C++11 status.

Discussion:

25.3.2 [alg.move], p6 says:

>     template<class BidirectionalIterator1, class BidirectionalIterator2>
>       BidirectionalIterator2
>         move_backward(BidirectionalIterator1 first,
>                       BidirectionalIterator1 last,
>                       BidirectionalIterator2 result);

> > ...

> > Requires: `result` shall not be in the range `[first,last)`.

This is essentially an "off-by-one" error.

When `result == last`, which is allowed by this specification, then the range `[first, last)` is being move assigned into the range `[first, last)`. The `move` (forward) algorithm doesn't allow self move assignment, and neither should `move_backward`. So `last` should be included in the range which `result` can not be in.

Conversely, when `result == first`, which is not allowed by this specification, then the range `[first, last)` is being move assigned into the range `[first - (last-first), first)`. I.e. into a non-overlapping range. Therefore `first` should not be included in the range which `result` can not be in.

The same argument applies to `copy_backward` though copy assigning elements to themselves (`result == last`) should be harmless (though is disallowed by `copy`).

[ 2010 Pittsburgh: Moved to Ready. ]

Proposed resolution:

Change 25.3.2 [alg.move], p6:

>     template<class BidirectionalIterator1, class BidirectionalIterator2>
>       BidirectionalIterator2
>         move_backward(BidirectionalIterator1 first,
>                       BidirectionalIterator1 last,
>                       BidirectionalIterator2 result);

> > ...

> > Requires: `result` shall not be in the range `[(first,last])`.

Change 25.3.1 [alg.copy], p13:

>     template<class BidirectionalIterator1, class BidirectionalIterator2>
>       BidirectionalIterator2
>         copy_backward(BidirectionalIterator1 first,
>                       BidirectionalIterator1 last,
>                       BidirectionalIterator2 result);

> > ...

> > Requires: `result` shall not be in the range `[(first,last])`.

### 1207. Underspecified `std::list` operations?

Section: 23.3.5.5 [list.ops] Status: C++11 Submitter: Loïc Joly Opened: 2009-09-13 Last modified: 2015-04-08

View all other issues in [list.ops].

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Discussion:

It looks to me like some operations of `std::list` (`sort`, `reverse`, `remove`, `unique` & `merge`) do not specify the validity of iterators, pointers & references to elements of the list after those operations. Is it implied by some other text in the standard?

I believe `sort` & `reverse` do not invalidating anything, `remove` & `unique` only invalidates what refers to erased elements, `merge` does not invalidate anything (with the same precision as `splice` for elements who changed of container). Are those assumptions correct ?

[ 2009-12-08 Jonathan Wakely adds: ]

> 23.2.1 [container.requirements.general] paragraph 11 says iterators aren't invalidated unless specified, so I don't think it needs to be repeated on every function that doesn't invalidate iterators. `list::unique` says it "eliminates" elements, that should probably be "erases" because IMHO that term is used elsewhere and so makes it clearer that iterators to the erased elements are invalidated.

> `list::merge` coud use the same wording as `list::splice` w.r.t iterators and references to moved elements.

> Suggested resolution:

> In 23.3.5.5 [list.ops] change paragraph 19

> >                                      void unique();
> >     template <class BinaryPredicate> void unique(BinaryPredicate binary_pred);

> > > Effects: Eliminates Erases all but the first element from every consecutive group ...

> Add to the end of paragraph 23

> >     void                          merge(list<T,Allocator>&& x);
> >     template <class Compare> void merge(list<T,Allocator>&& x, Compare comp);

> > > ...

> > > Effects: ... that is, for every iterator `i`, in the range other than the first, the condition `comp(*i, *(i - 1)` will be false. Pointers and references to the moved elements of `x` now refer to those same elements but as members of `*this`. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into `*this`, not into `x`.

[ 2009-12-12 Loïc adds wording. ]

[ 2010-02-10 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010-02-10 Alisdair opens: ]

> I object to the current resolution of #1207. I believe it is overly strict with regard to `list` end iterators, being the only mutating operations to require such stability.

> More importantly, the same edits need to be applied to `forward_list`, which uses slightly different words to describe some of these operations so may require subtly different edits (not checked.)

> I am prepared to pick up the `end()` iterator as a separate (new) issue, as part of the FCD ballot review (BSI might tell me 'no' first ;~) but I do want to see `forward_list` adjusted at the same time.

[ 2010-03-28 Daniel adds the first 5 bullets in an attempt to address Alisdair's concerns. ]

[ 2010 Rapperswil: ]

> The wording looks good. Move to Tentatively Ready.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

1. Change 23.3.4.6 [forwardlist.ops]/12 as indicated:

   >     void remove(const T& value);
   >     template <class Predicate> void remove_if(Predicate pred);

   > > 12 Effects: Erases all the elements in the list referred by a list iterator `i` for which the following conditions hold: `*i == value (for remove()), pred(*i)` is true (`for remove_if()`). This operation shall be stable: the relative order of the elements that are not removed is the same as their relative order in the original list. Invalidates only the iterators and references to the erased elements.

2. Change 23.3.4.6 [forwardlist.ops]/15 as indicated:

   >     template <class BinaryPredicate> void unique(BinaryPredicate pred);

   > > 15 Effects:: Eliminates Erases all but the first element from every consecutive group of equal elements referred to by the iterator `i` in the range `[first + 1,last)` for which `*i == *(i-1)` (for the version with no arguments) or `pred(*i, *(i - 1))` (for the version with a predicate argument) holds. Invalidates only the iterators and references to the erased elements.

3. Change 23.3.4.6 [forwardlist.ops]/19 as indicated:

   >     void merge(forward_list<T,Allocator>&& x);
   >     template <class Compare> void merge(forward_list<T,Allocator>&& x, Compare comp)

   > > [..]

   > > 19 Effects:: Merges `x` into `*this`. This operation shall be stable: for equivalent elements in the two lists, the elements from `*this` shall always precede the elements from `x`. `x` is empty after the merge. If an exception is thrown other than by a comparison there are no effects. Pointers and references to the moved elements of `x` now refer to those same elements but as members of `*this`. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into `*this`, not into `x`.

4. Change 23.3.4.6 [forwardlist.ops]/22 as indicated:

   >     void sort();
   >     template <class Compare> void sort(Compare comp);

   > > [..]

   > > 22 Effects:: Sorts the list according to the `operator<` or the `comp` function object. This operation shall be stable: the relative order of the equivalent elements is preserved. If an exception is thrown the order of the elements in `*this` is unspecified. Does not affect the validity of iterators and references.

5. Change 23.3.4.6 [forwardlist.ops]/24 as indicated:

   >     void reverse();

   > > 24 Effects:: Reverses the order of the elements in the list. Does not affect the validity of iterators and references.

6. Change 23.3.5.5 [list.ops], p15:

   >                                void remove(const T& value);
   >     template <class Predicate> void remove_if(Predicate pred);

   > > Effects: Erases all the elements in the list referred by a list iterator `i` for which the following conditions hold: `*i == value, pred(*i) != false`. Invalidates only the iterators and references to the erased elements.

7. Change 23.3.5.5 [list.ops], p19:

   >                                      void unique();
   >     template <class BinaryPredicate> void unique(BinaryPredicate binary_pred);

   > > Effects: Eliminates Erases all but the first element from every consecutive group of equal elements referred to by the iterator `i` in the range `[first + 1,last)` for which `*i == *(i-1)` (for the version of `unique` with no arguments) or `pred(*i, *(i - 1))` (for the version of `unique` with a predicate argument) holds. Invalidates only the iterators and references to the erased elements.

8. Change 23.3.5.5 [list.ops], p23:

   >     void                          merge(list<T,Allocator>&& x);
   >     template <class Compare> void merge(list<T,Allocator>&& x, Compare comp);

   > > Effects: If `(&x == this)` does nothing; otherwise, merges the two sorted ranges `[begin(), end())` and `[x.begin(), x.end())`. The result is a range in which the elements will be sorted in non-decreasing order according to the ordering defined by `comp`; that is, for every iterator `i`, in the range other than the first, the condition `comp(*i, *(i - 1)` will be false. Pointers and references to the moved elements of `x` now refer to those same elements but as members of `*this`. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into `*this`, not into `x`.

9. Change 23.3.5.5 [list.ops], p26:

   >     void reverse();

   > > Effects: Reverses the order of the elements in the list. Does not affect the validity of iterators and references.

10. Change 23.3.5.5 [list.ops], p30:

    >                              void sort();
    >     template <class Compare> void sort(Compare comp);

    > > Effects: Sorts the list according to the `operator<` or a `Compare` function object. Does not affect the validity of iterators and references.

### 1208. `valarray initializer_list` constructor has incorrect effects

Section: 26.6.2.2 [valarray.cons] Status: C++11 Submitter: Howard Hinnant Opened: 2009-09-23 Last modified: 2015-04-08

View all other issues in [valarray.cons].

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Discussion:

26.6.2.2 [valarray.cons] says:

>     valarray(initializer_list<T> il);

> > Effects: Same as `valarray(il.begin(), il.end())`.

But there is no `valarray` constructor taking two `const T*`.

[ 2009-10-29 Howard: ]

> Moved to Tentatively Ready after 6 positive votes on c++std-lib.

Proposed resolution:

Change 26.6.2.2 [valarray.cons]:

>     valarray(initializer_list<T> il);

> > Effects: Same as `valarray(il.begin(), il.endsize())`.

### 1209. `match_results` should be moveable

Section: 28.10.1 [re.results.const] Status: C++11 Submitter: Stephan T. Lavavej Opened: 2009-09-15 Last modified: 2015-04-08

View other active issues in [re.results.const].

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Discussion:

In Working Draft N2914, `match_results` lacks a move constructor and move assignment operator. Because it owns dynamically allocated memory, it should be moveable.

As far as I can tell, this isn't tracked by an active issue yet; Library Issue 723 doesn't talk about `match_results`.

[ 2009-09-21 Daniel provided wording. ]

[ 2009-11-18: Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

1. Add the following member declarations to 28.10 [re.results]/3:

   >     // 28.10.1, construct/copy/destroy:
   >     explicit match_results(const Allocator& a = Allocator());
   >     match_results(const match_results& m);
   >     match_results(match_results&& m);
   >     match_results& operator=(const match_results& m);
   >     match_results& operator=(match_results&& m);
   >     ~match_results();

2. Add the following new prototype descriptions to 28.10.1 [re.results.const] using the table numbering of N3000 (referring to the table titled "`match_results` assignment operator effects"):

   >     match_results(const match_results& m);

   > > 4 Effects: Constructs an object of class `match_results`, as a copy of `m`.

   >     match_results(match_results&& m);

   > > 5 Effects: Move-constructs an object of class `match_results` from `m` satisfying the same postconditions as Table 131. Additionally the stored `Allocator` value is move constructed from `m.get_allocator()`. After the initialization of `*this` sets `m` to an unspecified but valid state.

   > > 6 Throws: Nothing if the allocator's move constructor throws nothing.

   >     match_results& operator=(const match_results& m);

   > > 7 Effects: Assigns `m` to `*this`. The postconditions of this function are indicated in Table 131.

   >     match_results& operator=(match_results&& m);

   > > 8 Effects: Move-assigns `m` to `*this`. The postconditions of this function are indicated in Table 131. After the assignment, `m` is in a valid but unspecified state.

   > > 9 Throws: Nothing.

### 1210. Iterator reachability should not require a container

Section: 24.2 [iterator.requirements] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-09-18 Last modified: 2015-04-08

View all other issues in [iterator.requirements].

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Discussion:

p6 Iterator requirements 24.2 [iterator.requirements]

> An iterator `j` is called reachable from an iterator `i` if and only if there is a finite sequence of applications of the expression `++i` that makes `i == j`. If `j` is reachable from `i`, they refer to the same container.

A good example would be stream iterators, which do not refer to a container. Typically, the end iterator from a range of stream iterators will compare equal for many such ranges. I suggest striking the second sentence.

An alternative wording might be:

> If `j` is reachable from `i`, and both `i` and `j` are dereferencable iterators, then they refer to the same range.

[ 2010 Pittsburgh: Moved to NAD Editorial Resolved. Rationale added below. ]

Rationale:

Solved by N3066.

Proposed resolution:

Change 24.2 [iterator.requirements], p6:

> An iterator `j` is called reachable from an iterator `i` if and only if there is a finite sequence of applications of the expression `++i` that makes `i == j`. If `j` is reachable from `i`, they refer to the same container.

### 1211. Move iterators should be restricted as input iterators

Section: 24.5.3.1 [move.iterator] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-09-18 Last modified: 2015-04-08

View all other issues in [move.iterator].

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Discussion:

I contend that while we can support both bidirectional and random access traversal, the category of a move iterator should never be better than `input_iterator_tag`.

The contentious point is that you cannot truly have a multipass property when values are moved from a range. This is contentious if you view a moved-from object as still holding a valid value within the range.

The second reason comes from the Forward Iterator requirements table:

> Forward iterators 24.2.5 [forward.iterators]

> Table 102 — Forward iterator requirements

> > For expression `*a` the return type is: "`T&` if `X` is mutable, otherwise `const T&`"

There is a similar constraint on `a->m`.

There is no support for rvalue references, nor do I believe their should be. Again, opinions may vary but either this table or the definition of `move_iterator` need updating.

Note: this requirement probably need updating anyway if we wish to support proxy iterators but I am waiting to see a new working paper before filing that issue.

[ 2009-10 post-Santa Cruz: ]

> Move to Open. Howard to put his rationale mentioned above into the issue as a note.

[ 2009-10-26 Howard adds: ]

> `vector::insert(pos, iter, iter)` is significantly more effcient when `iter` is a random access iterator, as compared to when it is an input iterator.

> When `iter` is an input iterator, the best algorithm is to append the inserted range to the end of the `vector` using `push_back`. This may involve several reallocations before the input range is exhausted. After the append, then one can use `std::rotate` to place the inserted range into the correct position in the vector.

> But when `iter` is a random access iterator, the best algorithm is to first compute the size of the range to be inserted (`last - first`), do a buffer reallocation if necessary, scoot existing elements in the `vector` down to make the "hole", and then insert the new elements directly to their correct place.

> > The insert-with-random-access-iterators algorithm is considerably more efficient than the insert-with-input-iterators algorithm

> Now consider:

> >     vector<A> v;
> >     //  ... build up a large vector of A ...
> >     vector<A> temp;
> >     //  ... build up a large temporary vector of A to later be inserted ...
> >     typedef move_iterator<vector<A>::iterator> MI;
> >     //  Now insert the temporary elements:
> >     v.insert(v.begin() + N, MI(temp.begin()), MI(temp.end()));

> A major motivation for using `move_iterator` in the above example is the expectation that `A` is cheap to move but expensive to copy. I.e. the customer is looking for high performance. If we allow `vector::insert` to subtract two `MI`'s to get the distance between them, the customer enjoys substantially better performance, compared to if we say that `vector::insert` can not subtract two `MI`'s.

> I can find no rationale for not giving this performance boost to our customers. Therefore I am strongly against restricting `move_iterator` to the `input_iterator_tag` category.

> I believe that the requirement that forward iterators have a dereference that returns an lvalue reference to cause unacceptable pessimization. For example `vector<bool>::iterator` also does not return a `bool&` on dereference. Yet I am not aware of a single vendor that is willing to ship `vector<bool>::iterator` as an input iterator. Everyone classifies it as a random access iterator. Not only does this not cause any problems, it prevents significant performance problems.

[ 2010 Pittsburgh: Moved to NAD Editorial Resolved. Rationale added below. ]

Rationale:

Solved by N3066.

Proposed resolution:

Class template move_iterator 24.5.3.1 [move.iterator]

>     namespace std {
>     template <class Iterator>
>     class move_iterator {
>     public:
>      ...
>      typedef typename iterator_traits<Iterator>::iterator_category input_iterator_tag iterator_category;

### 1212. result of post-increment/decrement operator

Section: 24.2 [iterator.requirements] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-09-18 Last modified: 2015-04-08

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Discussion:

Forward iterator and bidirectional iterator place different requirements on the result of post-increment/decrement operator. The same form should be used in each case.

Merging row from:

>     Table 102 -- Forward iterator requirements
>     Table 103 -- Bidirectional iterator requirements

>         r++ : convertible to const X&
>         r-- : convertible to const X&

>         *r++ : T& if X is mutable, otherwise const T&
>         *r-- : convertible to T

[ 2010 Pittsburgh: Moved to NAD Editorial Resolved. Rationale added below. ]

Rationale:

Solved by N3066.

Proposed resolution:

### 1214. Insufficient/inconsistent key immutability requirements for associative containers

Section: 23.2.4 [associative.reqmts] Status: C++14 Submitter: Daniel Krügler Opened: 2009-09-20 Last modified: 2015-04-08

View other active issues in [associative.reqmts].

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Discussion:

Scott Meyers' mentions on a recent posting on c.s.c++ some arguments that point to an incomplete resolution of 103 and to an inconsistency of requirements on keys in ordered and unordered associative containers:

> 1) 103 introduced the term immutable without defining it in a unique manner in 23.2.4 [associative.reqmts]/5:

> > [..] Keys in an associative container are immutable.

> According to conventional dictionaries immutable is an unconditional way of saying that something cannot be changed. So without any further explicit allowance a user always runs into undefined behavior if (s)he attempts to modify such a key. IMO this was not the intend of the committee to resolve 103 in that way because the comments suggest an interpretation that should give any user the freedom to modify the key in an explicit way provided it would not affect the sort order in that container.

> 2) Another observation was that surprisingly no similar 'safety guards' exists against unintentional key changes for the unordered associative containers, specifically there is no such requirement as in 23.2.4 [associative.reqmts]/6 that "both `iterator` and `const_iterator` are constant iterators". But the need for such protection against unintentional changes as well as the constraints in which manner any explicit changes may be performed are both missing and necessary, because such changes could potentially change the equivalence of keys that is measured by the `hasher` and `key_equal`.

> I suggest to fix the unconditional wording involved with "immutable keys" by at least adding a hint for the reader that users may perform such changes in an explicit manner and to perform similar wording changes as 103 did for the ordered associative containers also for the unordered containers.

[ 2010-03-27 Daniel provides wording. ]

> This update attempts to provide normative wording that harmonizes the key and function object constraints of associative and unordered containers.

[ 2010 Batavia: ]

We're uncomfortable with the first agenda item, and we can live with the second agenda item being applied before or after Madrid.

[ 2011 Bloomington ]

Further discussion persuades us this issue is Ready (and so moved). We may need a further issue clarifying the notion of key value vs. key object, as object identity appears to be important to this wording.

Proposed resolution:

1. Change 23.2.4 [associative.reqmts]/2 as indicated: [This ensures that associative containers make better clear what this "arbitrary" type is, as the unordered containers do in 23.2.5 [unord.req]/3]

   > 2 Each associative container is parameterized on `Key` and an ordering relation `Compare` that induces a strict weak ordering (25.4) on elements of `Key`. In addition, `map` and `multimap` associate an arbitrary mapped type type `T` with the `Key`. The object of type `Compare` is called the comparison object of a container.

2. Change 23.2.4 [associative.reqmts]/5 as indicated: [This removes the too strong requirement that keys must not be changed at all and brings this line in sync with 23.2.5 [unord.req]/7. We take care about the real constraints by the remaining suggested changes. The rationale provided by LWG 103 didn't really argue why that addition is necessary, and I believe the remaining additions make it clear that any user changes have strong restrictions]:

   > 5 For `set` and `multiset` the value type is the same as the key type. For `map` and `multimap` it is equal to `pair<const Key, T>`. Keys in an associative container are immutable.

3. Change 23.2.5 [unord.req]/3+4 as indicated: [The current sentence of p.4 has doesn't say something really new and this whole subclause misses to define the concepts of the container-specific hasher object and predicate object. We introduce the term key equality predicate which is already used in the requirements table. This change does not really correct part of this issue, but is recommended to better clarify the nomenclature and the difference between the function objects and the function object types, which is important, because both can potentially be stateful.]

   > 3 Each unordered associative container is parameterized by `Key`, by a function object type `Hash` that meets the `Hash` requirements (20.2.4) and acts as a hash function for argument values of type `Key`, and by a binary predicate `Pred` that induces an equivalence relation on values of type `Key`. Additionally, `unordered_map` and `unordered_multimap` associate an arbitrary mapped type `T` with the `Key`.

   > 4 The container's object of type `Hash` - denoted by `hash` - is called the hash function of the container. The container's object of type `Pred` - denoted by `pred` - is called the key equality predicate of the container.A hash function is a function object that takes a single argument of type `Key` and returns a value of type `std::size_t`.

4. Change 23.2.5 [unord.req]/5 as indicated: [This adds a similar safe-guard as the last sentence of 23.2.4 [associative.reqmts]/3]

   > 5 Two values `k1` and `k2` of type `Key` are considered equivalent if the container's key equality predicate `key_equal` function object returns `true` when passed those values. If `k1` and `k2` are equivalent, the container's hash function shall return the same value for both. [Note: thus, when an unordered associative container is instantiated with a non-default `Pred` parameter it usually needs a non-default `Hash` parameter as well. — end note] For any two keys `k1` and `k2` in the same container, calling `pred(k1, k2)` shall always return the same value. For any key `k` in a container, calling `hash(k)` shall always return the same value.

5. After 23.2.5 [unord.req]/7 add the following new paragraph: [This ensures the same level of compile-time protection that we already require for associative containers. It is necessary for similar reasons, because any change in the stored key which would change it's equality relation to others or would change it's hash value such that it would no longer fall in the same bucket, would break the container invariants]

   > 7 For `unordered_set` and `unordered_multiset` the value type is the same as the key type. For `unordered_map` and `unordered_multimap` it is `std::pair<const Key, T>`.

   > For unordered containers where the value type is the same as the key type, both `iterator` and `const_iterator` are constant iterators. It is unspecified whether or not `iterator` and `const_iterator` are the same type. [Note: `iterator` and `const_iterator` have identical semantics in this case, and `iterator` is convertible to `const_iterator`. Users can avoid violating the One Definition Rule by always using `const_iterator` in their function parameter lists. — end note]

### 1215. `list::merge` with unequal allocators

Section: 23.3.5.5 [list.ops] Status: C++11 Submitter: Pablo Halpern Opened: 2009-09-24 Last modified: 2015-04-08

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Discussion:

In Bellevue (I think), we passed N2525, which, among other things, specifies that the behavior of `list::splice` is undefined if the allocators of the two lists being spliced do not compare equal. The same rationale should apply to `list::merge`. The intent of `list::merge` (AFAIK) is to move nodes from one sorted `list` into another sorted `list` without copying the elements. This is possible only if the allocators compare equal.

Proposed resolution:

Relative to the August 2009 WP, N2857, change 23.3.5.5 [list.ops], paragraphs 22-25 as follows:

>     void merge(list&& x);
>     template <class Compare> void merge(list&& x, Compare comp);

> > Requires: both the list and the argument list shall be sorted according to operator< or comp.

> > Effects: If `(&x == this)` does nothing; otherwise, merges the two sorted ranges `[begin(), end())` and `[x.begin(), x.end())`. The result is a range in which the elements will be sorted in non-decreasing order according to the ordering defined by `comp`; that is, for every iterator `i`, in the range other than the `first`, the condition `comp(*i, *(i - 1))` will be `false`.

> > Remarks: Stable. If `(&x != this)` the range `[x.begin(), x.end())` is empty after the merge. No elements are copied by this operation. The behavior is undefined if `this->get_allocator() != x.get_allocator()`.

> > Complexity: At most `size() + x.size() - 1` applications of `comp` if `(&x != this)`; otherwise, no applications of `comp` are performed. If an exception is thrown other than by a comparison there are no effects.

### 1216. LWG 1066 Incomplete?

Section: 18.8.6 [except.nested] Status: C++11 Submitter: Pete Becker Opened: 2009-09-25 Last modified: 2015-04-08

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Discussion:

LWG 1066 adds `[[noreturn]]` to a bunch of things. It doesn't add it to `rethrow_nested()`, which seems like an obvious candidate. I've made the changes indicated in the issue, and haven't changed `rethrow_nested()`.

[ 2009 Santa Cruz: ]

> Move to Ready.

Proposed resolution:

Add `[[noreturn]]` to `rethrow_nested()` in 18.8.6 [except.nested].

### 1218. mutex destructor synchronization

Section: 30.4.1 [thread.mutex.requirements] Status: C++11 Submitter: Jeffrey Yasskin Opened: 2009-09-30 Last modified: 2015-04-08

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Discussion:

If an object `*o` contains a mutex `mu` and a correctly-maintained reference count `c`, is the following code safe?

>     o->mu.lock();
>     bool del = (--(o->c) == 0);
>     o->mu.unlock();
>     if (del) { delete o; }

If the implementation of `mutex::unlock()` can touch the mutex's memory after the moment it becomes free, this wouldn't be safe, and "Construction and destruction of an object of a Mutex type need not be thread-safe" 30.4.1 [thread.mutex.requirements] may imply that it's not safe. Still, it's useful to allow mutexes to guard reference counts, and if it's not allowed, users are likely to write bugs.

[ 2009-11-18: Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

- Add a new paragraph after 30.4.1.2.1 [thread.mutex.class] p1:

  > 1 The class `mutex` provides a non-recursive mutex ...

  > [Note: After a thread `A` has called `unlock()`, releasing the mutex, it is possible for another thread `B` to lock the same mutex, observe that it is no longer in use, unlock and destroy it, before thread `A` appears to have returned from its unlock call. Implementations are required to handle such scenarios correctly, as long as thread `A` doesn't access the mutex after the unlock call returns. These cases typically occur when a reference-counted object contains a mutex that is used to protect the reference count. — end note]

### 1220. What does `condition_variable` wait on?

Section: 30.5 [thread.condition] Status: C++11 Submitter: Jeffrey Yasskin Opened: 2009-09-30 Last modified: 2015-04-08

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Discussion:

"Class `condition_variable` provides a condition variable that can only wait on an object of type `unique_lock`" should say "...object of type `unique_lock<mutex>`"

[ 2009-11-06 Howard adds: ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

Proposed resolution:

Change 30.5 [thread.condition], p1:

> Condition variables provide synchronization primitives used to block a thread until notified by some other thread that some condition is met or until a system time is reached. Class `condition_variable` provides a condition variable that can only wait on an object of type `unique_lock<mutex>`, allowing maximum efficiency on some platforms. Class `condition_variable_any` provides a general condition variable that can wait on objects of user-supplied lock types.

### 1221. `condition_variable` wording

Section: 30.5.1 [thread.condition.condvar] Status: C++11 Submitter: Jeffrey Yasskin Opened: 2009-09-30 Last modified: 2015-04-08

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Discussion:

30.5.1 [thread.condition.condvar] says:

>     ~condition_variable();

> > Precondition: There shall be no thread blocked on `*this`. [Note: That is, all threads shall have been notified; they may subsequently block on the lock specified in the wait. Beware that destroying a `condition_variable` object while the corresponding predicate is `false` is likely to lead to undefined behavior. — end note]

The text hasn't introduced the notion of a "corresponding predicate" yet.

[ 2010-02-11 Anthony provided wording. ]

[ 2010-02-12 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Modify 30.5.1 [thread.condition.condvar]p4 as follows:

>     ~condition_variable();

> > 4 Precondition: There shall be no thread blocked on `*this`. [Note: That is, all threads shall have been notified; they may subsequently block on the lock specified in the wait. Beware that destroying a `condition_variable` object while the corresponding predicate is false is likely to lead to undefined behavior. The user must take care to ensure that no threads wait on `*this` once the destructor has been started, especially when the waiting threads are calling the wait functions in a loop or using the overloads of `wait`, `wait_for` or `wait_until` that take a predicate. — end note]

### 1222. condition_variable incorrect effects for exception safety

Section: 30.5 [thread.condition] Status: C++11 Submitter: Jeffrey Yasskin Opened: 2009-09-30 Last modified: 2015-04-08

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Discussion:

30.5.1 [thread.condition.condvar] says:

>     void wait(unique_lock<mutex>& lock);

> > ...

> > Effects:

> > - ...
> > - If the function exits via an exception, `lock.unlock()` shall be called prior to exiting the function scope.

Should that be `lock.lock()`?

[ 2009-11-17 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Change 30.5.1 [thread.condition.condvar] p10:

>     void wait(unique_lock<mutex>& lock);

> > ...

> > Effects:

> > - ...
> > - If the function exits via an exception, `lock.unlock()` shall be called prior to exiting the function scope.

And make a similar change in p16, and in 30.5.2 [thread.condition.condvarany], p8 and p13.

### 1225. C++0x `result_of` issue

Section: X [func.ret] Status: Resolved Submitter: Sebastian Gesemann Opened: 2009-10-05 Last modified: 2015-04-08

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Discussion:

I think the text about `std::result_of` could be a little more precise. Quoting from N2960...

> X [func.ret] Function object return types

>     template<class> class result_of;

>     template<class Fn, class... ArgTypes>
>     class result_of<Fn(ArgTypes...)> {
>     public:
>       typedef see below type;
>     };

> Given an rvalue `fn` of type `Fn` and values `t1, t2, ..., tN` of types `T1, T2, ... TN` in `ArgTypes` respectivly, the `type` member is the result type of the expression `fn(t1,t2,...,tN)`. the values `ti` are lvalues when the corresponding type `Ti` is an lvalue-reference type, and rvalues otherwise.

This text doesn't seem to consider lvalue reference types for `Fn`. Also, it's not clear whether this class template can be used for "SFINAE" like `std::enable_if`. Example:

>     template<typename Fn, typename... Args>
>     typename std::result_of<Fn(Args...)>::type
>     apply(Fn && fn, Args && ...args)
>     {
>       // Fn may be an lvalue reference, too
>       return std::forward<Fn>(fn)(std::forward<Args>(args)...);
>     }

Either `std::result_of<...>` can be instantiated and simply may not have a typedef "`type`" (-->SFINAE) or instantiating the class template for some type combinations will be a "hard" compile-time error.

[ 2010-02-14 Daniel adds: ]

> This issue should be considered resolved by 1255 and 1270. The wish to change `result_of` into a compiler-support trait was beyond the actual intention of the submitter Sebastian.

[ 2010 Pittsburgh: Moved to NAD Editorial Resolved, rationale added below. ]

Rationale:

Solved by 1270.

Proposed resolution:

[ These changes will require compiler support ]

Change X [func.ret]:

>     template<class> class result_of; // undefined

>     template<class Fn, class... ArgTypes>
>     class result_of<Fn(ArgTypes...)> {
>     public:
>       typedef see below type;
>     };

> Given an rvalue `fn` of type `Fn` and values `t1, t2, ..., tN` of types `T1, T2, ... TN` in `ArgTypes` respectivly, the `type` member is the result type of the expression `fn(t1,t2,...,tN)`. the values `ti` are lvalues when the corresponding type `Ti` is an lvalue-reference type, and rvalues otherwise.

> The class template `result_of` shall meet the requirements of a TransformationTrait: Given the types `Fn`, `T1`, `T2`, ..., `TN` every template specialization `result_of<Fn(T1,T2,...,TN)>` shall define the member typedef type equivalent to `decltype(RE)` if and only if the expression `RE`

> >     value<Fn>() ( value<T1>(), value<T2>(), ... value<TN>()  )

> would be well-formed. Otherwise, there shall be no member typedef `type` defined.

[ The `value<>` helper function is a utility Daniel Krügler proposed in N2958. ]

### 1226. Incomplete changes of #890

Section: 30.6.2 [futures.errors] Status: Resolved Submitter: Daniel Krügler Opened: 2009-10-05 Last modified: 2015-04-08

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Discussion:

Defect issue 890 overlooked to adapt the `future_category` from 30.6.1 [futures.overview] and 30.6.2 [futures.errors]:

>     extern const error_category* const future_category;

which should be similarly transformed into function form.

[ 2009-10-27 Howard: ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ 2009-11-11 Daniel adds: ]

> I just observe that the proposed resolution of this issue is incomplete and needs to reworded. The problem is that the corresponding declarations

> >     constexpr error_code make_error_code(future_errc e);
> >     constexpr error_condition make_error_condition(future_errc e);

> as constexpr functions are incompatible to the requirements of constexpr functions given their specified implementation. Note that the incompatibility is not a result of the modifications proposed by the issue resolution, but already existed within the N2960 state where we have

> >     extern const error_category* const future_category;

> combined with

> >     constexpr error_code make_error_code(future_errc e);

> > > 3 Returns: `error_code(static_cast<int>(e), *future_category)`.

> >     constexpr error_code make_error_condition(future_errc e);

> > > 4 Returns: `error_condition(static_cast<int>(e), *future_category)`.

> Neither is any of the constructors of `error_code` and `error_condition` constexpr, nor does the expression `*future_category` satisfy the requirements for a constant expression (5.20 [expr.const]/2 bullet 6 in N3000).

> The simple solution is just to remove the constexpr qualifiers for both functions, which makes sense, because none of the remaining `make_error_*` overloads in the library is constexpr. One might consider to realize that those `make_*` functions could satisfy the constexpr requirements, but this looks not like an easy task to me, because it would need to rely on a not yet existing language feature. If such a change is wanted, a new issue should be opened after the language extension approval (if at all) [1].

> If no-one complaints I would like to ask Howard to add the following modifications to this issue, alternatively a new issue could be opened but I don't know what the best solution is that would cause as little overhead as possible.

> What-ever the route is, the following is my proposed resolution for this issue interaction part of the story:

> > In 30.6.1 [futures.overview]/1, Header `<future>` synopsis and in 30.6.2 [futures.errors]/3+4 change as indicated:

> > >     constexpr error_code make_error_code(future_errc e);
> > >     constexpr error_condition make_error_condition(future_errc e);

> [1] Let me add that we have a related NAD issue here: 832 so the chances for realization are little IMO.

> [ Howard: I've updated the proposed wording as Daniel suggests and set to Review. ]

[ 2009-11-13 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010 Pittsburgh: ]

> Moved to NAD Editorial Resolved. Rationale added below.

Rationale:

Solved by N3058.

Proposed resolution:

1. Change in 30.6.1 [futures.overview], header `<future>` synopsis:

   >     extern const error_category&* const future_category();

2. In 30.6.1 [futures.overview]/1, Header `<future>` synopsis change as indicated:

   >     constexpr error_code make_error_code(future_errc e);
   >     constexpr error_condition make_error_condition(future_errc e);

3. Change in 30.6.2 [futures.errors]:

   >     extern const error_category&* const future_category();

   > > 1- `future_category` shall point to a statically initialized object of a type derived from class `error_category`.

   > > 1- Returns: A reference to an object of a type derived from class `error_category`.

   >     constexpr error_code make_error_code(future_errc e);

   > > 3 Returns: `error_code(static_cast<int>(e), *future_category())`.

   >     constexpr error_codecondition make_error_condition(future_errc e);

   > > 4 Returns: `error_condition(static_cast<int>(e), *future_category())`.

### 1227. `<bitset>` synopsis overspecified

Section: 20.6 [template.bitset] Status: C++11 Submitter: Bo Persson Opened: 2009-10-05 Last modified: 2015-04-08

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Discussion:

The resolutions to some library defect reports, like 1178 requires that `#includes` in each synopsis should be taken literally. This means that the `<bitset>` header now must include `<stdexcept>`, even though none of the exceptions are mentioned in the `<bitset>` header.

Many other classes are required to throw exceptions like `invalid_argument` and `out_of_range`, without explicitly including `<stdexcept>` in their synopsis. It is totally possible for implementations to throw the needed exceptions from utility functions, whose implementations are not visible in the headers.

I propose that `<stdexcept>` is removed from the `<bitset>` header.

[ 2009-10 Santa Cruz: ]

> Moved to Ready.

Proposed resolution:

Change 20.6 [template.bitset]:

>     #include <cstddef>        // for size_t
>     #include <string>
>     #include <stdexcept>      // for invalid_argument,
>                               // out_of_range, overflow_error
>     #include <iosfwd>         // for istream, ostream
>     namespace std {
>     ...

### 1229. `error_code operator=` typo

Section: 19.5.2.3 [syserr.errcode.modifiers] Status: Resolved Submitter: Stephan T. Lavavej Opened: 2009-10-08 Last modified: 2015-04-08

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Discussion:

N2960 19.5.2.1 [syserr.errcode.overview] and 19.5.2.3 [syserr.errcode.modifiers] say:

>     template <class ErrorCodeEnum>
>       typename enable_if<is_error_code_enum<ErrorCodeEnum>::value>::type&
>         operator=(ErrorCodeEnum e);

They should say:

>     template <class ErrorCodeEnum>
>       typename enable_if<is_error_code_enum<ErrorCodeEnum>::value, error_code>::type&
>         operator=(ErrorCodeEnum e);

Or (I prefer this form):

>     template <class ErrorCodeEnum>
>       typename enable_if<is_error_code_enum<ErrorCodeEnum>::value, error_code&>::type
>         operator=(ErrorCodeEnum e);

This is because `enable_if` is declared as (20.10.7.6 [meta.trans.other]):

>     template <bool B, class T = void> struct enable_if;

So, the current wording makes `operator=` return `void&`, which is not good.

19.5.2.3 [syserr.errcode.modifiers]/4 says

> Returns: `*this`.

which is correct.

Additionally,

19.5.3.1 [syserr.errcondition.overview]/1 says:

>     template<typename ErrorConditionEnum>
>       typename enable_if<is_error_condition_enum<ErrorConditionEnum>, error_code>::type &
>         operator=( ErrorConditionEnum e );

Which contains several problems (`typename` versus `class` inconsistency, lack of `::value`, `error_code` instead of `error_condition`), while 19.5.3.3 [syserr.errcondition.modifiers] says:

>     template <class ErrorConditionEnum>
>       typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value>::type&
>         operator=(ErrorConditionEnum e);

Which returns `void&`. They should both say:

>     template <class ErrorConditionEnum>
>       typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value,
>       error_condition>::type&
>         operator=(ErrorConditionEnum e);

Or (again, I prefer this form):

>     template <class ErrorConditionEnum>
>       typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value,
>       error_condition&>::type
>         operator=(ErrorConditionEnum e);

Additionally, 19.5.3.3 [syserr.errcondition.modifiers] lacks a "Returns: `*this`." paragraph, which is presumably necessary.

[ 2009-10-18 Beman adds: ]

> The proposed resolution for issue 1237 makes this issue moot, so it should become NAD.

[ 2009-10 Santa Cruz: ]

> NAD Resolved, solved by 1237.

Proposed resolution:

Change 19.5.2.1 [syserr.errcode.overview] and 19.5.2.3 [syserr.errcode.modifiers]:

>     template <class ErrorCodeEnum>
>       typename enable_if<is_error_code_enum<ErrorCodeEnum>::value,
>       error_code&>::type&
>         operator=(ErrorCodeEnum e);

Change 19.5.3.1 [syserr.errcondition.overview]:

>     template<typename class ErrorConditionEnum>
>       typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value,
>       error_conditionde&>::type &
>         operator=( ErrorConditionEnum e );

Change 19.5.3.3 [syserr.errcondition.modifiers]:

>     template <class ErrorConditionEnum>
>       typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value,
>       error_condition&>::type&
>         operator=(ErrorConditionEnum e);

> > Postcondition: `*this == make_error_condition(e)`.

> > Returns: `*this`.

> > Throws: Nothing.

### 1231. `weak_ptr` comparisons incompletely resolved

Section: 20.8.2.3.5 [util.smartptr.weak.obs] Status: C++11 Submitter: Daniel Krügler Opened: 2009-10-10 Last modified: 2015-04-08

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Discussion:

The n2637 paper suggested several updates of the ordering semantics of `shared_ptr` and `weak_ptr`, among those the explicit comparison operators of `weak_ptr` were removed/deleted, instead a corresponding functor `owner_less` was added. The problem is that n2637 did not clearly enough specify, how the previous wording parts describing the comparison semantics of `weak_ptr` should be removed.

[ 2009-11-06 Howard adds: ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

Proposed resolution:

1. Change 20.8.2.3 [util.smartptr.weak]/2 as described, the intention is to fix the now no longer valid requirement that `weak_ptr` is `LessComparable` [Note the deleted comma]:

   > Specializations of `weak_ptr` shall be `CopyConstructible`, and `CopyAssignable`, and `LessThanComparable`, allowing their use in standard containers.

2. In 20.8.2.3.5 [util.smartptr.weak.obs] remove the paragraphs 9-11 including prototype:

   > template<class T, class U> bool operator<(const weak_ptr<T>& a, const weak_ptr<U>& b);

   > Returns: an unspecified value such that
   > - `operator<` is a strict weak ordering as described in 25.4;
   > - under the equivalence relation defined by `operator<`, `!(a < b) && !(b < a)`, two `weak_ptr` instances are equivalent if and only if they share ownership or are both empty.

   > Throws: nothing.

   > [Note: Allows `weak_ptr` objects to be used as keys in associative containers. — end note]

### 1234. "Do the right thing" and `NULL`

Section: 23.2.3 [sequence.reqmts] Status: C++11 Submitter: Matt Austern Opened: 2009-10-09 Last modified: 2015-04-08

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Discussion:

On g++ 4.2.4 (x86_64-linux-gnu), the following file gives a compile error:

>     #include <vector>
>     void foo() { std::vector<int*> v(500L, NULL); }

Is this supposed to work?

The issue: if `NULL` happens to be defined as `0L`, this is an invocation of the constructor with two arguments of the same integral type. 23.2.3 [sequence.reqmts]/14 (N3035) says that this will behave as if the overloaded constructor

>     X(size_type, const value_type& = value_type(),
>       const allocator_type& = allocator_type())

were called instead, with the arguments `static_cast<size_type>(first)`, `last` and `alloc`, respectively. However, it does not say whether this actually means invoking that constructor with the exact textual form of the arguments as supplied by the user, or whether the standard permits an implementation to invoke that constructor with variables of the same type and value as what the user passed in. In most cases this is a distinction without a difference. In this particular case it does make a difference, since one of those things is a null pointer constant and the other is not.

Note that an implementation based on forwarding functions will use the latter interpretation.

[ 2010 Pittsburgh: Moved to Open. ]

[ 2010-03-19 Daniel provides wording. ]

> - Adapts the numbering used in the discussion to the recent working paper N3035.
> - Proposes a resolution that requires implementations to use sfinae-like means to possibly filter away the too generic template c'tor. In fact this resolution is equivalent to that used for the `pair-NULL` problem (811), the only difference is, that issue 1234 was already a C++03 problem.

> This issue can be considered as a refinement of 438.

[ Post-Rapperswil ]

Wording was verified to match with the most recent WP. Jonathan Wakely and Alberto Barbati observed that the current WP has a defect that should be fixed here as well: The functions signatures `fx1` and `fx3` are incorrectly referring to `iterator` instead of `const_iterator`.

> Moved to Tentatively Ready with revised wording after 7 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 23.2.3 [sequence.reqmts]/14+15 as indicated:

> 14 For every sequence container defined in this Clause and in Clause 21:

> - If the constructor

>   >     template <class InputIterator>
>   >     X(InputIterator first, InputIterator last,
>   >       const allocator_type& alloc = allocator_type())

>   is called with a type `InputIterator` that does not qualify as an input iterator, then the constructor shall not participate in overload resolution.will behave as if the overloaded constructor:

>   >     X(size_type, const value_type& = value_type(),
>   >       const allocator_type& = allocator_type())

>   were called instead, with the arguments `static_cast<size_type>(first)`, `last` and `alloc`, respectively.

> - If the member functions of the forms:

>   >     template <class InputIterator> // such as insert()
>   >     rt fx1(const_iterator p, InputIterator first, InputIterator last);

>   >     template <class InputIterator> // such as append(), assign()
>   >     rt fx2(InputIterator first, InputIterator last);

>   >     template <class InputIterator> // such as replace()
>   >     rt fx3(const_iterator i1, const_iterator i2, InputIterator first, InputIterator last);

>   are called with a type `InputIterator` that does not qualify as an input iterator, then these functions shall not participate in overload resolution.will behave as if the overloaded member functions:

>   >     rt fx1(iterator, size_type, const value_type&);

>   >     rt fx2(size_type, const value_type&);

>   >     rt fx3(iterator, iterator, size_type, const value_type&);

>   were called instead, with the same arguments.

> 15 In the previous paragraph the alternative binding will fail if `first` is not implicitly convertible to `X::size_type` or if `last` is not implicitly convertible to `X::value_type`.

### 1237. Constrained `error_code`/`error_condition members`

Section: 19.5 [syserr] Status: C++11 Submitter: Daniel Krügler Opened: 2009-10-14 Last modified: 2015-04-08

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Discussion:

I'm just reflecting on the now SFINAE-constrained constructors and assignment operators of `error_code` and `error_condition`:

These are the only library components that are pro-actively announcing that they are using `std::enable_if` as constraining tool, which has IMO several disadvantages:

1. With the availability of template default arguments and decltype, using `enable_if` in the C++0x standard library, seems unnecessary restricting implementation freedom. E.g. there should be no need for a useless specification of a dummy default function argument, which only confuses the reader. A more reasonable implementation could e.g. be

   >     template <class ErrorCodeEnum
   >      class = typename enable_if<is_error_code_enum<ErrorCodeEnum>::value>::type>
   >     error_code(ErrorCodeEnum e);

   As currently specified, the function signatures are so unreadable, that errors quite easily happen, see e.g. 1229.

2. We have a lot of constrained functions in other places, that now have a standard phrase that is easily understandable:

   > Remarks: This constructor/function shall participate in overload resolution if and only if X.

   where X describes the condition. Why should these components deviate?

3. If `enable_if` would not be explicitly specified, the standard library is much better prepared for the future. It would also be possible, that libraries with partial support for not-yet-standard-concepts could provide a much better diagnostic as is possible with `enable_if`. This again would allow for experimental concept implementations in the wild, which as a result would make concept standardization a much more natural thing, similar to the way as templates were standardized in C++.

   In summary: I consider it as a library defect that `error_code` and `error_condition` explicitly require a dependency to `enable_if` and do limit implementation freedom and I volunteer to prepare a corresponding resolution.

[ 2009-10-18 Beman adds: ]

> I support this proposed resolution, and thank Daniel for writing it up.

[ 2009-10 Santa Cruz: ]

> Moved to Ready.

Proposed resolution:

[ Should this resolution be accepted, I recommend to resolve 1229 as NAD ]

1. In 19.5.2.1 [syserr.errcode.overview]/1, class `error_code`, change as indicated:

   >     // 19.5.2.2 constructors:
   >     error_code();
   >     error_code(int val, const error_category& cat);
   >     template <class ErrorCodeEnum>
   >       error_code(ErrorCodeEnum e,
   >         typename enable_if<is_error_code_enum<ErrorCodeEnum>::value>::type * = 0);

   >     // 19.5.2.3 modifiers:
   >     void assign(int val, const error_category& cat);
   >     template <class ErrorCodeEnum>
   >       typename enable_if<is_error_code_enum<ErrorCodeEnum>::value>::typeerror_code&
   >         operator=(ErrorCodeEnum e);
   >     void clear();

2. Change 19.5.2.2 [syserr.errcode.constructors] around the prototype before p. 7:

   >     template <class ErrorCodeEnum>
   >     error_code(ErrorCodeEnum e,
   >       typename enable_if<is_error_code_enum<ErrorCodeEnum>::value>::type * = 0);

   > > Remarks: This constructor shall not participate in overload resolution, unless `is_error_code_enum<ErrorCodeEnum>::value == true`.

3. Change 19.5.2.3 [syserr.errcode.modifiers] around the prototype before p. 3:

   >     template <class ErrorCodeEnum>
   >       typename enable_if<is_error_code_enum<ErrorCodeEnum>::value>::typeerror_code&
   >         operator=(ErrorCodeEnum e);

   > > Remarks: This operator shall not participate in overload resolution, unless `is_error_code_enum<ErrorCodeEnum>::value == true`.

4. In 19.5.3.1 [syserr.errcondition.overview]/1, class `error_condition`, change as indicated:

   >     // 19.5.3.2 constructors:
   >     error_condition();
   >     error_condition(int val, const error_category& cat);
   >     template <class ErrorConditionEnum>
   >       error_condition(ErrorConditionEnum e,
   >         typename enable_if<is_error_condition_enum<ErrorConditionEnum>::type* = 0);

   >     // 19.5.3.3 modifiers:
   >     void assign(int val, const error_category& cat);
   >     template<typenameclass ErrorConditionEnum>
   >       typename enable_if<is_error_condition_enum<ErrorConditionEnum>, error_code>::typeerror_condition &
   >         operator=( ErrorConditionEnum e );
   >     void clear();

5. Change 19.5.3.2 [syserr.errcondition.constructors] around the prototype before p. 7:

   >     template <class ErrorConditionEnum>
   >       error_condition(ErrorConditionEnum e,
   >         typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value>::type* = 0);

   > > Remarks: This constructor shall not participate in overload resolution, unless `is_error_condition_enum<ErrorConditionEnum>::value == true`.

6. Change 19.5.3.3 [syserr.errcondition.modifiers] around the prototype before p. 3:

   >     template <class ErrorConditionEnum>
   >       typename enable_if<is_error_condition_enum<ErrorConditionEnum>::value>::typeerror_condition&
   >         operator=(ErrorConditionEnum e);

   > > Remarks: This operator shall not participate in overload resolution, unless `is_error_condition_enum<ErrorConditionEnum>::value == true`.

   > > Postcondition: `*this == make_error_condition(e)`.

   > > Returns: `*this`

### 1240. Deleted comparison functions of `std::function` not needed

Section: 20.9.12.2 [func.wrap.func] Status: C++11 Submitter: Daniel Krügler Opened: 2009-10-18 Last modified: 2015-04-08

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Discussion:

The class template `std::function` contains the following member declarations:

>     // deleted overloads close possible hole in the type system
>     template<class R2, class... ArgTypes2>
>       bool operator==(const function<R2(ArgTypes2...)>&) = delete;
>     template<class R2, class... ArgTypes2>
>       bool operator!=(const function<R2(ArgTypes2...)>&) = delete;

The leading comment here is part of the history of `std::function`, which was introduced with N1402. During that time no explicit conversion functions existed, and the "safe-bool" idiom (based on pointers-to-member) was a popular technique. The only disadvantage of this idiom was that given two objects `f1` and `f2` of type `std::function` the expression

>     f1 == f2;

was well-formed, just because the built-in `operator==` for pointer to member was considered after a single user-defined conversion. To fix this, an overload set of undefined comparison functions was added, such that overload resolution would prefer those ending up in a linkage error. The new language facility of deleted functions provided a much better diagnostic mechanism to fix this issue.

The central point of this issue is, that with the replacement of the safe-bool idiom by explicit conversion to bool the original "hole in the type system" does no longer exist and therefore the comment is wrong and the superfluous function definitions should be removed as well. An explicit conversion function is considered in direct-initialization situations only, which indirectly contain the so-called "contextual conversion to bool" (4 [conv]/3). These conversions are not considered for `==` or `!=` as defined by the core language.

[ Post-Rapperswil ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

In 20.9.12.2 [func.wrap.func]/1, class function change as indicated:

>     // 20.7.15.2.3, function capacity:
>     explicit operator bool() const;

>     // deleted overloads close possible hole in the type system
>     template<class R2, class... ArgTypes2>
>       bool operator==(const function<R2(ArgTypes2...)>&) = delete;
>     template<class R2, class... ArgTypes2>
>       bool operator!=(const function<R2(ArgTypes2...)>&) = delete;

### 1241. `unique_copy` needs to require `EquivalenceRelation`

Section: 25.3.9 [alg.unique] Status: C++11 Submitter: Daniel Krügler Opened: 2009-10-17 Last modified: 2015-04-08

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Discussion:

A lot of fixes were silently applied during concept-time and we should not lose them again. The Requires clause of 25.3.9 [alg.unique]/5 doesn't mention that `==` and the predicate need to satisfy an `EquivalenceRelation`, as it is correctly said for `unique`. This was intentionally fixed during conceptification, were we had:

>     template<InputIterator InIter, class OutIter>
>       requires OutputIterator<OutIter, RvalueOf<InIter::value_type>::type>
>             && EqualityComparable<InIter::value_type>
>             && HasAssign<InIter::value_type, InIter::reference>
>             && Constructible<InIter::value_type, InIter::reference>
>       OutIter unique_copy(InIter first, InIter last, OutIter result);

>     template<InputIterator InIter, class OutIter,
>              EquivalenceRelation<auto, InIter::value_type> Pred>
>       requires OutputIterator<OutIter, RvalueOf<InIter::value_type>::type>
>             && HasAssign<InIter::value_type, InIter::reference>
>             && Constructible<InIter::value_type, InIter::reference>
>             && CopyConstructible<Pred>
>       OutIter unique_copy(InIter first, InIter last, OutIter result, Pred pred);

Note that EqualityComparable implied an equivalence relation.

[ N.B. `adjacent_find` was also specified to require `EquivalenceRelation`, but that was considered as a defect in concepts, see 1000 ]

[ 2009-10-31 Howard adds: ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

Proposed resolution:

Change 25.3.9 [alg.unique]/5 as indicated:

>     template<class InputIterator, class OutputIterator>
>       OutputIterator
>         unique_copy(InputIterator first, InputIterator last, OutputIterator result);

>     template<class InputIterator, class OutputIterator, class BinaryPredicate>
>       OutputIterator
>         unique_copy(InputIterator first, InputIterator last,
>                     OutputIterator result, BinaryPredicate pred);

> > Requires: The comparison function shall be an equivalence relation. The ranges `[first,last)` and `[result,result+(last-first))` shall not overlap. The expression `*result = *first` shall be valid. If neither `InputIterator` nor `OutputIterator` meets the requirements of forward iterator then the value type of `InputIterator` shall be `CopyConstructible` (34) and `CopyAssignable` (table 36). Otherwise `CopyConstructible` is not required.

### 1244. `wait_*()` in `*future` for synchronous functions

Section: 30.6 [futures] Status: Resolved Submitter: Detlef Vollmann Opened: 2009-10-22 Last modified: 2015-04-08

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Discussion:

With the addition of `async()`, a `future` might be associated with a function that is not running in a different thread but is stored to by run synchronously on the `get()` call. It's not clear what the `wait()` functions should do in this case.

Suggested resolution:

Throw an exception.

[ 2010 Pittsburgh: Moved to NAD Editorial Resolved. Rationale added below. ]

Rationale:

Solved by N3058.

Proposed resolution:

### 1245. `std::hash<string>` & co

Section: 20.9.13 [unord.hash] Status: C++11 Submitter: Paolo Carlini Opened: 2009-10-22 Last modified: 2015-04-08

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Discussion:

In 20.9.13 [unord.hash], `operator()` is specified as taking the argument by value. Moreover, it is said that `operator()` shall not throw exceptions.

However, for the specializations for class types, like `string`, `wstring`, etc, the former requirement seems suboptimal from the performance point of view (a specific PR has been filed about this in the GCC Bugzilla) and, together with the latter requirement, hard if not impossible to fulfill. It looks like pass by const reference should be allowed in such cases.

[ 2009-11-18: Ganesh updates wording. ]

> I've removed the list of types for which `hash` shall be instantiated because it's already explicit in the synopsis of header `<functional>` in 20.9 [function.objects]/2.

[ 2009-11-18: Original wording here: ]

> Add to 20.9.13 [unord.hash]/2:

> >     namespace std {
> >       template <class T>
> >       struct hash : public std::unary_function<T, std::size_t> {
> >         std::size_t operator()(T val) const;
> >       };
> >     }

> > The return value of `operator()` is unspecified, except that equal arguments shall yield the same result. `operator()` shall not throw exceptions. It is also unspecified whether `operator()` of `std::hash` specializations for class types takes its argument by value or const reference.

[ 2009-11-19 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2009-11-24 Ville Opens: ]

> I have received community requests to ask for this issue to be reopened. Some users feel that mandating the inheritance is overly constraining.

[ 2010-01-31 Alisdair: related to 978 and 1182. ]

[ 2010-02-07 Proposed resolution updated by Beman, Daniel and Ganesh. ]

[ 2010-02-09 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Insert a new subclause either before or after the current 17.6.3.5 [allocator.requirements]:

> ### `Hash` Requirements [hash.requirements]

> This subclause defines the named requirement `Hash`, used in several clauses of the C++ standard library. A type `H` meets the `Hash` requirement if

> - it is a function object type (20.9 [function.objects]).

> - it satisfies the requirements of `CopyConstructible`, and `Destructible` (17.6.3.1 [utility.arg.requirements]),

> - the expressions shown in the following table are valid and have the indicated semantics, and

> - it satisfies all other requirements of this subclause.

> Given `Key` is an argument type for function objects of type `H`, in the table below `h` is a value of type (possibly `const`) `H`, `u` is an lvalue of type `Key`, and `k` is a value of a type convertible to (possibly `const`) `Key`:

> Table ? — `Hash` requirements
> Expression | Return type | Requirement

> `h(k)`

> `size_t`

> Shall not throw exceptions. The value returned shall depend only on the argument `k`. [Note: Thus all evaluations of the expression `h(k)` with the same value for `k` yield the same result. — end note] [Note: For `t1` and `t2` of different values, the probability that `h(t1)` and `h(t2)` compare equal should be very small, approaching `(1.0/numeric_limits<size_t>::max())`. — end note] Comment (not to go in WP): The wording for the second note is based on a similar note in 22.4.4.1.2 [locale.collate.virtuals]/3

> `h(u)` | `size_t` | Shall not modify `u`.

Change 20.9.13 [unord.hash] as indicated:

> 1 The unordered associative containers defined in Clause 23.5 [unord] use specializations of the class template `hash` as the default hash function. For all object types `T` for which there exists a specialization `hash<T>`, the instantiation `hash<T>` shall:

> - satisfy the `Hash` requirements([hash.requirements]), with `T` as the function call argument type, the `DefaultConstructible` requirements ([defaultconstructible]), the `CopyAssignable` requirements ([copyassignable]), and the `Swappable` requirements ([swappable]),
> - provide two nested types `result_type` and `argument_type` which shall be synonyms for `size_t` and `T`, respectively,
> - satisfy the requirement that if `k1 == k2` is `true`, `h(k1) == h(k2)` is `true`, where `h` is an object of type `hash<T>`, and `k1`, `k2` are objects of type `T`.

> This class template is only required to be instantiable for integer types (3.9.1 [basic.fundamental]), floating-point types (3.9.1 [basic.fundamental]), pointer types (8.3.1 [dcl.ptr]), and `std::string`, `std::u16string`, `std::u32string`, `std::wstring`, `std::error_code`, `std::thread::id`, `std::bitset`, and `std::vector<bool>`.

> >     namespace std {
> >       template <class T>
> >       struct hash : public std::unary_function<T, std::size_t> {
> >         std::size_t operator()(T val) const;
> >       };
> >     }

> 2 The return value of `operator()` is unspecified, except that equal arguments shall yield the same result. `operator()` shall not throw exceptions.

Change Unordered associative containers 23.2.5 [unord.req] as indicated:

> Each unordered associative container is parameterized by `Key`, by a function object type `Hash`([hash.requirements]) that acts as a hash function for argument values of type `Key`, and by a binary predicate `Pred` that induces an equivalence relation on values of type `Key`. Additionally, `unordered_map` and `unordered_multimap` associate an arbitrary mapped type `T` with the `Key`.

> A hash function is a function object that takes a single argument of type `Key` and returns a value of type `std::size_t`.

> Two values `k1` and `k2` of type `Key` are considered equal if the container's equality function object returns `true` when passed those values. If `k1` and `k2` are equal, the hash function shall return the same value for both. [Note: Thus supplying a non-default `Pred` parameter usually implies the need to supply a non-default `Hash` parameter. — end note]

### 1247. `auto_ptr` is overspecified

Section: X [auto.ptr] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-10-24 Last modified: 2015-04-08

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Discussion:

This issue is extracted as the ongoing point-of-interest from earlier issue 463.

`auto_ptr` is overspecified as the `auto_ptr_ref` implementation detail is formally specified, and the technique is observable so workarounds for compiler defects can cause a working implementation of the primary `auto_ptr` template become non-conforming.

`auto_ptr_ref` is a documentation aid to describe a possible mechanism to implement the class. It should be marked exposition only, as per similar classes, e.g., `istreambuf_iterator::proxy`

[ 2009-10-25 Daniel adds: ]

> I wonder, whether the revised wording shouldn't be as straight as for `istream_buf` by adding one further sentence:

> > An implementation is permitted to provide equivalent functionality without providing a class with this name.

[ 2009-11-06 Alisdair adds Daniel's suggestion to the proposed wording. ]

[ 2009-11-06 Howard moves issue to Review. ]

[ 2009-11-14 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Add the term "exposition only" in the following two places:

Ammend X [auto.ptr]p2:

> The exposition only class T template `auto_ptr_ref` holds a reference to an `auto_ptr`. It is used by the `auto_ptr` conversions to allow `auto_ptr` objects to be passed to and returned from functions. An implementation is permitted to provide equivalent functionality without providing a class with this name.

> >     namespace std {
> >      template <class Y> struct auto_ptr_ref { }; // exposition only

### 1248. Equality comparison for unordered containers

Section: 23.5 [unord] Status: Resolved Submitter: Herb Sutter Opened: 2009-10-25 Last modified: 2015-04-08

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Discussion:

See N2986.

[ 2010-01-22 Alisdair Opens. ]

[ 2010-01-24 Alisdair provides wording. ]

[ 2010 Pittsburgh: Moved to NAD Editorial Resolved. Rationale added below. ]

Rationale:

Solved by N3068.

Proposed resolution:

Apply paper N2986.

### 1249. `basic_ios` default ctor

Section: 27.5.5.2 [basic.ios.cons] Status: C++11 Submitter: Martin Sebor Opened: 2009-10-25 Last modified: 2015-04-08

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Discussion:

The `basic_ios` default ctor is required to leave the objects members uninitialized (see below). The paragraph says the object must be initialized by calling `basic_ios::init()` before it's destroyed but I can't find a requirement that it be initialized before calling any of the class other member functions. Am I not looking in the right place or that an issue?

[ 2009-10-25 Daniel adds: ]

> I agree, that your wording makes that clearer, but suggest to write

> > ... calling `basic_ios::init()` before ...

> Doing so, I recommend to adapt that of `ios_base();` as well, where we have:

> > Effects: Each `ios_base` member has an indeterminate value after construction. These members shall be initialized by calling `basic_ios::init`. If an `ios_base` object is destroyed before these initializations have taken place, the behavior is undefined.

[ Post-Rapperswil: ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 27.5.3.7 [ios.base.cons] p1:

>     ios_base();

> > Effects: Each `ios_base` member has an indeterminate value after construction. These The object's members shall be initialized by calling `basic_ios::init` before the object's first use or before it is destroyed, whichever comes first; otherwise the behavior is undefined.. If an `ios_base` object is destroyed before these initializations have taken place, the behavior is undefined.

Change 27.5.5.2 [basic.ios.cons] p2:

>     basic_ios();

> > Effects: Constructs an object of class `basic_ios` (27.5.2.7) leaving its member objects uninitialized. The object shall be initialized by calling its `basic_ios::init` before its first use or before it is destroyed, whichever comes first; otherwise the behavior is undefined. member function. If it is destroyed before it has been initialized the behavior is undefined.

### 1250. `<bitset>` still overspecified

Section: 20.6 [template.bitset] Status: C++11 Submitter: Martin Sebor Opened: 2009-10-29 Last modified: 2015-04-08

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Discussion:

Issue 1227 — `<bitset>` synopsis overspecified makes the observation that `std::bitset`, and in fact the whole library, may be implemented without needing to `#include <stdexcept>` in any library header. The proposed resolution removes the `#include <stdexcept>` directive from the header.

I'd like to add that the `<bitset>` header (as well as the rest of the library) has also been implemented without #including the `<cstddef>` header in any library header. In the case of `std::bitset`, the template is fully usable (i.e., it may be instantiated and all its member functions may be used) without ever mentioning `size_t`. In addition, just like no library header except for `<bitset>` `#includes <stdexcept>` in its synopsis, no header but `<bitset>` `#includes <cstddef>` either.

Thus I suggest that the `#include <cstddef>` directive be similarly removed from the synopsis of `<bitset>`.

[ 2010-02-08 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

Proposed resolution:

Change 20.6 [template.bitset]:

>     #include <cstddef>        // for size_t
>     #include <string>
>     #include <iosfwd>         // for istream, ostream
>     namespace std {
>     ...

### 1252. `wbuffer_convert::state_type` inconsistency

Section: 22.3.3.2.3 [conversions.buffer] Status: C++11 Submitter: Bo Persson Opened: 2009-10-21 Last modified: 2015-04-08

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Discussion:

The synopsis for `wbuffer_convert` 22.3.3.2.3 [conversions.buffer]/2 contains

>     typedef typename Tr::state_type   state_type;

making `state_type` a synonym for (possibly) some `char_traits<x>::state_type`.

However, in paragraph 9 of the same section, we have

>     typedef typename Codecvt::state_type state_type;

> > The type shall be a synonym for `Codecvt::state_type`.

From what I can see, it might be hard to implement `wbuffer_convert` if the types were not both `std::mbstate_t`, but I cannot find a requirement that they must be the same type.

[ Batavia 2010: ]

Howard to draft wording, move to Review. Run it by Bill. Need to move this in Madrid.

[2011-03-06: Howard drafts wording]

[2011-03-24 Madrid meeting]

Moved to Immediate

Proposed resolution:

Modify the `state_type` typedef in the synopsis of 22.3.3.2.3 [conversions.buffer] p.2 as shown [This makes the synopsis consistent with 22.3.3.2.3 [conversions.buffer] p.9]:

>     namespace std {
>     template<class Codecvt,
>       class Elem = wchar_t,
>       class Tr = std::char_traits<Elem> >
>     class wbuffer_convert
>       : public std::basic_streambuf<Elem, Tr> {
>     public:
>       typedef typename TrCodecvt::state_type state_type;
>       […]
>     };
>     }

### 1253. invalidation of iterators and `emplace` vs. `insert` inconsistence in assoc. containers

Section: 23.2.4 [associative.reqmts] Status: C++11 Submitter: Boris Dušek Opened: 2009-10-24 Last modified: 2015-04-08

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Discussion:

In the latest published draft N2960, section 23.2.4 [associative.reqmts], paragraph 8, it is specified that that `insert` does not invalidate any iterators. As per 23.2.1 [container.requirements.general], paragraph 12, this holds true not only for `insert`, but `emplace` as well. This gives the `insert` member a special treatment w.r.t. `emplace` member in 23.2.4 [associative.reqmts], par. 8, since both modify the container. For the sake of consistency, in 23.2.4 [associative.reqmts], par. 8: either reference to `insert` should be removed (i.e. count on 23.2.1 [container.requirements.general], par. 12), or reference to `emplace` be added (i.e. mention all members of assoc. containers that modify it).

[ 2009-11-18 Chris provided wording. ]

> This suggested wording covers both the issue discussed, and a number of other identical issues (namely `insert` being discussed without `emplace`). I'm happy to go back and split and introduce a new issue if appropriate, but I think the changes are fairly mechanical and obvious.

[ 2010-01-23 Daniel Krügler and J. Daniel García updated wording to make the use of `hint` consistent with `insert`. ]

[ 2011-02-23 Daniel Krügler adapts wording to numbering changes to match the N3225 draft. During this action it was found that 23.2.5 [unord.req] had been changed considerably due to acceptance of N3068 during the Pittsburgh meeting, such that the suggested wording change to p. 6 can no longer be applied. The new wording is more general and should now include `insert()` and `emplace()` calls as well ("mutating operations"). ]

[2011-03-06 Daniel Krügler adapts wording to numbering changes to match the N3242 draft]

Proposed resolution:

1. Modify bullet 1 of 23.2.1 [container.requirements.general], p. 10:

   10 Unless otherwise specified (see [associative.reqmts.except], [unord.req.except], [deque.modifiers], and [vector.modifiers]) all container types defined in this Clause meet the following additional requirements:

   - if an exception is thrown by an `insert()` or `emplace()` function while inserting a single element, that function has no effects.
   - ...

2. Modify 23.2.4 [associative.reqmts], p. 4:

   > 4 An associative container supports unique keys if it may contain at most one element for each key. Otherwise, it supports equivalent keys. The `set` and `map` classes support unique keys; the `multiset` and `multimap` classes support equivalent keys. For `multiset` and `multimap`, `insert`, `emplace`, and `erase` preserve the relative ordering of equivalent elements.

3. Modify Table 102 — Associative container requirements in 23.2.4 [associative.reqmts]:

   > Table 102 — Associative container requirements (in addition to container)
   > Expression | Return type | Assertion/note pre-/post-condition | Complexity
   > --- | --- | --- | ---
   > […]
   > `a_eq.emplace(args)` | `iterator` | […] Effects: Inserts a `T` object `t` constructed with `std::forward<Args>(args)...` and returns the iterator pointing to the newly inserted element. If a range containing elements equivalent to `t` exists in `a_eq`, `t` is inserted at the end of that range. | logarithmic

   > `a.emplace_hint(p, args)`

   > `iterator`

   > equivalent to `a.emplace(std::forward<Args>(args)...)`. Return value is an iterator pointing to the element with the key equivalent to the newly inserted element. The `const_iterator p` is a hint pointing to where the search should start. The element is inserted as close as possible to the position just prior to `p`. Implementations are permitted to ignore the hint.

   > logarithmic in general, but amortized constant if the element is inserted right after before `p`

   > […]

4. Modify 23.2.4 [associative.reqmts], p. 9:

   > 9 The `insert` and `emplace` members shall not affect the validity of iterators and references to the container, and the `erase` members shall invalidate only iterators and references to the erased elements.

5. Modify 23.2.4.1 [associative.reqmts.except], p. 2:

   > 2 For associative containers, if an exception is thrown by any operation from within an `insert()` or `emplace()` function inserting a single element, the `insert()` function insertion has no effect.

6. Modify 23.2.5 [unord.req], p. 13 and p. 14:

   > 6 An unordered associative container supports unique keys if it may contain at most one element for each key. Otherwise, it supports equivalent keys. `unordered_set` and `unordered_map` support unique keys. `unordered_multiset` and `unordered_multimap` support equivalent keys. In containers that support equivalent keys, elements with equivalent keys are adjacent to each other in the iteration order of the container. Thus, although the absolute order of elements in an unordered container is not specified, its elements are grouped into equivalent-key groups such that all elements of each group have equivalent keys. Mutating operations on unordered containers shall preserve the relative order of elements within each equivalent-key group unless otherwise specified.

   > […]

   > 13 The `insert` and `emplace` members shall not affect the validity of references to container elements, but may invalidate all iterators to the container. The erase members shall invalidate only iterators and references to the erased elements.

   > 14 The `insert` and `emplace` members shall not affect the validity of iterators if `(N+n) < z * B`, where `N` is the number of elements in the container prior to the insert operation, `n` is the number of elements inserted, `B` is the container's bucket count, and `z` is the container's maximum load factor.

7. Modify 23.2.5.1 [unord.req.except], p. 2:

   > 2 For unordered associative containers, if an exception is thrown by any operation other than the container's hash function from within an `insert()` or `emplace()` function inserting a single element, the `insert()` insertion function has no effect.

### 1254. Misleading sentence in `vector<bool>::flip`

Section: 23.3.7 [vector.bool] Status: C++11 Submitter: Christopher Jefferson Opened: 2009-11-01 Last modified: 2015-04-08

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Discussion:

The effects of `vector<bool>::flip` has the line:

> It is unspecified whether the function has any effect on allocated but unused bits.

While this is technically true, it is misleading, as any member function in any standard container may change unused but allocated memory. Users can never observe such changes as it would also be undefined behaviour to read such memory.

[ 2009-11-14 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Strike second sentence from the definition of `vector<bool>::flip()`, 23.3.7 [vector.bool], paragraph 5.

> Effects: Replaces each element in the container with its complement. It is unspecified whether the function has any effect on allocated but unused bits.

### 1255. `declval` should be added to the library

Section: 20.2 [utility] Status: C++11 Submitter: Daniel Krügler Opened: 2009-11-03 Last modified: 2015-04-08

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Discussion:

During the Santa Cruz meeting it was decided to split off the provision of the library utility `value()` proposed in N2979 from the concrete request of the UK 300 comment. The provision of a new library component that allows the production of values in unevaluated expressions is considered as important to realize constrained templates in C++0x where concepts are not available.

The following proposed resolution is an improvement over that suggested in N2958, because the proposed component can now be defined without loss of general usefulness and any use by user-code will make the program ill-formed. A possible prototype implementation that satisfies the core language requirements can be written as:

>     template<class T>
>       struct declval_protector {
>         static const bool stop = false;
>         static typename std::add_rvalue_reference<T>::type delegate(); // undefined
>       };

>     template<class T>
>     typename std::add_rvalue_reference<T>::type declval() {
>       static_assert(declval_protector<T>::stop, "declval() must not be used!");
>       return declval_protector<T>::delegate();
>     }

Further-on the earlier suggested name `value()` has been changed to `declval()` after discussions with committee members.

Finally the suggestion shown below demonstrates that it can simplify existing standard wording by directly using it in the library specification, and that it also improves an overlooked corner case for `common_type` by adding support for `cv void`.

[ 2009-11-19 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

Proposed resolution:

[ The proposed resolution has been updated to N3000 numbering and wording ]

1. Change 20.2 [utility], header `<utility>` synopsis as indicated:

   >     // 20.3.3, forward/move:
   >     template <class T> struct identity;
   >     template <class T, class U> T&& forward(U&&);
   >     template <class T> typename remove_reference<T>::type&& move(T&&);

   >     // 20.3.4, declval:
   >     template <class T> typename add_rvalue_reference<T>::type declval(); // as unevaluated operand

2. Immediately after the current section 20.2.4 [forward] insert a new section:

   20.3.4 Function template declval [declval]

   The library provides the function template `declval` to simplify the definition of expressions which occur as unevaluated operands (5 [expr]). The template parameter `T` of `declval` may be an incomplete type.

       template <class T> typename add_rvalue_reference<T>::type declval(); // as unevaluated operand

   > Remarks: If this function is used according to 3.2 [basic.def.odr], the program is ill-formed.

   > [Example:

   > >     template<class To, class From>
   > >     decltype(static_cast<To>(declval<From>())) convert(From&&);

   > declares a function template `convert`, which only participates in overloading if the type `From` can be explicitly cast to type `To`. For another example see class template `common_type` (20.10.7.6 [meta.trans.other]). — end example]

3. This bullet just makes clear that after applying N2984, the changes in 20.10.4.3 [meta.unary.prop], before table Type property queries should not use `declval`, because the well-formedness requirement of the specification of `is_constructible` would become more complicated, because we would need to make sure that the expression CE is checked in an unevaluated context.

4. Also 20.10.6 [meta.rel]/4 is not modified similar to the previous bullet, because with the stricter requirements of not using `declval()` the well-formedness condition would be harder to specify. The following changes are only editorial ones (e.g. the removal of the duplicate declaration of `create()`):

   > Given the following function prototype:

   > >     template <class T>
   > >       typename add_rvalue_reference<T>::type create();

   > the predicate condition for a template specialization `is_convertible<From, To>` shall be satisfied if and only if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function:

   > >     template <class T>
   > >     typename add_rvalue_reference<T>::type create();
   > >     To test() {
   > >       return create<From>();
   > >     }

5. Change the entry in column "Comments" for `common_type` in Table 51 — Other transformations (20.10.7.6 [meta.trans.other]):

   [ NB: This wording change extends the type domain of `common_type` for `cv void => cv void` transformations and thus makes `common_type` usable for all binary type combinations that are supported by `is_convertible` ]

   > The member typedef `type` shall be defined as set out below. All types in the parameter pack `T` shall be complete or (possibly cv-qualified) `void`. A program may specialize this trait if at least one template parameter in the specialization is a user-defined type. [Note: Such specializations are needed when only explicit conversions are desired among the template arguments. — end note]

6. Change 20.10.7.6 [meta.trans.other]/3 as indicated:

   [ NB: This wording change is more than an editorial simplification of the definition of `common_type`: It also extends its usefulness for `cv void` types as outlined above ]

   > The nested typedef `common_type::type` shall be defined as follows:

   > > [..]

   > >     template <class T, class U>
   > >     struct common_type<T, U> {
   > >     private:
   > >       static T&& __t();
   > >       static U&& __u();
   > >     public:
   > >       typedef decltype(true ? __tdeclval<T>() : __udeclval<U>()) type;
   > >     };

7. Change X [func.ret]/1 as indicated [This part solves some main aspects of issue 1225]:

   >     namespace std {
   >       template <class> class result_of; // undefined

   >       template <class Fn, class... ArgTypes>
   >       class result_of<Fn(ArgTypes...)> {
   >       public :
   >         // types
   >         typedef see belowdecltype(declval<Fn>() ( declval<ArgTypes>()... )) type;
   >       };
   >     }

   > 1 Given an rvalue `fn` of type `Fn` and values `t1, t2, ..., tN` of types `T1, T2, ..., TN` in `ArgTypes`, respectively, the `type` member is the result type of the expression `fn(t1, t2, ...,tN)`. The values `ti` are lvalues when the corresponding type `Ti` is an lvalue-reference type, and rvalues otherwise.

### 1256. `weak_ptr` comparison functions should be removed

Section: 20.8.2.3 [util.smartptr.weak] Status: C++11 Submitter: Daniel Krügler Opened: 2009-11-04 Last modified: 2015-04-08

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Discussion:

Additional to the necessary cleanup of the description of the the `weak_ptr` component from 20.8.2.3 [util.smartptr.weak] described in 1231 it turns out that the currently deleted comparison functions of `weak_ptr` are not needed at all: There is no safe-bool conversion from `weak_ptr`, and it won't silently chose a conversion to `shared_ptr`.

[ 2009-11-14 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Change 20.8.2.3 [util.smartptr.weak]/1 as indicated:

>     namespace std {
>     template<class T> class weak_ptr {
>     public:
>     ...
>       // comparisons
>       template<class Y> bool operator<(weak_ptr<Y> const&) const = delete;
>       template<class Y> bool operator<=(weak_ptr<Y> const&) const = delete;
>       template<class Y> bool operator>(weak_ptr<Y> const&) const = delete;
>       template<class Y> bool operator>=(weak_ptr<Y> const&) const = delete;
>     };
>     ...

### 1257. Header <ios> still contains a `concept_map`

Section: 27.5 [iostreams.base] Status: C++11 Submitter: Beman Dawes Opened: 2009-11-04 Last modified: 2015-04-08

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Discussion:

The current WP still contains a `concept_map`.

[ 2009-11-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Change Iostreams base classes 27.5 [iostreams.base], Header <ios> synopsis, as indicated:

>     concept_map ErrorCodeEnum<io_errc> { };
>     template <> struct is_error_code_enum<io_errc> : true_type { }
>     error_code make_error_code(io_errc e);
>     error_condition make_error_condition(io_errc e);
>     const error_category& iostream_category();

### 1258. std::function Effects clause impossible to satisfy

Section: 20.9.12.2.2 [func.wrap.func.mod] Status: Resolved Submitter: Daniel Krügler Opened: 2009-11-05 Last modified: 2015-04-08

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Discussion:

As of 20.9.12.2.2 [func.wrap.func.mod]/2+ we have the following prototype description:

>     template<class F, Allocator Alloc>
>       void assign(F, const Alloc&);

> > Effects: `function(f, a).swap(*this)`

Two things: First the concept debris needs to be removed, second and much more importantly, the effects clause is now impossible to satisfy, because there is no constructor that would match the parameter sequence (`FunctionObject`, `Allocator`) [plus the fact that no `f` and no `a` is part of the signature]. The most probable candidate is

>     template<class F, class A> function(allocator_arg_t, const A&, F);

and the effects clause needs to be adapted to use this signature.

[ 2009-11-13 Daniel brought wording up to date. ]

[ 2009-11-15 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010-02-11 Moved to Tentatively NAD Editorial after 5 positive votes on c++std-lib. It was noted that this issue was in partial conflict with 1288, and the two issues were merged in 1288. ]

Rationale:

Addressed by 1288.

Proposed resolution:

Change in 20.9.12.2.2 [func.wrap.func.mod] the complete prototype description as indicated

[ Question to the editor: Shouldn't there a paragraph number in front of the Effects clause? ]

>     template<class F, Allocator Allocclass A>
>       void assign(F f, const Alloc& a);

> > 3 Effects: `function(f, aallocator_arg, a, f).swap(*this)`

### 1260. `is_constructible<int*,void*>` reports true

Section: 20.10.4.3 [meta.unary.prop] Status: Resolved Submitter: Peter Dimov Opened: 2009-11-07 Last modified: 2015-04-08

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Discussion:

The specification of `is_constructible<T,Args...>` in N3000 uses

>     static_cast<T>(create<Args>()...)

for the one-argument case, but `static_cast` also permits unwanted conversions such as `void*` to `T*` and `Base*` to `Derived*`.

[ Post-Rapperswil: ]

> Moved to NAD Editorial Resolved, this issue is addressed by paper n3047

Proposed resolution:

Change 20.10.4.3 [meta.unary.prop], p6:

> the predicate condition for a template specialization `is_constructible<T, Args>` shall be satisfied, if and only if the following expression CE variable definition would be well-formed:

> - if `sizeof...(Args) == 0 1`, the expression:

>   >     static_cast<T>(create<Args>()...)
>   >     T t;

> - otherwise the expression:

>   >     T t(create<Args>()...);

### 1261. Insufficent overloads for `to_string` / `to_wstring`

Section: 21.5 [string.conversions] Status: C++11 Submitter: Christopher Jefferson Opened: 2009-11-10 Last modified: 2015-04-08

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Discussion:

Reported on the gcc mailing list.

> The code "`int i; to_string(i);`" fails to compile, as '`int`' is ambiguous between '`long long`' and '`long long unsigned`'. It seems unreasonable to expect users to cast numbers up to a larger type just to use `to_string`.

[ 2009-11-14 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

21.3 [string.classes], change `to_string` and `to_wstring` to:

>     string to_string(int val);
>     string to_string(unsigned val);
>     string to_string(long val);
>     string to_string(unsigned long val);
>     string to_string(long long val);
>     string to_string(unsigned long long val);
>     string to_string(float val);
>     string to_string(double val);
>     string to_string(long double val);

>     wstring to_wstring(int val);
>     wstring to_wstring(unsigned val);
>     wstring to_wstring(long val);
>     wstring to_wstring(unsigned long val);
>     wstring to_wstring(long long val);
>     wstring to_wstring(unsigned long long val);
>     wstring to_wstring(float val);
>     wstring to_wstring(double val);
>     wstring to_wstring(long double val);

In 21.5 [string.conversions], paragraph 7, change to:

>     string to_string(int val);
>     string to_string(unsigned val);
>     string to_string(long val);
>     string to_string(unsigned long val);
>     string to_string(long long val);
>     string to_string(unsigned long long val);
>     string to_string(float val);
>     string to_string(double val);
>     string to_string(long double val);

> > 7 Returns: each function returns a `string` object holding the character representation of the value of its argument that would be generated by calling `sprintf(buf, fmt, val)` with a format specifier of `"%d"`, `"%u"`, `"%ld"`, `"%lu"`, `"%lld"`, `"%llu"`, `"%f"`, `"%f"`, or `"%Lf"`, respectively, where `buf` designates an internal character buffer of sufficient size.

In 21.5 [string.conversions], paragraph 14, change to:

>     wstring to_wstring(int val);
>     wstring to_wstring(unsigned val);
>     wstring to_wstring(long val);
>     wstring to_wstring(unsigned long val);
>     wstring to_wstring(long long val);
>     wstring to_wstring(unsigned long long val);
>     wstring to_wstring(float val);
>     wstring to_wstring(double val);
>     wstring to_wstring(long double val);

> > 14 Returns: Each function returns a `wstring` object holding the character representation of the value of its argument that would be generated by calling `swprintf(buf, buffsz, fmt, val)` with a format specifier of `L"%d"`, `L"%u"`, `L"%ld"`, `L"%lu"`, `L"%lld"`, `L"%llu"`, `L"%f"`, `L"%f"`, or `L"%Lf"`, respectively, where `buf` designates an internal character buffer of sufficient size `buffsz`.

### 1262. `std::less<std::shared_ptr<T>>` is underspecified

Section: 20.8.2.2.7 [util.smartptr.shared.cmp] Status: C++11 Submitter: Jonathan Wakely Opened: 2009-11-10 Last modified: 2015-04-08

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Discussion:

20.8.2.2.7 [util.smartptr.shared.cmp]/5 says:

> For templates `greater`, `less`, `greater_equal`, and `less_equal`, the partial specializations for `shared_ptr` shall yield a total order, even if the built-in operators `<`, `>`, `<=`, and `>=` do not. Moreover, `less<shared_ptr<T> >::operator()(a, b)` shall return `std::less<T*>::operator()(a.get(), b.get())`.

This is necessary in order to use `shared_ptr` as the key in associate containers because n2637 changed `operator<` on `shared_ptr`s to be defined in terms of `operator<` on the stored pointers (a mistake IMHO but too late now.) By 5.9 [expr.rel]/2 the result of comparing builtin pointers is unspecified except in special cases which generally do not apply to `shared_ptr`.

Earlier versions of the WP (n2798, n2857) had the following note on that paragraph:

> [Editor's note: It's not clear to me whether the first sentence is a requirement or a note. The second sentence seems to be a requirement, but it doesn't really belong here, under `operator<`.]

I agree completely - if partial specializations are needed they should be properly specified.

20.8.2.2.7 [util.smartptr.shared.cmp]/6 has a note saying the comparison operator allows `shared_ptr` objects to be used as keys in associative containers, which is misleading because something else like a `std::less` partial specialization is needed. If it is not correct that note should be removed.

20.8.2.2.7 [util.smartptr.shared.cmp]/3 refers to '`x`' and '`y`' but the prototype has parameters '`a`' and '`b`' - that needs to be fixed even if the rest of the issue is NAD.

I see two ways to fix this, I prefer the first because it removes the need for any partial specializations and also fixes `operator>` and other comparisons when defined in terms of `operator<`.

1. Replace 20.8.2.2.7 [util.smartptr.shared.cmp]/3 with the following and remove p5:

   >     template<class T, class U> bool operator<(const shared_ptr<T>& a, const shared_ptr<U>& b);

   > > 3 Returns: `x.get() < y.get()`. `std::less<V>()(a.get(), b.get())`, where `V` is the composite pointer type (5.9 [expr.rel]).

   > > 4 Throws: nothing.

   > > 5 For templates `greater`, `less`, `greater_equal`, and `less_equal`, the partial specializations for `shared_ptr` shall yield a total order, even if the built-in operators `<`, `>`, `<=`, and `>=` do not. Moreover, `less<shared_ptr<T> >::operator()(a, b)` shall return `std::less<T*>::operator()(a.get(), b.get())`.

   > > 6 [Note: Defining a comparison operator allows `shared_ptr` objects to be used as keys in associative containers. — end note]

2. Add to 20.8.2.2 [util.smartptr.shared]/1 (after the `shared_ptr` comparisons)

   >     template<class T> struct greater<shared_ptr<T>>;
   >     template<class T> struct less<shared_ptr<T>>;
   >     template<class T> struct greater_equal<shared_ptr<T>>;
   >     template<class T> struct less_equal<shared_ptr<T>>;

   Remove 20.8.2.2.7 [util.smartptr.shared.cmp]/5 and /6 and replace with:

   >     template<class T, class U> bool operator<(const shared_ptr<T>& a, const shared_ptr<U>& b);

   > > 3 Returns: `xa.get() < yb.get()`.

   > > 4 Throws: nothing.

   > > 5 For templates `greater`, `less`, `greater_equal`, and `less_equal`, the partial specializations for `shared_ptr` shall yield a total order, even if the built-in operators `<`, `>`, `<=`, and `>=` do not. Moreover, `less<shared_ptr<T> >::operator()(a, b)` shall return `std::less<T*>::operator()(a.get(), b.get())`.

   > > 6 [Note: Defining a comparison operator allows `shared_ptr` objects to be used as keys in associative containers. — end note]

   >     template<class T> struct greater<shared_ptr<T>> :
   >     binary_function<shared_ptr<T>, shared_ptr<T>, bool> {
   >       bool operator()(const shared_ptr<T>& a, const shared_ptr<T>& b) const;
   >     };

   > > `operator()` returns `greater<T*>()(a.get(), b.get())`.

   >     template<class T> struct less<shared_ptr<T>> :
   >     binary_function<shared_ptr<T>, shared_ptr<T>, bool> {
   >       bool operator()(const shared_ptr<T>& a, const shared_ptr<T>& b) const;
   >     };

   > > `operator()` returns `less<T*>()(a.get(), b.get())`.

   >     template<class T> struct greater_equal<shared_ptr<T>> :
   >     binary_function<shared_ptr<T>, shared_ptr<T>, bool> {
   >       bool operator()(const shared_ptr<T>& a, const shared_ptr<T>& b) const;
   >     };

   > > `operator()` returns `greater_equal<T*>()(a.get(), b.get())`.

   >     template<class T> struct less_equal<shared_ptr<T>> :
   >     binary_function<shared_ptr<T>, shared_ptr<T>, bool> {
   >       bool operator()(const shared_ptr<T>& a, const shared_ptr<T>& b) const;
   >     };

   > > `operator()` returns `less_equal<T*>()(a.get(), b.get())`.

[ 2009-11-18: Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Replace 20.8.2.2.7 [util.smartptr.shared.cmp]/3 with the following and remove p5:

>     template<class T, class U> bool operator<(const shared_ptr<T>& a, const shared_ptr<U>& b);

> > 3 Returns: `x.get() < y.get()`. `less<V>()(a.get(), b.get())`, where `V` is the composite pointer type (5.9 [expr.rel]) of `T*` and `U*`.

> > 4 Throws: nothing.

> > 5 For templates `greater`, `less`, `greater_equal`, and `less_equal`, the partial specializations for `shared_ptr` shall yield a total order, even if the built-in operators `<`, `>`, `<=`, and `>=` do not. Moreover, `less<shared_ptr<T> >::operator()(a, b)` shall return `std::less<T*>::operator()(a.get(), b.get())`.

> > 6 [Note: Defining a comparison operator allows `shared_ptr` objects to be used as keys in associative containers. — end note]

### 1264. `quick_exit` support for freestanding implementations

Section: 17.6.1.3 [compliance] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-11-12 Last modified: 2015-04-08

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Discussion:

Addresses UK 172

This issue is a response to NB comment UK-172

The functions `quick_exit` and `at_quick_exit` should be added to the required features of `<cstdlib>` in a freestanding implementation.

This comment was rejected in Summit saying neither `at_exit` nor `at_quick_exit` should be required. This suggests the comment was misread, as `atexit` is already required to be supported. If the LWG really did wish to not require the registration functions be supported, then a separate issue should be opened to change the current standard.

Given both `exit` and `atexit` are required, the UK panel feels it is appropriate to require the new `quick_exit` facility is similarly supported.

[ 2009-12-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Ammend p3 Freestanding implementations 17.6.1.3 [compliance]

> 3 The supplied version of the header `<cstdlib>` shall declare at least the functions `abort()`, `atexit()`, `at_quick_exit`, and `exit()`, and `quick_exit`(18.5 [support.start.term]). The other headers listed in this table shall meet the same requirements as for a hosted implementation.

### 1266. `shared_future::get` and deferred `async` functions

Section: 30.6.7 [futures.shared_future] Status: Resolved Submitter: Anthony Williams Opened: 2009-11-17 Last modified: 2015-04-08

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Discussion:

If a `shared_future` is constructed with the result of an `async` call with a deferred function, and two or more copies of that `shared_future` are created, with multiple threads calling `get()`, it is not clear which thread runs the deferred function. 30.6.7 [futures.shared_future]p22 from N3000 says (minus editor's note):

> Effects: if the associated state contains a deferred function, executes the deferred function. Otherwise, blocks until the associated state is ready.

In the absence of wording to the contrary, this implies that every thread that calls `wait()` will execute the deferred function.

[ 2010 Pittsburgh: Moved to NAD Editorial Resolved. Rationale added below. ]

Rationale:

Solved by N3058.

Proposed resolution:

Replace 30.6.7 [futures.shared_future]p22 with the following:

> Effects: If the associated state contains a deferred function, executes the deferred function. Otherwise, blocks until the associated state is ready. was created by a `promise` or `packaged_task` object, block until the associated state is ready. If the associated state is associated with a thread created for an `async` call (30.6.8 [futures.async]), as if `associated-thread.join()`.

> If the associated state contains a deferred function, calls to `wait()` on all `shared_future` objects that share the same associated state are serialized. The first call to `wait()` that shares a given associated state executes the deferred function and stores the return value or exception in the associated state.

> Synchronization: if the associated state was created by a `promise` object, the completion of `set_value()` or `set_exception()` to that `promise` happens before (1.10 [intro.multithread]) `wait()` returns. If the associated state was created by a `packaged_task` object, the completion of the associated task happens before `wait()` returns. If the associated state is associated with a thread created for an `async` call (30.6.8 [futures.async]), the completion of the associated thread happens-before `wait()` returns.

> If the associated state contained a deferred function, the invocation of the deferred function happens-before any call to `wait()` on a `future` that shares that state returns.

### 1267. Incorrect wording for `condition_variable_any::wait_for`

Section: 30.5.2 [thread.condition.condvarany] Status: C++11 Submitter: Anthony Williams Opened: 2009-11-17 Last modified: 2015-04-08

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Discussion:

30.5.2 [thread.condition.condvarany]p18 and 30.5.2 [thread.condition.condvarany]p27 specify incorrect preconditions for `condition_variable_any::wait_for`. The stated preconditions require that `lock` has a `mutex()` member function, and that this produces the same result for all concurrent calls to `wait_for()`. This is inconsistent with `wait()` and `wait_until()` which do not impose such a requirement.

[ 2009-12-24 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Remove 30.5.2 [thread.condition.condvarany]p18 and 30.5.2 [thread.condition.condvarany]p27.

>     template <class Lock, class Rep, class Period>
>       cv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);

> > 18 Precondition: lock is locked by the calling thread, and either

> > - no other thread is waiting on this `condition_variable` object or
> > - `lock.mutex()` returns the same value for each of the lock arguments supplied by all concurrently waiting (via `wait`, `wait_for`, or `wait_until`) threads.

> ...

>     template <class Lock, class Rep, class Period, class Predicate>
>       bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);

> > 27 Precondition: lock is locked by the calling thread, and either

> > - no other thread is waiting on this `condition_variable` object or
> > - `lock.mutex()` returns the same value for each of the lock arguments supplied by all concurrently waiting (via `wait`, `wait_for`, or `wait_until`) threads.

### 1268. The Mutex requirements in 30.4.1 and 30.4.2 are wrong

Section: 30.4 [thread.mutex] Status: Resolved Submitter: Anthony Williams Opened: 2009-11-17 Last modified: 2015-04-08

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Discussion:

The `Mutex` requirements in 30.4.1 [thread.mutex.requirements] and 30.4.1.3 [thread.timedmutex.requirements] confuse the requirements on the behaviour of `std::mutex` et al with the requirements on `Lockable` types for use with `std::unique_lock`, `std::lock_guard` and `std::condition_variable_any`.

[ 2010 Pittsburgh: ]

> Concepts of threads chapter and issue presentation are: Lockable < Mutex < TimedMutex and Lockable < TimedLockable < TimedMutex.

> Typo in failed deletion of Mutex in 30.4.4 p4 edits.

> Lockable requirements are too weak for condition_variable_any, but the Mutex requirements are too strong.

> Need subset of Lockable requirements for condition_variable_any that does not include try_lock. E.g. CvLockable < Lockable.

> Text needs updating to recent draft changes.

> Needs to specify exception behavior in Lockable.

> The current standard is fine for what it says, but it places requirements that are too strong on authors of mutexes and locks.

> Move to open status. Suggest Anthony look at condition_variable_any requirements. Suggest Anthony refine requirements/concepts categories.

> Related to 964 and 966

[ 2010-03-28 Daniel synced with N3092. ]

[ 2010-10-25 Daniel adds: ]

> Accepting n3130 would solve this issue.

[ 2010-11 Batavia: ]

> Resolved by adopting n3197.

Proposed resolution:

Add a new section to 30.2 [thread.req] after 30.2.4 [thread.req.timing] as follows:

> 30.2.5 Requirements for Lockable types

> The standard library templates `unique_lock` (30.4.2.2 [thread.lock.unique]), `lock_guard` (30.4.2.1 [thread.lock.guard]), `lock`, `try_lock` (30.4.3 [thread.lock.algorithm]) and `condition_variable_any` (30.5.2 [thread.condition.condvarany]) all operate on user-supplied `Lockable` objects. Such an object must support the member functions specified for either the `Lockable` Requirements or the `TimedLockable` requirements as appropriate to acquire or release ownership of a `lock` by a given `thread`. [Note: the nature of any lock ownership and any synchronization it may entail are not part of these requirements. — end note]

> 30.2.5.1 Lockable Requirements

> In order to qualify as a `Lockable` type, the following expressions must be supported, with the specified semantics, where `m` denotes a value of type `L` that supports the `Lockable`:

> The expression `m.lock()` shall be well-formed and have the following semantics:

> Effects:
>   Block until a lock can be acquired for the current thread.
> Return type:
>   `void`

> The expression `m.try_lock()` shall be well-formed and have the following semantics:

> Effects:
>   Attempt to acquire a lock for the current thread without blocking.
> Return type:
>   `bool`
> Returns:
>   `true` if the lock was acquired, `false` otherwise.

> The expression `m.unlock()` shall be well-formed and have the following semantics:

> Effects:
>   Release a lock on `m` held by the current thread.
> Return type:
>   `void`
> Throws:
>   Nothing if the current thread holds a lock on `m`.

> 30.2.5.2 `TimedLockable` Requirements

> For a type to qualify as `TimedLockable` it must meet the `Lockable` requirements, and additionally the following expressions must be well-formed, with the specified semantics, where `m` is an instance of a type `TL` that supports the `TimedLockable` requirements, `rel_time` denotes instantiation of `duration` (20.12.5 [time.duration]) and `abs_time` denotes an instantiation of `time_point` (20.12.6 [time.point])

> The expression `m.try_lock_for(rel_time)` shall be well-formed and have the following semantics:

> Effects:
>   Attempt to acquire a lock for the current thread within the specified time period.
> Return type:
>   `bool`
> Returns:
>   `true` if the lock was acquired, `false` otherwise.

> The expression `m.try_lock_until(abs_time)` shall be well-formed and have the following semantics:

> Effects:
>   Attempt to acquire a lock for the current thread before the specified point in time.
> Return type:
>   `bool`
> Returns:
>   `true` if the lock was acquired, `false` otherwise.

Replace 30.4.1 [thread.mutex.requirements] paragraph 2 with the following:

> 2 This section describes requirements on template argument types used to instantiate templates defined in the mutex types supplied by the C++ standard library. The template definitions in the C++ standard library refer These types shall conform to the named `Mutex` requirements whose details are set out below. In this description, `m` is an object of a `Mutex` type one of the standard library mutex types `std::mutex`, `std::recursive_mutex`, `std::timed_mutex` or `std::recursive_timed_mutex`..

Add the following paragraph after 30.4.1 [thread.mutex.requirements] paragraph 2:

> A `Mutex` type shall conform to the `Lockable` requirements (30.2.5.1).

Replace 30.4.1.3 [thread.timedmutex.requirements] paragraph 1 with the following:

> The C++ standard library `TimedMutex` types `std::timed_mutex` and `std::recursive_timed_mutex` A `TimedMutex` type shall meet the requirements for a `Mutex` type. In addition, it they shall meet the requirements set out in this Clause 30.4.2 below, where `rel_time` denotes an instantiation of `duration` (20.12.5 [time.duration]) and `abs_time` denotes an instantiation of `time_point` (20.12.6 [time.point]).

Add the following paragraph after 30.4.1.3 [thread.timedmutex.requirements] paragraph 1:

> A `TimedMutex` type shall conform to the `TimedLockable` requirements (30.2.5.1).

Add the following paragraph following 30.4.2.1 [thread.lock.guard] paragraph 1:

> The supplied `Mutex` type shall meet the `Lockable` requirements (30.2.5.1).

Add the following paragraph following 30.4.2.2 [thread.lock.unique] paragraph 1:

> The supplied `Mutex` type shall meet the `Lockable` requirements (30.2.5.1). `unique_lock<Mutex>` meets the `Lockable` requirements. If `Mutex` meets the `TimedLockable` requirements (30.2.5.2) then `unique_lock<Mutex>` also meets the `TimedLockable` requirements.

Replace the use of "mutex" or "mutex object" with "lockable object" throughout clause 30.4.2 [thread.lock] paragraph 1:

> 1 A lock is an object that holds a reference to a mutex lockable object and may unlock the mutex lockable object during the lock's destruction (such as when leaving block scope). A thread of execution may use a lock to aid in managing mutex ownership of a lockable object in an exception safe manner. A lock is said to own a mutex lockable object if it is currently managing the ownership of that mutex lockable object for a thread of execution. A lock does not manage the lifetime of the mutex lockable object it references. [ Note: Locks are intended to ease the burden of unlocking the mutex lockable object under both normal and exceptional circumstances. — end note ]

30.4.2 [thread.lock] paragaph 2:

> 2 Some lock constructors take tag types which describe what should be done with the mutex lockable object during the lock's constuction.

30.4.2.1 [thread.lock.guard] paragaph 1:

> 1 An object of type `lock_guard` controls the ownership of a mutex lockable object within a scope. A `lock_guard` object maintains ownership of a mutex lockable object throughout the `lock_guard` object's lifetime. The behavior of a program is undefined if the mutex lockable object referenced by `pm` does not exist for the entire lifetime (3.8) of the `lock_guard` object. `Mutex` shall meet the `Lockable` requirements (30.2.5.1).

30.4.2.2 [thread.lock.unique] paragaph 1:

> 1 An object of type `unique_lock` controls the ownership of a mutex lockable object within a scope. Mutex o Ownership of the lockable object may be acquired at construction or after construction, and may be transferred, after acquisition, to another `unique_lock` object. Objects of type `unique_lock` are not copyable but are movable. The behavior of a program is undefined if the contained pointer `pm` is not null and the mutex pointed to by `pm` does not exist for the entire remaining lifetime (3.8) of the `unique_lock` object. `Mutex` shall meet the `Lockable` requirements (30.2.5.1).

Add the following to the precondition of `unique_lock(mutex_type& m, const chrono::time_point<Clock, Duration>& abs_time)` in 30.4.2.2.1 [thread.lock.unique.cons] paragraph 18:

>     template <class Clock, class Duration>
>       unique_lock(mutex_type& m, const chrono::time_point<Clock, Duration>& abs_time);

> > 18 Requires: If `mutex_type` is not a recursive mutex the calling thread does not own the mutex. The supplied `mutex_type` type shall meet the `TimedLockable` requirements (30.2.5.2).

Add the following to the precondition of `unique_lock(mutex_type& m, const chrono::duration<Rep, Period>& rel_time)` in 30.4.2.2.1 [thread.lock.unique.cons] paragraph 22

> 22 Requires: If `mutex_type` is not a recursive mutex the calling thread does not own the mutex. The supplied `mutex_type` type shall meet the `TimedLockable` requirements (30.2.5.2).

Add the following as a precondition of `bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time)` before 30.4.2.2.2 [thread.lock.unique.locking] paragraph 8

>     template <class Clock, class Duration>
>       bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time);

> > Requires: The supplied `mutex_type` type shall meet the `TimedLockable` requirements (30.2.5.2).

Add the following as a precondition of `bool try_lock_for(const chrono::duration<Rep, Period>& rel_time)` before 30.4.2.2.2 [thread.lock.unique.locking] paragraph 12

>     template <class Rep, class Period>
>       bool try_lock_for(const chrono::duration<Rep, Period>& rel_time);

> > Requires: The supplied `mutex_type` type shall meet the `TimedLockable` requirements (30.2.5.2).

Replace 30.4.3 [thread.lock.algorithm] p1 with the following:

>     template <class L1, class L2, class... L3> int try_lock(L1&, L2&, L3&...);

> > 1 Requires: Each template parameter type shall meet the `Mutex Lockable` requirements (30.2.5.1)., except that a call to `try_lock()` may throw an exception. [Note: The `unique_lock` class template meets these requirements when suitably instantiated. — end note]

Replace 30.4.3 [thread.lock.algorithm] p4 with the following:

>     template <class L1, class L2, class... L3> void lock(L1&, L2&, L3&...);

> > 4 Requires: Each template parameter type shall meet the `MutexMutex Lockable` requirements (30.2.5.1)., except that a call to `try_lock()` may throw an exception. [Note: The `unique_lock` class template meets these requirements when suitably instantiated. — end note]

Replace 30.5.2 [thread.condition.condvarany] paragraph 1 with:

> 1 A `Lock` type shall meet the requirements for a `Mutex` type `Lockable` requirements (30.2.5.1), except that `try_lock` is not required. [Note: All of the standard mutex types meet this requirement. — end note]

### 1269. Associated state doesn't account for `async`

Section: 30.6.4 [futures.state] Status: Resolved Submitter: Anthony Williams Opened: 2009-11-18 Last modified: 2015-04-08

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Discussion:

The current description of the associated state in 30.6.4 [futures.state] does not allow for futures created by an `async` call. The description therefore needs to be extended to cover that.

[ 2010 Pittsburgh: Moved to NAD Editorial Resolved. Rationale added below. ]

Rationale:

Solved by N3058.

Proposed resolution:

Add a new sentence to 30.6.4 [futures.state] p2:

> 2 This associated state consists of some state information and some (possibly not yet evaluated) result, which can be a (possibly `void`) value or an exception. If the associated state was created by a call to `async` (30.6.8 [futures.async]) then it may also contain a deferred function or an associated `thread`.

Add an extra bullet to 30.6.4 [futures.state] p3:

> The result of an associated state can be set by calling:

> - `promise::set_value`,
> - `promise::set_exception`, or
> - packaged_task::operator()., or
> - a call to `async` (30.6.8 [futures.async]).

### 1270. `result_of` should be moved to `<type_traits>`

Section: X [func.ret] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-11-19 Last modified: 2015-04-08

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Discussion:

Addresses UK 198

NB Comment: UK-198 makes this request among others. It refers to a more detailed issue that BSI did not manage to submit by the CD1 ballot deadline though.

`result_of` is essentially a metafunction to return the type of an expression, and belongs with the other library metafunctions in `<type_traits>` rather than lurking in `<functional>`. The current definition in `<functional>` made sense when `result_of` was nothing more than a protocol to enable several components in `<functional>` to provide their own result types, but it has become a more general tool. For instance, `result_of` is now used in the threading and futures components.

Now that `<type_traits>` is a required header for free-standing implementations it will be much more noticeable (in such environments) that a particularly useful trait is missing, unless that implementation also chooses to offer `<functional>` as an extension.

The simplest proposal is to simply move the wording (editorial direction below) although a more consistent form for type_traits would reformat this as a table.

Following the acceptance of 1255, `result_of` now depends on the `declval` function template, tentatively provided in `<utility>` which is not (yet) required of a free-standing implementation.

This dependency is less of an issue when `result_of` continues to live in `<functional>`.

Personally, I would prefer to clean up the dependencies so both `result_of` and `declval` are available in a free-standing implementation, but that would require slightly more work than suggested here. A minimal tweak would be to require `<utility>` in a free-standing implementation, although there are a couple of subtle issues with `make_pair`, which uses `reference_wrapper` in its protocol and that is much harder to separate cleanly from `<functional>`.

An alternative would be to enact the other half of N2979 and create a new minimal header for the new C++0x library facilities to be added to the freestanding requirements (plus `swap`.)

I have a mild preference for the latter, although there are clearly reasons to consider better library support for free-standing in general, and adding the whole of `<utility>` could be considered a step in that direction. See NB comment JP-23 for other suggestions (`array`, `ratio`)

[ 2010-01-27 Beman updated wording. ]

> The original wording is preserved here:

> > Move X [func.ret] to a heading below 20.10 [meta]. Note that in principle we should not change the tag, although this is a new tag for 0x. If it has been stable since TR1 it is clearly immutable though.

> > This wording should obviously adopt any other changes currently in (Tentatively) Ready status that touch this wording, such as 1255.

[ 2010-02-09 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

From Function objects 20.9 [function.objects], Header `<functional>` synopsis, remove:

>     // 20.7.4 result_of:
>     template <class> class result_of; // undefined
>     template <class F, class... Args> class result_of<F(ArgTypes...)>;

Remove Function object return types X [func.ret] in its entirety. This sub-section reads:

>     namespace std {
>       template <class> class result_of; // undefined

>       template <class Fn, class... ArgTypes>
>       class result_of<Fn(ArgTypes...)> {
>       public :
>         // types
>         typedef see below type;
>       };
>     }

> Given an rvalue `fn` of type `Fn` and values `t1, t2, ..., tN` of types T1, T2, ..., TN in `ArgTypes`, respectively, the type member is the result type of the expression `fn(t1, t2, ...,tN)`. The values `ti` are lvalues when the corresponding type `Ti` is an lvalue-reference type, and rvalues otherwise.

To Header <type_traits> synopsis 20.10.2 [meta.type.synop], add at the indicated location:

>     template <class T> struct underlying_type;
>     template <class T> struct result_of; // not defined
>     template <class Fn, class... ArgTypes> struct result_of<Fn(ArgTypes...)>;

To Other transformations 20.10.7.6 [meta.trans.other], Table 51 — Other transformations, add:

> Template | Condition | Comments
> `template <class T> struct underlying_type;` | `T` shall be an enumeration type (7.2) | The member typedef `type` shall name the underlying type of `T`.
> `template <class Fn, class... ArgTypes> struct result_of<Fn(ArgTypes...)>;` | `Fn` shall be a function object type 20.9 [function.objects], reference to function, or reference to function object type. `decltype(declval<Fn>()(declval<ArgTypes>()...))` shall be well formed. | The member typedef `type` shall name the type `decltype(declval<Fn>()(declval<ArgTypes>()...))`.

At the end of Other transformations 20.10.7.6 [meta.trans.other] add:

> [Example: Given these definitions:

>     typedef bool(&PF1)();
>     typedef short(*PF2)(long);

>     struct S {
>       operator PF2() const;
>       double operator()(char, int&);
>      };

> the following assertions will hold:

>     static_assert(std::is_same<std::result_of<S(int)>::type, short>::value, "Error!");
>     static_assert(std::is_same<std::result_of<S&(unsigned char, int&)>::type, double>::value, "Error!");
>     static_assert(std::is_same<std::result_of<PF1()>::type, bool>::value, "Error!");

> — end example]

### 1271. `CR` undefined in duration operators

Section: 20.12.5.5 [time.duration.nonmember] Status: C++11 Submitter: Daniel Krügler Opened: 2009-11-21 Last modified: 2015-04-08

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Discussion:

IMO `CR` alone is not really defined (it should be `CR(Rep1, Rep2)`).

[ 2009-12-24 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Change 20.12.5.5 [time.duration.nonmember] paragraphs 9 and 12:

>     template <class Rep1, class Period, class Rep2>
>       duration<typename common_type<Rep1, Rep2>::type, Period>
>       operator/(const duration<Rep1, Period>& d, const Rep2& s);

> > 9 Returns: `duration<CR(Rep1, Rep2), Period>(d) /= s`.

>     template <class Rep1, class Period, class Rep2>
>       duration<typename common_type<Rep1, Rep2>::type, Period>
>       operator%(const duration<Rep1, Period>& d, const Rep2& s);

> > 12 Returns: `duration<CR(Rep1, Rep2), Period>(d) %= s`.

### 1272. confusing declarations of `promise::set_value`

Section: 30.6.5 [futures.promise] Status: Resolved Submitter: Jonathan Wakely Opened: 2009-11-22 Last modified: 2015-04-08

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Discussion:

The definitions of `promise::set_value` need tidying up, the synopsis says:

>     // setting the result
>     void set_value(const R& r);
>     void set_value(see below);

Why is the first one there? It implies it is always present for all specialisations of promise, which is not true.

The definition says:

>     void set_value(const R& r);
>     void promise::set_value(R&& r);
>     void promise<R&>::set_value(R& r);
>     void promise<void>::set_value();

The lack of qualification on the first one again implies it's present for all specialisations, again not true.

[ 2010 Pittsburgh: Moved to NAD Editorial Resolved. Rationale added below. ]

Rationale:

Solved by N3058.

Proposed resolution:

Change the synopsis in 30.6.5 [futures.promise]:

>     // setting the result
>     void set_value(const R& r);
>     void set_value(see below);

And the definition be changed by qualifying the first signature:

>     void promise::set_value(const R& r);
>     void promise::set_value(R&& r);
>     void promise<R&>::set_value(R& r);
>     void promise<void>::set_value();

### 1273. `future::valid` should be callable on an invalid future

Section: 30.6.6 [futures.unique_future] Status: Resolved Submitter: Jonathan Wakely Opened: 2009-11-22 Last modified: 2015-04-08

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Discussion:

30.6.6 [futures.unique_future]/3 says:

> The effect of calling any member function other than the destructor or the move-assignment operator on a `future` object for which `valid() == false` is undefined.

This means calling `future::valid()` is undefined unless it will return `true`, so you can only use it if you know the answer!

[ 2009-12-08 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010 Pittsburgh: ]

> Moved to NAD Editorial Resolved. Rationale added below.

Rationale:

Solved by N3058.

Proposed resolution:

Change 30.6.6 [futures.unique_future]/3:

> The effect of calling any member function other than the destructor, or the move-assignment operator, or `valid`, on a `future` object for which `valid() == false` is undefined.

### 1274. `atomic_future` constructor

Section: X [futures.atomic_future] Status: Resolved Submitter: Jonathan Wakely Opened: 2009-11-22 Last modified: 2015-04-08

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Discussion:

In X [futures.atomic_future] this constructor:

>     atomic_future(future<R>&&);

is declared in the synopsis, but not defined. Instead n2997 defines:

>     atomic_future(const future<R>&& rhs);

and n3000 defines

>     atomic_future(atomic_future<R>&& rhs);

both of which are wrong. The constructor definition should be changed to match the synopsis.

[ 2009-12-12 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010 Pittsburgh: ]

> Moved to NAD Editorial Resolved. Rationale added below.

Rationale:

Solved by N3058.

Proposed resolution:

Adjust the signature above X [futures.atomic_future]/6 like so:

>     atomic_future(atomic_future<R>&& rhs);

### 1275. Creating and setting futures

Section: 30.6 [futures] Status: Resolved Submitter: Jonathan Wakely Opened: 2009-11-22 Last modified: 2015-04-08

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Discussion:

30.6.6 [futures.unique_future]/1 should be updated to mention `async`.

30.6.7 [futures.shared_future]/1 should also be updated for `async`. That paragraph also says

> ... Its value or exception can be set by use of a `shared_future`, `promise` (30.6.5 [futures.promise]), or `packaged_task` (30.6.9 [futures.task]) object that shares the same associated state.

How can the value be set by a `shared_future`?

X [futures.atomic_future]/1 says

> An `atomic_future` object can only be created by use of a `promise` (30.6.5 [futures.promise]) or `packaged_task` (30.6.9 [futures.task]) object.

which is wrong, it's created from a `std::future`, which could have been default-constructed. That paragraph should be closer to the text of 30.6.7 [futures.shared_future]/1, and should also mention `async`.

[ 2010 Pittsburgh: Moved to NAD Editorial Resolved. Rationale added below. ]

Rationale:

Solved by N3058.

Proposed resolution:

### 1276. `forwardlist` missing allocator constructors

Section: 23.3.4 [forwardlist] Status: C++11 Submitter: Daniel Krügler Opened: 2009-12-12 Last modified: 2015-04-08

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Discussion:

I found that forward_list has only

>     forward_list(const forward_list<T,Allocator>& x);
>     forward_list(forward_list<T,Allocator>&& x);

but misses

>     forward_list(const forward_list& x, const Allocator&);
>     forward_list(forward_list&& x, const Allocator&);

Note to other reviewers: I also checked the container adaptors for similar inconsistencies, but as far as I can see these are already handled by the current active issues 1194 and 1199.

[ 2010-01-14 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

In 23.3.4 [forwardlist]/3, class template forward_list synopsis change as indicated:

>     forward_list(const forward_list<T,Allocator>& x);
>     forward_list(forward_list<T,Allocator>&& x);
>     forward_list(const forward_list&, const Allocator&);
>     forward_list(forward_list&&, const Allocator&);

### 1277. `std::thread::id` should be trivially copyable

Section: 30.3.1.1 [thread.thread.id] Status: C++11 Submitter: Anthony Williams Opened: 2009-11-24 Last modified: 2015-04-08

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Discussion:

The class definition of `std::thread::id` in N3000 is:

>     class thread::id {
>     public:
>       id();
>     };

Typically, I expect that the internal data members will either be pointers or integers, so that in practice the class will be trivially copyable. However, I don't think the current wording guarantees it, and I think it would be useful. In particular, I can see a use for `std::atomic<std::thread::id>` to allow a `thread` to claim ownership of a data structure atomicly, and `std::atomic<T>` requires that `T` is trivially copyable.

[ 2010-02-12 Moved to Tentatively Ready after 7 positive votes on c++std-lib. ]

Proposed resolution:

Add a new sentence to 30.3.1.1 [thread.thread.id] p1:

> 1 An object of type `thread::id` provides a unique identifier for each thread of execution and a single distinct value for all `thread` objects that do not represent a thread of execution (30.3.1 [thread.thread.class]). Each thread of execution has an associated `thread::id` object that is not equal to the `thread::id` object of any other thread of execution and that is not equal to the `thread::id` object of any `std::thread` object that does not represent threads of execution. The library may reuse the value of a `thread::id` of a terminated thread that can no longer be joined. `thread::id` shall be a trivially copyable class (9 [class]).

### 1278. Inconsistent return values for `forward_list::insert_after`

Section: 23.3.4.5 [forwardlist.modifiers] Status: C++11 Submitter: Bo Persson Opened: 2009-11-25 Last modified: 2015-04-08

View all other issues in [forwardlist.modifiers].

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Discussion:

After applying LDR149, `forward_list` now has 5 overloads of `insert_after`, all returning an iterator.

However, two of those - inserting a single object - return "An iterator pointing to a copy of `x` [the inserted object]" while the other three - inserting zero or more objects - return an iterator equivalent to the position parameter, pointing before any possibly inserted objects.

Is this the intended change?

I don't really know what `insert_after(position, empty_range)` should really return, but always returning `position` seems less than useful.

[ 2010-02-04 Howard adds: ]

> I agree this inconsistency will be error prone and needs to be fixed. Additionally `emplace_after`'s return value is unspecified.

[ 2010-02-04 Nico provides wording. ]

[ 2010 Pittsburgh: ]

> We prefer to return an iterator to the last inserted element. Modify the proposed wording and then set to Ready.

[ 2010-03-15 Howard adds: ]

> Wording updated and set to Ready.

Proposed resolution:

In `forward_list` modifiers 23.3.4.5 [forwardlist.modifiers] make the following modifications:

>     iterator insert_after(const_iterator position, size_type n, const T& x);

> > ...

> > 10 Returns: position. An iterator pointing to the last inserted copy of `x` or `position` if `n == 0`.

>     template <class InputIterator>
>       iterator insert_after(const_iterator position, InputIterator first, InputIterator last);

> > ...

> > 13 Returns: position. An iterator pointing to the last inserted element or `position` if `first == last`.

>     iterator insert_after(const_iterator position, initializer_list<T> il);

> > ...

> > 15 Returns: position. An iterator pointing to the last inserted element or `position` if `il` is empty.

>     template <class... Args>
>       iterator emplace_after(const_iterator position, Args&&... args);

> > ...

> > 17 ...

> > Returns: An iterator pointing to the new constructed element from args.

### 1279. forbid `[u|bi]nary_function` specialization

Section: X [depr.base] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2009-11-30 Last modified: 2015-04-08

View all other issues in [depr.base].

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Discussion:

A program should not be allowed to add specialization of class templates `unary_function` and `binary_function`, in force of 17.6.4.2.1 [namespace.std]/1. If a program were allowed to specialize these templates, the library could no longer rely on them to provide the intended typedefs or there might be other undesired interactions.

[ 2010-03-27 Daniel adds: ]

> Accepting issue 1290 would resolve this issue as NAD editorial.

[ 2010-10-24 Daniel adds: ]

> Accepting n3145 would resolve this issue as NAD editorial.

[ 2010 Batavia: ]

> Pete: Is this issue actually addressed by N3198, or did deprecating unary/binary_function?

> We determined that this issue is NOT resolved and that it must be resolved or else N3198 could break code that does specialize unary/binary function.

> Matt: don't move to NAD

> Howard: I suggest we go further and move 1279 to ready for Madrid.

> Group: Agrees move 1279 to ready for Madrid

Previous proposed resolution:

> 1 The following classes class templates are provided to simplify the typedefs of the argument and result types:. A program shall not declare specializations of these templates.

[2011-03-06 Daniel comments]

> This meeting outcome was not properly reflected in the proposed resolution. I also adapted the suggested wording to the N3242 numbering and content state. During this course of action it turned out that the first suggested wording change has already been applied.

Proposed resolution:

Change paragraph X [depr.base]/1 as follows:

> 1 The class templates `unary_function` and `binary_function` are deprecated. A program shall not declare specializations of these templates.

### 1280. Initialization of stream iterators

Section: 24.6.1.1 [istream.iterator.cons], 24.6.2.1 [ostream.iterator.cons.des] Status: C++11 Submitter: Jonathan Wakely Opened: 2009-12-04 Last modified: 2015-04-08

View all other issues in [istream.iterator.cons].

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Discussion:

24.6.1.1 [istream.iterator.cons] describes the effects in terms of:

>     basic_istream<charT,traits>* in_stream; // exposition only

> 3 Effects: Initializes in_stream with `s`.

That should be `&s` and similarly for 24.6.2.1 [ostream.iterator.cons.des].

[ 2009-12-23 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

Proposed resolution:

Change 24.6.1.1 [istream.iterator.cons] like so:

>     istream_iterator(istream_type& s);

> > 3 Effects: Initializes in_stream with `&s`. value ...

And 24.6.2.1 [ostream.iterator.cons.des] like so:

>     ostream_iterator(ostream_type& s);

> > 1 Effects: Initializes out_stream with `&s` and delim with null.

>     ostream_iterator(ostream_type& s, const charT* delimiter);

> > 2 Effects: Initializes out_stream with `&s` and delim with `delimiter`.

### 1281. CopyConstruction and Assignment between ratios having the same normalized form

Section: 20.11.3 [ratio.ratio] Status: Resolved Submitter: Vicente Juan Botet Escribá Opened: 2009-12-07 Last modified: 2015-04-08

View all other issues in [ratio.ratio].

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Discussion:

CopyConstruction and Assignment between `ratio`s having the same normalized form. Current N3000 do not allows to copy-construct or assign `ratio` instances of `ratio` classes having the same normalized form.

Two `ratio` classes `ratio<N1,D1>` and `ratio<N2,D2>` have the same normalized form if

>     ratio<N1, D1>::num == ratio<N2, D2>::num &&
>     ratio<N1, D1>::den == ratio<N2, D2>::den

This simple example

>     ratio<1,3> r1;
>     ratio<3,9> r2;
>     r1 = r2; // (1)

fails to compile in (1). Other example

>     ratio<1,3> r1;
>     ratio_subtract<ratio<2,3>, ratio<1,3>>::type r2;
>     r1 = r2;

The nested type of `ratio_subtract<ratio<2,3>, ratio<1,3>>` could be `ratio<3,9>` so the compilation could fail. It could also be `ratio<1,3>` and the compilation succeeds.

In 20.11.4 [ratio.arithmetic] 3 and similar clauses

> 3 The nested typedef `type` shall be a synonym for `ratio<T1, T2>` where `T1` has the value `R1::num * R2::den - R2::num * R1::den` and `T2` has the value `R1::den * R2::den`.

the meaning of synonym let think that the result shall be a normalized `ratio` equivalent to `ratio<T1, T2>`, but there is not an explicit definition of what synonym means in this context.

Additionally we should add a typedef for accessing the normalized `ratio`, and change 20.11.4 [ratio.arithmetic] to return only this normalized result.

[ 2010 Pittsburgh: ]

> There is no consensus to add the converting copy constructor or converting copy assignment operator. However there was consensus to add the typedef.

> Proposed wording modified. Original proposed wording preserved here. Moved to Review.

> > Make `ratio` default constructible, copy-constructible and assignable from any `ratio` which has the same reduced form.

> > Add to 20.11.3 [ratio.ratio] synopsis

> > >     template <intmax_t N, intmax_t D = 1>
> > >     class ratio {
> > >     public:
> > >       static constexpr intmax_t num;
> > >       static constexpr intmax_t den;

> > >       typedef ratio<num, den> type;

> > >       ratio() = default;
> > >       template <intmax_t N2, intmax_t D2>
> > >         ratio(const ratio<N2, D2>&);
> > >       template <intmax_t N2, intmax_t D2>
> > >         ratio& operator=(const ratio<N2, D2>&);
> > >     };

> > Add to 20.11.3 [ratio.ratio]:

> > > Two ratio classes `ratio<N1,D1>` and `ratio<N2,D2>` have the same reduced form if `ratio<N1,D1>::type` is the same type as `ratio<N2,D2>::type`

> > Add a new section: [ratio.cons]

> > > Construction and assignment [ratio.cons]

> > >     template <intmax_t N2, intmax_t D2>
> > >       ratio(const ratio<N2, D2>& r);

> > > > Effects: Constructs a `ratio` object.

> > > > Remarks: This constructor shall not participate in overload resolution unless `r` has the same reduced form as `*this`.

> > >     template <intmax_t N2, intmax_t D2>
> > >       ratio& operator=(const ratio<N2, D2>& r);

> > > > Effects: None.

> > > > Returns: `*this`.

> > > > Remarks: This operator shall not participate in overload resolution unless `r` has the same reduced form as `*this`.

> > Change 20.11.4 [ratio.arithmetic]

> > > Implementations may use other algorithms to compute these values. If overflow occurs, a diagnostic shall be issued.

> > >     template <class R1, class R2> struct ratio_add {
> > >       typedef see below type;
> > >     };

> > > > The nested typedef `type` shall be a synonym for `ratio<T1, T2>::type` where `T1` has the value `R1::num * R2::den + R2::num * R1::den` and `T2` has the value `R1::den * R2::den`.

> > >     template <class R1, class R2> struct ratio_subtract {
> > >       typedef see below type;
> > >     };

> > > > The nested typedef `type` shall be a synonym for `ratio<T1, T2>::type` where `T1` has the value `R1::num * R2::den - R2::num * R1::den` and `T2` has the value `R1::den * R2::den`.

> > >     template <class R1, class R2> struct ratio_multiply {
> > >       typedef see below type;
> > >     };

> > > > The nested typedef `type` shall be a synonym for `ratio<T1, T2>::type` where `T1` has the value `R1::num * R2::num` and `T2` has the value `R1::den * R2::den`.

> > >     template <class R1, class R2> struct ratio_divide {
> > >       typedef see below type;
> > >     };

> > > > The nested typedef `type` shall be a synonym for `ratio<T1, T2>::type` where `T1` has the value `R1::num * R2::den` and `T2` has the value `R1::den * R2::num`.

[ 2010-03-27 Howard adds: ]

> Daniel brought to my attention the recent addition of the typedef `type` to the FCD N3092:

> >     typedef ratio type;

> This issue was discussed in Pittsburgh, and the decision there was to accept the typedef as proposed and move to Review. Unfortunately the issue was accidently applied to the FCD, and incorrectly. The FCD version of the typedef refers to `ratio<N, D>`, but the typedef is intended to refer to `ratio<num, den>` which in general is not the same type.

> I've updated the wording to diff against N3092.

[Batavia: NAD Editorial Resolved - see rationale below]

Rationale:

Already fixed in working draft

Proposed resolution:

Add to 20.11.3 [ratio.ratio] synopsis

>     template <intmax_t N, intmax_t D = 1>
>     class ratio {
>     public:
>       static constexpr intmax_t num;
>       static constexpr intmax_t den;

>       typedef ratio<num, den> type;
>     };

### 1283. `MoveConstructible` and `MoveAssignable` need clarification of moved-from state

Section: 17.6.3.1 [utility.arg.requirements] Status: Resolved Submitter: Howard Hinnant Opened: 2009-12-12 Last modified: 2015-04-08

View all other issues in [utility.arg.requirements].

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Discussion:

Addresses UK 150

There is on going confusion over what one can and can not do with a moved-from object (e.g. UK 150, 910). This issue attempts to clarify that moved-from objects are valid objects with an unknown state.

[ 2010-01-22 Wording tweaked by Beman. ]

[ 2010-01-22 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010-01-23 Alisdair opens: ]

> I'm afraid I must register an objection.

> My primary objection is that I have always been opposed to this kind of a resolution as over-constraining. My preferred example is a call implementing the pImpl idiom via `unique_ptr`. Once the pImpl has been moved from, it is no longer safe to call the vast majority of the object's methods, yet I see no reason to make such a type unusable in the standard library. I would prefer a resolution along the lines suggested in the UK comment, which only requires that the object can be safely destroyed, and serve as the target of an assignment operator (if supported.)

> However, I will not hold the issue up if I am a lone dissenting voice on this (yes, that is a call to hear more support, or I will drop that objection in Pittsburgh)

> With the proposed wording, I'm not clear what the term 'valid object' means. In my example above, is a pImpl holding a null pointer 'valid'? What about a float holding a signalling NaN? What determines if an object is valid? Without a definition of a valid/invalid object, I don't think this wording adds anything, and this is an objection that I do want resolved.

> [ 2010-01-24 Alisdair removes his objection. ]

> [ 2010-01-24 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010-02-10 Reopened. The wording here has been merged into 1309. ]

[ 2010-02-10 Moved to Tentatively NAD Editorial Resolved after 5 postive votes on c++std-lib. Rationale added below. ]

Rationale:

This issue is now addressed by 1309.

Proposed resolution:

Change the follwing tables in 17.6.3.1 [utility.arg.requirements] as shown:

> Table 33 — `MoveConstructible` requirements [moveconstructible]
> Expression | Post-condition
> --- | ---
> `T t(rv)` | `t` is equivalent to the value of `rv` before the construction.
> [Note: There is no requirement on the value of `rv` after the construction. `rv` remains a valid object. Its state is unspecified. — end note]

> Table 35 — `MoveAssignable` requirements [moveassignable]
> Expression | Return type | Return value | Post-condition
> --- | --- | --- | ---
> `t = rv` | `T&` | `t` | `t` is equivalent to the value of `rv` before the assigment.
> [Note: There is no requirement on the value of `rv` after the assignment. `rv` remains a valid object. Its state is unspecified. — end note]

### 1284. `vector<bool> initializer_list` constructor missing an allocator argument

Section: 23.3.7 [vector.bool] Status: C++11 Submitter: Bo Persson Opened: 2009-12-09 Last modified: 2015-04-08

View all other issues in [vector.bool].

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Discussion:

The specialization for `vector<bool>` (23.3.7 [vector.bool]) has a constructor

>     vector(initializer_list<bool>);

which differs from the base template's constructor (and other containers) in that it has no `allocator` parameter.

[ 2009-12-16 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Change the signature in the synopsis of 23.3.7 [vector.bool] to

>     vector(initializer_list<bool>, const Allocator& = Allocator());

### 1285. `allocator_traits` call to `new`

Section: 20.7.8.2 [allocator.traits.members] Status: C++11 Submitter: Howard Hinnant Opened: 2009-12-10 Last modified: 2015-04-08

View all other issues in [allocator.traits.members].

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Discussion:

LWG issue 402 added "`::`" to the call to `new` within `allocator::construct`. I suspect we want to retain that fix.

[ 2009-12-13 Moved to Tentatively Ready after 7 positive votes on c++std-lib. ]

Proposed resolution:

Change 17.6.3.5 [allocator.requirements], table 40 "Allocator requirements":

> Table 40 — Allocator requirements
> Expression | Return type | Assertion/note pre-/post-condition | Default
> --- | --- | --- | ---
> `a.construct(c,args)` | (not used) | Effect: Constructs an object of type `C` at `c` | `::new ((void*)c) C(forward<Args>(args)...)`

Change 20.7.8.2 [allocator.traits.members], p4:

>     template <class T, class... Args>
>       static void construct(Alloc& a, T* p, Args&&... args);

> > 4 Effects: calls `a.construct(p, std::forward<Args>(args)...)` if that call is well-formed; otherwise, invokes `::new (static_cast<void*>(p)) T(std::forward<Args>(args)...)`.

### 1286. `allocator_traits::select_on_container_copy_construction` type-o

Section: 20.7.8.2 [allocator.traits.members] Status: C++11 Submitter: Howard Hinnant Opened: 2009-12-10 Last modified: 2015-04-08

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Discussion:

`allocator_traits::select_on_container_copy_construction` refers to an unknown "`a`":

>     static Alloc select_on_container_copy_construction(const Alloc& rhs);

> > 7 Returns: `rhs.select_on_container_copy_construction(a)` if that expression is well-formed; otherwise, `rhs`.

[ 2009-12-13 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Change 20.7.8.2 [allocator.traits.members], p7:

>     static Alloc select_on_container_copy_construction(const Alloc& rhs);

> > 7 Returns: `rhs.select_on_container_copy_construction(a)` if that expression is well-formed; otherwise, `rhs`.

### 1287. `std::function` requires `CopyConstructible` target object

Section: 20.9.12.2.1 [func.wrap.func.con] Status: C++11 Submitter: Jonathan Wakely Opened: 2009-12-13 Last modified: 2015-04-08

View all other issues in [func.wrap.func.con].

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Discussion:

I think `std::function` should require `CopyConstructible` for the target object.

I initially thought that `MoveConstructible` was enough, but it's not. If `F` is move-only then function's copy constructor cannot be called, but because function uses type erasure, `F` is not known and so the copy constructor cannot be disabled via `enable_if`. One option would be to throw an exception if you try to copy a function with a non-copyable target type, but I think that would be a terrible idea.

So although the constructors require that the target be initialised by `std::move(f)`, that's only an optimisation, and a copy constructor is required.

[ 2009-12-24 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Add to 20.9.12.2.1 [func.wrap.func.con] paragraph 9:

>     template<class F> function(F f);
>     template <class F, class A> function(allocator_arg_t, const A& a, F f);

> > 9 Requires: `F` shall be `CopyConstructible`. `f` shall be callable for argument types `ArgTypes` and return type `R`. The copy constructor and destructor of `A` shall not throw exceptions.

### 1288. `std::function` assignment from rvalues

Section: 20.9.12.2.1 [func.wrap.func.con] Status: C++11 Submitter: Jonathan Wakely Opened: 2009-12-13 Last modified: 2015-04-08

View all other issues in [func.wrap.func.con].

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Discussion:

In 20.9.12.2.1 [func.wrap.func.con]

>     template<class F> function& operator=(F f);

> > 20 Effects: `function(f).swap(*this);`

> > 21 Returns: `*this`

This assignment operator can be called such that `F` is an rvalue-reference e.g.

>     func.operator=<F&&>(f);

There are two issues with this.

1. the effects mean that `f` is passed as an lvalue and so there will be an unnecessary copy. The argument should be forwarded, so that the copy can be avoided.
2. It should not be necessary to use that syntax to pass an rvalue. As `F` is a deduced context it can be made to work with either lvalues or rvalues.

The same issues apply to `function::assign`.

N.B. this issue is not related to 1287 and applies whether that issue is resolved or not. The wording below assumes the resolution of LWG 1258 has been applied.

[ 2009-12-16 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 201002-11 Opened by Alisdair for the purpose of merging 1258 into this issue as there is a minor conflict. ]

[ 2010-02-11 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

In 20.9.12.2.1 [func.wrap.func.con]

>     template<class F> function& operator=(F&& f);

> > 20 Effects: `function(std::forward<F>(f)).swap(*this);`

> > 21 Returns: `*this`

In 20.9.12.2.2 [func.wrap.func.mod]

>     template<class F, Allocator Allocclass A>
>       void assign(F&& f, const Alloc& a);

> > 3 Effects: `function(f, aallocator_arg, a, std::forward<F>(f)).swap(*this);`

Update member function signature for class template in 20.9.12.2 [func.wrap.func]

>     template<class F> function& operator=(F&&);

>     template<class F, class A> void assign(F&&, const A&);

### 1290. Don't require `[u|bi]nary_function` inheritance

Section: 20.9 [function.objects] Status: Resolved Submitter: Daniel Krügler Opened: 2009-12-14 Last modified: 2015-04-08

View all other issues in [function.objects].

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Discussion:

This issue is a follow-up of the discussion on issue 870 during the 2009 Santa Cruz meeting.

The class templates `unary_function` and `binary_function` are actually very simple typedef providers,

>     namespace std {

>     template <class Arg, class Result>
>     struct unary_function {
>      typedef Arg argument_type;
>      typedef Result result_type;
>     };

>     template <class Arg1, class Arg2, class Result>
>     struct binary_function {
>      typedef Arg1 first_argument_type;
>      typedef Arg2 second_argument_type;
>      typedef Result result_type;
>     };

>     }

which may be used as base classes (similarly to the iterator template), but were originally not intended as a customization point. The SGI documentation introduced the concept Adaptable Unary Function as function objects "with nested typedefs that define its argument type and result type" and a similar definition for Adaptable Binary Function related to `binary_function`. But as of TR1 a protocol was introduced that relies on inheritance relations based on these types. 20.9.4 [refwrap]/3 b. 3 requires that a specialization of `reference_wrapper<T>` shall derive from `unary_function`, if type `T` is "a class type that is derived from `std::unary_function<T1, R>`" and a similar inheritance-based rule for `binary_function` exists as well.

As another disadvantage it has been pointed out in the TR1 issue list, N1837 (see section 10.39), that the requirements of `mem_fn` 20.9.11 [func.memfn]/2+3 to derive from `std::unary_function/std::binary_function` under circumstances, where the provision of corresponding typedefs would be sufficient, unnecessarily prevent implementations that take advantage of empty-base-class optimizations.

Both requirements should be relaxed in the sense that the `reference_wrapper` should provide typedef's `argument_type`, `first_argument_type`, and `second_argument_type` based on similar rules as the weak result type rule (20.9.2 [func.require]/3) does specify the presence of `result_type` member types.

For a related issue see also 1279.

[ 2010-10-24 Daniel adds: ]

> Accepting n3145 would resolve this issue as NAD editorial.

[ 2010-11 Batavia: Solved by N3198 ]

Resolved by adopting n3198.

Previous proposed resolution:

[ The here proposed resolution is an attempt to realize the common denominator of the reflector threads c++std-lib-26011, c++std-lib-26095, and c++std-lib-26124. ]

> 1. Change [base]/1 as indicated: [The intend is to provide an alternative fix for issue 1279 and some editorial harmonization with existing wording in the library, like 24.4.2 [iterator.basic]/1]

>    > 1 The following class templates are provided to simplify the definition of typedefs of the argument and result types for function objects. The behavior of a program that adds specializations for any of these templates is undefined.:

>    > >     namespace std {
>    > >      template <class Arg, class Result>
>    > >      struct unary_function {
>    > >        typedef Arg argument_type;
>    > >        typedef Result result_type;
>    > >      };
>    > >     }

>    > >     namespace std {
>    > >      template <class Arg1, class Arg2, class Result>
>    > >      struct binary_function {
>    > >        typedef Arg1 first_argument_type;
>    > >        typedef Arg2 second_argument_type;
>    > >        typedef Result result_type;
>    > >      };
>    > >     }

> 2. Change 20.9.4 [refwrap], class template `reference_wrapper` synopsis as indicated: [The intent is to remove the requirement that `reference_wrapper` derives from `unary_function` or `binary_function` if the situation requires the definition of the typedefs `argument_type`, `first_argument_type`, or `second_argument_type`. This change is suggested, because the new way of definition uses the same strategy as the weak result type specification applied to argument types, which provides the following advantages: It creates less potential conflicts between `[u|bi]nary_function` bases and typedefs in a function object and it ensures that user-defined function objects which provide typedefs but no such bases are handled as first class citizens.]

>    >     namespace std {
>    >      template <class T> class reference_wrapper
>    >        : public unary_function<T1, R> // see below
>    >        : public binary_function<T1, T2, R> // see below
>    >      {
>    >      public :
>    >        // types
>    >        typedef T type;
>    >        typedef see below result_type; // not always defined
>    >        typedef see below argument_type; // not always defined
>    >        typedef see below first_argument_type; // not always defined
>    >        typedef see below second_argument_type; // not always defined

>    >        // construct/copy/destroy
>    >        ...
>    >      };

> 3. Change 20.9.4 [refwrap]/3 as indicated: [The intent is to remove the requirement that `reference_wrapper` derives from `unary_function` if the situation requires the definition of the typedef `argument_type` and `result_type`. Note that this clause does concentrate on `argument_type` alone, because the `result_type` is already ruled by p. 2 via the weak result type specification. The new way of specifying `argument_type` is equivalent to the weak result type specification]

>    > 3 The template instantiation `reference_wrapper<T>` shall be derived from `std::unary_function<T1, R>`define a nested type named `argument_type` as a synonym for `T1` only if the type `T` is any of the following:

>    > - a function type or a pointer to function type taking one argument of type `T1` and returning `R`
>    > - a pointer to member function `R T0::f` cv (where cv represents the member function's cv-qualifiers); the type `T1` is cv `T0*`
>    > - a class type that is derived from `std::unary_function<T1, R>`with a member type `argument_type`; the type `T1` is `T::argument_type`

> 4. Change 20.9.4 [refwrap]/4 as indicated: [The intent is to remove the requirement that `reference_wrapper` derives from `binary_function` if the situation requires the definition of the typedef `first_argument_type`, `second_argument_type`, and `result_type`. Note that this clause does concentrate on `first_argument_type` and `second_argument_type` alone, because the `result_type` is already ruled by p. 2 via the weak result type specification. The new way of specifying `first_argument_type` and `second_argument_type` is equivalent to the weak result type specification]

>    > The template instantiation `reference_wrapper<T>` shall be derived from `std::binary_function<T1, T2, R>`define two nested types named `first_argument_type` and `second_argument_type` as a synonym for `T1` and `T2`, respectively, only if the type `T` is any of the following:

>    > - a function type or a pointer to function type taking two arguments of types `T1` and `T2` and returning `R`
>    > - a pointer to member function `R T0::f(T2)` cv (where cv represents the member function's cv-qualifiers); the type `T1` is cv `T0*`
>    > - a class type that is derived from `std::binary_function<T1, T2, R>`with member types `first_argument_type` and `second_argument_type`; the type `T1` is `T::first_argument_type` and the type `T2` is `T::second_argument_type`

> 5. Change 20.9.11 [func.memfn]/2+3 as indicated: [The intent is to remove the requirement that mem_fn's return type has to derive from `[u|bi]nary_function`. The reason for suggesting the change here is to better support empty-base-class optimization choices as has been pointed out in N1837]

>    > 2 The simple call wrapper shall be derived from `std::unary_function<cv T*, Ret>`define two nested types named `argument_type` and `result_type` as a synonym for `cv T*` and `Ret`, respectively, when `pm` is a pointer to member function with cv-qualifier cv and taking no arguments, where `Ret` is `pm`'s return type.

>    > 3 The simple call wrapper shall be derived from `std::binary_function<cv T*, T1, Ret>`define three nested types named `first_argument_type`, `second_argument_type`, and `result_type` as a synonym for `cv T*`, `T1`, and `Ret`, respectively, when `pm` is a pointer to member function with cv-qualifier cv and taking one argument of type `T1`, where `Ret` is `pm`'s return type.

Proposed resolution:

Addressed by paper n3198.

### 1291. Exceptions thrown during `promise::set_value`

Section: 30.6.5 [futures.promise] Status: Resolved Submitter: Jonathan Wakely Opened: 2009-12-18 Last modified: 2015-04-08

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Discussion:

In 30.6.5 [futures.promise]

Does `promise<R>::set_value` return normally if the copy/move constructor of `R` throws?

The exception could be caught and set using `promise<R>::set_exception`, or it could be allowed to leave the `set_value` call, but it's not clear which is intended. I suggest the exception should not be caught.

N.B. This doesn't apply to `promise<R&>::set_value` or `promise<void>::set_value` because they don't construct a new object.

[ 2010 Pittsburgh: Moved to NAD Editorial Resolved. Rationale added below. ]

Rationale:

Solved by N3058.

Proposed resolution:

Change 30.6.5 [futures.promise]/18:

> 18 Throws: `future_error` if its associated state is already ready or, for the first version an exception thrown by the copy constructor of `R`, or for the second version an exception thrown by the move constructor of `R`.

### 1292. `std::function` should support all callable types

Section: 20.9.12.2.1 [func.wrap.func.con] Status: C++11 Submitter: Daniel Krügler Opened: 2009-12-19 Last modified: 2015-04-08

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Discussion:

Some parts of the specification of `std::function` is unnecessarily restricted to a subset of all callable types (as defined in 20.9.1 [func.def]/3), even though the intent clearly is to be usable for all of them as described in 20.9.12.2 [func.wrap.func]/1. This argument becomes strengthened by the fact that current C++0x-compatible compilers work fine with them:

>     #include <functional>
>     #include <iostream>

>     struct A
>     {
>       int foo(int i) const {return i+1;}
>     };

>     struct B
>     {
>       int mem;
>     };

>     int main()
>     {
>       std::function<int(const A&, int)> f(&A::foo);
>       A a;
>       std::cout << f(a, 1) << '\n';
>       std::cout << f.target_type().name() << '\n';
>       typedef int (A::* target_t)(int) const;
>       target_t* p = f.target<target_t>();
>       std::cout << (p != 0) << '\n';
>       std::function<int(B&)> f2(&B::mem);
>       B b = { 42 };
>       std::cout << f2(b) << '\n';
>       std::cout << f2.target_type().name() << '\n';
>       typedef int (B::* target2_t);
>       target2_t* p2 = f2.target<target2_t>();
>       std::cout << (p2 != 0) << '\n';
>     }

The problematic passages are 20.9.12.2.1 [func.wrap.func.con]/10:

>     template<class F> function(F f);
>     template <class F, class A> function(allocator_arg_t, const A& a, F f);

> > ...

> > 10 Postconditions: `!*this` if any of the following hold:

> > - `f` is a `NULL` function pointer.
> > - `f` is a `NULL` member function pointer.
> > - `F` is an instance of the function class template, and `!f`

because it does not consider pointer to data member and all constraints based on function objects which like 20.9.12.2 [func.wrap.func]/2 or 20.9.12.2.5 [func.wrap.func.targ]/3. The latter two will be resolved by the proposed resolution of 870 and are therefore not handled here.

[ Post-Rapperswil: ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 20.9.12.2.1 [func.wrap.func.con]/10+11 as indicated:

>     template<class F> function(F f);
>     template <class F, class A> function(allocator_arg_t, const A& a, F f);

> > ...

> > 10 Postconditions: `!*this` if any of the following hold:

> > - `f` is a `NULL` function pointer.
> > - `f` is a `NULL` pointer to member function pointer.
> > - `F` is an instance of the function class template, and `!f`

> > 11 Otherwise, `*this` targets a copy of `f` or, initialized with `std::move(f)` if `f` is not a pointer to member function, and targets a copy of `mem_fn(f)` if `f` is a pointer to member function. [Note: implementations are encouraged to avoid the use of dynamically allocated memory for small function objects, for example, where `f`'s target is an object holding only a pointer or reference to an object and a member function pointer. — end note]

### 1293. `unique_ptr<T[], D>` needs to get rid of unspecified-pointer-type

Section: 20.8.1.3 [unique.ptr.runtime] Status: Resolved Submitter: Daniel Krügler Opened: 2009-12-20 Last modified: 2015-04-08

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Discussion:

Addresses UK 211

As a response to UK 211 LWG issue 1021 has replaced the unspecified-pointer-type by `nullptr_t` to allow assignment of type-safe null-pointer literals in the non-array form of `unique_ptr::operator=`, but did not the same for the specialization for arrays of runtime length. But without this parallel change of the signature we have a status quo, where `unique_ptr<T[], D>` declares a member function which is completely unspecified.

[ 2009-12-21 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010-03-14 Howard adds: ]

> We moved N3073 to the formal motions page in Pittsburgh which should obsolete this issue. I've moved this issue to NAD Editorial, solved by N3073.

Rationale:

Solved by N3073.

Proposed resolution:

In 20.8.1.3 [unique.ptr.runtime], class template `unique_ptr<T[], D>` synopsis, change as indicated:

>     // assignment
>     unique_ptr& operator=(unique_ptr&& u);
>     unique_ptr& operator=(unspecified-pointer-typenullptr_t);

### 1294. Difference between callable wrapper and forwarding call wrapper unclear

Section: 20.9.2 [func.require] Status: C++11 Submitter: Jens Maurer Opened: 2009-12-21 Last modified: 2015-04-08

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Discussion:

The current wording in the standard makes it hard to discriminate the difference between a "call wrapper" as defined in 20.9.1 [func.def]/5+6:

> 5 A call wrapper type is a type that holds a callable object and supports a call operation that forwards to that object.

> 6 A call wrapper is an object of a call wrapper type.

and a "forwarding call wrapper" as defined in 20.9.2 [func.require]/4:

> 4 [..] A forwarding call wrapper is a call wrapper that can be called with an argument list. [Note: in a typical implementation forwarding call wrappers have an overloaded function call operator of the form

> >     template<class... ArgTypes>
> >     R operator()(ArgTypes&&... args) cv-qual;

> — end note]

Reason for this lack of clear difference seems to be that the wording adaption to variadics and rvalues that were applied after it's original proposal in N1673:

> [..] A forwarding call wrapper is a call wrapper that can be called with an argument list `t1, t2, ..., tN` where each `ti` is an lvalue. The effect of calling a forwarding call wrapper with one or more arguments that are rvalues is implementation defined. [Note: in a typical implementation forwarding call wrappers have overloaded function call operators of the form

> >     template<class T1, class T2, ..., class TN>
> >     R operator()(T1& t1, T2& t2, ..., TN& tN) cv-qual;

> — end note]

combined with the fact that the word "forward" has two different meanings in this context. This issue attempts to clarify the difference better.

[ 2010-09-14 Daniel provides improved wording and verified that it is correct against N3126. Previous resolution is shown here: ]

> 4 [..] A forwarding call wrapper is a call wrapper that can be called with an arbitrary argument list and uses perfect forwarding to deliver the arguments to the wrapped callable object. [Note: in a typical implementation forwarding call wrappers have an overloaded function call operator of the form

> >     template<class... ArgTypes>
> >     R operator()(ArgTypes&&... args) cv-qual;

> — end note]

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 20.9.2 [func.require]/4 as indicated:

> [..] A forwarding call wrapper is a call wrapper that can be called with an arbitrary argument list and delivers the arguments as references to the wrapped callable object. This forwarding step shall ensure that rvalue arguments are delivered as rvalue-references and lvalue arguments are delivered as lvalue-references. [Note: in a typical implementation forwarding call wrappers have an overloaded function call operator of the form

> >     template<class... UnBoundArgs>
> >     R operator()(UnBoundArgs&&... unbound_args) cv-qual;

> — end note ]

### 1295. Contradictory call wrapper requirements

Section: 20.9.2 [func.require] Status: C++11 Submitter: Daniel Krügler Opened: 2009-12-22 Last modified: 2015-04-08

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Discussion:

20.9.2 [func.require]/3 b 1 says

> 3 If a call wrapper (20.9.1 [func.def]) has a weak result type the type of its member type `result_type` is based on the type `T` of the wrapper's target object (20.9.1 [func.def]):

> - if `T` is a function, reference to function, or pointer to function type, `result_type` shall be a synonym for the return type of `T`;
> - [..]

The first two enumerated types (function and reference to function) can never be valid types for `T`, because

20.9.1 [func.def]/7

> 7 A target object is the callable object held by a call wrapper.

and 20.9.1 [func.def]/3

> 3 A callable type is a pointer to function, a pointer to member function, a pointer to member data, or a class type whose objects can appear immediately to the left of a function call operator.

exclude functions and references to function as "target objects".

[ Post-Rapperswil: ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 20.9.2 [func.require]/3 b 1 as indicated:

> 3 If a call wrapper (20.9.1 [func.def]) has a weak result type the type of its member type `result_type` is based on the type `T` of the wrapper's target object (20.9.1 [func.def]):

> - if `T` is a function, reference to function, or pointer to function type, `result_type` shall be a synonym for the return type of `T`;
> - [..]

### 1297. `unique_ptr`'s relational operator functions should induce a total order

Section: 20.8.1.5 [unique.ptr.special] Status: Resolved Submitter: Daniel Krügler Opened: 2009-12-23 Last modified: 2015-04-08

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Discussion:

The comparison functions of `unique_ptr` currently directly delegate to the underlying comparison functions of `unique_ptr<T, D>::pointer`. This is disadvantageous, because this would not guarantee to induce a total ordering for native pointers and it is hard to define a total order for mixed types anyway.

The currently suggested resolution for `shared_ptr` comparison as of 1262 uses a normalization strategy: They perform the comparison on the composite pointer type (5.9 [expr.rel]). This is not exactly possible for `unique_ptr` in the presence of user-defined pointer-like types but the existing definition of `std::duration` comparison as of 20.12.5.6 [time.duration.comparisons] via `common_type` of both argument types demonstrates a solution of this problem. The approach can be seen as the general way to define a composite pointer type and this is the approach which is used for here suggested wording change.

For consistency reasons I would have preferred the same normalization strategy for `==` and `!=`, but Howard convinced me not to do so (now).

[ 2010-11-03 Daniel comments and adjustes the currently proposed wording changes: ]

Issue 1401 is remotely related. Bullet A of its proposed resolution provides an alternative solution for issue discussed here and addresses NB comment GB-99. Additionally I updated the below suggested wording in regard to the following: It is an unncessary requirement that the below defined effective composite pointer-like type `CT` satisfies the `LessThanComparable` requirements. All what is needed is, that the function object type `less<CT>` induces a strict weak ordering on the pointer values.

[2011-03-24 Madrid meeting]

Resolved by 1401

Proposed resolution:

Change 20.8.1.5 [unique.ptr.special]/4-7 as indicated: [The implicit requirements and remarks imposed on the last three operators are the same as for the first one due to the normative "equivalent to" usage within a Requires element, see 17.5.1.4 [structure.specifications]/4. The effects of this change are that all real pointers wrapped in a `unique_ptr` will order like `shared_ptr` does.]

>     template <class T1, class D1, class T2, class D2>
>       bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

> > ? Requires: Let `CT` be `common_type<unique_ptr<T1, D1>::pointer, unique_ptr<T2, D2>::pointer>::type`. Then the specialization `less<CT>` shall be a function object type ([function.objects]) that induces a strict weak ordering ([alg.sorting]) on the pointer values.

> > 4 Returns: `less<CT>()(x.get(), y.get())x.get() < y.get()`.

> > ? Remarks: If `unique_ptr<T1, D1>::pointer` is not implicitly convertible to `CT` or `unique_ptr<T2, D2>::pointer` is not implicitly convertible to `CT`, the program is ill-formed.

>     template <class T1, class D1, class T2, class D2>
>       bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

> > 5 Effects: Equivalent to `return !(y < x)` Returns: `x.get() <= y.get()`.

>     template <class T1, class D1, class T2, class D2>
>       bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

> > 6 Effects: Equivalent to `return (y < x)` Returns: `x.get() > y.get()`.

>     template <class T1, class D1, class T2, class D2>
>       bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

> > 7 Effects: Equivalent to `return !(x < y)` Returns: `x.get() >= y.get()`.

### 1298. Missing specialization of `ctype_byname<char>`

Section: 22.2 [locale.syn] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-12-25 Last modified: 2015-04-08

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Discussion:

The `<locale>` synopsis in 22.2 [locale.syn] calls out an explicit specialization for `ctype_byname<char>`, however no such specialization is defined in the standard. The only reference I can find to `ctype_byname<char>` is 22.3.1.1.2 [locale.facet]:Table 77 — Required specializations (for facets) which also refers to `ctype_byname<wchar_t>` which has no special consideration.

Is the intent an explicit instantiation which would use a slightly different syntax? Should the explicit specialization simply be struck?

[ 2010-01-31 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

22.2 [locale.syn]

> Strike the explicit specialization for `ctype_byname<char>` from the `<locale>` synopsis

> >     ...
> >     template <class charT> class ctype_byname;
> >     template <>            class ctype_byname<char>;  // specialization
> >     ...

### 1299. Confusing typo in specification for `get_time`

Section: 27.7.5 [ext.manip] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-12-25 Last modified: 2015-04-08

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Discussion:

Extended Manipulators 27.7.5 [ext.manip] p8 defines the semantics of `get_time` in terms of a function `f`.

>     template <class charT, class traits>
>     void f(basic_ios<charT, traits>& str, struct tm* tmb, const charT* fmt) {
>        typedef istreambuf_iterator<charT, traits> Iter;
>        typedef time_get<charT, Iter> TimeGet;

>        ios_base::iostate err = ios_base::goodbit;
>        const TimeGet& tg = use_facet<TimeGet>(str.getloc());

>        tm.get(Iter(str.rdbuf()), Iter(), str, err, tmb, fmt, fmt + traits::length(fmt));

>        if (err != ios_base::goodbit)
>            str.setstate(err):
>     }

Note the call to `tm.get`. This is clearly an error, as `tm` is a type and not an object. I believe this should be `tg.get`, rather than `tm`, but this is not my area of expertise.

[ 2010-01-14 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Change 27.7.5 [ext.manip] p8:

>     template <class charT, class traits>
>     void f(basic_ios<charT, traits>& str, struct tm* tmb, const charT* fmt) {
>        typedef istreambuf_iterator<charT, traits> Iter;
>        typedef time_get<charT, Iter> TimeGet;

>        ios_base::iostate err = ios_base::goodbit;
>        const TimeGet& tg = use_facet<TimeGet>(str.getloc());

>        tgm.get(Iter(str.rdbuf()), Iter(), str, err, tmb, fmt, fmt + traits::length(fmt));

>        if (err != ios_base::goodbit)
>            str.setstate(err):
>     }

### 1300. Circular definition of `promise::swap`

Section: 30.6.5 [futures.promise] Status: Resolved Submitter: Jonathan Wakely Opened: 2009-12-26 Last modified: 2015-04-08

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Discussion:

30.6.5 [futures.promise]/12 defines the effects of `promise::swap(promise&)` as

>     void swap(promise& other);

> > 12 Effects: `swap(*this, other)`

and 30.6.5 [futures.promise]/25 defines `swap(promise<R&>, promise<R>&)` as

>     template <class R>
>       void swap(promise<R>& x, promise<R>& y);

> > 25 Effects: `x.swap(y)`.

[ 2010-01-13 Daniel added "Throws: Nothing." ]

[ 2010-01-14 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010 Pittsburgh: ]

> Moved to NAD Editorial Resolved. Rationale added below.

Rationale:

Solved by N3058.

Proposed resolution:

Change 30.6.5 [futures.promise] paragraph 12

>     void swap(promise& other);

> > 12 Effects: `swap(*this, other)` Exchanges the associated states of `*this` and `other`.

> > 13 ...

> > Throws: Nothing.

### 1303. `shared_ptr`, `unique_ptr`, and rvalue references v2

Section: 20.8.1.2 [unique.ptr.single], 20.8.2.2 [util.smartptr.shared] Status: C++11 Submitter: Stephan T. Lavavej Opened: 2010-01-23 Last modified: 2015-04-08

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Discussion:

N3000 20.8.2.2 [util.smartptr.shared]/1 still says:

>     template <class Y, class D> explicit shared_ptr(const unique_ptr<Y, D>& r) = delete;
>     template <class Y, class D> shared_ptr& operator=(const unique_ptr<Y, D>& r) = delete;

I believe that this is unnecessary now that "rvalue references v2" prevents rvalue references from binding to lvalues, and I didn't see a Library Issue tracking this.

[ 2010-02-12 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Strike from 20.8.1.2 [unique.ptr.single]:

>     template <class T, class D = default_delete<T>> class unique_ptr {
>       ...
>       unique_ptr(const unique_ptr&) = delete;
>       template <class U, class E> unique_ptr(const unique_ptr<U, E>&) = delete;
>       unique_ptr& operator=(const unique_ptr&) = delete;
>       template <class U, class E> unique_ptr& operator=(const unique_ptr<U, E>&) = delete;
>     };

Strike from 20.8.2.2 [util.smartptr.shared]:

>     template<class T> class shared_ptr {
>       ...
>       template <class Y, class D> explicit shared_ptr(const unique_ptr<Y, D>& r) = delete;
>       ...
>       template <class Y, class D> shared_ptr& operator=(const unique_ptr<Y, D>& r) = delete;
>       ...
>     };

### 1304. Missing preconditions for `shared_future`

Section: 30.6.7 [futures.shared_future] Status: Resolved Submitter: Alisdair Meredith Opened: 2010-01-23 Last modified: 2015-04-08

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Discussion:

The revised futures package in the current working paper simplified the `is_ready/has_exception/has_value` set of APIs, replacing them with a single 'valid' method. This method is used in many places to signal pre- and post- conditions, but that edit is not complete. Each method on a `shared_future` that requires an associated state should have a pre-condition that `valid() == true`.

[ 2010-01-28 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010 Pittsburgh: ]

> Moved to NAD Editorial Resolved. Rationale added below.

Rationale:

Solved by N3058.

Proposed resolution:

Insert the following extra paragraphs:

In 30.6.7 [futures.shared_future]

>     shared_future();

> > 4 Effects: constructs ...

> > Postcondition: `valid() == false`.

> > Throws: nothing.

>     void wait() const;

> > Requires: `valid() == true`.

> > 22 Effects: if the associated ...

>     template <class Rep, class Period>
>       bool wait_for(const chrono::duration<Rep, Period>& rel_time) const;

> > Requires: `valid() == true`.

> > 23 Effects: if the associated ...

>     template <class Clock, class Duration>
>       bool wait_until(const chrono::time_point<Clock, Duration>& abs_time) const;

> > Requires: `valid() == true`.

> > 25 Effects: blocks until ...

### 1305. preconditions for `atomic_future`

Section: X [futures.atomic_future] Status: Resolved Submitter: Alisdair Meredith Opened: 2010-01-23 Last modified: 2015-04-08

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Discussion:

The revised futures package in the current working paper simplified the `is_ready/has_exception/has_value` set of APIs, replacing them with a single 'valid' method. This method is used in many places to signal pre- and post- conditions, but that edit is not complete.

Atomic future retains the extended earlier API, and provides defined, synchronized behaviour for all calls. However, some preconditions and throws clauses are missing, which can easily be built around the new `valid()` api. Note that for consistency, I suggest `is_ready/has_exception/has_value throw` an exception if `valid()` is not `true`, rather than return `false`. I think this is implied by the existing pre-condition on `is_ready`.

[ 2010-01-23 See discussion starting with Message c++std-lib-26666. ]

[ 2010 Pittsburgh: Moved to NAD Editorial Resolved. Rationale added below. ]

Rationale:

Solved by N3058.

Proposed resolution:

Insert the following extra paragraphs:

In X [futures.atomic_future]

>     bool is_ready() const;

> > 17 Precondition Requires: `valid() == true`.

> > 18 Returns: `true` only if the associated state is ready.

> > Throws: `future_error` with an error condition of `no_state` if the precondition is not met.

>     bool has_exception() const;

> > Requires: `valid() == true`.

> > 19 Returns: `true` only if the associated state is ready and contains an exception.

> > Throws: `future_error` with an error condition of `no_state` if the precondition is not met.

>     bool has_value() const;

> > Requires: `valid() == true`.

> > 20 Returns: `true` only if the associated state is ready and contains a value.

> > Throws: `future_error` with an error condition of `no_state` if the precondition is not met.

>     void wait() const;

> > Requires: `valid() == true`.

> > 22 Effects: blocks until ...

> > Throws: `future_error` with an error condition of `no_state` if the precondition is not met.

>     template <class Rep, class Period>
>       bool wait_for(const chrono::duration<Rep, Period>& rel_time) const;

> > Requires: `valid() == true`.

> > 23 Effects: blocks until ...

> > 24 Returns: `true` only if ...

> > Throws: `future_error` with an error condition of `no_state` if the precondition is not met.

>     template <class Clock, class Duration>
>       bool wait_until(const chrono::time_point<Clock, Duration>& abs_time) const;

> > Requires: `valid() == true`.

> > 25 Effects: blocks until ...

> > 26 Returns: `true` only if ...

> > Throws: `future_error` with an error condition of `no_state` if the precondition is not met.

### 1306. `pointer` and `const_pointer` for `<array>`

Section: 23.3.2 [array] Status: C++11 Submitter: Nicolai Josuttis Opened: 2010-01-24 Last modified: 2015-04-08

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Discussion:

Class `<array>` is the only sequence container class that has no types `pointer` and `const_pointer` defined. You might argue that this makes no sense because there is no allocator support, but on the other hand, types `reference` and `const_reference` are defined for `array`.

[ 2010-02-11 Moved to Tentatively Ready after 6 positive votes on c++std-lib. ]

Proposed resolution:

Add to Class template array 23.3.2 [array]:

>     namespace std {
>       template <class T, size_t N >
>       struct array {
>         ...
>         typedef T value_type;
>         typedef T * pointer;
>         typedef const T * const_pointer;
>         ...
>       };
>     }

### 1307. `exception_ptr` and `allocator` pointers don't understand !=

Section: 18.8.5 [propagation] Status: Resolved Submitter: Daniel Krügler Opened: 2010-01-26 Last modified: 2015-04-08

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Discussion:

The current requirements for a conforming implementation of `std::exception_ptr` (18.8.5 [propagation]/1-6) does not clarify whether the expression

>     e1 != e2
>     e1 != nullptr

with `e1` and `e2` being two values of type `std::exception_ptr` are supported or not. Reason for this oddity is that the concept `EqualityComparable` does not provide operator `!=`.

For the same reason programmers working against the types `X::pointer`, `X::const_pointer`, `X::void_pointer`, and `X::const_void_pointer` of any allocator concept `X` (17.6.3.5 [allocator.requirements]/4 + Table 40) in a generic context can not rely on the availability of the != operation, which is rather unnatural and error-prone.

[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]

Rationale:

Solved by N3073.

Proposed resolution:

### 1309. Missing expressions for `Move/CopyConstructible`

Section: 17.6.3.1 [utility.arg.requirements] Status: C++11 Submitter: Daniel Krügler Opened: 2010-02-03 Last modified: 2015-04-08

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Discussion:

Table 33 — MoveConstructible requirements [moveconstructible] and Table 34 — CopyConstructible requirements [copyconstructible] support solely the following expression:

>     T t(rv)

where `rv` is defined to be as "non-const rvalue of type `T`" and `t` as a "modifiable lvalue of type `T`" in 17.6.3.1 [utility.arg.requirements]/1.

This causes two different defects:

1. We cannot move/copy-initialize a const lvalue of type `T` as in:

   >     int get_i();

   >     const int i1(get_i());

   both in Table 33 and in Table 34.

2. The single support for

   >     T t(rv)

   in case of `CopyConstructible` means that we cannot provide an lvalue as a source of a copy as in

   >     const int& get_lri();

   >     int i2(get_lri());

I believe this second defect is due to the fact that this single expression supported both initialization situations according to the old (implicit) lvalue reference -> rvalue reference conversion rules.

Finally [copyconstructible] refers to some name `u` which is not part of the expression, and both [copyconstructible] and [moveconstructible] should support construction expressions from temporaries - this would be a stylistic consequence in the light of the new `DefaultConstructible` requirements and compared with existing requirements (see e.g. Container requirements or the output/forward iterator requirements)..

[ 2010-02-09 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

[ 2010-02-10 Reopened. The proposed wording of 1283 has been merged here. ]

[ 2010-02-10 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

1. Change 17.6.3.1 [utility.arg.requirements]/1 as indicated: [This change suggestion is motivated to make type descriptions clearer: First, `a`, `b`, and `c` may also be non-`const T`. Second, `u` is described in a manner consistent with the container requirements tables.]

   > 1 The template definitions in the C++ standard library refer to various named requirements whose details are set out in tables 31-38. In these tables, `T` is an object or reference type to be supplied by a C++ program instantiating a template; `a`, `b`, and `c` are values of type (possibly `const) T`; `s` and `t` are modifiable lvalues of type `T`; `u` denotes an identifier; is a value of type (possibly `const`) `T`; and `rv` is an non-const rvalue of type `T`; and `v` is an lvalue of type (possibly `const`) `T` or an rvalue of type `const T`.

2. In 17.6.3.1 [utility.arg.requirements] Table 33 ([moveconstructible]) change as indicated [Note: The symbol `u` is defined to be either a const or a non-const value and is the right one we need here]:

   > Table 33 — `MoveConstructible` requirements [moveconstructible]
   > Expression | Post-condition
   > --- | ---
   > `T tu(rv);` | `tu` is equivalent to the value of `rv` before the construction
   > `T(rv)` | `T(rv)` is equivalent to the value of `rv` before the construction
   > [Note: There is no requirement on the value of `rv` after the construction. `rv` remains a valid object. Its state is unspecified. — end note]

3. In 17.6.3.1 [utility.arg.requirements] Table 34 ([copyconstructible]) change as indicated [Note: The symbol `u` is defined to be either a const or a non-const value and is the right one we need here. The expressions using `a` are recommended to ensure that lvalues are supported as sources of the copy expression]:

   > Table 34 — `CopyConstructible` requirements [copyconstructible]
   > (in addition to `MoveConstructible`)
   > Expression | Post-condition
   > --- | ---
   > `T tu(rv);` | the value of `uv` is unchanged and is equivalent to `tu`
   > `T(v)` | the value of `v` is unchanged and is equivalent to `T(v)`
   > [Note: A type that satisfies the `CopyConstructible` requirements also satisfies the `MoveConstructible` requirements. — end note]

4. In Table 35 — MoveAssignable requirements [moveassignable] change as indicated:

   > Table 35 — `MoveAssignable` requirements [moveassignable]
   > Expression | Return type | Return value | Post-condition
   > --- | --- | --- | ---
   > `t = rv` | `T&` | `t` | `t` is equivalent to the value of `rv` before the assigment.
   > [Note: There is no requirement on the value of `rv` after the assignment. `rv` remains a valid object. Its state is unspecified. — end note]

5. In 17.6.3.1 [utility.arg.requirements] change Table 36 as indicated:

   > Table 36 — `CopyAssignable` requirements [copyassignable]
   > (in addition to `MoveAssignable`)
   > Expression | Return type | Return value | Post-condition
   > --- | --- | --- | ---
   > `t = uv` | `T&` | `t` | `t` is equivalent to `uv`, the value of `uv` is unchanged
   > [Note: A type that satisfies the `CopyAssignable` requirements also satisfies the `MoveAssignable` requirements. — end note]

### 1310. `forward_list splice_after` from lvalues

Section: 23.3.4.6 [forwardlist.ops] Status: C++11 Submitter: Howard Hinnant Opened: 2010-02-05 Last modified: 2015-04-08

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Discussion:

We've moved 1133 to Tentatively Ready and I'm fine with that.

1133 adds lvalue-references to the `splice` signatures for `list`. So now `list` can `splice` from lvalue and rvalue lists (which was the intent of the original move papers btw). During the discussion of this issue it was mentioned that if we want to give the same treatment to `forward_list`, that should be a separate issue.

This is that separate issue.

Consider the following case where you want to splice elements from one place in a `forward_list` to another. Currently this must be coded like so:

>     fl.splice_after(to_here, std::move(fl), from1, from2);

This looks pretty shocking to me. I would expect to be able to code instead:

>     fl.splice_after(to_here, fl, from1, from2);

but we currently don't allow it.

When I say `move(fl)`, I consider that as saying that I don't care about the value of `fl` any more (until I assign it a new value). But in the above example, this simply isn't true. I do care about the value of `fl` after the move, and I'm not assigning it a new value. I'm merely permuting its current value.

I propose adding `forward_list&` overloads to the 3 `splice_after` members. For consistency's sake (principal of least surprise) I'm also proposing to overload `merge` this way as well.

Proposed resolution:

Add to the synopsis of 23.3.4.1 [forwardlist.overview]:

>     template <class T, class Allocator = allocator<T> >
>     class forward_list {
>     public:
>       ...
>       // [forwardlist.ops], forward_list operations:
>       void splice_after(const_iterator p, forward_list& x);
>       void splice_after(const_iterator p, forward_list&& x);
>       void splice_after(const_iterator p, forward_list& x, const_iterator i);
>       void splice_after(const_iterator p, forward_list&& x, const_iterator i);
>       void splice_after(const_iterator p, forward_list& x,
>                         const_iterator first, const_iterator last);
>       void splice_after(const_iterator p, forward_list&& x,
>                         const_iterator first, const_iterator last);
>       ...
>       void merge(forward_list& x);
>       void merge(forward_list&& x);
>       template <class Compare> void merge(forward_list& x, Compare comp);
>       template <class Compare> void merge(forward_list&& x, Compare comp);
>       ...
>     };

Add to the signatures of 23.3.4.6 [forwardlist.ops]:

>     void splice_after(const_iterator p, forward_list& x);
>     void splice_after(const_iterator p, forward_list&& x);

> > 1 ...

>     void splice_after(const_iterator p, forward_list& x, const_iterator i);
>     void splice_after(const_iterator p, forward_list&& x, const_iterator i);

> > 4 ...

>     void splice_after(const_iterator p, forward_list& x,
>                     const_iterator first, const_iterator last);
>     void splice_after(const_iterator p, forward_list&& x,
>                     const_iterator first, const_iterator last);

> > 7 ...

>     void merge(forward_list& x);
>     void merge(forward_list&& x);
>     template <class Compare> void merge(forward_list& x, Compare comp);
>     template <class Compare> void merge(forward_list&& x, Compare comp);

> > 16 ...

### 1311. multi-pass property of Forward Iterator underspecified

Section: 24.2.5 [forward.iterators] Status: Resolved Submitter: Alisdair Meredith Opened: 2010-02-07 Last modified: 2015-04-08

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Discussion:

The following example demonstrates code that would meet the guarantees of a Forward Iterator, but only permits a single traversal of the underlying sequence:

>     template< typename ForwardIterator>
>     struct bad_iterator {
>       shared_ptr<ForwardIterator> impl;

>       bad_iterator( ForwardIterator iter ) {
>          : impl{new ForwardIterator{iter} }
>          {
>       }

>       auto operator*() const -> decltype(*ForwardIterator{}) {
>          return **impl;
>       }

>       auto operator->() const -> ForwardIterator {
>          return *impl;
>       }

>       auto operator==(bad_iterator const & rhs) const -> bool {
>          return impl == rhs.impl;
>       }

>       auto operator++() -> bad_iterator& {
>          ++(*impl);
>          return *this;
>       }
>       // other operations as necessary...
>     };

Here, we use `shared_ptr` to wrap a forward iterator, so all iterators constructed from the same original iterator share the same 'value', and incrementing any one copy increments all others.

There is a missing guarantee, expressed by the following code sequence

>     FwdIter x = seq.begin();  // obtain forward iterator from a sequence
>     FwdIter y = x;            // copy the iterator
>     assert(x == y);           // iterators must be the same
>     ++x;                      // increment *just one* iterator
>     assert(x != y);           // iterators *must now be different*
>     ++y;                      // increment the other iterator
>     assert(x == y);           // now the iterators must be the same again

That inequality in the middle is an essential guarantee. Note that this list is simplified, as each assertion should also note that they refer to exactly the same element `(&*x == &*y)` but I am not complicating the issue with tests to support proxy iterators, or value types overloading unary `operator+`.

I have not yet found a perverse example that can meet this additional constraint, and not meet the multi-pass expectations of a Forward Iterator without also violating other Forward Iterator requirements.

Note that I do not yet have standard-ready wording to resolve the problem, as saying this neatly and succinctly in 'standardese' is more difficult.

[ 2010 Pittsburgh: Moved to NAD Editorial Resolved. Rationale added below. ]

Rationale:

Solved by N3066.

Proposed resolution:

### 1312. `vector::data` no longer returns a raw pointer

Section: 23.3.6.4 [vector.data] Status: C++11 Submitter: Alisdair Meredith Opened: 2010-02-07 Last modified: 2015-04-08

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Discussion:

The original intent of `vector::data` was to match `array::data` in providing a simple API with direct access to the contiguous buffer of elements that could be passed to a "classic" C API. At some point, the return type became the '`pointer`' typedef, which is not derived from the `allocator` via allocator traits - it is no longer specified to precisely `T *`. The return type of this function should be corrected to no longer use the typedef.

[ 2010-02-10 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

23.3.6 [vector]

Update the class definition in p2:

>     // 23.3.6.3 data access
>     pointerT * data();
>     const_pointerconst T * data() const;

23.3.6.4 [vector.data]

Adjust signatures:

>     pointerT * data();
>     const_pointerconst T * data() const;

### 1316. `scoped_allocator_adaptor operator==` has no definition

Section: 20.13 [allocator.adaptor] Status: C++11 Submitter: Pablo Halpern Opened: 2009-02-11 Last modified: 2015-04-08

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Discussion:

The WP (N3000) contains these declarations:

>     template <class OuterA1, class OuterA2, class... InnerAllocs>
>       bool operator==(const scoped_allocator_adaptor<OuterA1, InnerAllocs...>& a,
>                       const scoped_allocator_adaptor<OuterA2, InnerAllocs...>& b);
>     template <class OuterA1, class OuterA2, class... InnerAllocs>
>       bool operator!=(const scoped_allocator_adaptor<OuterA1, InnerAllocs...>& a,
>                       const scoped_allocator_adaptor<OuterA2, InnerAllocs...>& b);

But does not define what the behavior of these operators are.

[ Post-Rapperswil: ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Add a new section after 20.13.4 [allocator.adaptor.members]:

> Scoped allocator operators [scoped.adaptor.operators]

>     template <class OuterA1, class OuterA2, class... InnerAllocs>
>       bool operator==(const scoped_allocator_adaptor<OuterA1, InnerAllocs...>& a,
>                       const scoped_allocator_adaptor<OuterA2, InnerAllocs...>& b);

> > Returns: `a.outer_allocator() == b.outer_allocator()` if `sizeof...(InnerAllocs)` is zero; otherwise, `a.outer_allocator() == b.outer_allocator() && a.inner_allocator() == b.inner_allocator()`.

>     template <class OuterA1, class OuterA2, class... InnerAllocs>
>       bool operator!=(const scoped_allocator_adaptor<OuterA1, InnerAllocs...>& a,
>                       const scoped_allocator_adaptor<OuterA2, InnerAllocs...>& b);

> > Returns: `!(a == b)`.

### 1319. Containers should require an iterator that is at least a Forward Iterator

Section: 23.2.1 [container.requirements.general] Status: C++11 Submitter: Alisdair Meredith Opened: 2010-02-16 Last modified: 2015-04-08

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Discussion:

The requirements on container iterators are spelled out in 23.2.1 [container.requirements.general], table 91.

> Table 91 — Container requirements
> Expression | Return type | Operational semantics | Assertion/note pre-/post-condition | Complexity
> --- | --- | --- | --- | ---
> `...`
> `X::iterator` | iterator type whose value type is `T` |  | any iterator category except output iterator. Convertible to `X::const_iterator`. | compile time
> `X::const_iterator` | constant iterator type whose value type is `T` |  | any iterator category except output iterator | compile time
> `...`

As input iterators do not have the multi-pass guarantee, they are not suitable for iterating over a container. For example, taking two calls to `begin()`, incrementing either iterator might invalidate the other. While data structures might be imagined where this behaviour produces interesting and useful results, it is very unlikely to meet the full set of requirements for a standard container.

[ Post-Rapperswil: ]

Daniel notes: I changed the currently suggested P/R slightly, because it is not robust in regard to new fundamental iterator catagories. I recommend to say instead that each container::iterator shall satisfy (and thus may refine) the forward iterator requirements.

> Moved to Tentatively Ready with revised wording after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

1. Change Table 93 — Container requirements in [container.requirements.general] as indicated:

   > Table 93 — Container requirements
   > Expression | Return type | Operational semantics | Assertion/note pre-/post-condition | Complexity
   > --- | --- | --- | --- | ---
   > `...`
   > `X::iterator` | iterator type whose value type is `T` |  | any iterator category except output iterator that meets the forward iterator requirements. convertible to `X::const_iterator` | compile time
   > `X::const_iterator` | constant iterator type whose value type is `T` |  | any iterator category except output iterator that meets the forward iterator requirements. | compile time
   > `...`

### 1321. `scoped_allocator_adaptor construct` and `destroy` don't use `allocator_traits`

Section: 20.13.4 [allocator.adaptor.members] Status: Resolved Submitter: Howard Hinnant Opened: 2010-02-16 Last modified: 2015-04-08

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Discussion:

20.13.4 [allocator.adaptor.members] p8-9 says:

>     template <class T, class... Args>
>       void construct(T* p, Args&&... args);

> > 8 Effects: let `OUTERMOST(x)` be `x` if `x` does not have an `outer_allocator()` function and `OUTERMOST(x.outer_allocator())` otherwise.

> > - If `uses_allocator<T, inner_allocator_type>::value` is `false` and `is_constructible<T, Args...>::value` is `true`, calls `OUTERMOST(*this).construct(p, std::forward<Args>(args)...)`.
> > - Otherwise, if `uses_allocator<T, inner_allocator_type>::value` is `true` and `is_constructible<T, allocator_arg_t, inner_allocator_type, Args...>::value` is `true`, calls `OUTERMOST(*this).construct(p, allocator_arg, inner_allocator(),std::forward<Args>(args)...)`.
> > - Otherwise, if `uses_allocator<T, inner_allocator_type>::value` is `true` and `is_constructible<T, Args..., inner_allocator_type>::value` is `true`, calls `OUTERMOST(*this).construct(p, std::forward<Args>(args)..., inner_allocator())`.
> > - Otherwise, the program is ill-formed. [Note: an error will result if `uses_allocator` evaluates to `true` but the specific constructor does not take an allocator. This definition prevents a silent failure to pass an inner allocator to a contained element. — end note]

>     template <class T>
>       void destroy(T* p);

> > 9 Effects: calls `outer_allocator().destroy(p)`.

In all other calls where applicable `scoped_allocator_adaptor` does not call members of an allocator directly, but rather does so indirectly via `allocator_traits`. For example:

>     size_type max_size() const;

> > 7 Returns: `allocator_traits<OuterAlloc>::max_size(outer_allocator())`.

Indeed, without the indirection through `allocator_traits` the definitions for `construct` and `destroy` are likely to fail at compile time since the `outer_allocator()` may not have the members `construct` and `destroy`.

[ The proposed wording is a product of Pablo, Daniel and Howard. ]

[ 2010 Pittsburgh: Moved to NAD Editorial. Rationale added below. ]

Rationale:

Solved by N3059.

Proposed resolution:

In 20.13.4 [allocator.adaptor.members] move and change p8 as indicated, and change p9 as indicated:

> Let `OUTERMOST(x)` be `x` if `x` does not have an `outer_allocator()` member function and `OUTERMOST(x.outer_allocator())` otherwise. Let `OUTERMOST_ALLOC_TRAITS(x)` be `allocator_traits<decltype(OUTERMOST(x))>`. [Note: `OUTERMOST(x)` and `OUTERMOST_ALLOC_TRAITS(x)` are recursive operations. It is incumbent upon the definition of `outer_allocator()` to ensure that the recursion terminates. It will terminate for all instantiations of `scoped_allocator_adaptor`. — end note]

>     template <class T, class... Args>
>       void construct(T* p, Args&&... args);

> > 8 Effects: let `OUTERMOST(x)` be `x` if `x` does not have an `outer_allocator()` function and `OUTERMOST(x.outer_allocator())` otherwise.

> > - If `uses_allocator<T, inner_allocator_type>::value` is `false` and `is_constructible<T, Args...>::value` is `true`, calls `OUTERMOST(*this). OUTERMOST_ALLOC_TRAITS(outer_allocator())::construct( OUTERMOST(outer_allocator()), p, std::forward<Args>(args)... )`.
> > - Otherwise, if `uses_allocator<T, inner_allocator_type>::value` is `true` and `is_constructible<T, allocator_arg_t, inner_allocator_type, Args...>::value` is `true`, calls `OUTERMOST(*this). OUTERMOST_ALLOC_TRAITS(outer_allocator())::construct( OUTERMOST(outer_allocator()), p, allocator_arg, inner_allocator(), std::forward<Args>(args)... )`.
> > - Otherwise, if `uses_allocator<T, inner_allocator_type>::value` is `true` and `is_constructible<T, Args..., inner_allocator_type>::value` is `true`, calls `OUTERMOST(*this). OUTERMOST_ALLOC_TRAITS(outer_allocator())::construct( OUTERMOST(outer_allocator()), p, std::forward<Args>(args)..., inner_allocator() )`.
> > - Otherwise, the program is ill-formed. [Note: an error will result if `uses_allocator` evaluates to `true` but the specific constructor does not take an allocator. This definition prevents a silent failure to pass an inner allocator to a contained element. — end note]

>     template <class T>
>       void destroy(T* p);

> > 9 Effects: calls `outer_allocator(). OUTERMOST_ALLOC_TRAITS(outer_allocator())::destroy( OUTERMOST(outer_allocator()), p)`.

### 1322. Explicit `CopyConstructible` requirements are insufficient

Section: 17.6.3.1 [utility.arg.requirements] Status: Resolved Submitter: Daniel Krügler Opened: 2010-02-16 Last modified: 2015-04-08

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Discussion:

With the acceptance of library defect 822 only direct-initialization is supported, and not copy-initialization in the requirement sets `MoveConstructible` and `CopyConstructible`. This is usually a good thing, if only the library implementation needs to obey these restrictions, but the Empire strikes back quickly:

1. Affects user-code: `std::exception_ptr` is defined purely via requirements, among them `CopyConstructible`. A strict reading of the standard would make implementations conforming where `std::exception_ptr` has an explicit copy-c'tor and user-code must code defensively. This is a very unwanted effect for such an important component like `std::exception_ptr`.

2. Wrong re-use: Recently proposed requirement sets (`NullablePointer` as of N3025, Hash) or cleanup of existing requirement sets (e.g. iterator requirements as of N3046) tend to reuse existing requirement sets, so reusing `CopyConstructible` is attempting, even in cases, where the intend is to support copy-initialization as well.

3. Inconsistency: The current iterator requirements set Table 102 (output iterator requirements) and Table 103 (forward iterator requirements) demonstrate quite clearly a strong divergence of copy-semantics: The specified semantics of

   >     X u(a);
   >     X u = a;

   are underspecified compared to the most recent clarifications of the `CopyConstructible` requirements, c.f. issue 1309 which is very unsatisfactory. This will become worse for each further issue that involves the `CopyConstructible` specification (for possible directions see 1173).

The suggested resolution is to define two further requirements `implicit-MoveConstructible` and `implicit-CopyConstructible` (or any other reasonable name like `MoveConvertible` and `CopyConvertible`) each with a very succinct but precise meaning solving all three problems mentioned above.

[Batavia: Resolved by accepting n3215.]

Proposed resolution:

1. Add the following new table ?? after Table 34 — `MoveConstructible` requirements [moveconstructible]:

   > Table ?? — `Implicit MoveConstructible` requirements [implicit.moveconstructible] (in addition to `MoveConstructible`)
   > Expression | Operational Semantics
   > --- | ---
   > `T u = rv;` | Equivalent to: `T u(rv);`

2. Add the following new table ?? after Table 35 — `CopyConstructible` requirements [copyconstructible]:

   > Table ?? — `Implicit CopyConstructible` requirements [implicit.copyconstructible] (in addition to `CopyConstructible`)
   > Expression | Operational Semantics
   > --- | ---
   > `T u = v;` | Equivalent to: `T u(v);`

3. Change 17.6.3.3 [nullablepointer.requirements]/1 as follows:

   > A `NullablePointer` type is a pointer-like type that supports null values. A type `P` meets the requirements of `NullablePointer` if:

   > - `P` satisfies the requirements of `EqualityComparable`, `DefaultConstructible`, `implicit CopyConstructible`, `CopyAssignable`, and `Destructible`,
   > - [..]

4. Change 17.6.3.4 [hash.requirements]/1 as indicated: [explicit copy-constructible functors could not be provided as arguments to any algorithm that takes these by value. Also a typo is fixed.]

   > 1 A type `H` meets the Hash requirements if:

   > - it is a function object type (20.8),
   > - it satisfies ifes the requirements of `implicit CopyConstructible` and `Destructible` (20.2.1),
   > - [..]

5. Change 20.10.1 [meta.rqmts]/1+2 as indicated:

   > 1 A UnaryTypeTrait describes a property of a type. It shall be a class template that takes one template type argument and, optionally, additional arguments that help define the property being described. It shall be `DefaultConstructible`, `implicit CopyConstructible`, [..]

   > 2 A `BinaryTypeTrait` describes a relationship between two types. It shall be a class template that takes two template type arguments and, optionally, additional arguments that help define the relationship being described. It shall be `DefaultConstructible`, `implicit CopyConstructible`, and [..]

6. Change 20.9.2 [func.require]/4 as indicated: [explicit copy-constructible functors could not be provided as arguments to any algorithm that takes these by value]

   > 4 Every call wrapper (20.8.1) shall be `implicit MoveConstructible`. A simple call wrapper is a call wrapper that is `implicit CopyConstructible` and `CopyAssignable` and whose copy constructor, move constructor, and assignment operator do not throw exceptions. [..]

7. Change 20.9.4 [refwrap]/1 as indicated:

   > 1 `reference_wrapper<T>` is an `implicit` `CopyConstructible` and `CopyAssignable` wrapper around a reference to an object or function of type `T`.

8. Change 20.9.10.3 [func.bind.bind]/5+9 as indicated:

   > 5 Remarks: The return type shall satisfy the requirements of `implicit MoveConstructible`. If all of `FD` and `TiD` satisfy the requirements of `CopyConstructible`, then the return type shall satisfy the requirements of `implicit CopyConstructible`. [Note: this implies that all of `FD` and `TiD` are `MoveConstructible`. — end note]

   > [..]

   > 9 Remarks: The return type shall satisfy the requirements of `implicit MoveConstructible`. If all of `FD` and `TiD` satisfy the requirements of `CopyConstructible`, then the return type shall satisfy the requirements of `implicit CopyConstructible`. [Note: this implies that all of `FD` and `TiD` are `MoveConstructible`. — end note]

9. Change 20.9.10.4 [func.bind.place] as indicated:

   > 1 All placeholder types shall be `DefaultConstructible` and `implicit CopyConstructible`, and [..]

10. Change 20.8.1 [unique.ptr]/5 as indicated:

    > 5 Each object of a type `U` instantiated form the `unique_ptr` template specified in this subclause has the strict ownership semantics, specified above, of a unique pointer. In partial satisfaction of these semantics, each such `U` is `implicit MoveConstructible` and `MoveAssignable`, but is not `CopyConstructible` nor `CopyAssignable`. The template parameter `T` of `unique_ptr` may be an incomplete type.

11. Change 20.8.2.2 [util.smartptr.shared]/2 as indicated:

    > 2 Specializations of `shared_ptr` shall be `implicit CopyConstructible`, `CopyAssignable`, and `LessThanComparable`, [..]

12. Change 20.8.2.3 [util.smartptr.weak]/2 as indicated:

    > 2 Specializations of `weak_ptr` shall be `implicit CopyConstructible` and `CopyAssignable`, allowing their use in standard containers. The template parameter `T` of `weak_ptr` may be an incomplete type.

13. Change 24.2.2 [iterator.iterators]/2 as indicated: [This fixes a defect in the Iterator requirements. None of the usual algorithms accepting iterators would be usable with iterators with explicit copy-constructors]

    > 2 A type `X` satisfies the Iterator requirements if:

    > - `X` satisfies the `implicit CopyConstructible`, `CopyAssignable`, and `Destructible` requirements (20.2.1) and lvalues of type `X` are swappable (20.2.2), and [..]
    > - ...

14. Change X [auto.ptr]/3 as indicated:

    > 3 [..] Instances of `auto_ptr` meet the requirements of `implicit MoveConstructible` and `MoveAssignable`, but do not meet the requirements of `CopyConstructible` and `CopyAssignable`. — end note]

### 1323. `basic_string::replace` should use `const_iterator`

Section: 21.4.6.6 [string::replace] Status: C++11 Submitter: Daniel Krügler Opened: 2010-02-19 Last modified: 2015-04-08

View all other issues in [string::replace].

View all issues with C++11 status.

Discussion:

In contrast to all library usages of purely positional iterator values several overloads of `std::basic_string::replace` still use iterator instead of `const_iterator` arguments. The paper N3021 quite nicely visualizes the purely positional responsibilities of the function arguments.

This should be fixed to make the library consistent, the proposed changes are quite mechanic.

[ Post-Rapperswil: ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

1. In 21.4 [basic.string], class template `basic_string` synopsis change as indicated:

   >     // 21.4.6 modifiers:
   >     ...
   >     basic_string& replace(const_iterator i1, const_iterator i2,
   >                           const basic_string& str);
   >     basic_string& replace(const_iterator i1, const_iterator i2,
   >                           const charT* s, size_type n);
   >     basic_string& replace(const_iterator i1, const_iterator i2,
   >                           const charT* s);
   >     basic_string& replace(const_iterator i1, const_iterator i2,
   >                           size_type n, charT c);
   >     template<class InputIterator>
   >       basic_string& replace(const_iterator i1, const_iterator i2,
   >                             InputIterator j1, InputIterator j2);
   >     basic_string& replace(const_iterator, const_iterator,
   >                           initializer_list<charT>);

2. In 21.4.6.6 [string::replace] before p.18, change the following signatures as indicated:

   >     basic_string& replace(const_iterator i1, const_iterator i2, const basic_string& str);

3. In 21.4.6.6 [string::replace] before p.21, change the following signatures as indicated:

   >     basic_string&
   >       replace(const_iterator i1, const_iterator i2, const charT* s, size_type n);

4. In 21.4.6.6 [string::replace] before p.24, change the following signatures as indicated:

   >     basic_string& replace(const_iterator i1, const_iterator i2, const charT* s);

5. In 21.4.6.6 [string::replace] before p.27, change the following signatures as indicated:

   >     basic_string& replace(const_iterator i1, const_iterator i2, size_type n,
   >                           charT c);

6. In 21.4.6.6 [string::replace] before p.30, change the following signatures as indicated:

   >     template<class InputIterator>
   >       basic_string& replace(const_iterator i1, const_iterator i2,
   >                             InputIterator j1, InputIterator j2);

7. In 21.4.6.6 [string::replace] before p.33, change the following signatures as indicated:

   >     basic_string& replace(const_iterator i1, const_iterator i2,
   >                           initializer_list<charT> il);

### 1324. Still too many implicit conversions for `pair` and `tuple`

Section: 20.3.2 [pairs.pair], 20.4.2.1 [tuple.cnstr] Status: Resolved Submitter: Daniel Krügler Opened: 2010-03-20 Last modified: 2015-04-08

View all other issues in [pairs.pair].

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Discussion:

In analogy to library defect 811, `tuple`'s variadic constructor

>     template <class... UTypes>
>     explicit tuple(UTypes&&... u);

creates the same problem as pair:

>     #include <tuple>

>     int main()
>     {
>       std::tuple<char*> p(0);
>     }

produces a similar compile error for a recent gcc implementation.

I suggest to follow the same resolution path as has been applied to `pair`'s corresponding c'tor, that is require that these c'tors should not participate in overload resolution, if the arguments are not implicitly convertible to the element types.

Further-on both `pair` and `tuple` provide converting constructors from different `pairs`/`tuples` that should be not available, if the corresponding element types are not implicitly convertible. It seems astonishing that in the following example

>     struct A {
>       explicit A(int);
>     };

>     A  a = 1; // Error

>     std::tuple<A> ta = std::make_tuple(1); // # OK?

the initialization marked with # could be well-formed.

[ Only constraints on constructors are suggested. Adding similar constraints on assignment operators is considered as QoI, because the assigments wouldn't be well-formed anyway. ]

1. Following 20.3.2 [pairs.pair]/5 add a new Remarks element:

   >     template<class U, class V> pair(const pair<U, V>& p);

   > > 5 Effects: Initializes members from the corresponding members of the argument, performing implicit conversions as needed.

   > > Remarks: This constructor shall not participate in overload resolution unless `U` is implicitly convertible to `first_type` and `V` is implicitly convertible to `second_type`.

2. Following 20.3.2 [pairs.pair]/6 add a new Remarks element:

   >     template<class U, class V> pair(pair<U, V>&& p);

   > > 6 Effects: The constructor initializes `first` with `std::move(p.first)` and second with `std::move(p.second)`.

   > > Remarks: This constructor shall not participate in overload resolution unless `U` is implicitly convertible to `first_type` and `V` is implicitly convertible to `second_type`.

3. Following 20.4.2.1 [tuple.cnstr]/7 add a new Remarks element:

   >     template <class... UTypes>
   >     explicit tuple(UTypes&&... u);

   > > 6 Requires: Each type in `Types` shall satisfy the requirements of `MoveConstructible` (Table 33) from the corresponding type in `UTypes`. `sizeof...(Types) == sizeof...(UTypes)`.

   > > 7 Effects: Initializes the elements in the `tuple` with the corresponding value in `std::forward<UTypes>(u)`.

   > > Remarks: This constructor shall not participate in overload resolution unless each type in `UTypes` is implicitly convertible to its corresponding type in `Types`.

4. Following 20.4.2.1 [tuple.cnstr]/13 add a new Remarks element:

   >     template <class... UTypes> tuple(const tuple<UTypes...>& u);

   > > 12 Requires: Each type in `Types` shall be constructible from the corresponding type in `UTypes`. `sizeof...(Types) == sizeof...(UTypes)`.

   > > 13 Effects: Constructs each element of `*this` with the corresponding element of `u`.

   > > Remarks: This constructor shall not participate in overload resolution unless each type in `UTypes` is implicitly convertible to its corresponding type in `Types`.

   > > 14 [Note: `enable_if` can be used to make the converting constructor and assignment operator exist only in the cases where the source and target have the same number of elements. — end note]

5. Following 20.4.2.1 [tuple.cnstr]/16 add a new Remarks element:

   >     template <class... UTypes> tuple(tuple<UTypes...>&& u);

   > > 15 Requires: Each type in `Types` shall shall satisfy the requirements of `MoveConstructible` (Table 33) from the corresponding type in `UTypes`. `sizeof...(Types) == sizeof...(UTypes)`.

   > > 16 Effects: Move-constructs each element of `*this` with the corresponding element of `u`.

   > > Remarks: This constructor shall not participate in overload resolution unless each type in `UTypes` is implicitly convertible to its corresponding type in `Types`.

   > > [Note: `enable_if` can be used to make the converting constructor and assignment operator exist only in the cases where the source and target have the same number of elements. — end note]

6. Following 20.4.2.1 [tuple.cnstr]/18 add a new Remarks element:

   >     template <class U1, class U2> tuple(const pair<U1, U2>& u);

   > > 17 Requires: The first type in `Types` shall be constructible from `U1` and the second type in `Types` shall be constructible from `U2`. `sizeof...(Types) == 2`.

   > > 18 Effects: Constructs the first element with `u.first` and the second element with `u.second`.

   > > Remarks: This constructor shall not participate in overload resolution unless `U1` is implicitly convertible to the first type in `Types` and `U2` is implicitly convertible to the second type in `Types`.

7. Following 20.4.2.1 [tuple.cnstr]/20 add a new Remarks element:

   >     template <class U1, class U2> tuple(pair<U1, U2>&& u);

   > > 19 Requires: The first type in `Types` shall shall satisfy the requirements of `MoveConstructible`(Table 33) from `U1` and the second type in `Types` shall be move-constructible from `U2`. `sizeof...(Types) == 2`.

   > > 20 Effects: Constructs the first element with `std::move(u.first)` and the second element with `std::move(u.second)`

   > > Remarks: This constructor shall not participate in overload resolution unless `U1` is implicitly convertible to the first type in `Types` and `U2` is implicitly convertible to the second type in `Types`.

[ 2010-10-24 Daniel adds: ]

> Accepting n3140 would solve this issue.

Proposed resolution:

See n3140.

### 1325. `bitset`

Section: 20.6.1 [bitset.cons] Status: C++11 Submitter: Christopher Jefferson Opened: 2010-03-07 Last modified: 2015-04-08

View all other issues in [bitset.cons].

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Discussion:

As mentioned on the boost mailing list:

The following code, valid in C++03, is broken in C++0x due to ambiguity between the "`unsigned long long`" and "`char*`" constructors.

>     #include <bitset>
>     std::bitset<10> b(0);

[ The proposed resolution has been reviewed by Stephan T. Lavavej. ]

[ Post-Rapperswil ]

The proposed resolution has two problems:

- it fails to provide support for non-terminated strings, which could be easily added and constitutes an important use-case. For example, the following code would invoke UB with the current P/R:

  >     char s[4] = { '0', '1', '0', '1' }; // notice: not null-terminated!
  >     bitset<4> b(s, 0, 4);

  because it requires the evaluation (under the as-if rule, to be fair, but it doesn't matter) of `basic_string<char>(s)`

- it promotes a consistency between the two `bitset` constructors that take a `const std::string&` and a `const char*`, respectively, while practice established by `std::basic_string` would recommend a different set of parameters. In particular, the constructor of `std::basic_string` that takes a `const char*` does not have a `pos` parameter

> Moved to Tentatively Ready with revised wording provided by Alberto Ganesh Babati after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

1. In the synopsis of header `<bitset>` in 20.6 [template.bitset]/1, replace the fourth bitset constructor:

   >     explicit bitset(const char *str);
   >     template <class charT>
   >       explicit bitset(
   >         const charT *str,
   >         typename basic_string<charT>::size_type n = basic_string<charT>::npos,
   >         charT zero = charT('0'), charT one = charT('1'));

2. In 20.6.1 [bitset.cons]/8:

   >     explicit bitset(const char *str);
   >     template <class charT>
   >     explicit
   >     bitset(const charT *str,
   >            typename basic_string<charT>::size_type n = basic_string<charT>::npos,
   >            charT zero = charT('0'), charT one = charT('1'));

   Effects: Constructs an object of class `bitset<N>` as if by bitset(string(str)).

   >     bitset(
   >       n == basic_string<charT>::npos
   >         ? basic_string<charT>(str)
   >         : basic_string<charT>(str, n),
   >       0, n, zero, one)

### 1326. Missing/wrong preconditions for `pair` and `tuple` functions

Section: 20.3.2 [pairs.pair], 20.4.2.1 [tuple.cnstr] Status: Resolved Submitter: Daniel Krügler Opened: 2010-03-07 Last modified: 2015-04-08

View all other issues in [pairs.pair].

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Discussion:

There are several constructors and creation functions of std::tuple that impose requirements on it's arguments, that are unnecessary restrictive and don't match the intention for the supported argument types. This is related to the fact that tuple is supposed to accept both object types and lvalue-references and the usual MoveConstructible and CopyConstructible requirements are bad descriptions for non-const references. Some examples:

1. 20.4.2.1 [tuple.cnstr] before p.4 and p.8, resp.:

   >     explicit tuple(const Types&...);

   > > 4 Requires: Each type in `Types` shall be copy constructible.

   >     tuple(const tuple& u) = default;

   > > 8 Requires: Each type in `Types` shall satisfy the requirements of `CopyConstructible` (Table 34).

   A tuple that contains lvalue-references to non-const can never satisfy the `CopyConstructible` requirements. `CopyConstructible` requirements refine the `MoveConstructible` requirements and this would require that these lvalue-references could bind rvalues. But the core language does not allow that. Even, if we would interpret that requirement as referring to the underlying non-reference type, this requirement would be wrong as well, because there is no reason to disallow a type such as

   >     struct NoMoveNoCopy {
   >       NoMoveNoCopy(NoMoveNoCopy&&) = delete;
   >       NoMoveNoCopy(const NoMoveNoCopy&) = delete;
   >       ...
   >     }:

   for the instantiation of `std::tuple<NoMoveNoCopy&>` and that of it's copy constructor.

   A more reasonable requirement for this example would be to require that "`is_constructible<Ti, const Ti&>::value` shall evaluate to true for all `Ti` in `Types`". In this case the special reference-folding and const-merging rules of references would make this well-formed in all cases. We could also add the further constraint "if `Ti` is an object type, it shall satisfy the `CopyConstructible` requirements", but this additional requirement seems not really to help here. Ignoring it would only mean that if a user would provide a curious object type `C` that satisfies the `std::is_constructible<C, const C&>` test, but not the "`C` is `CopyConstructible`" test would produce a `tuple<C>` that does not satisfy the `CopyConstructible` requirements as well.

2. 20.4.2.1 [tuple.cnstr] before p.6 and p.10, resp.:

   >     template <class... UTypes>
   >     explicit tuple(UTypes&&... u);

   > > 6 Requires: Each type in `Types` shall satisfy the requirements of `MoveConstructible` (Table 33) from the corresponding type in `UTypes`. `sizeof...(Types) == sizeof...(UTypes)`.

   >     tuple(tuple&& u);

   > > 10 Requires: Each `type` in `Types` shall shall satisfy the requirements of `MoveConstructible` (Table 33).

   We have a similar problem as in (a): Non-const lvalue-references are intended template arguments for `std::tuple`, but cannot satisfy the `MoveConstructible` requirements. In this case the correct requirements would be

   > `is_constructible<Ti, Ui>::value` shall evaluate to true for all `Ti` in `Types` and for all `Ui` in `UTypes`

   and

   > `is_constructible<Ti, Ti>::value` shall evaluate to true for all `Ti` in `Types`

   respectively.

Many `std::pair` member functions do not add proper requirements, e.g. the default c'tor does not require anything. This is corrected within the suggested resolution. Further-on the P/R has been adapted to the FCD numbering.

[ 2010-03-25 Daniel updated wording: ]

> The issue became updated to fix some minor inconsistencies and to ensure a similarly required fix for `std::pair`, which has the same specification problem as `std::tuple`, since `pair` became extended to support reference members as well.

[Original proposed resolution:]

1. Change 20.3.2 [pairs.pair]/1 as indicated [The changes for the effects elements are not normative changes, they just ensure harmonization with existing wording style]:

   >     constexpr pair();

   > > Requires: `first_type` and `second_type` shall satisfy the `DefaultConstructible` requirements.

   > > 1 Effects: Value-initializes `first` and `second`.Initializes its members as if implemented: `pair() : first(), second() { }`.

2. Change 20.3.2 [pairs.pair]/2 as indicated:

   >     pair(const T1& x, const T2& y);

   > > Requires: `is_constructible<T1, const T1&>::value` is `true` and `is_constructible<T2, const T2&>::value` is `true`.

   > > 2 Effects: The constructor initializes `first` with `x` and `second` with `y`.

3. Change 20.3.2 [pairs.pair]/3 as indicated:

   >     template<class U, class V> pair(U&& x, V&& y);

   > > Requires: `is_constructible<first_type, U>::value` is `true` and `is_constructible<second_type, V>::value` is `true`.

   > > 3 Effects: The constructor initializes `first` with `std::forward<U>(x)` and `second` with `std::forward<V>(y)`.

   > > 4 Remarks: If `U` is not implicitly convertible to `first_type` or `V` is not implicitly convertible to `second_type` this constructor shall not participate in overload resolution.

4. Change 20.3.2 [pairs.pair]/5 as indicated [The change in the effects element should be non-normatively and is in compatible to the change suggestion of 1324]:

   >     template<class U, class V> pair(const pair<U, V>& p);

   > > Requires: `is_constructible<first_type, const U&>::value` is `true` and `is_constructible<second_type, const V&>::value` is `true`.

   > > 5 Effects: Initializes members from the corresponding members of the argument, performing implicit conversions as needed.

5. Change 20.3.2 [pairs.pair]/6 as indicated:

   >     template<class U, class V> pair(pair<U, V>&& p);

   > > Requires: `is_constructible<first_type, U>::value` is `true` and `is_constructible<second_type, V>::value` is `true`.

   > > 6 Effects: The constructor initializes `first` with `std::moveforward<U>(p.first)` and `second` with `std::moveforward<V>(p.second)`.

6. Change 20.3.2 [pairs.pair]/7+8 as indicated [The deletion in the effects element should be non-normatively]:

   >     template<class... Args1, class... Args2>
   >       pair(piecewise_construct_t,
   >            tuple<Args1...> first_args, tuple<Args2...> second_args);

   > > 7 Requires: `is_constructible<first_type, Args1...>::value` is `true` and `is_constructible<second_type, Args2...>::value` is `true`. All the types in `Args1` and `Args2` shall be `CopyConstructible` (Table 35). `T1` shall be constructible from `Args1`. `T2` shall be constructible from `Args2`.

   > > 8 Effects: The constructor initializes `first` with arguments of types `Args1...` obtained by forwarding the elements of `first_args` and initializes `second` with arguments of types `Args2...` obtained by forwarding the elements of `second_args`. (Here, forwarding an element `x` of type `U` within a `tuple` object means calling `std::forward<U>(x)`.) This form of construction, whereby constructor arguments for `first` and `second` are each provided in a separate `tuple` object, is called piecewise construction.

7. Change 20.3.2 [pairs.pair] before 12 as indicated:

   >     pair& operator=(pair&& p);

   > > Requires: `first_type` and `second_type` shall satisfy the `MoveAssignable` requirements.

   > > 12 Effects: Assigns to `first` with `std::move(p.first)` and to `second` with `std::move(p.second)`.

   > > 13 Returns: `*this`.

8. Change [pairs.pair] before 14 as indicated: [The heterogeneous usage of MoveAssignable is actually not defined, but the library uses it at several places, so we follow this tradition until a better term has been agreed on. One alternative could be to write "first_type shall be assignable from an rvalue of U [..]"]

   >     template<class U, class V> pair& operator=(pair<U, V>&& p);

   > > Requires: `first_type` shall be `MoveAssignable` from `U` and `second_type` shall be `MoveAssignable` from `V`.

   > > 14 Effects: Assigns to `first` with `std::move(p.first)` and to `second` with `std::move(p.second)`.

   > > 15 Returns: `*this`.

9. Change 20.4.2.1 [tuple.cnstr]/4+5 as indicated:

   >     explicit tuple(const Types&...);

   > > 4 Requires: `is_constructible<Ti, const Ti&>::value == true` for e Each type `Ti` in `Types` shall be copy constructible.

   > > 5 Effects: Copy i Initializes each element with the value of the corresponding parameter.

10. Change 20.4.2.1 [tuple.cnstr]/6 as indicated:

    >     template <class... UTypes>
    >     explicit tuple(UTypes&&... u);

    > > 6 Requires: `is_constructible<Ti, Ui>::value == true` for e Each type `Ti` in `Types` shall satisfy the requirements of `MoveConstructible` (Table 33) from and for the corresponding type `Ui` in `UTypes`. `sizeof...(Types) == sizeof...(UTypes)`.

    > > 7 Effects: Initializes the elements in the `tuple` with the corresponding value in `std::forward<UTypes>(u)`.

11. Change 20.4.2.1 [tuple.cnstr]/8+9 as indicated:

    >     tuple(const tuple& u) = default;

    > > 8 Requires: `is_constructible<Ti, const Ti&>::value == true` for e Each type `Ti` in `Types` shall satisfy the requirements of `CopyConstructible`(Table 34).

    > > 9 Effects: Initializes Copy constructs each element of `*this` with the corresponding element of `u`.

12. Change 20.4.2.1 [tuple.cnstr]/10+11 as indicated:

    >     tuple(tuple&& u);

    > > 10 Requires: Let `i` be in `[0, sizeof...(Types))` and let `Ti` be the `i`th type in `Types`. Then `is_constructible<Ti, Ti>::value` shall be `true` for all `i`. Each type in `Types` shall shall satisfy the requirements of `MoveConstructible` (Table 34).

    > > 11 Effects: For each `Ti` in `Types`, initializes the `i`th Move-constructs each element of `*this` with the corresponding element of `std::forward<Ti>(get<i>(``u``))`.

13. Change 20.4.2.1 [tuple.cnstr]/15+16 as indicated:

    >     template <class... UTypes> tuple(tuple<UTypes...>&& u);

    > > 15 Requires: Let `i` be in `[0, sizeof...(Types))`, `Ti` be the `i`th type in `Types`, and `Ui` be the `i`th type in `UTypes`. Then `is_constructible<Ti, Ui>::value` shall be `true` for all `i`. Each type in `Types` shall shall satisfy the requirements of `MoveConstructible` (Table 34) from the corresponding type in `UTypes`. `sizeof...(Types) == sizeof...(UTypes)`.

    > > 16 Effects: For each type `Ti`, initializes the `i`th Move-constructs each element of `*this` with the corresponding element of `std::forward<Ui>(get<i>(``u``))`.

14. Change 20.4.2.1 [tuple.cnstr]/19+20 as indicated:

    >     template <class U1, class U2> tuple(pair<U1, U2>&& u);

    > > 19 Requires: `is_constructible<T1, U1>::value == true` for `t` `T`he first type `T1` in `Types` shall shall satisfy the requirements of `MoveConstructible`(Table 33) from `U1` and `is_constructible<T2, U2>::value == true` for the second type `T2` in `Types` shall be move-constructible from `U2`. `sizeof...(Types) == 2`.

    > > 20 Effects: Initializes Constructs the first element with `std::forward<U1>move(u.first)` and the second element with `std::forward<U2>move(u.second)`.

15. Change 20.4.2.4 [tuple.creation]/9-16 as indicated:

    >     template <class... TTypes, class... UTypes>
    >     tuple<TTypes..., UTypes...> tuple_cat(const tuple<TTypes...>& t, const tuple<UTypes...>& u);

    > > 9 Requires: `is_constructible<Ti, const Ti&>::value == true` for each type `Ti` All the types in `TTypes` shall be `CopyConstructible` (Table 34). `is_constructible<Ui, const Ui&>::value == true` for each type `Ui` All the types in `UTypes` shall be `CopyConstructible` (Table 34).

    > > 10 Returns: A `tuple` object constructed by initializing copy constructing its first `sizeof...(TTypes)` elements from the corresponding elements of `t` and initializing copy constructing its last `sizeof...(UTypes)` elements from the corresponding elements of `u`.

    >     template <class... TTypes, class... UTypes>
    >     tuple<TTypes..., UTypes...> tuple_cat(tuple<TTypes...>&& t, const tuple<UTypes...>& u);

    > > 11 Requires: Let `i` be in `[0, sizeof...(TTypes))`, `Ti` be the `i`th type in `Types`, `j` be in `[0, sizeof...(UTypes))`, and `Uj` be the `j`th type in `UTypes`. `is_constructible<Ti, Ti>::value` shall be `true` for each type `Ti` and `is_constructible<Uj, const Uj&>::value` shall be `true` for each type `Uj` All the types in `TTypes` shall be `MoveConstructible` (Table 34). All the types in `UTypes` shall be `CopyConstructible` (Table 35).

    > > 12 Returns: A `tuple` object constructed by initializing the `i`th element with `std::forward<Ti>(get<i>(t))` for all `Ti` in `TTypes` and initializing the `(j+sizeof...(TTypes))`th element with `get<j>(u)` for all `Uj` in `UTypes`. move constructing its first `sizeof...(TTypes)` elements from the corresponding elements of `t` and copy constructing its last `sizeof...(UTypes)` elements from the corresponding elements of `u`.

    >     template <class... TTypes, class... UTypes>
    >     tuple<TTypes..., UTypes...> tuple_cat(const tuple<TTypes...>& t, tuple<UTypes...>&& u);

    > > 13 Requires: Let `i` be in `[0, sizeof...(TTypes))`, `Ti` be the `i`th type in `Types`, `j` be in `[0, sizeof...(UTypes))`, and `Uj` be the `j`th type in `UTypes`. `is_constructible<Ti, const Ti&>::value` shall be `true` for each type `Ti` and `is_constructible<Uj, Uj>::value` shall be `true` for each type `Uj` All the types in `TTypes` shall be `CopyConstructible` (Table 35). All the types in `UTypes` shall be `MoveConstructible` (Table 34).

    > > 14 Returns: A `tuple` object constructed by initializing the `i`th element with `get<i>(t)` for each type `Ti` and initializing the `(j+sizeof...(TTypes))`th element with `std::forward<Uj>(get<j>(u))` for each type `Uj` copy constructing its first `sizeof...(TTypes)` elements from the corresponding elements of `t` and move constructing its last `sizeof...(UTypes)` elements from the corresponding elements of `u`.

    >     template <class... TTypes, class... UTypes>
    >     tuple<TTypes..., UTypes...> tuple_cat(tuple<TTypes...>&& t, tuple<UTypes...>&& u);

    > > 15 Requires: Let `i` be in `[0, sizeof...(TTypes))`, `Ti` be the `i`th type in `Types`, `j` be in `[0, sizeof...(UTypes))`, and `Uj` be the `j`th type in `UTypes`. `is_constructible<Ti, Ti>::value` shall be `true` for each type `Ti` and `is_constructible<Uj, Uj>::value` shall be `true` for each type `Uj` All the types in `TTypes` shall be `MoveConstructible` (Table 34). All the types in `UTypes` shall be `MoveConstructible` (Table 34).

    > > 16 Returns: A `tuple` object constructed by initializing the `i`th element with `std::forward<Ti>(get<i>(t))` for each type `Ti` and initializing the `(j+sizeof...(TTypes))`th element with `std::forward<Uj>(get<j>(u))` for each type `Uj` move constructing its first `sizeof...(TTypes)` elements from the corresponding elements of `t` and move constructing its last `sizeof...(UTypes)` elements from the corresponding elements of `u`.

[ 2010-10-24 Daniel adds: ]

> Accepting n3140 would solve this issue.

Proposed resolution:

See n3140.

### 1327. templates defined in `<cmath>` replacing C macros with the same name

Section: 26.8 [c.math] Status: Resolved Submitter: Michael Wong Opened: 2010-03-07 Last modified: 2015-04-08

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Discussion:

In 26.8 [c.math]p12 The templates defined in `<cmath>` replace the C99 macros with the same names. The templates have the following declarations:

>     namespace std {
>     template <class T> bool signbit(T x);
>     template <class T> int fpclassify(T x);
>     template <class T> bool isfinite(T x);
>     template <class T> bool isinf(T x);
>     template <class T> bool isnan(T x);
>     template <class T> bool isnormal(T x);
>     template <class T> bool isgreater(T x, T y);
>     template <class T> bool isgreaterequal(T x, T y);
>     template <class T> bool isless(T x, T y);
>     template <class T> bool islessequal(T x, T y);
>     template <class T> bool islessgreater(T x, T y);
>     template <class T> bool isunordered(T x, T y);
>     }

and p13:

> 13 The templates behave the same as the C99 macros with corresponding names defined in C99 7.12.3, Classification macros, and C99 7.12.14, Comparison macros in the C standard.

The C Std versions look like this:

> 7.12.14.1/p1:

> > Synopsis

> > 1 `#include <math.h>`

> >     int isgreaterequal(real-floating x, real-floating y);

which is not necessarily the same types as is required by C++ since the two parameters may be different. Would it not be better if it were truly aligned with C?

[ 2010 Pittsburgh: Bill to ask WG-14 if heterogeneous support for the two-parameter macros is intended. ]

[ 2010-09-13 Daniel comments: ]

> I recommend to resolve this issue as NAD Editorial because the accepted resolution for NB comment US-136 by motion 27 does address this.

[ 2010-09-14 Bill comments: ]

> Motion 27 directly addresses LWG 1327 and solves the problem presented there. Moreover, the solution has been aired before WG14 with no dissent. These functions now behave the same for mixed-mode calls in both C and C++

Proposed resolution:

Apply proposed resolution for US-136

### 1328. istream extractors not setting `failbit` if `eofbit` is already set

Section: 27.7.2.1.3 [istream::sentry] Status: Resolved Submitter: Paolo Carlini Opened: 2010-02-17 Last modified: 2015-04-08

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Discussion:

Basing on the recent discussion on the library reflector, see c++std-lib-27728 and follow ups, I hereby formally ask for LWG 419 to be re-opened, the rationale being that according to the current specifications, per n3000, it seems actually impossible to seek away from end of file, contrary to the rationale which led 342 to its closure as NAD. My request is also supported by Martin Sebor, and I'd like also to add, as tentative proposed resolution for the re-opened issue, the wording suggested by Sebor, thus, change the beginning of 27.7.2.1.3 [istream::sentry]/2, to:

> 2 Effects: If `(!noskipws && !is.good())` is `false true`, calls `is.setstate(failbit)`. Otherwise prepares for formatted or unformatted input. ...

[ 2010-10 Batavia ]

Resolved by adopting n3168.

Previous proposed resolution:

Change 27.7.2.1.3 [istream::sentry] p.2:

> 2 Effects: If `(!noskipws && !is.good())` is `false true`, calls `is.setstate(failbit)`. Otherwise prepares for formatted or unformatted input. ...

Proposed resolution:

Addressed by paper n3168.

### 1329. Data races on `vector<bool>`

Section: 23.2.2 [container.requirements.dataraces] Status: Resolved Submitter: Jeffrey Yaskin Opened: 2010-03-09 Last modified: 2015-04-08

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Discussion:

The common implementation of `vector<bool>` is as an unsynchronized bitfield. The addition of 23.2.2 [container.requirements.dataraces]/2 would require either a change in representation or a change in access synchronization, both of which are undesireable with respect to compatibility and performance.

[ 2010 Pittsburgh: Moved to NAD Editorial Resolved. Rationale added below. ]

Rationale:

Solved by N3069.

Proposed resolution:

Container data races 23.2.2 [container.requirements.dataraces]

Paragraph 1 is unchanged as follows:

> 1 For purposes of avoiding data races (17.6.4.8), implementations shall consider the following functions to be `const`: `begin`, `end`, `rbegin`, `rend`, `front`, `back`, `data`, `find`, `lower_bound`, `upper_bound`, `equal_range`, and, except in associative containers, `operator[]`.

Edit paragraph 2 as follows:

> 2 Notwithstanding (17.6.4.8), implementations are required to avoid data races when the contents of the contained object in different elements in the same sequence, excepting `vector<bool>`, are modified concurrently.

Edit paragraph 3 as follows:

> 3 [Note: For a `vector<int> x` with a size greater than one, `x[1] = 5` and `*x.begin() = 10` can be executed concurrently without a data race, but `x[0] = 5` and `*x.begin() = 10` executed concurrently may result in a data race. As an exception to the general rule, for a `vector<bool> y`, `y[i] = true` may race with `y[j] = true`. —end note]

### 1332. Let Hash objects throw!

Section: 17.6.3.4 [hash.requirements] Status: C++11 Submitter: Daniel Krügler Opened: 2010-03-26 Last modified: 2015-04-08

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Discussion:

The currently added `Hash` requirements demand in Table 40 — `Hash` requirements [hash]:

> Table 40 — Hash requirements [hash]
> Expression | Return type | Requirement
> --- | --- | ---
> `h(k)` | `size_t` | Shall not throw exceptions. [..]

While it surely is a generally accepted idea that hash function objects should not throw exceptions, this basic constraint for such a fundamental requirement set does neither match the current library policy nor real world cases:

1. There are little known situations where a swap or move operation may throw an exception and in some popular domains such functions are required not to throw. But the library invested already efforts for good reasons to require "working" container implementations in the presence of throwing move or swap operations, see e.g. 23.2.4.1 [associative.reqmts.except], 23.2.5.1 [unord.req.except].
2. The container library is already specified to cope with potentially throwing comparers, predicates, and hash function objects, see above.

3. The new definition goes beyond the original hash requirements as specified by SGI library in regard to the exception requirement:

   > http://www.sgi.com/tech/stl/HashFunction.html

4. There are indeed real-world examples of potentially throwing hash functions, typically when the proxy pattern is used and when the to-be hashed proxied instance is some volatile object, e.g. a file or internet resource, that might suddenly be unavailable at the time of hashing.
5. With the new `noexcept` language facility libraries can still take advantage of no-throw guarantees of hasher functions with stricter guarantees.

Even though the majority of all known move, swap, and hash functions won't throw and in some cases must not throw, it seems like unnecessary over-constraining the definition of a Hash functor not to propagate exceptions in any case and it contradicts the general principle of C++ to impose such a requirement for this kind of fundamental requirement.

[ 2010-11-11 Daniel asks the working group whether they would prefer a replacement for the second bullet of the proposed resolution (a result of discussing this with Alberto) of the form: ]

Add to 20.9.13 [unord.hash]/1 a new bullet:

> 1 The unordered associative containers defined in Clause 23.5 use specializations of the class template `hash` as the default hash function. For all object types `Key` for which there exists a specialization `hash<Key>`, the instantiation `hash<Key>` shall:

> - satisfy the `Hash` requirements (20.2.4), with `Key` as the function call argument type, the `DefaultConstructible` requirements (33), the `CopyAssignable` requirements (37),
> - be swappable (20.2.2) for lvalues,
> - provide two nested types `result_type` and `argument_type` which shall be synonyms for `size_t` and `Key`, respectively,
> - satisfy the requirement that if `k1 == k2` is true, `h(k1) == h(k2)` is also true, where `h` is an object of type `hash<Key>` and `k1` and `k2` are objects of type `Key`,.
> - satisfy the requirement `noexcept(h(k)) == true`, where `h` is an object of type `hash<Key>` and `k` is an object of type `Key`, unless `hash<Key>` is a user-defined specialization that depends on at least one user-defined type.

[Batavia: Closed as NAD Future, then reopened. See the wiki for Tuesday.]

Proposed resolution:

1. Change Table 26 — `Hash` requirements [tab:hash] as indicated:

   > Table 26 — `Hash` requirements [tab:hash]
   > Expression | Return type | Requirement
   > --- | --- | ---
   > `h(k)` | `size_t` | Shall not throw exceptions. […]

2. Add to 20.9.13 [unord.hash] p. 1 a new bullet:

   > 1 The unordered associative containers defined in Clause 23.5 [unord] use specializations of the class template `hash` as the default hash function. For all object types `Key` for which there exists a specialization `hash<Key>`, the instantiation `hash<Key>` shall:

   > - satisfy the `Hash` requirements ([hash.requirements]), with `Key` as the function call argument type, the `DefaultConstructible` requirements (Table [defaultconstructible]), the `CopyAssignable` requirements (Table [copyassignable]),
   > - be swappable ([swappable.requirements]) for lvalues,
   > - provide two nested types `result_type` and `argument_type` which shall be synonyms for `size_t` and `Key`, respectively,
   > - satisfy the requirement that if `k1 == k2` is true, `h(k1) == h(k2)` is also true, where `h` is an object of type `hash<Key>` and `k1` and `k2` are objects of type `Key`,.
   > - satisfy the requirement that the expression `h(k)`, where `h` is an object of type `hash<Key>` and `k` is an object of type `Key`, shall not throw an exception, unless `hash<Key>` is a user-defined specialization that depends on at least one user-defined type.

### 1333. Missing forwarding during `std::function` invocation

Section: 20.9.12.2.4 [func.wrap.func.inv] Status: C++11 Submitter: Daniel Krügler Opened: 2010-03-26 Last modified: 2015-04-08

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Discussion:

The current wording of 20.9.12.2.4 [func.wrap.func.inv]/1:

>     R operator()(ArgTypes... args) const

> > Effects: `INVOKE(f, t1, t2, ..., tN, R)` (20.8.2), where `f` is the target object (20.8.1) of `*this` and `t1, t2, ..., tN` are the values in `args...`.

uses an unclear relation between the actual args and the used variables `ti`. It should be made clear, that `std::forward` has to be used to conserve the expression lvalueness.

[ Post-Rapperswil: ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 20.9.12.2.4 [func.wrap.func.inv]/1+2 as indicated:

>     R operator()(ArgTypes... args) const

> > 1 Effects:: `INVOKE(f, std::forward<ArgTypes>(args)...t1, t2, ..., tN, R)` (20.8.2), where `f` is the target object (20.8.1) of `*this` and `t1, t2, ..., tN` are the values in `args...`.

> > 2 Returns: Nothing if `R` is `void`, otherwise the return value of `INVOKE(f, std::forward<ArgTypes>(args)...t1, t2, ..., tN, R)`.

> > 3 Throws: `bad_function_call` if `!*this`; otherwise, any exception thrown by the wrapped callable object.

### 1334. Insert iterators are broken for some proxy containers compared to C++03

Section: 24.5.2.2.2 [back.insert.iter.op=], 24.5.2.4.2 [front.insert.iter.op=], X [insert.insert.iter.op=] Status: C++11 Submitter: Daniel Krügler Opened: 2010-03-28 Last modified: 2015-04-08

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Discussion:

In C++03 this was valid code:

>     #include <vector>
>     #include <iterator>

>     int main() {
>       typedef std::vector<bool> Cont;
>       Cont c;
>       std::back_insert_iterator<Cont> it = std::back_inserter(c);
>       *it = true;
>     }

In C++0x this code does no longer compile because of an ambiguity error for this `operator=` overload pair:

>     back_insert_iterator<Container>&
>     operator=(typename Container::const_reference value);

>     back_insert_iterator<Container>&
>     operator=(typename Container::value_type&& value);

This is so, because for proxy-containers like `std::vector<bool>` the `const_reference` usually is a non-reference type and in this case it's identical to `Container::value_type`, thus forming the ambiguous overload pair

>     back_insert_iterator<Container>&
>     operator=(bool value);

>     back_insert_iterator<Container>&
>     operator=(bool&& value);

The same problem exists for `std::back_insert_iterator`, `std::front_insert_iterator`, and `std::insert_iterator`.

One possible fix would be to require that `const_reference` of a proxy container must not be the same as the `value_type`, but this would break earlier valid code. The alternative would be to change the first signature to

>     back_insert_iterator<Container>&
>     operator=(const typename Container::const_reference& value);

This would have the effect that this signature always expects an lvalue or rvalue, but it would not create an ambiguity relative to the second form with rvalue-references. [For all non-proxy containers the signature will be the same as before due to reference-collapsing and const folding rules]

[ Post-Rapperswil ]

This problem is not restricted to the unspeakable `vector<bool>`, but is already existing for other proxy containers like gcc's `rope` class. The following code does no longer work ([Bug libstdc++/44963]):

>     #include <iostream>
>     #include <ext/rope>

>     using namespace std;

>     int main()
>     {
>          __gnu_cxx::crope line("test");
>          auto ii(back_inserter(line));

>          *ii++ = 'm'; // #1
>          *ii++ = 'e'; // #2

>          cout << line << endl;
>     }

Both lines marked with #1 and #2 issue now an error because the library has properly implemented the current wording state (Thanks to Paolo Calini for making me aware of this real-life example).

The following P/R is a revision of the orignal P/R and was initially suggested by Howard Hinnant. Paolo verified that the approach works in gcc.

> Moved to Tentatively Ready with revised wording after 6 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

The wording refers to N3126.

1. Change [back.insert.iterator], class `back_insert_iterator` synopsis as indicated:

   >     template <class Container>
   >     class back_insert_iterator :
   >      public iterator<output_iterator_tag,void,void,void,void> {
   >     protected:
   >      Container* container;
   >     public:
   >      [..]
   >      back_insert_iterator<Container>&
   >        operator=(const typename Container::const_referencevalue_type& value);
   >      back_insert_iterator<Container>&
   >        operator=(typename Container::value_type&& value);
   >      [..]
   >     };

2. Change [back.insert.iter.op=] before p. 1 as indicated:

   >     back_insert_iterator<Container>&
   >        operator=(const typename Container::const_referencevalue_type& value);

   > > 1 Effects: `container->push_back(value)`;
   > > 2 Returns: `*this`.

3. Change [front.insert.iterator], class `front_insert_iterator` synposis as indicated:

   >     template <class Container>
   >     class front_insert_iterator :
   >      public iterator<output_iterator_tag,void,void,void,void> {
   >     protected:
   >      Container* container;
   >     public:
   >      [..]
   >      front_insert_iterator<Container>&
   >        operator=(const typename Container::const_referencevalue_type& value);
   >      front_insert_iterator<Container>&
   >        operator=(typename Container::value_type&& value);
   >      [..]
   >     };

4. Change [front.insert.iter.op=] before p.1 as indicated:

   >     front_insert_iterator<Container>&
   >        operator=(const typename Container::const_referencevalue_type& value);

   > > 1 Effects: `container->push_front(value)`;
   > > 2 Returns: `*this`.

5. Change [insert.iterator], class insert_iterator synopsis as indicated:

   >     template <class Container>
   >        class insert_iterator :
   >          public iterator<output_iterator_tag,void,void,void,void> {
   >        protected:
   >          Container* container;
   >          typename Container::iterator iter;
   >        public:
   >          [..]
   >          insert_iterator<Container>&
   >            operator=(const typename Container::const_referencevalue_type& value);
   >          insert_iterator<Container>&
   >            operator=(typename Container::value_type&& value);
   >          [..]
   >        };

6. Change [insert.iter.op=] before p. 1 as indicated:

   >     insert_iterator<Container>&
   >         operator=(const typename Container::const_referencevalue_type& value);

   > > 1 Effects:

   > > >       iter = container->insert(iter, value);
   > > >       ++iter;

   > > 2 Returns: `*this`.

### 1335. Insufficient requirements for `tuple::operator<()`

Section: 20.4.2.7 [tuple.rel] Status: C++11 Submitter: Joe Gottman Opened: 2010-05-15 Last modified: 2015-04-08

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Discussion:

The requirements section for `std::tuple` says the following:

> Requires: For all `i`, where `0 <= i and i < sizeof...(Types)`, `get<i>(t) < get<i>(u)` is a valid expression returning a type that is convertible to `bool`. `sizeof...(TTypes) == sizeof...(UTypes)`.

This is necessary but not sufficient, as the algorithm for comparing `tuple`s also computes `get<i>(u) < get<i>(t)` (note the order)

[ Post-Rapperswil ]

> Moved to Tentatively Ready with updated wording correcting change-bars after 6 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

1. Change [tuple.rel] before p. 4 as indicated [Remark to the editor: This paragraph doesn't have a number yet, but it seems to me as if it should have one]:

   >     template<class... TTypes, class... UTypes>
   >     bool operator<(const tuple<TTypes...>& t, const tuple<UTypes...>& u);

   > > Requires: For all `i`, where `0 <= i` and `i < sizeof...(Types)`, `get<i>(t) < get<i>(u)` and `get<i>(u) < get<i>(t)`is a valid expression returning a type that is are valid expressions returning types that are convertible to `bool`. `sizeof...(TTypes) == sizeof...(UTypes)`.

### 1337. Swapped arguments in `regex_traits::isctype`

Section: 28.7 [re.traits] Status: C++11 Submitter: Howard Hinnant Opened: 2010-06-21 Last modified: 2015-04-08

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Discussion:

28.7 [re.traits]/12 describes `regex_traits::isctype` in terms of `ctype::is(c, m)`, where `c` is a `charT` and `m` is a `ctype_base::mask`. Unfortunately 22.4.1.1.1 [locale.ctype.members] specifies this function as `ctype::is(m, c)`

[ Post-Rapperswil: ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 28.7 [re.traits] p.12:

>     bool isctype(charT c, char_class_type f) const;

> > 11 ...

> > 12 Returns: Converts `f` into a value `m` of type `std::ctype_base::mask` in an unspecified manner, and returns `true` if `use_facet<ctype<charT> >(getloc()).is(cm, mc)` is `true`. Otherwise returns `true` if `f` bitwise or'ed with the result of calling `lookup_classname` with an iterator pair that designates the character sequence `"w"` is not equal to 0 and `c == '_'`, or if `f` bitwise or'ed with the result of calling `lookup_classname` with an iterator pair that designates the character sequence `"blank"` is not equal to 0 and `c` is one of an implementation-defined subset of the characters for which `isspace(c, getloc())` returns `true`, otherwise returns `false`.

### 1338. LWG 1205 incorrectly applied

Section: 25.2.13 [alg.search] Status: C++11 Submitter: Howard Hinnant Opened: 2010-06-25 Last modified: 2015-04-08

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Discussion:

LWG 1205 (currently in WP) clarified the return value of several algorithms when dealing with empty ranges. In particular it recommended for 25.2.13 [alg.search]:

>     template<class ForwardIterator1, class ForwardIterator2>
>       ForwardIterator1
>         search(ForwardIterator1 first1, ForwardIterator1 last1,
>                ForwardIterator2 first2, ForwardIterator2 last2);

>     template<class ForwardIterator1, class ForwardIterator2,
>              class BinaryPredicate>
>       ForwardIterator1
>         search(ForwardIterator1 first1, ForwardIterator1 last1,
>                ForwardIterator2 first2, ForwardIterator2 last2,
>                BinaryPredicate pred);

> > 1 Effects: ...

> > 2 Returns: ... Returns `last1` if no such iterator is found.

> > 3 Remarks: Returns `first1` if `[first2,last2)` is empty.

Unfortunately this got translated to an incorrect specification for what gets returned when no such iterator is found (N3092):

>     template<class ForwardIterator1, class ForwardIterator2>
>       ForwardIterator1
>         search(ForwardIterator1 first1, ForwardIterator1 last1,
>                ForwardIterator2 first2, ForwardIterator2 last2);

>     template<class ForwardIterator1, class ForwardIterator2,
>              class BinaryPredicate>
>       ForwardIterator1
>         search(ForwardIterator1 first1, ForwardIterator1 last1,
>                ForwardIterator2 first2, ForwardIterator2 last2,
>                BinaryPredicate pred);

> > 1 Effects: ...

> > 2 Returns: ... Returns `first1` if `[first2,last2)` is empty or if no such iterator is found.

LWG 1205 is correct and N3092 is not equivalent nor correct.

I have not reviewed the other 10 recommendations of 1205.

[ Post-Rapperswil ]

> It was verified that none of the remaining possibly affected algorithms does have any similar problems and a concrete P/R was added that used a similar style as has been applied to the other cases.

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

1. Change [alg.search] p.2 as indicated:

   >     template<class ForwardIterator1, class ForwardIterator2>
   >       ForwardIterator1
   >         search(ForwardIterator1 first1, ForwardIterator1 last1,
   >                ForwardIterator2 first2, ForwardIterator2 last2);

   >     template<class ForwardIterator1, class ForwardIterator2,
   >                 class BinaryPredicate>
   >       ForwardIterator1
   >         search(ForwardIterator1 first1, ForwardIterator1 last1,
   >                ForwardIterator2 first2, ForwardIterator2 last2,
   >                BinaryPredicate pred);

   > > 1 - [...]

   > > 2 - Returns: The first iterator `i` in the range `[first1,last1 - (last2-first2))` such that for any nonnegative integer `n` less than `last2 - first2` the following corresponding conditions hold: `*(i + n) == *(first2 + n)`, `pred(*(i + n), *(first2 + n)) != false`. Returns `first1` if `[first2,last2)` is empty or, otherwise returns `last1` if no such iterator is found.

### 1339. `uninitialized_fill_n` should return the end of its range

Section: 20.7.12.4 [uninitialized.fill.n] Status: C++11 Submitter: Jared Hoberock Opened: 2010-07-14 Last modified: 2015-04-08

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Discussion:

N3092's specification of `uninitialized_fill_n` discards useful information and is inconsistent with other algorithms such as `fill_n` which accept an iterator and a size. As currently specified, `unintialized_fill_n` requires an additional linear traversal to find the end of the range.

Instead of returning `void`, `unintialized_fill_n` should return one past the last iterator it dereferenced.

[ Post-Rapperswil: ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

In section 20.7 [memory] change:,

>     template <class ForwardIterator, class Size, class T>
>       void ForwardIterator uninitialized_fill_n(ForwardIterator first, Size n, const T& x);

In section 20.7.12.4 [uninitialized.fill.n] change,

>     template <class ForwardIterator, class Size, class T>
>       void ForwardIterator uninitialized_fill_n(ForwardIterator first, Size n, const T& x);

> > 1 Effects:

> > >     for (; n--; ++first)
> > >       ::new (static_cast<void*>(&*first))
> > >         typename iterator_traits<ForwardIterator>::value_type(x);
> > >     return first;

### 1340. Why does `forward_list::resize` take the object to be copied by value?

Section: 23.3.4.5 [forwardlist.modifiers] Status: C++11 Submitter: James McNellis Opened: 2010-07-16 Last modified: 2015-04-08

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Discussion:

In N3092 23.3.4.5 [forwardlist.modifiers], the `resize()` member function is declared as:

>     void resize(size_type sz, value_type c);

The other sequence containers (`list`, `deque`, and `vector`) take `'c'` by const reference.

Is there a reason for this difference? If not, then `resize()` should be declared as:

>     void resize(size_type sz, const value_type& c);

The declaration would need to be changed both at its declaration in the class definition at 23.3.4 [forwardlist]/3 and where its behavior is specified at 23.3.4.5 [forwardlist.modifiers]/22.

This would make `forward_list` consistent with the CD1 issue 679.

[ Post-Rapperswil ]

Daniel changed the P/R slightly, because one paragraph number has been changed since the issue had been submitted. He also added a similar Requires element that exists in all other containers with a `resize` member function. He deliberately did not touch the wrong usage of "default-constructed" because that will be taken care of by LWG issue 868.

> Moved to Tentatively Ready with revised wording after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

1. Change [forwardlist]/3, class template `forward_list` synopsis, as indicated:

   >     ...
   >     void resize(size_type sz);
   >     void resize(size_type sz, const value_type& c);
   >     void clear();
   >     ...

2. Change [forwardlist.modifiers]/27 as indicated:

   >     void resize(size_type sz);
   >     void resize(size_type sz, const value_type& c);

   > > 27 Effects: If `sz < distance(begin(), end())`, erases the last `distance(begin(), end()) - sz` elements from the list. Otherwise, inserts `sz - distance(begin(), end())` elements at the end of the list. For the first signature the inserted elements are default constructed, and for the second signature they are copies of `c`.

   > > 28 - Requires: `T` shall be `DefaultConstructible` for the first form and it shall be `CopyConstructible` for the second form.

### 1344. Replace `throw()` with `noexcept`

Section: 17 [library] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Duplicate of: 1351

Discussion:

Addresses GB-60, CH-16

Dynamic exception specifications are deprecated; the library should recognise this by replacing all non-throwing exception specifications of the form `throw()` with the `noexcept` form.

[ Resolution proposed by ballot comment: ]

Replace all non-throwing exception specifications of the form 'throw()' with the 'noexcept' form.

[ 2010-10-31 Daniel comments: ]

> The proposed resolution of n3148 would satisfy this request.

[ 2010-Batavia: ]

> Resolved by adopting n3148.

Proposed resolution:

See n3148 See n3150 See n3195 See n3155 See n3156

### 1345. Library classes should have `noexcept` move operations

Section: 17 [library] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-61

All library types should have non-throwing move constructors and move-assignment operators unless wrapping a type with a potentially throwing move operation. When such a type is a class-template, these operations should have a conditional `noexcept` specification.

There are many other places where a `noexcept` specification may be considered, but the move operations are a special case that must be called out, to effectively support the `move_if_noexcept` function template.

[ Resolution proposed by ballot comment: ]

Review every class and class template in the library. If `noexcept` move constructor/assignment operators can be implicitly declared, then they should be implicitly declared, or explicitly defaulted. Otherwise, a move constructor/move assignment operator with a `noexcept` exception specification should be provided.

[ 2010-10-31 Daniel comments: ]

> The proposed resolution of n3157 would satisfy this request.

[2011-03-24 Madrid meeting]

Resolved by papers to be listed later

Proposed resolution:

See n3157

### 1346. Apply `noexcept` where library specification does not permit exceptions

Section: 17 [library] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Duplicate of: 1352

Discussion:

Addresses GB-62, CH-17

Issues with efficiency and unsatisfactory semantics mean many library functions document they do not throw exceptions with a Throws: Nothing clause, but do not advertise it with an exception specification. The semantic issues are largely resolved with the new 'noexcept' specifications, and the noexcept operator means we will want to detect these guarantees programatically in order to construct programs taking advantage of the guarantee.

[ Resolution proposed by ballot comment: ]

Add a `noexcept` exception specification on each libary API that offers an unconditional Throws: Nothing guarantee. Where the guarantee is conditional, add the appropriate `noexcept(constant-expression)` if an appropriate constant expression exists.

[ 2010-10-31 Daniel comments: ]

> The proposed resolution of n3149 would satisfy this request.

[ 2010-Batavia: ]

> Resolved by adopting n3149.

Proposed resolution:

This issue is resolved by the adoption of n3195

### 1347. Apply `noexcept` judiciously throughout the library

Section: 17 [library] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-63, US-80

Since the newly introduced operator `noexcept` makes it easy (easier than previously) to detect whether or not a function has been declared with the empty exception specification (including `noexcept`) library functions that cannot throw should be decorated with the empty exception specification. Failing to do so and leaving it as a matter of QoI would be detrimental to portability and efficiency.

[ Resolution proposed by ballot comment ]

Review the whole library, and apply the `noexcept` specification where it is appropriate.

[ 2010-10-31 Daniel comments: ]

> The proposed resolution of the combination of n3155, n3156, n3157, n3167 would satisfy this request. The paper n3150 is related to this as well.

[ 2010 Batavia: ]

While the LWG expects to see further papers in this area, sufficient action was taken in Batavia to close the issue as Resolved by the listed papers.

Proposed resolution:

See n3155, n3156, n3157, n3167 and remotely n3150

### 1349. `swap` should not throw

Section: 17 [library] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-65

Nothrowing `swap` operations are key to many C++ idioms, notably the common copy/swap idiom to provide the strong exception safety guarantee.

[ Resolution proposed by ballot comment ]

Where possible, all library types should provide a `swap` operation with an exception specification guaranteeing no exception shall propagate. Where `noexcept(true)` cannot be guaranteed to not terminate the program, and the `swap` in questions is a template, an exception specification with the appropriate conditional expression could be specified.

[2011-03-13: Daniel comments and drafts wording]

During a survey of the library some main categories for potential `noexcept` `swap` function could be isolated:

1. Free `swap` functions that are specified in terms of already `noexcept` `swap` member functions, like that of `valarray`.
2. Free `swap` of `std::function`, and member and free `swap` functions of stream buffers and streams where considered but rejected as good candidates, because of the danger to potentially impose requirements of existing implementations. These functions could be reconsidered as candidates in the future.

Negative list:

1. Algorithms related to swap, like `iter_swap`, have not been touched, because there are no fundamental exceptions constraints on iterator operations in general (only for specific types, like library container iterators)

While evaluating the current state of `swap` functions of library components it was observed that several conditional `noexcept` functions have turned into unconditional ones, e.g. in the header `<utility>` synopsis:

>     template<class T> void swap(T& a, T& b) noexcept;

The suggested resolution shown below also attempts to fix these cases.

[2011-03-22 Daniel redrafts to satisfy new criteria for applying `noexcept`. Parts resolved by N3263-v2 and D3267 are not added here.]

Proposed resolution:

1. Edit 20.2 [utility] p. 2, header `<utility>` synopsis and 20.2.2 [utility.swap] before p. 1, as indicated (The intent is to fix an editorial omission):

   >     template<class T> void swap(T& a, T& b) noexcept(see below);

2. Edit the prototype declaration in 20.3.2 [pairs.pair] before p. 34 as indicated (The intent is to fix an editorial omission):

   >     void swap(pair& p) noexcept(see below);

3. Edit 20.4.1 [tuple.general] p. 2 header `<tuple>` synopsis and 20.4.2.9 [tuple.special] before p. 1 as indicated (The intent is to fix an editorial omission):

   >     template <class... Types>
   >     void swap(tuple<Types...>& x, tuple<Types...>& y) noexcept(see below);

4. Edit 20.4.2 [tuple.tuple], class template `tuple` synopsis and 20.4.2.3 [tuple.swap] before p. 1 as indicated (The intent is to fix an editorial omission):

   >     void swap(tuple&) noexcept(see below);

5. Edit 20.7.2 [memory.syn] p. 1, header `<memory>` synopsis as indicated (The intent is to fix an editorial omission of the proposing paper N3195).

   >     template<class T> void swap(shared_ptr<T>& a, shared_ptr<T>& b) noexcept;

6. Edit header `<valarray>` synopsis, 26.6.1 [valarray.syn] and 26.6.3.4 [valarray.special] before p. 1 as indicated [Drafting comment: The corresponding member swap is already noexcept]:

   >     template<class T> void swap(valarray<T>&, valarray<T>&) noexcept;

### 1353. Clarify the state of a moved-from object

Section: 17 [library] Status: Resolved Submitter: Switzerland Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses CH-18

The general approach on moving is that a library object after moving out is in a "valid but unspecified state". But this is stated at the single object specifications, which is error prone (especially if the move operations are implicit) and unnecessary duplication.

[ Resolution proposed by ballot comment ]

Consider putting a general statement to the same effect into clause 17.

[2010-11-05 Beman provides exact wording. The wording was inspired by Dave Abrahams' message c++std-lib-28958, and refined with help from Alisdair, Daniel, and Howard. ]

[2011-02-25 P/R wording superseded by N3241. ]

Proposed resolution:

Resolved by N3241

### 1354. The definition of deadlock excludes cases involving a single thread

Section: 17.3 [defns.deadlock] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-52

The definition of deadlock in 17.3.7 excludes cases involving a single thread making it incorrect.

[ Resolution proposed by ballot comment ]

The definition should be corrected.

[ 2010 Batavia Concurrency group provides a Proposed Resolution ]

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 17.3 [defns.deadlock] as indicated:

> deadlock

> two one or more threads are unable to continue execution because each is blocked waiting for one or more of the others to satisfy some condition.

### 1355. The definition of move-assignment operator is redundant

Section: X [defns.move.assign.op] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-50

This definition of move-assignment operator is redundant and confusing now that the term move-assignment operator is defined by the core language in subclause 12.8p21.

[ 2010-10-24 Daniel adds: ]

> Accepting n3142 provides a superior resolution.

[ Resolution proposed by ballot comment ]

Strike subclause X [defns.move.assign.op]. Add a cross-reference to (12.8) to 17.3.12.

Proposed resolution:

Resolved by paper n3142.

### 1356. The definition of move-constructor is redundant

Section: X [defns.move.ctor] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-51

This definition of move-constructor is redundant and confusing now that the term constructor is defined by the core language in subclause 12.8p3.

[ 2010-10-24 Daniel adds: ]

> Accepting n3142 provides a superior resolution.

[ 2010 Batavia: resolved as NAD Editorial by adopting paper n3142. ]

Original proposed resolution preserved for reference:

> Strike subclause 17.3.14, [defns.move.ctor]

> > 17.3.14 [defns.move.ctor]
> > move constructor a constructor which accepts only an rvalue argument of the type being constructed and might modify the argument as a side effect during construction.

Proposed resolution:

Resolved by paper n3142.

### 1357. Library bitmask types to not satisfy the bimask type requirements

Section: 17.5.2.1.3 [bitmask.types] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-53

The bitmask types defined in 27.5.2 and 28.5 contradict the bitmask type requirements in 17.5.2.1.3, and have missing or incorrectly defined operators.

[ Resolution proposed by ballot comment ]

See Appendix 1 - Additional Details

[ 2010 - Rapperswil ]

The paper n3110 was made available during the meeting to resolve this comment, but withdrawn from formal motions to give others time to review the document. There was no technical objection, and it is expected that this paper will go forward at the next meeting.

Proposed resolution:

See n3110.

### 1360. Add `<atomic>` to free-standing implementations

Section: 17.6.1.3 [compliance] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-57

The atomic operations facility is closely tied to clause 1 and the memory model. It is not easily supplied as an after-market extension, and should be trivial to implement of a single-threaded serial machine. The consequence of not having this facility will be poor interoperability with future C++ libraries that memory model concerns seriously, and attempt to treat them in a portable way.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Add `<atomic>` to table 15, headers required for a free-standing implementation.

### 1362. Description of binding to rvalue-references should use the new 'xvalue' vocabulary

Section: 17.6.4.9 [res.on.arguments] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-82

17.6.4.9 [res.on.arguments] p.1. b.3: The second Note can benefit by adopting recent nomenclature.

[ Resolution proposed by the ballot comment: ]

Rephrase the Note in terms of xvalue.

[ Pre-Batavia: ]

Walter Brown provides wording.

[Batavia: Immediate]

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Amend the note in 17.6.3.9 [res.on.arguments] p1 bullet 3.

> [ Note: If a program casts an lvalue to an rvalue xvalue while passing that lvalue to a library function (e.g. by calling the function with the argument `move(x)`), the program is effectively asking that function to treat that lvalue as a temporary. The implementation is free to optimize away aliasing checks which might be needed if the argument was anlvalue. — end note]

### 1363. `offsetof` should be marked `noexcept`

Section: 18.2 [support.types] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-68

There is no reason for the offsetof macro to invoke potentially throwing operations, so the result of noexcept(offsetof(type,member-designator)) should be true.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Add to the end of 18.2p4:

> No operation invoked by the offsetof macro shall throw an exception, and `noexcept(offsetof(type,member-designator))` shall be true.

### 1364. It is not clear how `exception_ptr` is synchronized

Section: 18.8.5 [propagation] Status: Resolved Submitter: Switzerland Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses CH-19

It is not clear how `exception_ptr` is synchronized.

[ Resolution proposed by ballot comment ]

Make clear that accessing in different threads multiple `exception_ptr` objects that all refer to the same exception introduce a race.

[2011-03-08: Lawrence comments and drafts wording]

I think fundamentally, this issue is NAD, but clarification would not hurt.

> Proposed resolution

> Add a new paragraph to 18.8.5 [propagation] after paragraph 6 as follows:

> [Note: Exception objects have no synchronization requirements, and expressions using them may conflict (1.10 [intro.multithread]). — end note]

Proposed resolution:

Resolved 2011-03 Madrid meeting by paper N3278

### 1365. Thread-safety of handler functions

Section: 18.6.2 [alloc.errors] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-71

The thread safety of `std::set_new_handler()`, `std::set_unexpected()`, `std::set_terminate()`, is unspecified making the the functions impossible to use in a thread safe manner.

[ Resolution proposed by ballot comment: ]

> The thread safety guarantees for the functions must be specified and new interfaces should be provided to make it possible to query and install handlers in a thread safe way.

[ 2010-10-31 Daniel comments: ]

> The proposed resolution of n3122 partially addresses this request. This issue is related to 1366.

[ 2010-Batavia: ]

> Resolved by adopting n3189.

Proposed resolution:

Resolved in Batavia by accepting n3189.

### 1366. New-handler and data races

Section: 18.6.1.4 [new.delete.dataraces] Status: Resolved Submitter: DIN Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses DE-14

It is unclear how a user replacement function can simultaneously satisfy the race-free conditions imposed in this clause and query the new-handler in case of a failed allocation with the only available, mutating interface `std::set_new_handler`.

[ Resolution proposed by ballot comment: ]

Offer a non-mutating interface to query the current new-handler.

[ 2010-10-24 Daniel adds: ]

> Accepting n3122 would solve this issue. This issue is related to 1365.

[ 2010-Batavia: ]

> Resolved by adopting n3189.

Proposed resolution:

Resolved in Batavia by accepting n3189.

### 1367. Deprecate library support for checking dynamic exception specifications

Section: D.8 [exception.unexpected] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-72

Dynamic exception specifications are deprecated, so clause 18.8.2 that describes library support for this facility should move to Annex D, with the exception of the `bad_exception` class which is retained to indicate other failures in the exception dispatch mechanism (e.g. calling `current_exception()`).

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

With the exception of 18.8.2.1 [bad.exception], move clause 18.8.2 directly to Annex D. [bad.exception] should simply become the new 18.8.2.

### 1368. Thread safety of `std::uncaught_exception()`

Section: X [uncaught] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-73

The thread safety `std::uncaught_exception()` and the result of the function when multiple threads throw exceptions at the same time are unspecified. To make the function safe to use in the presence of exceptions in multiple threads the specification needs to be updated.

[ Resolution proposed by ballot comment ]

Update this clause to support safe calls from multiple threads without placing synchronization requirements on the user.

[ 2010 Batavia Concurrency group provides a Proposed Resolution ]

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change X [uncaught] p. 1 as follows:

Returns: `true` after the current thread has initialized initializing an exception object (15.1) until a handler for the exception (including `unexpected()` or `terminate()`) is activated (15.3). [ Note: This includes stack unwinding (15.2). — end note ]

### 1370. `throw_with_nested` should not use perfect forwarding

Section: 18.8.6 [except.nested] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-84

The `throw_with_nested` specification passes in its argument as `T&&` (perfect forwarding pattern), but then discusses requirements on `T` without taking into account that `T` may be an lvalue-reference type. It is also not clear in the spec that `t` is intended to be perfectly forwarded.

[ Resolution proposed by ballot comment ]

Patch [except.nested] p6-7 to match the intent with regards to requirements on `T` and the use of `std::forward<T>(t)`.

[ 2010-10-24 Daniel adds: ]

> Accepting n3144 would solve this issue.

[2010-11-10 Batavia: LWG accepts Howard's updated wording with corrected boo boos reported by Sebastian Gesemann and Pete Becker, which is approved for Immediate adoption this meeting.]

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 18.8.7 nested_exception [except.nested] as indicated:

>     [[noreturn]] template <class T> void throw_with_nested(T&& t);

> > Let `U` be `remove_reference<T>::type`

> > 6 Requires: `T U` shall be `CopyConstructible`.

> > 7 Throws: If `T U` is a non-union class type not derived from `nested_exception`, an exception of unspecified type that is publicly derived from both `T U` and `nested_exception` and constructed from `std::forward<T>(t)`, otherwise throws `std::forward<T>(t)`.

### 1372. Adopt recommended practice for standard error categories

Section: 19.5.1.5 [syserr.errcat.objects] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-76

The C++0x FCD recommends, in a note (see 19.5.1.1/1), that users create a single `error_category` object for each user defined error category and specifies `error_category` equality comparsions based on equality of addresses (19.5.1.3). The Draft apparently ignores this when specifying standard error category objects in section 19.5.1.5, by allowing the `generic_category()` and `system_category()` functions to return distinct objects for each invocation.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Append a new sentence to 19.5.1.5 [syserr.errcat.objects]/1, and append the same sentence to 19.5.1.5/3.

> All calls of this function return references to the same object.

### 1377. The revised `forward` is not compatible with access-control

Section: 20.2 [utility] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-90

In n3090, at variance with previous iterations of the idea discussed in papers and incorporated in WDs, `std::forward` is constrained via `std::is_convertible`, thus is not robust wrt access control. This causes problems in normal uses as implementation detail of member functions. For example, the following snippet leads to a compile time failure, whereas that was not the case for an implementation along the lines of n2835 (using `enable_if`s instead of concepts for the constraining, of course)

    #include <utility>
    struct Base { Base(Base&&); };

    struct Derived
      : private Base
    {
      Derived(Derived&& d)
        : Base(std::forward<Base>(d)) { }
    };

In other terms, LWG 1054 can be resolved in a better way, the present status is not acceptable.

[ 2010-10-24 Daniel adds: ]

> Accepting n3143 would solve this issue.

Proposed resolution:

Resolved as NAD Editorial by paper n3143.

### 1378. `pair` and `tuple` have too many conversions

Section: 20.3 [pairs] Status: Resolved Submitter: DIN Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses DE-15

Several function templates of `pair` and `tuple` allow for too many implicit conversions, for example:

    #include <tuple>
    std::tuple<char*> p(0); // Error?

    struct A { explicit A(int){} };
    A a = 1; // Error
    std::tuple<A> ta = std::make_tuple(1); // OK?

[ Resolution proposed by ballot comment ]

Consider to add wording to constrain these function templates.

[ 2010-10-24 Daniel adds: ]

> Accepting n3140 would solve this issue.

Proposed resolution:

See n3140.

### 1379. `pair` copy-assignment not consistent for references

Section: 20.3.2 [pairs.pair] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-95

Copy-assignment for `pair` is defaulted and does not work for pairs with reference members. This is inconsistent with conversion-assignment, which deliberately succeeds even if one or both elements are reference types, just as for `tuple`. The copy-assignment operator should be consistent with the conversion-assignment operator and with `tuple`'s assignment operators.

[ 2010-10-24 Daniel adds: ]

> Accepting n3140 would provide a superior resolution, because `pair` does not depend on the semantic requirements of `CopyAssignable`.

[ 2010-11 Batavia ]

Resolved by adopting n3140.

Proposed resolution:

Add to `pair` synopsis:

> `pair& operator=(const pair& p);`

Add before paragraph 9:

> `pair& operator=(const pair& p);`

> > Requires: `T1` and `T2` shall satisfy the requirements of `CopyAssignable`.

> > Effects: Assigns `p.first` to `first` and `p.second` to `second`. Returns: `*this`.

### 1380. `pair` and `tuple` of references need to better specify move-semantics

Section: 20.3 [pairs] Status: Resolved Submitter: DIN Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses DE-16

Several `pair` and `tuple` functions in regard to move operations are incorrectly specified if the member types are references, because the result of a `std::move` cannot be assigned to lvalue-references. In this context the usage of the requirement sets `MoveConstructible` and `CopyConstructible` also doesn't make sense, because non-const lvalue-references cannot satisfy these requirements.

[ Resolution proposed by ballot comment ]

Replace the usage of `std::move` by that of `std::forward` and replace `MoveConstructible` and `CopyConstructible` requirements by other requirements.

[ 2010-10-24 Daniel adds: ]

> Accepting n3140 would solve this issue.

[ 2010-11 Batavia: ]

> Resolved by adopting n3140.

Proposed resolution:

See n3140.

### 1381. Replace `pair`'s range support by proper range facility

Section: X [pair.range] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-85

While `std::pair` may happen to hold a pair of iterators forming a valid range, this is more likely a coincidence than a feature guaranteed by the semantics of the `pair` template. A distinct range-type should be supplied to enable the new for-loop syntax rather than overloading an existing type with a different semantic.

If a replacement facility is required for C++0x, consider n2995.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Strike 20.3.5.4 and the matching declarations in 20.3 header synopsis.

### 1382. `pair` and `tuple` constructors should `forward` arguments

Section: 20.3 [pairs] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-96

`pair` and `tuple` constructors and assignment operators use `std::move` when they should use `std::forward`. This causes lvalue references to be erroneously converted to rvalue references. Related requirements clauses are also wrong.

[ Resolution proposed by ballot comment ]

> See Appendix 1 - Additional Details

[ 2010-10-24 Daniel adds: ]

> Accepting n3140 would solve this issue.

[ 2010-11 Batavia ]

Resolved by adopting n3140.

Proposed resolution:

See n3140.

### 1383. Inconsistent defaulted move/copy members in `pair` and `tuple`

Section: 20.3 [pairs], 20.4 [tuple] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-97

`pair`'s class definition in N3092 20.3.2 [pairs.pair] contains "`pair(const pair&) = default;`" and "`pair& operator=(pair&& p);`". The latter is described by 20.3.2 [pairs.pair] p.12-13.

"`pair(const pair&) = default;`" is a user-declared explicitly defaulted copy constructor. According to 12.8 [class.copy]/10, this inhibits the implicitly-declared move constructor. `pair` should be move constructible. (12.8 [class.copy]/7 explains that "`pair(pair<U, V>&& p)`" will never be instantiated to move `pair<T1, T2>` to `pair<T1, T2>`.)
"`pair& operator=(pair&& p);`" is a user-provided move assignment operator (according to 8.4.2 [dcl.fct.def.default]/4: "A special member function is user-provided if it is user-declared and not explicitly defaulted on its first declaration."). According to 12.8 [class.copy]/20, this inhibits the implicitly-declared copy assignment operator. `pair` should be copy assignable, and was in C++98/03. (Again, 12.8 [class.copy]/7 explains that "`operator=(const pair<U, V>& p)`" will never be instantiated to copy `pair<T1, T2>` to `pair<T1, T2>`.)
Additionally, "`pair& operator=(pair&& p);`" is unconditionally defined, whereas according to 12.8 [class.copy]/25, defaulted copy/move assignment operators are defined as deleted in several situations, such as when non-static data members of reference type are present.

If "`pair(const pair&) = default;`" and "`pair& operator=(pair&& p);`" were removed from `pair`'s class definition in 20.3.2 [pairs.pair] and from 20.3.2 [pairs.pair]/12-13, `pair` would receive implicitly-declared copy/move constructors and copy/move assignment operators, and 12.8 [class.copy]/25 would apply. The implicitly-declared copy/move constructors would be trivial when `T1` and `T2` have trivial copy/move constructors, according to 12.8 [class.copy]/13, and similarly for the assignment operators, according to12.8 [class.copy]/27. Notes could be added as a reminder that these functions would be implicitly-declared, but such notes would not be necessary (the Standard Library specification already assumes a high level of familiarity with the Core Language, and casual readers will simply assume that `pair` is copyable and movable).

Alternatively, `pair` could be given explicitly-defaulted copy/move constructors and copy/move assignment operators. This is a matter of style.

`tuple` is also affected. `tuple`'s class definition in 20.4 [tuple] contains:

    tuple(const tuple&) = default;
    tuple(tuple&&);
    tuple& operator=(const tuple&);
    tuple& operator=(tuple&&);

They should all be removed or all be explicitly-defaulted, to be consistent with `pair`. Additionally, 20.4.2.1 [tuple.cnstr]/8-9 specifies the behavior of an explicitly defaulted function, which is currently inconsistent with `pair`.

[ Resolution proposed by ballot comment: ]

> Either remove "`pair(const pair&) = default;`" and "`pair& operator=(pair&& p);`" from `pair`'s class definition in 20.3.2 [pairs.pair] and from 20.3.2 [pairs.pair] p.12-13, or give pair explicitly-defaulted copy/move constructors and copy/move assignment operators.
> Change `tuple` to match.

[ 2010-10-24 Daniel adds: ]

> Accepting n3140 would solve this issue: The move/copy constructor will be defaulted, but the corresponding assignment operators need a non-default implementation because they are supposed to work for references as well.

Proposed resolution:

See n3140.

### 1384. Function `pack_arguments` is poorly named

Section: 20.4.2.4 [tuple.creation] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-98

`pack_arguments` is poorly named. It does not reflect the fact that it is a tuple creation function and that it forwards arguments.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Rename `pack_arguments` to `forward_as_tuple` throughout the standard.

### 1385. `tuple_cat` should be a single variadic signature

Section: 20.4.2.4 [tuple.creation] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-88

The `tuple_cat` template consists of four overloads and that can concatenate only two `tuple`s. A single variadic signature that can concatenate an arbitrary number of `tuple`s would be preferred.

[ Resolution proposed by ballot comment: ]

> Adopt a simplified form of the proposal in n2975, restricted to `tuple`s and neither requiring nor outlawing support for other `tuple`-like types.

[ 2010 Rapperswil: Alisdair to provide wording. ]

[ 2010-11-06: Daniel comments and proposes some alternative wording: ]

There are some problems in the wording: First, even though the result type `tuple<see below>` implies it, the specification of the contained tuple element types is missing. Second, the term "`tuple` protocol" is not defined anywhere and I see no reason why this normative wording should not be a non-normative note. We could at least give a better approximation, maybe "tuple-like protocol" as indicated from header `<utility>` synopsis. Further, it seems to me that the effects need to contain a combination of `std::forward` with the call of `get`. Finally I suggest to replace the requirements `Move/CopyConstructible` by proper usage of `is_constructible`, as indicated by n3140.

[ 2010 Batavia ]

Moved to Ready with Daniel's improved wording.

Proposed resolution:

Note: This alternate proposed resolution works only if 1191 has been accepted.

1. Change 20.4.1 [tuple.general] p. 2, header `<tuple>` synopsis, as indicated:

   >     namespace std {

   >     ...

   >     // 20.4.2.4, tuple creation functions:
   >     const unspecified ignore;

   >     template <class... Types>
   >       tuple<VTypes...> make_tuple(Types&&...);
   >       template <class... Types>
   >       tuple<ATypes...> forward_as_tuple(Types&&...);

   >     template<class... Types>
   >       tuple<Types&...> tie(Types&...);

   >     template <class... TTypes, class... UTypes>
   >       tuple<TTypes..., UTypes...> tuple_cat(const tuple<TTypes...>&, const tuple<UTypes...>&);
   >     template <class... TTypes, class... UTypes>
   >       tuple<TTypes..., UTypes...> tuple_cat(tuple<TTypes...>&&, const tuple<UTypes...>&);
   >     template <class... TTypes, class... UTypes>
   >       tuple<TTypes..., UTypes...> tuple_cat(const tuple<TTypes...>&, tuple<UTypes...>&&);
   >     template <class... TTypes, class... UTypes>
   >       tuple<TTypes..., UTypes...> tuple_cat(tuple<TTypes...>&&, tuple<UTypes...>&&);
   >     template <class... Tuples>
   >       tuple<CTypes...> tuple_cat(Tuples&&...);

   >     ...

2. Change 20.4.2.4 [tuple.creation] as indicated:

   >     template <class... TTypes, class... UTypes>
   >       tuple<TTypes..., UTypes...> tuple_cat(const tuple<TTypes...>& t, const tuple<UTypes...>& u);

   > > 8 Requires: All the types in `TTypes` shall be `CopyConstructible` (Table 35). All the types in `UTypes` shall be `CopyConstructible` (Table 35).

   > > 9 Returns: A `tuple` object constructed by copy constructing its first `sizeof...(TTypes)` elements from the corresponding elements of `t` and copy constructing its last `sizeof...(UTypes)` elements from the corresponding elements of `u`.

   >     template <class... TTypes, class... UTypes>
   >       tuple<TTypes..., UTypes...> tuple_cat(tuple<TTypes...>&& t, const tuple<UTypes...>& u);

   > > 10 Requires: All the types in `TTypes` shall be `MoveConstructible` (Table 34). All the types in `UTypes` shall be `CopyConstructible` (Table 35).

   > > 11 Returns: A `tuple` object constructed by move constructing its first `sizeof...(TTypes)` elements from the corresponding elements of `t` and copy constructing its last `sizeof...(UTypes)` elements from the corresponding elements of `u`.

   >     template <class... TTypes, class... UTypes>
   >       tuple<TTypes..., UTypes...> tuple_cat(const tuple<TTypes...>& t, tuple<UTypes...>&& u);

   > > 12 Requires: All the types in `TTypes` shall be `CopyConstructible` (Table 35). All the types in `UTypes` shall be `MoveConstructible` (Table 34).

   > > 13 Returns: A `tuple` object constructed by copy constructing its first `sizeof...(TTypes)` elements from the corresponding elements of `t` and move constructing its last `sizeof...(UTypes)` elements from the corresponding elements of `u`.

   >     template <class... TTypes, class... UTypes>
   >       tuple<TTypes..., UTypes...> tuple_cat(tuple<TTypes...>&& t, tuple<UTypes...>&& u);

   > > 14 Requires: All the types in `TTypes` shall be `MoveConstructible` (Table 34). All the types in `UTypes` shall be `MoveConstructible` (Table 34).

   > > 15 Returns: A `tuple` object constructed by move constructing its first `sizeof...(TTypes)` elements from the corresponding elements of `t` and move constructing its last `sizeof...(UTypes)` elements from the corresponding elements of `u`.

   >     template <class... Tuples>
   >       tuple<CTypes...> tuple_cat(Tuples&&... tpls);

   > > 8 Let `Ti` be the `i`th type in `Tuples`, `Ui` be `remove_reference<Ti>::type`, and `tpi` be the `i`th parameter in the function parameter pack `tpls`, where all indexing is zero-based in the following paragraphs of this sub-clause [tuple.creation].

   > > 9 Requires: For all `i`, `Ui` shall be the type cv`i` `tuple<Argsi...>`, where cv`i` is the (possibly empty) `i`th cv-qualifier-seq, and `Argsi` is the parameter pack representing the element types in `Ui`. Let `Aik` be the `ki`th type in `Argsi`, then for all `Aik` the following requirements shall be satisfied: If `Ti` is deduced as an lvalue reference type, then `is_constructible<Aik, cvi Aik&>::value == true`, otherwise `is_constructible<Aik, cvi Aik&&>::value == true`.

   > > 10 Remarks: The types in `CTypes` shall be equal to the ordered sequence of the expanded types `Args0..., Args1..., Argsn-1...`, where `n` equals `sizeof...(Tuples)`. Let `ei...` be the `i`th ordered sequence of tuple elements of the result `tuple` object corresponding to the type sequence `Argsi`.

   > > 11 Returns: A `tuple` object constructed by initializing the `ki`th type element `eik` in `ei...` with `get<ki>(std::forward<Ti>(tpi))` for each valid `ki` and each element group `ei` in order.

   > > 12 [Note: An implementation may support additional types in the parameter pack `Tuples`, such as `pair` and `array` that support the `tuple`-like protocol. — end note]

### 1386. `pack_arguments` overly complex

Section: 20.4.2.4 [tuple.creation] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-99

`pack_arguments` is overly complex.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

This issue resulted from a lack of understanding of how references are forwarded. The definition of `pack_arguments` should be simply:
`template <class... Types> tuple<ATypes&&> pack_arguments(Types&&...t);`
Types: Let `Ti` be each type in `Types`....
Effects: ...
Returns:
`tuple<ATypes&&...>(std::forward<Types>(t)...)`
The synopsis should also change to reflect this simpler signature.

### 1387. Range support by `tuple` should be removed

Section: X [tuple.range] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-87

There is no compelling reason to assume a heterogeneous tuple of two elements holds a pair of iterators forming a valid range. Unlike `std::pair`, there are no functions in the standard library using this as a return type with a valid range, so there is even less reason to try to adapt this type for the new for-loop syntax.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Strike 20.4.2.10 and the matching declarations in the header synopsis in 20.4.

### 1388. LWG 1281 incorrectly accepted

Section: 20.11.3 [ratio.ratio] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-100

LWG 1281 was discussed in Pittsburgh, and the decision there was to accept the typedef as proposed and move to Review. Unfortunately the issue was accidentally applied to the FCD, and incorrectly. The FCD version of the typedef refers to `ratio<N, D>`, but the typedef is intended to refer to `ratio<num, den>` which in general is not the same type.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Accept the current proposed wording of LWG 1281 which adds:
`typedef ratio<num, den> type;`

### 1389. Compile-time rational arithmetic and overflow

Section: 20.11.4 [ratio.arithmetic] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-89

The alias representations of the `ratio` arithmetic templates do not allow implementations to avoid overflow, since they explicitly specify the form of the aliased template instantiation. For example `ratio_multiply`, `ratio<2, LLONG_MAX>` is required to alias `ratio<2*LLONG_MAX, LLONG_MAX*2>`, which overflows, so is ill-formed. However, this is trivially equal to `ratio<1, 1>`. It also contradicts the opening statement of 20.11.4 [ratio.arithmetic] p. 1 "implementations may use other algorithms to compute these values".

[ 2010-10-25 Daniel adds: ]

> Accepting n3131 would solve this issue.

[Batavia: Resolved by accepting n3210.]

Proposed resolution:

Change the wording in 20.11.4 [ratio.arithmetic] p. 2-5 as follows:

>     template <class R1, class R2> using ratio_add = see below;

> > 2 The type `ratio_add<R1, R2>` shall be a synonym for `ratio<T1, T2>` `ratio<U, V>` such that `ratio<U, V>::num` and `ratio<U, V>::den` are the same as the corresponding members of `ratio<T1, T2>` would be in the absence of arithmetic overflow where `T1` has the value `R1::num * R2::den + R2::num * R1::den` and `T2` has the value `R1::den * R2::den`. If the required values of `ratio<U, V>::num` and `ratio<U, V>::den` cannot be represented in `intmax_t` then the program is ill-formed.

>     template <class R1, class R2> using ratio_subtract = see below;

> > 3 The type `ratio_subtract<R1, R2>` shall be a synonym for `ratio<T1, T2>` `ratio<U, V>` such that `ratio<U, V>::num` and `ratio<U, V>::den` are the same as the corresponding members of `ratio<T1, T2>` would be in the absence of arithmetic overflow where `T1` has the value `R1::num * R2::den - R2::num * R1::den` and `T2` has the value `R1::den * R2::den`. If the required values of `ratio<U, V>::num` and `ratio<U, V>::den` cannot be represented in `intmax_t` then the program is ill-formed.

>     template <class R1, class R2> using ratio_multiply = see below;

> > 4 The type `ratio_multiply<R1, R2>` shall be a synonym for `ratio<T1, T2>` `ratio<U, V>` such that `ratio<U, V>::num` and `ratio<U, V>::den` are the same as the corresponding members of `ratio<T1, T2>` would be in the absence of arithmetic overflow where `T1` has the value `R1::num * R2::num` and `T2` has the value `R1::den * R2::den`. If the required values of `ratio<U, V>::num` and `ratio<U, V>::den` cannot be represented in `intmax_t` then the program is ill-formed.

>     template <class R1, class R2> using ratio_divide = see below;

> > 5 The type `ratio_divide<R1, R2>` shall be a synonym for `ratio<T1, T2>` `ratio<U, V>` such that `ratio<U, V>::num` and `ratio<U, V>::den` are the same as the corresponding members of `ratio<T1, T2>` would be in the absence of arithmetic overflow where `T1` has the value `R1::num * R2::den` and `T2` has the value `R1::den * R2::num`. If the required values of `ratio<U, V>::num` and `ratio<U, V>::den` cannot be represented in `intmax_t` then the program is ill-formed.

### 1390. Limit speculative compilation for constructible/convertible traits

Section: 20.10 [meta] Status: Resolved Submitter: DIN Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses DE-17

Speculative compilation for `std::is_constructible` and `std::is_convertible` should be limited, similar to the core language (see 14.8.2 paragraph 8).

[ 2010-10-24 Daniel adds: ]

> Accepting n3142 would solve this issue.

Proposed resolution:

Resolved by paper n3142.

### 1391. constructible/convertible traits and access control

Section: 20.10 [meta] Status: Resolved Submitter: DIN Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses DE-18

Several type traits require compiler support, e.g. `std::is_constructible` or `std::is_convertible`. Their current specification seems to imply, that the corresponding test expressions should be well-formed, even in absense of access:

    class X { X(int){} };
    constexpr bool test = std::is_constructible<X, int>::value;

The specification does not clarify the context of this test and because it already goes beyond normal language rules, it's hard to argue by means of normal language rules what the context and outcome of the test should be.

[ Resolution proposed by ballot comment ]

Specify that `std::is_constructible` and `std::is_convertible` will return `true` only for public constructors/conversion functions.

[ 2010-10-24 Daniel adds: ]

> Accepting n3142 would solve this issue.

Proposed resolution:

Resolved by paper n3142.

### 1392. `result_of` should support pointer-to-data-member

Section: 20.10.4 [meta.unary] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-102

Despite Library Issue 520's ("`result_of` and pointers to data members") resolution of CD1, the FCD's `result_of` supports neither pointers to member functions nor pointers to data members. It should.

[ Resolution proposed by ballot comment ]

Ensure `result_of` supports pointers to member functions and pointers to data members.

[ 2010-10-24 Daniel adds: ]

> Accepting n3123 would solve this issue.

Proposed resolution:

Resolved by n3123.

### 1393. Trivial traits imply `noexcept`

Section: 20.10.4.3 [meta.unary.prop] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-92

Trivial functions implicitly declare a `noexcept` exception specification, so the references to `has_trivial_*` traits in the `has_nothrow_*` traits are redundant, and should be struck for clarity.

[ Resolution proposed by ballot comment ]

For each of the `has_nothrow_something` traits, remove all references to the matching `has_trivial_something` traits.

[ 2010-10-24 Daniel adds: ]

> Accepting n3142 would solve this issue.

Proposed resolution:

Resolved by n3142.

### 1394. `is_constructible` reports false positives

Section: 20.10.4.3 [meta.unary.prop] Status: Resolved Submitter: DIN Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses DE-19

The fundamental trait `is_constructible` reports false positives, e.g.

    is_constructible<char*, void*>::value

evaluates to true, even though a corresponding variable initialization would be ill-formed.

[ Resolved in Rapperswil by paper N3047. ]

Proposed resolution:

Remove all false positives from the domain of `is_constructible`.

### 1397. Deprecate '98 binders

Section: 20.9 [function.objects] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-95

The adaptable function protocol supported by `unary_function/binary_function` has been superceded by lambda expressions and `std::bind`. Despite the name, the protocol is not very adaptable as it requires intrusive support in the adaptable types, rather than offering an external traits-like adaption mechanism. This protocol and related support functions should be deprecated, and we should not make onerous requirements for the specification to support this protocol for callable types introduced in this standard revision, including those adopted from TR1. It is expected that high-quality implementations will provide such support, but we should not have to write robust standard specifications mixing this restricted support with more general components such as `function`, `bind` and `reference_wrapper`.

[ Resolution proposed by ballot comment ]

Move clauses 20.8.3, 20.8.9, 20.8.11 and 20.8.12 to Annex D.

Remove the requirements to conditionally derive from `unary/binary_function` from `function`, `reference_wrapper`, and the results of calling `mem_fn` and `bind`.

[ 2010-10-24 Daniel adds: ]

> Accepting n3145 would solve this issue.

Proposed resolution:

Resolved by paper N3198.

### 1399. `function` does not need an `explicit` default constructor

Section: 20.9.12.2 [func.wrap.func] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses JP-3

`explicit` default contructor is defined in `std::function`. Although it is allowed according to 12.3.1, it seems unnecessary to qualify the constructor as `explicit`. If it is `explicit`, there will be a limitation in `initializer_list`.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Remove `explicit`.

    namespace std {
    template<class> class function;
    // undefined
    template<class R, class... ArgTypes>
    class function<R(ArgTypes...)>
    : public unary_function<T1, R>
    // iff sizeof...(ArgTypes) == 1 and ArgTypes contains T1
    : public binary_function<T1, T2, R>
    // iff sizeof...(ArgTypes) == 2 and ArgTypes contains T1 andT2
    {
    public:typedef R result_type;
    // 20.8.14.2.1, construct/copy/destroy:
      explicit function();

### 1400. FCD `function` does not need an `explicit` default constructor

Section: 20.9.12.2.1 [func.wrap.func.con] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses JP-4

Really does the `function` require that default constructor is `explicit`?

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Remove `explicit`.

    function();
    template <class A>
    function(allocator_arg_t, const A& a);

### 1401. Provide support for `unique_ptr<T> == nullptr`

Section: 20.7 [memory] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-99

One reason that the `unique_ptr` constructor taking a `nullptr_t` argument is not `explicit` is to allow conversion of `nullptr` to `unique_ptr` in contexts like equality comparison. Unfortunately `operator==` for `unique_ptr` is a little more clever than that, deducing template parameters for both arguments. This means that `nullptr` does not get deduced as `unique_ptr` type, and there are no other comparison functions to match.

[ Resolution proposed by ballot comment: ]

> Add the following signatures to 20.7 [memory] p.1, `<memory>` header synopsis:

> >     template<typename T, typename D>
> >     bool operator==(const unique_ptr<T, D> & lhs, nullptr_t);
> >     template<typename T, typename D>
> >     bool operator==(nullptr_t, const unique_ptr<T, D> & rhs);
> >     template<typename T, typename D>
> >     bool operator!=(const unique_ptr<T, D> & lhs, nullptr_t);
> >     template<typename T, typename D>
> >     bool operator!=(nullptr_t, const unique_ptr<T, D> & rhs);

[ 2010-11-02 Daniel comments and provides a proposed resolution: ]

> The same problem applies to `shared_ptr` as well: In both cases there are no conversions considered because the comparison functions are templates. I agree with the direction of the proposed resolution, but I believe it would be very surprising and inconsistent, if given a smart pointer object `p`, the expression `p == nullptr` would be provided, but not `p < nullptr` and the other relational operators. According to 5.9 [expr.rel] they are defined if null pointer values meet other pointer values, even though the result is unspecified for all except some trivial ones. But null pointer values are nothing special here: The Library already defines the relational operators for both `unique_ptr` and `shared_ptr` and the outcome of comparing non-null pointer values will be equally unspecified. If the idea of supporting `nullptr_t` arguments for relational operators is not what the committee prefers, I suggest at least to consider to remove the existing relational operators for both `unique_ptr` and `shared_ptr` for consistency. But that would not be my preferred resolution of this issue.

> The number of overloads triple the current number, but I think it is much clearer to provide them explicitly instead of adding wording that attempts to say that "sufficient overloads" are provided. The following proposal makes the declarations explicit.

> Additionally, the proposal adds the missing declarations for some `shared_ptr` comparison functions for consistency.

[ 2010-11-03 Daniel adds: ]

Issue 1297 is remotely related. The following proposed resolution splits this bullet into sub-bullets A and B. Sub-bullet A would also solve 1297, but sub-bullet B would not.

A further remark in regard to the proposed semantics of the ordering of `nullptr` against other pointer(-like) values: One might think that the following definition might be superior because of simplicity:

>     template<class T>
>     bool operator<(const shared_ptr<T>& a, nullptr_t);
>     template<class T>
>     bool operator>(nullptr_t, const shared_ptr<T>& a);

> > Returns: `false`.

The underlying idea behind this approach is the assumption that nullptr corresponds to the least ordinal pointer value. But this assertion does not hold for all supported architectures, therefore this approach was not followed because it would lead to the inconsistency, that the following assertion could fire:

>     shared_ptr<int> p(new int);
>     shared_ptr<int> null;
>     bool v1 = p < nullptr;
>     bool v2 = p < null;
>     assert(v1 == v2);

[2011-03-06: Daniel comments]

The current issue state is not acceptable, because the Batavia meeting did not give advice whether choice A or B of bullet 3 should be applied. Option B will now be removed and if this resolution is accepted, issue 1297 should be declared as resolved by 1401. This update also resyncs the wording with N3242.

Proposed resolution:

Wording changes are against N3242.

1. Change 20.7.2 [memory.syn] p. 1, header `<memory>` synopsis as indicated. `noexcept` specifications are only added, where the guarantee exists, that the function shall no throw an exception (as replacement of "Throws: Nothing". Note that the `noexcept` additions to the `shared_ptr` comparisons are editorial, because they are already part of the accepted paper n3195:

   >     namespace std {
   >       […]
   >       // [unique.ptr] Class unique_ptr:
   >       template <class T> class default_delete;
   >       template <class T> class default_delete<T[]>;
   >       template <class T, class D = default_delete<T>> class unique_ptr;
   >       template <class T, class D> class unique_ptr<T[], D>;

   >       template <class T1, class D1, class T2, class D2>
   >       bool operator==(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
   >       template <class T1, class D1, class T2, class D2>
   >       bool operator!=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
   >       template <class T1, class D1, class T2, class D2>
   >       bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
   >       template <class T1, class D1, class T2, class D2>
   >       bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
   >       template <class T1, class D1, class T2, class D2>
   >       bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
   >       template <class T1, class D1, class T2, class D2>
   >       bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

   >       template <class T, class D>
   >       bool operator==(const unique_ptr<T, D>& x, nullptr_t) noexcept;
   >       template <class T, class D>
   >       bool operator==(nullptr_t, const unique_ptr<T, D>& x) noexcept;
   >       template <class T, class D>
   >       bool operator!=(const unique_ptr<T, D>& x, nullptr_t) noexcept;
   >       template <class T, class D>
   >       bool operator!=(nullptr_t, const unique_ptr<T, D>& x) noexcept;
   >       template <class T, class D>
   >       bool operator<(const unique_ptr<T, D>& x, nullptr_t);
   >       template <class T, class D>
   >       bool operator<(nullptr_t, const unique_ptr<T, D>& x);
   >       template <class T, class D>
   >       bool operator<=(const unique_ptr<T, D>& x, nullptr_t);
   >       template <class T, class D>
   >       bool operator<=(nullptr_t, const unique_ptr<T, D>& x);
   >       template <class T, class D>
   >       bool operator>(const unique_ptr<T, D>& x, nullptr_t);
   >       template <class T, class D>
   >       bool operator>(nullptr_t, const unique_ptr<T, D>& x);
   >       template <class T, class D>
   >       bool operator>=(const unique_ptr<T, D>& x, nullptr_t);
   >       template <class T, class D>
   >       bool operator>=(nullptr_t, const unique_ptr<T, D>& x);

   >       // [util.smartptr.weakptr], Class bad_weak_ptr:
   >       class bad_weak_ptr;

   >       // [util.smartptr.shared], Class template shared_ptr:
   >       template<class T> class shared_ptr;

   >       // [util.smartptr.shared.cmp], shared_ptr comparisons:
   >       template<class T, class U>
   >       bool operator==(shared_ptr<T> const& a, shared_ptr<U> const& b)  noexcept;
   >       template<class T, class U>
   >       bool operator!=(shared_ptr<T> const& a, shared_ptr<U> const& b)  noexcept;
   >       template<class T, class U>
   >       bool operator<(shared_ptr<T> const& a, shared_ptr<U> const& b)  noexcept;
   >       template<class T, class U>
   >       bool operator>(shared_ptr<T> const& a, shared_ptr<U> const& b)  noexcept;
   >       template<class T, class U>
   >       bool operator<=(shared_ptr<T> const& a, shared_ptr<U> const& b)  noexcept;
   >       template<class T, class U>
   >       bool operator>=(shared_ptr<T> const& a, shared_ptr<U> const& b)  noexcept;

   >       template<class T>
   >       bool operator==(shared_ptr<T> const& a, nullptr_t) noexcept;
   >       template<class T>
   >       bool operator==(nullptr_t, shared_ptr<T> const& a) noexcept;
   >       template<class T>
   >       bool operator!=(shared_ptr<T> const& a, nullptr_t) noexcept;
   >       template<class T>
   >       bool operator!=(nullptr_t, shared_ptr<T> const& a) noexcept;
   >       template<class T>
   >       bool operator<(shared_ptr<T> const& a, nullptr_t) noexcept;
   >       template<class T>
   >       bool operator<(nullptr_t, shared_ptr<T> const& a) noexcept;
   >       template>class T>
   >       bool operator>(shared_ptr<T> const& a, nullptr_t) noexcept;
   >       template>class T>
   >       bool operator>(nullptr_t, shared_ptr<T> const& a) noexcept;
   >       template<class T>
   >       bool operator<=(shared_ptr<T> const& a, nullptr_t) noexcept;
   >       template<class T>
   >       bool operator<=(nullptr_t, shared_ptr<T> const& a) noexcept;
   >       template>class T>
   >       bool operator>=(shared_ptr<T> const& a, nullptr_t) noexcept;
   >       template>class T>
   >       bool operator>=(nullptr_t, shared_ptr<T> const& a) noexcept;

   >       […]
   >     }

2. Change the synopsis just after 20.8.1 [unique.ptr] p. 6 as indicated:

   >     namespace std {
   >       […]

   >       template <class T1, class D1, class T2, class D2>
   >       bool operator==(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
   >       template <class T1, class D1, class T2, class D2>
   >       bool operator!=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
   >       template <class T1, class D1, class T2, class D2>
   >       bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
   >       template <class T1, class D1, class T2, class D2>
   >       bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
   >       template <class T1, class D1, class T2, class D2>
   >       bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);
   >       template <class T1, class D1, class T2, class D2>
   >       bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

   >       template <class T, class D>
   >       bool operator==(const unique_ptr<T, D>& x, nullptr_t) noexcept;
   >       template <class T, class D>
   >       bool operator==(nullptr_t, const unique_ptr<T, D>& x) noexcept;
   >       template <class T, class D>
   >       bool operator!=(const unique_ptr<T, D>& x, nullptr_t) noexcept;
   >       template <class T, class D>
   >       bool operator!=(nullptr_t, const unique_ptr<T, D>& x) noexcept;
   >       template <class T, class D>
   >       bool operator<(const unique_ptr<T, D>& x, nullptr_t);
   >       template <class T, class D>
   >       bool operator<(nullptr_t, const unique_ptr<T, D>& x);
   >       template <class T, class D>
   >       bool operator<=(const unique_ptr<T, D>& x, nullptr_t);
   >       template <class T, class D>
   >       bool operator<=(nullptr_t, const unique_ptr<T, D>& x);
   >       template <class T, class D>
   >       bool operator>(const unique_ptr<T, D>& x, nullptr_t);
   >       template <class T, class D>
   >       bool operator>(nullptr_t, const unique_ptr<T, D>& x);
   >       template <class T, class D>
   >       bool operator>=(const unique_ptr<T, D>& x, nullptr_t);
   >       template <class T, class D>
   >       bool operator>=(nullptr_t, const unique_ptr<T, D>& x);

   >     }

3. This bullet does now only suggest the first approach:

   Change 20.8.1.5 [unique.ptr.special] p. 4-7 as indicated and add a series of prototype descriptions:

   >     template <class T1, class D1, class T2, class D2>
   >       bool operator<(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

   > > ? Requires: Let `CT` be `common_type<unique_ptr<T1, D1>::pointer, unique_ptr<T2, D2>::pointer>::type`. Then the specialization `less<CT>` shall be a function object type ([function.objects]) that induces a strict weak ordering ([alg.sorting]) on the pointer values.

   > > 4 Returns: `less<CT>()(x.get(), y.get())x.get() < y.get()`.

   > > ? Remarks: If `unique_ptr<T1, D1>::pointer` is not implicitly convertible to `CT` or `unique_ptr<T2, D2>::pointer` is not implicitly convertible to `CT`, the program is ill-formed.

   >     template <class T1, class D1, class T2, class D2>
   >       bool operator<=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

   > > 5 Returns: `!(y < x)x.get() <= y.get()`.

   >     template <class T1, class D1, class T2, class D2>
   >       bool operator>(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

   > > 6 Returns: `(y < x)x.get() > y.get()`.

   >     template <class T1, class D1, class T2, class D2>
   >       bool operator>=(const unique_ptr<T1, D1>& x, const unique_ptr<T2, D2>& y);

   > > 7 Returns: `!(x < y)x.get() >= y.get()`.

   >     template <class T, class D>
   >     bool operator==(const unique_ptr<T, D>& x, nullptr_t) noexcept;
   >     template <class T, class D>
   >     bool operator==(nullptr_t, const unique_ptr<T, D>& x) noexcept;

   > > ? Returns: `!x`.

   >     template <class T, class D>
   >     bool operator!=(const unique_ptr<T, D>& x, nullptr_t) noexcept;
   >     template <class T, class D>
   >     bool operator!=(nullptr_t, const unique_ptr<T, D>& x) noexcept;

   > > ? Returns: `(bool) x`.

   >     template <class T, class D>
   >     bool operator<(const unique_ptr<T, D>& x, nullptr_t);
   >     template <class T, class D>
   >     bool operator>(nullptr_t, const unique_ptr<T, D>& x);

   > > ? Requires: The specialization `less<unique_ptr<T, D>::pointer>` shall be a function object type ([function.objects]) that induces a strict weak ordering ([alg.sorting]) on the pointer values.

   > > ? Returns: `less<unique_ptr<T, D>::pointer>()(x.get(), nullptr)`.

   >     template <class T, class D>
   >     bool operator<(nullptr_t, const unique_ptr<T, D>& x);
   >     template <class T, class D>
   >     bool operator>(const unique_ptr<T, D>& x, nullptr_t);

   > > ? Requires: The specialization `less<unique_ptr<T, D>::pointer>` shall be a function object type ([function.objects]) that induces a strict weak ordering ([alg.sorting]) on the pointer values.

   > > ? Returns: `less<unique_ptr<T, D>::pointer>()(nullptr, x.get())`.

   >     template <class T, class D>
   >     bool operator<=(const unique_ptr<T, D>& x, nullptr_t);
   >     template <class T, class D>
   >     bool operator>=(nullptr_t, const unique_ptr<T, D>& x);

   > > ? Returns: `!(nullptr < x)`.

   >     template <class T, class D>
   >     bool operator<=(nullptr_t, const unique_ptr<T, D>& x);
   >     template <class T, class D>
   >     bool operator>=(const unique_ptr<T, D>& x, nullptr_t);

   > > ? Returns: `!(x < nullptr)`.

4. Change 20.8.2.2 [util.smartptr.shared] p. 1, class template `shared_ptr` synopsis as indicated. For consistency reasons the remaining normal relation operators are added as well:

   >     namespace std {

   >       […]

   >       // [util.smartptr.shared.cmp], shared_ptr comparisons:
   >       template<class T, class U>
   >       bool operator==(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
   >       template<class T, class U>
   >       bool operator!=(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
   >       template<class T, class U>
   >       bool operator<(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
   >       template<class T, class U>
   >       bool operator>(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
   >       template<class T, class U>
   >       bool operator<=(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;
   >       template<class T, class U>
   >       bool operator>=(const shared_ptr<T>& a, const shared_ptr<U>& b) noexcept;

   >       template<class T>
   >       bool operator==(const shared_ptr<T>& a, nullptr_t) noexcept;
   >       template<class T>
   >       bool operator==(nullptr_t, const shared_ptr<T>& a) noexcept;
   >       template<class T>
   >       bool operator!=(const shared_ptr<T>& a, nullptr_t) noexcept;
   >       template<class T>
   >       bool operator!=(nullptr_t, const shared_ptr<T>& a) noexcept;
   >       template<class T>
   >       bool operator<(const shared_ptr<T>& a, nullptr_t) noexcept;
   >       template<class T>
   >       bool operator<(nullptr_t, const shared_ptr<T>& a) noexcept;
   >       template>class T>
   >       bool operator>(const shared_ptr<T>& a, nullptr_t) noexcept;
   >       template>class T>
   >       bool operator>(nullptr_t, const shared_ptr<T>& a) noexcept;
   >       template<class T>
   >       bool operator<=(const shared_ptr<T>& a, nullptr_t) noexcept;
   >       template<class T>
   >       bool operator<=(nullptr_t, const shared_ptr<T>& a) noexcept;
   >       template>class T>
   >       bool operator>=(const shared_ptr<T>& a, nullptr_t) noexcept;
   >       template>class T>
   >       bool operator>=(nullptr_t, const shared_ptr<T>& a) noexcept;

   >       […]
   >     }

5. Add the following series of prototype specifications at the very end of 20.8.2.2.7 [util.smartptr.shared.cmp]. For mixed comparison the general "generation" rule of 20.2.1 [operators] p. 10 does not apply, therefore all of them are defined. Below wording takes advantage of the simplified definition of the composite pointer type if one partner is a null pointer constant:

   >     template<class T>
   >     bool operator==(const shared_ptr<T>& a, nullptr_t) noexcept;
   >     template<class T>
   >     bool operator==(nullptr_t, const shared_ptr<T>& a) noexcept;

   > > ? Returns: `!a`.

   >     template<class T>
   >     bool operator!=(const shared_ptr<T>& a, nullptr_t) noexcept;
   >     template<class T>
   >     bool operator!=(nullptr_t, const shared_ptr<T>& a) noexcept;

   > > ? Returns: `(bool) a`.

   >     template<class T>
   >     bool operator<(const shared_ptr<T>& a, nullptr_t) noexcept;
   >     template<class T>
   >     bool operator>(nullptr_t, const shared_ptr<T>& a) noexcept;

   > > ? Returns: `less<T*>()(a.get(), nullptr)`.

   >     template<class T>
   >     bool operator<(nullptr_t, const shared_ptr<T>& a) noexcept;
   >     template<class T>
   >     bool operator>(const shared_ptr<T>& a, nullptr_t) noexcept;

   > > ? Returns: `less<T*>()(nullptr, a.get())`.

   >     template<class T>
   >     bool operator<=(const shared_ptr<T>& a, nullptr_t) noexcept;
   >     template<class T>
   >     bool operator>=(nullptr_t, const shared_ptr<T>& a) noexcept;

   > > ? Returns: `!(nullptr < a)`.

   >     template<class T>
   >     bool operator<=(nullptr_t, const shared_ptr<T>& a) noexcept;
   >     template<class T>
   >     bool operator>=(const shared_ptr<T>& a, nullptr_t) noexcept;

   > > ? Returns: `!(a < nullptr)`.

### 1402. `nullptr` constructors for smart pointers should be `constexpr`

Section: 20.7 [memory] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

View all other issues in [memory].

View all issues with C++11 status.

Discussion:

Addresses GB-100

The `unique_ptr` and `shared_ptr` constructors taking `nullptr_t` delegate to a `constexpr` constructor, and could be `constexpr` themselves.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

In the 20.8.1.2 [unique.ptr.single] synopsis add "constexpr" to `unique_ptr(nullptr_t)`.
In the 20.8.1.3 [unique.ptr.runtime] synopsis add "constexpr" to `unique_ptr(nullptr_t)`.
In the 20.8.2.2 [util.smartptr.shared] synopsis add "constexpr" to `shared_ptr(nullptr_t)`.

### 1403. Inconsistent definitions for `allocator_arg`

Section: 20.7.6 [allocator.tag] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2015-04-08

View all issues with C++11 status.

Discussion:

Addresses JP-85

There are inconsistent definitions for `allocator_arg`. In 20.7 [memory] paragraph 1,

    constexpr allocator_arg_t allocator_arg = allocator_arg_t();

and in 20.9.1,

    const allocator_arg_t allocator_arg = allocator_arg_t();

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Change "const" to "constexpr" in 20.9.1 as follows.

    constexpr allocator_arg_t allocator_arg = allocator_arg_t();

### 1404. `pointer_traits` should have a `size_type` member

Section: 20.7.3 [pointer.traits] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

View all issues with C++11 status.

Discussion:

Addresses US-106

`pointer_traits` should have a `size_type` member for completeness.

Add `typedef see below size_type;` to the generic `pointer_traits` template and `typedef size_t size_type;` to `pointer_traits<T*>`. Use `pointer_traits::size_type` and `pointer_traits::difference_type` as the defaults for `allocator_traits::size_type` and `allocator_traits::difference_type`.

See Appendix 1 - Additional Details

[ Post-Rapperswil, Pablo provided wording: ]

The original ballot comment reads simply: "`pointer_traits` should have a `size_type` for completeness." The additional details reveal, however, that the desire for a `size_type` is actually driven by the needs of `allocator_traits`. The `allocator_traits` template should get its default `difference_type` from `pointer_traits` but if it did, it should get its `size_type` from the same source. Unfortunately, there is no obvious meaning for `size_type` in `pointer_traits`.

Alisdair suggested, however, that the natural relationship between `difference_type` and `size_type` can be expressed simply by the `std::make_unsigned<T>` metafunction. Using this metafunction, we can easily define `size_type` for `allocator_traits` without artificially adding `size_type` to `pointer_traits`.

> Moved to Tentatively Ready after 6 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

In [allocator.requirements], Table 42, change two rows as follows:

> `X::size_type` | unsigned integral type | a type that can represent the size of the largest object in the allocation model | `size_t make_unsigned<X::difference_type>::type`
> `X::difference_type` | signed integral type | a type that can represent the difference between any two pointers in the allocation model | `ptrdiff_t pointer_traits<X::pointer>::difference_type`

In [allocator.traits.types], Change the definition of `difference_type` and `size_type` as follows:

> `typedef` see below `difference_type;`

> > Type: `Alloc::difference_type` if such a type exists, else `ptrdiff_t pointer_traits<pointer>::difference_type`.

> `typedef` see below `size_type;`

> > Type: `Alloc::size_type` if such a type exists, else `size_t make_unsigned<difference_type>::type`.

### 1405. Move `scoped_allocator_adaptor` into separate header

Section: 20.13 [allocator.adaptor] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

View all other issues in [allocator.adaptor].

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Discussion:

Addresses US-107

`scoped_allocator_adaptor` should have its own header.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

See Appendix 1 - Additional Details

### 1407. Synch `shared_ptr` constructors taking movable types

Section: 20.8.2.2.1 [util.smartptr.shared.const] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

View all other issues in [util.smartptr.shared.const].

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Discussion:

Addresses US-108

`shared_ptr` should have the same policy for constructing from `auto_ptr` as `unique_ptr`. Currently it does not.

[ Resolved in Rapperswil by paper N3109. ]

Proposed resolution:

Add

    template <class Y> explicit shared_ptr(auto_ptr<Y>&);

to [util.smartptr.shared.const] (and to the synopsis).

### 1408. Allow recycling of pointers after `undeclare_no_pointers`

Section: 20.7.4 [util.dynamic.safety] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

View all other issues in [util.dynamic.safety].

View all issues with C++11 status.

Discussion:

Addresses GB-103

The precondition to calling `declare_no_pointers` is that no bytes in the range "have been previously registered" with this call. As written, this precondition includes bytes in ranges, even after they have been explicitly unregistered with a later call to `undeclare_no_pointers`.

Proposed resolution:

Update 20.7.4 [util.dynamic.safety] p.9:

>     void declare_no_pointers(char *p, size_t n);

> > `9` Requires: No bytes in the specified range have been previously registered are currently registered with `declare_no_pointers()`. If the specified range is in an allocated object, then it must be entirely within a single allocated object. The object must be live until the corresponding `undeclare_no_pointers()` call. [..]

### 1409. Specify whether `monotonic_clock` is a distinct type or a typedef

Section: X [time.clock.monotonic] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

View all other issues in [time.clock.monotonic].

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Discussion:

Addresses US-111

What it means for `monotonic_clock` to be a synonym is undefined. If it may or may not be a typedef, then certain classes of programs become unportable.

[ Resolution proposed in ballot comment: ]

Require that it be a distinct class type.

[ 2010-11-01 Daniel comments: ]

> Paper n3128 addresses this issue by replacing `monotonic_clock` with `steady_clock`, which is not a typedef.

Proposed resolution:

This is resolved by n3191.

### 1410. Add a feature-detect macro for `monotonic_clock`

Section: X [time.clock.monotonic] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

View all other issues in [time.clock.monotonic].

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Duplicate of: 1411

Discussion:

Addresses GB-107, DE-20

1.4 [intro.compliance] p.9 states that which conditionally supported constructs are available should be provided in the documentation for the implementation. This doesn't help programmers trying to write portable code, as they must then rely on implementation-specific means to determine the availability of such constructs. In particular, the presence or absence of `std::chrono::monotonic_clock` may require different code paths to be selected. This is the only conditionally-supported library facility, and differs from the conditionally-supported language facilities in that it has standard-defined semantics rather than implementation-defined semantics.

[ Resolution proposed in ballot comment: ]

Provide feature test macro for determining the presence of `std::chrono::monotonic_clock`. Add `_STDCPP_HAS_MONOTONIC_CLOCK` to the `<chrono>` header, which is defined if `monotonic_clock` is present, and not defined if it is not present.

[ 2010-11-01 Daniel comments: ]

> Paper n3128 addresses this issue by replacing `monotonic_clock` with `steady_clock`, which is not conditionally supported, so there is no need to detect it.

Proposed resolution:

This is resolved by n3191.

### 1412. Make monotonic clocks mandatory

Section: X [time.clock.monotonic] Status: Resolved Submitter: Switzerland Opened: 2010-08-25 Last modified: 2015-04-08

View all other issues in [time.clock.monotonic].

View all issues with Resolved status.

Discussion:

Addresses CH-21

Monotonic clocks are generally easy to provide on all systems and are implicitely required by some of the library facilities anyway.

[ 2010-11-01 Daniel comments: ]

> Paper n3128 addresses this issue by replacing `monotonic_clock` with `steady_clock`, which is mandatory.

[ 2010-11-13 Batavia meeting: ]

This is resolved by adopting n3191. The original resolution is preserved for reference:

> Make monotonic clocks mandatory.

> Strike X [time.clock.monotonic] p.2

> > `2` The class `monotonic_clock` is conditionally supported.

> Change 30.2.4 [thread.req.timing] p.2 accordingly

> > The member functions whose names end in `_for` take an argument that specifies a relative time. Implementations should use a monotonic clock to measure time for these functions. [ Note: Implementations are not required to use a monotonic clock because such a clock may not be available. — end note ]

Proposed resolution:

This is resolved by n3191.

### 1414. Fixing remaining dead links to `POS_T` and `OFF_T`

Section: 21.2.3.2 [char.traits.specializations.char16_t] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Duplicate of: 1444

Discussion:

Addresses GB-109, GB-123

It is not clear what the specification means for `u16streampos`, `u32streampos` or `wstreampos` when they refer to the requirements for `POS_T` in 21.2.2, as there are no longer any such requirements. Similarly the annex D.7 refers to the requirements of type `POS_T` in 27.3 that no longer exist either.

Clarify the meaning of all cross-reference to the removed type `POS_T`.

[ Post-Rapperswil, Daniel provides the wording. ]

When preparing the wording for this issue I first thought about adding both `u16streampos` and `u32streampos` to the [iostream.forward] header `<iosfwd>` synopsis similar to `streampos` and `wstreampos`, but decided not to do so, because the IO library does not yet actively support the `char16_t` and `char32_t` character types. Adding those would misleadingly imply that they would be part of the iostreams. Also, the addition would make them also similarly equal to a typedef to `fpos<mbstate_t>`, as for `streampos` and `wstreampos`, so there is no loss for users that would like to use the proper `fpos` instantiation for these character types.

Additionally the way of referencing was chosen to follow the style suggested by NB comment GB 108.

> Moved to Tentatively Ready with proposed wording after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

The following wording changes are against N3126.

1. Change [char.traits.specializations.char16_t]p.1 as indicated:

   > 1 - The type `u16streampos` shall be an implementation-defined type that satisfies the requirements for `POS_T` in 21.2.2 `pos_type` in [iostreams.limits.pos].

2. Change [char.traits.specializations.char32_t]p.1 as indicated:

   > 1 - The type `u32streampos` shall be an implementation-defined type that satisfies the requirements for `POS_T` in 21.2.2 `pos_type` in [iostreams.limits.pos].

3. Change [char.traits.specializations.wchar.t]p.2 as indicated:

   > 2 - The type `wstreampos` shall be an implementation-defined type that satisfies the requirements for `POS_T` in 21.2.2 `pos_type` in [iostreams.limits.pos].

4. Change [fpos.operations], Table 124 — Position type requirements as indicated:

   > Table 124 — Position type requirements
   > Expression | Return type | `...`
   > --- | --- | ---
   > `...` | `...` | `...`
   > `O(p)` | `OFF_T` `streamoff` | ...
   > `...` | `...` | `...`
   > `o = p - q` | `OFF_T` `streamoff` | `...`
   > `streamsize(o)` `O(sz)` | `streamsize` `OFF_T` `streamoff` | `...`

5. Change [depr.ios.members]p.1 as indicated:

   >     namespace std {
   >      class ios_base {
   >      public:
   >        typedef T1 io_state;
   >        typedef T2 open_mode;
   >        typedef T3 seek_dir;
   >        typedef OFF_Timplementation-defined streamoff;
   >        typedef POS_Timplementation-defined streampos;
   >        // remainder unchanged
   >      };
   >     }

6. Change [depr.ios.members]p.5+6 as indicated:

   > 5 - The type `streamoff` is an implementation-defined type that satisfies the requirements of type `OFF_T` (27.5.1)`off_type` in [iostreams.limits.pos].

   > 6 - The type `streampos` is an implementation-defined type that satisfies the requirements of type `POS_T` (27.3)`pos_type` in [iostreams.limits.pos].

### 1416. `forward_list::erase_after` should not be allowed to throw

Section: 23.2 [container.requirements] Status: C++11 Submitter: DIN Opened: 2010-08-25 Last modified: 2015-04-08

View other active issues in [container.requirements].

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Discussion:

Addresses DE-21

23.2.1/11 provides a general no-throw guarantee for erase() container functions, exceptions from this are explicitly mentioned for individual containers. Because of its different name, forward_list's erase_after() function is not ruled by this but should so.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Add a "Throws: Nothing" clause to both `erase_after` overloads in 23.3.3.4, [forwardlist.modifiers].

### 1417. `front/back` on a zero-sized `array` should be undefined

Section: 23.3.2.8 [array.zero] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-112

Should the effect of calling `front/back` on a zero-sized `array` really be implementation defined i.e. require the implementor to define behaviour?

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Change "implementation defined" to "undefined"

### 1418. Effects of `resize(size())` on a `deque`

Section: 23.3.3.3 [deque.capacity] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-113

There is no mention of what happens if `sz==size()`. While it obviously does nothing I feel a standard needs to say this explicitely.

[ 2010 Batavia ]

Accepted with a simplified resolution turning one of the `<` comparisons into `<=.`

Proposed resolution:

Ammend [deque.capacity]

`void resize(size_type sz);`

Effects: If `sz <= size()`, equivalent to `erase(begin() + sz, end());`. If `size() < sz`, appends `sz - size()` default constructed value initialized elements to the sequence.

### 1420. Effects of `resize(size())` on a `list`

Section: 23.3.5.3 [list.capacity] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-115

There is no mention of what happens if `sz==size()`. While it obviously does nothing I feel a standard needs to say this explicitely.

[ Resolution proposed in ballot comment ]

Express the semantics as pseudo-code similarly to the way it is done for the copying overload that follows (in p3). Include an else clause that does nothing and covers the `sz==size()` case.

[ 2010 Batavia ]

Accepted with a simplified resolution turning one of the `<` comparisons into `<=.`

Proposed resolution:

Ammend [list.capacity] p1:

> `void resize(size_type sz);`

> > Effects: If `sz <= size()`, equivalent to `list<T>::iterator it = begin(); advance(it, sz); erase(it, end());`. If `size() < sz`, appends `sz - size()` default constructed value initialized elements to the sequence.

### 1421. Accidental move-only library types due to new core language rules

Section: 23.6 [container.adaptors] Status: Resolved Submitter: DIN Opened: 2010-08-25 Last modified: 2015-04-08

View all other issues in [container.adaptors].

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Duplicate of: 1350

Discussion:

Addresses DE-22, CH-15

With the final acceptance of move operations as special members and introduction of corresponding suppression rules of implicitly generated copy operations the some library types that were copyable in C++03 are no longer copyable (only movable) in C++03, among them `queue`, `priority_queue`, and `stack`.

[ 2010-10-26: Daniel comments: ]

Accepting n3112 should fix this.

[2011-02-17: Lawrence comments:]

The only open issue in CH 15 with respect to the concurrency group was the treatment of `atomic_future`. Since we removed `atomic_future` in Batavia, I think all that remains is to remove the open issue from N3112 and adopt it.

[2011-03-23 Madrid meeting]

Resolved by N3264

Proposed resolution:

See n3112

### 1423. `map` constructor accepting an allocator as single parameter should be explicit

Section: 23.4.4 [map] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses JP-6

Constructor accepting an allocator as a single parameter should be qualified as explicit.

    namespace std {
    template <class Key, class T, class Compare =
    less<Key>,
    class Allocator = allocator<pair<const Key, T> > >
    class map {
    public:
    ...
    map(const Allocator&);

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Add explicit.

    namespace std {
    template <class Key, class T, class Compare =
    less<Key>,
    class Allocator = allocator<pair<const Key, T> > >
    class map {
    public:
    ...
    explicit map(const Allocator&);

### 1424. `multimap` constructor accepting an allocator as a single parameter should be explicit

Section: 23.4.5 [multimap] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses JP-7

Constructor accepting an allocator as a single parameter should be qualified as explicit.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Add explicit.

    namespace std {
    template <class Key, class T, class Compare =
    less<Key>,
    class Allocator = allocator<pair<const Key, T> > >
    class multimap {
    public:
    ...
    explicit multimap(const Allocator&);

### 1425. `set` constructor accepting an allocator as a single parameter should be explicit

Section: 23.4.6 [set] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses JP-8

Constructor accepting an allocator as a single parameter should be qualified as explicit.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Add explicit.

    namespace std {
    template <class Key, class Compare = less<Key>,
    class Allocator = allocator<Key> >
    class set {
    public:
    ...
    explicit set(const Allocator&);

### 1426. `multiset` constructor accepting an allocator as a single parameter should be explicit

Section: 23.4.7 [multiset] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses JP-9

Constructor accepting an allocator as a single parameter should be qualified as explicit.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Add explicit.

    namespace std {
    template <class Key, class Compare = less<Key>,
    class Allocator = allocator<Key> >
    class multiset {
    public:
    ...
    explicit multiset(const Allocator&);

### 1427. `unordered_map` constructor accepting an allocator as a single parameter should be explicit

Section: 23.5.4 [unord.map] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses JP-10

Constructor accepting an allocator as a single parameter should be qualified as explicit.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Add explicit.

    namespace std {
    template <class Key,
    template <class Key,
    class T,
    class Hash = hash<Key>,
    class Pred = std::equal_to<Key>,
    class Alloc = std::allocator<std::pair<const Key,
    T> > >
    class unordered_map
    {
    public:
    ...
    explicit unordered_map(const Allocator&);

### 1428. `unordered_multimap` constructor accepting an allocator as a single parameter should be explicit

Section: 23.5.5 [unord.multimap] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses JP-11

Constructor accepting an allocator as a single parameter should be qualified as explicit.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Add explicit.

    namespace std {
    template <class Key,
    class T,
    class Hash = hash<Key>,
    class Pred = std::equal_to<Key>,
    class Alloc = std::allocator<std::pair<const Key,
    T> > >
    class unordered_multimap
    {
    public:
    ...
    explicit unordered_multimap(const Allocator&);

### 1429. `unordered_set` constructor accepting an allocator as a single parameter should be explicit

Section: 23.5.6 [unord.set] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses JP-12

Constructor accepting an allocator as a single parameter should be qualified as explicit.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Add explicit.

    namespace std {
    template <class Key,
    class Hash = hash<Key>,
    class Pred = std::equal_to<Key>,
    class Alloc = std::allocator<Key> >
    class unordered_set
    {
    public:
    ...
    explicit unordered_set(const Allocator&);

### 1430. `unordered_multiset` constructor accepting an allocator as a single parameter should be explicit

Section: 23.5.7 [unord.multiset] Status: C++11 Submitter: Japan Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses JP-13

Constructor accepting an allocator as a single parameter should be qualified as explicit.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Add explicit.

    namespace std {
    template <class Key,
    class Hash = hash<Key>,
    class Pred = std::equal_to<Key>,
    class Alloc = std::allocator<Key> >
    class unordered_multiset
    {
    public:
    ...
    explicit unordered_multiset(const Allocator&);

### 1431. `is_permutation` must be more restrictive

Section: 25.2.12 [alg.is_permutation] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-120

`is_permutation` is underspecified for anything but the simple case where both ranges have the same value type and the comparison function is an equivalence relation.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

Restrict `is_permutation` to the case where it is well specified. See Appendix 1 - Additional Details

### 1432. `random_shuffle` signatures are inconsistent

Section: 25.3.12 [alg.random.shuffle] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Duplicate of: 1433

Discussion:

Addresses US-121, GB-119

`random_shuffle` and `shuffle` should be consistent in how they accept their source of randomness: either both by rvalue reference or both by lvalue reference.

[ Post-Rapperswil, Daniel provided wording ]

The signatures of the `shuffle` and `random_shuffle` algorithms are different in regard to the support of rvalues and lvalues of the provided generator:

>     template<class RandomAccessIterator, class RandomNumberGenerator>
>     void random_shuffle(RandomAccessIterator first,
>     RandomAccessIterator last,
>     RandomNumberGenerator&& rand);

>     template<class RandomAccessIterator, class UniformRandomNumberGenerator>
>     void shuffle(RandomAccessIterator first,
>     RandomAccessIterator last,
>     UniformRandomNumberGenerator& g);

The first form uses the perfect forwarding signature and that change compared to `C++03` was done intentionally as shown in the first rvalue proposal papers.

While it is true, that random generators are excellent examples of stateful functors, there still exist good reasons to support rvalues as arguments:

1. If one of the shuffle algorithms is called with the intention to shuffle items with a reproducible ordering from a given generator class, it makes sense to create a generator exactly at the call point.
2. Other algorithms with similar need for stateful functors (like `std::generate` and `std::generate_n`) accept both rvalues and lvalues as well.
3. Given the deduction rules for perfect forwarding it is hard for a user to produce code that does the wrong thing unintentionally. Any lvalue generator will deduce an lvalue-reference and behave as in `C++03`. In the specific cases, where rvalues are provided, the argument will be accepted instead of being rejected.

Arguments have been raised that accepting rvalues is error-prone or even fundamentally wrong. The author of this proposal disagrees with that position for two additional reasons:

1. Enforcing lvalues as arguments won't prevent user code to enforce what they want. So given

   >     my_generator get_generator(int size);

   instead of writing

   >     std::vector<int> v = ...;
   >     std::shuffle(v.begin(), v.end(), get_generator(v.size()));

   they will just write

   >     std::vector<int> v = ...;
   >     auto gen = get_generator(v.size());
   >     std::shuffle(v.begin(), v.end(), gen);

   and feel annoyed about the need for it.
2. Generators may be copyable and movable, and random number engines are required to be `CopyConstructible` and this is obviously a generally useful property for such objects. It is also useful and sometimes necessary to start a generator with exactly a specific seed again and again and thus to provide a new generator (or a copy) for each call. The `CopyConstructible` requirements allow providing rvalues of generators and thus this idiom must be useful as well. Therefore preventing `[random_]shuffle` to accept rvalues is an unnecessary restriction which doesn't prevent any user-error, if there would exist one.

Thus this proposal recommends to make both `shuffle` functions consistent and perfectly forward-able.

> Moved to Tentatively Ready after 6 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

1. Change [algorithms.general], header `<algorithm>` synopsis as indicated:

   >     template<class RandomAccessIterator, class UniformRandomNumberGenerator>
   >     void shuffle(RandomAccessIterator first, RandomAccessIterator last,
   >     UniformRandomNumberGenerator&& rand);

2. Change the prototype description of [alg.random.shuffle] as indicated:

   >     template<class RandomAccessIterator, class UniformRandomNumberGenerator>
   >     void shuffle(RandomAccessIterator first, RandomAccessIterator last,
   >     UniformRandomNumberGenerator&& rand);

### 1435. Unclear returns specifications for C99 complex number functions

Section: 26.4.7 [complex.value.ops] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

View all other issues in [complex.value.ops].

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Discussion:

Addresses GB-120

The complex number functions added for compatibility with the C99 standard library are defined purely as a cross-reference, with no hint of what they should return. This is distinct from the style of documentation for the functions in the earlier standard. In the case of the inverse-trigonometric and hyperbolic functions, a reasonable guess of the functionality may be made from the name, this is not true of the cproj function, which apparently returns the projection on the Reimann Sphere. A single line description of each function, associated with the cross-reference, will greatly improve clarity.

[2010-11-06 Beman provides proposed resolution wording.]

[ 2010 Batavia: The working group concurred with the issue's Proposed Resolution ]

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 26.4.7 complex value operations [complex.value.ops] as indicated:

> `template<class T> complex<T> proj(const complex<T>& x);`

> > Returns: the projection of `x` onto the Riemann sphere.

> > Effects: Remarks: Behaves the same as the C function `cproj`, defined in 7.3.9.4.

Change 26.4.8 complex transcendentals [complex.transcendentals] as indicated:

> `template<class T> complex<T> acos(const complex<T>& x);`

> > Returns: the complex arc cosine of `x`.

> > Effects: Remarks: Behaves the same as the C function `cacos`, defined in 7.3.5.1.

Change 26.4.8 complex transcendentals [complex.transcendentals] as indicated:

> `template<class T> complex<T> asin(const complex<T>& x);`

> > Returns: the complex arc sine of `x`.

> > Effects: Remarks: Behaves the same as the C function `casin`, defined in 7.3.5.2.

Change 26.4.8 complex transcendentals [complex.transcendentals] as indicated:

> `template<class T> complex<T> atan(const complex<T>& x);`

> > Returns: the complex arc tangent of `x`.

> > Effects: Remarks: Behaves the same as the C function `catan`, defined in 7.3.5.3.

Change 26.4.8 complex transcendentals [complex.transcendentals] as indicated:

> `template<class T> complex<T> acosh(const complex<T>& x);`

> > Returns: the complex arc hyperbolic cosine of `x`.

> > Effects: Remarks: Behaves the same as the C function `cacosh`, defined in 7.3.6.1.

Change 26.4.8 complex transcendentals [complex.transcendentals] as indicated:

> `template<class T> complex<T> asinh(const complex<T>& x);`

> > Returns: the complex arc hyperbolic sine of `x`.

> > Effects: Remarks: Behaves the same as the C function `casinh`, defined in 7.3.6.2.

Change 26.4.8 complex transcendentals [complex.transcendentals] as indicated:

> `template<class T> complex<T> atanh(const complex<T>& x);`

> > Returns: the complex arc hyperbolic tangent of `x`.

> > Effects: Remarks: Behaves the same as the C function `catanh`, defined in 7.3.6.2.

### 1436. Random number engine constructor concerns

Section: 26.5.3 [rand.eng] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

View all other issues in [rand.eng].

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Discussion:

Addresses GB-121

All the random number engine types in this clause have a constructor taking an unsigned integer type, and a constructor template for seed sequences. This means that an attempt to create a random number engine seeded by an integer literal must remember to add the appropriate unsigned suffix to the literal, as a signed integer will attempt to use the seed sequence template, yielding undefined behaviour, as per 26.5.1.1p1a. It would be helpful if at least these anticipated cases produced a defined behaviour, either an erroneous program with diagnostic, or a conversion to unsigned int forwarding to the appropriate constructor.

[ 2010-11-03 Daniel comments and provides a proposed resolution: ]

I suggest to apply a similar solution as recently suggested for 1234. It is basically a requirement for an implementation to constrain the template.

[ 2010-11-04 Howard suggests to use `!is_convertible<Sseq, result_type>::value` as minimum requirement instead of the originally proposed `!is_scalar<Sseq>::value`. This would allow for a user-defined type `BigNum`, that is convertible to `result_type`, to be used as argument for a seed instead of a seed sequence. The wording has been updated to reflect this suggestion. ]

[ 2010 Batavia: There were some initial concerns regarding the portability and reproducibility of results when seeded with a negative signed value, but these concerns were allayed after discussion. Thus, after reviewing the issue, the working group concurred with the issue's Proposed Resolution. ]

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Add the following paragraph at the end of 26.5.3 [rand.eng]:

> > 5 Each template specified in this section [rand.eng] requires one or more relationships, involving the value(s) of its non-type template parameter(s), to hold. A program instantiating any of these templates is ill-formed if any such required relationship fails to hold.

> > ? For every random number engine and for every random number engine adaptor `X` defined in this sub-clause [rand.eng] and in sub-clause [rand.adapt]:

> > - If the constructor

> >   >     template<class Sseq> explicit X(Sseq& q);

> >   is called with a type `Sseq` that does not qualify as a seed sequence, then this constructor shall not participate in overload resolution.

> > - If the member function

> >   >     template<class Sseq> void seed(Sseq& q);

> >   is called with a type `Sseq` that does not qualify as a seed sequence, then this function shall not participate in overload resolution.

> > The extent to which an implementation determines that a type cannot be a seed sequence is unspecified, except that as a minimum a type shall not qualify as seed sequence, if it is implicitly convertible to `X::result_type`.

### 1437. Mersenne twister meaningless for word sizes less than two

Section: 26.5.3.2 [rand.eng.mers] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-124

The Mersenne twister algorithm is meaningless for word sizes less than two, as there are then insufficient bits available to be “twisted”.

[ Resolution proposed by ballot comment: ]

> Insert the following among the relations that are required to hold: `2u < w`.

[ 2010 Batavia: The working group concurred with the issue's Proposed Resolution ]

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 26.5.3.2 [rand.eng.mers] p. 4 as indicated:

> 4 The following relations shall hold: `0 < m`, `m <= n`, `2u < w`, `r <= w`, `u <= w`, `s <= w`, `t <= w`, `l <= w`, `w <= numeric_limits<UIntType>::digits`, `a <= (1u<<w) - 1u`, `b <= (1u<<w) - 1u`, `c <= (1u<<w) - 1u`, `d <= (1u<<w) - 1u`, and `f <= (1u<<w) - 1u`.

### 1438. No definition for `base()`

Section: 26.5.4.2 [rand.adapt.disc] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-126

Each adaptor has a member function called `base()` which has no definition.

[ Resolution proposed by ballot comment: ]

> Give it the obvious definition.

[ 2010-11-03 Daniel comments and provides a proposed resolution: ]

The following proposal adds `noexcept` specifiers to the declarations of the `base()` functions as replacement for a "Throws: Nothing" element.

[ 2010 Batavia: The working group reviewed this issue, and recommended to add the following to the Proposed Resolution. ]

- Append to paragraph 1 of [rand.req.adapt] (or at the Editor's discretion insert as a new paragraph following that paragraph): The expression `a.base()` shall be valid and shall return a const reference to `a`'s base engine.

After further review, the working group concurred with the Proposed Resolution.

[Batavia: waiting for WEB to review wording]

Proposed resolution:

1. Add the following sentence to the end of 26.5.1.5 [rand.req.adapt]/1:

   > A random number engine adaptor (commonly shortened to adaptor) `a` of type `A` is a random number engine that takes values produced by some other random number engine, and applies an algorithm to those values in order to deliver a sequence of values with different randomness properties. An engine `b` of type `B` adapted in this way is termed a base engine in this context. The expression `a.base()` shall be valid and shall return a const reference to `a`'s base engine.

2. Change in [rand.adapt.disc]/3, class template `discard_block_engine` synopsis, the following declaration:

   >     // property functions
   >     const Engine& base() const noexcept;

3. Add the following new prototype description at the end of sub-clause 26.5.4.2 [rand.adapt.disc]:

   >     const Engine& base() const noexcept;

   > > ? Returns: `e`.

4. Change in [rand.adapt.ibits]/4, class template `independent_bits_engine` synopsis, the following declaration:

   >     // property functions
   >     const Engine& base() const noexcept;

5. Add the following new prototype description at the end of sub-clause 26.5.4.3 [rand.adapt.ibits]:

   >     const Engine& base() const noexcept;

   > > ? Returns: `e`.

6. Change in 26.5.4.4 [rand.adapt.shuf]/3, class template `shuffle_order_engine` synopsis, the following declaration:

   >     // property functions
   >     const Engine& base() const noexcept;

7. Add the following new prototype description at the end of sub-clause 26.5.4.4 [rand.adapt.shuf]:

   >     const Engine& base() const noexcept;

   > > ? Returns: `e`.

### 1439. Return from `densities()` functions?

Section: 26.5.8.6.2 [rand.dist.samp.pconst] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-134

These two distributions have a member function called `densities()` which returns a `vector<double>`. The distribution is templated on `RealType`. The distribution also has another member called `intervals()` which returns a `vector<RealType>`. Why doesn't densities return `vector<RealType>` as well? If `RealType` is `long double`, the computed densities property isn't being computed to the precision the client desires. If `RealType` is `float`, the densities vector is taking up twice as much space as the client desires.

[ Resolution proposed by ballot comment: ]

> Change the piecewise constant and linear distributions to hold / return the densities in a `vector<result_type>`.

> If this is not done, at least correct 26.5.8.6.2 [rand.dist.samp.pconst] p. 13 which describes the return of densities as a `vector<result_type>`.

[ Batavia 2010: After reviewing this issue, the working group concurred with the first of the suggestions proposed by the NB comment: "Change the piecewise constant and linear distributions to hold/return the densities in a vector. " ]

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

1. Change 26.5.8.6.2 [rand.dist.samp.pconst] p. 2, class template `piecewise_constant_distribution` synopsis and the prototype description 26.5.8.6.2 [rand.dist.samp.pconst] before p. 13 as indicated:

   >     vector<doubleresult_type> densities() const;

2. Change 26.5.8.6.3 [rand.dist.samp.plinear] p. 2, class template `piecewise_linear_distribution` synopsis and the prototype description 26.5.8.6.3 [rand.dist.samp.plinear] before p. 13 as indicated:

   >     vector<doubleresult_type> densities() const;

### 1440. Incorrect specification for `piecewise_linear_distribution`

Section: 26.5.8.6.3 [rand.dist.samp.plinear] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-135

This paragraph says: Let bk = xmin+k·δ for k = 0,...,n, and wk = fw(bk +δ) for k = 0,...,n. However I believe that fw(bk) would be far more desirable. I strongly suspect that this is nothing but a type-o.

[ Resolution proposed by ballot comment: ]

> Change p. 10 to read:
> Let bk = xmin+k·δ for k = 0,...,n, and wk = fw(bk) for k = 0,...,n.

[ 2010-11-02 Daniel translates into a proposed resolution ]

[ 2010 Batavia: The working group concurred with the issue's Proposed Resolution ]

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Change 26.5.8.6.3 [rand.dist.samp.plinear] p. 10 as indicated:

> 10 Effects: Constructs a `piecewise_linear_distribution` object with parameters taken or calculated from the following values: Let `bk = xmin+k·δ` for `k = 0, . . . , n`, and `wk = fw(bk +δ)` for `k = 0, . . . , n`.

### 1441. Floating-point test functions are incorrectly specified

Section: 26.8 [c.math] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-136

Floating-point test functions are incorrectly specified.

[ Resolved in Rapperswil by a motion to directly apply the words from the ballot comment in N3102. ]

Proposed resolution:

See Appendix 1 - Additional Details

### 1445. Several iostreams member functions incorrectly specified

Section: 27.7 [iostream.format] Status: Resolved Submitter: INCITS/PJ Plauger Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-137

Several iostreams member functions are incorrectly specified.

[ Resolution proposed by ballot comment: ]

See Appendix 1 - Additional Details

[ 2010-10-24 Daniel adds: ]

> Accepting n3168 would solve this issue.

Proposed resolution:

Addressed by paper n3168.

### 1447. Request to resolve issue LWG 1328

Section: 27.7 [iostream.format] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-139

Resolve issue LWG 1328 one way or the other, but preferably in the direction outlined in the proposed resolution, which, however, is not complete as-is: in any case, the sentry must not set `ok_ = false` if `is.good() == false`, otherwise `istream::seekg`, being an unformatted input function, does not take any action because the sentry object returns false when converted to type `bool`. Thus, it remains impossible to seek away from end of file.

Proposed resolution:

Addressed by paper n3168.

### 1448. Concerns about `basic_stringbuf::str(basic_string)` postconditions

Section: 27.8.2.3 [stringbuf.members] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-124

N3092 27.8.2.3 [stringbuf.members] contains this text specifying the postconditions of `basic_stringbuf::str(basic_string)`:

> Postconditions: If `mode & ios_base::out` is `true`, `pbase()` points to the first underlying character and `epptr() >= pbase() + s.size()` holds; in addition, if `mode & ios_base::in` is `true`, `pptr() == pbase() + s.data()` holds, otherwise `pptr() == pbase()` is `true`. [...]

Firstly, there's a simple mistake: It should be `pbase() + s.length()`, not `pbase() + s.data()`.

Secondly, it doesn't match existing implementations. As far as I can tell, GCC 4.5 does not test for `mode & ios_base::in` in the second part of that sentence, but for `mode & (ios_base::app | ios_base_ate)`, and Visual C++ 9 for `mode & ios_base::app`. Besides, the wording of the C++0x draft doesn't make any sense to me. I suggest changing the second part of the sentence to one of the following:

Replace `ios_base::in` with `(ios_base::ate | ios_base::app)`, but this would require Visual C++ to change (replacing only with `ios_base::ate` would require GCC to change, and would make `ios_base::app` completely useless with `stringstreams`):

in addition, if `mode & (ios_base::ate | ios_base::app)` is `true`, `pptr() == pbase() + s.length()` holds, otherwise `pptr() == pbase()` is `true`.

Leave `pptr()` unspecified if `mode & ios_base::app`, but not `mode & ios_base::ate` (implementations already differ in this case, and it is always possible to use `ios_base::ate` to get the effect of appending, so it is not necessary to require any implementation to change):

in addition, if `mode & ios_base::ate` is `true`, `pptr() == pbase() + s.length()` holds, if neither `mode & ios_base::ate` nor `mode & ios_base::app` is `true`, `pptr() == pbase()` holds, otherwise `pptr() >= pbase() && pptr() <= pbase() + s.length()` (which of the values in this range is unspecified).

Slightly stricter:

in addition, if `mode & ios_base::ate` is `true`, `pptr() == pbase() + s.length()` holds, if neither `mode & ios_base::ate` nor `mode & ios_base::app` is `true`, `pptr() == pbase()` holds, otherwise `pptr() == pbase() || pptr() == pbase() + s.length()` (which of these two values is unspecified). A small table might help to better explain the three cases. BTW, at the end of the postconditions is this text: "`egptr() == eback() + s.size()` hold". Is there a perference for `basic_string::length` or `basic_string::size`? It doesn't really matter, but it looks a bit inconsistent.

[2011-03-09: Nicolai Josuttis comments and drafts wording]

First, it seems the current wording is just an editorial mistake. When we added issue 432 to the draft standard (in n1733), the wording in the issue:

> If `mode & ios_base::out` is true, initializes the output sequence such that `pbase()` points to the first underlying character, `epptr()` points one past the last underlying character, and if `(mode & ios_base::ate)` is true, `pptr()` is set equal to `epptr()` else `pptr()` is set equal to `pbase()`.

became:

> If `mode & ios_base::out` is true, initializes the output sequence such that `pbase()` points to the first underlying character, `epptr()` points one past the last underlying character, and `pptr()` is equal to `epptr()` if `mode & ios_base::in` is true, otherwise `pptr()` is equal to `pbase()`.

which beside some changes of the order of words changed

>     ios_base::ate

into

>     ios_base::in

So, from this point of view, clearly `mode & ios_base::ate` was meant.

Nevertheless, with this proposed resolution we'd have no wording regarding `ios_base::app`. Currently the only statements about `app` in the Standard are just in two tables:

- Table 125 — "`openmode` effects" says that the effect of `app` is "seek to end before each write"
- Table 132 — "File open modes" says that the stdio equivalent for `app` is `"a"`

Indeed we seem to have different behavior currently in respect to `app`: For

>     stringstream s2(ios_base::out|ios_base::in|ios_base::app);
>     s2.str("s2 hello");
>     s1 << "more";

- Visual C++ 10 does overwrite (=> `"moreello"`)
- G++ 4.5 does append (=> `"s2 hellomore"`)

BTW, for fstreams, both implementations append when `app` is set: If `f2.txt` has contents `"xy"`,

>     fstream f2("f2.txt",ios_base::out|ios_base::in|ios_base::app);
>     f1 << "more";

appends `"more"` so that the contents is `"xymore"`.

So IMO `app` should set the write pointer to the end so that each writing appends.

I don't know whether what the standard says is enough. You can argue the statement in Table 125 clearly states that such a buffer should always append, which of course also applies to `str()` of stringbuffer.

Nevertheless, it doesn't hurt IMO if we clarify the behavior of `str()` here in respect to `app`.

[2011-03-10: P.J.Plauger comments:]

I think we should say nothing special about `app` at construction time (thus leaving the write pointer at the beginning of the buffer). Leave implementers wiggle room to ensure subsequent append writes as they see fit, but don't change existing rules for initial seek position.

[Madrid meeting: It was observed that a different issue should be opened that clarifies the meaning of `app` for `stringstream`]

Proposed resolution:

Change 27.8.2.3 [stringbuf.members] p. 3 as indicated:

>     void str(const basic_string<charT,traits,Allocator>& s);

> > 2 Effects: Copies the content of `s` into the `basic_stringbuf` underlying character sequence and initializes the input and output sequences according to `mode`.

> > 3 Postconditions: If `mode & ios_base::out` is true, `pbase()` points to the first underlying character and `epptr() >= pbase() + s.size()` holds; in addition, if `mode & ios_base::inios_base::ate` is true, `pptr() == pbase() + s.data()s.size()` holds, otherwise `pptr() == pbase()` is true. If `mode & ios_base::in` is true, `eback()` points to the first underlying character, and both `gptr() == eback() and egptr() == eback() + s.size()` hold.

### 1449. Incomplete specification of header `<cinttypes>`

Section: 27.8.3 [istringstream] Status: C++11 Submitter: Canada Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses CA-4

Subclause 27.9.2 [c.files] specifies that <cinttypes> has declarations for abs() and div(); however, the signatures are not present in this subclause. The signatures proposed under TR1 ([tr.c99.inttypes]) are not present in FCD (unless if intmax_t happened to be long long). It is unclear as to which, if any of the abs() and div() functions in [c.math] are meant to be declared by <cinttypes>. This subclause mentions imaxabs() and imaxdiv(). These functions, among other things, are not specified in FCD to be the functions from Subclause 7.8 of the C Standard. Finally, <cinttypes> is not specified in FCD to include <cstdint> (whereas <inttypes.h> includes <stdint.h> in C).

[ Post-Rapperswil, Daniel provides wording ]

Subclause [c.files] specifies that `<cinttypes>` has declarations for `abs()` and `div()`; however, the signatures are not present in this subclause. The signatures proposed under TR1 ([tr.c99.inttypes]) are not present in FCD (unless if `intmax_t` happened to be `long long`). It is unclear as to which, if any of the `abs()` and `div()` functions in [c.math] are meant to be declared by `<cinttypes>`. This subclause mentions `imaxabs()` and `imaxdiv()`. These functions, among other things, are not specified in FCD to be the functions from subclause 7.8 of the `C` Standard. Finally, `<cinttypes>` is not specified in FCD to include `<cstdint>` (whereas `<inttypes.h>` includes `<stdint.h>` in `C`).

> Moved to Tentatively Ready with proposed wording after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

The wording refers to N3126.

1. Add the following series of new paragraphs following [c.files] p.1:

   > Table 132 describes header `<cinttypes>`. [Note: The macros defined by `<cinttypes>` are provided unconditionally. In particular, the symbol `__STDC_FORMAT_MACROS`, mentioned in footnote 182 of the `C` standard, plays no role in `C++`. — end note ]

   > 2 - The contents of header `<cinttypes>` are the same as the Standard `C` library header `<inttypes.h>`, with the following changes:

   > 3 - The header `<cinttypes>` includes the header `<cstdint>` instead of `<stdint.h>`.

   > 4 - If and only if the type `intmax_t` designates an extended integer type ([basic.fundamental]), the following function signatures are added:

   > >     intmax_t abs(intmax_t);
   > >     imaxdiv_t div(intmax_t, intmax_t);

   > which shall have the same semantics as the function signatures `intmax_t imaxabs(intmax_t)` and `imaxdiv_t imaxdiv(intmax_t, intmax_t)`, respectively.

### 1450. Contradiction in `regex_constants`

Section: 28.5.2 [re.matchflag] Status: C++14 Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-127

The Bitmask Type requirements in 17.5.2.1.3 [bitmask.types] p.3 say that all elements on a bitmask type have distinct values, but 28.5.2 [re.matchflag] defines `regex_constants::match_default` and `regex_constants::format_default` as elements of the bitmask type `regex_constants::match_flag_type`, both with value 0. This is a contradiction.

[ Resolution proposed by ballot comment: ]

> One of the bitmask elements should be removed from the declaration and should be defined separately, in the same manner as `ios_base::adjustfield`, `ios_base::basefield` and `ios_base::floatfield` are defined by 27.5.3.1.2 [ios::fmtflags] p.2 and Table 120. These are constants of a bitmask type, but are not distinct elements, they have more than one value set in the bitmask. `regex_constants::format_default` should be specified as a constant with the same value as `regex_constants::match_default`.

[ 2010-10-31 Daniel comments: ]

Strictly speaking, a bitmask type cannot have any element of value 0 at all, because any such value would contradict the requirement expressed in 17.5.2.1.3 [bitmask.types] p. 3:

> for any pair Ci and Cj, Ci & Ci is nonzero

So, actually both `regex_constants::match_default` and `regex_constants::format_default` are only constants of the type `regex_constants::match_flag_type`, and no bitmask elements.

[ 2010-11-03 Daniel comments and provides a proposed resolution: ]

The proposed resolution is written against N3126 and considered as a further improvement of the fixes suggested by n3110.

> Add the following sentence to 28.5.2 [re.matchflag] paragraph 1:

> > 1 The type `regex_constants::match_flag_type` is an implementation-defined bitmask type (17.5.2.1.3). Matching a regular expression against a sequence of characters [first,last) proceeds according to the rules of the grammar specified for the regular expression object, modified according to the effects listed in Table 136 for any bitmask elements set. Type `regex_constants::match_flag_type` also defines the constants `regex_constants::match_default` and `regex_constants::format_default`.

[ 2011 Bloomington ]

It appears the key problem is the phrasing of the bitmask requirements. Jeremiah supplies updated wording.

Pete Becker has also provided an alternative resolution.

Ammend 17.5.2.1.3 [bitmask.types]:

Change the list of values for "enum bit mask" in p2 from

`V0 = 1 << 0, V1 = 1 << 1, V2 = 1 << 2, V3 = 1 << 3, ...`.

to

`V0 = 0, V1 = 1 << 0, V2 = 1 << 1, V3 = 1 << 2, ...`.

Here, the names C0, C1, etc. represent bitmask elements for this particular bitmask type. All such non-zero elements have distinct values such that, for any pair Ci and Cj where i != j, Ci & Ci is nonzero and Ci & Cj is zero.

Change bullet 3 of paragraph 4:

The A non-zero value Y is set in the object X if the expression X & Y is nonzero.

[2014-02-13 Issaquah:]

Proposed resolution:

Ammend 17.5.2.1.3 [bitmask.types] p3:

Here, the names C0, C1, etc. represent bitmask elements for this particular bitmask type. All such elements have distinct, nonzero values such that, for any pair Ci and Cj where i != j, Ci & Ci is nonzero and Ci & Cj is zero. Additionally, the value 0 is used to represent an empty bitmask, in which no bitmask elements are set.

Add the following sentence to 28.5.2 [re.matchflag] paragraph 1:

> 1 The type `regex_constants::match_flag_type` is an implementation-defined bitmask type (17.5.2.1.3). The constants of that type, except for `match_default` and `format_default`, are bitmask elements. The `match_default` and `format_default` constants are empty bitmasks. Matching a regular expression against a sequence of characters [first,last) proceeds according to the rules of the grammar specified for the regular expression object, modified according to the effects listed in Table 136 for any bitmask elements set.

### 1453. Default constructed `match_results` behavior for certain operations

Section: 28.10.4 [re.results.acc] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-126

It's unclear how match_results should behave if it has been default-constructed. The sub_match objects returned by operator[], prefix and suffix cannot point to the end of the sequence that was searched if no search was done. The iterators held by unmatched sub_match objects might be singular.

[ Resolution proposed by ballot comment: ]

> Add to match_results::operator[], match_results::prefix and match_results::suffix:
> Requires: !empty()

[ 2010-10-24 Daniel adds: ]

> Accepting n3158 would solve this issue.

Proposed resolution:

Addressed by paper n3158.

### 1455. C language compatibility for atomics

Section: 29 [atomics] Status: Resolved Submitter: Switzerland Opened: 2010-08-25 Last modified: 2015-04-08

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Duplicate of: 1454

Discussion:

Addresses CH-22, GB-128

WG14 currently plans to introduce atomic facilities that are intended to be compatible with the facilities of clause 29. They should be compatible.

[ Resolution proposed by ballot comment ]

Make sure the headers in clause 29 are defined in a way that is compatible with the planned C standard.

[ 2010 Batavia ]

Resolved by adoption of n3193.

Proposed resolution:

Solved by n3193.

### 1457. Splitting lock-free properties

Section: 29.2 [atomics.syn] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-130

The synopsis for the `<atomic>` header lists the macros `ATOMIC_INTEGRAL_LOCK_FREE` and `ATOMIC_ADDRESS_LOCK_FREE`.

The `ATOMIC_INTEGRAL_LOCK_FREE` macro has been replaced with a set of macros for each integral type, as listed in 29.4 [atomics.lockfree].

[Proposed resolution as of comment]

> Against FCD, N3092:

> In [atomics.syn], header `<atomic>` synopsis replace as indicated:

> >     // 29.4, lock-free property
> >     #define ATOMIC_INTEGRAL_LOCK_FREE unspecified
> >     #define ATOMIC_CHAR_LOCK_FREE implementation-defined
> >     #define ATOMIC_CHAR16_T_LOCK_FREE implementation-defined
> >     #define ATOMIC_CHAR32_T_LOCK_FREE implementation-defined
> >     #define ATOMIC_WCHAR_T_LOCK_FREE implementation-defined
> >     #define ATOMIC_SHORT_LOCK_FREE implementation-defined
> >     #define ATOMIC_INT_LOCK_FREE implementation-defined
> >     #define ATOMIC_LONG_LOCK_FREE implementation-defined
> >     #define ATOMIC_LLONG_LOCK_FREE implementation-defined
> >     #define ATOMIC_ADDRESS_LOCK_FREE unspecified

[ 2010-10-26: Daniel adds: ]

The proposed resolution below is against the FCD working draft. After application of the editorial issues US-144 and US-146 the remaining difference against the working draft is the usage of implementation-defined instead of unspecified, effectively resulting in this delta:

>     // 29.4, lock-free property
>     #define ATOMIC_CHAR_LOCK_FREE unspecifiedimplementation-defined
>     #define ATOMIC_CHAR16_T_LOCK_FREE unspecifiedimplementation-defined
>     #define ATOMIC_CHAR32_T_LOCK_FREE unspecifiedimplementation-defined
>     #define ATOMIC_WCHAR_T_LOCK_FREE unspecifiedimplementation-defined
>     #define ATOMIC_SHORT_LOCK_FREE unspecifiedimplementation-defined
>     #define ATOMIC_INT_LOCK_FREE unspecifiedimplementation-defined
>     #define ATOMIC_LONG_LOCK_FREE unspecifiedimplementation-defined
>     #define ATOMIC_LLONG_LOCK_FREE unspecifiedimplementation-defined
>     #define ATOMIC_ADDRESS_LOCK_FREE unspecified

It is my understanding that the intended wording should be unspecified as for `ATOMIC_ADDRESS_LOCK_FREE` but if this is right, we need to use the same wording in 29.4 [atomics.lockfree], which consequently uses the term implementation-defined. I recommend to keep 29.2 [atomics.syn] as it currently is and to fix 29.4 [atomics.lockfree] instead as indicated:

[2011-02-24 Reflector discussion]

Moved to Tentatively Ready after 5 votes.

[2011-02-20: Daniel adapts the proposed wording to N3225 and fixes an editorial omission of applying N3193]

[2011-03-06: Daniel adapts the wording to N3242]

[Proposed Resolution]

> Change 29.4 [atomics.lockfree] as indicated:

> >     #define ATOMIC_CHAR_LOCK_FREE implementation-definedunspecified
> >     #define ATOMIC_CHAR16_T_LOCK_FREE implementation-definedunspecified
> >     #define ATOMIC_CHAR32_T_LOCK_FREE implementation-definedunspecified
> >     #define ATOMIC_WCHAR_T_LOCK_FREE implementation-definedunspecified
> >     #define ATOMIC_SHORT_LOCK_FREE implementation-definedunspecified
> >     #define ATOMIC_INT_LOCK_FREE implementation-definedunspecified
> >     #define ATOMIC_LONG_LOCK_FREE implementation-definedunspecified
> >     #define ATOMIC_LLONG_LOCK_FREE implementation-definedunspecified

Proposed resolution:

Resolved 2011-03 Madrid meeting by paper N3278

### 1460. Missing lock-free property for type `bool` should be added

Section: 29.4 [atomics.lockfree] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-154

There is no `ATOMIC_BOOL_LOCK_FREE` macro.

Proposed resolution suggested by the NB comment:

Add `ATOMIC_BOOL_LOCK_FREE` to 29.4 [atomics.lockfree] and to 29.2 [atomics.syn]:

>     [..]
>     #define ATOMIC_BOOL_LOCK_FREE unspecified
>     #define ATOMIC_CHAR_LOCK_FREE unspecified
>     #define ATOMIC_CHAR16_T_LOCK_FREE unspecified
>     #define ATOMIC_CHAR32_T_LOCK_FREE unspecified
>     [..]

[2011-03-12: Lawrence comments and drafts wording]

Point: We were missing a macro test for `bool`.

Comment: The `atomic<bool>` type is the easiest to make lock-free. There is no harm in providing a macro.

Action: Add an `ATOMIC_BOOL_LOCK_FREE`.

Point: We were missing a macro test for pointers.

Comment: The national body comment noting the missing macro for `bool` did not note the lack of a macro for pointers because `ATOMIC_ADDRESS_LOCK_FREE` was present at the time of the comment. Its removal appears to be an overzealous consequence of removing `atomic_address`.

Action: Add an `ATOMIC_POINTER_LOCK_FREE`.

Point: Presumably `atomic_is_lock_free()` will be an inline function producing a constant in those cases in which the macros are useful.

Comment: The point is technically correct, but could use some exposition. Systems providing forward binary compatibility, e.g. mainstream desktop and server systems, would likely have these functions as inline constants only when the answer is true. Otherwise, the function should defer to a platform-specific dynamic library to take advantage of future systems that do provide lock-free support.

Comment: Such functions are not useful in the preprocessor, and not portably useful in `static_assert`.

Action: Preserve the macros.

Point: The required explicit instantiations are `atomic<X>` for each of the types `X` in Table 145. Table 145 does not list `bool`, so `atomic<bool>` is not a required instantiation.

Comment: I think specialization was intended in the point instead of instantiation. In any event, 29.5 [atomics.types.generic] paragraph 5 does indirectly require a specialization for `atomic<bool>`. Confusion arises because the specialization for other integral types have a wider interface than the generic `atomic<T>`, but `atomic<bool>` does not.

Action: Add clarifying text.

Point: The name of Table 145, "atomic integral typedefs", is perhaps misleading, since the types listed do not contain all of the "integral" types.

Comment: Granted, though the table describe those with extra operations.

Action: Leave the text as is.

Point: The macros correspond to the types in Table 145, "with the signed and unsigned variants grouped together". That's a rather inartful way of saying that `ATOMIC_SHORT_LOCK_FREE` applies to `signed short` and `unsigned short`. Presumably this also means that `ATOMIC_CHAR_LOCK_FREE` applies to all three char types.

Comment: Yes, it is inartful.

Comment: Adding additional macros to distinguish signed and unsigned would provide no real additional information given the other constraints in the language.

Comment: Yes, it applies to all three `char` types.

Action: Leave the text as is.

Point: The standard says that "There are full specializations over the integral types (`char`, `signed char`, ...)" `bool` is not in the list. But this text is not normative. It simply tells you that "there are" specializations, not "there shall be" specializations, which would impose a requirement. The requirement, to the extent that there is one, is in the header synopsis, which, in N3242, sort of pulls in the list of types in Table 145.

Comment: The intent was for the specializations to be normative. Otherwise the extra member functions could not be present.

Action: Clarify the text.

[Proposed Resolution]

> 1. Edit header `<atomic>` synopsis 29.2 [atomics.syn]:

>    >     namespace std {
>    >       // 29.3, order and consistency
>    >       enum memory_order;
>    >       template <class T>
>    >       T kill_dependency(T y);

>    >       // 29.4, lock-free property
>    >       #define ATOMIC_BOOL_LOCK_FREE unspecified
>    >       #define ATOMIC_CHAR_LOCK_FREE unspecified
>    >       #define ATOMIC_CHAR16_T_LOCK_FREE unspecified
>    >       #define ATOMIC_CHAR32_T_LOCK_FREE unspecified
>    >       #define ATOMIC_WCHAR_T_LOCK_FREE unspecified
>    >       #define ATOMIC_SHORT_LOCK_FREE unspecified
>    >       #define ATOMIC_INT_LOCK_FREE unspecified
>    >       #define ATOMIC_LONG_LOCK_FREE unspecified
>    >       #define ATOMIC_LLONG_LOCK_FREE unspecified
>    >       #define ATOMIC_POINTER_LOCK_FREE unspecified

>    >       // 29.5, generic types
>    >       template<class T> struct atomic;
>    >       template<> struct atomic<integral>;
>    >       template<class T> struct atomic<T*>;

>    >       // 29.6.1, general operations on atomic types
>    >       // In the following declarations, atomic_type is either
>    >       // atomic<T> or a named base class for T from
>    >       // Table 145 or inferred from
>    >       // Table 146 or from bool.

>    >       […]
>    >     }

> 2. Edit the synopsis of 29.4 [atomics.lockfree] and paragraph 1 as follows:

>    >     #define ATOMIC_BOOL_LOCK_FREE unspecified
>    >     #define ATOMIC_CHAR_LOCK_FREE implementation-definedunspecified
>    >     #define ATOMIC_CHAR16_T_LOCK_FREE implementation-definedunspecified
>    >     #define ATOMIC_CHAR32_T_LOCK_FREE implementation-definedunspecified
>    >     #define ATOMIC_WCHAR_T_LOCK_FREE implementation-definedunspecified
>    >     #define ATOMIC_SHORT_LOCK_FREE implementation-definedunspecified
>    >     #define ATOMIC_INT_LOCK_FREE implementation-definedunspecified
>    >     #define ATOMIC_LONG_LOCK_FREE implementation-definedunspecified
>    >     #define ATOMIC_LLONG_LOCK_FREE implementation-definedunspecified
>    >     #define ATOMIC_POINTER_LOCK_FREE unspecified

>    > > 1 The `ATOMIC_…_LOCK_FREE` macros indicate the lock-free property of the corresponding atomic types, with the signed and unsigned variants grouped together. The properties also apply to the corresponding (partial) specializations of the `atomic` template. A value of 0 indicates that the types are never lock-free. A value of 1 indicates that the types are sometimes lock-free. A value of 2 indicates that the types are always lock-free.

> 3. Edit 29.5 [atomics.types.generic] paragraph 3, 4, and 6-8 as follows:

>    > 2 The semantics of the operations on specializations of `atomic` are defined in 29.6 [atomics.types.operations].

>    > 3 Specializations and instantiations of the `atomic` template shall have a deleted copy constructor, a deleted copy assignment operator, and a `constexpr` value constructor.

>    > 4 There are shall be full specializations over for the integral types ( `char`, `signed char`, `unsigned char`, `short`, `unsigned short`, `int`, `unsigned int`, `long`, `unsigned long`, `long long`, `unsigned long long`, `char16_t`, `char32_t`, and `wchar_t`, and any other types needed by the typedefs in the header `<cstdint>`) on the `atomic` class template. For each integral type integral, the specialization `atomic<integral>` provides additional atomic operations appropriate to integral types. [Editor's note: I'm guessing that this is the correct rendering of the text in the paper; if this sentence was intended to impose a requirement, rather than a description, it will have to be changed.] There shall be a specialization `atomic<bool>` which provides the general atomic operations as specified in 29.6.1 [atomics.types.operations.general].

>    > 5 The atomic integral specializations and the specialization `atomic<bool>` shall have standard layout. They shall each have a trivial default constructor and a trivial destructor. They shall each support aggregate initialization syntax.

>    > 6 There are shall be pointer partial specializations on of the `atomic` class template. These specializations shall have trivial default constructors and trivial destructors.

>    > 7 There are shall be named types corresponding to the integral specializations of `atomic`, as specified in Table 145. In addition, there shall be named type `atomic_bool` corresponding to the specialization `atomic<bool>`. Each named type is either a typedef to the corresponding specialization or a base class of the corresponding specialization. If it is a base class, it shall support the same member functions as the corresponding specialization.

>    > 8 There are shall be atomic typedefs corresponding to the typedefs in the header `<inttypes.h>` as specified in Table 146.

Proposed resolution:

Resolved 2011-03 Madrid meeting by paper N3278

### 1462. Ambiguous value assignment to `atomic_bool`

Section: X [atomics.types.integral] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Duplicate of: 1463

Discussion:

Addresses GB-132, US-157

The `atomic_itype` types and `atomic_address` have two overloads of `operator=`; one is `volatile` qualified, and the other is not. `atomic_bool` only has the `volatile` qualified version:

>     bool operator=(bool) volatile;

On a non-`volatile`-qualified object this is ambiguous with the deleted copy-assignment operator

>     atomic_bool& operator=(atomic_bool const&) = delete;

due to the need for a single standard conversion in each case when assigning a bool to an `atomic_bool` as in:

>     atomic_bool b;
>     b = true;

The conversions are:

>     atomic_bool& → atomic_bool volatile&

vs

>     bool → atomic_bool

[ Proposed resolution as of NB comment: ]

Change X [atomics.types.integral] as indicated:

>     namespace std {
>       typedef struct atomic_bool {
>         [..]
>         bool operator=(bool) volatile;
>         bool operator=(bool);
>       } atomic_bool;
>       [..]
>     }

[ 2010-10-27 Daniel adds: ]

> Accepting n3164 would solve this issue by replacing `atomic_bool` by `atomic<bool>`.

[ 2010 Batavia ]

Resolved by adoption of n3193.

Proposed resolution:

Solved by n3193.

### 1464. Underspecified typedefs for atomic integral types

Section: X [atomics.types.integral] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-160

The last sentence of X [atomics.types.integral] p.1 says:

> Table 143 shows typedefs to atomic integral classes and the corresponding `<cstdint>` typedefs.

That's nice, but nothing says these are supposed to be part of the implementation, and they are not listed in the synopsis.

[ Proposed resolution as of NB comment ]

1. Remove Table 143 — Atomics for standard typedef types.

2. Change X [atomics.types.integral] p.1 as indicated:

   > 1 The name `atomic_itype` and the functions operating on it in the preceding synopsis are placeholders for a set of classes and functions. Throughout the preceding synopsis, `atomic_itype` should be replaced by each of the class names in Table 142 and integral should be replaced by the integral type corresponding to the class name. Table 143 shows typedefs to atomic integral classes and the corresponding `<cstdint>` typedefs.

[ 2010-10-27 Daniel adds: ]

> Accepting n3164 would solve this issue.

[ 2010-11 Batavia ]

Resolved by adopting n3193.

Proposed resolution:

Solved by n3193.

### 1465. Missing arithmetic operators for `atomic_address`

Section: X [atomics.types.address] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-161

`atomic_address` has `operator+=` and `operator-=`, but no `operator++` or `operator--`. The template specialization `atomic<Ty*>` has all of them.

[ 2010-10-27 Daniel adds: ]

> Accepting n3164 would solve this issue by replacing `atomic_address` by `atomic<void*>`.

[ Resolved in Batavia by accepting n3193. ]

Proposed resolution:

Change X [atomics.types.address], class `atomic_address` synopsis, as indicated:

>     namespace std {
>       typedef struct atomic_address {
>         […]
>         void* operator=(const void*) volatile;
>         void* operator=(const void*);
>         void* operator++(int) volatile;
>         void* operator++(int);
>         void* operator--(int) volatile;
>         void* operator--(int);
>         void* operator++() volatile;
>         void* operator++();
>         void* operator--() volatile;
>         void* operator--();
>         void* operator+=(ptrdiff_t) volatile;
>         void* operator+=(ptrdiff_t);
>         void* operator-=(ptrdiff_t) volatile;
>         void* operator-=(ptrdiff_t);
>       } atomic_address;
>       […]
>     }

### 1466. Silent `const` breakage by `compare_exchange_*` member functions

Section: X [atomics.types.address] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-162

The `compare_exchange_weak` and `compare_exchange_strong` member functions that take `const void*` arguments lead to a silent removal of `const`, because the `load` member function and other acessors return the stored value as a `void*`.

[ Proposed resolution as of NB comment: ]

Change X [atomics.types.address], class `atomic_address` synopsis, as indicated:

>     namespace std {
>       typedef struct atomic_address {
>         [..]
>         bool compare_exchange_weak(const void*&, const void*,
>           memory_order, memory_order) volatile;
>         bool compare_exchange_weak(const void*&, const void*,
>           memory_order, memory_order);
>         bool compare_exchange_strong(const void*&, const void*,
>           memory_order, memory_order) volatile;
>         bool compare_exchange_strong(const void*&, const void*,
>           memory_order, memory_order);
>         bool compare_exchange_weak(const void*&, const void*,
>           memory_order = memory_order_seq_cst) volatile;
>         bool compare_exchange_weak(const void*&, const void*,
>           memory_order = memory_order_seq_cst);
>         bool compare_exchange_strong(const void*&, const void*,
>           memory_order = memory_order_seq_cst) volatile;
>         bool compare_exchange_strong(const void*&, const void*,
>           memory_order = memory_order_seq_cst);
>         [..]
>       } atomic_address;
>       [..]
>     }

[ 2010-10-27 Daniel adds: ]

> Accepting n3164 would solve this issue by replacing `atomic_address` by `atomic<void*>`.

[ Resolved in Batavia by accepting n3193. ]

Proposed resolution:

Solved by n3193.

### 1467. Deriving `atomic<T*>` from `atomic_address` breaks type safety

Section: X [atomics.types.address] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-163

Requiring `atomic<T*>` to be derived from `atomic_address` breaks type safety:

>     atomic<double*> ip;
>     char ch;
>     atomic_store(&ip, &ch);
>     *ip.load() = 3.14159;

The last line overwrites `ch` with a value of type `double`.

[ 2010-10-27 Daniel adds: ]

> Resolving this issue will also solve 1469

> Accepting n3164 would solve this issue by removing `atomic_address`.

[ Resolved in Batavia by accepting n3193. ]

Proposed resolution:

1. Change 29.5 [atomics.types.generic], class template specialization `atomic<T*>` synopsis, as indicated:

   >     namespace std {
   >       template <class T> struct atomic<T*> : atomic_address {
   >         [..]
   >       };
   >       [..]
   >     }

2. Change 29.5 [atomics.types.generic] p. 4 as indicated:

   > 4 There are pointer partial specializations on the `atomic` class template. These specializations shall be publicly derived from `atomic_address`. The unit of addition/subtraction for these specializations shall be the size of the referenced type. These specializations shall have trivial default constructors and trivial destructors.

### 1468. `atomic_address::compare_exchange_*` member functions should match `atomic_compare_exchange_*` free functions

Section: X [atomics.types.address] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-164

`atomic_address` has member functions `compare_exchange_weak` and `compare_exchange_strong` that take arguments of type `const void*`, in addition to the `void*` versions. If these member functions survive, there should be corresponding free functions.

[ 2010-10-27 Daniel adds: ]

> Accepting n3164 would solve this issue differently by removing the overloads with `const void*` arguments, because they break type-safety.

[ Resolved in Batavia by accepting n3193. ]

Proposed resolution:

Extend the synopsis around `atomic_address` in X [atomics.types.address] as indicated:

>     namespace std {
>       [..]
>       bool atomic_compare_exchange_weak(volatile atomic_address*, void**, void*);
>       bool atomic_compare_exchange_weak(atomic_address*, void**, void*);
>       bool atomic_compare_exchange_strong(volatile atomic_address*, void**, void*);
>       bool atomic_compare_exchange_strong(atomic_address*, void**, void*);
>       bool atomic_compare_exchange_weak_explicit(volatile atomic_address*, void**, void*,
>         memory_order, memory_order);
>       bool atomic_compare_exchange_weak_explicit(atomic_address*, void**, void*,
>         memory_order, memory_order);
>       bool atomic_compare_exchange_strong_explicit(volatile atomic_address*, void**, void*,
>         memory_order, memory_order);
>       bool atomic_compare_exchange_strong_explicit(atomic_address*, void**, void*,
>         memory_order, memory_order);
>       bool atomic_compare_exchange_weak(volatile atomic_address*, const void**, const void*);
>       bool atomic_compare_exchange_weak(atomic_address*, const void**, const void*);
>       bool atomic_compare_exchange_strong(volatile atomic_address*, const void**, const void*);
>       bool atomic_compare_exchange_strong(atomic_address*, const void**, const void*);
>       bool atomic_compare_exchange_weak_explicit(volatile atomic_address*, const void**, const void*,
>         memory_order, memory_order);
>       bool atomic_compare_exchange_weak_explicit(atomic_address*, const void**, const void*,
>         memory_order, memory_order);
>       bool atomic_compare_exchange_strong_explicit(volatile atomic_address*, const void**, const void*,
>         memory_order, memory_order);
>       bool atomic_compare_exchange_strong_explicit(volatile atomic_address*, const void**, const void*,
>         memory_order, memory_order);
>       bool atomic_compare_exchange_strong_explicit(atomic_address*, const void**, const void*,
>         memory_order, memory_order);
>       [..]
>     }

### 1469. `atomic<T*>` inheritance from `atomic_address` breaks type safety

Section: 29.5 [atomics.types.generic] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-133

The free functions that operate on `atomic_address` can be used to store a pointer to an unrelated type in an `atomic<T*>` without a cast. e.g.

>     int i;
>     atomic<int*> ai(&i);
>     string s;
>     atomic_store(&ai,&s);

Overload the `atomic_store`, `atomic_exchange` and `atomic_compare_exchange_[weak/strong]` operations for `atomic<T*>` to allow storing only pointers to `T`.

[ 2010-10-27 Daniel adds: ]

> Resolving this issue will also solve 1467

> Accepting n3164 would solve this issue by removing `atomic_address`.

[Resolved in Batavia by accepting n3193. ]

Proposed resolution:

Add the following overloads to 29.5 [atomics.types.generic], the synopsis around the specialization `atomic<T*>`, as indicated:

>     namespace std {
>       [..]
>       template <class T> struct atomic<T*> : atomic_address {
>         [..]
>       };

>       template<typename T>
>       void atomic_store(atomic<T*>&,T*);
>       template<typename T>
>       void atomic_store(atomic<T*>&,void*) = delete;
>       template<typename T>
>       void atomic_store_explicit(atomic<T*>&,T*,memory_order);
>       template<typename T>
>       void atomic_store_explicit(atomic<T*>&,void*,memory_order) = delete;
>       template<typename T>
>       T* atomic_exchange(atomic<T*>&,T*);
>       template<typename T>
>       T* atomic_exchange(atomic<T*>&,void*) = delete;
>       template<typename T>
>       T* atomic_exchange_explicit(atomic<T*>&,T*,memory_order);
>       template<typename T>
>       T* atomic_exchange_explicit(atomic<T*>&,void*,memory_order) = delete;
>       template<typename T>
>       T* atomic_compare_exchange_weak(atomic<T*>&,T**,T*);
>       template<typename T>
>       T* atomic_compare_exchange_weak(atomic<T*>&,void**,void*) = delete;
>       template<typename T>
>       T* atomic_compare_exchange_weak_explicit(atomic<T*>&,T**,T*,memory_order);
>       template<typename T>
>       T* atomic_compare_exchange_weak_explicit(atomic<T*>&,void**,void*,memory_order) = delete;
>       template<typename T>
>       T* atomic_compare_exchange_strong(atomic<T*>&,T**,T*);
>       template<typename T>
>       T* atomic_compare_exchange_strong(atomic<T*>&,void**,void*) = delete;
>       template<typename T>
>       T* atomic_compare_exchange_strong_explicit(atomic<T*>&,T**,T*,memory_order);
>       template<typename T>
>       T* atomic_compare_exchange_strong_explicit(atomic<T*>&,void**,void*,memory_order) = delete;

>     }

### 1474. weak compare-and-exchange confusion

Section: 29.6.5 [atomics.types.operations.req] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Duplicate of: 1470, 1475, 1476, 1477

Discussion:

Addresses US-175, US-165, CH-23, GB-135

29.6.5 [atomics.types.operations.req] p. 25: The first sentence is grammatically incorrect.

[ 2010-10-28 Daniel adds: ]

Duplicate issue 1475 also has a proposed resolution, but both issues are resolved with below proposed resolution.

[ 2011-02-15 Howard fixes numbering, Hans improves the wording ]

[2011-02-24 Reflector discussion]

Moved to Tentatively Ready after 6 votes.

Proposed resolution:

1. Change 29.6.5 [atomics.types.operations.req] p. 23 as indicated:

   > [ Note: For example, t The effect of the compare-and-exchange operations `atomic_compare_exchange_strong` is

   > >     if (memcmp(object, expected, sizeof(*object)) == 0)
   > >       memcpy(object, &desired, sizeof(*object));
   > >     else
   > >       memcpy(expected, object, sizeof(*object));

   > — end note ] [..]

2. Change 29.6.5 [atomics.types.operations.req] p. 25 as indicated:

   > 25 Remark: The weak compare-and-exchange operations may fail spuriously, that is, return false while leaving the contents of memory pointed to by `expected` before the operation is the same that same as that of the `object` and the same as that of `expected` after the operation A weak compare-and-exchange operation may fail spuriously. That is, even when the contents of memory referred to by `expected` and `object` are equal, it may return false and store back to `expected` the same memory contents that were originally there.. [ Note: This spurious failure enables implementation of compare-and-exchange on a broader class of machines, e.g., loadlocked store-conditional machines. A consequence of spurious failure is that nearly all uses of weak compare-and-exchange will be in a loop.

   > When a compare-and-exchange is in a loop, the weak version will yield better performance on some platforms. When a weak compare-and-exchange would require a loop and a strong one would not, the strong one is preferable. — end note ]

### 1478. Clarify race conditions in atomics initialization

Section: 29.6 [atomics.types.operations] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-136

GB requests normative clarification in 29.6 [atomics.types.operations] p.4 that concurrent access constitutes a race, as already done on p.6 and p.7.

[ Resolution proposed in ballot comment: ]

> Initialisation of atomics:

> We believe the intent is that for any atomics there is a distinguished initialisation write, but that this need not happens-before all the other operations on that atomic - specifically so that the initialisation write might be non-atomic and hence give rise to a data race, and hence undefined behaviour, in examples such as this (from Hans):

> >     atomic<atomic<int> *> p
> >     f()                      |
> >     { atomic<int>x;          | W_na x
> >       p.store(&x,mo_rlx);    | W_rlx p=&x
> >     }                        |

> (where na is nonatomic and rlx is relaxed). We suspect also that no other mixed atomic/nonatomic access to the same location is intended to be permitted. Either way, a note would probably help.

[2011-02-26: Hans comments and drafts wording]

I think the important point here is to clarify that races on atomics are possible, and can be introduced as a result of non-atomic initialization operations. There are other parts of this that remain unclear to me, such as whether there are other ways to introduce data races on atomics, or whether the races with initialization also introduce undefined behavior by the 3.8 lifetime rules. But I don't think that it is necessary to resolve those issues before releasing the standard. That's particularly true since we've introduced `atomic_init`, which allows easier ways to construct initialization races.

[2011-03 Madrid]

Accepted to be applied immediately to the WP

Proposed resolution:

1. Update 29.6.5 [atomics.types.operations.req] p. 5 as follows:

   >     constexpr A::A(C desired);

   > > 5 Effects: Initializes the object with the value `desired`. [ Note: Construction is not atomic. — end note ] Initialization is not an atomic operation (1.10) [intro.multithread]. [Note: It is possible to have an access to an atomic object `A` race with its construction, for example by communicating the address of the just-constructed object `A` to another thread via `memory_order_relaxed` atomic operations on a suitable atomic pointer variable, and then immediately accessing `A` in the receiving thread. This results in undefined behavior. — end note]

2. In response to the editor comment to 29.6.5 [atomics.types.operations.req] p. 8: The first Effects element is the correct and intended one:

   >     void atomic_init(volatile A *object, C desired);
   >     void atomic_init(A *object, C desired);

   > > 8 Effects: Non-atomically initializes `*object` with value `desired`. This function shall only be applied to objects that have been default constructed, and then only once. [ Note: these semantics ensure compatibility with C. — end note ] [ Note: Concurrent access from another thread, even via an atomic operation, constitutes a data race. — end note ] [Editor's note: The preceding text is from the WD as amended by N3196. N3193 makes different changes, marked up in the paper as follows:] Effects: Dynamically initializes an atomic variable. Non-atomically That is, non-atomically assigns the value desired to `*object`. [ Note: this operation may need to initialize locks. — end note ] Concurrent access from another thread, even via an atomic operation, constitutes a data race.

### 1479. Fence functions should be `extern "C"`

Section: 29.8 [atomics.fences] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-179

The fence functions (29.8 [atomics.fences] p.5 + p.6) should be `extern "C"`, for `C` compatibility.

[2011-02-16 Reflector discussion]

Moved to Tentatively Ready after 6 votes.

Proposed resolution:

1. Change 29.2 [atomics.syn], header `<atomic>` synopsis as indicated:

   >     namespace std {
   >       [..]
   >       // 29.8, fences
   >       extern "C" void atomic_thread_fence(memory_order);
   >       extern "C" void atomic_signal_fence(memory_order);
   >     }

2. Change 29.8 [atomics.fences], p. 5 and p. 6 as indicated:

   >     extern "C" void atomic_thread_fence(memory_order);

   > > 5 Effects: depending on the value of `order`, this operation: [..]

   >     extern "C" void atomic_signal_fence(memory_order);

   > > 6 Effects: equivalent to `atomic_thread_fence(order)`, except that synchronizes with relationships are established only between a thread and a signal handler executed in the same thread.

### 1480. Atomic fences don't have synchronizes with relation

Section: 29.8 [atomics.fences] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-137

Thread fence not only establish synchronizes with relationships, there are semantics of fences that are expressed not in terms of synchronizes with relationships (for example see 29.3 [atomics.order] p.5). These semantics also need to apply to the use of `atomic_signal_fence` in a restricted way.

[Batavia: Concurrency group discussed issue, and is OK with the proposed resolution.]

[2011-02-26 Reflector discussion]

Moved to Tentatively Ready after 5 votes.

Proposed resolution:

Change 29.8 [atomics.fences] p. 6 as indicated:

>     void atomic_signal_fence(memory_order);

> > 6 Effects: equivalent to `atomic_thread_fence(order)`, except that synchronizes with relationships the resulting ordering constraints are established only between a thread and a signal handler executed in the same thread.

### 1481. Missing Lockable requirements

Section: 30.2 [thread.req] Status: Resolved Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-138

The FCD combines the requirements for lockable objects with those for the standard mutex objects. These should be separate. This is LWG issue 1268.

[ Resolution proposed by ballot comment: ]

> See attached Appendix 1 - Additional Details

[ 2010-11-01 Daniel comments: ]

> Paper n3130 addresses this issue.

Proposed resolution:

Resolved by n3197.

### 1482. Timeout operations are under-specified

Section: 30.2.4 [thread.req.timing] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-181

The timeout operations are under-specified.

[ Resolution proposed by ballot comment: ]

> Define precise semantics for `timeout_until` and `timeout_for`. See n3141 page 193 - Appendix 1 - Additional Details

[ 2010-11-01 Daniel comments: ]

> Accepting n3128 would solve this issue.

Proposed resolution:

Resolved by n3191.

### 1487. Clock related operations exception specifications conflict

Section: 30.3.2 [thread.thread.this] Status: C++11 Submitter: Switzerland Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses CH-25

Clock related operations are currently not required not to throw. So "Throws: Nothing." is not always true.

[ Resolution proposed by ballot comment: ]

> Either require clock related operations not to throw (in 20.10) or change the Throws clauses in 30.3.2. Also possibly add a note that `abs_time` in the past or negative `rel_time` is allowed.

[2011-02-10: Howard Hinnant provides a resolution proposal]

[Previous proposed resolution:]

1. Change the Operational semantics of `C1::now()` in 20.12.3 [time.clock.req], Table 59 — `Clock` requirements as follows:

   > Table 59 — `Clock` requirements
   > Expression | Return type | Operational semantics
   > --- | --- | ---
   > `C1::now()` | `C1::time_point` | Returns a `time_point` object representing the current point in time. Shall not throw an exception.

[2011-02-19: Daniel comments and suggests an alternative wording]

Imposing the no-throw requirement on `C1::now()` of any clock time is an overly radical step: It has the indirect consequences that representation types for `C1::rep` can never by types with dynamic memory managment, e.g. my `big_int`, which are currently fully supported by the time utilities. Further-on this strong constraint does not even solve the problem described in the issue, because we are still left with the fact that any of the arithmetic operations of `C1::rep`, `C1::duration`, and `C1::time_point` may throw exceptions.

The alternative proposal uses the following strategy: The general `Clock` requirements remain untouched, but we require that any functions of the library-provided clocks from sub-clause 20.12.7 [time.clock] and their associated types shall not throw exceptions. Second, we replace existing `noexcept` specifications of functions from Clause 30 that depend on durations, clocks, or time points by wording that clarifies that these functions can only throw, if the operations of user-provided durations, clocks, or time points used as arguments to these functions throw exceptions.

[2011-03-23 Daniel and Peter check and simplify the proposed resolution resulting in this paper]

There is an inherent problem with `std::time_point` that it doesn't seem to have an equivalent value for `((time_t)-1)` that gets returned by C's `time()` function to signal a problem, e.g., because the underlying hardware is unavailable. After a lot of thinking and checks we came to the resolution that `timepoint::max()` should be the value to serve as a value signaling errors in cases where we prefer to stick with no-throw conditions. Of-course, user-provided representation types don't need to follow this approach if they prefer exceptions to signal such failures.

the functions `now()` and `from_time_t()` can remain `noexcept` with the solution to return `timepoint::max()` in case the current time cannot be determined or `(time_t)-1` is passed in, respectively.

Based on the previous proposed solution to LWG 1487 we decided that the new `TrivialClock` requirements should define that `now()` mustn't throw and return `timepoint::max()` to signal a problem. That is in line with the C standard where `(time_t)-1` signals a problem. Together with a fix to a - we assumed - buggy specifcation in 20.11.3 p2 which uses "happens-before" relationship with something that isn't any action:

> 2 In Table 59 `C1` and `C2` denote clock types. `t1` and `t2` are values returned by `C1::now()` where the call returning `t1` happens before (1.10) the call returning `t2` and both of these calls happen before `C1::time_point::max()`.

[2011-03-23 Review with Concurrency group suggested further simplifications and Howard pointed out, that we do not need time_point::max() as a special value.]

also the second "happens before" will be changed to "occurs before" in the english meaning. this is to allow a steady clock to wrap.

Peter updates issue accordingly to discussion.

[Note to the editor: we recommend removal of the following redundant paragraphs in 30.5.2 [thread.condition.condvarany] p. 18 to p. 21, p. 27, p. 28, p. 30, and p. 31 that are defining details for the wait functions that are given by the Effects element. ]

[Note to the editor: we recommend removal of the following redundant paragraphs in 30.5.1 [thread.condition.condvar]: p24-p26, p33-p34, and p36-p37 that are defining details for the `wait_for` functions. We believe these paragraphs are redundant with respect to the Effects clauses that define semantics based on `wait_until`. An example of such a specification is the `wait()` with a predicate. ]

Proposed resolution:

1. Change p2 in 20.11.3 [time.clock.req] as follows

   > 2 In Table 59 `C1` and `C2` denote clock types. `t1` and `t2` are values returned by `C1::now()` where the call returning `t1` happens before (1.10) the call returning `t2` and both of these calls happen occur before `C1::time_point::max()`. [ Note: This means `C1` didn't wrap around between `t1` and `t2` — end note ]

2. Add the following new requirement set at the end of sub-clause 20.12.3 [time.clock.req]: [Comment: This requirement set is intentionally incomplete. The reason for this incompleteness is the based on the fact, that if we would make it right for C++0x, we would end up defining something like a complete `ArithmeticLike` concept for `TC::rep`, `TC::duration`, and `TC::time_point`. But this looks out-of scope for C++0x to me. The effect is that we essentially do not exactly say, which arithmetic or comparison operations can be used in the time-dependent functions from Clause 30, even though I expect that all declared functions of `duration` and `time_point` are well-formed and well-defined. — end comment]

   > 3 [ Note: the relative difference in durations between those reported by a given clock and the SI definition is a measure of the quality of implementation. — end note ]

   > ? A type `TC` meets the `TrivialClock` requirements if:

   > - `TC` satisfies the `Clock` requirements (20.12.3 [time.clock.req]),

   > - the types `TC::rep`, `TC::duration`, and `TC::time_point` satisfy the requirements of `EqualityComparable` ( [equalitycomparable]), `LessThanComparable` ( [lessthancomparable]), `DefaultConstructible` ( [defaultconstructible]), `CopyConstructible` ( [copyconstructible]), `CopyAssignable` ( [copyassignable]), `Destructible` ( [destructible]), and of numeric types ([numeric.requirements]) [Note: This means in particular, that operations of these types will not throw exceptions — end note ],

   > - lvalues of the types `TC::rep`, `TC::duration`, and `TC::time_point` are swappable (17.6.3.2 [swappable.requirements]),

   > - the function `TC::now()` does not throw exceptions, and

   > - the type `TC::time_point::clock` meets the `TrivialClock` requirements, recursively.

3. Modify 20.12.7 [time.clock] p. 1 as follows:

   > 1 - The types defined in this subclause shall satisfy the `TrivialClock` requirements (20.11.1).

4. Modify 20.12.7.1 [time.clock.system] p. 1, class `system_clock` synopsis, as follows:

   >     class system_clock {
   >     public:
   >       typedef see below rep;
   >       typedef ratio<unspecified , unspecified > period;
   >       typedef chrono::duration<rep, period> duration;
   >       typedef chrono::time_point<system_clock> time_point;
   >       static const bool is_monotonic is_steady = unspecified;
   >       static time_point now() noexcept;
   >       // Map to C API
   >       static time_t to_time_t (const time_point& t) noexcept;
   >       static time_point from_time_t(time_t t) noexcept;
   >     };

5. Modify the prototype declarations in 20.12.7.1 [time.clock.system] p. 3 + p. 4 as indicated (This edit also fixes the miss of the `static` specifier in these prototype declarations):

   >     static time_t to_time_t(const time_point& t) noexcept;

   >     static time_point from_time_t(time_t t) noexcept;

6. Modify 20.12.7.2 [time.clock.steady] p. 1, class `steady_clock` synopsis, as follows:

   >     class steady_clock {
   >     public:
   >       typedef unspecified rep;
   >       typedef ratio<unspecified , unspecified > period;
   >       typedef chrono::duration<rep, period> duration;
   >       typedef chrono::time_point<unspecified, duration> time_point;
   >       static const bool is_monotonic is_steady = true;

   >       static time_point now() noexcept;
   >     };

7. Modify 20.12.7.3 [time.clock.hires] p. 1, class `high_resolution_clock` synopsis, as follows:

   >     class high_resolution_clock {
   >     public:
   >       typedef unspecified rep;
   >       typedef ratio<unspecified , unspecified > period;
   >       typedef chrono::duration<rep, period> duration;
   >       typedef chrono::time_point<unspecified, duration> time_point;
   >       static const bool is_monotonic is_steady = unspecified;

   >       static time_point now() noexcept;
   >     };

8. Add a new paragraph at the end of 30.2.4 [thread.req.timing]:

   > 6 The resolution of timing provided by an implementation depends on both operating system and hardware. The finest resolution provided by an implementation is called the native resolution.

   > ? Implementation-provided clocks that are used for these functions shall meet the `TrivialClock` requirements (20.12.3 [time.clock.req]).

9. Edit the synopsis of 30.3.2 [thread.thread.this] before p. 1. [Note: this duplicates edits also in D/N3267]:

   >     template <class Clock, class Duration>
   >     void sleep_until(const chrono::time_point<Clock, Duration>& abs_time) noexcept;
   >     template <class Rep, class Period>
   >     void sleep_for(const chrono::duration<Rep, Period>& rel_time) noexcept;

10. Modify the prototype specifications in 30.3.2 [thread.thread.this] before p. 4 and p. 6 and re-add a Throws element following the Synchronization elements at p. 5 and p. 7:

    >     template <class Clock, class Duration>
    >     void sleep_until(const chrono::time_point<Clock, Duration>& abs_time) noexcept;

    > > 4 - [...]

    > > 5 - Synchronization: None.

    > > ? - Throws: Nothing if `Clock` satisfies the `TrivialClock` requirements (20.12.3 [time.clock.req]) and operations of `Duration` do not throw exceptions. [Note: Instantiations of time point types and clocks supplied by the implementation as specified in 20.12.7 [time.clock] do not throw exceptions. — end note]

    >     template <class Rep, class Period>
    >     void sleep_for(const chrono::duration<Rep, Period>& rel_time) noexcept;

    > > 6 [...]

    > > 7 Synchronization: None.

    > > ? Throws: Nothing if operations of `chrono::duration<Rep, Period>` do not throw exceptions. [Note: Instantiations of duration types supplied by the implementation as specified in 20.12.7 [time.clock] do not throw exceptions. — end note]

11. Fix a minor incorrectness in p. 5: Duration types need to compare against `duration<>::zero()`, not `0`:

    > 3 The expression `m.try_lock_for(rel_time)` shall be well-formed and have the following semantics:

    > [...]

    > 5 Effects: The function attempts to obtain ownership of the mutex within the relative timeout (30.2.4) specified by `rel_time`. If the time specified by `rel_time` is less than or equal to `0` `rel_time.zero()`, the function attempts to obtain ownership without blocking (as if by calling `try_lock()`). The function shall return within the timeout specified by `rel_time` only if it has obtained ownership of the mutex object. [ Note: As with `try_lock()`, there is no guarantee that ownership will be obtained if the lock is available, but implementations are expected to make a strong effort to do so. — end note ]

12. Modify the class `timed_mutex` synopsis in 30.4.1.3.1 [thread.timedmutex.class] as indicated: [Note: this duplicates edits also in D/N3267]:

    >     class timed_mutex {
    >     public:
    >       [...]
    >       template <class Rep, class Period>
    >         bool try_lock_for(const chrono::duration<Rep, Period>& rel_time) noexcept;
    >       template <class Clock, class Duration>
    >         bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time) noexcept;
    >       [...]
    >     };

13. Modify the class `recursive_timed_mutex` synopsis in 30.4.1.3.2 [thread.timedmutex.recursive] as indicated: [Note: this duplicates edits also in D/N3267]:

    >     class recursive_timed_mutex {
    >     public:
    >       [...]
    >       template <class Rep, class Period>
    >         bool try_lock_for(const chrono::duration<Rep, Period>& rel_time) noexcept;
    >       template <class Clock, class Duration>
    >         bool try_lock_until(const chrono::time_point<Clock, Duration>& abs_time) noexcept;
    >       [...]
    >     };

14. Modify the class template `unique_lock` synopsis in 30.4.2.2 [thread.lock.unique] as indicated. [Note: this duplicates edits also in D/N3267]:

    >     template <class Mutex>
    >     class unique_lock {
    >     public:
    >       [...]
    >       template <class Clock, class Duration>
    >         unique_lock(mutex_type& m, const chrono::time_point<Clock, Duration>& abs_time) noexcept;
    >       template <class Rep, class Period>
    >         unique_lock(mutex_type& m, const chrono::duration<Rep, Period>& rel_time) noexcept;
    >       [...]
    >     };

15. Modify the constructor prototypes in 30.4.2.2.1 [thread.lock.unique.cons] before p. 14 and p. 17 [Note: this duplicates edits also in D/N3267]:

    >     template <class Clock, class Duration>
    >       unique_lock(mutex_type& m, const chrono::time_point<Clock, Duration>& abs_time) noexcept;

    >     template <class Rep, class Period>
    >       unique_lock(mutex_type& m, const chrono::duration<Rep, Period>& rel_time) noexcept;

### 1490. Mutex requirements too stringent

Section: 30.4.1 [thread.mutex.requirements] Status: Resolved Submitter: Switzerland Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses CH-27

The mutex requirements force `try_lock` to be `noexcept(true)`. However, where they are used by the generic algorithms, those relax this requirement and say that `try_lock` may throw. This means the requirement is too stringent, also a non-throwing `try_lock` does not allow for a diagnostic such as `system_error` that `lock()` will give us.

[ Resolution proposed by ballot comment: ]

> delete p18, adjust 30.4.4 p1 and p4 accordingly

[ 2010-11-01 Daniel comments: ]

> Accepting n3130 would solve this issue.

Proposed resolution:

Resolved by n3197.

### 1491. `try_lock` does not guarantee forward progress

Section: 30.4.1 [thread.mutex.requirements] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-186

`try_lock` does not provide a guarantee of forward progress because it is allowed to spuriously fail.

[ Resolution proposed by ballot comment: ]

> The standard mutex types must not fail spuriously in `try_lock`. See n3141 page 205 - Appendix 1 - Additional Details

[ 2010-11-01 Daniel comments: ]

> Paper n3152 addresses this issue.

Proposed resolution:

Resolved by n3209.

### 1492. Mutex requirements should not be bound to threads

Section: 30.4.1 [thread.mutex.requirements] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-188

Mutex requirements should not be bound to threads.

[ Resolution proposed by ballot comment: ]

> See Appendix 1 of n3141 - Additional Details, p. 208.

[ 2010-10-24 Daniel adds: ]

> Accepting n3130 would solve this issue.

Proposed resolution:

Resolved by n3197.

### 1494. Term "are serialized" not defined

Section: 30.4.4.2 [thread.once.callonce] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-190

The term "are serialized" is never defined (30.4.4.2 [thread.once.callonce] p. 2).

[ Resolution proposed by ballot comment: ]

Remove the sentence with "are serialized" from paragraph 2. Add "Calls to `call_once` on the same `once_flag` object shall not introduce data races (17.6.4.8)." to paragraph 3.

[ 2010-11-01 Daniel translates NB comment into wording ]

[ 2011-02-17: Hans proposes an alternative resolution ]

[ 2011-02-25: Hans, Clark, and Lawrence update the suggested wording ]

[2011-02-26 Reflector discussion]

Moved to Tentatively Ready after 5 votes.

Proposed resolution:

Change 30.4.4.2 [thread.once.callonce] p.2+3 as indicated:

>     template<class Callable, class ...Args>
>     void call_once(once_flag& flag, Callable&& func, Args&&... args);

> > [..]

> > 2 Effects: Calls to `call_once` on the same `once_flag` object are serialized. If there has been a prior effective call to `call_once` on the same `once_flag object`, the call to `call_once` returns without invoking `func`. If there has been no prior effective call to `call_once` on the same `once_flag` object, `INVOKE(decay_copy( std::forward<Callable>(func)), decay_copy(std::forward<Args>(args))...)` is executed. The call to `call_once` is effective if and only if `INVOKE(decay_copy( std::forward<Callable>(func)), decay_copy(std::forward<Args>(args))...)` returns without throwing an exception. If an exception is thrown it is propagated to the caller. An execution of `call_once` that does not call its `func` is a passive execution. An execution of `call_once` that calls its `func` is an active execution. An active execution shall call `INVOKE(decay_copy(std::forward<Callable>(func)), decay_copy(std::forward<Args>(args))...)`. If such a call to `func` throws an exception, the execution is exceptional, otherwise it is returning. An exceptional execution shall propagate the exception to the caller of `call_once`. Among all executions of `call_once` for any given `once_flag`: at most one shall be a returning execution; if there is a returning execution, it shall be the last active execution; and there are passive executions only if there is a returning execution. [Note: Passive executions allow other threads to reliably observe the results produced by the earlier returning execution. — end note]

> > 3 Synchronization: The completion of an effective call to `call_once` on a `once_flag` object synchronizes with (1.10 [intro.multithread]) all subsequent calls to `call_once` on the same `once_flag` object.For any given `once_flag`: all active executions occur in a total order; completion of an active execution synchronizes with (1.10 [intro.multithread]) the start of the next one in this total order; and the returning execution synchronizes with the return from all passive executions.

### 1497. `lock()` postcondition can not be generally achieved

Section: 30.5 [thread.condition] Status: C++11 Submitter: Switzerland Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses CH-30

If `lock.lock()` throws an exception, the postcondition can not be generally achieved.

[ Resolution proposed by ballot comment: ]

> Either state that the postcondition might not be achieved, depending on the error condition, or state that `terminate()` is called in this case.

[ 2010-08-13 Peter Sommerlad comments and provides wording ]

> 30.5.1 [thread.condition.condvar], 30.5.2 [thread.condition.condvarany]

> p. 13, last bullet, and corresponding paragraphs in all wait functions

> Problem:
> Condition variable wait might fail, because the lock cannot be acquired when notified. CH-30 says: "If lock.lock() throws an exception, the postcondition can not be generally achieved." CH-30 proposes: "Either state that the postcondition might not be achieved, depending on the error condition, or state that terminate() is called in this case."

> The discussion in Rapperswil concluded that calling `terminate()` might be too drastic in this case and a corresponding exception should be thrown/passed on and one should use a lock type that allows querying its status, which `unique_lock` allows for `std::condition_variable`

> We also had some additional observations while discussing in Rapperswil:
> - in 30.5.1 [thread.condition.condvar] `wait` with predicate and `wait_until` with predicate lack the precondition, postcondition and Error conditions sections. the lack of the precondition would allow to call `pred()` without holding the lock.
> - in 30.5.1 [thread.condition.condvar] `wait_until` and `wait_for` and 30.5.2 [thread.condition.condvarany] `wait_for` still specify an error condition for a violated precondition. This should be removed.

> and add the following proposed solution:

[2011-02-27: Daniel adapts numbering to n3225]

Proposed resolution:

1. Change 30.5.1 [thread.condition.condvar] as indicated:

   >     void wait(unique_lock<mutex>& lock);

   > 9 Requires: `lock``.owns_lock()` is `true` and `lock.mutex()` is locked by the calling thread, and either

   > - no other thread is waiting on this `condition_variable` object or
   > - `lock.mutex()` returns the same value for each of the `lock` arguments supplied by all concurrently waiting (via `wait` or `timed_wait`) threads.

   [..]

   > 11 Postcondition: `lock``.owns_lock()` is `true` and `lock.mutex()` is locked by the calling thread.

   [..]

   >     template <class Predicate>
   >     void wait(unique_lock<mutex>& lock, Predicate pred);

   > ?? Requires: `lock.owns_lock()` is `true` and `lock.mutex()` is locked by the calling thread, and either

   > - no other thread is waiting on this `condition_variable` object or
   > - `lock.mutex()` returns the same value for each of the `lock` arguments supplied by all concurrently waiting (via `wait` or `timed_wait`) threads.

   > 14 Effects:

   > >     while (!pred())
   > >       wait(lock);

   > ?? Postcondition: `lock.owns_lock()` is `true` and `lock.mutex()` is locked by the calling thread.

   > ?? Throws: `std::system_error` when an exception is required (30.2.2).

   > ?? Error conditions:

   > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

   >     template <class Clock, class Duration>
   >     cv_status wait_until(unique_lock<mutex>& lock,
   >       const chrono::time_point<Clock, Duration>& abs_time);

   > 15 Requires: `lock``.owns_lock()` is `true` and `lock.mutex()` is locked by the calling thread, and either

   > - no other thread is waiting on this `condition_variable` object or
   > - `lock.mutex()` returns the same value for each of the `lock` arguments supplied by all concurrently waiting (via `wait`, `wait_for`, or `wait_until`) threads.

   [..]

   > 17 Postcondition: `lock``.owns_lock()` is `true` and `lock.mutex()` is locked by the calling thread.

   [..]

   > 20 Error conditions:

   > - `operation_not_permitted` — if the thread does not own the lock.
   > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

   >     template <class Rep, class Period>
   >     cv_status wait_for(unique_lock<mutex>& lock,
   >       const chrono::duration<Rep, Period>& rel_time);

   > 21 Requires: `lock``.owns_lock()` is `true` and `lock.mutex()` is locked by the calling thread, and either

   > - no other thread is waiting on this `condition_variable` object or
   > - `lock.mutex()` returns the same value for each of the `lock` arguments supplied by all concurrently waiting (via `wait`, `wait_for`, or `wait_until`) threads.

   [..]

   > 24 Postcondition: `lock``.owns_lock()` is `true` and `lock.mutex()` is locked by the calling thread.

   [..]

   > 26 Error conditions:

   > - `operation_not_permitted` — if the thread does not own the lock.
   > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

   >     template <class Clock, class Duration, class Predicate>
   >     bool wait_until(unique_lock<mutex>& lock,
   >       const chrono::time_point<Clock, Duration>& abs_time,
   >         Predicate pred);

   > ?? Requires: `lock.owns_lock()` is `true` and `lock.mutex()` is locked by the calling thread, and either

   > - no other thread is waiting on this `condition_variable` object or
   > - `lock.mutex()` returns the same value for each of the `lock` arguments supplied by all concurrently waiting (via `wait` or `timed_wait`) threads.

   > 27 Effects:

   > >     while (!pred())
   > >       if (wait_until(lock, abs_time) == cv_status::timeout)
   > >         return pred();
   > >     return true;

   > 28 Returns: `pred()`

   > ?? Postcondition: `lock.owns_lock()` is `true` and `lock.mutex()` is locked by the calling thread.

   > 29 [ Note: The returned value indicates whether the predicate evaluates to true regardless of whether the timeout was triggered. — end note ]

   > ?? Throws: `std::system_error` when an exception is required (30.2.2).

   > ?? Error conditions:

   > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

   >     template <class Rep, class Period, class Predicate>
   >     bool wait_for(unique_lock<mutex>& lock,
   >       const chrono::duration<Rep, Period>& rel_time,
   >         Predicate pred);

   > 30 Requires: `lock``.owns_lock()` is `true` and `lock.mutex()` is locked by the calling thread, and either

   > - no other thread is waiting on this `condition_variable` object or
   > - `lock.mutex()` returns the same value for each of the `lock` arguments supplied by all concurrently waiting (via `wait`, `wait_for`, or `wait_until`) threads.

   [..]

   > 33 Postcondition: `lock``.owns_lock()` is `true` and `lock.mutex()` is locked by the calling thread.

   [..]

   > 37 Error conditions:

   > - `operation_not_permitted` — if the thread does not own the lock.
   > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

2. Change 30.5.2 [thread.condition.condvarany] as indicated:

   [..]

   >     template <class Lock, class Predicate>
   >     void wait(Lock& lock, Predicate pred);

   > [Note: if any of the wait functions exits with an exception it is indeterminate if the `Lock` is held. One can use a `Lock` type that allows to query that, such as the `unique_lock` wrapper. — end note]

   > 11 Effects:

   > >     while (!pred())
   > >       wait(lock);

   [..]

   > 31 Error conditions:

   > - `operation_not_permitted` — if the thread does not own the lock.
   > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

### 1498. Unclear specification for [thread.condition]

Section: 30.5 [thread.condition] Status: Resolved Submitter: Switzerland Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses CH-29

It is unclear if a spurious wake-up during the loop and reentering of the blocked state due to a repeated execution of the loop will adjust the timer of the blocking with the respect to the previously specified `rel_time` value.

[ Resolution proposed by ballot comment: ]

> Make it clear (e.g. by a note) that when reexecuting the loop the waiting time when blocked will be adjusted with respect to the elapsed time of the previous loop executions.

[ 2010-08-13 Peter Sommerlad comments and provides wording: ]

> Problem: It is unclear if a spurious wake-up during the loop and re-entering of the blocked state due to a repeated execution of the loop will adjust the timer of the blocking with the respect to the previously specified `rel_time` value.

> Proposed Resolution from CH29:

> Make it clear (e.g. by a note) that when re-executing the loop the waiting time when blocked will be adjusted with respect to the elapsed time of the previous loop executions.

> Discussion in Rapperswil:

> Assuming the introduction of a mandatory `steady_clock` proposed by US-181 to the FCD the specification of `condition_variable::wait_for` can be defined in terms of `wait_until` using the `steady_clock`. This is also interleaving with US-181, because that touches the same paragraph (30.5.1 p 25, p34 and 30.5.2 p 20, p 28 in n3092.pdf)

> (The "as if" in the proposed solutions should be confirmed by the standardization terminology experts)

[ 2010-11 Batavia: Resolved by applying n3191 ]

> 1. Change 30.5.1 [thread.condition.condvar] paragraph 25, `wait_for` Effects as indicated:

>    >     template <class Rep, class Period>
>    >     cv_status wait_for(unique_lock<mutex>& lock,
>    >       const chrono::duration<Rep, Period>& rel_time);

>    > [..]

>    > 25 Effects: as if

>    > >     return wait_until(lock, chrono::steady_clock::now() + rel_time);

>    > - Atomically calls `lock.unlock()` and blocks on `*this`.
>    > - When unblocked, calls `lock.lock()` (possibly blocking on the lock), then returns.
>    > - The function will unblock when signaled by a call to `notify_one()` or a call to `notify_all()`, by the elapsed time `rel_time` passing (30.2.4), or spuriously.
>    > - If the function exits via an exception, `lock.lock()` shall be called prior to exiting the function scope.

> 2. Change 30.5.1 [thread.condition.condvar] paragraph 34, `wait_for` with predicate Effects as indicated:

>    >     template <class Rep, class Period, class Predicate>
>    >     bool wait_for(unique_lock<mutex>& lock,
>    >       const chrono::duration<Rep, Period>& rel_time,
>    >       Predicate pred);

>    > [..]

>    > 34 Effects: as if

>    > >     return wait_until(lock, chrono::steady_clock::now() + rel_time, std::move(pred));

>    > - Executes a loop: Within the loop the function first evaluates `pred()` and exits the loop if the result is `true`.
>    > - Atomically calls `lock.unlock()` and blocks on `*this`.
>    > - When unblocked, calls `lock.lock()` (possibly blocking on the lock).
>    > - The function will unblock when signaled by a call to `notify_one()` or a call to `notify_all()`, by the elapsed time `rel_time` passing (30.2.4), or spuriously.
>    > - If the function exits via an exception, `lock.lock()` shall be called prior to exiting the function scope.
>    > - The loop terminates when `pred()` returns `true` or when the time duration specified by `rel_time` has elapsed.

> 3. Change 30.5.2 [thread.condition.condvarany] paragraph 20, `wait_for` Effects as indicated:

>    >     template <class Lock, class Rep, class Period>
>    >     cv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);

>    > 20 Effects: as if

>    > >     return wait_until(lock, chrono::steady_clock::now() + rel_time);

>    > - Atomically calls `lock.unlock()` and blocks on `*this`.
>    > - When unblocked, calls `lock.lock()` (possibly blocking on the lock), then returns.
>    > - The function will unblock when signaled by a call to `notify_one()` or a call to `notify_all()`, by the elapsed time `rel_time` passing (30.2.4), or spuriously.
>    > - If the function exits via an exception, `lock.unlock()` shall be called prior to exiting the function scope.

> 4. Change 30.5.2 [thread.condition.condvarany] paragraph 28, `wait_for` with predicate Effects as indicated:

>    >     template <class Lock, class Rep, class Period, class Predicate>
>    >     bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);

>    > 28 Effects: as if

>    > >     return wait_until(lock, chrono::steady_clock::now() + rel_time, std::move(pred));

>    > - Executes a loop: Within the loop the function first evaluates `pred()` and exits the loop if the result is `true`.
>    > - Atomically calls `lock.unlock()` and blocks on `*this`.
>    > - When unblocked, calls `lock.lock()` (possibly blocking on the lock).
>    > - The function will unblock when signaled by a call to `notify_one()` or a call to `notify_all()`, by the elapsed time `rel_time` passing (30.2.4), or spuriously.
>    > - If the function exits via an exception, `lock.unlock()` shall be called prior to exiting the function scope.
>    > - The loop terminates when `pred()` returns `true` or when the time duration specified by `rel_time` has elapsed.

Proposed resolution:

Resolved by n3191.

### 1501. Specification for managing associated asynchronous state has problems

Section: 30.6 [futures] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-194

The specification for managing associated asynchronous state is confusing, sometimes omitted, and redundantly specified.

[ Resolution proposed by ballot comment: ]

> Define terms-of-art for releasing, making ready, and abandoning an associated asynchronous state. Use those terms where appropriate. See Appendix 1 - Additional Details

Proposed resolution:

Resolved in Batavia by accepting n3192.

### 1502. Specification of [futures.state] unclear

Section: 30.6.4 [futures.state] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-195

The intent and meaning of 30.6.4 [futures.state] p10 is not apparent.

> 10 Accesses to the same shared state conflict (1.10).

[ 2011-03-07 Jonathan Wakely adds: ]

> It's not clear which paragraph this refers to, I had to go to the ballot comments where US-195 reveals it's para 8, which in the FCD (N3092) says:

> > Accesses to the same associated asynchronous state conflict (1.10).

> This is now para 10 in N3242:

> > Accesses to the same shared state conflict (1.10).

[2011-03-07: Lawrence comments and drafts wording]

The intent of this paragraph is to deal with operations, such as `shared_future::get()`, that return a reference to a value held in the shared state. User code could potentially conflict when accessing that value.

Lawrence proposed resolution:

Modify 30.6.4 [futures.state] p10 as follows:

> 10 Accesses to the same shared state conflict (1.10 [intro.multithread]).Some operations, e.g. `shared_future::get()` (30.6.7 [futures.shared_future]), may return a reference to a value held in their shared state. Accesses and modifications through those references by concurrent threads to the same shared state may potentially conflict (1.10 [intro.multithread]). [Note: As a consequence, accesses must either use read-only operations or provide additional synchronization. — end note]

[2011-03-19: Detlef suggests an alternative resolution, shown below.]

> Proposed Resolution

> Modify 30.6.4 [futures.state] p10 as follows:

> > 10 Accesses to the same shared state conflict (1.10 [intro.multithread]). [Note: This explicitely specifies that the shared state is visible in the objects that reference this state in the sense of data race avoidance 17.6.5.9 [res.on.data.races]. — end note]

Proposed resolution:

Resolved 2011-03 Madrid meeting by paper N3278

### 1504. Term "are serialized" is not defined

Section: 30.6.5 [futures.promise] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-196

The term "are serialized" is not defined (30.6.5 [futures.promise] p. 21, 25).

[ Resolution proposed by ballot comment: ]

> Replace "are serialized" with "shall not introduce a data race (17.6.4.8)".

[ 2010-11-02 Daniel translates proposal into proper wording changes ]

[2011-03-19: Detlef comments]

The proposed resolution for 1507 would cover this issue as well.

[Proposed Resolution]

> 1. Change 30.6.5 [futures.promise] p. 21 as indicated:

>    > 21 Synchronization: calls to `set_value` and `set_exception` on a single `promise` object are serialized shall not introduce a data race ([res.on.data.races]). [ Note: and they synchronize and serialize with other functions through the referred associated asynchronous state. — end note ]

> 2. Change 30.6.5 [futures.promise] p. 25 as indicated:

>    > 25 Synchronization: calls to `set_value` and `set_exception` on a single `promise` object are serialized shall not introduce a data race ([res.on.data.races]). [ Note: and they synchronize and serialize with other functions through the referred associated asynchronous state. — end note ]

Proposed resolution:

Resolved 2001-03 Madrid by issue 1507.

### 1505. Synchronization between `promise::set_value` and `future::get`

Section: 30.6.5 [futures.promise] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-197

There is no defined synchronization between `promise::set_value` and `future::get` (30.6.5 [futures.promise] p. 21, 25).

[ Resolution proposed by ballot comment: ]

> Replace "[Note: and they synchronize and serialize with other functions through the referred associated asynchronous state. — end note]" with the normative "They synchronize with (1.10) any operation on a future object with the same associated asynchronous state marked ready."

[ 2010-11-02 Daniel translates proposal into proper wording changes ]

[2011-03-19: Detlef comments]

The proposed resolution for 1507 would cover this issue as well. Effectively it will reject the request but a clarification is added that the normative wording is already in 30.6.4 [futures.state].

> 1. Change 30.6.5 [futures.promise] p. 21 as indicated:

>    > 21 Synchronization: calls to `set_value` and `set_exception` on a single `promise` object are serialized. [ Note: and they synchronize and serialize with other functions through the referred associated asynchronous state. — end note ]They synchronize with ([intro.multithread]) any operation on a future object with the same associated asynchronous state marked ready.

> 2. Change 30.6.5 [futures.promise] p. 25 as indicated:

>    > 25 Synchronization: calls to `set_value` and `set_exception` on a single `promise` object are serialized. [ Note: and they synchronize and serialize with other functions through the referred associated asynchronous state. — end note ]They synchronize with ([intro.multithread]) any operation on a future object with the same associated asynchronous state marked ready.

Proposed resolution:

Resolved 2001-03 Madrid by issue 1507.

### 1507. `promise::XXX_at_thread_exit` functions have no synchronization requirements

Section: 30.6.5 [futures.promise] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-199

`promise::XXX_at_thread_exit` functions have no synchronization requirements. Specifying synchronization for these member functions requires coordinating with the words in 30.6.5 [futures.promise]/21 and 25, which give synchronization requirements for `promise::set_value` and `promise::set_exception` (30.6.5 [futures.promise] p. 26 ff., p. 29 ff.).

[ Resolution proposed by ballot comment: ]

> Change 30.6.5 [futures.promise]/21 to mention `set_value_at_thread_exit` and `set_exception_at_thread_exit;` with this text, replace 30.6.5 [futures.promise]/25 and add two new paragraphs, after 30.6.5 [futures.promise]/28 and 30.6.5 [futures.promise]/31.

[2011-03-8: Lawrence comments and drafts wording]

This comment applies as well to other `*_at_thread_exit` functions. The following resolution adds synchronization paragraphs to all of them and edits a couple of related synchronization paragraphs.

[2011-03-09: Hans and Anthony add some improvements]

[2011-03-19: Detlef comments]

In regard to the suggested part:

> These operations do not provide any ordering guarantees with respect to other operations, except through operations on futures that reference the same shared state.

I would like this to change to:

> These operations do not provide any ordering guarantees with respect to other operations on the same promise object. [Note: They synchronize with calls to operations on objects that refer to the same shared state according to 30.6.4 [futures.state]. — end note]

The current proposed resolution has exactly the same paragraph at for places. I propose to have it only once as new paragraph 2.

This also covers 1504 (US-196) and 1505 (US-197). US-197 is essentially rejected with this resolution, but a clarification is added that the normative wording is already in 30.6.4 [futures.state].

> Proposed Resolution

> 1. Edit 30.4.1.2 [thread.mutex.requirements.mutex] paragraph 5 as follows:

>    > 5 The implementation shall provide lock and unlock operations, as described below. The implementation shall serialize those operations.For purposes of determining the existence of a data race, these behave as atomic operations (1.10 [intro.multithread]). The lock and unlock operations on a single mutex shall appear to occur in a single total order. [Note: this can be viewed as the modification order (1.10 [intro.multithread]) of the mutex. — end note] [ Note: Construction and destruction of an object of a mutex type need not be thread-safe; other synchronization should be used to ensure that mutex objects are initialized and visible to other threads. — end note ]

> 2. Edit 30.5 [thread.condition] paragraphs 6-9 as follows:

>    >     void notify_all_at_thread_exit(condition_variable& cond, unique_lock<mutex> lk);

>    > > -6- Requires: `lk` is locked by the calling thread and either

>    > > - no other thread is waiting on `cond`, or
>    > > - `lk.mutex()` returns the same value for each of the lock arguments supplied by all concurrently waiting (via `wait`, `wait_for`, or `wait_until`) threads.

>    > > -7- Effects: transfers ownership of the lock associated with `lk` into internal storage and schedules `cond` to be notified when the current thread exits, after all objects of thread storage duration associated with the current thread have been destroyed. This notification shall be as if

>    > > >     lk.unlock();
>    > > >     cond.notify_all();

>    > > -?- Synchronization: The call to `notify_all_at_thread_exit` and the completion of the destructors for all the current thread's variables of thread storage duration synchronize with (1.10 [intro.multithread]) calls to functions waiting on `cond`.

>    > > -8- Note: The supplied lock will be held until the thread exits, and care must be taken to ensure that this does not cause deadlock due to lock ordering issues. After calling `notify_all_at_thread_exit` it is recommended that the thread should be exited as soon as possible, and that no blocking or time-consuming tasks are run on that thread.

>    > > -9- Note: It is the user's responsibility to ensure that waiting threads do not erroneously assume that the thread has finished if they experience spurious wakeups. This typically requires that the condition being waited for is satisfied while holding the lock on `lk`, and that this lock is not released and reacquired prior to calling `notify_all_at_thread_exit`.

> 3. Edit 30.6.5 [futures.promise], paragraphs 14-27 as follows:

>    >     void promise::set_value(const R& r);
>    >     void promise::set_value(R&& r);
>    >     void promise<R&>::set_value(R& r);
>    >     void promise<void>::set_value();

>    > > -14- Effects: atomically stores the value `r` in the shared state and makes that state ready (30.6.4 [futures.state]).

>    > > -15- Throws:
>    > > - `future_error` if its shared state already has a stored value or exception, or
>    > > - for the first version, any exception thrown by the copy constructor of `R`, or
>    > > - for the second version, any exception thrown by the move constructor of `R`.

>    > > -16- Error conditions:

>    > > - `promise_already_satisfied` if its shared state already has a stored value or exception.
>    > > - `no_state` if `*this` has no shared state.

>    > > -17- Synchronization: calls to `set_value` and `set_exception` on a single `promise` object are serialized. [ Note: And they synchronize and serialize with other functions through the referred shared state. — end note ]For purposes of determining the existence of a data race, `set_value`, `set_exception`, `set_value_at_thread_exit`, and `set_exception_at_thread_exit` behave as atomic operations (1.10 [intro.multithread]) on the memory location associated with the `promise`. Calls to these operations on a single promise shall appear to occur in a single total order. [Note: this can be viewed as the modification order (1.10 [intro.multithread]) of the promise. — end note] These operations do not provide any ordering guarantees with respect to other operations, except through operations on futures that reference the same shared state.

>    >     void set_exception(exception_ptr p);

>    > > -18- Effects: atomically stores the exception pointer `p` in the shared state and makes that state ready (30.6.4 [futures.state]).

>    > > -19- Throws: `future_error` if its shared state already has a stored value or exception.

>    > > -20- Error conditions:

>    > > - `promise_already_satisfied` if its shared state already has a stored value or exception.
>    > > - `no_state` if `*this` has no shared state.

>    > > -21- Synchronization: calls to `set_value` and `set_exception` on a single `promise` object are serialized. [ Note: And they synchronize and serialize with other functions through the referred shared state. — end note ]For purposes of determining the existence of a data race, `set_value`, `set_exception`, `set_value_at_thread_exit`, and `set_exception_at_thread_exit` behave as atomic operations (1.10 [intro.multithread]) on the memory location associated with the `promise`. Calls to these operations on a single promise shall appear to occur in a single total order. [Note: this can be viewed as the modification order (1.10 [intro.multithread]) of the promise. — end note] These operations do not provide any ordering guarantees with respect to other operations, except through operations on futures that reference the same shared state.

>    >     void promise::set_value_at_thread_exit(const R& r);
>    >     void promise::set_value_at_thread_exit(R&& r);
>    >     void promise<R&>::set_value_at_thread_exit(R& r);
>    >     void promise<void>::set_value_at_thread_exit();

>    > > -22- Effects: Stores the value `r` in the shared state without making that state ready immediately. Schedules that state to be made ready when the current thread exits, after all objects of thread storage duration associated with the current thread have been destroyed.

>    > > -23- Throws: `future_error` if an error condition occurs.

>    > > -24- Error conditions:
>    > > - `promise_already_satisfied` if its shared state already has a stored value or exception.
>    > > - `no_state` if `*this` has no shared state.

>    > > -??- Synchronization: For purposes of determining the existence of a data race, `set_value`, `set_exception`, `set_value_at_thread_exit`, and `set_exception_at_thread_exit` behave as atomic operations (1.10 [intro.multithread]) on the memory location associated with the `promise`. Calls to these operations on a single promise shall appear to occur in a single total order. [Note: this can be viewed as the modification order (1.10 [intro.multithread]) of the promise. — end note] These operations do not provide any ordering guarantees with respect to other operations, except through operations on futures that reference the same shared state.

>    >     void promise::set_exception_at_thread_exit(exception_ptr p);

>    > > -25- Effects: Stores the exception pointer `p` in the shared state without making that state ready immediately. Schedules that state to be made ready when the current thread exits, after all objects of thread storage duration associated with the current thread have been destroyed.

>    > > -26- Throws: `future_error` if an error condition occurs.

>    > > -27- Error conditions:
>    > > - `promise_already_satisfied` if its shared state already has a stored value or exception.
>    > > - `no_state` if `*this` has no shared state.

>    > > -??- Synchronization: For purposes of determining the existence of a data race, `set_value`, `set_exception`, `set_value_at_thread_exit`, and `set_exception_at_thread_exit` behave as atomic operations (1.10 [intro.multithread]) on the memory location associated with the `promise`. Calls to these operations on a single promise shall appear to occur in a single total order. [Note: this can be viewed as the modification order (1.10 [intro.multithread]) of the promise. — end note] These operations do not provide any ordering guarantees with respect to other operations, except through operations on futures that reference the same shared state.

> 4. Edit 30.6.9.1 [futures.task.members], paragraph 15-21 as follows:

>    >     void operator()(ArgTypes... args);

>    > > -15- Effects: `INVOKE(f, t1, t2, ..., tN, R)`, where `f` is the stored task of `*this` and `t1`, `t2`, `...`, `tN` are the values in `args...`. If the task returns normally, the return value is stored as the asynchronous result in the shared state of `*this`, otherwise the exception thrown by the task is stored. The shared state of `*this` is made ready, and any threads blocked in a function waiting for the shared state of `*this` to become ready are unblocked.

>    > > -16- Throws: a `future_error` exception object if there is no shared state or the stored task has already been invoked.

>    > > -17- Error conditions:
>    > > - `promise_already_satisfied` if the shared state is already ready.
>    > > - `no_state` if `*this` has no shared state.

>    > > -18- Synchronization: a successful call to `operator()` synchronizes with (1.10 [intro.multithread]) a call to any member function of a `future` or `shared_future` object that shares the shared state of `*this`. The completion of the invocation of the stored task and the storage of the result (whether normal or exceptional) into the shared state synchronizes with (1.10 [intro.multithread]) the successful return from any member function that detects that the state is set to ready. [ Note: `operator()` synchronizes and serializes with other functions through the shared state. — end note ]

>    >     void make_ready_at_thread_exit(ArgTypes... args);

>    > > -19- Effects: `INVOKE(f, t1, t2, ..., tN, R)`, where `f` is the stored task and `t1`, `t2`, `...`, `tN` are the values in `args...`. If the task returns normally, the return value is stored as the asynchronous result in the shared state of `*this`, otherwise the exception thrown by the task is stored. In either case, this shall be done without making that state ready (30.6.4 [futures.state]) immediately. Schedules the shared state to be made ready when the current thread exits, after all objects of thread storage duration associated with the current thread have been destroyed.

>    > > -20- Throws: `future_error` if an error condition occurs.

>    > > -21- Error conditions:
>    > > - `promise_already_satisfied` if the shared state already has a stored value or exception.
>    > > - `no_state` if `*this` has no shared state.

>    > > -??- Synchronization: a successful call to `make_ready_at_thread_exit` synchronizes with (1.10 [intro.multithread]) a call to any member function of a `future` or `shared_future` object that shares the shared state of `*this`. The completion of

>    > > - the invocation of the stored task and the storage of the result (whether normal or exceptional) into the shared state

>    > > - the destructors for all the current thread's variables of thread storage duration

>    > > synchronize with (1.10 [intro.multithread]) the successful return from any member function that detects that the state is set to ready. [Note: `make_ready_at_thread_exit` synchronizes and serializes with other functions through the shared state. — end note]

Proposed resolution:

Resolved 2011-03 Madrid meeting by paper N3278

### 1508. Rename `packaged_task::operator bool()`

Section: 30.6.9 [futures.task] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-201

`packaged_task` provides `operator bool()` to check whether an object has an associated asynchronous state. The various `future` types provide a member function `valid()` that does the same thing. The names of these members should be the same.

[ Resolution proposed by ballot comment: ]

> Replaced the name `packaged_task::operator bool()` with `packaged_task::valid()` in the synopsis (30.6.9 [futures.task]/2) and the member function specification (before 30.6.9.1 [futures.task.members]/15).

[ 2010-11-02 Daniel translates proposed wording changes into a proper proposed resolution and verified that no other places implicitly take advantage of `packaged_task` conversion to bool. ]

[Resolved in Batavia by accepting n3194. ]

Proposed resolution:

1. Change 30.6.9 [futures.task]/2, class template `packaged_task` synopsis as indicated:

   >     template<class R, class... ArgTypes>
   >     class packaged_task<R(ArgTypes...)> {
   >     public:
   >       typedef R result_type;
   >       [..]
   >       explicit operator bool valid() const;
   >       [..]
   >     };

2. Change 30.6.9 [futures.task] before p. 15 as indicated:

   >     explicit operator bool valid() const;

   > > 15 Returns: true only if `*this` has an associated asynchronous state.

   > > 16 Throws: nothing.

### 1513. 'launch' enum too restrictive

Section: 30.6 [futures] Status: Resolved Submitter: Switzerland Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses CH-36

Providing only three different possible values for the enum `launch` and saying that `launch::any` means either `launch::sync` or `launch::async` is very restricting. This hinders future implementors to provide clever infrastructures that can simply by used by a call to `async(launch::any,...)`. Also there is no hook for an implementation to provide additional alternatives to `launch` enumeration and no useful means to combine those (i.e. interpret them like flags). We believe something like `async(launch::sync | launch::async, ...)` should be allowed and can become especially useful if one could say also something like `async(launch::any & ~launch::sync, ....)` respectively. This flexibility might limit the features usable in the function called through `async()`, but it will allow a path to effortless profit from improved hardware/software without complicating the programming model when just using `async(launch::any,...)`

[ Resolution proposed by ballot comment: ]

Change in 30.6.1 [futures.overview] 'enum class launch' to allow further implementation defined values and provide the following bit-operators on the launch values (`operator|`, `operator&`, `operator~` delivering a `launch` value).

Note: a possible implementation might use an unsigned value to represent the `launch` enums, but we shouldn't limit the standard to just 32 or 64 available bits in that case and also should keep the launch enums in their own enum namespace.

Change [future.async] p3 according to the changes to `enum launch`. change --`launch::any` to "the implementation may choose any of the policies it provides." Note: this can mean that an implementation may restrict the called function to take all required information by copy in case it will be called in a different address space, or even, on a different processor type. To ensure that a call is either performed like `launch::async` or `launch::sync` describe one should call `async(launch::sync|launch::async,...)`

[ 2010-11-02 Daniel comments: ]

> The new paper n3113 provides concrete wording.

Proposed resolution:

Resolved by n3188.

### 1514. `packaged_task` constructors need review

Section: 30.6.9.1 [futures.task.members] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-207

The constructor that takes `R(*)(ArgTypes...)` is not needed; the constructor that takes a callable type works for this argument type. More generally, the constructors for packaged_task should parallel those for function.

[ US-207 Suggested Resolution: ]

> Review the constructors for packaged_task and provide the same ones as function, except where inappropriate.

[ 2010-10-22 Howard provides wording, as requested by the LWG in Rapperswil. ]

[2011-02-10 Reflector discussion]

Moved to Tentatively Ready after 5 votes.

Proposed resolution:

Alter the list of constructors in both 30.6.9 [futures.task] and in 30.6.9.1 [futures.task.members] as indicated:

>     template <class F>
>     explicit packaged_task(F f);
>     template <class F, class Allocator>
>     explicit packaged_task(allocator_arg_t, const Allocator& a, F f);
>     explicit packaged_task(R(*f)(ArgTypes...));
>     template <class F>
>     explicit packaged_task(F&& f);
>     template <class F, class Allocator>
>     explicit packaged_task(allocator_arg_t, const Allocator& a, F&& f);

### 1515. `packaged_task::make_ready_at_thread_exit` has no synchronization requirements

Section: 30.6.9.1 [futures.task.members] Status: Resolved Submitter: INCITS Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses US-208

`packaged_task::make_ready_at_thread_exit` has no synchronization requirements.

[ Resolution proposed by ballot comment: ]

Figure out what the synchronization requirements should be and write them.

[2011-02-09 Anthony provides a proposed resolution]

[2011-02-19 Additional edits by Hans, shown in the proposed resolution section]

[2011-02-22 Reflector discussion]

Moved to Tentatively Ready after 5 votes.

> Proposed Resolution

> Add a new paragraph following 30.6.9.1 [futures.task.members] p. 19:

> >     void make_ready_at_thread_exit(ArgTypes... args);

> > 19 - ...

> > ?? - Synchronization: Following a successful call to `make_ready_at_thread_exit`, the destruction of all objects with thread storage duration associated with the current thread happens before the associated asynchronous state is made ready. The marking of the associated asynchronous state as ready synchronizes with (1.10 [intro.multithread]) the successful return from any function that detects that the state is set to ready.

Proposed resolution:

Resolved 2011-03 Madrid meeting by paper N3278

### 1516. No specification for which header contains `auto_ptr`

Section: X [depr.auto.ptr] Status: C++11 Submitter: BSI Opened: 2010-08-25 Last modified: 2015-04-08

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Discussion:

Addresses GB-142

`auto_ptr` does not appear in the `<memory>` synopsis and [depr.auto.ptr] doesn't say which header declares it. Conversely, the deprecated binders `bind1st` etc. are in the `<functional>` synopsis, this is inconsistent

Either `auto_ptr` should be declared in the `<memory>` synopsis, or the deprecated binders should be removed from the `<functional>` synopsis and appendix D should say which header declares the binders and `auto_ptr`.

[ Post-Rapperswil ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Add the following lines to the synopsis of header `<memory>` in [memory]/1:

>     // [depr.auto.ptr], Class auto_ptr (deprecated):
>     template <class X> class auto_ptr;

### 1517. default_delete's default constructor should be trivial

Section: 20.8.1.1.2 [unique.ptr.dltr.dflt] Status: C++11 Submitter: Daniel Krügler Opened: 2010-09-12 Last modified: 2015-04-08

View all other issues in [unique.ptr.dltr.dflt].

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Discussion:

The current working draft does specify the default c'tor of `default_delete` in a manner to guarantee static initialization for default-constructed objects of static storage duration as a consequence of the acceptance of the proposal n2976 but this paper overlooked the fact that the suggested declaration does not ensure that the type will be a trivial type. The type `default_delete` was always considered as a simple wrapper for calling `delete` or `delete[]`, respectivly and should be a trivial type.

In agreement with the new settled core language rules this easy to realize by just changing the declaration to

>     constexpr default_delete() = default;

This proposal also automatically solves the problem, that the semantics of the default constructor of the partial specialization `default_delete<T[]>` is not specified at all. By defaulting its default constructor as well, the semantics are well-defined.

[ Post-Rapperswil ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

The following wording changes are against N3126.

1. Change the synopsis of the primary template definition of `default_delete` in [unique.ptr.dltr.dflt] as indicated:

   >     namespace std {
   >       template <class T> struct default_delete {
   >         constexpr default_delete() = default;
   >         template <class U> default_delete(const default_delete<U>&);
   >         void operator()(T*) const;
   >       };
   >     }

2. Remove the prototype specification of the `default_delete` default constructor in [unique.ptr.dltr.dflt]/1. This brings it in harmony with the style used in the partial specialization `default_delete<T[]>`. Since there are neither implied nor explicit members, there is no possibility to misinterpret what the constructor does:

   >     constexpr default_delete();

   > > 1 Effects: Default constructs a `default_delete` object.

3. Change the synopsis of the partial specialization of `default_delete` in [unique.ptr.dltr.dflt1] as indicated:

   >     namespace std {
   >       template <class T> struct default_delete<T[]> {
   >         constexpr default_delete() = default;
   >         void operator()(T*) const;
   >         template <class U> void operator()(U*) const = delete;
   >       };
   >     }

### 1518. Waiting for deferred functions

Section: 30.6 [futures] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2010-09-14 Last modified: 2015-04-08

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Discussion:

The current WP N3126 contains ambiguous statements about the behaviour of functions `wait_for`/`wait_until` in case the future refers to a deferred function. Moreover, I believe it describes a disputable intent, different from the one contained in the original async proposals, that may have been introduced inadvertently during the "async cleanup" that occurred recently. Consider the following case:

>     int f();
>     future<int> x = async(launch::deferred, f);
>     future_status s = x.wait_for(chrono::milliseconds(100));

This example raises two questions:

1. is `f` invoked?
2. what is the value of `s`?

According to the current WP, the answer to question 1 is yes, because 30.6.9/3 says "The first call to a function waiting for the associated asynchronous state created by this async call to become ready shall invoke the deferred function in the thread that called the waiting function". The answer to question 2, however, is not as clear. According to 30.6.6/23, s should be `future_status::deferred` because `x` refers to a deferred function that is not running, but it should also be `future_status::ready` because after executing `f` (and we saw that `f` is always executed) the state becomes ready. By the way, the expression "deferred function that is not running" is very unfortunate in itself, because it may apply to both the case where the function hasn't yet started, as well as the case where it was executed and completed.

While we clearly have a defect in the WP answering to question 2, it is my opinion that the answer to question 1 is wrong, which is even worse. Consider that the execution of the function `f` can take an arbitrarily long time. Having `wait_for()` invoke `f` is a potential violation of the reasonable expectation that the execution of `x.wait_for(chrono::milliseconds(100))` shall take at most 100 milliseconds plus a delay dependent on the quality of implementation and the quality of management (as described in paper N3128). In fact, previous versions of the WP clearly specified that only function `wait()` is required to execute the deferred function, while `wait_for()` and `wait_until()` shouldn't.

The proposed resolution captures the intent that `wait_for()` and `wait_until()` should never attempt to invoke the deferred function. In other words, the P/R provides the following answers to the two questions above:

1. no
2. `future_status::deferred`

In order to simplify the wording, the definition of deferred function has been tweaked so that the function is no longer considered deferred once its evaluation has started, as suggested by Howard.

Discussions in the reflector questioned whether `wait_for()` and `wait_until()` should return immediately or actually wait hoping for a second thread to execute the deferred function. I believe that waiting could be useful only in a very specific scenario but detrimental in the general case and would introduce another source of ambiguity: should `wait_for()` return `future_status::deferred` or `future_status::timeout` after the wait? Therefore the P/R specifies that `wait_for`/`wait_until` shall return immediately, which is simpler, easier to explain and more useful in the general case.

[ Post-Rapperswil ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

The proposed wording changes are relative to the Final Committee Draft, N3126.

Note to the editor: the proposed wording is meant not be in conflict with any change proposed by paper N3128 "C++ Timeout Specification". Ellipsis are deliberately used to avoid any unintended overlapping.

1. In [futures.unique_future] 30.6.6/22:

   Effects: none if the associated asynchronous state contains a deferred function (30.6.9), otherwise blocks until the associated asynchronous state is ready or [...].

2. In [futures.unique_future] 30.6.6/23 first bullet:

   — future_status::deferred if the associated asynchronous state contains a deferred function that is not running.

3. In [futures.unique_future] 30.6.6/25:

   Effects: none if the associated asynchronous state contains a deferred function (30.6.9), otherwise blocks until the associated asynchronous state is ready or [...].

4. In [futures.unique_future] 30.6.6/26 first bullet:

   — future_status::deferred if the associated asynchronous state contains a deferred function that is not running.

5. In [futures.shared_future] 30.6.7/27

   Effects: none if the associated asynchronous state contains a deferred function (30.6.9), otherwise blocks until the associated asynchronous state is ready or [...].

6. In [futures.unique_future] 30.6.7/28 first bullet:

   — future_status::deferred if the associated asynchronous state contains a deferred function that is not running.

7. In [futures.shared_future] 30.6.6/30:

   Effects: none if the associated asynchronous state contains a deferred function (30.6.9), otherwise blocks until the associated asynchronous state is ready or [...].

8. In [futures.unique_future] 30.6.7/31 first bullet:

   — future_status::deferred if the associated asynchronous state contains a deferred function that is not running.

9. In [futures.atomic_future] 30.6.8/23

   Effects: none if the associated asynchronous state contains a deferred function (30.6.9), otherwise blocks until the associated asynchronous state is ready or [...].

10. In [futures.unique_future] 30.6.8/24 first bullet:

    — future_status::deferred if the associated asynchronous state contains a deferred function that is not running.

11. In [futures.atomic_future] 30.6.8/27:

    Effects: none if the associated asynchronous state contains a deferred function (30.6.9), otherwise blocks until the associated asynchronous state is ready or [...].

12. In [futures.unique_future] 30.6.8/28 first bullet:

    — future_status::deferred if the associated asynchronous state contains a deferred function that is not running.

13. In [futures.async] 30.6.9/3 second bullet:

    [...] The first call to a function waiting requiring a non-timed wait for the associated asynchronous state created by this async call to become ready shall invoke the deferred function in the thread that called the waiting function; once evaluation of `INVOKE(g, xyz)` begins, the function is no longer considered deferred all other calls waiting for the same associated asynchronous state to become ready shall block until the deferred function has completed.

### 1519. bucketsize() const only for unordered set

Section: 23.5.4 [unord.map], 23.5.5 [unord.multimap], 23.5.7 [unord.multiset] Status: C++11 Submitter: Nicolai Josuttis Opened: 2010-10-09 Last modified: 2015-04-08

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Discussion:

While `bucket_size()` is const for `unordered_set`, for all other unordered containers it is not defined as constant member function.

[ Post-Rapperswil ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

The wording refers to N3126.

1. Change 23.7.1 Class template unordered_map [unord.map]/3, as indicated:

   >       namespace std {
   >         template <class Key,
   >           class T,
   >           class Hash = hash<Key>,
   >           class Pred = std::equal_to<Key>,
   >           class Alloc = std::allocator<std::pair<const Key, T> > >
   >         class unordered_map
   >         {
   >         public:
   >           [..]
   >           // bucket interface
   >           size_type bucket_count() const;
   >           size_type max_bucket_count() const;
   >           size_type bucket_size(size_type n) const;
   >           [..]

2. Change 23.7.2 Class template unordered_multimap [unord.multimap]/3, as indicated:

   >       namespace std {
   >         template <class Key,
   >           class T,
   >           class Hash = hash<Key>,
   >           class Pred = std::equal_to<Key>,
   >           class Alloc = std::allocator<std::pair<const Key, T> > >
   >         class unordered_multimap
   >         {
   >         public:
   >           [..]
   >           // bucket interface
   >           size_type bucket_count() const;
   >           size_type max_bucket_count() const;
   >           size_type bucket_size(size_type n) const;
   >           [..]

3. Change 23.7.4 Class template unordered_multiset [unord.multiset]/3, as indicated:

   >       namespace std {
   >         template <class Key,
   >           class Hash = hash<Key>,
   >           class Pred = std::equal_to<Key>,
   >           class Alloc = std::allocator<Key> >
   >         class unordered_multiset
   >         {
   >         public:
   >           [..]
   >           // bucket interface
   >           size_type bucket_count() const;
   >           size_type max_bucket_count() const;
   >           size_type bucket_size(size_type n) const;
   >           [..]

### 1520. `INVOKE` on member data pointer with too many arguments

Section: 20.9.2 [func.require] Status: C++11 Submitter: Howard Hinnant Opened: 2010-10-10 Last modified: 2015-04-08

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Discussion:

20.8.2 [func.require] p1 says:

> 1 Define `INVOKE(f, t1, t2, ..., tN)` as follows:

> - `(t1.*f)(t2, ..., tN)` when `f` is a pointer to a member function of a class `T` and `t1` is an object of type `T` or a reference to an object of type `T` or a reference to an object of a type derived from `T`;
> - `((*t1).*f)(t2, ..., tN)` when `f` is a pointer to a member function of a class `T` and `t1` is not one of the types described in the previous item;
> - `t1.*f` when `f` is a pointer to member data of a class `T` and `t1` is an object of type `T` or a reference to an object of type `T` or a reference to an object of a type derived from `T`;
> - `(*t1).*f` when `f` is a pointer to member data of a class `T` and `t1` is not one of the types described in the previous item;
> - `f(t1, t2, ..., tN)` in all other cases.

The question is: What happens in the 3rd and 4th bullets when `N > 1`?

Does the presence of `t2, ..., tN` get ignored, or does it make the `INVOKE` ill formed?

Here is sample code which presents the problem in a concrete example:

>     #include <functional>
>     #include <cassert>

>     struct S {
>        char data;
>     };

>     typedef char S::*PMD;

>     int main()
>     {
>        S s;
>        PMD pmd = &S::data;
>        std::reference_wrapper<PMD> r(pmd);
>        r(s, 3.0) = 'a';  // well formed?
>        assert(s.data == 'a');
>     }

Without the "`3.0`" the example is well formed.

[Note: Daniel provided wording to make it explicit that the above example is ill-formed. — end note ]

[ Post-Rapperswil ]

> Moved to Tentatively Ready after 5 positive votes on c++std-lib.

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

The wording refers to N3126.

Change 20.8.2 [func.require]/1 as indicated:

> 1 Define `INVOKE(f, t1, t2, ..., tN)` as follows:

> - ...
> - ...
> - `t1.*f` when `N == 1` and `f` is a pointer to member data of a class `T` and `t1` is an object of type `T` or a reference to an object of type `T` or a reference to an object of a type derived from `T`;
> - `(*t1).*f` when `N == 1` and `f` is a pointer to member data of a class `T` and `t1` is not one of the types described in the previous item;
> - ...

### 1522. `conj` specification is now nonsense

Section: 26.4.9 [cmplx.over] Status: C++11 Submitter: P.J. Plauger Opened: 2010-10-14 Last modified: 2015-04-08

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Discussion:

In Pittsburgh, we accepted the resolution of library issue 1137, to add a sentence 3 to [cmplx.over]:

> All the specified overloads shall have a return type which is the nested `value_type` of the effectively cast arguments.

This was already true for four of the six functions except `conj` and `proj`. It is not completely unreasonable to make `proj` return the real value only, but the IEC specification does call for an imaginary part of -0 in some circumstances. The people who care about these distinctions really care, and it is required by an international standard.

Making `conj` return just the real part breaks it horribly, however. It is well understood in mathematics that `conj(re + i*im)` is `(re - i*im)`, and it is widely used. The accepted new definition makes `conj` useful only for pure real operations. This botch absolutely must be fixed.

[ 2010 Batavia: The working group concurred with the issue's Proposed Resolution ]

[ Adopted at 2010-11 Batavia ]

Proposed resolution:

Remove the recently added paragraph 3 from [cmplx.over]:

> 3 All the specified overloads shall have a return type which is the nested `value_type` of the effectively cast arguments.

### 1523. `noexcept` for Clause 29

Section: 29 [atomics] Status: Resolved Submitter: Hans Boehm Opened: 2010-11-13 Last modified: 2015-04-08

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Discussion:

Addresses GB-63 for Clause 29

Clause 29 does not specify noexcept for any of the atomic operations. It probably should, though that's not completely clear. In particular, atomics may want to throw in implementations that support transactional memory.

Proposed resolution:

Apply paper N3251, noexcept for the Atomics Library.

### 1524. Allocation functions are missing happens-before requirements and guarantees

Section: 18.6.1.4 [new.delete.dataraces] Status: C++11 Submitter: Hans Boehm Opened: 2011-02-26 Last modified: 2015-04-08

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Discussion:

Addresses US-34

Technical details:

When the same unit of storage is allocated and deallocated repeatedly, operations on it can't be allowed to race between the allocator and the user program. But I don't see any mention of happens-before in the descriptions of allocation and deallocation functions.

Proposed resolution (not wording yet):

- The call to an allocation function returning a pointer `P` must happen-before the matching deallocation call with `P` as a parameter. Otherwise the behavior is undefined. I don't know whether receiving `P` with `memory_order_consume` fits this requirement. `memory_order_relaxed` does not.

- If some memory is passed to a deallocation function, the implementation must ensure that the deallocation call happens-before any allocation call that returns the same memory address.

[2011-02-26: Hans comments and drafts wording]

The second requirement already exists, almost verbatim, as 18.6.1.4 [new.delete.dataraces] p. 1. I think this is where the statement belongs. However, this paragraph requires work to correctly address the first part of the issue.

[Adopted at Madrid, 2011-03]

Proposed resolution:

Change 18.6.1.4 [new.delete.dataraces] p. 1 as follows:

> 1 The library versions of `operator new` and `operator delete`, user replacement versions of global `operator new` and `operator delete`, and the C standard library functions `calloc`, `malloc`, `realloc`, and `free` shall not introduce data races (1.10 [intro.multithread]) as a result of concurrent calls from different threads. For purposes of determining the existence of data races, the library versions of `operator new`, user replacement versions of global `operator new`, and the C standard library functions `calloc` and `malloc` shall behave as though they accessed and modified only the storage referenced by the return value. The library versions of `operator delete`, user replacement versions of `operator delete`, and the C standard library function `free` shall behave as though they accessed and modified only the storage referenced by their first argument. The C standard library `realloc` function shall behave as though it accessed and modified only the storage referenced by its first argument and by its return value. Calls to these functions that allocate or deallocate a particular unit of storage shall occur in a single total order, and each such deallocation call shall happen before the next allocation (if any) in this order.

### 1525. Effects of `resize(size())` on a `vector`

Section: 23.3.6.3 [vector.capacity] Status: C++11 Submitter: BSI Opened: 2011-03-24 Last modified: 2015-04-08

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Discussion:

Addresses GB-117

23.3.6.3 [vector.capacity] p. 9 (Same as for 23.3.3.3 [deque.capacity] p. 1 i.e. `deque::resize`). There is no mention of what happens if `sz==size()`. While it obviously does nothing I feel a standard needs to say this explicitely.

Suggested resolution:

Append "If `sz == size()`, does nothing" to the effects.

[2011-03-24 Daniel comments]

During the edit of this issue some non-conflicting overlap with 2033 became obvious. `CopyInsertable` should be `MoveInsertable` and there is missing the `DefaultConstructible` requirements, but this should be fixed by 2033.

Proposed resolution:

Change 23.3.6.3 [vector.capacity] p. 9 as follows:

>     void resize(size_type sz);

> > 9 Effects: If `sz <= size()`, equivalent to `erase(begin() + sz, end());`. If `size() < sz`, appends `sz - size()` value-initialized elements to the sequence.

> > 10 Requires: `T` shall be `CopyInsertable` into `*this`.

### 1526. C++ should not impose thread safety requirements on C99 library implementations

Section: 17.6.5.9 [res.on.data.races] Status: Resolved Submitter: BSI Opened: 2011-03-24 Last modified: 2015-05-22

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Discussion:

Addresses GB-111

Section 17.6.5.9 [res.on.data.races], Data Race Avoidance, requires the C++ Standard Library to avoid data races that might otherwise result from two threads making calls to C++ Standard Library functions on distinct objects. The C standard library is part of the C++ Standard Library and some C++ Standary library functions (parts of the Localization library, as well as Numeric Conversions in 21.5), are specified to make use of the C standard library. Therefore, the C++ standard indirectly imposes a requirement on the thread safety of the C standard library. However, since the C standard does not address the concept of thread safety conforming C implementations exist that do no provide such guarantees. This conflict needs to be reconciled.

Suggested resolution by national body comment:

> remove the requirement to make use of `strtol()` and `sprintf()` since these functions depend on the global C locale and thus cannot be made thread safe.

[2011-03-24 Madrid meeting]

Deferred

[ 2011 Bloomington ]

Alisdair: PJ, does this cause a problem in C?

PJ: Every implementation know of is thread safe.

Pete: There a couple of effects that are specified on strtol() and sprintf() which is a problem.

PJ: When C++ talks about C calls it should be "as if" calling the function.

Pete: Culprit is to string stuff. My fault.

PJ: Not your fault. You did what you were told. Distinct resolution to change wording.

Dietmar: What would we break if we change it back?

Pete: Nothing. If implemented on top of thread safe C library you are just fine.

Alisdair: Anyone want to clean up wording and put it back to what Pete gave us?

Alisdair: No volunteers. Do we want to mark as NAD? We could leave it as deferred.

Stefanus: Did original submitter care about this?

Lawrence: There is some work to make local calls thread safe. The resolution would be to call those thread safe version.

Pete: "As if called under single threaded C program"

Action Item (Alisdair): Write wording for this issue.

[2012, Kona]

Re-opened at the request of the concurrency subgroup, who feel there is an issue that needs clarifying for the (planned) 2017 standard.

Rationale:

No consensus to make a change at this time

[2012, Portland]

The concurrency subgroup decided to encourage the LWG to consider a change to 17.2 [library.c] or thereabouts to clarify that we are requiring C++-like thread-safety for `setlocale`, so that races are not introduced by C locale accesses, even when the C library allows it. This would require e.g. adding "and data race avoidance" at the end of 17.2 [library.c] p1:

"The C++ standard library also makes available the facilities of the C standard library, suitably adjusted to ensure static type safety and data race avoidance.",

with some further clarifications in the sections mentioned in 1526.

This seems to be consistent with existing implementations. This would technically not be constraining C implementation, but it would be further constraining C libraries used for both C and C++.

[Lenexa 2015-05-05: Move to Resolved]

JW: it's a bit odd that the issue title says sould not impose requirements on C libs, then the P/R does exactly that. Does make sense though, previously we imposed an implicit requirement which would not have been met. Now we say it explicitly and require it is met.

STL: I think this is Resolved, it has been fixed in the working paper [support.runtime]/6 is an example where we call out where things can race. That implies that for everything else they don't create races.

JW: I'm not sure, I think we still need the "and data race avoidance" to clarify that the features from C avoid races, even though C99 says no such thing.

STL: [library.c] says that something like sqrt is part of the C++ Standard LIbrary. [res.on.data.races] then applies to them. Would be OK with a note there, but am uncomfortable with "and data race avoidance" which sounds like it's making a very strong guarantee.

ACTION ITEM JW to editorially add note to [library.c] p1: "Unless otherwise specified, the C Standard Library functions shall meet the requirements for data race avoidance (xref [res.on.data.races])"

Move to Resolved?

10 in favor, 0 opposed, 3 abstentions

Proposed resolution:

This wording is relative to N3376.

1. Change 17.2 [library.c] p1 as indicated:

   > -1- The C++ standard library also makes available the facilities of the C standard library, suitably adjusted to ensure static type safety and data race avoidance.

### 2000. Missing definition of `packaged_task` specialization of `uses_allocator`

Section: 30.6.9.2 [futures.task.nonmembers] Status: C++11 Submitter: Howard Hinnant Opened: 2010-08-29 Last modified: 2015-04-08

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Discussion:

[futures.task.nonmembers]/3 says:

>        template <class R, class Alloc>
>          struct uses_allocator<packaged_task<R>, Alloc>;

This is a declaration, but should be a definition.

Proposed resolution:

Change [futures.task.nonmembers]/3:

>        template <class R, class Alloc>
>          struct uses_allocator<packaged_task<R>, Alloc>;
>             : true_type {};

### 2001. Class template `basic_regex` uses non existent `string_type`

Section: 28.8.3 [re.regex.assign] Status: C++11 Submitter: Volker Lukas Opened: 2010-10-21 Last modified: 2015-04-08

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Discussion:

In working draft N3126, subclause 28.8.3 [re.regex.assign], paragraphs 12, 13 and 19, the name `string_type` is used. This is presumably a typedef for `basic_string<value_type>`, where `value_type` is the character type used by `basic_regex`. The `basic_regex` template however defines no such typedef, and neither does the `<regex>` header or the `<initializer_list>` header included by `<regex>`.

[ 2010-11-03 Daniel comments and suggests alternative wording: ]

> The proposed resolution needs to use `basic_string<charT>` instead of `basic_string<char>`

Previous Proposed Resolution:

Make the following changes to [re.regex.assign]:

>     basic_regex& assign(const charT* ptr, flag_type f = regex_constants::ECMAScript);

> > 12 Returns: `assign(string_typebasic_string<charT>(ptr), f)`.

>     basic_regex& assign(const charT* ptr, size_t len,
>       flag_type f = regex_constants::ECMAScript);

> > 13 Returns: `assign(string_typebasic_string<charT>(ptr, len), f)`.

>     [..]

>     template <class InputIterator>
>       basic_regex& assign(InputIterator first, InputIterator last,
>                               flag_type f = regex_constants::ECMAScript);

> > 18 Requires: The type `InputIterator` shall satisfy the requirements for an Input Iterator (24.2.3).

> > 19 Returns: `assign(string_typebasic_string<charT>(first, last), f)`.

[ 2010 Batavia ]

Unsure if we should just give `basic_regex` a `string_type` typedef. Looking for when `string_type` was introduced into `regex`. Howard to draft wording for `typedef typename traits::string_type string_type`, then move to Review.

[ 2011-02-16: Daniel comments and provides an alternative resolution. ]

I'm strongly in favour with the Batavia idea to provide a separate `string_type` within `basic_regex`, but it seems to me that the issue resultion should add one more important typedef, namely that of the traits type! Currently, `basic_regex` is the only template that does not publish the type of the associated traits type. Instead of opening a new issue, I added this suggestion as part of the proposed wording.

[2011-02-24 Reflector discussion]

Moved to Tentatively Ready after 6 votes.

Proposed resolution:

Change the class template `basic_regex` synopsis, 28.8 [re.regex] p. 3, as indicated:

>     namespace std {
>       template <class charT,
>                 class traits = regex_traits<charT> >
>       class basic_regex {
>       public:
>         // types:
>         typedef charT value_type;
>         typedef traits traits_type;
>         typedef typename traits::string_type string_type;
>         typedef regex_constants::syntax_option_type flag_type;
>         typedef typename traits::locale_type locale_type;

>         [..]
>       };
>     }

### 2002. Class template `match_results` does not specify the semantics of `operator==`

Section: 28.10.8 [re.results.nonmember] Status: Resolved Submitter: Daniel Krügler Opened: 2010-10-24 Last modified: 2015-04-08

View all issues with Resolved status.

Discussion:

The Returns element of `operator==` says:

> `true` only if the two objects refer to the same match

It is not really clear what this means: The current specification would allow for an implementation to return `true`, only if the address values of `m1` and `m2` are the same. While this approach is unproblematic in terms of used operations this is also a bit unsatisfactory. With identity equality alone there seems to be no convincing reason to provide this operator at all. It could for example also refer to an comparison based on iterator values. In this case a user should better know that this will be done, because there is no guarantee at all that inter-container comparison of iterators is a feasible operation. This was a clear outcome of the resolution provided in N3066 for LWG issue 446. It could also mean that a character-based comparison of the individual `sub_match` elements should be done - this would be equivalent to applying `operator==` to the subexpressions, prefix and suffix.

Proposed resolution:

Addressed by paper n3158.

### 2003. String exception inconsistency in erase.

Section: 21.4.1 [string.require] Status: C++14 Submitter: José Daniel García Sánchez Opened: 2010-10-21 Last modified: 2015-04-08

View all other issues in [string.require].

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Discussion:

Clause 21.4.1 [string.require]p3 states:

> No `erase()` or `pop_back()` member function shall throw any exceptions.

However in 21.4.6.5 [string::erase] p2 the first version of `erase` has

> Throws: `out_of_range` if `pos > size()`.

[2011-03-24 Madrid meeting]

Beman: Don't want to just change this, can we just say "unless otherwise specified"?

Alisdair: Leave open, but update proposed resolution to say something like "unless otherwise specified".

General agreement that it should be corrected but not a stop-ship.

Action: Update proposed wording for issue 2003 as above, but leave Open.

[2014-02-12 Issaquah meeting]

Jeffrey: Madrid meeting's proposed wording wasn't applied, and it's better than the original proposed wording. However, this sentence is only doing 3 functions' worth of work, unlike the similar paragraphs in 23.2.1 [container.requirements.general]. Suggest just putting "Throws: Nothing" on the 3 functions.

[2014-02-13 Issaquah meeting]

Move as Immmediate

Proposed resolution:

Remove [string.require]p/3:

> 3 No `erase()` or `pop_back()` member function shall throw any exceptions.

Add to the specifications of `iterator erase(const_iterator p);`, `iterator erase(const_iterator first, const_iterator last);`, and `void pop_back();` in 21.4.6.5 [string::erase]:

> Throws: Nothing

### 2004. `duration::operator*` has template parameters in funny order

Section: 20.12.5.5 [time.duration.nonmember] Status: C++11 Submitter: P.J. Plauger Opened: 2010-10-14 Last modified: 2015-04-08

View all other issues in [time.duration.nonmember].

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Discussion:

In [time] and [time.duration.nonmember] we have:

>     template <class Rep1, class Period, class Rep2>
>         duration<typename common_type<Rep1, Rep2>::type, Period>
>             operator*(const Rep1& s, const duration<Rep2, Period>& d);

Everywhere else, we always have `<rep, period>` in that order for a given type. But here, we have `Period` and `Rep2` in reverse order for `<Rep2, Period>`. This is probably of little importance, since the template parameters are seldom spelled out for a function like this. But changing it now will eliminate a potential source of future errors and confusion.

Proposed resolution:

Change the signature in [time] and [time.duration.nonmember] to:

>     template <class Rep1, class PeriodRep2, class Rep2Period>
>         duration<typename common_type<Rep1, Rep2>::type, Period>
>             operator*(const Rep1& s, const duration<Rep2, Period>& d);

### 2005. `unordered_map::insert(T&&)` protection should apply to `map` too

Section: 23.4.4.4 [map.modifiers], 23.4.5.3 [multimap.modifiers], 23.5.4.4 [unord.map.modifiers], 23.5.5.3 [unord.multimap.modifiers] Status: C++14 Submitter: P.J. Plauger Opened: 2010-10-14 Last modified: 2015-04-08

View all other issues in [map.modifiers].

View all issues with C++14 status.

Discussion:

In [unord.map.modifiers], the signature:

>     template <class P>
>         pair<iterator, bool> insert(P&& obj);

now has an added Remarks paragraph:

> Remarks: This signature shall not participate in overload resolution unless `P` is implicitly convertible to `value_type`.

The same is true for `unordered_multimap`.

But neither `map` nor `multimap` have this constraint, even though it is a Good Thing(TM) in those cases as well.

[ The submitter suggests: Add the same Remarks clause to [map.modifiers] and [multimap.modifiers]. ]

[ 2010-10-29 Daniel comments: ]

I believe both paragraphs need more cleanup: First, the current Requires element conflict with the Remark; second, it seems to me that the whole single Requires element is intended to be split into a Requires and an Effects element; third, the reference to `tuple` is incorrect (noticed by Paolo Carlini); fourth, it refers to some non-existing `InputIterator` parameter relevant for a completely different overload; sixth, the return type of the overload with hint is wrong. The following proposed resolution tries to solve these issues as well and uses similar wording as for the corresponding unordered containers. Unfortunately it has some redundancy over Table 99, but I did not remove the specification because of the more general template parameter `P` - the Table 99 requirements apply only for an argument identical to `value_type`.

Daniel's Proposed resolution (not current):

> 1. Change 23.4.4.4 [map.modifiers] around p. 1 as indicated:

>    >     template <class P> pair<iterator, bool> insert(P&& x);
>    >     template <class P> pair<iterator, bool> insert(const_iterator position, P&& x);

>    > > 1 Requires: `P` shall be convertible to `value_type` is constructible from `std::forward<P>(x)`..

>    > > If `P` is instantiated as a reference type, then the argument `x` is copied from. Otherwise `x` is considered to be an rvalue as it is converted to `value_type` and inserted into the map. Specifically, in such cases `CopyConstructible` is not required of `key_type` or `mapped_type` unless the conversion from `P` specifically requires it (e.g., if `P` is a `tuple<const key_type, mapped_type>`, then `key_type` must be `CopyConstructible`). The signature taking `InputIterator` parameters does not require `CopyConstructible` of either `key_type` or `mapped_type` if the dereferenced `InputIterator` returns a non-const rvalue `pair<key_type,mapped_type>`. Otherwise `CopyConstructible` is required for both `key_type` and `mapped_type`.
>    > > ? Effects: Inserts `x` converted to `value_type` if and only if there is no element in the container with key equivalent to the key of `value_type(x)`. For the second form, the iterator `position` is a hint pointing to where the search should start.

>    > > ? Returns: For the first form, the `bool` component of the returned `pair` object indicates whether the insertion took place and the iterator component - or for the second form the returned iterator - points to the element with key equivalent to the key of `value_type(x)`.

>    > > ? Complexity: Logarithmic in general, but amortized constant if `x` is inserted right before `position`.

>    > > ? Remarks: These signatures shall not participate in overload resolution unless `P` is implicitly convertible to `value_type`.

> 2. Change 23.4.5.3 [multimap.modifiers] around p. 1 as indicated:

>    >     template <class P> iterator insert(P&& x);
>    >     template <class P> iterator insert(const_iterator position, P&& x);

>    > > 1 Requires: `P` shall be convertible to `value_type` is constructible from `std::forward<P>(x)`.

>    > > If `P` is instantiated as a reference type, then the argument `x` is copied from. Otherwise `x` is considered to be an rvalue as it is converted to `value_type` and inserted into the map. Specifically, in such cases `CopyConstructible` is not required of `key_type` or `mapped_type` unless the conversion from `P` specifically requires it (e.g., if `P` is a `tuple<const key_type, mapped_type>`, then `key_type` must be `CopyConstructible`). The signature taking `InputIterator` parameters does not require `CopyConstructible` of either `key_type` or `mapped_type` if the dereferenced `InputIterator` returns a non-const rvalue `pair<key_type, mapped_type>`. Otherwise `CopyConstructible` is required for both `key_type` and `mapped_type`.
>    > > ? Effects: Inserts `x` converted to `value_type`. For the second form, the iterator `position` is a hint pointing to where the search should start.

>    > > ? Returns: An iterator that points to the element with key equivalent to the key of `value_type(x)`.

>    > > ? Complexity: Logarithmic in general, but amortized constant if `x` is inserted right before `position`.

>    > > ? Remarks: These signatures shall not participate in overload resolution unless `P` is implicitly convertible to `value_type`.

[ 2010 Batavia: ]

We need `is_convertible`, not `is_constructible`, both in ordered and unordered containers.

[ 2011 Bloomington ]

The effects of these inserts can be concisely stated in terms of emplace(). Also, the correct term is "EmplaceConstructible", not "constructible".

New wording by Pablo, eliminating duplicate requirements already implied by the effects clause. Move to Review.

[ 2011-10-02 Daniel comments and refines the proposed wording ]

> Unfortunately the template constraints expressed as "`P` is implicitly convertible to `value_type`" reject the intended effect to support move-only key types, which was the original intention when the library became move-enabled through the rvalue-reference proposals by Howard (This can clearly be deduced from existing carefully selected wording that emphasizes that `CopyConstructible` is only required for special situations involving lvalues or const rvalues as arguments). The root of the problem is based on current core rules, where an "implicitly converted" value has copy-initialization semantics. Consider a move-only key type `KM`, some mapped type `T`, and a source value `p` of type `P` equal to `std::pair<KM, T>`, this is equivalent to:

> >     std::pair<const KM, T> dest = std::move(p);

> Now 8.5 [dcl.init] p16 b6 sb2 says that the effects of this heterogeneous copy-initialization (`p` has a different type than `dest`) are as-if a temporary of the target type `std::pair<const KM, T>` is produced from the rvalue `p` of type `P` (which is fine), and this temporary is used to initialize `dest`. This second step cannot succeed, because we cannot move from `const KM` to `const KM`. This means that `std::is_convertible<P, std::pair<const KM, T>>::value` is false.

> But the actual code that is required (with the default allocator) is simply a direct-initialization from `P` to `value_type`, so imposing an implicit conversion is more than necessary. Therefore I strongly recommend to reduce the "overload participation" constraint to `std::is_constructible<std::pair<const KM, T>, P>::value` instead. This change is the only change that has been performed to the previous proposed wording from Pablo shown below.

[2012, Kona]

Moved to Tentatively Ready by the post-Kona issues processing subgroup, after much discussion on Daniel's analysis of Copy Initialization and move semantics, which we ultimately agreed with.

[2012, Portland: applied to WP]

Proposed resolution:

1. Change 23.4.4.4 [map.modifiers] around p. 1 as indicated:

   >     template <class P> pair<iterator, bool> insert(P&& x);
   >     template <class P> pair<iterator, bool> insert(const_iterator position, P&& x);

   > > 1 Requires: `P` shall be convertible to `value_type`.

   > > If `P` is instantiated as a reference type, then the argument `x` is copied from. Otherwise `x` is considered to be an rvalue as it is converted to `value_type` and inserted into the map. Specifically, in such cases `CopyConstructible` is not required of `key_type` or `mapped_type` unless the conversion from `P` specifically requires it (e.g., if `P` is a `tuple<const key_type, mapped_type>`, then `key_type` must be `CopyConstructible`). The signature taking `InputIterator` parameters does not require `CopyConstructible` of either `key_type` or `mapped_type` if the dereferenced `InputIterator` returns a non-const rvalue `pair<key_type,mapped_type>`. Otherwise `CopyConstructible` is required for both `key_type` and `mapped_type`.
   > > ? Effects: The first form is equivalent to `return emplace(std::forward<P>(x))`. The second form is equivalent to `return emplace_hint(position, std::forward<P>(x))`.

   > > ? Remarks: These signatures shall not participate in overload resolution unless `std::is_constructible<value_type, P&&>::value` is true.

2. Change 23.4.5.3 [multimap.modifiers] around p. 1 as indicated:

   >     template <class P> iterator insert(P&& x);
   >     template <class P> iterator insert(const_iterator position, P&& x);

   > > 1 Requires: `P` shall be convertible to `value_type`.

   > > If `P` is instantiated as a reference type, then the argument `x` is copied from. Otherwise `x` is considered to be an rvalue as it is converted to `value_type` and inserted into the map. Specifically, in such cases `CopyConstructible` is not required of `key_type` or `mapped_type` unless the conversion from `P` specifically requires it (e.g., if `P` is a `tuple<const key_type, mapped_type>`, then `key_type` must be `CopyConstructible`). The signature taking `InputIterator` parameters does not require `CopyConstructible` of either `key_type` or `mapped_type` if the dereferenced `InputIterator` returns a non-const rvalue `pair<key_type, mapped_type>`. Otherwise `CopyConstructible` is required for both `key_type` and `mapped_type`.
   > > ? Effects: The first form is equivalent to `return emplace(std::forward<P>(x))`. The second form is equivalent to `return emplace_hint(position, std::forward<P>(x))`.

   > > ? Remarks: These signatures shall not participate in overload resolution unless `std::is_constructible<value_type, P&&>::value` is true.

3. Change [unord.map.modifers] around p. 1 as indicated:

   >     template <class P>
   >     pair<iterator, bool> insert(P&& obj);

   > > 1 Requires: `value_type` is constructible from `std::forward<P>(obj)`.

   > > 2 Effects: equivalent to `return emplace(std::forward<P>(obj))`. Inserts obj converted to `value_type` if and only if there is no element in the container with key equivalent to the key of `value_type(obj)`.

   > > 3 Returns: The bool component of the returned pair object indicates whether the insertion took place and the iterator component points to the element with key equivalent to the key of `value_type(obj)`.

   > > 4 Complexity: Average case O(1), worst case O(size()).

   > > 5 3 Remarks: This signature shall not participate in overload resolution unless `P` is implicitly convertible to `value_type` `std::is_constructible<value_type, P&&>::value` is true.

   >     template <class P>
   >     iterator insert(const_iterator hint, P&& obj);

   > > 6 Requires: `value_type` is constructible from `std::forward<P>(obj)`.

   > > 7? Effects: equivalent to `return emplace_hint(hint, std::forward<P>(obj))`. Inserts obj converted to `value_type` if and only if there is no element in the container with key equivalent to the key of `value_type(obj)`. The iterator hint is a hint pointing to where the search should start.

   > > 8 Returns: An iterator that points to the element with key equivalent to the key of `value_type(obj)`.

   > > 9 Complexity: Average case O(1), worst case O(size()).

   > > 10? Remarks: This signature shall not participate in overload resolution unless `P` is implicitly convertible to `value_type` `std::is_constructible<value_type, P&&>::value` is true.

4. Change [unord.multimap.modifers] around p. 1 as indicated:

   >     template <class P>
   >     iterator insert(P&& obj);

   > > 1 Requires: `value_type` is constructible from `std::forward<P>(obj)`.

   > > 2 Effects: equivalent to `return emplace(std::forward<P>(obj))`. Inserts obj converted to `value_type`.

   > > 3 Returns: An iterator that points to the element with key equivalent to the key of `value_type(obj)`.

   > > 4 Complexity: Average case O(1), worst case O(size()).

   > > 5 3 Remarks: This signature shall not participate in overload resolution unless `P` is implicitly convertible to `value_type` `std::is_constructible<value_type, P&&>::value` is true.

   >     template <class P>
   >     iterator insert(const_iterator hint, P&& obj);

   > > 6 Requires: `value_type` is constructible from `std::forward<P>(obj)`.

   > > 7? Effects: equivalent to `return emplace_hint(hint, std::forward<P>(obj))`. Inserts obj converted to `value_type`. The iterator hint is a hint pointing to where the search should start.

   > > 8 Returns: An iterator that points to the element with key equivalent to the key of `value_type`(obj).

   > > 9 Complexity: Average case O(1), worst case O(size()).

   > > 10? Remarks: This signature shall not participate in overload resolution unless `P` is implicitly convertible to `value_type` `std::is_constructible<value_type, P&&>::value` is true.

### 2007. Incorrect specification of return value for `map<>::at()`

Section: 23.4.4.3 [map.access] Status: C++11 Submitter: Matt Austern Opened: 2010-11-01 Last modified: 2015-04-08

View all other issues in [map.access].

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Discussion:

In [map.access]/9, the Returns clause for `map<Key, T>::at(x)` says that it returns "a reference to the element whose key is equivalent to `x`." That can't be right. The signature for `at()` says that its return type is `T`, but the elements of `map<Key, T>` have type `pair<const K, T>`. (I checked [unord.map.elem] and found that its specification of `at()` is correct. This is a problem for `map` only.)

Proposed resolution:

Change the wording in [map.access]/9 so it's identical to what we already say for `operator[]`, which is unambiguous and correct.

> Returns: A reference to the element whose key is equivalent `mapped_type` corresponding to `x` in `*this`.

### 2008. Conflicting Error Conditions for `packaged_task::operator()`

Section: 30.6.9.1 [futures.task.members] Status: C++11 Submitter: Pete Becker Opened: 2010-06-21 Last modified: 2015-04-08

View all other issues in [futures.task.members].

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Discussion:

The Throws clause for `packaged_task::operator()` says that it throws "a `future_error` exception object if there is no associated asynchronous state or the stored task has already been invoked." However, the Error Conditions clause does not define an error condition when the stored task has already been invoked, only when the associated state is already ready (i.e. the invocation has completed).

[2011-02-17 Anthony provides an alternative resolution]

Previous proposed resolution:

Change the first bullet item in 30.6.9.1 [futures.task.members] /22:

>     void operator()(ArgTypes... args);

> > 20 ...

> > 21 ...

> > 22 Error conditions:

> > - `promise_already_satisfied` if the associated asynchronous state is already ready `operator()` has already been called.
> > - `no_state` if `*this` has no associated asynchronous state.

[Adopted at Madrid, 2011-03]

Proposed resolution:

1. Change the first bullet item in 30.6.9.1 [futures.task.members] p. 17:

   >     void operator()(ArgTypes... args);

   > > 15 ...

   > > 16 ...

   > > 17 Error conditions:

   > > - `promise_already_satisfied` if the associated asynchronous state is already ready stored task has already been invoked.
   > > - `no_state` if `*this` has no associated asynchronous state.

2. Change the first bullet item in 30.6.9.1 [futures.task.members] p. 21:

   >     void make_ready_at_thread_exit(ArgTypes... args);

   > > 19 ...

   > > 20 ...

   > > 21 Error conditions:

   > > - `promise_already_satisfied` if the associated asynchronous state already has a stored value or exception stored task has already been invoked.
   > > - `no_state` if `*this` has no associated asynchronous state.

### 2009. Reporting out-of-bound values on numeric string conversions

Section: 21.5 [string.conversions] Status: C++14 Submitter: Alisdair Meredith Opened: 2010-07-19 Last modified: 2015-04-08

View all other issues in [string.conversions].

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Discussion:

The functions (`w`)`stoi` and (`w`)`stof` are specified in terms of calling C library APIs for potentially wider types. The integer and floating-point versions have subtly different behaviour when reading values that are too large to convert. The floating point case will throw `out_of_bound` if the read value is too large to convert to the wider type used in the implementation, but behaviour is undefined if the converted value cannot narrow to a float. The integer case will throw `out_of_bounds` if the converted value cannot be represented in the narrower type, but throws `invalid_argument`, rather than `out_of_range`, if the conversion to the wider type fails due to overflow.

Suggest that the Throws clause for both specifications should be consistent, supporting the same set of fail-modes with the matching set of exceptions.

Proposed resolution:

21.5p3 [string.conversions]

>     int stoi(const string& str, size_t *idx = 0, int base = 10);
>     long stol(const string& str, size_t *idx = 0, int base = 10);
>     unsigned long stoul(const string& str, size_t *idx = 0, int base = 10);
>     long long stoll(const string& str, size_t *idx = 0, int base = 10);
>     unsigned long long stoull(const string& str, size_t *idx = 0, int base = 10);

> > ...

> > 3 Throws: `invalid_argument` if `strtol`, `strtoul`, `strtoll`, or `strtoull` reports that no conversion could be performed. Throws `out_of_range` if `strtol`, `strtoul`, `strtoll` or `strtoull` sets `errno` to `ERANGE`, or if the converted value is outside the range of representable values for the return type.

21.5p6 [string.conversions]

>     float stof(const string& str, size_t *idx = 0);
>     double stod(const string& str, size_t *idx = 0);
>     long double stold(const string& str, size_t *idx = 0);

> > ...

> > 6 Throws: `invalid_argument` if `strtod` or `strtold` reports that no conversion could be performed. Throws `out_of_range` if `strtod` or `strtold` sets `errno` to `ERANGE` or if the converted value is outside the range of representable values for the return type.

### 2010. `is_* traits` for binding operations can't be meaningfully specialized

Section: 20.9.10.1 [func.bind.isbind] Status: C++14 Submitter: Sean Hunt Opened: 2010-07-19 Last modified: 2015-04-08

View all other issues in [func.bind.isbind].

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Discussion:

20.9.10.1 [func.bind.isbind] says for `is_bind_expression`:

> Users may specialize this template to indicate that a type should be treated as a subexpression in a `bind` call.

But it also says:

> If `T` is a type returned from `bind`, `is_bind_expression<T>` shall be publicly derived from `integral_constant<bool, true>`, otherwise from `integral_constant<bool, false>`.

This means that while the user is free to specialize, any specialization would have to be `false` to avoid violating the second requirement. A similar problem exists for `is_placeholder`.

[ 2010 Batavia (post meeting session) ]

Alisdair recognises this is clearly a bug introduced by some wording he wrote, the sole purpose of this metafunction is as a customization point for users to write their own `bind`-expression types that participate in the standard library `bind` protocol. The consensus was that this should be fixed in Madrid, moved to Open.

[2011-05-13 Jonathan Wakely comments and provides proposed wording]

The requirements are that `is_bind_expression<T>::value` is true when `T` is a type returned from `bind`, false for any other type, except when there's a specialization involving a user-defined type (N.B. 17.6.4.2.1 [namespace.std] means we don't need to say e.g. `is_bind_expression<string>` is false.)

The obvious way to meet the requirements is for the primary template to derive from `integral_constant<bool, false>` and for implementations to provide specializations for the unspecified types returned from `bind`. User-defined specializations can do whatever they like, as long as `is_bind_expression::value` is sane. There's no reason to forbid users from defining `is_bind_expression<user_defined_type>::value=false` if that's what they want to do.

Similar reasoning applies to `is_placeholder`, but a further issue is that 20.9.10.1 [func.bind.isbind] contains wording for `is_placeholder` but contains no definition of it and the sub-clause name only refers to `is_bind_expression`. The wording below proposes splitting paragraphs 3 and 4 of 20.9.10.1 [func.bind.isbind] into a new sub-clause covering `is_placeholder`.

If the template specializations added by the proposed wording are too vague then they could be preceded by "for exposition only" comments

[2011-05-18 Daniel comments and provides some refinements to the P/R]

Both `bind`-related type traits should take advantage of the UnaryTypeTrait requirements. Additionally, the updated wording does not imply that the implementation provides several specializations. Wording was used similar to the specification of the `uses_allocator` type trait (which unfortunately is not expressed in terms of BinaryTypeTrait requirements).

[Bloomington, 2011]

Move to Ready

Proposed resolution:

This wording is relative to the FDIS.

1. Change 20.9.10.1 [func.bind.isbind] to:

   >     namespace std {
   >       template<class T> struct is_bind_expression; // see below
   >         : integral_constant<bool, see below> { };
   >     }

   > > -1- `is_bind_expression` can be used to detect function objects generated by `bind`. `bind` uses `is_bind_expression` to detect subexpressions. Users may specialize this template to indicate that a type should be treated as a subexpression in a `bind` call.

   > > -2- If `T` is a type returned from `bind`, `is_bind_expression<T>` shall be publicly derived from `integral_constant<bool, true>`, otherwise from `integral_constant<bool, false>`Instantiations of the `is_bind_expression` template shall meet the UnaryTypeTrait requirements ([meta.rqmts]). The implementation shall provide a definition that has a BaseCharacteristic of `true_type` if `T` is a type returned from `bind`, otherwise it shall have a BaseCharacteristic of `false_type`. A program may specialize this template for a user-defined type `T` to have a BaseCharacteristic of `true_type` to indicate that `T` should be treated as a subexpression in a `bind` call..

   > > -3- `is_placeholder` can be used to detect the standard placeholders `_1`, `_2`, and so on. `bind` uses `is_placeholder` to detect placeholders. Users may specialize this template to indicate a placeholder type.

   > > -4- If `T` is the type of `std::placeholders::_J`, `is_placeholder<T>` shall be publicly derived from `integral_constant<int, J>`, otherwise from `integral_constant<int, 0>`.

2. Insert a new sub-clause immediately following sub-clause 20.9.10.1 [func.bind.isbind], the suggested sub-clause tag is [func.bind.isplace]:

   ### 20.8.9.1.? Class template `is_placeholder` [func.bind.isplace]

   >     namespace std {
   >       template<class T> struct is_placeholder; // see below
   >     }

   > > -?- `is_placeholder` can be used to detect the standard placeholders `_1`, `_2`, and so on. `bind` uses `is_placeholder` to detect placeholders.

   > > -?- Instantiations of the `is_placeholder` template shall meet the UnaryTypeTrait requirements ([meta.rqmts]). The implementation shall provide a definition that has a BaseCharacteristic of `integral_constant<int, J>` if `T` is the type of `std::placeholders::_J`, otherwise it shall have a BaseCharacteristic of `integral_constant<int, 0>`. A program may specialize this template for a user-defined type `T` to have a BaseCharacteristic of `integral_constant<int, N>` with `N > 0` to indicate that `T` should be treated as a placeholder type.

### 2011. Unexpected output required of strings

Section: 21.4.8.9 [string.io] Status: C++14 Submitter: James Kanze Opened: 2010-07-23 Last modified: 2015-04-08

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Discussion:

What should the following code output?

>     #include <string>
>     #include <iostream>
>     #include <iomanip>

>     int main()
>     {
>        std::string test("0X1Y2Z");
>        std::cout.fill('*');
>        std::cout.setf(std::ios::internal, std::ios::adjustfield);
>        std::cout << std::setw(8) << test << std::endl;
>     }

I would expect "`**0X1Y2Z`", and this is what the compilers I have access to (VC++, g++ and Sun CC) do. But according to the standard, it should be "`0X**1Y2Z`":

21.4.8.9 [string.io]/5:

>     template<class charT, class traits, class Allocator>
>       basic_ostream<charT, traits>&
>         operator<<(basic_ostream<charT, traits>& os, const basic_string<charT,traits,Allocator>& str);

> > Effects: Behaves as a formatted output function (27.7.3.6.1 [ostream.formatted.reqmts]). After constructing a `sentry` object, if this object returns `true` when converted to a value of type `bool`, determines padding as described in 22.4.2.2.2 [facet.num.put.virtuals], then inserts the resulting sequence of characters seq as if by calling `os.rdbuf()->sputn(seq, n)`, where `n` is the larger of `os.width()` and `str.size()`; then calls `os.width(0)`.

22.4.2.2.2 [facet.num.put.virtuals]/5:

> […]

> Stage 3: A local variable is initialized as

> >     fmtflags adjustfield= (flags & (ios_base::adjustfield));

> The location of any padding is determined according to Table 88.

> If `str.width()` is nonzero and the number of `charT`'s in the sequence after stage 2 is less than `str.width()`, then enough fill characters are added to the sequence at the position indicated for padding to bring the length of the sequence to `str.width()`. `str.width(0)` is called.

> Table 88 — Fill padding
> State | Location
> --- | ---
> `adjustfield == ios_base::left` | pad after
> `adjustfield == ios_base::right` | pad before
> `adjustfield == internal` and a sign occurs in the representation | pad after the sign
> `adjustfield == internal` and representation after stage 1 began with 0x or 0X | pad after x or X
> otherwise | pad before

Although it's not 100% clear what "the sequence after stage 2" should mean here, when there is no stage 2, the only reasonable assumption is that it is the contents of the string being output. In the above code, the string being output is "`0X1Y2Z`", which starts with "`0X`", so the padding should be inserted "after x or X", and not before the string. I believe that this is a defect in the standard, and not in the three compilers I tried.

[ 2010 Batavia (post meeting session) ]

Consensus that all known implementations are consistent, and disagree with the standard. Preference is to fix the standard before implementations start trying to conform to the current spec, as the current implementations have the preferred form. Howard volunteered to drught for Madrid, move to Open.

[2011-03-24 Madrid meeting]

Daniel Krügler volunteered to provide wording, interacting with Dietmar and Bill.

[2011-06-24 Daniel comments and provides wording]

The same problem applies to the output provided by `const char*` and similar character sequences as of 27.7.3.6.4 [ostream.inserters.character] p. 5. and even for single character output (!) as described in 27.7.3.6.4 [ostream.inserters.character] p. 1, just consider the character value '-' where '-' is the sign character. In this case Table 91 — "Fill padding" requires to pad after the sign, i.e. the output for the program

>     #include <iostream>
>     #include <iomanip>

>     int main()
>     {
>        char c = '-';
>        std::cout.fill('*');
>        std::cout.setf(std::ios::internal, std::ios::adjustfield);
>        std::cout << std::setw(2) << c << std::endl;
>     }

According to the current wording this program should output "`-*`", but all tested implementations output "`*-`" instead.

I suggest to replace the reference to 22.4.2.2.2 [facet.num.put.virtuals] in all three places. It is not very complicated to describe the padding rules for simple character sequences "inline". A similar approach is used as for the `money_put` functions.

[ 2011 Bloomington ]

Move to Review, the resolution seems correct but it would be nice if some factoring of the common words were proposed.

[2012, Kona]

Moved to Tentatively Ready by the post-Kona issues processing subgroup.

While better factoring of the common words is desirable, it is also editorial and should not hold up the progress of this issue. As the edits impact two distinct clauses, it is not entirely clear what a better factoring should look like.

[2012, Portland: applied to WP]

Proposed resolution:

The new wording refers to the FDIS numbering.

1. Change 21.4.8.9 [string.io]/5 as indicated:

   >     template<class charT, class traits, class Allocator>
   >       basic_ostream<charT, traits>&
   >         operator<<(basic_ostream<charT, traits>& os,
   >                    const basic_string<charT,traits,Allocator>& str);

   > > -5- Effects: Behaves as a formatted output function ([ostream.formatted.reqmts]). After constructing a sentry object, if this object returns `true` when converted to a value of type `bool`, determines padding as described in [facet.num.put.virtuals],follows: A `charT` character sequence is produced, initially consisting of the elements defined by the range `[str.begin(), str.end())`. If `str.size()` is less than `os.width()`, then enough copies of `os.fill()` are added to this sequence as necessary to pad to a width of `os.width()` characters. If `(os.flags() & ios_base::adjustfield) == ios_base::left` is `true`, the fill characters are placed after the character sequence; otherwise, they are placed before the character sequence. T then inserts the resulting sequence of characters `seq` as if by calling `os.rdbuf()->sputn(seq, n)`, where `n` is the larger of `os.width()` and `str.size()`; then calls `os.width(0)`.

2. Change 27.7.3.6.4 [ostream.inserters.character]/1 as indicated (An additional editorial fix is suggested for the first prototype declaration):

   >     template<class charT, class traits>
   >       basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out,
   >                                               charT c});
   >     template<class charT, class traits>
   >       basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out,
   >                                               char c);
   >     // specialization
   >     template<class traits>
   >       basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out,
   >                                              char c);
   >     // signed and unsigned
   >     template<class traits>
   >       basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out,
   >                                              signed char c);
   >     template<class traits>
   >       basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out,
   >                                              unsigned char c);

   > > -1- Effects: Behaves like a formatted inserter (as described in [ostream.formatted.reqmts]) of `out`. After a sentry object is constructed it inserts characters. In case `c` has type `char` and the character type of the stream is not `char`, then the character to be inserted is `out.widen(c)`; otherwise the character is `c`. Padding is determined as described in [facet.num.put.virtuals]follows: A character sequence is produced, initially consisting of the insertion character. If `out.width()` is greater than one, then enough copies of `out.fill()` are added to this sequence as necessary to pad to a width of `out.width()` characters. If `(out.flags() & ios_base::adjustfield) == ios_base::left` is `true`, the fill characters are placed after the insertion character; otherwise, they are placed before the insertion character. `width(0)` is called. The insertion character and any required padding are inserted into `out`; then calls `os.width(0)`.

3. Change 27.7.3.6.4 [ostream.inserters.character]/5 as indicated:

   >     template<class charT, class traits>
   >       basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out,
   >                                               const charT* s);
   >     template<class charT, class traits>
   >       basic_ostream<charT,traits>& operator<<(basic_ostream<charT,traits>& out,
   >                                               const char* s);
   >     template<class traits>
   >       basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out,
   >                                              const char* s);
   >     template<class traits>
   >       basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out,
   >                                              const signed char* s);
   >     template<class traits>
   >       basic_ostream<char,traits>& operator<<(basic_ostream<char,traits>& out,
   >                                              const unsigned char* s);

   > > […]

   > > -5- Padding is determined as described in [facet.num.put.virtuals]. The `n` characters starting at `s` are widened using `out.widen` ([basic.ios.members])follows: A character sequence is produced, initially consisting of the elements defined by the `n` characters starting at `s` widened using `out.widen` ([basic.ios.members]). If `n` is less than `out.width()`, then enough copies of `out.fill()` are added to this sequence as necessary to pad to a width of `out.width()` characters. If `(out.flags() & ios_base::adjustfield) == ios_base::left` is `true`, the fill characters are placed after the character sequence; otherwise, they are placed before the character sequence. The widened characters and any required padding are inserted into `out`. Calls `width(0)`.

### 2012. Associative maps should insert `pair`, not `tuple`

Section: 23.4 [associative] Status: Resolved Submitter: Paolo Carlini Opened: 2010-10-29 Last modified: 2015-04-08

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Discussion:

I'm seeing something strange in the paragraphs 23.4.4.4 [map.modifiers] and 23.4.5.3 [multimap.modifiers]: they both talk about `tuple<const key_type, mapped_type>` but I think they should be talking about `pair<const key_type, mapped_type>` because, among other reasons, a `tuple` is not convertible to a `pair`. If I replace `tuple` with `pair` everything makes sense to me.

The proposed resolution is obvious.

[ 2010-11-07 Daniel comments ]

This is by far not the only necessary fix within both sub-clauses. For details see the 2010-10-29 comment in 2005.

[2011-03-24 Madrid meeting]

Paolo: Don't think we can do it now.

Daniel K: Agrees.

[ 2011 Bloomington ]

Consensus that this issue will be resolved by 2005, but held open until that issue is resolved.

Proposed resolution:

Apply the resolution proposed by the 2010-10-29 comment in 2005.

### 2013. Do library implementers have the freedom to add `constexpr`?

Section: 17.6.5.6 [constexpr.functions] Status: C++14 Submitter: Matt Austern Opened: 2010-11-12 Last modified: 2015-04-08

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Discussion:

Suppose that a particular function is not tagged as constexpr in the standard, but that, in some particular implementation, it is possible to write it within the constexpr constraints. If an implementer tags such a function as constexpr, is that a violation of the standard or is it a conforming extension?

There are two questions to consider. First, is this allowed under the as-if rule? Second, if it does not fall under as-if, is there (and should there be) any special license granted to implementers to do this anyway, sort of the way we allow elision of copy constructors even though it is detectable by users?

I believe that this does not fall under "as-if", so implementers probably don't have that freedom today. I suggest changing the WP to grant it. Even if we decide otherwise, however, I suggest that we make it explicit.

[ 2011 Bloomington ]

General surprise this was not already in 'Ready' status, and so moved.

[ 2012 Kona ]

Some concern expressed when presented to full committee for the vote to WP status that this issue had been resolved without sufficient thought of the consequences for diverging library implementations, as users may use SFINAE to observe different behavior from otherwise identical code. Issue moved back to Review status, and will be discussed again in Portland with a larger group. Note for Portland: John Spicer has agreed to represent Core's concerns during any such discussion within LWG.

[2013-09 Chicago]

Straw poll: LWG strongly favoured to remove from implementations the freedom to add `constexpr`.

Matt provides new wording.

[2013-09 Chicago]

Move to Immediate after reviewing Matt's new wording, apply the new wording to the Working Paper.

Proposed resolution:

In 17.6.5.6 [constexpr.functions], change paragraph 1 to:

> This standard explicitly requires that certain standard library functions are `constexpr` [dcl.constexpr]. An implementation shall not declare any standard library function signature as `constexpr` except for those where it is explicitly required. Within any header that provides any non-defining declarations of `constexpr` functions or constructors an implementation shall provide corresponding definitions.

### 2014. More restrictions on macro names

Section: 17.6.4.3.1 [macro.names] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2010-11-16 Last modified: 2015-04-08

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Discussion:

A program is currently forbidden to use keywords as macro names. This restriction should be strengthened to include all identifiers that could be used by the library as attribute-tokens (for example `noreturn`, which is used by header `<cstdlib>`) and the special identifiers introduced recently for override control (these are not currently used in the library public interface, but could potentially be used by the implementation or in future revisions of the library).

[2011-02-10 Reflector discussion]

Moved to Tentatively Ready after 5 votes.

Proposed resolution:

Modify 17.6.4.3.1 [macro.names] paragraph 2 as follows:

> A translation unit shall not `#define` or `#undef` names lexically identical to keywords, to the identifiers listed in Table X [Identifiers with special meaning], or to the attribute-tokens described in clause 7.6 [dcl.attr].

### 2015. Incorrect pre-conditions for some type traits

Section: 20.10.4 [meta.unary] Status: C++14 Submitter: Nikolay Ivchenkov Opened: 2010-11-08 Last modified: 2015-04-08

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Discussion:

According to N3126 ‑ 3.9/9,

"Scalar types, trivial class types (Clause 9), arrays of such types and cv‑qualified versions of these types (3.9.3) are collectively called trivial types."

Thus, an array (possibly of unknown bound) can be trivial type, non‑trivial type, or an array type whose triviality cannot be determined because its element type is incomplete.

According to N3126 ‑ Table 45, preconditions for `std::is_trivial` are defined as follows:

"`T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown bound"

It seems that "an array of unknown bound" should be changed to "an array of unknown bound of a complete element type". Preconditions for some other templates (e.g., `std::is_trivially_copyable`, `std::is_standard_layout`, `std::is_pod`, and `std::is_literal_type`) should be changed similarly.

On the other hand, some preconditions look too restrictive. For example, `std::is_empty` and `std::is_polymorphic` might accept any incomplete non‑class type.

[2011-02-18: Daniel provides wording proposal]

While reviewing the individual preconditions I could find three different groups of either too weakening or too strengthening constraints:

1.     is_empty/is_polymorphic/is_abstract/has_virtual_destructor:

   These traits can only apply for non‑union class types, otherwise the result must always be false

2.     is_base_of:

   Similar to the previous bullet, but the current wording comes already near to that ideal, it only misses to add the non‑union aspect.

3.     is_trivial/is_trivially_copyable/is_standard_layout/is_pod/is_literal_type:

   These traits always require that `std::remove_all_extents<T>::type` to be `cv void` or a complete type.

[Bloomington, 2011]

Move to Ready

Proposed resolution:

1. Modify the pre-conditions of the following type traits in 20.10.4.3 [meta.unary.prop], Table 48 — Type property predicates:

   > Table 48 — Type property predicates
   > Template | Condition | Preconditions
   > --- | --- | ---
   > ...
   > `template <class T> struct is_trivial;` | `T` is a trivial type (3.9) | `remove_all_extents<T>::type` shall be a complete type, or (possibly cv-qualified) `void`, or an array of unknown bound.
   > `template <class T> struct is_trivially_copyable;` | `T` is a trivially copyable type (3.9) | `remove_all_extents<T>::type` shall be a complete type, or (possibly cv-qualified) `void`, or an array of unknown bound.
   > `template <class T> struct is_standard_layout;` | `T` is a standard-layout type (3.9) | `remove_all_extents<T>::type` shall be a complete type, or (possibly cv-qualified) `void`, or an array of unknown bound.
   > `template <class T> struct is_pod;` | `T` is a POD type (3.9) | `remove_all_extents<T>::type` shall be a complete type, or (possibly cv-qualified) `void`, or an array of unknown bound.
   > `template <class T> struct is_literal_type;` | `T` is a literal type (3.9) | `remove_all_extents<T>::type` shall be a complete type, or (possibly cv-qualified) `void`, or an array of unknown bound.

   > `template <class T>
   > struct is_empty;`

   > `T` is a class type, but not a
   > union type, with no
   > non-static data members
   > other than bit-fields of
   > length 0, no virtual
   > member functions, no
   > virtual base classes, and
   > no base class B for which
   > `is_empty<B>::value` is
   > false.

   > `T` shall be a complete type,
   > (possibly cv-qualified) `void`, or
   > an array of unknown bound If `T`
   > is a non‑union class type, `T`
   > shall be a complete type.

   > `template <class T> struct is_polymorphic;` | `T` is a polymorphic class (10.3) | `T` shall be a complete type, type, (possibly cv-qualified) `void`, or an array of unknown bound If `T` is a non‑union class type, `T` shall be a complete type.
   > `template <class T> struct is_abstract;` | `T` is an abstract class (10.4) | `T` shall be a complete type, type, (possibly cv-qualified) `void`, or an array of unknown bound If `T` is a non‑union class type, `T` shall be a complete type.
   > ...
   > `template <class T> struct has_virtual_destructor;` | `T` has a virtual destructor (12.4) | `T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown bound If `T` is a non‑union class type, `T` shall be a complete type.

2. Modify the pre-conditions of the following type traits in 20.10.6 [meta.rel], Table 50 — Type relationship predicates:

   > Table 50 — Type relationship predicates
   > Template | Condition | Comments
   > --- | --- | ---
   > ...

   > `template <class Base, class
   > Derived>
   > struct is_base_of;`

   > `Base` is a base class of
   > `Derived` (10) without
   > regard to cv-qualifiers
   > or `Base` and `Derived`
   > are not unions and
   > name the same class
   > type without regard to
   > cv-qualifiers

   > If `Base` and `Derived` are
   > non‑union class types
   > and are different types
   > (ignoring possible cv-qualifiers)
   > then `Derived` shall be a complete
   > type. [ Note: Base classes that
   > are private, protected, or
   > ambigious are, nonetheless, base
   > classes. — end note ]

   > ...

### 2016. `Allocators` must be no-throw swappable

Section: 17.6.3.5 [allocator.requirements] Status: WP Submitter: Daniel Krügler Opened: 2010-11-17 Last modified: 2015-04-08

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Discussion:

During the Batavia meeting it turned out that there is a definition hole for types satisfying the `Allocators` requirements: The problem became obvious when it was discussed whether all `swap` functions of `Containers` with internal data handles can be safely tagged with `noexcept` or not. While it is correct that the implicit `swap` function of an allocator is required to be a no-throw operation (because move/copy-constructors and assignment operators are required to be no-throw functions), there are no such requirements for specialized `swap` overloads for a particular allocator.

But this requirement is essential because the `Containers` are required to support swappable `Allocators`, when the value `allocator_traits<>::propagate_on_container_swap` evaluates to `true`.

[2011-02-10 Alberto, Daniel, and Pablo collaborated on the proposed wording]

The proposed resolution (based on N3225) attempts to solve the following problems:

1. Table 44 — Allocator requirements, expression rows `X::propagate_on_container_copy_assignment`, `X::propagate_on_container_move_assignment`, and `X::propagate_on_container_swap` only describe operations, but no requirements. In fact, if and only if these compile-time predicates evaluate to `true`, the additional requirements `CopyAssignable`, no-throw `MoveAssignable`, and no-throw lvalue `Swappable`, respectively, are imposed on the allocator types.
2. 23.2.1 [container.requirements.general] p. 9 misses to refer to the correct swap conditions: The current wording does not relate to 17.6.3.2 [swappable.requirements] as it should and omits to mention that lvalues shall be swapped. Additional there is one situation described twice in p. 8 and p. 9 (undefined behaviour unless `a.get_allocator() == b.get_allocator()` or `allocator_traits<allocator_type>::propagate_on_container_swap::value == true`), which should be cleaned up.

[2011-04-08 Pablo comments]

I'm implementing a version of list now and I actually do find it impossible to write an exception-safe assignment operator unless I can assume that allocator assignment does not throw. (The problem is that I use a sentinel node and I need to allocate a new sentinel using the new allocator without destroying the old one -- then swap the allocator and sentinel pointer in atomically, without risk of an exception leaving one inconsistent with the other.

Please update the proposed resolution to add the nothrow requirement to copy-assignment.

[2014-02-14 Issaquah: Move to Ready]

Fix a couple of grammar issues related to calling `swap` and move to Ready.

Proposed resolution:

1. Adapt the following three rows from Table 44 — Allocator requirements:

   > Table 44 — Allocator requirements
   > Expression | Return type | Assertion/note pre-/post-condition | Default
   > --- | --- | --- | ---
   > `X::propagate_on_container_copy_assignment` | Identical to or derived from `true_type` or `false_type` | `true_type` only if an allocator of type `X` should be copied when the client container is copy-assigned. See Note B, below. | `false_type`
   > `X::propagate_on_container_move_assignment` | Identical to or derived from `true_type` or `false_type` | `true_type` only if an allocator of type `X` should be moved when the client container is move-assigned. See Note B, below. | `false_type`
   > `X::propagate_on_container_swap` | Identical to or derived from `true_type` or `false_type` | `true_type` only if an allocator of type `X` should be swapped when the client container is swapped. See Note B, below. | `false_type`

2. Following 17.6.3.5 [allocator.requirements] p. 3 insert a new normative paragraph:

   > Note B: If `X::propagate_on_container_copy_assignment::value` is true, `X` shall satisfy the `CopyAssignable` requirements (Table 39 [copyassignable]) and the copy operation shall not throw exceptions. If `X::propagate_on_container_move_assignment::value` is true, `X` shall satisfy the `MoveAssignable` requirements (Table 38 [moveassignable]) and the move operation shall not throw exceptions. If `X::propagate_on_container_swap::value` is true, lvalues of `X` shall be swappable (17.6.3.2 [swappable.requirements]) and the `swap` operation shall not throw exceptions.

3. Modify 23.2.1 [container.requirements.general] p. 8 and p. 9 as indicated:

   > 8 - [..] The allocator may be replaced only via assignment or `swap()`. Allocator replacement is performed by copy assignment, move assignment, or swapping of the allocator only if `allocator_traits<allocator_type>::propagate_on_container_copy_assignment::value`, `allocator_traits<allocator_type>::propagate_on_container_move_assignment::value`, or `allocator_traits<allocator_type>::propagate_on_container_swap::value` is true within the implementation of the corresponding container operation. The behavior of a call to a container's `swap` function is undefined unless the objects being swapped have allocators that compare equal or `allocator_traits<allocator_type>::propagate_on_container_swap::value` is true. In all container types defined in this Clause, the member `get_allocator()` returns a copy of the allocator used to construct the container or, if that allocator has been replaced, a copy of the most recent replacement.

   > 9 - The expression `a.swap(b)`, for containers `a` and `b` of a standard container type other than `array`, shall exchange the values of `a` and `b` without invoking any move, copy, or swap operations on the individual container elements. Lvalues of a Any `Compare`, `Pred`, or `Hash` objects types belonging to `a` and `b` shall be swappable and shall be exchanged by unqualified calls to non-member calling `swap` as described in 17.6.3.2 [swappable.requirements]. If `allocator_traits<allocator_type>::propagate_on_container_swap::value` is `true`, then lvalues of `allocator_type` shall be swappable and the allocators of `a` and `b` shall also be exchanged using an unqualified call to non-member by calling `swap` as described in 17.6.3.2 [swappable.requirements]. Otherwise, they the allocators shall not be swapped, and the behavior is undefined unless `a.get_allocator() == b.get_allocator()`. Every iterator referring to an element in one container before the swap shall refer to the same element in the other container after the swap. It is unspecified whether an iterator with value `a.end()` before the swap will have value `b.end()` after the swap.

### 2017. `std::reference_wrapper` makes incorrect usage of `std::result_of`

Section: 20.9.4 [refwrap] Status: C++11 Submitter: Nikolay Ivchenkov Opened: 2010-11-15 Last modified: 2015-04-08

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Discussion:

`std::reference_wrapper`'s function call operator uses wrong type encoding for rvalue-arguments. An rvalue-argument of type `T` must be encoded as `T&&`, not as just `T`.

>     #include <functional>
>     #include <iostream>
>     #include <string>
>     #include <type_traits>
>     #include <utility>

>     template <class F, class... Types>
>          typename std::result_of<F (Types...)>::type
>              f1(F f, Types&&... params)
>     {
>          return f(std::forward<Types...>(params...));
>     }

>     template <class F, class... Types>
>          typename std::result_of<F (Types&&...)>::type
>              f2(F f, Types&&... params)
>     {
>          return f(std::forward<Types...>(params...));
>     }

>     struct Functor
>     {
>          template <class T>
>              T&& operator()(T&& t) const
>          {
>              return static_cast<T&&>(t);
>          }
>     };

>     int main()
>     {
>          typedef std::string const Str;
>          std::cout << f1(Functor(), Str("1")) << std::endl; // (1)
>          std::cout << f2(Functor(), Str("2")) << std::endl; // (2)
>     }

Lets consider the function template `f1` (which is similar to `std::reference_wrapper`'s function call operator). In the invocation (1) `F` is deduced as '`Functor`' and `Types` is deduced as type sequence which consists of one type '`std::string const`'. After the substitution we have the following equivalent:

>     template <>
>         std::result_of<F (std::string const)>::type
>             f1<Functor, std::string const>(Functor f, std::string const && params)
>     {
>         return f(std::forward<const std::string>(params));
>     }

The top-level cv-qualifier in the parameter type of '`F (std::string const)`' is removed, so we have

>     template <>
>         std::result_of<F (std::string)>::type
>             f1<Functor, std::string const>(Functor f, std::string const && params)
>     {
>         return f(std::forward<const std::string>(params));
>     }

Let `r` be an rvalue of type '`std::string`' and `cr` be an rvalue of type '`std::string const`'. The expression `Str("1")` is `cr`. The corresponding return type for the invocation

>     Functor().operator()(r)

is '`std::string &&`'. The corresponding return type for the invocation

>     Functor().operator()(cr)

is '`std::string const &&`'.

`std::result_of<Functor (std::string)>::type` is the same type as the corresponding return type for the invocation `Functor().operator()(r)`, i.e. it is '`std::string &&`'. As a consequence, we have wrong reference binding in the return statement in `f1`.

Now lets consider the invocation (2) of the function template `f2`. When the template arguments are substituted we have the following equivalent:

>     template <>
>         std::result_of<F (std::string const &&)>::type
>             f2<Functor, std::string const>(Functor f, std::string const && params)
>     {
>         return f(std::forward<const std::string>(params));
>     }

`std::result_of<F (std::string const &&)>::type` is the same type as '`std::string const &&`'. This is correct result.

[ 2010-12-07 Jonathan Wakely comments and suggests a proposed resolution ]

I agree with the analysis and I think this is a defect in the standard, it would be a shame if it can't be fixed.

In the following example one would expect `f(Str("1"))` and `std::ref(f)(Str("2"))` to be equivalent but the current wording makes the invocation through `reference_wrapper` ill-formed:

>     #include <functional>
>     #include <string>

>     struct Functor
>     {
>        template <class T>
>            T&& operator()(T&& t) const
>            {
>                return static_cast<T&&>(t);
>            }
>     };

>     int main()
>     {
>        typedef std::string const Str;
>        Functor f;
>        f( Str("1") );
>        std::ref(f)( Str("2") );  // error
>     }

[ 2010-12-07 Daniel comments and refines the proposed resolution ]

There is one further defect in the usage of `result_of` within `reference_wrapper`'s function call operator: According to 20.9.4.4 [refwrap.invoke] p. 1 the invokable entity of type `T` is provided as lvalue, but `result_of` is fed as if it were an rvalue. This does not only lead to potentially incorrect result types, but it will also have the effect that we could never use the function call operator with a function type, because the type encoding used in `result_of` would form an invalid function type return a function type. The following program demonstrates this problem:

>     #include <functional>

>     void foo(int) {}

>     int main()
>     {
>        std::ref(foo)(0);  // error
>     }

The correct solution is to ensure that `T` becomes `T&` within `result_of`, which solves both problems at once.

[2011-02-24 Reflector discussion]

Moved to Tentatively Ready after 5 votes.

Proposed resolution:

1. Change the synopsis in 20.9.4 [refwrap] paragraph 1:

   >     namespace std {
   >       template <class T> class reference_wrapper
   >       {
   >       public :
   >         [...]
   >         // invocation
   >         template <class... ArgTypes>
   >         typename result_of<T&(ArgTypes&&...)>::type
   >         operator() (ArgTypes&&...) const;
   >       };
   >     }

2. Change the signature in 20.9.4.4 [refwrap.invoke] before paragraph 1

   >     template <class... ArgTypes>
   >     typename result_of<T&(ArgTypes&&... )>::type
   >     operator()(ArgTypes&&... args) const;

   > > 1 Returns: `INVOKE(get(), std::forward<ArgTypes>(args)...)`. (20.8.2)

### 2018. [CD] `regex_traits::isctype` Returns clause is wrong

Section: 28.7 [re.traits] Status: C++14 Submitter: Jonathan Wakely Opened: 2010-11-16 Last modified: 2015-04-08

View all other issues in [re.traits].

View all issues with C++14 status.

Discussion:

Addresses GB 10

28.7 [re.traits] p. 12 says:

> returns true if `f` bitwise or'ed with the result of calling `lookup_classname` with an iterator pair that designates the character sequence "w" is not equal to `0` and `c == '_'`

If the bitmask value corresponding to "w" has a non-zero value (which it must do) then the bitwise or with any value is also non-zero, and so `isctype('_', f)` returns true for any `f`. Obviously this is wrong, since `'_'` is not in every `ctype` category.

There's a similar problem with the following phrases discussing the "blank" char class.

[2011-05-06: Jonathan Wakely comments and provides suggested wording]

DR 2019 added `isblank` support to `<locale>` which simplifies the definition of `regex_traits::isctype` by removing the special case for the "blank" class.

My suggestion for 2018 is to add a new table replacing the lists of recognized names in the Remarks clause of `regex_traits::lookup_classname`. I then refer to that table in the Returns clause of `regex_traits::isctype` to expand on the "in an unspecified manner" wording which is too vague. The conversion can now be described using the "is set" term defined by 17.5.2.1.3 [bitmask.types] and the new table to convey the intented relationship between e.g. [[:digit:]] and `ctype_base::digit`, which is not actually stated in the FDIS.

The effects of `isctype` can then most easily be described in code, given an "exposition only" function prototype to do the not-quite-so-unspecified conversion from `char_class_type` to `ctype_base::mask`.

The core of LWG 2018 is the "bitwise or'ed" wording which gives the wrong result, always evaluating to true for all values of `f`. That is replaced by the condition `(f&x) == x` where `x` is the result of calling `lookup_classname` with "w". I believe that's necessary, because the "w" class could be implemented by an internal "underscore" class i.e. `x = _Alnum|_Underscore` in which case `(f&x) != 0` would give the wrong result when `f==_Alnum`.

The proposed resolution also makes use of `ctype::widen` which addresses the problem that the current wording only talks about "w" and '_' which assumes `charT` is char. There's still room for improvement here: the regex grammar in 28.13 [re.grammar] says that the class names in the table should always be recognized, implying that e.g. U"digit" should be recognized by `regex_traits<char32_t>`, but the specification of `regex_traits::lookup_classname` doesn't cover that, only mentioning `char` and `wchar_t`. Maybe the table should not distinguish narrow and wide strings, but should just have one column and add wording to say that `regex_traits` widens the name as if by using `use_facet<ctype<charT>>::widen()`.

Another possible improvement would be to allow additional implementation-defined extensions in `isctype`. An implementation is allowed to support additional class names in `lookup_classname`, e.g. [[:octdigit:]] for [0-7] or [[:bindigit:]] for [01], but the current definition of isctype provides no way to use them unless `ctype_base::mask` also supports them.

[2011-05-10: Alberto and Daniel perform minor fixes in the P/R]

[ 2011 Bloomington ]

Consensus that this looks to be a correct solution, and the presentation as a table is a big improvement.

Concern that the middle section wording is a little muddled and confusing, Stefanus volunteered to reword.

[ 2013-09 Chicago ]

Stefanus provides improved wording (replaced below)

[ 2013-09 Chicago ]

Move as Immediate after reviewing Stefanus's revised wording, apply the new wording to the Working Paper.

Proposed resolution:

This wording is relative to the FDIS.

1. Modify 28.7 [re.traits] p. 10 as indicated:

   >     template <class ForwardIterator>
   >       char_class_type lookup_classname(
   >         ForwardIterator first, ForwardIterator last, bool icase = false) const;

   > > -9- Returns: an unspecified value that represents the character classification named by the character sequence designated by the iterator range [`first`,`last`). If the parameter `icase` is true then the returned mask identifies the character classification without regard to the case of the characters being matched, otherwise it does honor the case of the characters being matched.(footnote 335) The value returned shall be independent of the case of the characters in the character sequence. If the name is not recognized then returns a value that compares equal to `0`.

   > > -10- Remarks: For `regex_traits<char>`, at least the names "d", "w", "s", "alnum", "alpha", "blank", "cntrl", "digit", "graph", "lower", "print", "punct", "space", "upper" and "xdigit"narrow character names in Table X shall be recognized. For `regex_traits<wchar_t>`, at least the names L"d", L"w", L"s", L"alnum", L"alpha", L"blank", L"cntrl", L"digit", L"graph", L"lower", L"print", L"punct", L"space", L"upper" and L"xdigit"wide character names in Table X shall be recognized.

2. Modify 28.7 [re.traits] p. 12 as indicated:

   >     bool isctype(charT c, char_class_type f) const;

   > > -11- Effects: Determines if the character `c` is a member of the character classification represented by `f`.

   > > -12- Returns: Converts `f` into a value `m` of type `std::ctype_base::mask` in an unspecified manner, and returns true if `use_facet<ctype<charT> >(getloc()).is(m, c)` is true. Otherwise returns true if `f` bitwise or'ed with the result of calling `lookup_classname` with an iterator pair that designates the character sequence "w" is not equal to `0` and `c == '_'`, or if `f` bitwise or'ed with the result of calling `lookup_classname` with an iterator pair that designates the character sequence "blank" is not equal to `0` and `c` is one of an implementation-defined subset of the characters for which `isspace(c, getloc())` returns true, otherwise returns false. Given an exposition-only function prototype

   > > >       template<class C>
   > > >        ctype_base::mask convert(typename regex_traits<C>::char_class_type f);

   > > that returns a value in which each `ctype_base::mask` value corresponding to a value in `f` named in Table X is set, then the result is determined as if by:

   > > >     ctype_base::mask m = convert<charT>(f);
   > > >     const ctype<charT>& ct = use_facet<ctype<charT>>(getloc());
   > > >     if (ct.is(m, c)) {
   > > >       return true;
   > > >     } else if (c == ct.widen('_')) {
   > > >       charT w[1] = { ct.widen('w') };
   > > >       char_class_type x = lookup_classname(w, w+1);

   > > >       return (f&x) == x;
   > > >     } else {
   > > >       return false;
   > > >     }

   > > [Example:

   > > >     regex_traits<char> t;
   > > >     string d("d");
   > > >     string u("upper");
   > > >     regex_traits<char>::char_class_type f;
   > > >     f = t.lookup_classname(d.begin(), d.end());
   > > >     f |= t.lookup_classname(u.begin(), u.end());
   > > >     ctype_base::mask m = convert<char>(f); // m == ctype_base::digit|ctype_base::upper

   > > — end example]

   > > [Example:

   > > >     regex_traits<char> t;
   > > >     string w("w");
   > > >     regex_traits<char>::char_class_type f;
   > > >     f = t.lookup_classname(w.begin(), w.end());
   > > >     t.isctype('A', f); // returns true
   > > >     t.isctype('_', f); // returns true
   > > >     t.isctype(' ', f); // returns false

   > > — end example]

3. At the end of 28.7 [re.traits] add a new "Table X — Character class names and corresponding ctype masks":

   > Table X — Character class names and corresponding ctype masks
   > Narrow character name | Wide character name | Corresponding `ctype_base::mask` value
   > --- | --- | ---
   > `"alnum"` | `L"alnum"` | `ctype_base::alnum`
   > `"alpha"` | `L"alpha"` | `ctype_base::alpha`
   > `"blank"` | `L"blank"` | `ctype_base::blank`
   > `"cntrl"` | `L"cntrl"` | `ctype_base::cntrl`
   > `"digit"` | `L"digit"` | `ctype_base::digit`
   > `"d"` | `L"d"` | `ctype_base::digit`
   > `"graph"` | `L"graph"` | `ctype_base::graph`
   > `"lower"` | `L"lower"` | `ctype_base::lower`
   > `"print"` | `L"print"` | `ctype_base::print`
   > `"punct"` | `L"punct"` | `ctype_base::punct`
   > `"space"` | `L"space"` | `ctype_base::space`
   > `"s"` | `L"s"` | `ctype_base::space`
   > `"upper"` | `L"upper"` | `ctype_base::upper`
   > `"w"` | `L"w"` | `ctype_base::alnum`
   > `"xdigit"` | `L"xdigit"` | `ctype_base::xdigit`

### 2019. `isblank` not supported by `std::locale`

Section: 22.3.3.1 [classification] Status: C++11 Submitter: Jonathan Wakely Opened: 2010-11-16 Last modified: 2015-04-08

View all issues with C++11 status.

Discussion:

C99 added `isblank` and `iswblank` to `<locale.h>` but `<locale>` does not provide any equivalent.

[2011-02-24 Reflector discussion]

Moved to Tentatively Ready after 6 votes.

Proposed resolution:

Add to 22.3.3.1 [classification] synopsis:

>     template <class charT> bool isgraph (charT c, const locale& loc);
>     template <class charT> bool isblank (charT c, const locale& loc);

Add to 22.4.1 [category.ctype] synopsis:

>     static const mask xdigit = 1 << 8;
>     static const mask blank = 1 << 9;
>     static const mask alnum = alpha | digit;
>     static const mask graph = alnum | punct;

### 2020. Time utility arithmetic `constexpr` functions have invalid effects

Section: 20.12.5.5 [time.duration.nonmember] Status: C++11 Submitter: Daniel Krügler Opened: 2010-12-06 Last modified: 2015-04-08

View all other issues in [time.duration.nonmember].

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Discussion:

As of issue 1171 several time-utility functions have been marked `constexpr`. Alas this was done without adapting the corresponding return elements, which has the effect that none of current arithmetic functions of class template `duration` marked as `constexpr` can ever be `constexpr` functions (which makes them ill-formed, no diagnostics required as of recent core rules), because they invoke a non-constant expression, e.g. 20.12.5.5 [time.duration.nonmember]/2:

>     template <class Rep1, class Period1, class Rep2, class Period2>
>     constexpr typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>{>}::type
>     operator+(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);

>     2 Returns: CD(lhs) += rhs.

The real problem is, that we cannot defer to as-if rules here: The returns element specifies an indirect calling contract of a potentially user-defined function. This cannot be the `+=` assignment operator of such a user-defined type, but must be the corresponding immutable binary `operator+` (unless we require that `+=` shall be an immutable function which does not really makes sense).

[2011-02-17 Reflector discussion]

Moved to Tentatively Ready after 5 votes.

Proposed resolution:

The suggested wording changes are against the working draft N3242. Additional to the normative wording changes some editorial fixes are suggested.

1. In 20.12.5.5 [time.duration.nonmember], change the following arithmetic function specifications as follows:

   >     template <class Rep1, class Period1, class Rep2, class Period2>
   >     constexpr typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>{>}::type
   >     operator+(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);

   > > 2 Returns: `CD(lhs) += rhs` `CD(CD(lhs).count() + CD(rhs).count())`.

   >     template <class Rep1, class Period1, class Rep2, class Period2>
   >     constexpr typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>{>}::type
   >     operator-(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);

   > > 3 Returns: `CD(lhs) -= rhs` `CD(CD(lhs).count() - CD(rhs).count())`.

   >     template <class Rep1, class Period, class Rep2>
   >     constexpr duration<typename common_type<Rep1, Rep2>::type, Period>
   >     operator*(const duration<Rep1, Period>& d, const Rep2& s);

   > > 4 Remarks: This operator shall not participate in overload resolution unless `Rep2` is implicitly convertible to `CR(Rep1, Rep2)`.

   > > 5 Returns: `duration<CR(Rep1, Rep2), Period>(d) *= s` `CD(CD(d).count() * s)`.

   > `[...]`

   >     template <class Rep1, class Period, class Rep2>
   >     constexpr duration<typename common_type<Rep1, Rep2>::type, Period>
   >     operator/(const duration<Rep1, Period>& d, const Rep2& s);

   > > 8 Remarks: This operator shall not participate in overload resolution unless `Rep2` is implicitly convertible to `CR(Rep1, Rep2)` and `Rep2` is not an instantiation of `duration`.

   > > 9 Returns: `duration<CR(Rep1, Rep2), Period>(d) /= s` `CD(CD(d).count() / s)`.

   > `[...]`

   >     template <class Rep1, class Period, class Rep2>
   >     constexpr duration<typename common_type<Rep1, Rep2>::type, Period>
   >     operator%(const duration<Rep1, Period>& d, const Rep2& s);

   > > 11 Remarks: This operator shall not participate in overload resolution unless `Rep2` is implicitly convertible to `CR(Rep1, Rep2)` and `Rep2` is not an instantiation of `duration`.

   > > 12 Returns: `duration<CR(Rep1, Rep2), Period>(d) %= s` `CD(CD(d).count() % s)`

   >     template <class Rep1, class Period1, class Rep2, class Period2>
   >     constexpr typename common_type<duration<Rep1, Period1>, duration<Rep2, Period2>>::type
   >     operator%(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);

   > > 13 Returns: `common_type<duration<Rep1, Period1>, duration<Rep2, Period2> >::type(lhs) %= rhs` `CD(CD(lhs).count() % CD(rhs).count())`.

### 2021. Further incorrect usages of `result_of`

Section: 20.9.10.3 [func.bind.bind], 30.6.1 [futures.overview], 30.6.8 [futures.async] Status: C++14 Submitter: Daniel Krügler Opened: 2010-12-07 Last modified: 2015-04-08

View all other issues in [func.bind.bind].

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Discussion:

Issue 2017 points out some incorrect usages of `result_of` in the declaration of the function call operator overload of `reference_wrapper`, but there are more such specification defects:

1. According to 20.9.10.3 [func.bind.bind] p. 3:

   > [..] The effect of `g(u1, u2, ..., uM)` shall be `INVOKE(fd, v1, v2, ..., vN, result_of<FD cv (V1, V2, ..., VN)>::type)` [..]

   but `fd` is defined as "an lvalue of type `FD` constructed from `std::forward<F>(f)`". This means that the above usage must refer to `result_of<FD cv & (V1, V2, ..., VN)>` instead.

2. Similar in 20.9.10.3 [func.bind.bind] p. 10 bullet 2 we have:

   > if the value of `is_bind_expression<TiD>::value` is true, the argument is `tid(std::forward<Uj>(uj)...)` and its type `Vi` is `result_of<TiD cv (Uj...)>::type`

   Again, `tid` is defined as "lvalue of type `TiD` constructed from `std::forward<Ti>(ti)`". This means that the above usage must refer to `result_of<TiD cv & (Uj...)>` instead. We also have similar defect as in 2017 in regard to the argument types, this leads us to the further corrected form `result_of<TiD cv & (Uj&&...)>`. This is not the end: Since the `Vi` are similar sensitive to the argument problem, the last part must say:

   "[..] its type `Vi` is `result_of<TiD cv & (Uj&&...)>::type &&"`

   (The bound arguments `Vi` can never be `void` types, therefore we don't need to use the more defensive `std::add_rvalue_reference` type trait)

3. The function template `async` is declared as follows (the other overload has the same problem):

   >     template <class F, class... Args>
   >     future<typename result_of<F(Args...)>::type>
   >     async(F&& f, Args&&... args);

   This usage has the some same problems as we have found in `reference_wrapper` (2017) and more: According to the specification in 30.6.8 [futures.async] the effective result type is that of the call of

   >     INVOKE(decay_copy(std::forward<F>(f)), decay_copy(std::forward<Args>(args))...)

   First, `decay_copy` potentially modifies the effective types to `decay<F>::type` and `decay<Args>::type...`. Second, the current specification is not really clear, what the value category of callable type or the arguments shall be: According to the second bullet of 30.6.8 [futures.async] p. 3:

   > Invocation of the deferred function evaluates `INVOKE(g, xyz)` where `g` is the stored value of `decay_copy(std::forward<F>(f))` and `xyz` is the stored copy of `decay_copy(std::forward<Args>(args))...`.

   This seems to imply that lvalues are provided in contrast to the direct call expression of 30.6.8 [futures.async] p. 2 which implies rvalues instead. The specification needs to be clarified.

[2011-06-13: Daniel comments and refines the proposed wording changes]

The feedback obtained following message c++std-lib-30745 and follow-ups point to the intention, that the implied provision of lvalues due to named variables in `async` should be provided as rvalues to support move-only types, but the functor type should be forwarded as lvalue in `bind`.

If `bind` were newly invented, the value strategy could be improved, because now we have a preference of ref `&` qualified function call operator overloads. But such a change seems to be too late now. User-code that needs to bind a callable object with an ref `&&` qualified function call operator (or conversion function to function pointer) needs to use a corresponding wrapper similar to `reference_wrapper` that forwards the reference as rvalue-reference instead.

The wording has been adapted to honor these observations and to fit to FDIS numbering as well.

[Bloomington, 2011]

Move to Ready

Proposed resolution:

The suggested wording changes are against the FDIS.

1. Change 20.9.10.3 [func.bind.bind] p. 3 as indicated:

   >     template<class F, class... BoundArgs>
   >       unspecified bind(F&& f, BoundArgs&&... bound_args);

   > > -2- Requires: `is_constructible<FD, F>::value` shall be true. For each `Ti` in `BoundArgs`, `is_constructible<TiD, Ti>::value` shall be true. `INVOKE(fd, w1, w2, ..., wN)` (20.8.2) shall be a valid expression for some values `w1`, `w2`, ..., `wN`, where `N == sizeof...(bound_args)`.

   > > -3- Returns: A forwarding call wrapper `g` with a weak result type (20.8.2). The effect of `g(u1, u2, ..., uM)` shall be `INVOKE(fd, std::forward<V1>(v1), std::forward<V2>(v2), ..., std::forward<VN>(vN), result_of<FD cv & (V1, V2, ..., VN)>::type)`, where cv represents the cv-qualifiers of `g` and the values and types of the bound arguments `v1`, `v2`, ..., `vN` are determined as specified below. […]

2. Change 20.9.10.3 [func.bind.bind] p. 7 as indicated:

   >     template<class R, class F, class... BoundArgs>
   >        unspecified bind(F&& f, BoundArgs&&... bound_args);

   > > -6- Requires: `is_constructible<FD, F>::value` shall be true. For each `Ti` in `BoundArgs`, `is_constructible<TiD, Ti>::value` shall be true. `INVOKE(fd, w1, w2, ..., wN)` shall be a valid expression for some values `w1`, `w2`, ..., `wN`, where `N == sizeof...(bound_args)`.

   > > -7- Returns: A forwarding call wrapper `g` with a nested type `result_type` defined as a synonym for `R`. The effect of `g(u1, u2, ..., uM)` shall be `INVOKE(fd, std::forward<V1>(v1), std::forward<V2>(v2), ..., std::forward<VN>(vN), R)`, where the values and types of the bound arguments `v1`, `v2`, ..., `vN` are determined as specified below. […]

3. Change 20.9.10.3 [func.bind.bind] p. 10 as indicated:

   > -10- The values of the bound arguments `v1`, `v2`, ..., `vN` and their corresponding types `V1`, `V2`, ..., `VN` depend on the types `TiD` derived from the call to bind and the cv-qualifiers cv of the call wrapper `g` as follows:

   > - if `TiD` is `reference_wrapper<T>`, the argument is `tid.get()` and its type `Vi` is `T&`;
   > - if the value of `is_bind_expression<TiD>::value` is `true`, the argument is `tid(std::forward<Uj>(uj)...)` and its type `Vi` is `result_of<TiD cv & (Uj&&...)>::type&&`;
   > - if the value `j` of `is_placeholder<TiD>::value` is not zero, the argument is `std::forward<Uj>(uj)` and its type `Vi` is `Uj&&`;
   > - otherwise, the value is `tid` and its type `Vi` is `TiD cv &`.

4. This resolution assumes that the wording of 30.6.8 [futures.async] is intended to provide rvalues as arguments of `INVOKE`.

   Change the function signatures in header `<future>` synopsis 30.6.1 [futures.overview] p. 1 and in 30.6.8 [futures.async] p. 1 as indicated:

   >     template <class F, class... Args>
   >     future<typename result_of<typename decay<F>::type(typename decay<Args>::type...)>::type>
   >     async(F&& f, Args&&... args);
   >     template <class F, class... Args>
   >     future<typename result_of<typename decay<F>::type(typename decay<Args>::type...)>::type>
   >     async(launch policy, F&& f, Args&&... args);

5. Change 30.6.8 [futures.async] as indicated: (Remark: There is also a tiny editorial correction in p. 4 that completes one `::` scope specifier)

   > -3- Effects: […]

   > - […]
   > - if `policy & launch::deferred` is non-zero — Stores `DECAY_COPY(std::forward<F>(f))` and `DECAY_COPY(std::forward<Args>(args))...` in the shared state. These copies of `f` and `args` constitute a deferred function. Invocation of the deferred function evaluates `INVOKE(std::move(g), std::move(xyz))` where `g` is the stored value of `DECAY_COPY(std::forward<F>(f))` and `xyz` is the stored copy of `DECAY_COPY(std::forward<Args>(args))...`. The shared state is not made ready until the function has completed. The first call to a non-timed waiting function (30.6.4) on an asynchronous return object referring to this shared state shall invoke the deferred function in the thread that called the waiting function. Once evaluation of `INVOKE(std::move(g), std::move(xyz))` begins, the function is no longer considered deferred. [ Note: If this policy is specified together with other policies, such as when using a `policy` value of `launch::async | launch::deferred`, implementations should defer invocation or the selection of the policy when no more concurrency can be effectively exploited. — end note ]

   > -4- Returns: an object of type `future<typename result_of<typename decay<F>::type(typename decay<Args>::type...)>::type>` that refers to the associated asynchronous state created by this call to `async`.

### 2022. `reference_wrapper<T>::result_type` is underspecified

Section: 20.9.4 [refwrap] Status: C++11 Submitter: Daniel Krügler Opened: 2010-12-08 Last modified: 2015-04-08

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Discussion:

Issue 1295 correctly removed function types and references to function types from the bullet 1 of 20.9.2 [func.require] p. 3 because neither function types nor function references satisfy the requirements for a target object which is defined to be an object of a callable type. This has the effect that the reference in 20.9.4 [refwrap] p. 2

> `reference_wrapper` has a weak result type (20.8.2).

is insufficient as a reference to define the member type `result_type` when the template argument `T` is a function type.

There are basically two approaches to solve the problem:

1. Extend the definition of a weak result type in 20.9.2 [func.require] p. 3 to both function types and references thereof. This extension must be specified independend from the concept of a call wrapper, though.

2. Add one extra sentence to 20.9.4 [refwrap] p. 2 that simply defines the member type `result_type` for `reference_wrapper<T>`, when `T` is a function type.

I checked the current usages of weak result type to have a base to argue for one or the other approach. It turns out, that there is no further reference to this definition in regard to function types or references thereof. The only other reference can be found in 20.9.10.3 [func.bind.bind] p. 3, where `g` is required to be a class type.

[2011-02-23 Reflector discussion]

Moved to Tentatively Ready after 5 votes.

Proposed resolution:

The suggested wording changes are against the working draft N3242.

1. Change 20.9.4 [refwrap] p. 2 as indicated:

   > 2 `reference_wrapper<T>` has a weak result type (20.8.2). If `T` is a function type, `result_type` shall be a synonym for the return type of `T`.

### 2023. Incorrect requirements for `lock_guard` and `unique_lock`

Section: 30.4.2.1 [thread.lock.guard], 30.4.2.2 [thread.lock.unique] Status: Resolved Submitter: Daniel Krügler Opened: 2010-12-08 Last modified: 2015-04-08

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Discussion:

There are two different `*Lockable` requirements imposed on template arguments of the class template `lock_guard` as of 30.4.2.1 [thread.lock.guard] p. 1+2:

> 1 [..] The supplied `Mutex` type shall meet the `BasicLockable` requirements (30.2.5.2).

> 2 The supplied `Mutex` type shall meet the `Lockable` requirements (30.2.5.3).

The `Lockable` requirements include the availability of a member function `try_lock()`, but there is no operational semantics in the specification of `lock_guard` that would rely on such a function. It seems to me that paragraph 2 should be removed.

Similarly, 30.4.2.2 [thread.lock.unique] p. 1+2 refer to exactly the same two requirements. In this case it seems as if the intention was that the template arguement `Mutex` should always provide the `try_lock()` member function, because several member functions of `unique_lock` (`unique_lock(mutex_type& m, try_to_lock_t)` or `bool try_lock()`) take advantage of such a function without adding extra requirements for this. It seems that the requirement subset `BasicLockable` should be removed.

I searched for further possible misusages of the `*Lockable` requirements, but could not find any more.

[2011-02-23]

Howard suggests an alternative approach in regard to `unique_lock`: The current minimum requirements on its template argument should better be reduced to `BasicLockable` instead of the current `Lockable`, including ammending member-wise constraints where required. This suggestions was supported by Anthony, Daniel, and Pablo.

Daniel drafts wording that follows this separation strategy.

[2011-02-24 Reflector discussion]

Moved to Tentatively Ready after 5 votes.

> The suggested wording changes are against the working draft N3242.

> 1. Remove 30.4.2.1 [thread.lock.guard] p. 2 completely:

>    > 2 The supplied `Mutex` type shall meet the `Lockable` requirements (30.2.5.3).

> 2. Change 30.4.2.2 [thread.lock.unique] p. 1-3 as indicated. The intend is to make `BasicLockable` the fundamental requirement for `unique_lock`. We also update the note to reflect these changes and synchronize one remaining reference of 'mutex' by the proper term 'lockable object' in sync to the wording changes of `lock_guard`:

>    > 1 [..] The behavior of a program is undefined if the contained pointer `pm` is not null and the mutex lockable object pointed to by `pm` does not exist for the entire remaining lifetime (3.8) of the `unique_lock` object. The supplied `Mutex` type shall meet the `BasicLockable` requirements (30.2.5.2). [Editor's note: BasicLockable is redundant, since the following additional paragraph requires Lockable.]

>    > 2 The supplied `Mutex` type shall meet the `Lockable` requirements (30.2.5.3).

>    > 3 [ Note: `unique_lock<Mutex>` meets the `BasicLockable` requirements. If `Mutex` meets the `Lockable` requirements ([thread.req.lockable.req]), `unique_lock<Mutex>` also meets the `Lockable` requirements and if `Mutex` meets the `TimedLockable` requirements (30.2.5.4), `unique_lock<Mutex>` also meets the `TimedLockable` requirements. — end note ]

> 3. Modify 30.4.2.2.1 [thread.lock.unique.cons] to add the now necessary member-wise additional constraints for `Lockable`:

>    >     unique_lock(mutex_type& m, try_to_lock_t) noexcept;

>    > > 8 Requires: The supplied `Mutex` type shall meet the `Lockable` requirements ([thread.req.lockable.req]). If `mutex_type` is not a recursive mutex the calling thread does not own the mutex.

>    > > 9 Effects: Constructs an object of type `unique_lock` and calls `m.try_lock()`.

> 4. Modify 30.4.2.2.2 [thread.lock.unique.locking] to add the now necessary member-wise additional constraints for `Lockable`:

>    >     bool try_lock();

>    > > ? Requires: The supplied `Mutex` type shall meet the `Lockable` requirements ([thread.req.lockable.req]).

>    > > 4 Effects: `pm->try_lock()`

Proposed resolution:

Resolved 2011-03 Madrid meeting by paper N3278

### 2024. Inconsistent implementation requirements for `atomic<integral>` and `atomic<T*>`

Section: 29.5 [atomics.types.generic] Status: Resolved Submitter: Daniel Krügler Opened: 2010-12-08 Last modified: 2015-04-08

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Discussion:

Paragraph 5 and 6 of 29.5 [atomics.types.generic] impose different requirements on implementations for specializations of the `atomic` class template for integral types and for pointer types:

> 5 The atomic integral specializations and the specialization `atomic<bool>` shall have standard layout. They shall each have a trivial default constructor and a trivial destructor. They shall each support aggregate initialization syntax.

> 6 There are pointer partial specializations on the `atomic` class template. These specializations shall have trivial default constructors and trivial destructors.

It looks like an oversight to me, that for pointer specializations the requirements for standard layout and support for aggregate initialization syntax are omitted. In fact, this been confirmed by the N3193 proposal author. I suggest to impose the same implementation requirements for pointer types as for integral types, this should not impose unrealistic requirements on implementations.

[2011-02-10 Reflector discussion]

Moved to Tentatively Ready after 5 votes.

> Proposed Resolution

> The suggested wording changes are against the working draft N3242.

> 1. Change 29.5 [atomics.types.generic] p. 6 as indicated:

>    > 6 There are pointer partial specializations on the `atomic` class template. These specializations shall have standard layout, trivial default constructors, and trivial destructors. They shall each support aggregate initialization syntax.

Proposed resolution:

Resolved 2011-03 Madrid meeting by paper N3278

### 2025. Incorrect semantics of move assignment operator of `packaged_task`

Section: 30.6.9.1 [futures.task.members] Status: Resolved Submitter: Daniel Krügler Opened: 2010-12-08 Last modified: 2015-04-08

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Discussion:

According to 30.6.9.1 [futures.task.members] p. 7 bullet 2:

>     packaged_task& operator=(packaged_task&& other);

> > 7 Effects:

> > - [...]

> > - `packaged_task<R, ArgTypes...>(other).swap(*this)`.

The argument `other` given to the move constructor is an lvalue and must be converted into an rvalue via appropriate usage of `std::move`.

> Proposed Resolution

> The suggested wording changes are against the working draft N3242.

> 1. Change 30.6.9.1 [futures.task.members] p. 7 bullet 2 as indicated:

>    >     packaged_task& operator=(packaged_task&& other);

>    > > 7 Effects:

>    > > - [...]

>    > > - `packaged_task(std::move(other)).swap(*this)`.

Proposed resolution:

Resolved 2011-03 Madrid meeting by paper N3278

### 2027. Initialization of the stored task of a `packaged_task`

Section: 30.6.9.1 [futures.task.members] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2011-02-09 Last modified: 2015-04-08

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Discussion:

Related with LWG issue 1514.

The move constructor of `packaged_task` does not specify how the stored task is constructed. The obvious way is to move-construct it using the task stored in the argument. Moreover, the constructor should be provided with a throws clause similar to one used for the other constructors, as the move constructor of the stored task is not required to be nothrow.

As for the other constructors, the terms "stores a copy of `f`" do not reflect the intent, which is to allow `f` to be moved when possible.

[2011-02-25: Alberto updates wording]

[2011-02-26 Reflector discussion]

Moved to Tentatively Ready after 5 votes.

Proposed resolution:

(wording written assuming LWG 1514 is also accepted)

1. Change 30.6.9.1 [futures.task.members] paragraph 3:

   > 3 Effects: constructs a new `packaged_task` object with an associated asynchronous state and stores a copy of `f` as the object's stored task initializes the object's stored task with `std::forward<F>(f)`. The constructors that take an `Allocator` argument use it to allocate memory needed to store the internal data structures.

2. Change 30.6.9.1 [futures.task.members] paragraph 5:

   > 5 Effects: constructs a new `packaged_task` object and transfers ownership of `other`'s associated asynchronous state to `*this`, leaving `other` with no associated asynchronous state. Moves the stored task from `other` to `*this`.

### 2028. `messages_base::catalog` overspecified

Section: 22.4.7.1 [locale.messages] Status: C++14 Submitter: Howard Hinnant Opened: 2011-02-14 Last modified: 2015-04-08

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Discussion:

In 22.4.7.1 [locale.messages], `messages_base::catalog` is specified to be a typedef to `int`. This type is subsequently used to open, access and close catalogs.

However, an OS may have catalog/messaging services that are indexed and managed by types other than `int`. For example `POSIX`, publishes the following messaging API:

>     typedef unspecified nl_catd;

>     nl_catd catopen(const char* name , int oflag);
>     char*   catgets(nl_catd catd, int set_id, int msg_id, const char* s);
>     int     catclose(nl_catd catd);

I.e., the catalog is managed with an unspecified type, not necessarily an `int`. Mac OS uses a `void*` for `nl_catd` (which is conforming to the `POSIX` standard). The current `messages_base` spec effectively outlaws using the built-in OS messaging service supplied for this very purpose!

[2011-02-24: Chris Jefferson updates the proposed wording, changing unspecified to unspecified signed integral type]

[2011-03-02: Daniel updates the proposed wording, changing unspecified signed integral type to unspecified signed integer type (We don't want to allow for `bool` or `char`)]

[2011-03-24 Madrid meeting]

Consensus that this resolution is the direction we would like to see.

Proposed resolution:

1. Modify 22.4.7.1 [locale.messages]:

   >     namespace std {
   >       class messages_base {
   >       public:
   >         typedef intunspecified signed integer type catalog;
   >       };
   >       ...
   >     }

### 2029. Missing '`noexcept`' on `basic_regex` move-assignment operator

Section: 28.8 [re.regex] Status: C++11 Submitter: Jonathan Wakely Opened: 2011-02-16 Last modified: 2015-04-08

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Discussion:

N3149 replaced the "`Throws: nothing`" clause on `basic_regex::assign(basic_regex&&)` with the `noexcept` keyword. The effects of the move-assignment operator are defined in terms of the `assign()` function, so the "`Throws: nothing`" applied there too, and a `noexcept`-specification should be added there too.

[2011-02-24 Reflector discussion]

Moved to Tentatively Ready after 7 votes.

Proposed resolution:

1. Modify the `basic_regex` synopsis in 28.8 [re.regex] p. 3:

   >     namespace std {
   >       template <class charT,
   >                 class traits = regex_traits<charT> >
   >       class basic_regex {
   >       public:
   >         ...
   >         basic_regex& operator=(const basic_regex&);
   >         basic_regex& operator=(basic_regex&&) noexcept;
   >         basic_regex& operator=(const charT* ptr);
   >         ...
   >       };
   >     }

2. Modify 28.8.3 [re.regex.assign] p. 2:

   >     basic_regex& operator=(basic_regex&& e) noexcept;

   > > 2 Effects: returns `assign(std::move(e))`.

### 2030. `packaged_task::result_type` should be removed

Section: 30.6.9 [futures.task] Status: C++11 Submitter: Anthony Williams Opened: 2010-11-12 Last modified: 2015-04-08

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Discussion:

`packaged_task::operator()` always returns `void`, regardless of the return type of the wrapped task. However, `packaged_task::result_type` is a typedef to the return type of the wrapped task. This is inconsistent with other uses of `result_type` in the standard, where it matches the return type of `operator()` (e.g. `function`, `owner_less`). This is confusing.

It also violates the TR1 `result_of` protocol, and thus makes `packaged_task` harder to use with anything that respects that protocol.

Finally, it is of little use anyway.

`packaged_task::result_type` should therefore be removed.

[2011-02-24 Reflector discussion]

Moved to Tentatively Ready after 5 votes.

Proposed resolution:

Alter the class definition of `packaged_task` in 30.6.9 [futures.task] p. 2 as follows:

>     template<class R, class... ArgTypes>
>     class packaged_task<R(ArgTypes...)> {
>     public:
>       typedef R result_type;
>       [...]
>     };

### 2031. `std::future<>::share()` only applies to rvalues

Section: 30.6.6 [futures.unique_future] Status: C++11 Submitter: Anthony Williams Opened: 2011-02-17 Last modified: 2015-04-08

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Discussion:

As specified, `future<>::share()` has the signature

>     shared_future<R> share() &&;

This means that it can only be applied to rvalues. One of the key benefits of `share()` is that it can be used with the new `auto` facility:

>     std::promise<some_long_winded_type_name> some_promise;
>     auto f = some_promise.get_future(); // std::future
>     auto sf = std::move(f).share();

`share()` is sufficiently explicit that the move should not be required. We should be able to write:

>     auto sf = f.share();

[2011-02-22 Reflector discussion]

Moved to Tentatively Ready after 5 votes.

Proposed resolution:

Alter the declaration of `share()` to remove the "&&" rvalue qualifier in 30.6.6 [futures.unique_future] p. 3, and 30.6.6 [futures.unique_future] p. 11:

>     shared_future<R> share() &&;

### 2032. Incorrect synchronization clause of `async` function

Section: 30.6.8 [futures.async] Status: C++11 Submitter: Alberto Ganesh Barbati Opened: 2011-02-17 Last modified: 2015-04-08

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Discussion:

Clause 30.6.8 [futures.async] has undergone significant rewording in Batavia. Due to co-presence of at least three different sources of modification there is a part where changes have overlapped (marked by an Editor's note), which should be reconciled. Moreover, I believe that a few non-overlapping sentences are now incorrect and should be fixed, so the problem cannot be handled editorially. (See c++std-lib-29667.)

[Adopted in Madrid, 2011-03]

Proposed resolution:

1. Edit 30.6.4 [futures.state], paragraph 3 as follows.

   > An asynchronous return object is an object that reads results from an associated asynchronous state. A waiting function of an asynchronous return object is one that potentially blocks to wait for the associated asynchronous state to be made ready. If a waiting function can return before the state is made ready because of a timeout (30.2.5), then it is a timed waiting function, otherwise it is a non-timed waiting function.

2. Edit within 30.6.8 [futures.async] paragraph 3 bullet 2 as follows.

   > Effects: [...]

   > - if `policy & launch::deferred` is non-zero — [...] The associated asynchronous state is not made ready until the function has completed. The first call to a non-timed waiting function (30.6.4 [futures.state]) requiring a non-timed wait on an asynchronous return object referring to the this associated asynchronous state created by this `async` call to become ready shall invoke the deferred function in the thread that called the waiting function;. once Once evaluation of `INVOKE(g, xyz)` begins, the function is no longer considered deferred. [...]

3. Edit 30.6.8 [futures.async] paragraph 5 as follows.

   > Synchronization: Regardless of the provided `policy` argument,

   > - the invocation of `async` synchronizes with (1.10) the invocation of `f`. [Note: this statement applies even when the corresponding future object is moved to another thread. —end note]; and
   > - the completion of the function `f` is sequenced before (1.10) the associated asynchronous state is made ready. [Note: `f` might not be called at all, so its completion might never happen. —end note]

   > If `policy & launch::async` is non-zero, If the implementation chooses the `launch::async` policy,

   > - a call to a waiting function on an asynchronous return object that shares the associated asynchronous state created by this `async` call shall block until the associated thread has completed, as if joined (30.3.1.5);
   > - the `join()` on the created thread object the associated thread completion synchronizes with (1.10) the return from the first function that successfully detects the ready status of the associated asynchronous state or with the return from the last function that releases the associated asynchronous state returns, whichever happens first. [Editor's note: N3196 changes the following sentence as indicated. N3188 removes the sentence. Please pick one.] If the invocation is deferred, the completion of the invocation of the deferred function synchronizes with the successful return from a call to a waiting function on the associated asynchronous state.

### 2033. Preconditions of `reserve`, `shrink_to_fit`, and `resize` functions

Section: 23.3.6.3 [vector.capacity], 23.3.3.3 [deque.capacity] Status: C++14 Submitter: Nikolay Ivchenkov Opened: 2011-02-20 Last modified: 2015-04-08

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Discussion:

I have several questions with regard to the working paper N3225 (C++0x working draft):

1. Where the working draft specifies preconditions for `shrink_to_fit` member function of `std::vector` and `std::deque`?

2. Where the working draft specifies preconditions for '`void reserve(size_type n)`' member function of `std::vector`?

3. Does a call to '`void resize(size_type sz)`' of `std::vector` require the element type to be `DefaultConstructible`? If yes, why such requirement is not listed in the Requires paragraph?

4. Does a call to '`void resize(size_type sz)`' of `std::vector` require the element type to be `MoveAssignable` because the call `erase(begin() + sz, end())` mentioned in the Effects paragraph would require the element type to be `MoveAssignable`?

5. Why `CopyInsertable` requirement is used for '`void resize(size_type sz)`' of `std::vector` instead of `MoveInsertable` requirement?

[2011-06-12: Daniel comments and provides wording]

According to my understanding of the mental model of vector (and to some parts for deque) the some requirements are missing in the standard as response to above questions:

1. The preconditions of `shrink_to_fit` for both `std::vector` and `std::deque` should impose the `MoveInsertable` requirements. The reason for this is, that these containers can host move-only types. For a container type `X` the C++03 idiom `X(*this).swap(*this)` imposes the `CopyInsertable` requirements which would make the function call ill-formed, which looks like an unacceptable restriction to me. Assuming the committee decides to support the move-only case, further wording has to be added for the situation where such a move-only type could throw an exception, because this can leave the object in an unspecified state. This seems consistent with the requirements of `reserve`, which seems like a very similar function to me (for `vector`). And this brings us smoothly to the following bullet:

2. I agree that we are currently missing to specify the preconditions of the `reserve` function. My interpretation of the mental model of this function is that it should work for move-only types, which seems to be supported by the wording used in 23.3.6.3 [vector.capacity] p2:

   > […] If an exception is thrown other than by the move constructor of a non-CopyInsertable type, there are no effects.

   Given this statement, the appropriate requirement is `MoveInsertable` into the `vector`.

3. I agree that `vector::resize(size_type)` misses to list the `DefaultConstructible` requirements.
4. Unfortunately the current specification in terms of `erase` implies the `MoveAssignable` requirements. I don't think that this implication is intended. This function requires "append" and "pop-back" effects, respectively, where the former can be realized in terms of `MoveInsertable` requirements. The same fix in regard to using `pop_back` instead of `erase` is necessary for the two argument overload of `resize` as well (no `MoveAssignable` is required).
5. The `CopyInsertable` requirement is incorrect and should be `MoveInsertable` instead.

In addition to above mentioned items, the proposed resolution adds a linear complexity bound for `shrink_to_fit` and attempts to resolve the related issue 2066.

[ 2011 Bloomington ]

Move to Ready.

Note for editor: we do not normally refer to 'linear time' for complexity requirements, but there is agreement that any clean-up of such wording is editorial.

Proposed resolution:

This wording is relative to the FDIS.

1. Edit 23.3.3.3 [deque.capacity] as indicated [Remark: The suggested change of p4 is not redundant, because `CopyInsertable` is not necessarily a refinement of `MoveInsertable` in contrast to the fact that `CopyConstructible` is a refinement of `MoveConstructible`]:

   >     void resize(size_type sz);

   > > -1- Effects: If `sz <= size()`, equivalent to `erase(begin() + sz, end());`calling `pop_back()` `size() - sz` times. If `size() < sz`, appends `sz - size()` value-initialized elements to the sequence.

   > > -2- Requires: `T` shall be `MoveInsertable` into `*this` and `DefaultConstructible`.

   >     void resize(size_type sz, const T& c);

   > > -3- Effects: If `sz <= size()`, equivalent to calling `pop_back()` `size() - sz` times. If `size() < sz`, appends `sz - size()` copies of `c` to the sequence.

   > > >     if (sz > size())
   > > >       insert(end(), sz-size(), c);
   > > >     else if (sz < size())
   > > >       erase(begin()+sz, end());
   > > >     else
   > > >       ; // do nothing

   > > -4- Requires: `T` shall be `MoveInsertable` into `*this` and `CopyInsertable` into `*this`.

   >     void shrink_to_fit();

   > > -?- Requires: `T` shall be `MoveInsertable` into `*this`.

   > > -?- Complexity: Takes at most linear time in the size of the sequence.

   > > -5- Remarks: `shrink_to_fit` is a non-binding request to reduce memory use but does not change the size of the sequence. [ Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note ]

2. Edit 23.3.6.3 [vector.capacity] as indicated including edits that also resolve 2066 [Remark: The combined listing of `MoveInsertable` and `CopyInsertable` before p12 is not redundant, because `CopyInsertable` is not necessarily a refinement of `MoveInsertable` in contrast to the fact that `CopyConstructible` is a refinement of `MoveConstructible`]:

   […]

   >     void reserve(size_type n);

   > > -?- Requires: `T` shall be `MoveInsertable` into `*this`.

   > > -2- Effects: A directive that informs a vector of a planned change in size, so that it can manage the storage allocation accordingly. After `reserve()`, `capacity()` is greater or equal to the argument of reserve if reallocation happens; and equal to the previous value of `capacity()` otherwise. Reallocation happens at this point if and only if the current capacity is less than the argument of `reserve()`. If an exception is thrown other than by the move constructor of a non-`CopyInsertable` type, there are no effects.

   > > -3- Complexity: It does not change the size of the sequence and takes at most linear time in the size of the sequence.

   > > -4- Throws: `length_error` if `n > max_size()`.[footnote 266]

   > > -5- Remarks: Reallocation invalidates all the references, pointers, and iterators referring to the elements in the sequence. It is guaranteed that no reallocation takes place during insertions that happen after a call to `reserve()` until the time when an insertion would make the size of the vector greater than the value of `capacity()`.

   >     void shrink_to_fit();

   > > -?- Requires: `T` shall be `MoveInsertable` into `*this`.

   > > -?- Complexity: Takes at most linear time in the size of the sequence.

   > > -6- Remarks: `shrink_to_fit` is a non-binding request to reduce `capacity()` to `size()`. [ Note: The request is non-binding to allow latitude for implementation-specific optimizations. — end note ] If an exception is thrown other than by the move constructor of a non-`CopyInsertable` `T` there are no effects.

   […]

   >     void resize(size_type sz);

   > > -9- Effects: If `sz <= size()`, equivalent to `erase(begin() + sz, end());`calling `pop_back()` `size() - sz` times. If `size() < sz`, appends `sz - size()` value-initialized elements to the sequence.

   > > -10- Requires: `T` shall be `CopyMoveInsertable` into `*this` and `DefaultConstructible`.

   > > -??- Remarks: If an exception is thrown other than by the move constructor of a non-`CopyInsertable` `T` there are no effects.

   >     void resize(size_type sz, const T& c);

   > > -11- Effects: If `sz <= size()`, equivalent to calling `pop_back()` `size() - sz` times. If `size() < sz`, appends `sz - size()` copies of `c` to the sequence.

   > > >     if (sz > size())
   > > >       insert(end(), sz-size(), c);
   > > >     else if (sz < size())
   > > >       erase(begin()+sz, end());
   > > >     else
   > > >       ; // do nothing

   > > -??- Requires: `T` shall be `MoveInsertable` into `*this` and `CopyInsertable` into `*this`.

   > > -12- Requires Remarks: If an exception is thrown other than by the move constructor of a non-`CopyInsertable` `T` there are no effects.

### 2034. Initialization of atomics is misspecified so that it doesn't preserve sequential consistency

Section: 29.3 [atomics.order] Status: Resolved Submitter: Hans Boehm Opened: 2011-02-26 Last modified: 2015-04-08

View other active issues in [atomics.order].

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Discussion:

This violates the core intent of the memory model, as stated in the note in 1.10 [intro.multithread] p. 21.

This was discovered by Mark Batty, and pointed out in their POPL 2011 paper, "Mathematizing C++ Concurrency", section 4, "Sequential consistency of SC atomics". The problem is quite technical, but well-explained in that paper.

This particular issue was not understood at the time the FCD comments were generated. But it is closely related to a number of FCD comments. It should have arisen from US-171, though that's not the actual history.

This issue has been under discussion for several months in a group that included a half dozen or so of the most interested committee members. The P/R represents a well-considered consensus among us:

[2011-03-16: Jens updates wording]

> Modify 29.3 [atomics.order] p.3, so that the normative part reads:

> > 3 There shall be a single total order S on all `memory_order_seq_cst` operations, consistent with the "happens before" order and modification orders for all affected locations, such that each `memory_order_seq_cst` operation that loads a value observes either the last preceding modification according to this order S, A (if any), or the result of an operation X that is not does not happen before A and that is not `memory_order_seq_cst`. [ Note: Although it is not explicitly required that S include locks, it can always be extended to an order that does include lock and unlock operations, since the ordering between those is already included in the "happens before" ordering. — end note ]

Proposed resolution:

Resolved 2011-03 Madrid meeting by paper N3278

### 2037. `atomic` free functions incorrectly specified

Section: 29.2 [atomics.syn] Status: Resolved Submitter: Pete Becker Opened: 2011-03-01 Last modified: 2015-04-08

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Discussion:

In earlier specifications of atomics the template specialization `atomic<integer>` was derived from `atomic_integer` (e.g. `atomic<int>` was derived from `atomic_int`), and the working draft required free functions such as

>     int atomic_load(const atomic_int*)

for each of the `atomic_integer` types. This worked fine with normal function overloading.

For the post-Batavia working draft, N3193 removed the requirement that `atomic<integer>` be derived from `atomic_integer` and replaced the free functions taking pointers to `atomic_integer` with template functions taking `atomic_type*`, such as

>     template <class T> T atomic_load(const atomic_type*);

and a code comment explaining that `atomic_type` can be either `atomic<T>` or a named base class of `atomic<T>`. The latter possibility is supposed to allow existing implementations based on the previous specification to continue to conform.

From history, this allowance seems to imply that functions like `atomic_load` can be non-template free functions, as they were before. The explicit requirements do not allow this, and, by requiring that they be templates, make them far more complicated. As the specification is currently written, code that uses an implementation that uses a base class would have to provide an explicit template type:

>     atomic<int> my_atomic_int;
>     atomic_load<int>(&my_atomic_int);

That type argument isn't needed when `atomic_type` is `atomic<T>`, but cautious users would always provide it to make their code portable across different implementations of the standard library.

One possibility for the implementor would be to do some template meta-programming to infer the type `T` when there are no function parameters of type `T`, but without running afoul of the prohibition on adding parameters with default values (17.6.5.4 [global.functions]/3).

So the promise that implementations of the previous specification continue to conform has not been met. The specification of these free functions should be rewritten to support library code written to the previous specification or the vacuous promise should be removed.

[2011-03-08: Lawrence comments and drafts wording:]

One of the goals is to permit atomics code to compile under both C and C++. Adding explicit template arguments would defeat that goal.

The intent was to permit the normal function overloads for `atomic_int` when `atomic_int` is distinct from `atomic<int>`. That intent was not reflected in the wording.

> Proposed Resolution

> Explicitly permit free functions.

> 1. Edit within the header `<atomic>` synopsis 29.2 [atomics.syn] as follows:

>    >     // 29.6.1, general operations on atomic types
>    >     // In the following declarations, atomic_type is either
>    >     // atomic<T> or a named base class for T from
>    >     // Table 145 or inferred from
>    >     // Table 146.
>    >     // In the following declarations, atomic-type is either
>    >     // atomic<T> or a named base class for T from
>    >     // Table 145 or inferred from
>    >     // Table 146.
>    >     // If it is atomic<T>, then the declaration is a template
>    >     // declaration prefixed with template <class T>
>    >     template <class T>
>    >     bool atomic_is_lock_free(const volatile atomic_typeatomic-type*);
>    >     template <class T>
>    >     bool atomic_is_lock_free(const atomic_typeatomic-type*);
>    >     template <class T>
>    >     void atomic_init(volatile atomic_typeatomic-type*, T);
>    >     template <class T>
>    >     void atomic_init(atomic_typeatomic-type*, T);
>    >     template <class T>
>    >     void atomic_store(volatile atomic_typeatomic-type*, T);
>    >     template <class T>
>    >     void atomic_store(atomic_typeatomic-type*, T);
>    >     template <class T>
>    >     void atomic_store_explicit(volatile atomic_typeatomic-type*, T, memory_order);
>    >     template <class T>
>    >     void atomic_store_explicit(atomic_typeatomic-type*, T, memory_order);
>    >     template <class T>
>    >     T atomic_load(const volatile atomic_typeatomic-type*);
>    >     template <class T>
>    >     T atomic_load(const atomic_typeatomic-type*);
>    >     template <class T>
>    >     T atomic_load_explicit(const volatile atomic_typeatomic-type*, memory_order);
>    >     template <class T>
>    >     T atomic_load_explicit(const atomic_typeatomic-type*, memory_order);
>    >     template <class T>
>    >     T atomic_exchange(volatile atomic_typeatomic-type*, T);
>    >     template <class T>
>    >     T atomic_exchange(atomic_typeatomic-type*, T);
>    >     template <class T>
>    >     T atomic_exchange_explicit(volatile atomic_typeatomic-type*, T, memory_order);
>    >     template <class T>
>    >     T atomic_exchange_explicit(atomic_typeatomic-type*, T, memory_order);
>    >     template <class T>
>    >     bool atomic_compare_exchange_weak(volatile atomic_typeatomic-type*, T*, T);
>    >     template <class T>
>    >     bool atomic_compare_exchange_weak(atomic_typeatomic-type*, T*, T);
>    >     template <class T>
>    >     bool atomic_compare_exchange_strong(volatile atomic_typeatomic-type*, T*, T);
>    >     template <class T>
>    >     bool atomic_compare_exchange_strong(atomic_typeatomic-type*, T*, T);
>    >     template <class T>
>    >     bool atomic_compare_exchange_weak_explicit(volatile atomic_typeatomic-type*, T*, T,
>    >       memory_order, memory_order);
>    >     template <class T>
>    >     bool atomic_compare_exchange_weak_explicit(atomic_typeatomic-type*, T*, T.
>    >       memory_order, memory_order);
>    >     template <class T>
>    >     bool atomic_compare)exchange_strong_explicit(volatile atomic_typeatomic-type*, T*, T,
>    >       memory_order, memory_order);
>    >     template <class T>
>    >     bool atomic_compare_exchange_strong_explicit(atomic_typeatomic-type*, T*, T,
>    >       memory_order, memory_order);

>    >     // 29.6.2, templated operations on atomic types
>    >     // In the following declarations, atomic_type is either
>    >     // atomic<T> or a named base class for T from
>    >     // Table 145 or inferred from
>    >     // Table 146.
>    >     template <class T>
>    >     T atomic_fetch_add(volatile atomic-typeatomic<T>*, T);
>    >     template <class T>
>    >     T atomic_fetch_add(atomic-typeatomic<T>*, T);
>    >     template <class T>
>    >     T atomic_fetch_add_explicit(volatile atomic-typeatomic<T>*, T, memory_order);
>    >     template <class T>
>    >     T atomic_fetch_add_explicit(atomic-typeatomic<T>*, T, memory_order);
>    >     template <class T>
>    >     T atomic_fetch_sub(volatile atomic-typeatomic<T>*, T);
>    >     template <class T>
>    >     T atomic_fetch_sub(atomic-typeatomic<T>*, T);
>    >     template <class T>
>    >     T atomic_fetch_sub_explicit(volatile atomic-typeatomic<T>*, T, memory_order);
>    >     template <class T>
>    >     T atomic_fetch_sub_explicit(atomic-typeatomic<T>*, T, memory_order);
>    >     template <class T>
>    >     T atomic_fetch_and(volatile atomic-typeatomic<T>*, T);
>    >     template <class T>
>    >     T atomic_fetch_and(atomic-typeatomic<T>*, T);
>    >     template <class T>
>    >     T atomic_fetch_and_explicit(volatile atomic-typeatomic<T>*, T, memory_order);
>    >     template <class T>
>    >     T atomic_fetch_and_explicit(atomic-typeatomic<T>*, T, memory_order);
>    >     template <class T>
>    >     T atomic_fetch_or(volatile atomic-typeatomic<T>*, T);
>    >     template <class T>
>    >     T atomic_fetch_or(atomic-typeatomic<T>*, T);
>    >     template <class T>
>    >     T atomic_fetch_or_explicit(volatile atomic-typeatomic<T>*, T, memory_order);
>    >     template <class T>
>    >     T atomic_fetch_or_explicit(atomic-typeatomic<T>*, T, memory_order);
>    >     template <class T>
>    >     T atomic_fetch_xor(volatile atomic-typeatomic<T>*, T);
>    >     template <class T>
>    >     T atomic_fetch_xor(atomic-typeatomic<T>*, T);
>    >     template <class T>
>    >     T atomic_fetch_xor_explicit(volatile atomic-typeatomic<T>*, T, memory_order);
>    >     template <class T>
>    >     T atomic_fetch_xor_explicit(atomic-typeatomic<T>*, T, memory_order);

>    >     // 29.6.3, arithmetic operations on atomic types
>    >     // In the following declarations, atomic-integral is either
>    >     // atomic<T> or a named base class for T from
>    >     // Table 145 or inferred from
>    >     // Table 146.
>    >     // If it is atomic<T>,
>    >     // then the declaration is a template specialization declaration prefixed with
>    >     // template <>
>    >     template <>
>    >     integral atomic_fetch_add(volatile atomic-integral*, integral);
>    >     template <>
>    >     integral atomic_fetch_add(atomic-integral*, integral);
>    >     template <>
>    >     integral atomic_fetch_add_explicit(volatile atomic-integral*, integral, memory_order);
>    >     template <>
>    >     integral atomic_fetch_add_explicit(atomic-integral*, integral, memory_order);
>    >     template <>
>    >     integral atomic_fetch_sub(volatile atomic-integral*, integral);
>    >     template <>
>    >     integral atomic_fetch_sub(atomic-integral*, integral);
>    >     template <>
>    >     integral atomic_fetch_sub_explicit(volatile atomic-integral*, integral, memory_order);
>    >     template <>
>    >     integral atomic_fetch_sub_explicit(atomic-integral*, integral, memory_order);
>    >     template <>
>    >     integral atomic_fetch_and(volatile atomic-integral*, integral);
>    >     template <>
>    >     integral atomic_fetch_and(atomic-integral*, integral);
>    >     template <>
>    >     integral atomic_fetch_and_explicit(volatile atomic-integral*, integral, memory_order);
>    >     template <>
>    >     integral atomic_fetch_and_explicit(atomic-integral*, integral, memory_order);
>    >     template <>
>    >     integral atomic_fetch_or(volatile atomic-integral*, integral);
>    >     template <>
>    >     integral atomic_fetch_or(atomic-integral*, integral);
>    >     template <>
>    >     integral atomic_fetch_or_explicit(atomic-integral*, integral, memory_order);
>    >     template <>
>    >     integral atomic_fetch_or_explicit(atomic-integral*, integral, memory_order);
>    >     template <>
>    >     integral atomic_fetch_xor(volatile atomic-integral*, integral);
>    >     template <>
>    >     integral atomic_fetch_xor(atomic-integral*, integral);
>    >     template <>
>    >     integral atomic_fetch_xor_explicit(volatile atomic-integral*, integral, memory_order);
>    >     template <>
>    >     integral atomic_fetch_xor_explicit(atomic-integral*, integral, memory_order);

> 2. Edit 29.6.1 [atomics.types.operations.general] paragraph 1+2 as follows:

>    > -1- The implementation shall provide the functions and function templates identified as "general operations on atomic types" in 29.2 [atomics.syn].

>    > -2- In the declarations of these functions and function templates, the name atomic-type refers to either `atomic<T>` or to a named base class for `T` from Table 145 or inferred from Table 146.

> 3. In 29.6.2 [atomics.types.operations.templ] delete paragraph 2:

>    > -1- The implementation shall declare but not define the function templates identified as "templated operations on atomic types" in 29.2 [atomics.syn].

>    > -2- In the declarations of these templates, the name atomic-type refers to either `atomic<T>` or to a named base class for `T` from Table 145 or inferred from Table 146.

> 4. Edit 29.6.3 [atomics.types.operations.arith] paragraph 1+2 as follows:

>    > -1- The implementation shall provide the functions and function template specializations identified as "arithmetic operations on atomic types" in 29.2 [atomics.syn].

>    > -2- In the declarations of these functions and function template specializations, the name integral refers to an integral type and the name atomic-integral refers to either `atomic<`integral`>` or to a named base class for integral from Table 145 or inferred from Table 146.

Proposed resolution:

Resolved 2011-03 Madrid meeting by paper N3278

### 2039. Issues with `std::reverse` and `std::copy_if`

Section: 25.3.1 [alg.copy], 25.3.10 [alg.reverse] Status: C++14 Submitter: Nikolay Ivchenkov Opened: 2011-03-02 Last modified: 2015-04-08

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Discussion:

1. In the description of `std::reverse`

   > Effects: For each non-negative integer `i <= (last - first)/2`, applies `iter_swap` to all pairs of iterators `first + i`, `(last - i) - 1`.

   should be changed to

   > Effects: For each non-negative integer `i < (last - first)/2`, applies `iter_swap` to all pairs of iterators `first + i`, `(last - i) - 1`.

   Here `i` shall be strictly less than `(last - first)/2`.

2. In the description of `std::copy_if` Returns paragraph is missing.

[2011-03-02: Daniel drafts wording]

Proposed resolution:

1. Modify 25.3.10 [alg.reverse] p. 1 as indicated:

   > 1 Effects: For each non-negative integer `i <= (last - first)/2`, applies `iter_swap` to all pairs of iterators `first + i`, `(last - i) - 1`.

2. Add the following Returns element after 25.3.1 [alg.copy] p. 9:

   >     template<class InputIterator, class OutputIterator, class Predicate>
   >     OutputIterator copy_if(InputIterator first, InputIterator last,
   >        OutputIterator result, Predicate pred);

   > > 8 Requires: The ranges `[first,last)` and `[result,result + (last - first))` shall not overlap.

   > > 9 Effects: Copies all of the elements referred to by the iterator `i` in the range `[first,last)` for which `pred(*i)` is true.

   > > ?? Returns: The end of the resulting range.

   > > 10 Complexity: Exactly `last - first` applications of the corresponding predicate.

   > > 11 Remarks: Stable.

### 2041. Stage 2 accumulate incompatibilty

Section: 22.4.2.1.2 [facet.num.get.virtuals] Status: C++11 Submitter: Howard Hinnant Opened: 2011-03-09 Last modified: 2015-04-08

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Discussion:

`num_get` Stage 2 accumulation changed between C++03 and the current C++0x working draft. The sentences:

> If it is not discarded, then a check is made to determine if `c` is allowed as the next character of an input field of the conversion specifier returned by stage 1. If so it is accumulated.

have been dropped from 22.4.2.1.2 [facet.num.get.virtuals], Stage 2, paragraph 3 that begins:

> If `discard` is true, […]

Consider this code:

>     #include <sstream>
>     #include <iostream>

>     int main(void)
>     {
>        std::istringstream s("8cz");
>        long i = 0;
>        char c;
>        s >> i;
>        if (!s.fail())
>            std::cout << "i = " << i << '\n';
>        else
>        {
>            std::cout << "s >> i failed\n";
>            s.clear();
>        }
>        s >> c;
>        if (!s.fail())
>            std::cout << "c = " << c << '\n';
>        else
>            std::cout << "s >> c failed\n";
>     }

C++0x currently prints out:

>     s >> i failed
>     c = z

However C++03 conforming implementations will output:

>     i = 8
>     c = c

I believe we need to restore C++03 compatibility.

Proposed resolution:

Add to 22.4.2.1.2 [facet.num.get.virtuals], Stage 2:

> If `discard` is true, then if `'.'` has not yet been accumulated, then the position of the character is remembered, but the character is otherwise ignored. Otherwise, if `'.'` has already been accumulated, the character is discarded and Stage 2 terminates. If it is not discarded, then a check is made to determine if `c` is allowed as the next character of an input field of the conversion specifier returned by stage 1. If so it is accumulated.

> If the character is either discarded or accumulated then in is advanced by `++in` and processing returns to the beginning of stage 2.

### 2042. Comparing `forward_list::before_begin()` to `forward_list::end()`

Section: 23.3.4.3 [forwardlist.iter] Status: C++11 Submitter: Joe Gottman Opened: 2011-03-13 Last modified: 2015-04-08

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Discussion:

For an object `c` of type `forward_list<X, Alloc>`, the iterators `c.before_begin()` and `c.end()` are part of the same underlying sequence, so the expression `c.before_begin() == c.end()` must be well-defined. But the standard says nothing about what the result of this expression should be. The forward iterator requirements says no dereferenceable iterator is equal to a non-dereferenceable iterator and that two dereferenceable iterators are equal if and only if they point to the same element. But since `before_begin()` and `end()` are both non-dereferenceable, neither of these rules applies.

Many `forward_list` methods, such as `insert_after()`, have a precondition that the iterator passed to them must not be equal to `end()`. Thus, user code might look like the following:

>     void foo(forward_list<int>& c, forward_list<int>::iterator it)
>     {
>       assert(it != c.end());
>       c.insert_after(it, 42);
>     }

Conversely, `before_begin()` was specifically designed to be used with methods like `insert_after()`, so if `c.before_begin()` is passed to this function the assertion must not fail.

[2011-03-14: Daniel comments and updates the suggested wording]

The suggested wording changes are necessary but not sufficient. Since there does not exist an equivalent semantic definition of `cbefore_begin()` as we have for `cbegin()`, this still leaves the question open whether the normative remark applies to `cbefore_begin()` as well. A simple fix is to define the operational semantics of `cbefore_begin()` in terms of `before_begin()`.

[2011-03-24 Madrid meeting]

General agreement that this is a serious bug.

Pablo: Any objections to moving 2042 to Immediate?

No objections.

Proposed resolution:

Add to the definition of `forward_list::before_begin()` 23.3.4.3 [forwardlist.iter] the following:

>     iterator before_begin();
>     const_iterator before_begin() const;
>     const_iterator cbefore_begin() const;

> > -1- Returns: A non-dereferenceable iterator that, when incremented, is equal to the iterator returned by `begin()`.

> > -?- Effects: `cbefore_begin()` is equivalent to `const_cast<forward_list const&>(*this).before_begin()`.

> > -?- Remarks: `before_begin() == end()` shall equal false.

### 2044. No definition of "Stable" for copy algorithms

Section: 17.6.5.7 [algorithm.stable] Status: C++14 Submitter: Pablo Halpern Opened: 2011-03-24 Last modified: 2015-04-08

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Discussion:

17.6.5.7 [algorithm.stable] specified the meaning of "stable" when applied to the different types of algorithms. The second bullet says:

> — For the remove algorithms the relative order of the elements that are not removed is preserved.

There is no description of what "stable" means for copy algorithms, even though the term is applied to `copy_if` (and perhaps others now or in the future). Thus, `copy_if` is using the term without a precise definition.

[Bloomington, 2011]

Move to Ready

Proposed resolution:

This wording is relative to the FDIS.

1. In 17.6.5.7 [algorithm.stable] p. 1 change as indicated:

   > When the requirements for an algorithm state that it is “stable” without further elaboration, it means:

   > - For the sort algorithms the relative order of equivalent elements is preserved.
   > - For the remove and copy algorithms the relative order of the elements that are not removed is preserved.
   > - For the merge algorithms, for equivalent elements in the original two ranges, the elements from the first range precede the elements from the second range.

### 2045. `forward_list::merge` and `forward_list::splice_after` with unequal allocators

Section: 23.3.4.6 [forwardlist.ops] Status: C++14 Submitter: Pablo Halpern Opened: 2011-03-24 Last modified: 2015-04-08

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Discussion:

See also: 1215

`list::merge` and `list::splice` have the requirement that the two lists being merged or spliced must use the same allocator. Otherwise, moving list nodes from one container to the other would corrupt the data structure. The same requirement is needed for `forward_list::merge` and `forward_list::splice_after`.

[ 2011 Bloomington ]

Move to Ready.

Proposed resolution:

This wording is relative to the FDIS.

1. In 23.3.4.6 [forwardlist.ops] p. 1 change as indicated:

   >     void splice_after(const_iterator position, forward_list<T,Allocator>& x);
   >     void splice_after(const_iterator position, forward_list<T,Allocator>&& x);

   > 1 - Requires: `position` is `before_begin()` or is a dereferenceable iterator in the range [`begin()`,`end()`). `get_allocator() == x.get_allocator()`. `&x != this`.

2. In 23.3.4.6 [forwardlist.ops] p. 5 change as indicated:

   >     void splice_after(const_iterator position, forward_list<T,Allocator>& x, const_iterator i);
   >     void splice_after(const_iterator position, forward_list<T,Allocator>&& x, const_iterator i);

   > 5 - Requires: `position` is `before_begin()` or is a dereferenceable iterator in the range [`begin()`,`end()`). The iterator following `i` is a dereferenceable iterator in `x`. `get_allocator() == x.get_allocator()`.

3. In 23.3.4.6 [forwardlist.ops] p. 9 change as indicated:

   >     void splice_after(const_iterator position, forward_list<T,Allocator>& x,
   >                       const_iterator first, const_iterator last);
   >     void splice_after(const_iterator position, forward_list<T,Allocator>&& x,
   >                       const_iterator first, const_iterator last);

   > 9 - Requires: `position` is `before_begin()` or is a dereferenceable iterator in the range [`begin()`,`end()`). (`first`,`last`) is a valid range in `x`, and all iterators in the range (`first`,`last`) are dereferenceable. `position` is not an iterator in the range (`first`,`last`). `get_allocator() == x.get_allocator()`.

4. In 23.3.4.6 [forwardlist.ops] p. 18 change as indicated:

   >     void merge(forward_list<T,Allocator>& x);
   >     void merge(forward_list<T,Allocator>&& x);
   >     template <class Compare> void merge(forward_list<T,Allocator>& x, Compare comp);
   >     template <class Compare> void merge(forward_list<T,Allocator>&& x, Compare comp);

   > 18 - Requires: `comp` defines a strict weak ordering ([alg.sorting]), and `*this` and `x` are both sorted according to this ordering. `get_allocator() == x.get_allocator()`.

### 2047. Incorrect "mixed" move-assignment semantics of `unique_ptr`

Section: 20.8.1.2.3 [unique.ptr.single.asgn] Status: C++14 Submitter: Daniel Krügler Opened: 2011-04-16 Last modified: 2015-04-08

View all other issues in [unique.ptr.single.asgn].

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Discussion:

The semantics described in 20.8.1.2.3 [unique.ptr.single.asgn] p. 6

> Effects: Transfers ownership from `u` to `*this` as if […] followed by an assignment from `std::forward<D>(u.get_deleter())`.

contradicts to the pre-conditions described in p. 4:

> Requires: If E is not a reference type, assignment of the deleter from an rvalue of type `E` shall be well-formed and shall not throw an exception. Otherwise, `E` is a reference type and assignment of the deleter from an lvalue of type `E` shall be well-formed and shall not throw an exception.

Either the pre-conditions are incorrect or the semantics should be an assignment from `std::forward<E>(u.get_deleter())`, instead.

It turns out that this contradiction is due to an incorrect transcription from the proposed resolution of 983 to the finally accepted proposal n3073 (see bullet 12) as confirmed by Howard Hinnant, thus the type argument provided to `std::forward` must be fixed as indicated.

[Bloomington, 2011]

Move to Ready

Proposed resolution:

This wording is relative to the FDIS.

1. Edit 20.8.1.2.3 [unique.ptr.single.asgn] p. 6 as indicated:

   >     template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;

   > > 4 - Requires: If `E` is not a reference type, assignment of the deleter from an rvalue of type `E` shall be well-formed and shall not throw an exception. Otherwise, `E` is a reference type and assignment of the deleter from an lvalue of type `E` shall be well-formed and shall not throw an exception.

   > > 5 - Remarks: This operator shall not participate in overload resolution unless:
   > > - `unique_ptr<U, E>::pointer` is implicitly convertible to `pointer` and
   > > - `U` is not an array type.

   > > 6 - Effects: Transfers ownership from `u` to `*this` as if by calling `reset(u.release())` followed by an assignment from `std::forward<DE>(u.get_deleter())`.

   > > 7 - Returns: `*this`.

### 2048. Unnecessary `mem_fn` overloads

Section: 20.9 [function.objects], 20.9.11 [func.memfn] Status: C++14 Submitter: Jonathan Wakely Opened: 2011-04-18 Last modified: 2015-04-08

View all other issues in [function.objects].

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Discussion:

The `mem_fn` overloads for member functions are redundant and misleading and should be removed from the post-C++11 WP.

I believe the history of the overloads is as follows:

In TR1 and in C++0x prior to the N2798 draft, `mem_fn` was specified by a single signature:

>     template<class R, class T> unspecified mem_fn(R T::* pm);

and was accompanied by the remark "Implementations may implement `mem_fn` as a set of overloaded function templates." This remark predates variadic templates and was presumably to allow implementations to provide overloads for a limited number of function parameters, to meet the implementation-defined limit on numbers of template parameters.

N2770 "Concepts for the C++0x Standard Library: Utilities" added separate overloads for pointers to member functions, apparently so that function parameters would require the `CopyConstructible` concept (those overloads first appeared in N2322.) The overloads failed to account for varargs member functions (i.e. those declared with an ellipsis in the parameter-declaration-clause) e.g.

>     struct S {
>      int f(int, ...);
>     };

Syntactically such a function would be handled by the original `mem_fn(R T::* pm)` signature, the only minor drawback being that there would be no `CopyConstructible` requirement on the parameter list. (Core DR 547 clarifies that partial specializations can be written to match cv-qualified and ref-qualified functions to support the case where `R T::*` matches a pointer to member function type.)

LWG issue 920 pointed out that additional overloads were missing for member functions with ref-qualifiers. These were not strictly necessary, because such functions are covered by the `mem_fn(R T::* pm)` signature.

Concepts were removed from the draft and N3000 restored the original single signature and accompanying remark.

LWG 1230 was opened to strike the remark again and to add an overload for member functions (this overload was unnecessary for syntactic reasons and insufficient as it didn't handle member functions with cv-qualifiers and/or ref-qualifiers.)

920 (and 1230) were resolved by restoring a full set of (non-concept-enabled) overloads for member functions with cv-qualifiers and ref-qualifiers, but as in the concept-enabled draft there were no overloads for member functions with an ellipsis in the parameter-declaration-clause. This is what is present in the FDIS.

Following the thread beginning with message c++std-lib-30675, it is my understanding that all the `mem_fn` overloads for member functions are unnecessary and were only ever added to allow concept requirements. I'm not aware of any reason implementations cannot implement `mem_fn` as a single function template. Without concepts the overloads are redundant, and the absence of overloads for varargs functions can be interpreted to imply that varargs functions are not intended to work with `mem_fn`. Clarifying the intent by adding overloads for varargs functions would expand the list of 12 redundant overloads to 24, it would be much simpler to remove the 12 redundant overloads entirely.

[Bloomington, 2011]

Move to Review.

The issue and resolution appear to be correct, but there is some concern that the wording of INVOKE may be different depending on whether you pass a pointer-to-member-data or pointer-to-member-function. That might make the current wording necessary after all, and then we might need to add the missing elipsis overloads.

There was some concern that the Remark confirming implementors had freedom to implement this as a set of overloaded functions may need to be restored if we delete the specification for these overloads.

[2012, Kona]

Moved to Tentatively Ready by the post-Kona issues processing subgroup.

[2012, Portland: applied to WP]

Proposed resolution:

This wording is relative to the FDIS.

1. Change the `<functional>` synopsis 20.9 [function.objects] p. 2 as follows:

   >     namespace std {
   >       […]
   >       // [func.memfn], member function adaptors:
   >       template<class R, class T> unspecified mem_fn(R T::*);
   >       template<class R, class T, class... Args>
   >       unspecified mem_fn(R (T::*)(Args...));
   >       template<class R, class T, class... Args>
   >       unspecified mem_fn(R (T::*)(Args...) const);
   >       template<class R, class T, class... Args>
   >       unspecified mem_fn(R (T::*)(Args...) volatile);
   >       template<class R, class T, class... Args>
   >       unspecified mem_fn(R (T::*)(Args...) const volatile);
   >       template<class R, class T, class... Args>
   >       unspecified mem_fn(R (T::*)(Args...) &);
   >       template<class R, class T, class... Args>
   >       unspecified mem_fn(R (T::*)(Args...) const &);
   >       template<class R, class T, class... Args>
   >       unspecified mem_fn(R (T::*)(Args...) volatile &);
   >       template<class R, class T, class... Args>
   >       unspecified mem_fn(R (T::*)(Args...) const volatile &);
   >       template<class R, class T, class... Args>
   >       unspecified mem_fn(R (T::*)(Args...) &&);
   >       template<class R, class T, class... Args>
   >       unspecified mem_fn(R (T::*)(Args...) const &&);
   >       template<class R, class T, class... Args>
   >       unspecified mem_fn(R (T::*)(Args...) volatile &&);
   >       template<class R, class T, class... Args>
   >       unspecified mem_fn(R (T::*)(Args...) const volatile &&);

   >       […]
   >     }

2. Change 20.9.11 [func.memfn] as follows:

   >     template<class R, class T> unspecified mem_fn(R T::*);
   >     template<class R, class T, class... Args>
   >     unspecified mem_fn(R (T::*)(Args...));
   >     template<class R, class T, class... Args>
   >     unspecified mem_fn(R (T::*)(Args...) const);
   >     template<class R, class T, class... Args>
   >     unspecified mem_fn(R (T::*)(Args...) volatile);
   >     template<class R, class T, class... Args>
   >     unspecified mem_fn(R (T::*)(Args...) const volatile);
   >     template<class R, class T, class... Args>
   >     unspecified mem_fn(R (T::*)(Args...) &);
   >     template<class R, class T, class... Args>
   >     unspecified mem_fn(R (T::*)(Args...) const &);
   >     template<class R, class T, class... Args>
   >     unspecified mem_fn(R (T::*)(Args...) volatile &);
   >     template<class R, class T, class... Args>
   >     unspecified mem_fn(R (T::*)(Args...) const volatile &);
   >     template<class R, class T, class... Args>
   >     unspecified mem_fn(R (T::*)(Args...) &&);
   >     template<class R, class T, class... Args>
   >     unspecified mem_fn(R (T::*)(Args...) const &&);
   >     template<class R, class T, class... Args>
   >     unspecified mem_fn(R (T::*)(Args...) volatile &&);
   >     template<class R, class T, class... Args>
   >     unspecified mem_fn(R (T::*)(Args...) const volatile &&);

### 2049. `is_destructible` is underspecified

Section: 20.10.4.3 [meta.unary.prop] Status: C++14 Submitter: Daniel Krügler Opened: 2011-04-18 Last modified: 2015-04-08

View other active issues in [meta.unary.prop].

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Discussion:

The conditions for the type trait `is_destructible` to be true are described in Table 49 — Type property predicates:

> For a complete type `T` and given
> `template <class U> struct test { U u; };`,
> `test<T>::~test()` is not deleted.

This specification does not say what the result would be for function types or for abstract types:

1. For an abstract type `X` the instantiation `test<X>` is already ill-formed, so we cannot say anything about whether the destructor would be deleted or not.
2. In regard to function types, there exists a special rule in the core language, 14.3.1 [temp.arg.type] p. 3, which excludes member functions to be declared via the type of the template parameter:

   > If a declaration acquires a function type through a type dependent on a template-parameter and this causes a declaration that does not use the syntactic form of a function declarator to have function type, the program is ill-formed.

   > [ Example:

   > >     template<class T> struct A {
   > >       static T t;
   > >     };
   > >     typedef int function();
   > >     A<function> a; // ill-formed: would declare A<function>::t
   > >                    // as a static member function

   > — end example ]

   which has the same consequence as for abstract types, namely that the corresponding instantiation of `test` is already ill-formed and we cannot say anything about the destructor.

To solve this problem, I suggest to specify function types as trivially and nothrowing destructible, because above mentioned rule is very special for templates. For non-templates, a typedef can be used to introduce a member as member function as clarified in 8.3.5 [dcl.fct] p. 10.

For abstract types, two different suggestions have been brought to my attention: Either declare them as unconditionally non-destructible or check whether the expression

>     std::declval<T&>().~T()

is well-formed in an unevaluated context. The first solution is very easy to specify, but the second version has the advantage for providing more information to user-code. This information could be quite useful, if generic code is supposed to invoke the destructor of a reference to a base class indirectly via a delete expression, as suggested by Howard Hinnant:

>     template <class T>
>     my_pointer<T>::~my_pointer() noexcept(is_nothrow_destructible<T>::value)
>     {
>        delete ptr_;
>     }

Additional to the `is_destructible` traits, its derived forms `is_trivially_destructible` and `is_nothrow_destructible` are similarly affected, because their wording refers to "the indicated destructor" and probably need to be adapted as well.

[ 2011 Bloomington ]

After discussion about to to handle the exceptional cases of reference types, function types (available by defererencing a function pointer) and `void` types, Howard supplied proposed wording.

[ 2011-08-20 Daniel comments and provides alternatives wording ]

The currently proposed wording would have the consequence that every array type is not destructible, because the pseudo-destructor requires a scalar type with the effect that the expression

>     std::declval<T&>().~T()

is not well-formed for e.g. `T` equal to `int[3]`. The intuitive solution to fix this problem would be to adapt the object type case to refer to the expression

>     std::declval<U&>().~U()

with `U` equal to `remove_all_extents<T>::type`, but that would have the effect that arrays of unknown bounds would be destructible, if the element type is destructible, which was not the case before (This was intentionally covered by the special "For a complete type T" rule in the FDIS).

Suggestion: Use the following definition instead:

> Let `U` be `remove_all_extents<T>::type`.
> For incomplete types and function types, `is_destructible<T>::value` is `false`.
> For object types, if the expression `std::declval<U&>().~U()` is well-formed
> when treated as an unevaluated operand (Clause 5), then `is_destructible<T>::value`
> is `true`, otherwise it is `false`.
> For reference types, `is_destructible<T>::value` is `true`.

This wording also harmonizes with the "unevaluated operand" phrase used in other places, there does not exist the definition of an "unevaluated context"

Note: In the actually proposed wording this wording has been slightly reordered with the same effects.

Howard's (old) proposed resolution:

> Update 20.10.4.3 [meta.unary.prop], table 49:

> `template <class T> struct is_destructible;`

> For a complete type `T` and given `template <class U> struct test { U u; };`, `test<T>::~test()` is not deleted.
> For object types, if the expression: `std::declval<T&>().~T()` is well-formed in an unevaluated context then `is_destructible<T>::value` is `true`, otherwise it is `false`.
> For `void` types, `is_destructible<T>::value` is `false`.
> For reference types, `is_destructible<T>::value` is `true`.
> For function types, `is_destructible<T>::value` is `false`.

> `T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown bound.

[2012, Kona]

Moved to Tentatively Ready by the post-Kona issues processing subgroup.

[2012, Portland: applied to WP]

Proposed resolution:

Update 20.10.4.3 [meta.unary.prop], table 49:

`template <class T> struct is_destructible;`

For a complete type `T` and given `template <class U> struct test { U u; };`, `test<T>::~test()` is not deleted.
For reference types, `is_destructible<T>::value` is `true`.
For incomplete types and function types, `is_destructible<T>::value` is `false`.
For object types and given `U` equal to `remove_all_extents<T>::type`,
if the expression `std::declval<U&>().~U()` is well-formed when treated as an
unevaluated operand (Clause 5 [expr]), then `is_destructible<T>::value` is `true`,
otherwise it is `false`.

`T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown bound.

### 2050. Unordered associative containers do not use `allocator_traits` to define member types

Section: 23.5 [unord] Status: C++14 Submitter: Tom Zieberman Opened: 2011-04-29 Last modified: 2015-04-08

View all other issues in [unord].

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Discussion:

The unordered associative containers define their member types `reference`, `const_reference`, `pointer`, `const_pointer` in terms of their template parameter `Allocator` (via `allocator_type` typedef). As a consequence, only the allocator types, that provide sufficient typedefs, are usable as allocators for unordered associative containers, while other containers do not have this deficiency. In addition to that, the definitions of said typedefs are different from ones used in the other containers. This is counterintuitive and introduces a certain level of confusion. These issues can be fixed by defining `pointer` and `const_pointer` typedefs in terms of `allocator_traits<Allocator>` and by defining `reference` and `const_reference` in terms of `value_type` as is done in the other containers.

[ 2011 Bloomington. ]

Move to Ready.

Proposed resolution:

This wording is relative to the FDIS.

- Change 23.5.4.1 [unord.map.overview] paragraph 3 as indicated:

  >     namespace std {
  >       template <class Key,
  >                 class T,
  >                 class Hash = hash<Key>,
  >                 class Pred = std::equal_to<Key>,
  >                 class Allocator = std::allocator<std::pair<const Key, T> > >
  >       class unordered_map
  >       {
  >       public:
  >         // types
  >         typedef Key key_type;
  >         typedef std::pair<const Key, T> value_type;
  >         typedef T mapped_type;
  >         typedef Hash hasher;
  >         typedef Pred key_equal;
  >         typedef Allocator allocator_type;
  >         typedef typename allocator_typeallocator_traits<Allocator>::pointer pointer;
  >         typedef typename allocator_typeallocator_traits<Allocator>::const_pointer const_pointer;
  >         typedef typename allocator_type::referencevalue_type& reference;
  >         typedef typename allocator_type::const_referenceconst value_type& const_reference;
  >         typedef implementation-defined size_type;
  >         typedef implementation-defined difference_type;

  >         […]
  >       };
  >     }

- Change 23.5.5.1 [unord.multimap.overview] paragraph 3 as indicated:

  >     namespace std {
  >       template <class Key,
  >                 class T,
  >                 class Hash = hash<Key>,
  >                 class Pred = std::equal_to<Key>,
  >                 class Allocator = std::allocator<std::pair<const Key, T> > >
  >       class unordered_multimap
  >       {
  >       public:
  >         // types
  >         typedef Key key_type;
  >         typedef std::pair<const Key, T> value_type;
  >         typedef T mapped_type;
  >         typedef Hash hasher;
  >         typedef Pred key_equal;
  >         typedef Allocator allocator_type;
  >         typedef typename allocator_typeallocator_traits<Allocator>::pointer pointer;
  >         typedef typename allocator_typeallocator_traits<Allocator>::const_pointer const_pointer;
  >         typedef typename allocator_type::referencevalue_type& reference;
  >         typedef typename allocator_type::const_referenceconst value_type& const_reference;
  >         typedef implementation-defined size_type;
  >         typedef implementation-defined difference_type;

  >         […]
  >       };
  >     }

- Change 23.5.6.1 [unord.set.overview] paragraph 3 as indicated:

  >     namespace std {
  >       template <class Key,
  >                 class Hash = hash<Key>,
  >                 class Pred = std::equal_to<Key>,
  >                 class Allocator = std::allocator<Key> >
  >       class unordered_set
  >       {
  >       public:
  >         // types
  >         typedef Key key_type;
  >         typedef Key value_type;
  >         typedef Hash hasher;
  >         typedef Pred key_equal;
  >         typedef Allocator allocator_type;
  >         typedef typename allocator_typeallocator_traits<Allocator>::pointer pointer;
  >         typedef typename allocator_typeallocator_traits<Allocator>::const_pointer const_pointer;
  >         typedef typename allocator_type::referencevalue_type& reference;
  >         typedef typename allocator_type::const_referenceconst value_type& const_reference;
  >         typedef implementation-defined size_type;
  >         typedef implementation-defined difference_type;

  >         […]
  >       };
  >     }

- Change 23.5.7.1 [unord.multiset.overview] paragraph 3 as indicated:

  >     namespace std {
  >       template <class Key,
  >                 class Hash = hash<Key>,
  >                 class Pred = std::equal_to<Key>,
  >                 class Allocator = std::allocator<Key> >
  >       class unordered_multiset
  >       {
  >       public:
  >         // types
  >         typedef Key key_type;
  >         typedef Key value_type;
  >         typedef Hash hasher;
  >         typedef Pred key_equal;
  >         typedef Allocator allocator_type;
  >         typedef typename allocator_typeallocator_traits<Allocator>::pointer pointer;
  >         typedef typename allocator_typeallocator_traits<Allocator>::const_pointer const_pointer;
  >         typedef typename allocator_type::referencevalue_type& reference;
  >         typedef typename allocator_type::const_referenceconst value_type& const_reference;
  >         typedef implementation-defined size_type;
  >         typedef implementation-defined difference_type;

  >         […]
  >       };
  >     }

### 2052. Mixup between `mapped_type` and `value_type` for associative containers

Section: 23.2.4 [associative.reqmts] Status: Resolved Submitter: Marc Glisse Opened: 2011-05-04 Last modified: 2015-04-08

View other active issues in [associative.reqmts].

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Discussion:

(this is basically reopening the first part of issue 2006, as discussed in the thread starting at c++std-lib-30698 )

Section 23.2.4 [associative.reqmts]

In Table 102, several uses of `T` (which means `mapped_type` here) should be `value_type` instead. This is almost editorial. For instance:

>     a_uniq.emplace(args)

> Requires: `T` shall be `EmplaceConstructible` into `X` from args.

> Effects: Inserts a `T` object `t` constructed with `std::forward<Args>(args)...` if and only if there is no element in the container with key equivalent to the key of `t`. The `bool` component of the returned pair is true if and only if the insertion takes place, and the iterator component of the pair points to the element with key equivalent to the key of `t`.

[ 2011 Bloomington ]

Not even an exhaustive list of problem locations. No reason to doubt issue.

Pablo agrees to provide wording.

[ 2011-09-04 Pablo Halpern provides improved wording ]

[2014-02-15 post-Issaquah session : move to Resolved]

AJM to replace this note with a 'Resolved By...' after tracking down the exact sequence of events, but clearly resolved in C++14 DIS.

Proposed resolution:

In both section 23.2.4 [associative.reqmts] Table 102 and 23.2.5 [unord.req], Table 103, make the following text replacements:

Original text, in FDIS | Replacement text
`T` is CopyInsertable into `X` and `CopyAssignable`. | `value_type` is `CopyInsertable` into `X`, `key_type` is `CopyAssignable`, and `mapped_type` is `CopyAssignable` (for containers having a `mapped_type`)
`T` is `CopyInsertable` | `value_type` is CopyInsertable
`T` shall be `CopyInsertable` | `value_type` shall be CopyInsertable
`T` shall be `MoveInsertable` | `value_type` shall be MoveInsertable
`T` shall be `EmplaceConstructible` | `value_type` shall be EmplaceConstructible
`T` object | `value_type` object

[ Notes to the editor: The above are carefully selected phrases that can be used for global search-and-replace within the specified sections without accidentally making changes to correct uses `T`. ]

### 2053. Errors in `regex` bitmask types

Section: 28.5 [re.const] Status: C++14 Submitter: Jonathan Wakely Opened: 2011-05-09 Last modified: 2015-04-08

View all issues with C++14 status.

Discussion:

When N3110 was applied to the WP some redundant "static" keywords were added and one form of initializer which isn't valid for enumeration types was replaced with another form of invalid initializer.

[ 2011 Bloomington. ]

Move to Ready.

Proposed resolution:

This wording is relative to the FDIS.

1. Change 28.5.1 [re.synopt] as indicated:

   >     namespace std {
   >       namespace regex_constants {
   >         typedef T1 syntax_option_type;
   >         static constexpr syntax_option_type icase = unspecified ;
   >         static constexpr syntax_option_type nosubs = unspecified ;
   >         static constexpr syntax_option_type optimize = unspecified ;
   >         static constexpr syntax_option_type collate = unspecified ;
   >         static constexpr syntax_option_type ECMAScript = unspecified ;
   >         static constexpr syntax_option_type basic = unspecified ;
   >         static constexpr syntax_option_type extended = unspecified ;
   >         static constexpr syntax_option_type awk = unspecified ;
   >         static constexpr syntax_option_type grep = unspecified ;
   >         static constexpr syntax_option_type egrep = unspecified ;
   >       }
   >     }

2. Change 28.5.2 [re.matchflag] as indicated:

   >     namespace std {
   >       namespace regex_constants {
   >         typedef T2 match_flag_type;
   >         static constexpr match_flag_type match_default = 0{};
   >         static constexpr match_flag_type match_not_bol = unspecified ;
   >         static constexpr match_flag_type match_not_eol = unspecified ;
   >         static constexpr match_flag_type match_not_bow = unspecified ;
   >         static constexpr match_flag_type match_not_eow = unspecified ;
   >         static constexpr match_flag_type match_any = unspecified ;
   >         static constexpr match_flag_type match_not_null = unspecified ;
   >         static constexpr match_flag_type match_continuous = unspecified ;
   >         static constexpr match_flag_type match_prev_avail = unspecified ;
   >         static constexpr match_flag_type format_default = 0{};
   >         static constexpr match_flag_type format_sed = unspecified ;
   >         static constexpr match_flag_type format_no_copy = unspecified ;
   >         static constexpr match_flag_type format_first_only = unspecified ;
   >       }
   >     }

3. Change 28.5.3 [re.err] as indicated:

   >     namespace std {
   >       namespace regex_constants {
   >         typedef T3 error_type;
   >         static constexpr error_type error_collate = unspecified ;
   >         static constexpr error_type error_ctype = unspecified ;
   >         static constexpr error_type error_escape = unspecified ;
   >         static constexpr error_type error_backref = unspecified ;
   >         static constexpr error_type error_brack = unspecified ;
   >         static constexpr error_type error_paren = unspecified ;
   >         static constexpr error_type error_brace = unspecified ;
   >         static constexpr error_type error_badbrace = unspecified ;
   >         static constexpr error_type error_range = unspecified ;
   >         static constexpr error_type error_space = unspecified ;
   >         static constexpr error_type error_badrepeat = unspecified ;
   >         static constexpr error_type error_complexity = unspecified ;
   >         static constexpr error_type error_stack = unspecified ;
   >       }
   >     }

### 2054. `time_point` constructors need to be `constexpr`

Section: 20.12.6 [time.point] Status: Resolved Submitter: Anthony Williams Opened: 2011-05-13 Last modified: 2015-04-08

View all issues with Resolved status.

Discussion:

In 20.12.6 [time.point], `time_point::min()` and `time_point::max()` are listed as `constexpr`. However, `time_point` has no `constexpr` constructors, so is not a literal type, and so these functions cannot be `constexpr` without adding a `constexpr` constructor for implementation purposes.

Proposed resolution: Add `constexpr` to the constructors of `time_point`. The effects of the constructor template basically imply that the member function `time_since_epoch()` is intended to be `constexpr` as well.

[2012, Portland]

Resolved by adopting paper n3469.

Proposed resolution:

This wording is relative to the FDIS.

1. Alter the class template definition in 20.12.6 [time.point] as follows:

   >     template <class Clock, class Duration = typename Clock::duration>
   >     class time_point {
   >       […]
   >     public:
   >       // 20.11.6.1, construct:
   >       constexpr time_point(); // has value epoch
   >       constexpr explicit time_point(const duration& d); // same as time_point() + d
   >       template <class Duration2>
   >         constexpr time_point(const time_point<clock, Duration2>& t);

   >       // 20.11.6.2, observer:
   >       constexpr duration time_since_epoch() const;
   >       […]
   >     };

2. Alter the declarations in 20.12.6.1 [time.point.cons]:

   >     constexpr time_point();

   > > -1- Effects: Constructs an object of type `time_point`, initializing `d_` with `duration::zero()`. Such a `time_point` object represents the epoch.

   >     constexpr explicit time_point(const duration& d);

   > > -2- Effects: Constructs an object of type `time_point`, initializing `d_` with `d`. Such a `time_point` object represents the epoch `+ d`.

   >     template <class Duration2>
   >       constexpr time_point(const time_point<clock, Duration2>& t);

   > > -3- Remarks: This constructor shall not participate in overload resolution unless `Duration2` is implicitly convertible to `duration`.

   > > -4- Effects: Constructs an object of type `time_point`, initializing `d_` with `t.time_since_epoch()`.

3. Alter the declaration in 20.12.6.2 [time.point.observer]:

   >     constexpr duration time_since_epoch() const;

   > > -1- Returns: d_.

### 2056. `future_errc` enums start with value 0 (invalid value for `broken_promise`)

Section: 30.6.1 [futures.overview] Status: C++14 Submitter: Nicolai Josuttis Opened: 2011-05-18 Last modified: 2015-04-08

View all other issues in [futures.overview].

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Discussion:

In 30.6.1 [futures.overview] `enum class future_errc` is defined as follows:

>     enum class future_errc {
>       broken_promise,
>       future_already_retrieved,
>       promise_already_satisfied,
>       no_state
>     };

With this declaration `broken_promise` has value 0, which means that for a `future_error f` with this code

>     f.code().operator bool()

yields false, which makes no sense. 0 has to be reserved for "no error". So, the enums defined here have to start with 1.

Howard, Anthony, and Jonathan have no objections.

[Discussion in Bloomington 2011-08-16]

Previous resolution:

> This wording is relative to the FDIS.

> 1. In 30.6.1 [futures.overview], header `<future>` synopsis, fix the declaration of `future_errc` as follows:

>    >     namespace std {
>    >       enum class future_errc {
>    >         broken_promise,
>    >         future_already_retrieved = 1,
>    >         promise_already_satisfied,
>    >         no_state,
>    >         broken_promise
>    >       };
>    >       […]
>    >     }

Is this resolution overspecified? These seem to be all implementation-defined. How do users add new values and not conflict with established error codes?

PJP proxy says: over-specified. boo.

Other error codes: look for `is_error_code_enum` specializations. Only one exists `io_errc`

Peter: I don't see any other parts of the standard that specify error codes where we have to do something similar.

Suggest that for every place where we add an error code, the following:

1. no zero values
2. all implementation defined values, so future_already_retrieved = implementation_defined
3. values are distinct

[2012, Kona]

Moved to Tentatively Ready by the post-Kona issues processing subgroup.

[2012, Portland: applied to WP]

Proposed resolution:

This wording is relative to the FDIS.

In 30.6.1 [futures.overview], header `<future>` synopsis, fix the declaration of `future_errc` as follows:

>     namespace std {
>       enum class future_errc {
>         broken_promise = implementation defined,
>         future_already_retrieved = implementation defined,
>         promise_already_satisfied = implementation defined,
>         no_state = implementation defined
>       };
>       […]
>     }

In 30.6.1 [futures.overview], header `<future>` synopsis, add a paragraph after paragraph 2 as follows:

The enum values of `future_errc` are distinct and not zero.

### 2057. `time_point + duration` semantics should be made `constexpr` conforming

Section: 20.12.6.5 [time.point.nonmember] Status: Resolved Submitter: Daniel Krügler Opened: 2011-05-21 Last modified: 2015-04-08

View all issues with Resolved status.

Discussion:

It has been observed by LWG 2054 that the specification of some `time_point` member functions already imply that `time_point` needs to be a literal type and suggests to specify the constructors and the member function `time_since_epoch()` as `constexpr` functions at the minimum necessary. Adding further `constexpr` specifier to other operations should clearly be allowed and should probably be done as well. But to allow for further `constexpr` functions in the future requires that their semantics is compatible to operations allowed in `constexpr` functions. This is already fine for all operations, except this binary plus operator:

>     template <class Clock, class Duration1, class Rep2, class Period2>
>     time_point<Clock, typename common_type<Duration1, duration<Rep2, Period2>>::type>
>     operator+(const time_point<Clock, Duration1>& lhs, const duration<Rep2, Period2>& rhs);

> > -1- Returns: `CT(lhs) += rhs`, where `CT` is the type of the return value.

for similar reasons as those mentioned in 2020. The semantics should be fixed to allow for making them `constexpr`. This issue should also be considered as a placeholder for a request to make the remaining `time_point` operations similarly `constexpr` as had been done for `duration`.

[2012, Portland]

Resolved by adopting paper n3469.

Proposed resolution:

This wording is relative to the FDIS.

1. In 20.12.6.5 [time.point.nonmember], p.1 change the Returns element semantics as indicated:

   >     template <class Clock, class Duration1, class Rep2, class Period2>
   >     time_point<Clock, typename common_type<Duration1, duration<Rep2, Period2>>::type>
   >     operator+(const time_point<Clock, Duration1>& lhs, const duration<Rep2, Period2>& rhs);

   > > -1- Returns: `CT(lhs) += rhsCT(lhs.time_since_epoch() + rhs)`, where `CT` is the type of the return value.

### 2058. `valarray` and `begin/end`

Section: 26.6 [numarray] Status: C++14 Submitter: Gabriel Dos Reis Opened: 2011-05-17 Last modified: 2015-04-08

View all other issues in [numarray].

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Discussion:

It was just brought to my attention that the pair of functions `begin/end` were added to `valarray` component. Those additions strike me as counter to the long standing agreement that `valarray<T>` is not yet another container. Valarray values are in general supposed to be treated as a whole, and as such has a loose specification allowing expression template techniques.

The addition of these functions sound to me as making it much harder (or close to impossible) to effectively use expression templates as implementation techniques, for no clear benefits.

My recommendation would be to drop `begin/end` - or at least for the `const valarray<T>&` version. I strongly believe those are defects.

[This issue was discussed on the library reflector starting from c++std-lib-30761. Some of the key conclusions of this discussion were:]

1. The `begin/end` members were added to allow `valarray` to participate in the new range-based for-loop by n2930 and not to make them container-like.
2. It is currently underspecified when the iterator values returned from `begin/end` become invalidated. To fix this, these invalidation rules need at least to reflect the invalidation rules of the references returned by the `operator[]` overloads of `valarray` (26.6.2.4 [valarray.access]).
3. A further problem is that the requirements expressed in 26.6.1 [valarray.syn] p.3-5 enforce an implementation to provide further overloads of `begin/end`, if the replacement type technique is used (which was clearly part of the design of `valarray`). Providing such additional overloads would also lead to life-time problems in examples like `begin(x + y)` where `x` and `y` are expressions involving `valarray` objects. To fix this, the `begin/end` overloads could be explicitly excluded from the general statements of 26.6.1 [valarray.syn] p.3-5. This would make it unspecified whether the expression `begin(x + y)` would be well-formed, portable code would need to write this as `begin(std::valarray<T>(x + y))`.

[ 2011 Bloomington ]

The intent of these overloads is entirely to support the new for syntax, and not to create new containers.

Stefanus provides suggested wording.

[2012, Kona]

Moved to Tenatively Ready by post-meeting issues processing group, after confirmation from Gaby.

[2012, Portland: applied to WP]

Proposed resolution:

In 26.6.1 [valarray.syn]/4, make the following insertion:

4 Implementations introducing such replacement types shall provide additional functions and operators as follows:

- for every function taking a `const valarray<T>&` other than `begin` and `end` (26.6.10 [valarray.range]), identical functions taking the replacement types shall be added;
- for every function taking two `const valarray<T>&` arguments, identical functions taking every combination of `const valarray<T>&` and replacement types shall be added.

In 26.6.10 [valarray.range], make the following insertion:

1 In the `begin` and `end` function templates that follow, unspecified1 is a type that meets the requirements of a mutable random access iterator (24.2.7) whose `value_type` is the template parameter `T` and whose `reference` type is `T&`. unspecified2 is a type that meets the requirements of a constant random access iterator (24.2.7) whose `value_type` is the template parameter `T` and whose `reference` type is `const T&`.

2 The iterators returned by `begin` and `end` for an array are guaranteed to be valid until the member function `resize(size_t, T)` (26.6.2.8 [valarray.members]) is called for that array or until the lifetime of that array ends, whichever happens first.

### 2059. C++0x ambiguity problem with `map::erase`

Section: 23.4.4 [map] Status: WP Submitter: Christopher Jefferson Opened: 2011-05-18 Last modified: 2015-05-22

View all other issues in [map].

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Discussion:

`map::erase` (and several related methods) took an iterator in C++03, but take a `const_iterator` in C++0x. This breaks code where the map's `key_type` has a constructor which accepts an iterator (for example a template constructor), as the compiler cannot choose between `erase(const key_type&)` and `erase(const_iterator)`.

>     #include <map>

>     struct X
>     {
>       template<typename T>
>       X(T&) {}
>     };

>     bool operator<(const X&, const X&) { return false; }

>     void erasor(std::map<X,int>& s, X x)
>     {
>       std::map<X,int>::iterator it = s.find(x);
>       if (it != s.end())
>         s.erase(it);
>     }

[ 2011 Bloomington ]

This issue affects only associative container `erase` calls, and is not more general, as these are the only functions that are also overloaded on another single arguement that might cause confusion - the `erase` by key method. The complete resolution should simply restore the `iterator` overload in addition to the `const_iterator` overload for all eight associative containers.

Proposed wording supplied by Alan Talbot, and moved to Review.

[2012, Kona]

Moved back to Open by post-meeting issues processing group.

Pablo very unhappy about case of breaking code with ambiguous conversion between both iterator types.

Alisdair strongly in favor of proposed resolution, this change from C++11 bit Chris in real code, and it took a while to track down the cause.

Move to open, bring in front of a larger group

Proposed wording from Jeremiah: `erase(key)` shall not participate in overload resolution if `iterator` is convertible to `key`. Note that this means making `erase(key)` a template-method

Poll Chris to find out if he already fixed his code, or fixed his library

Jeremiah - allow both overloads, but `enable_if` the `const_iterator` form as a template, requiring `is_same` to match only `const_iterator`.

Poll PJ to see if he has already applied this fix?

[2015-02 Cologne]

AM: To summarize, we changed a signature and code broke. At what point do we stop and accept breakage in increasingly obscure code? VV: libc++ is still broken, but libstdc++ works, so they've fixed this — perhaps using this PR? [Checks] Yes, libstdc++ uses this solution, and has a comment pointing to LWG 2059. AM: This issue hasn't been looked at since Kona. In any case, we already have implementation experience now.

AM: I'd say let's ship it. We already have implementation experience (libstdc++ and MSVS). MC: And "tentatively ready" lets me try to implement this and see how it works.

Proposed resolution:

Editorial note: The following things are different between 23.2.4 [associative.reqmts] p.8 and 23.2.5 [unord.req] p.10. These should probably be reconciled.

> 1. First uses the convention "denotes"; second uses the convention "is".
> 2. First redundantly says: "If no such element exists, returns a.end()." in erase table entry, second does not.

23.2.4 [associative.reqmts] Associative containers

8 In Table 102, `X` denotes an associative container class, `a` denotes a value of `X`, `a_uniq` denotes a value of `X` when `X` supports unique keys, `a_eq` denotes a value of `X` when `X` supports multiple keys, `u` denotes an identifier, `i` and `j` satisfy input iterator requirements and refer to elements implicitly convertible to `value_type`, `[i,j)` denotes a valid range, `p` denotes a valid const iterator to `a`, `q` denotes a valid dereferenceable const iterator to `a`, `r` denotes a valid dereferenceable iterator to a, `[q1, q2)` denotes a valid range of const iterators in `a`, `il` designates an object of type `initializer_list<value_type>`, `t` denotes a value of `X::value_type`, `k` denotes a value of `X::key_type` and `c` denotes a value of type `X::key_compare`. `A` denotes the storage allocator used by `X`, if any, or `std::allocator<X::value_type>` otherwise, and `m` denotes an allocator of a type convertible to `A`.

23.2.4 [associative.reqmts] Associative containers Table 102

Add row:

`a.erase(r)` | `iterator` | erases the element pointed to by `r`. Returns an iterator pointing to the element immediately following `r` prior to the element being erased. If no such element exists, returns `a.end()`. | amortized constant

23.2.5 [unord.req] Unordered associative containers

10 In table 103: `X` is an unordered associative container class, `a` is an object of type `X`, `b` is a possibly const object of type `X`, `a_uniq` is an object of type `X` when `X` supports unique keys, `a_eq` is an object of type `X` when `X` supports equivalent keys, `i` and `j` are input iterators that refer to `value_type`, `[i, j)` is a valid range, `p` and `q2` are valid const iterators to `a`, `q` and `q1` are valid dereferenceable const iterators to `a`, `r` is a valid dereferenceable iterator to a, `[q1,q2)` is a valid range in `a`, `il` designates an object of type `initializer_list<value_type>`, `t` is a value of type `X::value_type`, `k` is a value of type `key_type`, `hf` is a possibly const value of type `hasher`, `eq` is a possibly const value of type `key_equal`, `n` is a value of type `size_type`, and `z` is a value of type `float`.

23.2.5 [unord.req] Unordered associative containers Table 103

Add row:

`a.erase(r)` | `iterator` | Erases the element pointed to by `r`. Returns the iterator immediately following `r` prior to the erasure. | Average case O(1), worst case O(`a.size()`).

23.4.4.1 [map.overview] Class template map overview p. 2

    iterator erase(iterator position);
    iterator erase(const_iterator position);
    size_type erase(const key_type& x);
    iterator erase(const_iterator first, const_iterator last);

23.4.5.1 [multimap.overview] Class template multimap overview p. 2

    iterator erase(iterator position);
    iterator erase(const_iterator position);
    size_type erase(const key_type& x);
    iterator erase(const_iterator first, const_iterator last);

23.4.6.1 [set.overview] Class template set overview p. 2

    iterator erase(iterator position);
    iterator erase(const_iterator position);
    size_type erase(const key_type& x);
    iterator erase(const_iterator first, const_iterator last);

23.4.7.1 [multiset.overview] Class template multiset overview

    iterator erase(iterator position);
    iterator erase(const_iterator position);
    size_type erase(const key_type& x);
    iterator erase(const_iterator first, const_iterator last);

23.5.4.1 [unord.map.overview] Class template unordered_map overview p. 3

    iterator erase(iterator position);
    iterator erase(const_iterator position);
    size_type erase(const key_type& x);
    iterator erase(const_iterator first, const_iterator last);

23.5.5.1 [unord.multimap.overview] Class template unordered_multimap overview p. 3

    iterator erase(iterator position);
    iterator erase(const_iterator position);
    size_type erase(const key_type& x);
    iterator erase(const_iterator first, const_iterator last);

23.5.6.1 [unord.set.overview] Class template unordered_set overview p. 3

    iterator erase(iterator position);
    iterator erase(const_iterator position);
    size_type erase(const key_type& x);
    iterator erase(const_iterator first, const_iterator last);

23.5.7.1 [unord.multiset.overview] Class template unordered_multiset overview p. 3

    iterator erase(iterator position);
    iterator erase(const_iterator position);
    size_type erase(const key_type& x);
    iterator erase(const_iterator first, const_iterator last);

C.2.13 [diff.cpp03.containers] C.2.12 Clause 23: containers library

23.2.3, 23.2.4

Change: Signature changes: from iterator to const_iterator parameters

Rationale: Overspecification. Effects: The signatures of the following member functions changed from taking an iterator to taking a const_iterator:

- insert(iter, val) for vector, deque, list, set, multiset, map, multimap
- insert(pos, beg, end) for vector, deque, list, forward_list
- erase(iter) for set, multiset, map, multimap
- erase(begin, end) for set, multiset, map, multimap
- all forms of list::splice
- all forms of list::merge

Valid C++ 2003 code that uses these functions may fail to compile with this International Standard.

### 2061. `make_move_iterator` and arrays

Section: 24.3 [iterator.synopsis], 24.5.3 [move.iterators] Status: C++14 Submitter: Marc Glisse Opened: 2011-05-28 Last modified: 2015-04-08

View all other issues in [iterator.synopsis].

View all issues with C++14 status.

Discussion:

The standard library always passes template iterators by value and never by reference, which has the nice effect that an array decays to a pointer. There is one exception: `make_move_iterator`.

>     #include <iterator>
>     int main(){
>       int a[]={1,2,3,4};
>       std::make_move_iterator(a+4);
>       std::make_move_iterator(a); // fails here
>     }

[ 2011 Bloomington. ]

Move to Ready.

Proposed resolution:

This wording is relative to the FDIS.

1. Modify the header `<iterator>` synopsis in 24.3 [iterator.synopsis]:

   >     namespace std {
   >       […]
   >       template <class Iterator>
   >       move_iterator<Iterator> make_move_iterator(const Iterator&Iterator i);

   >       […]
   >     }

2. Modify the class template `move_iterator` synopsis in 24.5.3.1 [move.iterator]:

   >     namespace std {
   >       […]
   >       template <class Iterator>
   >       move_iterator<Iterator> make_move_iterator(const Iterator&Iterator i);
   >     }

3. Modify 24.5.3.3.14 [move.iter.nonmember]:

   >     template <class Iterator>
   >     move_iterator<Iterator> make_move_iterator(const Iterator&Iterator i);

   > > -3- Returns: `move_iterator<Iterator>(i)`.

### 2063. Contradictory requirements for string move assignment

Section: 21.4 [basic.string] Status: WP Submitter: Howard Hinnant Opened: 2011-05-29 Last modified: 2015-05-22

View other active issues in [basic.string].

View all other issues in [basic.string].

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Discussion:

21.4.1 [string.require]/p4 says that `basic_string` is an "allocator-aware" container and behaves as described in 23.2.1 [container.requirements.general].

23.2.1 [container.requirements.general] describes move assignment in p7 and Table 99.

If `allocator_traits<allocator_type>::propagate_on_container_move_assignment::value` is false, and if the allocators stored in the lhs and rhs sides are not equal, then move assigning a string has the same semantics as copy assigning a string as far as resources are concerned (resources can not be transferred). And in this event, the lhs may have to acquire resources to gain sufficient capacity to store a copy of the rhs.

However 21.4.2 [string.cons]/p22 says:

>     basic_string<charT,traits,Allocator>&
>     operator=(basic_string<charT,traits,Allocator>&& str) noexcept;

> > Effects: If `*this` and `str` are not the same object, modifies `*this` as shown in Table 71. [Note: A valid implementation is `swap(str)`. — end note ]

These two specifications for `basic_string::operator=(basic_string&&)` are in conflict with each other. It is not possible to implement a `basic_string` which satisfies both requirements.

Additionally assign from an rvalue `basic_string` is defined as:

>     basic_string& assign(basic_string&& str) noexcept;

> > Effects: The function replaces the string controlled by `*this` with a string of length `str.size()` whose elements are a copy of the string controlled by `str`. [ Note: A valid implementation is `swap(str)`. — end note ]

It seems contradictory that this member can be sensitive to `propagate_on_container_swap` instead of `propagate_on_container_move_assignment`. Indeed, there is a very subtle chance for undefined behavior here: If the implementation implements this in terms of `swap`, and if `propagate_on_container_swap` is false, and if the two allocators are unequal, the behavior is undefined, and will likely lead to memory corruption. That's a lot to go wrong under a member named "assign".

[ 2011 Bloomington ]

Alisdair: Can this be conditional `noexcept`?

Pablo: We said we were not going to put in many conditional `noexcept`s. Problem is not allocator, but non-normative definition. It says swap is a valid operation which it is not.

Dave: Move assignment is not a critical method.

Alisdair: Was confusing assignment and construction.

Dave: Move construction is critical for efficiency.

Kyle: Is it possible to test for `noexcept`.

Alisdair: Yes, query the `noexcept` operator.

Alisdair: Agreed there is a problem that we cannot unconditionally mark these operations as `noexcept`.

Pablo: How come swap is not defined in alloc

Alisdair: It is in utility.

Pablo: Swap has a conditional `noexcept`. Is no throw move constructable, is no throw move assignable.

Pablo: Not critical for strings or containers.

Kyle: Why?

Pablo: They do not use the default swap.

Dave: Important for deduction in other types.

Alisdair: Would change the policy we adopted during FDIS mode.

Pablo: Keep it simple and get some vendor experience.

Alisdair: Is this wording correct? Concerned with bullet 2.

Pablo: Where does it reference containers section.

Alisdair: String is a container.

Alisdair: We should not remove redundancy piecemeal.

Pablo: I agree. This is a deviation from rest of string. Missing forward reference to containers section.

Pablo: To fix section 2. Only the note needs to be removed. The rest needs to be a forward reference to containers.

Alisdair: That is a new issue.

Pablo: Not really. Talking about adding one sentence, saying that basic string is a container.

Dave: That is not just a forward reference, it is a semantic change.

PJ: We intended to make it look like a container, but it did not satisfy all the requirements.

Pablo: Clause 1 is correct. Clause 2 is removing note and `noexcept` (do not remove the rest). Clause 3 is correct.

Alisdair: Not sure data() is correct (in clause 2).

Conclusion: Move to open, Alisdair and Pablo volunteered to provide wording

[ originally proposed wording: ]

This wording is relative to the FDIS.

1. Modify the class template `basic_string` synopsis in 21.4 [basic.string]:

   >     namespace std {
   >       template<class charT, class traits = char_traits<charT>,
   >         class Allocator = allocator<charT> >
   >       class basic_string {
   >       public:
   >         […]
   >         basic_string& operator=(basic_string&& str) noexcept;
   >         […]
   >         basic_string& assign(basic_string&& str) noexcept;
   >         […]
   >       };
   >     }

2. Remove the definition of the `basic_string` move assignment operator from 21.4.2 [string.cons] entirely, including Table 71 — `operator=(const basic_string<charT, traits, Allocator>&&)`. This is consistent with how we define move assignment for the containers in Clause 23:

   >     basic_string<charT,traits,Allocator>&
   >     operator=(basic_string<charT,traits,Allocator>&& str) noexcept;

   > > -22- Effects: If `*this` and `str` are not the same object, modifies `*this` as shown in Table 71. [ Note: A valid implementation is `swap(str)`. — end note ]

   > > -23- If `*this` and `str` are the same object, the member has no effect.

   > > -24- Returns: `*this`

   > Table 71 — `operator=(const basic_string<charT, traits, Allocator>&&)`
   > Element | Value
   > --- | ---
   > `data()` | points at the array whose first element was pointed at by `str.data()`
   > `size()` | previous value of `str.size()`
   > `capacity()` | a value at least as large as `size()`

3. Modify the paragraphs prior to 21.4.6.3 [string::assign] p.3 as indicated (The first insertion recommends a separate paragraph number for the indicated paragraph):

   >     basic_string& assign(basic_string&& str) noexcept;

   > > -?- Effects: Equivalent to `*this = std::move(str)`. The function replaces the string controlled by `*this` with a string of length `str.size()` whose elements are a copy of the string controlled by `str`. [ Note: A valid implementation is `swap(str)`. — end note ]

   > > -3- Returns: `*this`

[ 2012-08-11 Joe Gottman observes: ]

> One of the effects of `basic_string`'s move-assignment operator (21.4.2 [string.cons], Table 71) is

> > Element | Value
> > --- | ---
> > `data()` | points at the array whose first element was pointed at by `str.data()`

> If a string implementation uses the small-string optimization and the input string `str` is small enough to make use of it, this effect is impossible to achieve. To use the small string optimization, a string has to be implemented using something like

> >     union
> >     {
> >        char buffer[SMALL_STRING_SIZE];
> >        char *pdata;
> >     };

> When the string is small enough to fit inside `buffer`, the `data()` member function returns `static_cast<const char *>(buffer)`, and since `buffer` is an array variable, there is no way to implement move so that the moved-to string's `buffer` member variable is equal to `this->buffer`.

> Resolution proposal:

> Change Table 71 to read:

> > Element | Value
> > --- | ---
> > `data()` | points at the array whose first element was pointed at by `str.data()` that contains the same characters in the same order as `str.data()` contained before `operator=()` was called

[2015-05-07, Lenexa]

Howard suggests improved wording

Move to Immediate

Proposed resolution:

This wording is relative to N4431.

1. Modify the class template `basic_string` synopsis in 21.4 [basic.string]:

   >     namespace std {
   >       template<class charT, class traits = char_traits<charT>,
   >         class Allocator = allocator<charT> >
   >       class basic_string {
   >       public:
   >         […]
   >         basic_string& assign(basic_string&& str) noexcept(
   >              allocator_traits<Allocator>::propagate_on_container_move_assignment::value ||
   >                allocator_traits<Allocator>::is_always_equal::value);
   >         […]
   >       };
   >     }

2. Change 21.4.2 [string.cons]/p21-23:

   >     basic_string&
   >     operator=(basic_string&& str) noexcept(
   >              allocator_traits<Allocator>::propagate_on_container_move_assignment::value ||
   >                allocator_traits<Allocator>::is_always_equal::value);

   > > -21- Effects: If `*this` and `str` are not the same object, modifies `*this` as shown in Table 71. [ Note: A valid implementation is `swap(str)`. — end note ] Move assigns as a sequence container ([container.requirements]), except that iterators, pointers and references may be invalidated.

   > > -22- If `*this` and `str` are the same object, the member has no effect.

   > > -23- Returns: `*this`

   > Table 71 — `operator=(basic_string&&)` effects
   > Element | Value
   > --- | ---
   > `data()` | points at the array whose first element was pointed at by `str.data()`
   > `size()` | previous value of `str.size()`
   > `capacity()` | a value at least as large as `size()`

3. Modify the paragraphs prior to 21.4.6.3 [string::assign] p.3 as indicated

   >     basic_string& assign(basic_string&& str) noexcept(
   >              allocator_traits<Allocator>::propagate_on_container_move_assignment::value ||
   >                allocator_traits<Allocator>::is_always_equal::value);

   > > -3- Effects: Equivalent to `*this = std::move(str)`. The function replaces the string controlled by `*this` with a string of length `str.size()` whose elements are a copy of the string controlled by `str`. [ Note: A valid implementation is `swap(str)`. — end note ]

   > > -4- Returns: `*this`

### 2064. More `noexcept` issues in `basic_string`

Section: 21.4 [basic.string] Status: C++14 Submitter: Howard Hinnant Opened: 2011-05-29 Last modified: 2015-04-08

View other active issues in [basic.string].

View all other issues in [basic.string].

View all issues with C++14 status.

Discussion:

The following inconsistencies regarding `noexcept` for `basic_string` are noted.

Member swap is not marked `noexcept`:

>     void swap(basic_string& str);

But the global swap is marked `noexcept`:

>     template<class charT, class traits, class Allocator>
>     void swap(basic_string<charT,traits,Allocator>& lhs,
>               basic_string<charT,traits,Allocator>& rhs) noexcept;

But only in the definition, not in the synopsis.

All comparison operators are marked `noexcept` in their definitions, but not in the synopsis.

The compare function that takes a pointer:

>     int compare(const charT *s) const;

is not marked `noexcept`. But some of the comparison functions which are marked `noexcept` (only in their definition) are specified to call the throwing compare operator:

>     template<class charT, class traits, class Allocator>
>     bool operator==(const basic_string<charT,traits,Allocator>& lhs,
>                     const charT* rhs) noexcept;

> > Returns: `lhs.compare(rhs) == 0`.

All functions with a narrow contract should not be declared as `noexcept` according to the guidelines presented in n3279. Among these narrow contract functions are the `swap` functions (23.2.1 [container.requirements.general] p. 8) and functions with non-`NULL` `const charT*` parameters.

[2011-06-08 Daniel provides wording]

[Bloomington, 2011]

Move to Ready

Proposed resolution:

This wording is relative to the FDIS. Both move-assignment operator and the moving `assign` function are not touched by this issue, because they are handled separately by issue 2063.

1. Modify the header `<string>` synopsis in 21.3 [string.classes] as indicated (Rationale: Adding `noexcept` to these specific overloads is in sync with applying the same rule to specific overloads of the member functions `find`, `compare`, etc. This approach deviates from that taken in n3279, but seems more consistent given similar application for comparable member functions):

   >     #include <initializer_list>

   >     namespace std {

   >       […]
   >       template<class charT, class traits, class Allocator>
   >         bool operator==(const basic_string<charT,traits,Allocator>& lhs,
   >                         const basic_string<charT,traits,Allocator>& rhs) noexcept;
   >       […]
   >       template<class charT, class traits, class Allocator>
   >         bool operator!=(const basic_string<charT,traits,Allocator>& lhs,
   >                         const basic_string<charT,traits,Allocator>& rhs) noexcept;
   >       […]

   >       template<class charT, class traits, class Allocator>
   >         bool operator<(const basic_string<charT,traits,Allocator>& lhs,
   >                        const basic_string<charT,traits,Allocator>& rhs) noexcept;
   >       […]
   >       template<class charT, class traits, class Allocator>
   >         bool operator>(const basic_string<charT,traits,Allocator>& lhs,
   >                        const basic_string<charT,traits,Allocator>& rhs) noexcept;
   >       […]

   >       template<class charT, class traits, class Allocator>
   >         bool operator<=(const basic_string<charT,traits,Allocator>& lhs,
   >                         const basic_string<charT,traits,Allocator>& rhs) noexcept;
   >       […]
   >       template<class charT, class traits, class Allocator>
   >         bool operator>=(const basic_string<charT,traits,Allocator>& lhs,
   >                         const basic_string<charT,traits,Allocator>& rhs) noexcept;
   >       […]
   >     }

2. Modify the class template `basic_string` synopsis in 21.4 [basic.string] as indicated (Remark 1: The `noexcept` at the move-constructor is fine, because even for a small-object optimization there is no problem here, because `basic_string::value_type` is required to be a non-array POD as of 21.1 [strings.general] p1, Remark 2: This proposal removes the `noexcept` at single character overloads of `find`, `rfind`, etc. because they are defined in terms of potentially allocating functions. It seems like an additional issue to me to change the semantics in terms of non-allocating functions and adding `noexcept` instead):

   >     namespace std {
   >       template<class charT, class traits = char_traits<charT>,
   >         class Allocator = allocator<charT> >
   >       class basic_string {
   >       public:
   >         […]
   >         // [string.ops], string operations:
   >         […]
   >         size_type find (charT c, size_type pos = 0) const noexcept;
   >         […]
   >         size_type rfind(charT c, size_type pos = npos) const noexcept;
   >         […]
   >         size_type find_first_of(charT c, size_type pos = 0) const noexcept;
   >         […]
   >         size_type find_last_of (charT c, size_type pos = npos) const noexcept;
   >         […]
   >         size_type find_first_not_of(charT c, size_type pos = 0) const noexcept;
   >         […]
   >         size_type find_last_not_of (charT c, size_type pos = npos) const noexcept;
   >         […]
   >       };
   >     }

3. Modify 21.4.7.2 [string::find] before p5 and before p7 as indicated:

   >     size_type find(const charT* s, size_type pos = 0) const noexcept;
   >     […]
   >     size_type find(charT c, size_type pos = 0) const noexcept;

   > > -7- Returns: `find(basic_string<charT,traits,Allocator>(1,c), pos)`.

4. Modify 21.4.7.3 [string::rfind] before p7 as indicated:

   >     size_type rfind(charT c, size_type pos = npos) const noexcept;

   > > -7- Returns: `rfind(basic_string<charT,traits,Allocator>(1,c),pos)`.

5. Modify 21.4.7.4 [string::find.first.of] before p7 as indicated:

   >     size_type find_first_of(charT c, size_type pos = 0) const noexcept;

   > > -7- Returns: `find_first_of(basic_string<charT,traits,Allocator>(1,c), pos)`.

6. Modify 21.4.7.5 [string::find.last.of] before p7 as indicated:

   >     size_type find_last_of(charT c, size_type pos = npos) const noexcept;

   > > -7- Returns: `find_last_of(basic_string<charT,traits,Allocator>(1,c),pos)`.

7. Modify 21.4.7.6 [string::find.first.not.of] before p7 as indicated:

   >     size_type find_first_not_of(charT c, size_type pos = 0) const noexcept;

   > > -7- Returns: `find_first_not_of(basic_string(1, c), pos)`.

8. Modify 21.4.7.7 [string::find.last.not.of] before p7 as indicated:

   >     size_type find_last_not_of(charT c, size_type pos = npos) const noexcept;

   > > -7- Returns: `find_last_not_of(basic_string(1, c), pos)`.

9. Modify 21.4.8.2 [string::operator==] before p2+p3 as indicated:

   >     template<class charT, class traits, class Allocator>
   >     bool operator==(const charT* lhs,
   >                     const basic_string<charT,traits,Allocator>& rhs) noexcept;

   >     […]

   >     template<class charT, class traits, class Allocator>
   >     bool operator==(const basic_string<charT,traits,Allocator>& lhs,
   >                     const charT* rhs) noexcept;

10. Modify 21.4.8.3 [string::op!=] before p2+p3 as indicated:

    >     template<class charT, class traits, class Allocator>
    >     bool operator!=(const charT* lhs,
    >                     const basic_string<charT,traits,Allocator>& rhs) noexcept;

    >     […]

    >     template<class charT, class traits, class Allocator>
    >     bool operator!=(const basic_string<charT,traits,Allocator>& lhs,
    >                     const charT* rhs) noexcept;

11. Modify 21.4.8.4 [string::op<] before p2+p3 as indicated:

    >     template<class charT, class traits, class Allocator>
    >     bool operator<(const charT* lhs,
    >                    const basic_string<charT,traits,Allocator>& rhs) noexcept;

    >     […]

    >     template<class charT, class traits, class Allocator>
    >     bool operator<(const basic_string<charT,traits,Allocator>& lhs,
    >                    const charT* rhs) noexcept;

12. Modify 21.4.8.5 [string::op>] before p2+p3 as indicated:

    >     template<class charT, class traits, class Allocator>
    >     bool operator>(const charT* lhs,
    >                    const basic_string<charT,traits,Allocator>& rhs) noexcept;

    >     […]

    >     template<class charT, class traits, class Allocator>
    >     bool operator>(const basic_string<charT,traits,Allocator>& lhs,
    >                    const charT* rhs) noexcept;

13. Modify 21.4.8.6 [string::op<=] before p2+p3 as indicated:

    >     template<class charT, class traits, class Allocator>
    >     bool operator<=(const charT* lhs,
    >                     const basic_string<charT,traits,Allocator>& rhs) noexcept;

    >     […]

    >     template<class charT, class traits, class Allocator>
    >     bool operator<=(const basic_string<charT,traits,Allocator>& lhs,
    >                     const charT* rhs) noexcept;

14. Modify 21.4.8.7 [string::op>=] before p2+p3 as indicated:

    >     template<class charT, class traits, class Allocator>
    >     bool operator>=(const charT* lhs,
    >                     const basic_string<charT,traits,Allocator>& rhs) noexcept;

    >     […]

    >     template<class charT, class traits, class Allocator>
    >     bool operator>=(const basic_string<charT,traits,Allocator>& lhs,
    >                     const charT* rhs) noexcept;

15. Modify 21.4.8.8 [string.special] as indicated (Remark: The change of the semantics guarantees as of 17.5.1.4 [structure.specifications] p4 that the "Throws: Nothing" element of member swap is implied):

    >     template<class charT, class traits, class Allocator>
    >       void swap(basic_string<charT,traits,Allocator>& lhs,
    >         basic_string<charT,traits,Allocator>& rhs) noexcept;

    > > -1- Effects: Equivalent to `lhs.swap(rhs);`

### 2065. Minimal allocator interface

Section: 17.6.3.5 [allocator.requirements] Status: C++14 Submitter: Jonathan Wakely Opened: 2011-06-06 Last modified: 2015-04-08

View other active issues in [allocator.requirements].

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Discussion:

The example in 17.6.3.5 [allocator.requirements] says `SimpleAllocator` satisfies the requirements of Table 28 — Allocator requirements, but it doesn't support comparison for equality/inequality.

[Bloomington, 2011]

Move to Ready

Proposed resolution:

This wording is relative to the FDIS.

1. Modify the example in 17.6.3.5 [allocator.requirements] p5 as indicated:

   > -5- […]

   > [ Example: the following is an allocator class template supporting the minimal interface that satisfies the requirements of Table 28:

   > >     template <class Tp>
   > >     struct SimpleAllocator {
   > >       typedef Tp value_type;
   > >       SimpleAllocator(ctor args);
   > >       template <class T> SimpleAllocator(const SimpleAllocator<T>& other);
   > >       Tp *allocate(std::size_t n);
   > >       void deallocate(Tp *p, std::size_t n);
   > >     };

   > >     template <class T, class U>
   > >     bool operator==(const SimpleAllocator<T>&, const SimpleAllocator<U>&);
   > >     template <class T, class U>
   > >     bool operator!=(const SimpleAllocator<T>&, const SimpleAllocator<U>&);

   > — end example ]

### 2066. Missing specification of `vector::resize(size_type)`

Section: 23.3.6.3 [vector.capacity] Status: Resolved Submitter: Rani Sharoni Opened: 2011-03-29 Last modified: 2015-04-08

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Discussion:

In C++1x (N3090) there are two version of `vector::resize` — 23.3.6.3 [vector.capacity]:

>     void resize(size_type sz);
>     void resize(size_type sz, const T& c);

The text in 23.3.6.3 [vector.capacity]/12 only mentions "no effects on throw" for the two args version of resize:

> Requires: If an exception is thrown other than by the move constructor of a non-`CopyConstructible` `T` there are no effects.

This seems like unintentional oversight since `resize(size)` is semantically the same as `resize(size, T())`. Additionally, the C++03 standard only specify single version of resize with default for the second argument - 23.2.4:

>     void resize(size_type sz, T c = T());

Therefore not requiring same guarantees for both version of resize is in fact a regression.

[2011-06-12: Daniel comments]

The proposed resolution for issue 2033 should solve this issue as well.

[ 2011 Bloomington ]

This issue will be resolved by issue 2033, and closed when this issue is applied.

[2012, Kona]

Resolved by adopting the resolution in issue 2033 at this meeting.

Proposed resolution:

Apply the proposed resolution of issue 2033

### 2067. `packaged_task` should have deleted copy c'tor with const parameter

Section: 30.6.9 [futures.task] Status: C++14 Submitter: Daniel Krügler Opened: 2011-06-16 Last modified: 2015-04-08

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Discussion:

Class template `packaged_task` is a move-only type with the following form of the deleted copy operations:

>     packaged_task(packaged_task&) = delete;
>     packaged_task& operator=(packaged_task&) = delete;

Note that the argument types are non-const. This does not look like a typo to me, this form seems to exist from the very first proposing paper on N2276. Using either of form of the copy-constructor did not make much difference before the introduction of defaulted special member functions, but it makes now an observable difference. This was brought to my attention by a question on a German C++ newsgroup where the question was raised why the following code does not compile on a recent gcc:

>     #include <utility>
>     #include <future>
>     #include <iostream>
>     #include <thread>

>     int main(){
>       std::packaged_task<void()> someTask([]{ std::cout << std::this_thread::get_id() << std::endl; });
>       std::thread someThread(std::move(someTask)); // Error here
>       // Remainder omitted
>     }

It turned out that the error was produced by the instantiation of some return type of `std::bind` which used a defaulted copy-constructor, which leads to a const declaration conflict with [class.copy] p8.

Some aspects of this problem are possibly core-language related, but I consider it more than a service to programmers, if the library would declare the usual form of the copy operations (i.e. those with const first parameter type) as deleted for `packaged_task` to prevent such problems.

A similar problem exists for class template `basic_ostream` in 27.7.3.1 [ostream]:

>     namespace std {
>       template <class charT, class traits = char_traits<charT> >
>       class basic_ostream : virtual public basic_ios<charT,traits> {
>         […]

>         // 27.7.3.3 Assign/swap
>         basic_ostream& operator=(basic_ostream& rhs) = delete;
>         basic_ostream& operator=(const basic_ostream&& rhs);
>         void swap(basic_ostream& rhs);
>     };

albeit this could be considered as an editorial swap of copy and move assignment operator, I suggest to fix this as part of this issue as well.

[ 2011 Bloomington. ]

Move to Ready.

Proposed resolution:

This wording is relative to the FDIS.

1. Modify the class template `basic_ostream` synopsis in 27.7.3.1 [ostream] as indicated (Note: The prototype signature of the move assignment operator in 27.7.3.3 [ostream.assign] is fine):

   >     namespace std {
   >       template <class charT, class traits = char_traits<charT> >
   >       class basic_ostream : virtual public basic_ios<charT,traits> {
   >         […]

   >         // 27.7.3.3 Assign/swap
   >         basic_ostream& operator=(const basic_ostream& rhs) = delete;
   >         basic_ostream& operator=(const basic_ostream&& rhs);
   >         void swap(basic_ostream& rhs);
   >     };

2. Modify the class template `packaged_task` synopsis in 30.6.9 [futures.task] p2 as indicated:

   >     namespace std {
   >       template<class> class packaged_task; // undefined

   >       template<class R, class... ArgTypes>
   >       class packaged_task<R(ArgTypes...)> {
   >       public:
   >         […]

   >         // no copy
   >         packaged_task(const packaged_task&) = delete;
   >         packaged_task& operator=(const packaged_task&) = delete;

   >         […]
   >       };
   >       […]
   >     }

### 2069. Inconsistent exception spec for `basic_string` move constructor

Section: 21.4.2 [string.cons] Status: C++14 Submitter: Bo Persson Opened: 2011-07-01 Last modified: 2015-04-08

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Discussion:

Sub-clause 21.4.2 [string.cons] contains these constructors in paragraphs 2 and 3:

>     basic_string(const basic_string<charT,traits,Allocator>& str);
>     basic_string(basic_string<charT,traits,Allocator>&& str) noexcept;

> > […]

> > -3- Throws: The second form throws nothing if the allocator's move constructor throws nothing.

How can it ever throw anything if it is marked `noexcept`?

[2011-07-11: Daniel comments and suggests wording changes]

Further, according to paragraph 18 of the same sub-clause:

>     basic_string(const basic_string& str, const Allocator& alloc);
>     basic_string(basic_string&& str, const Allocator& alloc);

> > […]

> > -18- Throws: The second form throws nothing if `alloc == str.get_allocator()` unless the copy constructor for `Allocator` throws.

The constraint "unless the copy constructor for `Allocator` throws" is redundant, because according to Table 28 — Allocator requirements, the expressions

>     X a1(a);
>     X a(b);

impose the requirement: "Shall not exit via an exception".

[ 2011 Bloomington. ]

Move to Ready.

Proposed resolution:

This wording is relative to the FDIS.

1. Change 21.4.2 [string.cons] p3 as indicated (This move constructor has a wide contract and is therefore safely marked as `noexcept`):

   >     basic_string(const basic_string<charT,traits,Allocator>& str);
   >     basic_string(basic_string<charT,traits,Allocator>&& str) noexcept;

   > > -2- Effects: Constructs an object of class `basic_string` as indicated in Table 64. In the second form, `str` is left in a valid state with an unspecified value.

   > > -3- Throws: The second form throws nothing if the allocator's move constructor throws nothing.

2. Change 21.4.2 [string.cons] p18 as indicated (This move-like constructor may throw, if the allocators don't compare equal, but not because of a potentially throwing allocator copy constructor, only because the allocation attempt may fail and throw an exception):

   >     basic_string(const basic_string& str, const Allocator& alloc);
   >     basic_string(basic_string&& str, const Allocator& alloc);

   > > […]

   > > -18- Throws: The second form throws nothing if `alloc == str.get_allocator()` unless the copy constructor for `Allocator` throws.

### 2071. `std::valarray` move-assignment

Section: 26.6.2.3 [valarray.assign] Status: C++14 Submitter: Paolo Carlini Opened: 2011-05-05 Last modified: 2015-04-08

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Discussion:

Yesterday I noticed that the language we have in the FDIS about `std::valarray` move assignment is inconsistent with the resolution of LWG 675. Indeed, we guarantee constant complexity (vs linear complexity). We also want it to be noexcept, that is more subtle, but again it's at variance with all the containers.

Also, even if we suppose that LWG 675 applies only to the containers proper, I don't think the current "as if by calling resize(v.size())" is internally consistent with the noexcept requirement.

So, what do we really want for `std::valarray`? Shall we maybe just strike or fix the as-if, consider it some sort of pasto from the copy-assignment text, thus keep the noexcept and constant complexity requirements (essentially the whole operation would boild down to a swap of POD data members). Or LWG 675 should be explicitly extended to `std::valarray` too? In that case both noexcept and constant complexity would go, I think, and the operation would boil down to the moral equivalent of `clear()` (which doesn't really exist in this case) + `swap`?

Howard: I agree the current wording is incorrect. The complexity should be linear in `size()` (not `v.size()`) because the first thing this operator needs to do is `resize(0)` (or `clear()` as you put it).

I think we can keep the `noexcept`.

As for proper wording, here's a first suggestion:

> Effects: `*this` obtains the value of `v`. The value of `v` after the assignment is not specified.

> Complexity: linear.

See also reflector discussion starting with c++std-lib-30690.

[2012, Kona]

Move to Ready.

Some discussion on the types supported by `valarray` concludes that the wording is trying to say something similar to the core wording for trivial types, but significantly predates it, and does allow for types with non-trivial destructors. Howard notes that the only reason for linear complexity, rather than constant, is to support types with non-trivial destructors.

AJM suggests replacing the word 'value' with 'state', but straw poll prefers moving forward with the current wording, 5 to 2.

[2012, Portland: applied to WP]

Proposed resolution:

This wording is relative to the FDIS.

In 26.6.2.3 [valarray.assign] update as follows:

>     valarray<T>& operator=(valarray<T>&& v) noexcept;

> > 3 Effects: `*this` obtains the value of `v`. If the length of `v` is not equal to the length of `*this`, resizes `*this` to make the two arrays the same length, as if by calling `resize(v.size())`, before performing the assignment.The value of `v` after the assignment is not specified.

> > 4 Complexity: Constant Linear.

### 2074. Off by one error in `std::reverse_copy`

Section: 25.3.10 [alg.reverse] Status: C++14 Submitter: Peter Miller Opened: 2011-08-17 Last modified: 2015-04-08

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Discussion:

The output of the program below should be:

>     "three two one null \n"

But when `std::reverse_copy` is implemented as described in N3291 25.3.10 [alg.reverse] it's:

>     "null three two one \n"

because there's an off by one error in 25.3.10 [alg.reverse]/4; the definition should read:

>     *(result + (last - first) - 1 - i) = *(first + i)

Test program:

>     #include <algorithm>
>     #include <iostream>

>     template <typename BiIterator, typename OutIterator>
>     auto
>     reverse_copy_as_described_in_N3291(
>       BiIterator first, BiIterator last, OutIterator result )
>     -> OutIterator
>     {
>       // 25.3.10/4 [alg.reverse]:
>       // "...such that for any non-negative integer i < (last - first)..."
>       for ( unsigned i = 0; i < ( last - first ); ++i )
>         // "...the following assignment takes place:"
>         *(result + (last - first) - i) = *(first + i);

>       // 25.3.10/6
>       return result + (last - first);
>     }

>     int main()
>     {
>       using std::begin;
>       using std::end;
>       using std::cout;

>       static const char*const in[3]  { "one", "two", "three" };
>       const char*             out[4] { "null", "null", "null", "null" };

>       reverse_copy_as_described_in_N3291( begin( in ), end( in ), out );

>       for ( auto s : out )
>         cout << s << ' ';

>       cout << std::endl;

>       return 0;
>     }

[2012, Kona]

Move to Ready.

[2012, Portland: applied to WP]

Proposed resolution:

This wording is relative to the FDIS.

Change 25.3.10 [alg.reverse] p4 as follows:

>     template<class BidirectionalIterator, class OutputIterator>
>       OutputIterator
>         reverse_copy(BidirectionalIterator first,
>                      BidirectionalIterator last, OutputIterator result);

> > -4- Effects: Copies the range [`first,last`) to the range [`result,result+(last-first)`) such that for any non-negative integer `i < (last - first)` the following assignment takes place: `*(result + (last - first) - 1 - i) = *(first + i)`.

> > -5- Requires: The ranges [`first,last`) and [`result,result+(last-first)`) shall not overlap.

> > -6- Returns: `result + (last - first)`.

> > -7- Complexity: Exactly `last - first` assignments.

### 2075. Progress guarantees, lock-free property, and scheduling assumptions

Section: 1.10 [intro.multithread], 29.4 [atomics.lockfree], 29.6.5 [atomics.types.operations.req] Status: Resolved Submitter: Torvald Riegel Opened: 2011-08-18 Last modified: 2015-04-08

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Discussion:

According to 1.10 [intro.multithread] p2:

> "Implementations should ensure that all unblocked threads eventually make progress."

- If taken literally, this cannot be achieved with lock-free atomics in general because they only guarantee that some thread makes progress (i.e., minimal progress, whereas 1.10 [intro.multithread] p2 seems to require maximal progress).
- What does it mean precisely to "make progress"? Does "unblocked threads" exclude live-locked threads (if so, lock-free atomics would be sufficient I suppose)?

- Which assumptions can an implementation make about the thread scheduling? This is relevant for how implementations implement compare-exchange with load-linked / store conditional (LL-SC), and atomic read-modifiy-write operations with load...compare-exchange-weak loops.

  - Do threads run long enough without being descheduled (e.g., OS timeslices are long enough, interrupt frequency is not too high, etc.)?
  - Or is this implementation-defined, and the sentence is just about stating that the progress guarantees will not hold on, for example, systems with unfair scheduling or thread priorities?

29.4 [atomics.lockfree] p2 declares the lock-free property for a particular object. However, "lock-free" is never defined, and in discussions that I had with committee members it seemed as if the standard's lock-free would be different from what lock-free means in other communities (eg, research, text books on concurrent programming, etc.).

- Originally, lock-freedom for an object requires minimal progress (ie, some thread makes progress, but other threads might never do) without any assumptions about the scheduling (threads could be stopped executing (so it is "nonblocking"), and threads are not guaranteed to execute in isolation, even for very small intervals of cycles).
- In contrast, obstruction-freedom, another nonblocking progress condition, guarantees progress for all threads that eventually get executed long enough in isolation (ie, without interference by other threads).
- Simple load...compare-exchange-weak loops (or LL-SC loops) to implement atomic read-modify-write operations can be just obstruction-free but not lock-free because they can livelock (depending on the hardware's LL-SC implementation, though). However, they effectively guarantee the same as lock-free iff threads will eventually run in isolation for long enough (that can be an assumption about the OS scheduler), or if the implementation adds this (e.g., probabilistically by employing randomized exponential back-off when contention is detected, in all operations that can create contention).
- Does the particular object has to be lock-free, or is it only required that threads make progress irrespective on which object? Again considering compare-exchange-weak or LL-SC here, what happens if the compare-exchange object shares a cacheline with an integer counter object that is constantly updated by other threads? The compare-exchange-weak can always fail, so the object would not be lock-free. However, if we consider progress to be overall progress for threads, it would be lock-free because other threads succeed updating the integer counter. I would have assumed the lock-free property is strictly about the atomic object, but in discussions with committee members it seemed as if progress for any object could be the intended guarantee.

Following 29.6.5 [atomics.types.operations.req] p7 `is_lock_free()` returns "true if the object is lock-free". What is returned if the object is only sometimes lock-free?

Basically, I would like to see clarifications for the progress guarantees so that users know what they can expect from implementations (and what they cannot expect!), and to give implementors a clearer understanding of which user expectations they have to implement.

1. Elaborate on the intentions of the progress guarantee in 1.10 [intro.multithread] p2. As I don't know about your intentions, it's hard to suggest a resolution.

   - Is it for straightforward, non-synchronizing code only?
   - Is it for blocking code only? (Is "unblocked" more than blocked on external I/O or on deadlocks?)
   - What does it mean to "make progress"?
   - Is this meant to only waive any progress guarantees if there are thread priorities?
   - Can an implementation make any assumptions about thread scheduling?

2. Define the lock-free property. The definition should probably include the following points:

   - Is it just nonblocking, or what is the distinction to just being nonblocking?
   - Does it make any assumptions about the scheduler?
   - What are the progress guarantees, minimal or maximal (some or all threads finish eventually).
   - Is progress guaranteed for all operations on the particular object, or do operations on other objects also count as "making progress"?
3. Add a note explaining that compare-exchange-weak is not necessarily lock-free (but is nonblocking)? Or is it indeed intended to be lock-free (only allowed to fail spuriously but guaranteed to not fail eventually)? Implementing the latter might be a challenge on LL-SC machines or lead to space overheads I suppose, see the cacheline sharing example above.

[2011-12-01: Hans comments]

1.10 [intro.multithread] p2 was an intentional compromise, and it was understood at the time that it was not a precise statement. The wording was introduced by N3209, which discusses some of the issues. There were additional reflector discussions.

This is somewhat separable from the question of what lock-free means, which is probably a more promising question to focus on.

[2012, Kona]

General direction: lock-free means obstruction-free. Leave the current "should" recommendation for progress. It would take a lot of effort to try to do better.

[2012, Portland: move to Open]

The current wording of 1.10 [intro.multithread] p2 doesn't really say very much. As far as we can tell the term lock-free is nowhere defined in the standard.

James: we would prefer a different way to phrase it.

Hans: the research literature includes the term abstraction-free which might be a better fit.

Detlef: does Posix define a meaning for blocking (or locking) that we could use?

Hans: things like compare-exchange-strong can wait indefinitely.

Niklas: what about spin-locks -- still making no progress.

Hans: suspect we can only give guidance, at best. The lock-free meaning from the theoretical commmunity (forard progress will be made) is probably too strong here.

Atrur: what about livelocks?

Hans: each atomic modification completes, even if the whole thing is blocked.

Moved to open.

[2013-11-06: Jason Hearne-McGuiness comments]

Related to this issue, the wording in 29.4 [atomics.lockfree] p2,

> In any given program execution, the result of the lock-free query shall be consistent for all pointers of the same type.

should be made clearer, because the object-specific (non-static) nature of the `is_lock_free()` functions from 29.5 [atomics.types.generic] and 29.6 [atomics.types.operations] imply that for different instances of pointers of the same type the returned value could be different.

Proposed resolution:

[2014-02-16 Issaquah: Resolved by paper n3927]

### 2076. Bad `CopyConstructible` requirement in set constructors

Section: 23.4.6.2 [set.cons] Status: WP Submitter: Jens Maurer Opened: 2011-08-20 Last modified: 2015-05-22

View all issues with WP status.

Discussion:

23.4.6.2 [set.cons] paragraph 4 says:

> Requires: If the iterator's dereference operator returns an lvalue or a non-const rvalue, then `Key` shall be `CopyConstructible`.

I'm confused why a "non-const rvalue" for the return value of the iterator would require `CopyConstructible`; isn't that exactly the situation when you'd want to apply the move constructor?

The corresponding requirement for `multimap` seems better in that regard ([multimap.cons] paragraph 3):

> Requires: If the iterator's dereference operator returns an lvalue or a const rvalue `pair<key_type, mapped_type>`, then both `key_type` and mapped_type shall be `CopyConstructible`.

Obviously, if I have a const rvalue, I can't apply the move constructor (which will likely attempt modify its argument).

Dave Abrahams:

I think you are right. Proposed resolution: drop "non-" from 23.4.6.2 [set.cons] paragraph 3.

[2012, Kona]

The wording is in this area will be affected by Pablo's paper being adopted at this meeting. Wait for that paper to be applied before visiting this issue - deliberately leave in New status until the next meeting.

Proposed resolution from Kona 2012:

> This wording is relative to the FDIS.

> Change 23.4.6.2 [set.cons] p3 as follows:

> >     template <class InputIterator>
> >       set(InputIterator first, InputIterator last,
> >         const Compare& comp = Compare(), const Allocator& = Allocator());

> > > -3- Effects: Constructs an empty set using the specified comparison object and allocator, and inserts elements from the range [`first,last`).

> > > -4- Requires: If the iterator's dereference operator returns an lvalue or a non-const rvalue, then `Key` shall be `CopyConstructible`.

> > > -5- Complexity: Linear in `N` if the range [`first,last`) is already sorted using `comp` and otherwise `N logN`, where `N` is `last - first`.

[2014-05-18, Daniel comments]

According to Pablo, the current P/R correctly incorporates the changes from his paper (which was adopted in Kona)

[2014-06-10, STL comments and suggests better wording]

N1858 was voted into WP N2284 but was "(reworded)", introducing the "non-const" damage.

N1858 wanted to add this for map:

> Requires: Does not require `CopyConstructible` of either `key_type` or `mapped_type` if the dereferenced `InputIterator` returns a non-const rvalue `pair<key_type, mapped_type>`. Otherwise `CopyConstructible` is required for both `key_type` and `mapped_type`.

And this for set:

> Requires: `Key` must be `CopyConstructible` only if the dereferenced `InputIterator` returns an lvalue or const rvalue.

(And similarly for multi.)

This was reworded to N2284 23.3.1.1 [map.cons]/3 and N2284 23.3.3.1 [set.cons]/4, and it slightly changed over the years into N3936 23.4.4.2 [map.cons]/3 and N3936 23.4.6.2 [set.cons]/4.

In 2005/2007, this was the best known way to say "hey, we should try to move this stuff", as the fine-grained element requirements were taking shape.

Then in 2010, N3173 was voted into WP N3225, adding the definition of `EmplaceConstructible` and modifying the container requirements tables to make the range constructors require `EmplaceConstructible`.

After looking at this history and double-checking our implementation (where `map`/`set` range construction goes through emplacement, with absolutely no special-casing for `map`'s pairs), I am convinced that N3173 superseded N1858 here. (Range-`insert()` and the unordered containers are unaffected.)

Previous resolution [SUPERSEDED]:

> This wording is relative to the N3936.

> Change 23.4.6.2 [set.cons] p4 as follows:

> >     template <class InputIterator>
> >       set(InputIterator first, InputIterator last,
> >         const Compare& comp = Compare(), const Allocator& = Allocator());

> > > -3- Effects: Constructs an empty `set` using the specified comparison object and allocator, and inserts elements from the range [`first,last`).

> > > -4- Requires: If the iterator's indirection operator returns an lvalue or a non-const rvalue, then `Key` shall be `CopyInsertible` into `*this`.

> > > -5- Complexity: Linear in `N` if the range [`first,last`) is already sorted using `comp` and otherwise `N logN`, where `N` is `last - first`.

[2015-02 Cologne]

GR: Do requirements supersede rather than compose [container requirements and per-function requirements]? AM: Yes, they supersede.

AM: This looks good. Ready? Agreement.

Proposed resolution:

This wording is relative to the N4296.

1. Remove 23.4.4.2 [map.cons] p3:

   >     template <class InputIterator>
   >       map(InputIterator first, InputIterator last,
   >           const Compare& comp = Compare(), const Allocator& = Allocator());

   > > -3- Requires: If the iterator's indirection operator returns an lvalue or a `const` rvalue `pair<key_type, mapped_type>`, then both `key_type` and `mapped_type` shall be `CopyInsertible` into `*this`.

   > > […]

2. Remove 23.4.5.2 [multimap.cons] p3:

   >     template <class InputIterator>
   >       multimap(InputIterator first, InputIterator last,
   >                const Compare& comp = Compare(),
   >                const Allocator& = Allocator());

   > > -3- Requires: If the iterator's indirection operator returns an lvalue or a `const` rvalue `pair<key_type, mapped_type>`, then both `key_type` and `mapped_type` shall be `CopyInsertible` into `*this`.

   > > […]

3. Remove 23.4.6.2 [set.cons] p4:

   >     template <class InputIterator>
   >       set(InputIterator first, InputIterator last,
   >           const Compare& comp = Compare(), const Allocator& = Allocator());

   > > […]

   > > -4- Requires: If the iterator's indirection operator returns an lvalue or a non-`const` rvalue, then `Key` shall be `CopyInsertible` into `*this`.

4. Remove 23.4.7.2 [multiset.cons] p3:

   >     template <class InputIterator>
   >       multiset(InputIterator first, InputIterator last,
   >                const Compare& comp = Compare(),
   >                const Allocator& = Allocator());

   > > -3- Requires: If the iterator's indirection operator returns an lvalue or a `const` rvalue, then `Key` shall be `CopyInsertible` into `*this`.

   > > […]

### 2078. Throw specification of `async()` incomplete

Section: 30.6.8 [futures.async] Status: C++14 Submitter: Nicolai Josuttis Opened: 2011-08-29 Last modified: 2015-04-08

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Discussion:

The current throw specification of `async()` does state:

> -6- Throws: `system_error` if policy is `launch::async` and the implementation is unable to start a new thread.

First it seems not clear whether this only applies if policy equals `launch::async` of if the `async` launch mode flag is set (if `policy|launch::async!=0`)

In the discussion Lawrence Crowl also wrote:

> More generally, I think what we want to say is that if the implementation cannot successfully execute on one of the policies allowed, then it must choose another. The principle would apply to implementation-defined policies as well.

Peter Sommerlad:

> Should not throw. That was the intent. "is async" meat exactly.

[2012, Portland: move to Tentatively NAD Editorial]

If no launch policy, it is undefined behavior.

Agree with Lawrence, should try all the allowed policies. We will rephrase so that the policy argument should be `lauch::async`. Current wording seems good enough.

We believe this choice of policy statement is really an editorial issue.

[2013-09 Chicago]

If all the implementors read it and can't get it right - it is not editorial. Nico to provide wording

No objections to revised wording, so moved to Immediate.

Accept for Working Paper

Proposed resolution:

This wording is relative to N3691.

1. Change 30.6.8 [futures.async] p6, p7 as indicated:

   > -6- Throws: `system_error` if `policy` is `==` `launch::async` and the implementation is unable to start a new thread.

   > -7- Error conditions:

   > - `resource_unavailable_try_again` — if `policy` is `==` `launch::async` and the system is unable to start a new thread.

### 2080. Specify when `once_flag` becomes invalid

Section: 30.4.4 [thread.once] Status: C++14 Submitter: Nicolai Josuttis Opened: 2011-08-30 Last modified: 2015-04-08

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Discussion:

In function `call_once` 30.4.4.2 [thread.once.callonce] paragraph 4 and 5 specify for `call_once()`:

> Throws: `system_error` when an exception is required (30.2.2 [thread.req.exception]), or any exception thrown by `func`.

> Error conditions:
> - `invalid_argument` — if the `once_flag` object is no longer valid.

However, nowhere in 30.4.4 [thread.once] is specified, when a once-flag becomes invalid.

As far as I know this happens if the flag is used for different functions. So we either have to have to insert a sentence/paragraph in

> 30.4.4.2 Function call_once [thread.once.callonce]

or

> 30.4.4 Call once [thread.once]

explaining when a `once_flag` becomes invalidated or we should state as error condition something like:

- `invalid_argument` — if the `func` used in combination with the `once_flag` is different from a previously passed `func` for the same `once_flag`

Anthony Williams:

> A `once_flag` is invalidated if you destroy it (e.g. it is an automatic object, or heap allocated and deleted, etc.)

> If the library can detect that this is the case then it will throw this exception. If it cannot detect such a case then it will never be thrown.

Jonathan Wakely:

> I have also wondered how that error can happen in C++, where the type system will reject a non-callable type being passed to `call_once()` and should prevent a `once_flag` being used after its destructor runs.

> If a `once_flag` is used after its destructor runs then it is indeed undefined behaviour, so implementations are already free to throw any exception (or set fire to a printer) without the standard saying so.

> My assumption was that it's an artefact of basing the API on pthreads, which says:

> > The `pthread_once()` function may fail if:

> > `[EINVAL]` If either `once_control` or `init_routine` is invalid.

Pete Becker:

> Yes, probably. We had to clean up several UNIXisms that were in the original design.

[2012, Kona]

Remove error conditions, move to Review.

[2012, Portland: move to Tentatively Ready]

Concurrency move to Ready, pending LWG review.

LWG did not have time to perform the final review in Portland, so moving to tentatively ready to reflect the Concurrency belief that the issue is ready, but could use a final inspection from library wordsmiths.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3337.

1. Change 30.4.4.2 [thread.once.callonce] as indicated:

   >     template<class Callable, class ...Args>
   >     void call_once(once_flag& flag, Callable&& func, Args&&... args);

   > > […]

   > > -4- Throws: `system_error` when an exception is required (30.2.2), or any exception thrown by `func`.

   > > -5- Error conditions:
   > > - `invalid_argument` — if the `once_flag` object is no longer valid.

### 2081. `Allocator` requirements should include `CopyConstructible`

Section: 17.6.3.5 [allocator.requirements] Status: C++14 Submitter: Jonathan Wakely Opened: 2011-08-30 Last modified: 2015-04-08

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Discussion:

As discussed in c++std-lib-31054 and c++std-lib-31059, the `Allocator` requirements implicitly require `CopyConstructible` because `a.select_on_container_copy_construction()` and `container.get_allocator()` both return a copy by value, but the requirement is not stated explicitly anywhere.

In order to clarify that allocators cannot have 'explicit' copy constructors, the requirements should include `CopyConstructible`.

[2012, Kona]

Move to Ready.

[2012, Portland: applied to WP]

Proposed resolution:

This wording is relative to the FDIS.

1. Change Table 28 — Allocator requirements in 17.6.3.5 [allocator.requirements]:

   Table 28 — Allocator requirements
   Expression | Return type | Assertion/note pre-/post-condition | Default
   --- | --- | --- | ---
   `X a1(a); X a1 = a;` |  | Shall not exit via an exception. post: `a1 == a` |
   `…`
   `X a1(move(a)); X a1 = move(a);` |  | Shall not exit via an exception. post: `a1` equals the prior value of `a`. |

2. Change 17.6.3.5 [allocator.requirements] paragraph 4:

   > An allocator type `X` shall satisfy the requirements of `CopyConstructible` (17.6.3.1 [utility.arg.requirements]). The `X::pointer`, `X::const_pointer`, `X::void_pointer`, and `X::const_void_pointer` types shall satisfy the requirements of `NullablePointer` (17.6.3.3 [nullablepointer.requirements]). No constructor, comparison operator, copy operation, move operation, or swap operation on these types shall exit via an exception. `X::pointer` and `X::const_pointer` shall also satisfy the requirements for a random access iterator (24.2 [iterator.requirements]).

### 2083. const-qualification on `weak_ptr::owner_before`

Section: 20.8.2.3 [util.smartptr.weak], 20.8.2.3.5 [util.smartptr.weak.obs] Status: C++14 Submitter: Ai Azuma Opened: 2011-09-06 Last modified: 2015-04-08

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Discussion:

Is there any reason why `weak_ptr::owner_before` member function templates are not const-qualified?

Daniel Krügler:

> I don't think so. To the contrary, without these to be const member function templates, the semantics of the specializations `owner_less<weak_ptr<T>>` and `owner_less<shared_ptr<T>>` described in 20.8.2.4 [util.smartptr.ownerless] is unclear.

> It is amusing to note that this miss has remain undetected from the accepted paper n2637 on. For the suggested wording changes see below.

[2012, Kona]

Move to Ready.

[2012, Portland: applied to WP]

Proposed resolution:

This wording is relative to the FDIS.

1. Change the class template `weak_ptr` synopsis in 20.8.2.3 [util.smartptr.weak] as indicated:

   >     namespace std {
   >       template<class T> class weak_ptr {
   >       public:
   >         typedef T element_type;
   >         […]
   >         template<class U> bool owner_before(shared_ptr<U> const& b) const;
   >         template<class U> bool owner_before(weak_ptr<U> const& b) const;
   >       };
   >       […]
   >     }

2. Change the prototypes in 20.8.2.3.5 [util.smartptr.weak.obs] before p6 as indicated:

   >     template<class U> bool owner_before(shared_ptr<U> const& b) const;
   >     template<class U> bool owner_before(weak_ptr<U> const& b) const;

### 2085. Wrong description of effect 1 of `basic_istream::ignore`

Section: 27.7.2.3 [istream.unformatted] Status: C++14 Submitter: Krzysztof Zelechowski Opened: 2011-09-11 Last modified: 2015-04-08

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Discussion:

27.7.2.3 [istream.unformatted] in N3242 currently has the following to say about the semantics of `basic_istream::ignore`:

> [..]. Characters are extracted until any of the following occurs:

> - if `n != numeric_limits<streamsize>::max()` (18.3.2), `n` characters are extracted

This statement, apart from being slightly ungrammatical, indicates that if (`n == numeric_limits<streamsize>::max()`), the method returns without extracting any characters.

The description intends to describe the observable behaviour of an implementation in terms of logical assertions. Logical assertions are not "bullets" that can be "entered" but need not; they are predicates that can evaluate to true or false.

The description contains two predicates, either of them causes extraction to terminate. In the incriminated case, the first predicate is evaluates to true because its premise is false, therefore no characters will be extracted.

The intended semantics would be described by the following statement:

> [..]. Characters are extracted until any of the following occurs:

> - `(n != numeric_limits<streamsize>::max())` (18.3.2) and (`n`) characters have been extracted so far.

[2013-04-20, Bristol]

Resolution: Ready.

[2013-09-29, Chicago]

Apply to Working Paper

Proposed resolution:

This wording is relative to the FDIS.

Change 27.7.2.3 [istream.unformatted] p25 as indicated:

>     basic_istream<charT,traits>&
>       ignore(streamsize n = 1, int_type delim = traits::eof());

> > -25- Effects: Behaves as an unformatted input function (as described in 27.7.2.3 [istream.unformatted], paragraph 1). After constructing a `sentry` object, extracts characters and discards them. Characters are extracted until any of the following occurs:

> > - if `n != numeric_limits<streamsize>::max()` (18.3.2.1 [limits.numeric]),and `n` characters are have been extracted so far
> > - end-of-file occurs on the input sequence (in which case the function calls `setstate(eofbit)`, which may throw `ios_base::failure` (27.5.5.4 [iostate.flags]));
> > - `traits::eq_int_type(traits::to_int_type(c), delim)` for the next available input character `c` (in which case `c` is extracted).

### 2086. Overly generic type support for math functions

Section: 26.8 [c.math] Status: C++14 Submitter: Daniel Krügler Opened: 2011-09-22 Last modified: 2015-04-08

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Discussion:

26.8 [c.math] ends with a description of a rule set for "sufficient overloads" in p11:

> Moreover, there shall be additional overloads sufficient to ensure:

> 1. If any argument corresponding to a `double` parameter has type `long double`, then all arguments corresponding to `double` parameters are effectively cast to `long double`.
> 2. Otherwise, if any argument corresponding to a `double` parameter has type `double` or an integer type, then all arguments corresponding to `double` parameters are effectively cast to `double`.
> 3. Otherwise, all arguments corresponding to `double` parameters are effectively cast to `float`.

My impression is that this rule set is probably more generic as intended, my assumption is that it is written to mimic the C99/C1x rule set in 7.25 p2+3 in the "C++" way:

> -2- Of the `<math.h>` and `<complex.h>` functions without an `f` (`float`) or `l` (`long double`) suffix, several have one or more parameters whose corresponding real type is `double`. For each such function, except `modf`, there is a corresponding type-generic macro. (footnote 313) The parameters whose corresponding real type is `double` in the function synopsis are generic parameters. Use of the macro invokes a function whose corresponding real type and type domain are determined by the arguments for the generic parameters. (footnote 314)

> -3- Use of the macro invokes a function whose generic parameters have the corresponding real type determined as follows:
> - First, if any argument for generic parameters has type `long double`, the type determined is `long double`.
> - Otherwise, if any argument for generic parameters has type `double` or is of integer type, the type determined is `double`.
> - Otherwise, the type determined is `float`.

where footnote 314 clarifies the intent:

> If the type of the argument is not compatible with the type of the parameter for the selected function, the behavior is undefined.

The combination of the usage of the unspecific term "cast" with otherwise no further constraints (note that C constraints the valid set to types that C++ describes as arithmetic types, but see below for one important difference) has the effect that it requires the following examples to be well-formed and well-defined:

>     #include <cmath>

>     enum class Ec { };

>     struct S { explicit operator long double(); };

>     void test(Ec e, S s) {
>      std::sqrt(e); // OK, behaves like std::sqrt((float) e);
>      std::sqrt(s); // OK, behaves like std::sqrt((float) s);
>     }

GCC 4.7 does not accept any of these examples.

I found another example where the C++ rule differs from the C set, but in this case I'm not so sure, which direction C++ should follow. The difference is located in the fact, that in C enumerated types are integer types as described in 6.2.5 p17 (see e.g. n1569 or n1256):

"The type char, the signed and unsigned integer types, and the enumerated types are collectively called integer types. The integer and real floating types are collectively called real types."

This indicates that in C the following code

>     #include <math.h>

>     enum E { e };

>     void test(void) {
>       sqrt(e); // OK, behaves like sqrt((double) e);
>     }

seems to be well-defined and `e` is cast to `double`, but in C++ referring to

>     #include <cmath>

>     enum E { e };

>     void test() {
>       std::sqrt(e); // OK, behaves like sqrt((float) e);
>     }

is also well-defined (because of our lack of constraints) but we must skip bullet 2 (because E is not an integer type) and effectively cast `e` to `float`. Accepting this, we would introduce a silent, but observable runtime difference for C and C++.

GCC 4.7 does not accept this example, but causes an ambiguity error among the three floating point overloads of sqrt.

My current suggestion to fix these problems would be to constrain the valid argument types of these functions to arithmetic types.

> Howard provided wording to solve the issue.

[2012, Kona]

Moved to Ready. The proposed wording reflects both original intent from TR1, and current implementations.

[2012, Portland: applied to WP]

Proposed resolution:

This wording is relative to the FDIS.

Change 26.8 [c.math] p11 as indicated:

> Moreover, there shall be additional overloads sufficient to ensure:

> 1. If any arithmetic argument corresponding to a `double` parameter has type `long double`, then all arithmetic arguments corresponding to `double` parameters are effectively cast to `long double`.
> 2. Otherwise, if any arithmetic argument corresponding to a `double` parameter has type `double` or an integer type, then all arithmetic arguments corresponding to `double` parameters are effectively cast to `double`.
> 3. Otherwise, all arithmetic arguments corresponding to `double` parameters are effectively cast to have type `float`.

### 2087. `iostream_category()` and `noexcept`

Section: 27.5 [iostreams.base] Status: C++14 Submitter: Nicolai Josuttis Opened: 2011-09-22 Last modified: 2015-04-08

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Discussion:

In `<system_error>` we have:

>     const error_category& generic_category() noexcept;
>     const error_category& system_category() noexcept;

In `<future>` we have:

>     const error_category& future_category() noexcept;

But in `<ios>` we have:

>     const error_category& iostream_category();

Is there any reason that `iostream_category()` is not declared with `noexcept` or is this an oversight?

Daniel:

This looks like an oversight to me. We made the above mentioned changes as part of noexcept-ifying the thread library but `iostream_category()` was skipped, so it seems to be forgotten. There should be no reason, why it cannot be `noexcept`. When doing so, we should also make these functions `noexcept` (similar to corresponding overloads):

>     error_code make_error_code(io_errc e);
>     error_condition make_error_condition(io_errc e);

Suggested wording provided by Daniel.

[2013-04-20, Bristol]

Unanimous.

Resolution: move to tentatively ready.

[2013-09-29, Chicago]

Apply to Working Paper

Proposed resolution:

This wording is relative to the FDIS.

1. Change 27.5.1 [iostreams.base.overview], header `<ios>` synopsis as indicated:

   >     #include <iosfwd>
   >     namespace std {
   >       […]
   >       error_code make_error_code(io_errc e) noexcept;
   >       error_condition make_error_condition(io_errc e) noexcept;
   >       const error_category& iostream_category() noexcept;
   >     }

2. Change the prototype declarations in 27.5.6.5 [error.reporting] as indicated:

   >     error_code make_error_code(io_errc e) noexcept;

   > -1- Returns: `error_code(static_cast<int>(e), iostream_category())`.

   >     error_condition make_error_condition(io_errc e) noexcept;

   > -2- Returns: `error_condition(static_cast<int>(e), iostream_category())`.

   >     const error_category& iostream_category() noexcept;

   > -3- Returns: A reference to an object of a type derived from class `error_category`.

   > -4- The object’s `default_error_condition` and `equivalent` virtual functions shall behave as specified for the class `error_category`. The object’s `name` virtual function shall return a pointer to the string `"iostream"`.

### 2091. Misplaced effect in `m.try_lock_for()`

Section: 30.4.1.3 [thread.timedmutex.requirements] Status: C++14 Submitter: Pete Becker Opened: 2011-10-18 Last modified: 2015-04-08

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Discussion:

30.4.1.3 [thread.timedmutex.requirements]/4 says, in part,

> "Requires: If the tick period of [the argument] is not exactly convertible … [it] shall be rounded up …"

This doesn't belong in the requires clause. It's an effect. It belongs in paragraph 5. Nitpickingly, this would be a technical change: as written it imposes an obligation on the caller, while moving it imposes an obligation on the callee. Although that's certainly not what was intended.

Peter Dimov comments:

Not to mention that it should round down, not up. :-)

Incidentally, I see that the wrong `try_lock` requirement that the caller shall not own the mutex has entered the standard, after all. Oh well. Let's hope that the real world continues to ignore it.

[2012, Kona]

Remove the offending sentence from the requirements clause. Do not add it back anywhere else. The implementation already must have license to wake up late, so the rounding is invisible.

Move to Review.

[2012, Portland]

Concurrency move to Ready.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3337.

1. Change 30.4.1.3 [thread.timedmutex.requirements]/4 as indicated:

   > -3- The expression `m.try_lock_for(rel_time)` shall be well-formed and have the following semantics:

   > -4- Requires: If the tick period of `rel_time` is not exactly convertible to the native tick period, the duration shall be rounded up to the nearest native tick period. If `m` is of type `std::timed_mutex`, the calling thread does not own the mutex.

### 2092. Vague Wording for `condition_variable_any`

Section: 30.5.2 [thread.condition.condvarany] Status: C++14 Submitter: Pete Becker Opened: 2011-10-20 Last modified: 2015-04-08

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Discussion:

30.5.2 [thread.condition.condvarany]/4 says, in part, that `condition_variable_any()` throws an exception "if any native handle type manipulated is not available".

I don't know what that means. Is this intended to say something different from the analogous words for `condition_variable()` [30.5.1 [thread.condition.condvar]/4], "if some non-memory resource limitation prevents initialization"? If not, it should be worded the same way.

[2012, Kona]

Copy the corresponding wording from the `condition_variable` constructor in 30.5.1 [thread.condition.condvar] p4.

Move to Review.

[2012, Portland]

Concurrency move to Ready.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3337.

1. Change 30.4.1.3 [thread.timedmutex.requirements]/4 as indicated:

   >     condition_variable_any();

   > > […]

   > > -4- Error conditions:
   > > - `resource_unavailable_try_again` — if any native handle type manipulated is not available if some non-memory resource limitation prevents initialization.
   > > - `operation_not_permitted` — if the thread does not have the privilege to perform the operation.

### 2093. Throws clause of `condition_variable::wait` with predicate

Section: 30.5.1 [thread.condition.condvar] Status: C++14 Submitter: Alberto Ganesh Barbati Opened: 2011-10-27 Last modified: 2015-04-08

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Discussion:

the Throws: clause of `condition_variable::wait/wait_xxx` functions that take a predicate argument is:

> Throws: `system_error` when an exception is required (30.2.2 [thread.req.exception]).

If executing the predicate throws an exception, I would expect such exception to propagate unchanged to the caller, but the throws clause seems to indicate that it gets mutated into a system_error. T hat's because of 17.5.1.4 [structure.specifications]/4:

"If F’s semantics contains a Throws:, Postconditions:, or Complexity: element, then that supersedes any occurrences of that element in the code sequence."

Is my interpretation correct? Does it match the intent?

Daniel comments:

I don't think that this interpretation is entirely correct, the wording does not say that `std::system_error` or a derived class must be thrown, it simply is underspecified in this regard (The extreme interpretation is that the behaviour would be undefined, but that would be too far reaching I think). We have better wording for this in 30.4.4.2 [thread.once.callonce] p4, where it says:

"Throws: `system_error` when an exception is required (30.2.2 [thread.req.exception]), or any exception thrown by `func`."

or in 30.3.2 [thread.thread.this] p6/p9:

"Throws: Nothing if `Clock` satisfies the `TrivialClock` requirements (20.12.3 [time.clock.req]) and operations of `Duration` do not throw exceptions. [ Note: instantiations of time point types and clocks supplied by the implementation as specified in 20.12.7 [time.clock] do not throw exceptions. — end note ]"

So, the here discussed Throws elements should add lines along the lines of

"Any exception thrown by operations of `pred`."

and similar wording for time-related operations:

"Any exception thrown by operations of `Duration`",

"Any exception thrown by operations of `chrono::duration<Rep, Period>`"

[2011-11-28: Ganesh comments and suggests wording]

As for the discussion about the exception thrown by the manipulation of time-related objects, I believe the argument applies to all functions declared in 30 [thread]. Therefore, instead of adding wording to each member, I would simply move those requirements from 30.3.2 [thread.thread.this] p6/p9 to a new paragraph in 30.2.4 [thread.req.timing].

As for 30.5.2 [thread.condition.condvarany], the member functions `wait()` and `wait_until()` are described only in terms of the Effects: clause (so strictly speaking, they need no changes), however, `wait_for()` is described with a full set of clauses including Throws: and Error conditions:. Either we should add those clauses to `wait/wait_until` with changes similar to the one above, or remove paragraphs 29 to 34 entirely. By the way, even paragraph 26 could be removed IMHO.

[2012, Kona]

We like the idea behind the proposed resolution.

Modify the first addition to read instead: "Functions that specify a timeout, will throw if an operation on a clock, time point, or time duration throws an exception."

In the note near the bottom change "even if the timeout has already expired" to "or if the timeout has already expired". (This is independent, but the original doesn't seem to make sense.)

Move to Review.

[2012, Portland]

Concurrency move to Ready with slightly ammended wording.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3337.

1. Add a new paragraph at the end of 30.2.4 [thread.req.timing]:

   > […]

   > -6- The resolution of timing provided by an implementation depends on both operating system and hardware. The finest resolution provided by an implementation is called the native resolution.

   > -7- Implementation-provided clocks that are used for these functions shall meet the `TrivialClock` requirements (20.12.3 [time.clock.req]).

   > -?- Functions that specify a timeout, will throw if, during the execution of this function, a clock, time point, or time duration throws an exception. [ Note: instantiations of clock, time point and duration types supplied by the implementation as specified in 20.12.7 [time.clock] do not throw exceptions. — end note]

2. Change 30.3.2 [thread.thread.this] as indicated:

   >     template <class Clock, class Duration>
   >       void sleep_until(const chrono::time_point<Clock, Duration>& abs_time);;

   > > -4- Effects: Blocks the calling thread for the absolute timeout (30.2.4 [thread.req.timing]) specified by `abs_time`.

   > > -5- Synchronization: None.

   > > -6- Throws: timeout-related exceptions (30.2.4 [thread.req.timing]).Nothing if `Clock` satisfies the `TrivialClock` requirements (20.12.3 [time.clock.req]) and operations of `Duration` do not throw exceptions. [ Note: instantiations of time point types and clocks supplied by the implementation as specified in 20.12.7 [time.clock] do not throw exceptions. — end note]

   >     template <class Rep, class Period>
   >       void sleep_for(const chrono::duration<Rep, Period>& rel_time);;

   > > -7- Effects: Blocks the calling thread for the relative timeout (30.2.4 [thread.req.timing]) specified by `rel_time`.

   > > -8- Synchronization: None.

   > > -9- Throws: timeout-related exceptions (30.2.4 [thread.req.timing]).Nothing if operations of `chrono::duration<Rep, Period>` do not throw exceptions. [ Note: instantiations of time point types and clocks supplied by the implementation as specified in 20.12.7 [time.clock] do not throw exceptions. — end note]

3. Change 30.4.1.3 [thread.timedmutex.requirements] as indicated:

   -3- The expression `m.try_lock_for(rel_time)` shall be well-formed and have the following semantics:

   […]

   -5- Effects: The function attempts to obtain ownership of the mutex within the relative timeout (30.2.4 [thread.req.timing]) specified by `rel_time`. If the time specified by `rel_time` is less than or equal to `rel_time.zero()`, the function attempts to obtain ownership without blocking (as if by calling `try_lock()`). The function shall return within the timeout specified by `rel_time` only if it has obtained ownership of the mutex object. [Note: As with `try_lock()`, there is no guarantee that ownership will be obtained if the lock is available, but implementations are expected to make a strong effort to do so. — end note]

   […]

   -8- Synchronization: If `try_lock_for()` returns `true`, prior `unlock()` operations on the same object synchronize with (1.10 [intro.multithread]) this operation.

   -9- Throws: timeout-related exceptions (30.2.4 [thread.req.timing]).Nothing.

   -10- The expression `m.try_lock_until(abs_time)` shall be well-formed and have the following semantics:

   […]

   -12- Effects: The function attempts to obtain ownership of the mutex. If `abs_time` has already passed, the function attempts to obtain ownership without blocking (as if by calling `try_lock()`). The function shall return before the absolute timeout (30.2.4 [thread.req.timing]) specified by `abs_time` only if it has obtained ownership of the mutex object. [Note: As with `try_lock()`, there is no guarantee that ownership will be obtained if the lock is available, but implementations are expected to make a strong effort to do so. — end note]

   […]

   -15- Synchronization: If `try_lock_until()` returns true, prior `unlock()` operations on the same object synchronize with (1.10 [intro.multithread]) this operation.

   -16- Throws: timeout-related exceptions (30.2.4 [thread.req.timing]).Nothing.

4. Change 30.5.1 [thread.condition.condvar] as indicated:

   >     template <class Predicate>
   >       void wait(unique_lock<mutex>& lock, Predicate pred);

   > > […]

   > > -15- Effects: Equivalent to:

   > > >     while (!pred())
   > > >       wait(lock);

   > > […]

   > > -17- Throws: `std::system_error` when an exception is required (30.2.2 [thread.req.exception]), timeout-related exceptions (30.2.4 [thread.req.timing]), or any exception thrown by `pred`.

   > > […]

   >     template <class Clock, class Duration>
   >       cv_status wait_until(unique_lock<mutex>& lock,
   >                            const chrono::time_point<Clock, Duration>& abs_time);

   > > […]

   > > -23- Throws: `system_error` when an exception is required (30.2.2 [thread.req.exception]) or timeout-related exceptions (30.2.4 [thread.req.timing]).

   > > […]

   >     template <class Rep, class Period>
   >       cv_status wait_for(unique_lock<mutex>& lock,
   >                          const chrono::duration<Rep, Period>& rel_time);

   > > […]

   > > -26- Effects: as if Equivalent to:

   > > >     return wait_until(lock, chrono::steady_clock::now() + rel_time);

   > > […]

   > > -29- Throws: `system_error` when an exception is required (30.2.2 [thread.req.exception]) or timeout-related exceptions (30.2.4 [thread.req.timing]).

   > > […]

   >     template <class Clock, class Duration, class Predicate>
   >       bool wait_until(unique_lock<mutex>& lock,
   >                       const chrono::time_point<Clock, Duration>& abs_time,
   >                       Predicate pred);

   > > […]

   > > -32- Effects: Equivalent to:

   > > >     while (!pred())
   > > >       if (wait_until(lock, abs_time) == cv_status::timeout)
   > > >         return pred();
   > > >     return true;

   > > -33- Returns: `pred()`

   > > […]

   > > -36- Throws: `std::system_error` when an exception is required (30.2.2 [thread.req.exception]), timeout-related exceptions (30.2.4 [thread.req.timing]), or any exception thrown by `pred`.

   > > […]

   >     template <class Rep, class Period, class Predicate>
   >       bool wait_for(unique_lock<mutex>& lock,
   >                     const chrono::duration<Rep, Period>& rel_time,
   >                     Predicate pred);

   > > […]

   > > -39- Effects: as if Equivalent to:

   > > >     return wait_until(lock, chrono::steady_clock::now() + rel_time, std::move(pred));

   > > […]

   > > -42- Returns: `pred()`

   > > […]

   > > -44- Throws: `system_error` when an exception is required (30.2.2 [thread.req.exception]), timeout-related exceptions (30.2.4 [thread.req.timing]), or any exception thrown by `pred`.

   > > […]

5. Change 30.5.2 [thread.condition.condvarany] as indicated:

   >     template <class Lock, class Predicate>
   >       void wait(Lock& lock, Predicate pred);

   > > -14- Effects: Equivalent to:

   > > >     while (!pred())
   > > >       wait(lock);

   >     template <class Lock, class Clock, class Duration>
   >       cv_status wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time);

   > > […]

   > > -18- Throws: `system_error` when an exception is required (30.2.2 [thread.req.exception]) or any timeout-related exceptions (30.2.4 [thread.req.timing]).

   > > […]

   >     template <class Lock, class Rep, class Period>
   >       cv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);

   > > […]

   > > -20- Effects: as if Equivalent to:

   > > >     return wait_until(lock, chrono::steady_clock::now() + rel_time);

   > > […]

   > > -23- Throws: `system_error` when an exception is required (30.2.2 [thread.req.exception]) or any timeout-related exceptions (30.2.4 [thread.req.timing]).

   > > […]

   >     template <class Lock, class Clock, class Duration, class Predicate>
   >       bool wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time, Predicate pred);

   > > -25- Effects: Equivalent to:

   > > >     while (!pred())
   > > >       if (wait_until(lock, abs_time) == cv_status::timeout)
   > > >         return pred();
   > > >     return true;

   > > -26- Returns: `pred()`[Note: There is no blocking if `pred()` is initially `true`, or if the timeout has already expired. — end note]

   > > -27- [Note: The returned value indicates whether the predicate evaluates to `true` regardless of whether the timeout was triggered. end note]

   >     template <class Lock, class Rep, class Period, class Predicate>
   >       bool wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time, Predicate pred);

   > > -28- Effects: as if Equivalent to:

   > > >     return wait_until(lock, chrono::steady_clock::now() + rel_time, std::move(pred));

   > > -29- [Note: There is no blocking if `pred()` is initially `true`, even if the timeout has already expired. — end note] -30- Postcondition: `lock` is locked by the calling thread.

   > > -31- Returns: `pred()`

   > > -32- [Note: The returned value indicates whether the predicate evaluates to `true` regardless of whether the timeout was triggered. — end note]

   > > -33- Throws: `system_error` when an exception is required (30.2.2 [thread.req.exception]).

   > > -34- Error conditions:
   > > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

### 2094. `duration` conversion overflow shouldn't participate in overload resolution

Section: 20.12.5.1 [time.duration.cons] Status: C++14 Submitter: Vicente J. Botet Escriba Opened: 2011-10-31 Last modified: 2015-04-08

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Discussion:

20.12.5.1 [time.duration.cons] says:

>     template <class Rep2, class Period2>
>       constexpr duration(const duration<Rep2, Period2>& d);

> > Remarks: This constructor shall not participate in overload resolution unless `treat_as_floating_point<rep>::value` is `true` or both `ratio_divide<Period2, period>::den` is `1` and `treat_as_floating_point<Rep2>::value` is `false`.

The evaluation of `ratio_divide<Period2, period>::den` could make `ratio_divide<Period2, period>::num` overflow.

This occur for example when we try to create a millisecond (`period`=`ratio<1,1000>`) from an exa-second (`Period2`=`ratio<1018>`).

`ratio_divide<ratio<1018>, ratio<1,1000>>::num` is `1021` which overflows which makes the compiler error.

If the function `f` is overloaded with milliseconds and seconds

>     void f(milliseconds);
>     void f(seconds);

The following fails to compile.

>     duration<int,exa> r(1);
>     f(r);

While the conversion to seconds work, the conversion to milliseconds make the program fail at compile time. In my opinion, this program should be well formed and the constructor from `duration<int,exa>` to milliseconds shouldn't participate in overload resolution as the result can not be represented.

I think the wording of the standard can be improved so no misinterpretations are possible by adding that "no overflow is induced by the conversion".

[2012, Kona]

Move to Review.

Pete: The wording is not right.

Howard: Will implement this to be sure it works.

Jeffrey: If ratio needs a new hook, should it be exposed to users for their own uses?

Pete: No.

Move to Review, Howard to implement in a way that mere mortals can understand.

[2013-04-18, Bristol]

Proposed resolution:

This wording is relative to the FDIS.

Change the following paragraphs of 20.12.5.1 [time.duration.cons] p4 indicated:

>     template <class Rep2, class Period2>
>       constexpr duration(const duration<Rep2, Period2>& d);

> > Remarks: This constructor shall not participate in overload resolution unless no overflow is induced in the conversion and `treat_as_floating_point<rep>::value` is `true` or both `ratio_divide<Period2, period>::den` is `1` and `treat_as_floating_point<Rep2>::value` is `false`. [ Note: This requirement prevents implicit truncation error when converting between integral-based duration types. Such a construction could easily lead to confusion about the value of the duration. — end note ]

### 2096. Incorrect constraints of `future::get` in regard to `MoveAssignable`

Section: 30.6.6 [futures.unique_future] Status: C++14 Submitter: Daniel Krügler Opened: 2011-11-02 Last modified: 2015-04-08

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Discussion:

30.6.6 [futures.unique_future] paragraph 15 says the following:

>     R future::get();

…

-15- Returns:

- `future::get()` returns the value stored in the object’s shared state. If the type of the value is `MoveAssignable` the returned value is moved, otherwise it is copied.

  …

There are some problems with the description:

"If the type of the value is `MoveAssignable` the returned value is moved, otherwise it is copied."

1. It seems to impose unrealistic constraints on implementations, because how could an implementor recognize whether a user-defined type satisfies the semantics of `MoveAssignable`? This should be based solely on a pure expression-based requirement, if this is an requirement for implementations.
2. Reducing `MoveAssignable` to the plain expression part `std::is_move_assignable` would solvs (1), but raises another question, namely why a move-assignment should be relevant for a function return based on the value stored in the future state? We would better fall back to `std::is_move_constructible` instead.

3. The last criticism I have is about the part

   "the returned value is moved, otherwise it is copied"

   because an implementation won't be able to recognize what the user-defined type will do during an expression that is prepared by the implementation. I think the wording is intended to allow a move by seeding with an rvalue expression via `std::move` (or equivalent), else the result will be an effective copy construction.

[2011-11-28 Moved to Tentatively Ready after 5 positive votes on c++std-lib.]

Proposed resolution:

This wording is relative to the FDIS.

Change 30.6.6 [futures.unique_future] paragraph 15 as indicated:

>     R future::get();

…

-15- Returns:

- `future::get()` returns the value `v` stored in the object’s shared state as `std::move(v)`. If the type of the value is `MoveAssignable` the returned value is moved, otherwise it is copied.

  …

### 2097. `packaged_task` constructors should be constrained

Section: 30.6.9.1 [futures.task.members] Status: C++14 Submitter: Jonathan Wakely Opened: 2011-11-02 Last modified: 2015-04-08

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Discussion:

With the proposed resolution of 2067, this no longer selects the copy constructor:

>     std::packaged_task<void()> p1;
>     std::packaged_task<void()> p2(p1);

Instead this constructor is a better match:

>     template <class F>
>      explicit packaged_task(F&& f);

This attempts to package a `packaged_task`, which internally tries to copy `p2`, which fails because the copy constructor is deleted. For at least one implementation the resulting error message is much less helpful than the expected "cannot call deleted function" because it happens after instantiating several more templates rather than in the context where the constructor is called.

I believe the solution is to constrain to the template constructors so the template argument `F` cannot be deduced as (possibly cv) `packaged_task&` or `packaged_task`. It could be argued this constraint is already implied because `packaged_task` is not copyable and the template constructors require that "invoking a copy of `f` shall behave the same as invoking `f`".

Daniel points out that the variadic constructor of `std::thread` described in 30.3.1.2 [thread.thread.constr] has a similar problem and suggests a similar wording change, which has been integrated below.

An alternative is to declare `thread(thread&)` and `packaged_task(packaged_task&)` as deleted.

[2012, Portland]

This issue appears to be more about library specification than technical concurrency issues, so should be handled in LWG.

[2013, Chicago]

Move to Immediate resolution.

Howard volunteered existing implementation experience with the first change, and saw no issue that the second would introduce any new issue.

Proposed resolution:

This wording is relative to the FDIS.

1. Insert a new Remarks element to 30.3.1.2 [thread.thread.constr] around p3 as indicated:

   >     template <class F, class ...Args> explicit thread(F&& f, Args&&... args);

   -3- Requires: `F` and each `Ti` in `Args` shall satisfy the `MoveConstructible` requirements. `INVOKE(DECAY_COPY ( std::forward<F>(f)), DECAY_COPY (std::forward<Args>(args))...)` (20.8.2) shall be a valid expression.

   -?- Remarks: This constructor shall not participate in overload resolution if `decay<F>::type` is the same type as `std::thread`.

2. Insert a new Remarks element to 30.6.9.1 [futures.task.members] around p2 as indicated:

   >     template <class F>
   >       packaged_task(F&& f);
   >     template <class F, class Allocator>
   >       explicit packaged_task(allocator_arg_t, const Allocator& a, F&& f);

   -2- Requires: `INVOKE(f, t1, t2, ..., tN, R)`, where `t1, t2, ..., tN` are values of the corresponding types in `ArgTypes...`, shall be a valid expression. Invoking a copy of `f` shall behave the same as invoking `f`.

   -?- Remarks: These constructors shall not participate in overload resolution if `decay<F>::type` is the same type as `std::packaged_task<R(ArgTypes...)>`.

### 2098. Minor Inconsistency between `promise::set_value` and `promise::set_value_at_thread_exit`

Section: 30.6.5 [futures.promise] Status: C++14 Submitter: Pete Becker Opened: 2011-11-14 Last modified: 2015-04-08

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Discussion:

30.6.5 [futures.promise]/16 says that `promise::set_value(const R&)` throws any exceptions thrown by `R`'s copy constructor, and that `promise_set_value(R&&)` throws any exceptions thrown by `R`'s move constructor.

30.6.5 [futures.promise]/22 is the Throws: clause for `promise::set_value_at_thread_exit`. It has no corresponding requirements, only that these functions throw "`future_error` if an error condition occurs."

Daniel suggests wording to fix this: The approach is a bit more ambitious and also attempts to fix wording glitches of 30.6.5 [futures.promise]/16, because it would be beyond acceptable efforts of implementations to determine whether a constructor call of a user-defined type will indeed call a copy constructor or move constructor (in the first case it might be a template constructor, in the second case it might also be a copy-constructor, if the type has no move constructor).

[2012, Portland: move to Review]

Moved to Review by the concurrency working group, with no further comments.

[2013-04-20, Bristol]

Accepted for the working paper

Proposed resolution:

This wording is relative to the FDIS.

1. Change 30.6.5 [futures.promise]/16 as indicated:

   >     void promise::set_value(const R& r);
   >     void promise::set_value(R&& r);
   >     void promise<R&>::set_value(R& r);
   >     void promise<void>::set_value();

   > > […]

   > > -16- Throws:
   > > - `future_error` if its shared state already has a stored value or exception, or
   > > - for the first version, any exception thrown by the copy constructor of constructor selected to copy an object of `R`, or
   > > - for the second version, any exception thrown by the move constructor of constructor selected to move an object of `R`.

2. Change 30.6.5 [futures.promise]/22 as indicated:

   >     void promise::set_value_at_thread_exit(const R& r);
   >     void promise::set_value_at_thread_exit(R&& r);
   >     void promise<R&>::set_value_at_thread_exit(R& r);
   >     void promise<void>::set_value_at_thread_exit();

   > > […]

   > > -16- Throws: `future_error` if an error condition occurs.
   > > - `future_error` if its shared state already has a stored value or exception, or
   > > - for the first version, any exception thrown by the constructor selected to copy an object of `R`, or
   > > - for the second version, any exception thrown by the constructor selected to move an object of `R`.

### 2099. Unnecessary constraints of `va_start()` usage

Section: 18.10 [support.runtime] Status: C++14 Submitter: Daniel Krügler Opened: 2011-11-12 Last modified: 2015-04-08

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Discussion:

In 18.10 [support.runtime] p3 we find (emphasis mine):

> The restrictions that ISO C places on the second parameter to the `va_start()` macro in header `<stdarg.h>` are different in this International Standard. The parameter `parmN` is the identifier of the rightmost parameter in the variable parameter list of the function definition (the one just before the ...).227 If the parameter `parmN` is declared with a function, array, or reference type, or with a type that is not compatible with the type that results when passing an argument for which there is no parameter, the behavior is undefined.

It seems astonishing that the constraints on function types and array types imposes these on the declared parameter `parmN`, not to the adjusted one (which would not require this extra wording, because that is implicit). This seems to say that a function definition of the form (Thanks to Johannes Schaub for this example)

>     #include <stdarg.h>

>     void f(char const paramN[], ...) {
>       va_list ap;
>       va_start(ap, paramN);
>       va_end(ap);
>     }

would produce undefined behaviour when used.

Similar wording exists in C99 and in the most recent C11 draft in 7.16.1.4 p4

In my opinion the constraints in regard to array types and function types are unnecessary and should be relaxed. Are there really implementations out in the wild that would (according to my understanding incorrectly) provide the declared and not the adjusted type of `paramN` as deduced type to `va_start()`?

[2012, Kona]

Move to Ready.

[2012, Portland: applied to WP]

Proposed resolution:

This wording is relative to the FDIS.

Change 18.10 [support.runtime] p3 as indicated:

> The restrictions that ISO C places on the second parameter to the `va_start()` macro in header `<stdarg.h>` are different in this International Standard. The parameter `parmN` is the identifier of the rightmost parameter in the variable parameter list of the function definition (the one just before the ...).227 If the parameter `parmN` is declared with of a function, array, or reference type, or with of a type that is not compatible with the type that results when passing an argument for which there is no parameter, the behavior is undefined.

### 2100. timed waiting functions cannot timeout if `launch::async` policy used

Section: 30.6.8 [futures.async] Status: C++14 Submitter: Jonathan Wakely Opened: 2011-11-14 Last modified: 2015-04-08

View all other issues in [futures.async].

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Discussion:

30.6.8 [futures.async] p5 says

> If the implementation chooses the `launch::async` policy,

> - a call to a waiting function on an asynchronous return object that shares the shared state created by this `async` call shall block until the associated thread has completed, as if joined (30.3.1.5 [thread.thread.member]);

That should say a non-timed waiting function, otherwise, calling a timed waiting function can block indefinitely waiting for the associated thread to complete, rather than timing out after the specified time.

Since `std::thread` does not provide a `timed_join()` function (nor does Pthreads, making it impossible on many platforms) there is no way for a timed waiting function to try to join but return early due to timeout, therefore timed waiting functions either cannot guarantee to timeout or cannot be used to meet the requirement to block until the thread is joined. In order to allow timed waiting functions to timeout the requirement should only apply to non-timed waiting functions.

[2012, Portland: move to Review]

Detlef: Do we actually need this fix — is it detectable?

Yes — you will never get a timeout. Should we strike the whole paragraph?

Hans: issue with thread local destruction.

Niklas: I have a strong expectation that a timed wait will respect the timeout

agreed

Detlef: we want a timed wait that does not time out to return like a non-timed wait; but is this implementable?

Pablo: Could we simply append ", or else time out"

Detlef: the time out on the shared state needs implementing anyway, even if the underlying O/S does not support a timed join.

Hans: the net effect is the timeout does not cover the thread local destruction... ah, I see what you're doing

Detlef: happy with Pablo's proposal

Wording proposed is to append after the word "joined" add ", or else time out"

Moved to review with this wording.

[2013, Bristol]

"Non-timed" made the new wording redundant and the result overly weak. Remove it.

Attempted to move to add this to the working paper (Concurrency motion 2) without the addition of "non-timed". Motion was withdrawn after Jonathan Wakely expressed implementability concerns.

[2013-09, Chicago]

Discussion of interaction with the absence of a Posix timed join.

Jonathan Wakely withdrew his objection, so moved to Immediate.

Accept for Working Paper

Proposed resolution:

[This wording is relative to the FDIS.]

Change 30.6.8 [futures.async] p5 as indicated:

> If the implementation chooses the `launch::async` policy,

> - a call to a waiting function on an asynchronous return object that shares the shared state created by this `async` call shall block until the associated thread has completed, as if joined, or else time out (30.3.1.5 [thread.thread.member]);

### 2102. Why is `std::launch` an implementation-defined type?

Section: 30.6.1 [futures.overview] Status: C++14 Submitter: Jonathan Wakely Opened: 2011-11-20 Last modified: 2015-04-08

View all other issues in [futures.overview].

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Discussion:

30.6.1 [futures.overview] says `std::launch` is an implementation-defined bitmask type, which would usually mean the implementation can choose whether to define an enumeration type, or a `bitset`, or an integer type. But in the case of `std::launch` it's required to be a scoped enumeration type,

>     enum class launch : unspecified {
>       async = unspecified,
>       deferred = unspecified,
>       implementation-defined
>     };

so what is implementation-defined about it, and what is an implementation supposed to document about its choice?

[2011-12-02 Moved to Tentatively Ready after 6 positive votes on c++std-lib.]

Proposed resolution:

This wording is relative to the FDIS.

Change 30.6.1 [futures.overview] paragraph 2 as indicated:

> The enum type `launch` is an implementation-defined a bitmask type (17.5.2.1.3 [bitmask.types]) with `launch::async` and `launch::deferred` denoting individual bits. [ Note: Implementations can provide bitmasks to specify restrictions on task interaction by functions launched by `async()` applicable to a corresponding subset of available launch policies. Implementations can extend the behavior of the first overload of `async()` by adding their extensions to the launch policy under the “as if” rule. — end note ]

### 2103. `std::allocator_traits<std::allocator<T>>::propagate_on_container_move_assignment`

Section: 20.7.9 [default.allocator] Status: C++14 Submitter: Ai Azuma Opened: 2011-11-08 Last modified: 2015-04-08

View all other issues in [default.allocator].

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Discussion:

"`std::allocator_traits<std::allocator<T>>::propagate_on_container_move_assignment::value`" is specified as "false", according to (20.7.9 [default.allocator]) and (20.7.8.1 [allocator.traits.types]). However, according to (23.2.1 [container.requirements.general]), this specification leads to the unneeded requirements (`MoveInsertable` and `MoveAssignable` of the value type) on the move assignment operator of containers with the default allocator.

Proposed resolution:

Either of the following two changes;

1. adding the nested typedef like "`typedef std::true_type propagate_on_container_move_assignment;`" in the definition of `std::allocator` class template,
2. adding the explicit partial specialization of "`std::allocator_traits`" class template for "`std::allocator`" class template, in which "`propagate_on_container_move_assignment`" nested typedef is specified as "`std::true_type`".

Pablo prefers the first resolution.

[2011-12-02: Pablo comments]

This issue has potentially some overlap with 2108. Should the trait `always_compare_equal` been added, this issue's resolution should be improved based on that.

[2012, Kona]

Move to Ready.

[2012, Portland: applied to WP]

Proposed resolution:

This wording is relative to the FDIS.

Change 20.7.9 [default.allocator], the class template `allocator` synopsis as indicated:

>     namespace std {
>       template <class T> class allocator;

>       // specialize for void:
>       template <> class allocator<void> {
>       public:
>         typedef void* pointer;
>         typedef const void* const_pointer;
>         // reference-to-void members are impossible.
>         typedef void value_type;
>         template <class U> struct rebind { typedef allocator<U> other; };
>       };

>       template <class T> class allocator {
>       public:
>         typedef size_t size_type;
>         typedef ptrdiff_t difference_type;
>         typedef T* pointer;
>         typedef const T* const_pointer;
>         typedef T& reference;
>         typedef const T& const_reference;
>         typedef T value_type;
>         template <class U> struct rebind { typedef allocator<U> other; };
>         typedef true_type propagate_on_container_move_assignment;

>         […]
>       };
>     }

### 2104. `unique_lock` move-assignment should not be `noexcept`

Section: 30.4.2.2 [thread.lock.unique] Status: C++14 Submitter: Anthony Williams Opened: 2011-11-27 Last modified: 2015-04-08

View all issues with C++14 status.

Discussion:

I just noticed that the `unique_lock` move-assignment operator is declared `noexcept`. This function may call `unlock()` on the wrapped mutex, which may throw.

Suggested change: remove the `noexcept` specification from `unique_lock::operator=(unique_lock&&)` in 30.4.2.2 [thread.lock.unique] and 30.4.2.2.1 [thread.lock.unique.cons].

Daniel:

I think the situation is actually a bit more complex as it initially looks.

First, the effects of the move-assignment operator are (emphasize mine):

> Effects: If `owns` calls `pm->unlock()`.

Now according to the `BasicLockable` requirements:

> `m.unlock()`

> 3 Requires: The current execution agent shall hold a lock on `m`.

> 4 Effects: Releases a lock on `m` held by the current execution agent.

> Throws: Nothing.

This shows that unlock itself is a function with narrow contract and for this reasons no unlock function of a mutex or lock itself does have a noexcept specifier according to our mental model.

Now the move-assignment operator attempts to satisfy these requirement of the function and calls it only when it assumes that the conditions are ok, so from the view-point of the caller of the move-assignment operator it looks as if the move-assignment operator would in total a function with a wide contract.

The problem with this analysis so far is, that it depends on the assumed correctness of the state "owns".

Looking at the construction or state-changing functions, there do exist several ones that depend on caller-code satisfying the requirements and there is one guy, who looks most suspicious:

> `unique_lock(mutex_type& m, adopt_lock_t);`

> 11 Requires: The calling thread own the mutex.
> […]
> 13 Postconditions: `pm == &m` and `owns == true`.

because this function does not even call `lock()` (which may, but is not required to throw an exception if the calling thread does already own the mutex). So we have in fact still a move-assignment operator that might throw an exception, if the mutex was either constructed or used (call of lock) incorrectly.

The correct fix seems to me to also add a "Throws: Nothing" element to the move-assignment operator, because using it correctly shall not throw an exception.

[Issaquah 20014-10-11: Move to Immediate after SG1 review]

Proposed resolution:

This wording is relative to the FDIS.

1. Change 30.4.2.2 [thread.lock.unique], class template `unique_lock` synopsis as indicated:

   >     namespace std {
   >       template <class Mutex>
   >       class unique_lock {
   >       public:
   >         typedef Mutex mutex_type;
   >         […]
   >         unique_lock(unique_lock&& u) noexcept;
   >         unique_lock& operator=(unique_lock&& u) noexcept;
   >         […]
   >       };
   >     }

2. Change 30.4.2.2.1 [thread.lock.unique.cons] around p22 as indicated:

   >     unique_lock& operator=(unique_lock&& u) noexcept;

   > > -22- Effects: If `owns` calls `pm->unlock()`.

   > > -23- Postconditions: `pm == u_p.pm` and `owns == u_p.owns` (where `u_p` is the state of `u` just prior to this construction), `u.pm == 0` and `u.owns == false`.

   > > -24- [Note: With a recursive mutex it is possible for both `*this` and `u` to own the same mutex before the assignment. In this case, `*this` will own the mutex after the assignment and `u` will not. — end note] -??- Throws: Nothing.

### 2105. Inconsistent requirements on `const_iterator`'s `value_type`

Section: 23.2.1 [container.requirements.general] Status: C++14 Submitter: Jeffrey Yasskin Opened: 2011-11-28 Last modified: 2015-04-08

View other active issues in [container.requirements.general].

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Discussion:

In the FDIS, Table 96 specifies `X::const_iterator` as a "constant iterator type whose value type is `T`". However, Table 97 specifies `X::const_reverse_iterator` as an "iterator type whose value type is `const T`" and which is defined as `reverse_iterator<const_iterator>`. But `reverse_iterator::value_type` is just "`typename iterator_traits<Iterator>::value_type`" 24.5.1.1 [reverse.iterator], so `const_iterator` and `const_reverse_iterator` must have the same `value_type`.

The resolution to issue 322 implies that `const_reverse_iterator` should change.

[2012, Kona]

Move to Ready.

[2012, Portland: applied to WP]

Proposed resolution:

This wording is relative to the FDIS.

Change Table 97 — "Reversible container requirements" as indicated

Table 97 — Reversible container requirements
Expression | Return type | Assertion/note pre-/post-condition | Complexity
--- | --- | --- | ---
`X::reverse_- iterator` | iterator type whose value type is `T` | `reverse_iterator<iterator>` | compile time
`X::const_- reverse_- iterator` | constant iterator type whose value type is `const T` | `reverse_iterator<const_iterator>` | compile time

### 2106. `move_iterator` wrapping iterators returning prvalues

Section: 24.5.3 [move.iterators] Status: WP Submitter: Dave Abrahams Opened: 2011-11-30 Last modified: 2015-04-08

View all other issues in [move.iterators].

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Discussion:

Shouldn't `move_iterator` be specialized so that if the iterator it wraps returns a prvalue when dereferenced, the `move_iterator` also returns by value? Otherwise, it creates a dangling reference.

Howard: I believe just changing `move_iterator<I>::reference` would do. A direction might be testing on `is_reference<iterator_traits<I>::reference>`, or `is_reference<decltype(*declval<I>())>`.

Daniel: I would prefer to use a consistent style among the iterator adaptors, so I suggest to keep with the `iterator_traits` typedefs if possible.

>     using reference = typename conditional<
>       is_reference<typename iterator_traits<Iterator>::reference>::value,
>       value_type&&,
>       value_type
>     >::type;

We might also want to ensure that if `Iterator`'s `reference` type is a reference, the referent is equal to `value_type` (after removal of cv-qualifiers). In general we have no such guarantee.

Marc: In the default case where we don't return `value_type&&`, should we use `value_type` or should we keep the `reference` type of the wrapped iterator?

Daniel: This suggestion looks appealing at first, but the problem here is that using this typedef can make it impossible for `move_iterator` to satisfy its contract, which means returning an rvalue of the value type (Currently it says rvalue-reference, but this must be fixed as of this issue anyway). I think that user-code can reasonably expect that when it has constructed an object `m` of `move_iterator<It>`, where `It` is a valid mutable iterator type, the expression

>     It::value_type&& rv = *m;

is well-formed.

Let's set `R` equal to `iterator_traits<Iterator>::reference` in the following. We can discuss the following situations:

1. `R` is a reference type: We can only return the corresponding xvalue of `R`, if `value_type` is reference-related to the referent type, else this is presumably no forward iterator and we cannot say much about it, except that it must be convertible to `value_type`, so it better should return a prvalue.
2. `R` is not a reference type: In this case we can rely on a conversion to `value_type` again, but not much more. Assume we would return `R` directly, this might turn out to have a conversion to an lvalue-reference type of the value type (for example). If that is the case, this would indirectly violate the contract of `move_iterator`.

In regard to the first scenario I suggest that implementations are simply required to check that `V2 = remove_cv<remove_reference<R>::type>::type` is equal to the value type `V1` as a criterion to return this reference as an xvalue, in all other cases it should return the value type directly as prvalue.

The additional advantage of this strategy is, that we always ensure that `reference` has the correct cv-qualification, if `R` is a real reference.

It is possible to improve this a bit by indeed supporting reference-related types, this would require to test `is_same<V1, V2>::value || is_base_of<V1, V2>::value` instead. I'm unsure whether (a) this additional effort is worth it and (b) a strict reading of the forward iterator requirements seems not to allow to return a reference-related type (Whether this is a defect or not is another question).

[2011-12-05: Marc Glisse comments and splits into two resolution alternatives]

I guess I am looking at the speed of:

>     value_type x;
>     x = *m;

(copy construction would likely trigger copy elision and thus be neutral) instead of the validity of:

>     value_type&& x = *m;

In this sense, Daniels earlier proposition that ignored `value_type` and just did `switch_lvalue_ref_to_rvalue_ref<reference>` was easier to understand (and it didn't require thinking about reference related types).

The currently proposed resolution has been split into two alternatives.

[2012, Kona]

Move to Review.

Alisdair: This only applies to input iterators, so keep that in mind when thinking about this.

STL: I see what B is doing, but not A.

Howard: I agree.

Alisdair: Should we use `add_rvalue_reference`?

STL: No, we do not want reference collapsing.

STL: Re A, messing with the CV qualification scares me.

Alisdair: Agree. That would break my intent.

STL: Actually I don't think it's actually wrong, but I still don't see what it's doing.

Alisdair: A is picking the value type, B is picking the proxy type.

Howard: I like returning the proxy type.

STL: Returning a reference (B) seems right, because the requirements say "reference". I suspect that B works correctly if you have a move iterator wrapping a move iterator wrapping a thing. I think that A would mess up the type in the middle.

Considerable discussion about which version is correct, checking various examples.

STL: Still think B is right. Still don't understand A. In A we are losing the proxyness.

Howard: Agree 100%. We don't want to lose the proxy. If it's const, so be it.

STL: B is also understandable by mortals.

Howard: Remove to review, keep A but move it out of the proposed resolution area (but keep it for rational).

Alisdair: Adding an explanatory note might be a good idea, if someone wants to write one.

Walter: Concerned about losing the word "reference" in p.1.

Howard: `move_iterator` will return an xvalue or a prvalue, both of which are rvalues.

[Proposed resolution A, rejected in preference to the currently proposed resolution (B)

1. Change 24.5.3 [move.iterators] p1 as indicated:

   > Class template `move_iterator` is an iterator adaptor with the same behavior as the underlying iterator except that its dereference operator implicitly converts the value returned by the underlying iterator's dereference operator to an rvalue reference of the value type. Some generic algorithms can be called with move iterators to replace copying with moving.

2. Change 24.5.3.1 [move.iterator], class template `move_iterator` synopsis, as indicated:

   >     namespace std {
   >       template <class Iterator>
   >       class move_iterator {
   >       public:
   >         typedef Iterator iterator_type;
   >         typedef typename iterator_traits<Iterator>::difference_type difference_type;
   >         typedef Iterator pointer;
   >         typedef typename iterator_traits<Iterator>::value_type value_type;
   >         typedef typename iterator_traits<Iterator>::iterator_category iterator_category;
   >         typedef value_type&&see below reference;
   >         […]
   >       };
   >     }

3. Immediately following the class template `move_iterator` synopsis in 24.5.3.1 [move.iterator] insert a new paragraph as indicated:

   > -?- Let `R` be `iterator_traits<Iterator>::reference` and let `V` be `iterator_traits<Iterator>::value_type`. If `is_reference<R>::value` is `true` and if `remove_cv<remove_reference<R>::type>::type` is the same type as `V`, the template instantiation `move_iterator<Iterator>` shall define the nested type named `reference` as a synonym for `remove_reference<R>::type&&`, otherwise as a synonym for `V`.

]

[2012, Portland: Move to Tentatively Ready]

AJM wonders if the implied trait might be useful elsewhere, and worth adding to type traits as a transformation type trait.

Suspicion that the Range SG might find such a trait useful, but wait until there is clear additional use of such a trait before standardizing.

Minor wording tweak to use `add_rvalue_reference` rather than manually adding the `&&`, then move to Tentatively Ready.

[2013-01-09 Howard Hinnant comments]

I believe the P/R for LWG 2106 is incorrect (item 3). The way it currently reads, `move_iterator<I>::reference` is always an lvalue reference. I.e. if `R` is an lvalue reference type, then reference becomes `add_rvalue_reference<R>::type` which is just `R`. And if `R` is not a reference type, then reference becomes `R` (which is also just `R` ;-)).

I believe the correct wording is what was there previously:

-?- Let `R` be `iterator_traits<Iterator>::reference`. If `is_reference<R>::value` is true, the template instantiation `move_iterator<Iterator>` shall define the nested type named `reference` as a synonym for `remove_reference<R>::type&&`, otherwise as a synonym for `R`.

Additionally Marc Glisse points out that `move_iterator<I>::operator*()` should return `static_cast<reference>(*current)`, not `std::move(*current)`.

Previous resolution:

> This wording is relative to the FDIS.

> 1. Change 24.5.3 [move.iterators] p1 as indicated:

>    > Class template `move_iterator` is an iterator adaptor with the same behavior as the underlying iterator except that its dereference operator implicitly converts the value returned by the underlying iterator's dereference operator to an rvalue reference. Some generic algorithms can be called with move iterators to replace copying with moving.

> 2. Change 24.5.3.1 [move.iterator], class template `move_iterator` synopsis, as indicated:

>    >     namespace std {
>    >       template <class Iterator>
>    >       class move_iterator {
>    >       public:
>    >         typedef Iterator iterator_type;
>    >         typedef typename iterator_traits<Iterator>::difference_type difference_type;
>    >         typedef Iterator pointer;
>    >         typedef typename iterator_traits<Iterator>::value_type value_type;
>    >         typedef typename iterator_traits<Iterator>::iterator_category iterator_category;
>    >         typedef value_type&&see below reference;
>    >         […]
>    >       };
>    >     }

> 3. Immediately following the class template `move_iterator` synopsis in 24.5.3.1 [move.iterator] insert a new paragraph as indicated:

>    > -?- Let `R` be `iterator_traits<Iterator>::reference`. If `is_reference<R>::value` is `true`, the template instantiation `move_iterator<Iterator>` shall define the nested type named `reference` as a synonym for `add_rvalue_reference<R>::type`, otherwise as a synonym for `R`.

[2014-05-19, Daniel comments]

The term instantiation has been changed to specialization in the newly added paragraph as suggested by STL and much preferred by myself.

[2014-05-19 Library reflector vote]

The issue has been identified as Tentatively Ready based on five votes in favour.

Proposed resolution:

This wording is relative to N3936.

1. Change 24.5.3 [move.iterators] p1 as indicated:

   > Class template `move_iterator` is an iterator adaptor with the same behavior as the underlying iterator except that its indirection operator implicitly converts the value returned by the underlying iterator's indirection operator to an rvalue reference. Some generic algorithms can be called with move iterators to replace copying with moving.

2. Change 24.5.3.1 [move.iterator], class template `move_iterator` synopsis, as indicated:

   >     namespace std {
   >       template <class Iterator>
   >       class move_iterator {
   >       public:
   >         typedef Iterator iterator_type;
   >         typedef typename iterator_traits<Iterator>::difference_type difference_type;
   >         typedef Iterator pointer;
   >         typedef typename iterator_traits<Iterator>::value_type value_type;
   >         typedef typename iterator_traits<Iterator>::iterator_category iterator_category;
   >         typedef value_type&&see below reference;
   >         […]
   >       };
   >     }

3. Immediately following the class template `move_iterator` synopsis in 24.5.3.1 [move.iterator] insert a new paragraph as indicated:

   > -?- Let `R` be `iterator_traits<Iterator>::reference`. If `is_reference<R>::value` is `true`, the template specialization `move_iterator<Iterator>` shall define the nested type named `reference` as a synonym for `remove_reference<R>::type&&`, otherwise as a synonym for `R`.

4. Edit 24.5.3.3.4 [move.iter.op.star] p1 as indicated:

   >     reference operator*() const;

   > > -1- Returns: `std::movestatic_cast<reference>(*current)`.

### 2108. No way to identify allocator types that always compare equal

Section: 17.6.3.5 [allocator.requirements] Status: Resolved Submitter: Jonathan Wakely Opened: 2011-12-01 Last modified: 2015-04-08

View other active issues in [allocator.requirements].

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Discussion:

Whether two allocator objects compare equal affects the complexity of container copy and move assignments and also the possibility of an exception being thrown by container move assignments. The latter point means container move assignment cannot be `noexcept` when `propagate_on_container_move_assignment` (POCMA) is false for the allocator because there is no way to detect at compile-time if two allocators will compare equal. LWG 2013 means this affects all containers using `std::allocator`, but even if that is resolved, this affects all stateless allocators which do not explicitly define POCMA to `true_type`.

One solution would be to add an "always_compare_equal" trait to `allocator_traits`, but that would be duplicating information that is already defined by the type's equality operator if that operator always returns true. Requiring users to write `operator==` that simply returns true and also explicitly override a trait to repeat the same information would be unfortunate and risk user errors that allow the trait and actual `operator==` to disagree.

Dave Abrahams suggested a better solution in message c++std-lib-31532, namely to allow `operator==` to return `true_type`, which is convertible to `bool` but also detectable at compile-time. Adopting this as the recommended way to identify allocator types that always compare equal only requires a slight relaxation of the allocator requirements so that `operator==` is not required to return `bool` exactly.

The allocator requirements do not make it clear that it is well-defined to compare non-const values, that should be corrected too.

In message c++std-lib-31615 Pablo Halpern suggested an `always_compare_equal` trait that could still be defined, but with a sensible default value rather than requiring users to override it, and using that to set sensible values for other allocator traits:

> Do we still need `always_compare_equal` if we can have an `operator==` that returns `true_type`? What would its default value be? `is_empty<A> || is_convertible<decltype(a == a), true_type>::value`, perhaps? One benefit I see to such a definition is that stateless C++03 allocators that don't use the `true_type` idiom will still benefit from the new trait.

> […]

> One point that I want to ensure doesn't get lost is that if we adopt some sort of `always_compare_equal`-like trait, then `propagate_on_container_swap` and `propagate_on_container_move_assignment` should default to `always_compare_equal`. Doing this will eliminate unnecessary requirements on the container element type, as per [LWG 2103].

Optionally, `operator==` for `std::allocator` could be made to return `true_type`, however if LWG 2103 is adopted that is less important.

Alberto Ganesh Barbati: Suggest either `always_compare_equal`, `all_objects_(are_)equivalent`, or `all_objects_compare_equal`.

[2014-11-07 Urbana]

Resolved by N4258

Proposed resolution:

This wording is relative to the FDIS.

1. Change Table 27 — "Descriptive variable definitions" in 17.6.3.5 [allocator.requirements]:

   Table 27 — Descriptive variable definitions
   Variable | Definition
   --- | ---
   `a3, a4` | an rvalue of values of (possibly `const`) type `X`
   `b` | a value of (possibly `const`) type `Y`

2. Change Table 28 — "Allocator requirements" in 17.6.3.5 [allocator.requirements]:

   Table 28 — Allocator requirements
   Expression | Return type | Assertion/note pre-/post-condition | Default
   --- | --- | --- | ---
   `a1 == a2a3 == a4` | convertible to `bool` | returns true only if storage allocated from each can be deallocated via the other. `operator==` shall be reflexive, symmetric, and transitive, and shall not exit via an exception. |
   `a1 != a2a3 != a4` | convertible to `bool` | same as `!(a1 == a2)!(a3 == a4)` |
   `a3 == b` | convertible to `bool` | same as `a3 == Y::rebind<T>::other(b)` |
   `a3 != b` | convertible to `bool` | same as `!(a3 == b)` |
   `[…]`
   `a.select_on_- container_copy_- construction()` | `X` | Typically returns either `a` or `X()` | `return a;`

   `X::always_compares_equal`

   Identical to or derived
   from `true_type` or
   `false_type`

   `true_type` if the expression `x1 == x2` is
   guaranteed to be `true` for any two (possibly
   `const`) values `x1, x2` of type `X`, when
   implicitly converted to `bool`. See Note B, below.

   `true_type`, if `is_empty<X>::value` is `true` or if
   `decltype(declval<const X&>() == declval<const X&>())`
   is convertible to `true_type`, otherwise `false_type`.

   `[…]`

   Note A: […]

   Note B: If `X::always_compares_equal::value` or `XX::always_compares_equal::value` evaluate to `true` and an expression equivalent to `x1 == x2` or `x1 != x2` for any two values `x1, x2` of type `X` evaluates to `false` or `true`, respectively, the behaviour is undefined.

3. Change class template `allocator_traits` synopsis, 20.7.8 [allocator.traits] as indicated:

   >     namespace std {
   >       template <class Alloc> struct allocator_traits {
   >         typedef Alloc allocator_type;
   >         […]
   >         typedef see below always_compares_equal;
   >         typedef see below propagate_on_container_copy_assignment;
   >         […]
   >       };
   >     }

4. Insert the following between 20.7.8.1 [allocator.traits.types] p6 and p7 as indicated:

   >     typedef see below always_compares_equal;

   > > -?- Type: `Alloc::always_compares_equal` if such a type exists; otherwise, `true_type` if `is_empty<Alloc>::value` is `true` or if `decltype(declval<const Alloc&>() == declval<const Alloc&>())` is convertible to `true_type`; otherwise, `false_type` .

   >     typedef see below propagate_on_container_copy_assignment;

   > > -7- Type: `Alloc::propagate_on_container_copy_assignment` if such a type exits, otherwise `false_type`.

5. Change class template `allocator` synopsis, 20.7.9 [default.allocator] as indicated:

   >     namespace std {
   >       template <class T> class allocator;

   >       // specialize for void:
   >       template <> class allocator<void> {
   >       public:
   >         typedef void* pointer;
   >         typedef const void* const_pointer;
   >         // reference-to-void members are impossible.
   >         typedef void value_type;
   >         template <class U> struct rebind { typedef allocator<U> other; };
   >       };

   >       template <class T> class allocator {
   >       public:
   >         typedef size_t size_type;
   >         typedef ptrdiff_t difference_type;
   >         typedef T* pointer;
   >         typedef const T* const_pointer;
   >         typedef T& reference;
   >         typedef const T& const_reference;
   >         typedef T value_type;
   >         template <class U> struct rebind { typedef allocator<U> other; };
   >         typedef true_type always_compares_equal;

   >         […]
   >       };
   >     }

### 2109. Incorrect requirements for `hash` specializations

Section: 19.5.5 [syserr.hash], 20.8.2.7 [util.smartptr.hash], 20.9.13 [unord.hash], 20.14.1 [type.index.synopsis], 21.6 [basic.string.hash], 23.3.7 [vector.bool], 30.3.1.1 [thread.thread.id] Status: C++14 Submitter: Daniel Krügler Opened: 2011-12-04 Last modified: 2015-04-08

View all issues with C++14 status.

Discussion:

20.8.2.7 [util.smartptr.hash] p2 is specified as follows:

> Requires: the template specializations shall meet the requirements of class template `hash` (20.8.12).

The problem here is the usage of a Requires element, which is actually a pre-condition that a user of a component has to satisfy. But the intent of this wording is actually to be a requirement on implementations. The Requires element should be removed here and the wording should be improved to say what it was intended for.

We have similar situations in basically all other places where the specification of library-provided `hash` specializations is defined. Usually, the Requires element is incorrect. In the special case of `hash<unique_ptr<T, D>>` the implementation depends on the behaviour of `hash` specializations, that could be user-provided. In this case the specification needs to separate the requirements on these specializations and those that are imposed on the implementation.

[2012, Kona]

Update wording and move to Review.

Believe a simpler formulation is to simply string the term Requires: and leave the current wording intact, rather than strike the whole clause and replace it.

[Originally proposed wording for reference

1. Change 19.5.5 [syserr.hash] as indicated:

   >     template <> struct hash<error_code>;

   > > -1- Requires: the template specialization shall meet the requirements of class template `hash` (20.9.13 [unord.hash])The header `<system_error>` provides a definition for a specialization of the template `hash<error_code>`. The requirements for the members of this specialization are given in sub-clause 20.9.13 [unord.hash].

2. Change 20.6.3 [bitset.hash] as indicated:

   >     template <size_t N> struct hash<bitset<N> >;

   > > -1- Requires: the template specialization shall meet the requirements of class template `hash` (20.9.13 [unord.hash])The header `<bitset>` provides a definition for a partial specialization of the `hash` class template for specializations of class template `bitset<N>`. The requirements for the members of instantiations of this specialization are given in sub-clause 20.9.13 [unord.hash].

3. Change 20.8.2.7 [util.smartptr.hash] as indicated:

   >     template <class T, class D> struct hash<unique_ptr<T, D> >;

   > > -1- Requires: the template specialization shall meet the requirements of class template `hash` (20.9.13 [unord.hash])The header `<memory>` provides a definition for a partial specialization of the `hash` class template for specializations of class template `unique_ptr<T, D>`. The requirements for the members of instantiations of this specialization are given in sub-clause 20.9.13 [unord.hash]. For an object `p` of type `UP`, where `UP` is `unique_ptr<T, D>`, `hash<UP>()(p)` shall evaluate to the same value as `hash<typename UP::pointer>()(p.get())`. The specialization `hash<typename UP::pointer>` shall be well-formed.

   > > -?- Requires: The specialization `hash<typename UP::pointer>` shall be well-formed and well-defined [Note: the general requirements of class template `hash` (20.9.13 [unord.hash]) are implied — end note].

   >     template <class T> struct hash<shared_ptr<T> >;

   > > -2- Requires: the template specialization shall meet the requirements of class template `hash` (20.9.13 [unord.hash])The header `<memory>` provides a definition for a partial specialization of the `hash` class template for specializations of class template `shared_ptr<T>`. The requirements for the members of instantiations of this specialization are given in sub-clause 20.9.13 [unord.hash]. For an object `p` of type `shared_ptr<T>`, `hash<shared_ptr<T> >()(p)` shall evaluate to the same value as `hash<T*>()(p.get())`.

4. Change 20.9.13 [unord.hash] p2 as indicated: [Comment: For unknown reasons the extended integer types are not mentioned here, which looks like an oversight to me and makes also the wording more complicated. See 2119 for this part of the problem. — end comment]

   >     template <> struct hash<bool>;
   >     template <> struct hash<char>;
   >     […]
   >     template <> struct hash<long double>;
   >     template <class T> struct hash<T*>;

   > > -2- Requires: the template specializations shall meet the requirements of class template `hash` (20.9.13 [unord.hash])The header `<functional>` provides definitions for specializations of the `hash` class template for each cv-unqualified arithmetic type except for the extended integer types. This header also provides a definition for a partial specialization of the `hash` class template for any pointer type. The requirements for the members of these specializations are given in sub-clause 20.9.13 [unord.hash].

5. Change 20.14.4 [type.index.hash] p1 as indicated:

   >     template <> struct hash<type_index>;

   > > -1- Requires: the template specialization shall meet the requirements of class template `hash` (20.9.13 [unord.hash])The header `<typeindex>` provides a definition for a specialization of the template `hash<type_index>`. The requirements for the members of this specialization are given in sub-clause 20.9.13 [unord.hash]. For an object `index` of type `type_index`, `hash<type_index>()(index)` shall evaluate to the same result as `index.hash_code()`.

6. Change 21.6 [basic.string.hash] p1 as indicated:

   >     template <> struct hash<string>;
   >     template <> struct hash<u16string>;
   >     template <> struct hash<u32string>;
   >     template <> struct hash<wstring>;

   > > -1- Requires: the template specializations shall meet the requirements of class template `hash` (20.9.13 [unord.hash])The header `<string>` provides definitions for specializations of the `hash` class template for the types `string`, `u16string`, `u32string`, and `wstring`. The requirements for the members of these specializations are given in sub-clause 20.9.13 [unord.hash].

7. Change 23.3.7 [vector.bool] p7 as indicated:

   >     template <class Allocator> struct hash<vector<bool, Allocator> >;

   > > -7- Requires: the template specialization shall meet the requirements of class template `hash` (20.9.13 [unord.hash])The header `<vector>` provides a definition for a partial specialization of the `hash` class template for specializations of class template `vector<bool, Allocator>`. The requirements for the members of instantiations of this specialization are given in sub-clause 20.9.13 [unord.hash].

8. Change 30.3.1.1 [thread.thread.id] p14 as indicated:

   >     template <> struct hash<thread::id>;

   > > -14- Requires: the template specialization shall meet the requirements of class template `hash` (20.9.13 [unord.hash])The header `<thread>` provides a definition for a specialization of the template `hash<thread::id>`. The requirements for the members of this specialization are given in sub-clause 20.9.13 [unord.hash].

]

[2012, Portland: Move to Tentatively Ready]

No further wording issues, so move to Tentatively Ready (post meeting issues processing).

[2013-04-20 Bristol]

Proposed resolution:

1. Change 19.5.5 [syserr.hash] as indicated:

   >     template <> struct hash<error_code>;

   > > -1- Requires: t The template specialization shall meet the requirements of class template `hash` (20.9.13 [unord.hash].

2. Change 20.6.3 [bitset.hash] as indicated:

   >     template <size_t N> struct hash<bitset<N> >;

   > > -1- Requires: t The template specialization shall meet the requirements of class template `hash` (20.9.13 [unord.hash]).

3. Change 20.8.2.7 [util.smartptr.hash] as indicated:

   >     template <class T, class D> struct hash<unique_ptr<T, D> >;

   > > -1- Requires: t The template specialization shall meet the requirements of class template `hash` (20.9.13 [unord.hash]). For an object `p` of type `UP`, where `UP` is `unique_ptr<T, D>`, `hash<UP>()(p)` shall evaluate to the same value as `hash<typename UP::pointer>()(p.get())`. The specialization `hash<typename UP::pointer>` shall be well-formed.

   > > -?- Requires: The specialization `hash<typename UP::pointer>` shall be well-formed and well-defined, and shall meet the requirements of class template `hash` (20.9.13 [unord.hash]).

   >     template <class T> struct hash<shared_ptr<T> >;

   > > -2- Requires: t The template specialization shall meet the requirements of class template `hash` (20.9.13 [unord.hash]). For an object `p` of type `shared_ptr<T>`, `hash<shared_ptr<T> >()(p)` shall evaluate to the same value as `hash<T*>()(p.get())`.

4. Change 20.9.13 [unord.hash] p2 as indicated: [Comment: For unknown reasons the extended integer types are not mentioned here, which looks like an oversight to me and makes also the wording more complicated. See 2119 for this part of the problem. — end comment]

   >     template <> struct hash<bool>;
   >     template <> struct hash<char>;
   >     […]
   >     template <> struct hash<long double>;
   >     template <class T> struct hash<T*>;

   > > -2- Requires: t The template specializations shall meet the requirements of class template `hash` (20.9.13 [unord.hash]).

5. Change 20.14.4 [type.index.hash] p1 as indicated:

   >     template <> struct hash<type_index>;

   > > -1- Requires: t The template specialization shall meet the requirements of class template `hash` (20.9.13 [unord.hash]). For an object `index` of type `type_index`, `hash<type_index>()(index)` shall evaluate to the same result as `index.hash_code()`.

6. Change 21.6 [basic.string.hash] p1 as indicated:

   >     template <> struct hash<string>;
   >     template <> struct hash<u16string>;
   >     template <> struct hash<u32string>;
   >     template <> struct hash<wstring>;

   > > -1- Requires: t The template specializations shall meet the requirements of class template `hash` (20.9.13 [unord.hash]).

7. Change 23.3.7 [vector.bool] p7 as indicated:

   >     template <class Allocator> struct hash<vector<bool, Allocator> >;

   > > -7- Requires: t The template specialization shall meet the requirements of class template `hash` (20.9.13 [unord.hash]).

8. Change 30.3.1.1 [thread.thread.id] p14 as indicated:

   >     template <> struct hash<thread::id>;

   > > -14- Requires: t The template specialization shall meet the requirements of class template `hash` (20.9.13 [unord.hash]).

### 2110. `remove` can't swap but note says it might

Section: 25.3.8 [alg.remove] Status: C++14 Submitter: Howard Hinnant Opened: 2011-12-07 Last modified: 2015-04-08

View all other issues in [alg.remove].

View all issues with C++14 status.

Discussion:

25.3.8 [alg.remove]/p1 says:

> 1 Requires: The type of `*first` shall satisfy the `MoveAssignable` requirements (Table 22).

This means that `remove`/`remove_if` can only use move assignment to permute the sequence. But then 25.3.8 [alg.remove]/p6 (non-normatively) contradicts p1:

> 6 Note: each element in the range `[ret,last)`, where `ret` is the returned value, has a valid but unspecified state, because the algorithms can eliminate elements by swapping with or moving from elements that were originally in that range.

[2012, Kona]

Move to Ready.

Alisdair notes we could extend permission to use `swap` if it is available, but there is no interest. Accept the proposed resolution as written.

[2012, Portland: applied to WP]

Proposed resolution:

This wording is relative to the FDIS.

Change 25.3.8 [alg.remove] as indicated:

>     template<class ForwardIterator, class T>
>       ForwardIterator remove(ForwardIterator first, ForwardIterator last,
>                              const T& value);

>     template<class ForwardIterator, class Predicate>
>       ForwardIterator remove_if(ForwardIterator first, ForwardIterator last,
>                                 Predicate pred);

> > […]

> > -6-Note: each element in the range `[ret,last)`, where `ret` is the returned value, has a valid but unspecified state, because the algorithms can eliminate elements by swapping with or moving from elements that were originally in that range.

### 2112. User-defined classes that cannot be derived from

Section: 17.6.5 [conforming], 20.7.8 [allocator.traits], 20.13.1 [allocator.adaptor.syn] Status: C++14 Submitter: Daniel Krügler Opened: 2011-11-30 Last modified: 2015-04-08

View all other issues in [conforming].

View all issues with C++14 status.

Discussion:

It is a very established technique for implementations to derive internally from user-defined class types that are used to customize some library component, e.g. deleters and allocators are typical candidates. The advantage of this approach is to possibly take advantage of the empty-base-class optimization (EBCO).

Whether or whether not libraries did take advantage of such a detail didn't much matter in C++03. Even though there did exist a portable idiom to prevent that a class type could be derived from, this idiom has never reached great popularity: The technique required to introduce a virtual base class and it did not really prevent the derivation, but only any construction of such a type. Further, such types are not empty as defined by the `std::is_empty` trait, so could easily be detected by implementations from TR1 on.

With the new C++11 feature of final classes and final member functions it is now very easy to define an empty, but not derivable from class type. From the point of the user it is quite natural to use this feature for types that he or she did not foresee to be derivable from.

On the other hand, most library implementations (including third-party libraries) often take advantage of EBCO applied to user-defined types used to instantiate library templates internally. As the time of submitting this issue the following program failed to compile on all tested library implementations:

>     #include <memory>

>     struct Noop final {
>      template<class Ptr>
>      void operator()(Ptr) const {}
>     };

>     std::unique_ptr<int, Noop> up;

In addition, many `std::tuple` implementations with empty, final classes as element types failed as well, due to a popular inheritance-based implementation technique. EBCO has also a long tradition to be used in library containers to efficiently store potentially stateless, empty allocators.

It seems that both user and library did the best they could: None of the affected types did impose explicit requirements on the corresponding user-defined types to be derivable from (This capability was not part of the required operations), and libraries did apply EBCO whereever possible to the convenience of the customer.

Nonetheless given the existence of non-derivable-from class types in C++11, libraries have to cope with failing derivations. How should that problem be solved?

It would certainly be possible to add weazel wording to the allocator requirements similar to what we had in C++03, but restricted to derivation-from requirements. I consider this as the bad solution, because it would add new requirements that never had existed before in this explicit form onto types like allocators.

Existing libraries presumably will need internal traits like `__is_final` or `__is_derivable` to make EBCO possible in the current form but excluding non-derivable class types. As of this writing this seems to happen already. Problem is that without a `std::is_derivable` trait, third-party libraries have no portable means to do the same thing as standard library implementations. This should be a good reason to make such a trait public available soon, but seems not essential to have now. Further, this issue should also be considered as a chance to recognice that EBCO has always been a very special corner case (There exist parallels to the previously existing odd core language rule that did make the interplay between `std::auto_ptr` and `std::auto_ptr_ref` possible) and that it would be better to provide explicit means for space-efficient storage, not necessarily restricted to inheritance relations, e.g. by marking data members with a special attribute.

At least two descriptions in the current standard should be fixed now for better clarification:

1. As mentioned by Ganesh, 20.7.8 [allocator.traits] p1 currently contains a (non-normative) note "Thus, it is always possible to create a derived class from an allocator." which should be removed.

2. As pointed out by Howard, the specification of `scoped_allocator_adaptor` as of 20.13.1 [allocator.adaptor.syn] already requires derivation from `OuterAlloc`, but only implies indirectly the same for the inner allocators due to the exposition-only description of member `inner`. This indirect implication should be normatively required for all participating allocators.

[2012, Kona]

What we really need is a type trait to indicate if a type can be derived from. Howard reports Clang and libc++ have had success with this approach.

Howard to provide wording, and AJM to alert Core that we may be wanting to add a new trait that requires compiler support.

[2014-02, Issaquah: Howard and Daniel comment and provide wording]

Several existing C++11 compilers do already provide an internal `__is_final` intrinsic (e.g. clang and gcc) and therefore we believe that this is evidence enough that this feature is implementable today.

We believe that both a simple and clear definition of the `is_final` query should result in a true outcome if and only if the current existing language definition holds that a complete class type (either union or non-union) has been marked with the class-virt-specifier `final` — nothing more.

The following guidelines lead to the design decision and the wording choice given below:

It has been expressed several times that a high-level trait such as "`is_derivable`" would be preferred and would be more useful for non-experts. One problem with that request is that it is astonishingly hard to find a common denominator for what the precise definition of this trait should be, especially regarding corner-cases. Another example of getting very differing points of view is to ask a bunch of C++ experts what the best definition of the `is_empty` trait should be (which can be considered as a kind of higher-level trait).

Once we have a fundamental trait like `is_final` available, we can easily define higher-level traits in the future on top of this by a proper logical combination of the low-level traits.

A critical question is whether providing such a low-level compile-time introspection might be considered as disadvantageous, because it could constrain the freedom of existing implementations even further and whether a high-level trait would solve this dilemma. We assert that since C++11 the static introspection capabilities are already very large and we believe that making the presence or absence of the `final` keyword testable does not make the current situation worse.

Below code example demonstrates the intention and the implementability of this feature:

>     #include <type_traits>

>     namespace std
>     {

>     template <class T>
>     struct is_final
>       : public integral_constant<bool, __is_final(T)>
>     {};

>     }  // std

>     // test it

>     union FinalUnion final { };

>     union NonFinalUnion { };

>     class FinalClass final { };

>     struct NonFinalClass { };

>     class Incomplete;

>     int main()
>     {
>       using std::is_final;
>       static_assert( is_final<const volatile FinalUnion>{}, "");
>       static_assert(!is_final<FinalUnion[]>{}, "");
>       static_assert(!is_final<FinalUnion[1]>{}, "");
>       static_assert(!is_final<NonFinalUnion>{}, "");
>       static_assert( is_final<FinalClass>{}, "");
>       static_assert(!is_final<FinalClass&>{}, "");
>       static_assert(!is_final<FinalClass*>{}, "");
>       static_assert(!is_final<NonFinalClass>{}, "");
>       static_assert(!is_final<void>{}, "");
>       static_assert(!is_final<int>{}, "");
>       static_assert(!is_final<Incomplete>{}, ""); // error incomplete type 'Incomplete' used in type trait expression
>     }

[2014-02-14, Issaquah: Move to Immediate]

This is an important issue, that we really want to solve for C++14.

Move to Immediate after polling LEWG, and then the NB heads of delegation.

Proposed resolution:

This wording is relative to N3797.

1. Change 20.10.2 [meta.type.synop], header `<type_traits>` synopsis, as indicated

   >     namespace std {
   >       […]
   >       // 20.10.4.3, type properties:
   >       […]
   >       template <class T> struct is_empty;
   >       template <class T> struct is_polymorphic;
   >       template <class T> struct is_abstract;
   >       template <class T> struct is_final;

   >       […]
   >     }

2. Change 20.10.4.3 [meta.unary.prop], Table 49 — Type property predicates, as indicated

   Table 49 — Type property predicates
   Template | Condition | Preconditions
   --- | --- | ---
   `template <class T>` `struct is_abstract;` | […] | […]
   `template <class T>` `struct is_final;` | `T` is a class type marked with the class-virt-specifier `final` (9 [class]). [Note: A union is a class type that can be marked with `final`. — end note] | If `T` is a class type, `T` shall be a complete type

3. After 20.10.4.3 [meta.unary.prop] p5 add one further example as indicated:

   > [Example:

   > >     // Given:
   > >     struct P final { };
   > >     union U1 { };
   > >     union U2 final { };

   > >     // the following assertions hold:
   > >     static_assert(!is_final<int>::value, "Error!");
   > >     static_assert( is_final<P>::value, "Error!");
   > >     static_assert(!is_final<U1>::value, "Error!");
   > >     static_assert( is_final<U2>::value, "Error!");

   > — end example]

### 2118. [CD] `unique_ptr` for array does not support cv qualification conversion of actual argument

Section: 20.8.1.3 [unique.ptr.runtime] Status: Resolved Submitter: Alf P. Steinbach Opened: 2011-12-16 Last modified: 2015-04-08

View all other issues in [unique.ptr.runtime].

View all issues with Resolved status.

Discussion:

Addresses US 16

N3290 20.8.1.3.1 [unique.ptr.runtime.ctor] "`unique_ptr` constructors":

> These constructors behave the same as in the primary template except that they do not accept pointer types which are convertible to `pointer`. [Note: One implementation technique is to create private templated overloads of these members. — end note]

This language excludes even `pointer` itself as type for the actual argument.

But of more practical concern is that both Visual C++ 10.0 and MinGW g++ 4.1.1 reject the code below, where only an implicit cv qualification is needed, which cv qualification is supported by the non-array version:

>     #include <memory>
>     using namespace std;

>     struct T {};

>     T* foo() { return new T; }
>     T const* bar() { return foo(); }

>     int main()
>     {
>        unique_ptr< T const >       p1( bar() );        // OK
>        unique_ptr< T const [] >    a1( bar() );        // OK

>        unique_ptr< T const >       p2( foo() );        // OK
>        unique_ptr< T const [] >    a2( foo() );        // ? this is line #15
>     }

The intent seems to be clearly specified in 20.8.1.3 [unique.ptr.runtime]/1 second bullet:

> — Pointers to types derived from `T` are rejected by the constructors, and by `reset`.

But the following language in 20.8.1.3.1 [unique.ptr.runtime.ctor] then rejects far too much...

Proposed new wording of N3290 20.8.1.3.1 [unique.ptr.runtime.ctor] "`unique_ptr` constructors":

> These constructors behave the same as in the primary template except that actual argument pointers `p` to types derived from `T` are rejected by the constructors. [Note: One implementation technique is to create private templated overloads of these members. — end note]

This will possibly capture the intent better, and avoid the inconsistency between the non-array and array versions of `unique_ptr`, by using nearly the exact same phrasing as for the paragraph explaining the intent.

[2012-08-25 Geoffrey Romer comments in c++std-lib-32978]

1. The current P/R seems to intend to support at least two different implementation techniques — additional unusable templates that catch forbidden arguments or replacing existing constructors by templates that ensure ill-formed code inside the template body, when the requirements are not met. It seems unclear whether the current wording allows the second approach, though. It should be considered to allow both strategies or if that is not possible the note should be clearer.

2. The very same problem exists for the `reset` member function, but even worse, because the current specification is more than clear that the deleted `reset` function will catch all cases not equal to `pointer`. It seems confusing at best to have different policies for the constructor and for the `reset` function. In this case, the question in regard to implementation freedom mentioned above is even more important.

3. It's awkward to refer to "the constructors" twice in the same sentence; I suggest revising the sentence as "...except that they do not accept argument pointers `p` to types derived from `T`"

[2012-12-20: Geoffrey Romer comments and provides a revised resolution]

The array specialization of `unique_ptr` differs from the primary template in several ways, including the following:

- `unique_ptr<T[], D>` cannot be constructed from a plain pointer whose type is not exactly `unique_ptr<T[], D>::pointer` or `nullptr_t`.

- `unique_ptr<T[], D>` cannot be constructed from a `unique_ptr<U[], E>&&` unless `U` is exactly `T` and `E` is exactly `D`.

- `unique_ptr<T[], D>` cannot be moveassigned from a `unique_ptr<U[], E>&&` unless `U` is exactly `T` and `E` is exactly `D`.

- `unique_ptr<T[], D>::reset` cannot take an argument whose type is not exactly `unique_ptr<T[], D>::pointer` or `nullptr_t`.

- `default_delete<T[]>` cannot be constructed from a `default_delete<U[]>` unless `U` is exactly `T`.

- `default_delete<T[]>::operator()` cannot be called on a pointer whose type is not exactly `T*`.

The common intent of all these restrictions appears to be to disallow implicit conversions from pointer-to-derived-class to pointer-to-base-class in contexts where the pointer is known to point to an array, because such conversions are inherently unsafe; deleting or subscripting the result of such a conversion leads to undefined behavior (see also CWG 1504). However, these restrictions have the effect of disallowing all implicit conversions in those contexts, including most notably cv-qualification, but also user-defined conversions, and possibly others. This PR narrows all those restrictions, to disallow only unsafe pointer-to-derived to pointer-to-base conversions, while allowing all others.

I removed the nebulous language stating that certain functions "will not accept" certain arguments. Instead I use explicitly deleted template functions, which participate in overload resolution only for pointer-to-derived to pointer-to-base conversions. This is more consistent with the existing text and easier to express correctly than an approach based on declaring certain types of calls to be ill-formed, but may produce inferior compiler diagnostics.

Wherever possible, this PR defines the semantics of template specializations in terms of their differences from the primary template. This improves clarity and minimizes the risk of unintended differences (e.g. LWG 2169, which this PR also fixes). This PR also makes it explicit that the specialization inherits the description of all members, not just member functions, from the primary template and, in passing, clarifies the default definition of pointer in the specialization.

This resolution only disallows pointer-to-derived to pointer-to-base conversions between ordinary pointer types; if user-defined pointer types provide comparable conversions, it is their responsibility to ensure they are safe. This is consistent with C++'s general preference for expressive power over safety, and for assuming the user knows what they're doing; furthermore, enforcing such a restriction on user-defined types appears to be impractical without cooperation from the user.

The "base class without regard to cv-qualifiers" language is intended to parallel the specification of `std::is_base_of`.

Jonathan Wakely has a working implementation of this PR patched into libstdc++.

Previous resolution:

> This wording is relative to the FDIS.

> Change 20.8.1.3.1 [unique.ptr.runtime.ctor] as indicated:

> >     explicit unique_ptr(pointer p) noexcept;
> >     unique_ptr(pointer p, see below d) noexcept;
> >     unique_ptr(pointer p, see below d) noexcept;

> > > These constructors behave the same as in the primary template except that they do not accept pointer types which are convertible to `pointer` argument pointers `p` to types derived from `T` are rejected by the constructors. [Note: One implementation technique is to create private templated overloads of these members. — end note]

[2014-02, Issaquah]

GR: want to prevent unsafe conversions. Standard is inconsistent how it does this. for `reset()` has deleted function template capturing everything except the known-safe cases. Other functions use SFINAE. Main reason this is hard is that `unique_ptr` supports fancy pointers. Have to figure out how to handle them and what requirements to put on them. Requirements are minimal, not even required to work with `pointer_traits`.

STL surprised `pointer_traits` doesn't work

GR: ways to get fancy pointers to work is to delegate responsibility for preventing unsafe conversions to the fancy pointers themselves. Howard doesn't like that, he wants even fancy pointers to be prevented from doing unsafe conversions in `unique_ptr` contexts.

AM: Howard says `unique_ptr` was meant to be very very safe under all conditions, this open a hole in that. Howard wants to eke forward and support more, but not if we open any holes in type safety.

GR: do we need to be typesafe even for fancy types with incorrect `pointer_traits`?

AM: that would mean it's only unsafe for people who lie by providing a broken specialization of `pointer_traits`

GR: probably can't continue with ambiguity between using SFINAE and ill-formedness. Would appreciate guidance on direction used for that.

STL: difference is observable in convertibility using type traits.

STL: for `reset()` which doesn't affect convertibility ill-formed allows `static_assert`, better diagnostic. For assignment it's detectable and has traits, constraining them is better.

EN: I strongly prefer constraints than `static_asserts`

STL: if we could rely on `pointer_traits` that might be good. Alternatively could we add more machinery to deleter? make deleter say conversions are allowed, otherwise we lock down all conversions. basically want to know if converting `U` to `T` is safe.

Previous resolution [SUPERSEDED]:

> This wording is relative to N3485.

> 1. Revise 20.8.1.1.3 [unique.ptr.dltr.dflt1] as follows

>    >     namespace std {
>    >       template <class T> struct default_delete<T[]> {
>    >         constexpr default_delete() noexcept = default;
>    >         template <class U> default_delete(const default_delete<U>&) noexcept;
>    >         void operator()(T*) const;
>    >         template <class U> void operator()(U*) const = delete;
>    >       };
>    >     }

>    -?- Descriptions are provided below only for member functions that have behavior different from the primary template.

>    >     template <class U> default_delete(const default_delete<U>&) noexcept;

>    > > -?- This constructor behaves the same as in the primary template except that it shall not participate in overload resolution unless:

>    > > - `U` is an array type, and

>    > > - `V*` is implicitly convertible to `T*`, and

>    > > - `T` is not a base class of `V` (without regard to cv-qualifiers),

>    > > where `V` is the array element type of `U`.

>    > >     void operator()(T* ptr) const;

>    > > > -1- Effects: calls `delete[]` on `ptr`.

>    > > > -2- Remarks: If `T` is an incomplete type, the program is ill-formed.

>    > >     template <class U> void operator()(U*) const = delete;

>    > > > -?- Remarks: This function shall not participate in overload resolution unless `T` is a base class of `U` (without regard to cv-qualifiers).

> 2. Revise 20.8.1.2 [unique.ptr.single]/3 as follows:

>    > If the type `remove_reference<D>::type::pointer` exists, then `unique_ptr<T, D>::pointer` shall be a synonym for `remove_reference<D>::type::pointer`. Otherwise `unique_ptr<T, D>::pointer` shall be a synonym for `Telement_type*`. The type `unique_ptr<T, D>::pointer` shall satisfy the requirements of `NullablePointer` (17.6.3.3 [nullablepointer.requirements]).

> 3. Revise 20.8.1.3 [unique.ptr.runtime] as follows:

>    >     namespace std {
>    >       template <class T, class D> class unique_ptr<T[], D> {
>    >       public:
>    >         typedef see below pointer;
>    >         typedef T element_type;
>    >         typedef D deleter_type;

>    >         // 20.7.1.3.1, constructors
>    >         constexpr unique_ptr() noexcept;
>    >         explicit unique_ptr(pointer p) noexcept;
>    >         template <class U> explicit unique_ptr(U* p) = delete;
>    >         unique_ptr(pointer p, see below d) noexcept;
>    >         template <class U> unique_ptr(U* p, see below d) = delete;
>    >         unique_ptr(pointer p, see below d) noexcept;
>    >         template <class U> unique_ptr(U* p, see below d) = delete;
>    >         unique_ptr(unique_ptr&& u) noexcept;
>    >         constexpr unique_ptr(nullptr_t) noexcept : unique_ptr() { }
>    >         template <class U, class E> unique_ptr(unique_ptr<U, E>&& u) noexcept;

>    >         // destructor
>    >         ~unique_ptr();

>    >         // assignment
>    >         unique_ptr& operator=(unique_ptr&& u) noexcept;
>    >         template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;
>    >         unique_ptr& operator=(nullptr_t) noexcept;

>    >         // 20.7.1.3.2, observers
>    >         T& operator[](size_t i) const;
>    >         pointer get() const noexcept;
>    >         deleter_type& get_deleter() noexcept;
>    >         const deleter_type& get_deleter() const noexcept;
>    >         explicit operator bool() const noexcept;

>    >         // 20.7.1.3.3 modifiers
>    >         pointer release() noexcept;
>    >         void reset(pointer p = pointer()) noexcept;
>    >         void reset(nullptr_t) noexcept;
>    >         template <class U> void reset(U*) = delete;
>    >         void swap(unique_ptr& u) noexcept;

>    >         // disable copy from lvalue
>    >         unique_ptr(const unique_ptr&) = delete;
>    >         unique_ptr& operator=(const unique_ptr&) = delete;
>    >       };
>    >     }

>    > -1- A specialization for array types is provided with a slightly altered interface.

>    > - Conversions between different types of `unique_ptr<T[], D>`from `unique_ptr<Derived[]>` to `unique_ptr<Base[]>`, where `Base` is a base class of `Derived`, from `auto_ptr`, or to or from the non-array forms of `unique_ptr` produce an ill-formed program.

>    > - Pointers to types derived from `T` are rejected by the constructors, and by `reset`.

>    > - The observers `operator*` and `operator->` are not provided.

>    > - The indexing observer `operator[]` is provided.

>    > - The default deleter will call `delete[]`.

>    > -2- Descriptions are provided below only for member functions that have behavior different members that differ from the primary template.

>    > -3- The template argument `T` shall be a complete type.

> 4. Revise 20.8.1.3.1 [unique.ptr.runtime.ctor] as follows:

>    >     explicit unique_ptr(pointer p) noexcept;
>    >     unique_ptr(pointer p, see below d) noexcept;
>    >     unique_ptr(pointer p, see below d) noexcept;
>    >     template <class U> explicit unique_ptr(U* p) = delete;
>    >     template <class U> unique_ptr(U* p, see below d) = delete;
>    >     template <class U> unique_ptr(U* p, see below d) = delete;

>    > > These constructors behave the same as in the primary template except that they do not accept pointer types which are convertible to pointer. [Note: One implementation technique is to create private templated overloads of these members. — end note]These constructors shall not participate in overload resolution unless:

>    > > - `pointer` is a pointer type, and

>    > > - `U*` is implicitly convertible to `pointer`, and

>    > > - `T` is a base class of `U` (without regard to cv-qualifiers).

>    > > The type of `d` is determined as in the corresponding non-deleted constructors.

>    >     template <class U, class E> unique_ptr(unique_ptr<U, E>&& u) noexcept;

>    > > -?- This constructor behaves the same as in the primary template, except that it shall not participate in overload resolution unless:

>    > > - `unique_ptr<U, E>::pointer` is implicitly convertible to `pointer`, and

>    > > - `U` is an array type, and

>    > > - either `D` is a reference type and `E` is the same type as `D`, or `D` is not a reference type and `E` is implicitly convertible to `D`, and

>    > > - either at least one of `pointer` and `unique_ptr<U, E>::pointer` is not a pointer type, or `T` is not a base class of the array element type of `U` (without regard to cv-qualifiers).

> 5. Insert a new sub-clause following 20.8.1.3.1 [unique.ptr.runtime.ctor] as follows:

>    ?? `unique_ptr` assignment [unique.ptr.runtime.asgn]

>    >     template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;

>    > > -?- This operator behaves the same as in the primary template, except that it shall not participate in overload resolution unless:

>    > > - `unique_ptr<U, E>::pointer` is implicitly convertible to `pointer`, and

>    > > - `U` is an array type, and

>    > > - either `D` is a reference type and `E` is the same type as `D`, or `D` is not a reference type and `E` is implicitly convertible to `D`, and

>    > > - either at least one of `pointer` and `unique_ptr<U, E>::pointer` is not a pointer type, or `T` is not a base class of the array element type of `U` (without regard to cv-qualifiers).

> 6. Revise 20.8.1.3.4 [unique.ptr.runtime.modifiers] as follows:

>    >     void reset(pointer p = pointer()) noexcept;
>    >     void reset(nullptr_t p) noexcept;
>    >     template <class U> void reset(U*) = delete;

>    > > -1- Effects: If `get() == nullptr` there are no effects. Otherwise `get_deleter()(get())`.

>    > > -2- Postcondition: `get() == p`.

>    > > -?- This function shall not participate in overload resolution unless:

>    > > - `pointer` is a pointer type, and

>    > > - `U*` is implicitly convertible to `pointer`, and

>    > > - `T` is a base class of `U` (without regard to cv-qualifiers).

[2014-06 Rapperswil]

Discussion of N4042 and general agreement that this paper resolves the substance of this issue and should be adopted with minor edits. Geoffrey Romer will provide an updated paper.

[2014-06 post-Rapperswil]

As described in N4089.

[2014-11-07 Urbana]

Resolved by N4089

Proposed resolution:

See proposed wording in N4089.

### 2120. What should `async` do if neither '`async`' nor '`deferred`' is set in policy?

Section: 30.6.8 [futures.async] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-01-01 Last modified: 2015-04-08

View all other issues in [futures.async].

View all issues with C++14 status.

Discussion:

Implementations already disagree, one returns an invalid future with no shared state, one chooses `policy == async` and one chooses `policy == deferred`, see c++std-lib-30839, c++std-lib-30840 and c++std-lib-30844. It's not clear if returning an invalid future is allowed by the current wording.

If the intention is to allow an empty future to be returned, then 30.6.8 [futures.async] p3 and p4 should be adjusted to clarify that a shared state might not be created and an invalid future might be returned.

If the intention is that a valid future is always returned, p3 should say something about the case where none of the conditions applies.

[2012, Portland: move to Review]

We could make it undefined if no launch policy is defined.

Hans: If no launch policy is specified the behaviour is undefined

Artur: or implementation defined?

Hans: no: we don't want people to do this

[Proposed wording]

This wording is relative to N3376

Add a third bullet to the end of the list in 30.6.8p3

> "if no valid launch policy is provided the behaviour is undefined"

Moved to review

[2013-04-19, Bristol]

Detlef provides new wording

Previous wording:

> [This wording is relative to N3376]

> Add a third bullet to the end of the list in 30.6.8 [futures.async]p3

> > – if no valid launch policy is provided the behaviour is undefined

[2013-09 Chicago]

If no policy is given, it should be undefined, so moved to Immediate.

Accept for Working Paper

Proposed resolution:

[This wording is relative to N3485]

Add a third bullet to the end of the list in 30.6.8 [futures.async]p3

> – If no value is set in the launch policy, or a value is set that is neither specified in this International Standard or by the implementation, the behaviour is undefined.

### 2122. `merge()` stability for lists versus forward lists

Section: 23.3.5.5 [list.ops], 23.3.4.6 [forwardlist.ops] Status: C++14 Submitter: Nicolai Josuttis Opened: 2012-01-15 Last modified: 2015-04-08

View all other issues in [list.ops].

View all issues with C++14 status.

Discussion:

`forward_list::merge()` is specified in 23.3.4.6 [forwardlist.ops], p19 as follows:

> This operation shall be stable: for equivalent elements in the two lists, the elements from `*this` shall always precede the elements from `x`.

But `list::merge()` is only specified in 23.3.5.5 [list.ops], p24 as follows:

> Remarks: Stable.

Note that in general we define "stable" only for algorithms (see 17.3 [defns.stable] and 17.6.5.7 [algorithm.stable]) so for member function we should explain it everywhere we use it.

Thus for lists we have to add:

> Stable: for equivalent elements in the two lists, the elements from the list always precede the elements from the argument list.

This, BTW, was the specification we had with C++03.

In addition, I wonder whether we also have some guarantees regarding stability saying that the order of equivalent elements of each list merged remains stable (which would be my interpretation of just saying "stable", BTW).

Thus, I'd expect that for equivalent elements we guarantee that

- we first have all element of `*this` (in the same order as on entry)
- and then all elements of the passed argument (in the same order as on entry).

[2012, Kona]

Move to Open.

STL says we need to fix up 17.6.5.7 to be stronger, and then the remarks for merge should just say "Remarks: Stable (see 17.6.5.7)"

Assigned to STL for word-smithing.

[ 2013-04-14 STL provides rationale and improved wording ]

Step 1: Centralize all specifications of stability to 17.6.5.7 [algorithm.stable].

Step 2: 17.3 [defns.stable] and 17.6.5.7 [algorithm.stable] talk about "algorithms", without mentioning "container member functions". There's almost no potential for confusion here, but there's a simple way to increase clarity without increasing verbosity: make the container member functions explicitly cite 17.6.5.7 [algorithm.stable]. For consistency, we can also update the non-member functions.

Step 3: Fix the "so obvious, we forgot to say it" bug in 17.6.5.7 [algorithm.stable]: a "stable" merge of equivalent elements A B C D and W X Y Z produces A B C D W X Y Z, never D C B A X W Z Y.

Step 3.1: Say "(preserving their original order)" to be consistent with "the relative order [...] is preserved" in 17.6.5.7 [algorithm.stable]'s other bullet points.

Step 4: Copy part of `list::merge()`'s wording to `forward_list::merge()`, in order to properly connect with 17.6.5.7 [algorithm.stable]'s "first range" and "second range".

[2013-04-18, Bristol]

Original wording saved here:

> This wording is relative to the FDIS.

> 1. Change 23.3.5.5 [list.ops] as indicated:

>    >     void                          merge(list<T,Allocator>& x);
>    >     void                          merge(list<T,Allocator>&& x);
>    >     template <class Compare> void merge(list<T,Allocator>& x, Compare comp);
>    >     template <class Compare> void merge(list<T,Allocator>&& x, Compare comp);

>    > > […]

>    > > -24- Remarks: Stable This operation shall be stable: for equivalent elements in the two lists, the elements from `*this` shall always precede the elements from `x` and the order of equivalent elements of `*this` and `x` remains stable. If `(&x != this)` the range `[x.begin(), x.end())` is empty after the merge. No elements are copied by this operation. The behavior is undefined if `this->get_allocator() != x.get_allocator()`.

> 2. Change 23.3.4.6 [forwardlist.ops] as indicated:

>    >     void merge(forward_list<T,Allocator>& x);
>    >     void merge(forward_list<T,Allocator>&& x);
>    >     template <class Compare> void merge(forward_list<T,Allocator>& x, Compare comp);
>    >     template <class Compare> void merge(forward_list<T,Allocator>&& x, Compare comp);

>    > > […]

>    > > -19- Effects: Merges `x` into `*this`. This operation shall be stable: for equivalent elements in the two lists, the elements from `*this` shall always precede the elements from `x` and the order of equivalent elements of `*this` and `x` remains stable. `x` is empty after the merge. If an exception is thrown other than by a comparison there are no effects. Pointers and references to the moved elements of `x` now refer to those same elements but as members of `*this`. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into `*this`, not into `x`.

Proposed resolution:

This wording is relative to the N3485.

1. Change 17.6.5.7 [algorithm.stable]/1 as indicated:

   > When the requirements for an algorithm state that it is “stable” without further elaboration, it means:

   > […]
   > - For the merge algorithms, for equivalent elements in the original two ranges, the elements from the first range (preserving their original order) precede the elements from the second range (preserving their original order).

2. Change 23.3.4.6 [forwardlist.ops] as indicated:

   >     void remove(const T& value);
   >     template <class Predicate> void remove_if(Predicate pred);

   > > -12- Effects: Erases all the elements in the list referred by a list iterator `i` for which the following conditions hold: `*i == value` (for `remove()`), `pred(*i)` is true (for `remove_if()`). This operation shall be stable: the relative order of the elements that are not removed is the same as their relative order in the original list. Invalidates only the iterators and references to the erased elements.

   > > -13- Throws: Nothing unless an exception is thrown by the equality comparison or the predicate.

   > > -??- Remarks: Stable (17.6.5.7 [algorithm.stable]).

   > > […]

   >     void merge(forward_list& x);
   >     void merge(forward_list&& x);
   >     template <class Compare> void merge(forward_list& x, Compare comp)
   >     template <class Compare> void merge(forward_list&& x, Compare comp)

   > > […]

   > > -19- Effects: Merges `x` into `*this` the two sorted ranges `[begin(), end())` and `[x.begin(), x.end())`. This operation shall be stable: for equivalent elements in the two lists, the elements from `*this` shall always precede the elements from `x`. `x` is empty after the merge. If an exception is thrown other than by a comparison there are no effects. Pointers and references to the moved elements of `x` now refer to those same elements but as members of `*this`. Iterators referring to the moved elements will continue to refer to their elements, but they now behave as iterators into `*this`, not into `x`.

   > > -20- Remarks: Stable (17.6.5.7 [algorithm.stable]). The behavior is undefined if `this->get_allocator() != x.get_allocator()`.

   > > […]

   >     void sort();
   >     template <class Compare> void sort(Compare comp);

   > > […]

   > > -23- Effects: Sorts the list according to the `operator<` or the `comp` function object. This operation shall be stable: the relative order of the equivalent elements is preserved. If an exception is thrown the order of the elements in `*this` is unspecified. Does not affect the validity of iterators and references.

   > > -??- Remarks: Stable (17.6.5.7 [algorithm.stable]).

   > > […]

3. Change 23.3.5.5 [list.ops] as indicated:

   >     void remove(const T& value);
   >     template <class Predicate> void remove_if(Predicate pred);

   > > […]

   > > -17- Remarks: Stable (17.6.5.7 [algorithm.stable]).

   > > […]

   >     void merge(list& x);
   >     void merge(list&& x);
   >     template <class Compare> void merge(list& x, Compare comp)
   >     template <class Compare> void merge(list&& x, Compare comp)

   > > […]

   > > -24- Remarks: Stable (17.6.5.7 [algorithm.stable]). […]

   > > […]

   >     void sort();
   >     template <class Compare> void sort(Compare comp);

   > > […]

   > > -30- Remarks: Stable (17.6.5.7 [algorithm.stable]).

   > > […]

4. Change 25.3.1 [alg.copy]/12 as indicated:

   >     template<class InputIterator, class OutputIterator, class Predicate>
   >     OutputIterator
   >     copy_if(InputIterator first, InputIterator last,
   >             OutputIterator result, Predicate pred);

   > > […]

   > > -12- Remarks: Stable (17.6.5.7 [algorithm.stable]).

5. Change 25.3.8 [alg.remove] as indicated:

   >     template<class ForwardIterator, class T>
   >     ForwardIterator
   >     remove(ForwardIterator first, ForwardIterator last, const T& value);
   >     template<class ForwardIterator, class Predicate>
   >     ForwardIterator
   >     remove_if(ForwardIterator first, ForwardIterator last, Predicate pred);

   > > […]

   > > -4- Remarks: Stable (17.6.5.7 [algorithm.stable]).

   > > […]

   >     template<class InputIterator, class OutputIterator, class T>
   >     OutputIterator
   >     remove_copy(InputIterator first, InputIterator last,
   >                 OutputIterator result, const T& value);
   >     template<class InputIterator, class OutputIterator, class Predicate>
   >     OutputIterator
   >     remove_copy_if(InputIterator first, InputIterator last,
   >                    OutputIterator result, Predicate pred);

   > > […]

   > > -11- Remarks: Stable (17.6.5.7 [algorithm.stable]).

6. Change 25.4.1.2 [stable.sort]/4 as indicated:

   >     template<class RandomAccessIterator>
   >     void stable_sort(RandomAccessIterator first, RandomAccessIterator last);
   >     template<class RandomAccessIterator, class Compare>
   >     void stable_sort(RandomAccessIterator first, RandomAccessIterator last,
   >     Compare comp);

   > > […]

   > > -4- Remarks: Stable (17.6.5.7 [algorithm.stable]).

7. Change 25.4.4 [alg.merge] as indicated:

   >     template<class InputIterator1, class InputIterator2,
   >              class OutputIterator>
   >     OutputIterator
   >     merge(InputIterator1 first1, InputIterator1 last1,
   >           InputIterator2 first2, InputIterator2 last2,
   >           OutputIterator result);
   >     template<class InputIterator1, class InputIterator2,
   >              class OutputIterator, class Compare>
   >     OutputIterator
   >     merge(InputIterator1 first1, InputIterator1 last1,
   >           InputIterator2 first2, InputIterator2 last2,
   >           OutputIterator result, Compare comp);

   > > […]

   > > -5- Remarks: Stable (17.6.5.7 [algorithm.stable]).

   > > […]

   >     template<class BidirectionalIterator>
   >     void inplace_merge(BidirectionalIterator first,
   >                        BidirectionalIterator middle,
   >                        BidirectionalIterator last);
   >     template<class BidirectionalIterator, class Compare>
   >     void inplace_merge(BidirectionalIterator first,
   >                        BidirectionalIterator middle,
   >                        BidirectionalIterator last, Compare comp);

   > > […]

   > > -9- Remarks: Stable (17.6.5.7 [algorithm.stable]).

### 2123. `merge()` allocator requirements for lists versus forward lists

Section: 23.3.4.6 [forwardlist.ops] Status: C++14 Submitter: Nicolai Josuttis Opened: 2012-01-15 Last modified: 2015-04-08

View all other issues in [forwardlist.ops].

View all issues with C++14 status.

Discussion:

Sub-clause 23.3.5.5 [list.ops], p24 states for lists:

> The behavior is undefined if `this->get_allocator() != x.get_allocator()`.

But there is nothing like that for forward lists in 23.3.4.6 [forwardlist.ops], although I would expect the same undefined behavior there.

[2012, Kona]

Move to Ready.

[2012, Portland: applied to WP]

Proposed resolution:

This wording is relative to the FDIS.

1. Add a new paragraph after 23.3.4.6 [forwardlist.ops] p19 as indicated:

   >     void merge(forward_list<T,Allocator>& x);
   >     void merge(forward_list<T,Allocator>&& x);
   >     template <class Compare> void merge(forward_list<T,Allocator>& x, Compare comp);
   >     template <class Compare> void merge(forward_list<T,Allocator>&& x, Compare comp);

   > > […]

   > > -19- Effects: […]

   > > -?- Remarks: The behavior is undefined if `this->get_allocator() != x.get_allocator()`.

### 2128. Absence of global functions `cbegin/cend`

Section: 24.3 [iterator.synopsis], 24.7 [iterator.range] Status: C++14 Submitter: Dmitry Polukhin Opened: 2012-01-23 Last modified: 2015-04-08

View all other issues in [iterator.synopsis].

View all issues with C++14 status.

Discussion:

All standard containers support `cbegin/cend` member functions but corresponding global functions are missing. Proposed resolution it to add global `cbegin/cend` functions by analogy with global `begin/end` functions. This addition will unify things for users.

[2012, Kona]

STL: Range-based for loops do not use global `begin`/`end` (anymore).

Alisdair: We will have to make sure these will be available through many headers.

STL: Do this, including `r` and `cr`. This won't add any additional work.

Matt: Users will find it strange if these are not all available.

Alisdair: Should we have these available everywhere begin/end are available?

Marshall: Yes. Not any extra work.

Howard: Adding all of these means we need all of `<iterator>`.

STL: We already need it all.

Matt: We have to be careful what we are requiring if we include the `r` versions.

Jeffrey: If we include `r`, should they adapt if the container does not define reverse iteration?

STL: No. No special behavior. Should fail to compile. Up to user to add the reverse code--it's easy.

Howard: Anyway it will SFINAE out.

Alisdair: Error messages due to SFINAE are harder to understand than simple failure to compile.

STL: Agrees that SFINAE makes error messages much worse.

Action: STL to provide additional wording for the `r` variants. Move to Review once that wording is availalbe.

[ 2013-04-14 STL provides rationale and improved wording ]

Step 1: Implement `std::cbegin/cend()` by calling `std::begin/end()`. This has numerous advantages:

- It automatically works with arrays, which is the whole point of these non-member functions.
- It works with C++98/03-era user containers, written before `cbegin/cend()` members were invented.
- It works with `initializer_list`, which is extremely minimal and lacks `cbegin/cend()` members.
- 23.2.1 [container.requirements.general] guarantees that this is equivalent to calling cbegin/cend() members.

Step 2: Like `std::begin/end()`, implement `std::rbegin/rend()` by calling `c.rbegin/rend()`. Note that C++98/03 had the `Reversible Container Requirements`.

Step 3: Also like `std::begin/end()`, provide overloads of `std::rbegin/rend()` for arrays.

Step 4: Provide overloads of `std::rbegin/rend()` for `initializer_list`, because it lacks `rbegin/rend()` members. These overloads follow 18.9.3 [support.initlist.range]'s signatures. Note that because these overloads return `reverse_iterator`, they aren't being specified in `<initializer_list>`.

Step 5: Like Step 1, implement `std::crbegin/crend()` by calling `std::rbegin/rend()`.

Original wording saved here:

> This wording is relative to N3337.

> 1. In 24.3 [iterator.synopsis], header iterator synopsis, add the following declarations:

>    >     namespace std {
>    >       […]
>    >       // 24.6.5, range access:
>    >       template <class C> auto begin(C& c) -> decltype(c.begin());
>    >       template <class C> auto begin(const C& c) -> decltype(c.begin());
>    >       template <class C> auto end(C& c) -> decltype(c.end());
>    >       template <class C> auto end(const C& c) -> decltype(c.end());
>    >       template <class C> auto cbegin(const C& c) -> decltype(c.cbegin());
>    >       template <class C> auto cend(const C& c) -> decltype(c.cend());
>    >       template <class T, size_t N> T* begin(T (&array)[N]);
>    >       template <class T, size_t N> T* end(T (&array)[N]);
>    >       template <class T, size_t N> const T* cbegin(T (&array)[N]);
>    >       template <class T, size_t N> const T* cend(T (&array)[N]);
>    >     }

> 2. In 24.7 [iterator.range] after p5 add the following series of paragraphs:

>    >     template <class C> auto cbegin(const C& c) -> decltype(c.cbegin());

>    > > -?- Returns: `c.cbegin()`.

>    >     template <class C> auto cend(const C& c) -> decltype(c.cend());

>    > > -?- Returns: `c.cend()`.

>    >     template <class T, size_t N> const T* cbegin(T (&array)[N]);

>    > > -?- Returns: `array`.

>    >     template <class T, size_t N> const T* cend(T (&array)[N]);

>    > > -?- Returns: `array + N`.

[2013-04-18, Bristol]

Proposed resolution:

This wording is relative to N3485.

1. In 24.3 [iterator.synopsis], header iterator synopsis, add the following declarations:

   >     namespace std {
   >       […]
   >       // 24.6.5, range access:
   >       template <class C> auto begin(C& c) -> decltype(c.begin());
   >       template <class C> auto begin(const C& c) -> decltype(c.begin());
   >       template <class C> auto end(C& c) -> decltype(c.end());
   >       template <class C> auto end(const C& c) -> decltype(c.end());
   >       template <class T, size_t N> T* begin(T (&array)[N]);
   >       template <class T, size_t N> T* end(T (&array)[N]);
   >       template <class C> auto cbegin(const C& c) -> decltype(std::begin(c));
   >       template <class C> auto cend(const C& c) -> decltype(std::end(c));
   >       template <class C> auto rbegin(C& c) -> decltype(c.rbegin());
   >       template <class C> auto rbegin(const C& c) -> decltype(c.rbegin());
   >       template <class C> auto rend(C& c) -> decltype(c.rend());
   >       template <class C> auto rend(const C& c) -> decltype(c.rend());
   >       template <class T, size_t N> reverse_iterator<T*> rbegin(T (&array)[N]);
   >       template <class T, size_t N> reverse_iterator<T*> rend(T (&array)[N]);
   >       template <class E> reverse_iterator<const E*> rbegin(initializer_list<E> il);
   >       template <class E> reverse_iterator<const E*> rend(initializer_list<E> il);
   >       template <class C> auto crbegin(const C& c) -> decltype(std::rbegin(c));
   >       template <class C> auto crend(const C& c) -> decltype(std::rend(c));
   >     }

2. At the end of 24.7 [iterator.range], add:

   >     template <class C> auto cbegin(const C& c) -> decltype(std::begin(c));

   > > -?- Returns: `std::begin(c)`.

   >     template <class C> auto cend(const C& c) -> decltype(std::end(c));

   > > -?- Returns: `std::end(c)`.

   >     template <class C> auto rbegin(C& c) -> decltype(c.rbegin());
   >     template <class C> auto rbegin(const C& c) -> decltype(c.rbegin());

   > > -?- Returns: `c.rbegin()`.

   >     template <class C> auto rend(C& c) -> decltype(c.rend());
   >     template <class C> auto rend(const C& c) -> decltype(c.rend());

   > > -?- Returns: `c.rend()`.

   >     template <class T, size_t N> reverse_iterator<T*> rbegin(T (&array)[N]);

   > > -?- Returns: `reverse_iterator<T*>(array + N)`.

   >     template <class T, size_t N> reverse_iterator<T*> rend(T (&array)[N]);

   > > -?- Returns: `reverse_iterator<T*>(array)`.

   >     template <class E> reverse_iterator<const E*> rbegin(initializer_list<E> il);

   > > -?- Returns: `reverse_iterator<const E*>(il.end())`.

   >     template <class E> reverse_iterator<const E*> rend(initializer_list<E> il);

   > > -?- Returns: `reverse_iterator<const E*>(il.begin())`.

   >     template <class C> auto crbegin(const C& c) -> decltype(std::rbegin(c));

   > > -?- Returns: `std::rbegin(c)`.

   >     template <class C> auto crend(const C& c) -> decltype(std::rend(c));

   > > -?- Returns: `std::rend(c)`.

### 2129. User specializations of `std::initializer_list`

Section: 17.6.4.2.1 [namespace.std], 18.9 [support.initlist] Status: WP Submitter: Richard Smith Opened: 2012-01-18 Last modified: 2015-04-08

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Discussion:

Since the implementation is intended to magically synthesize instances of `std::initializer_list` (rather than by a constructor call, for instance), user specializations of this type can't generally be made to work. I can't find any wording which makes such specializations ill-formed, though, which leads me to suspect that they're technically legal under the provisions of 17.6.4.2.1 [namespace.std] p1.

[2012, Kona]

This sounds correct, but we need wording for a resolution.

Marshall Clow volunteers to produce wording.

[2014-02-19, Jonathan Wakely provides proposed wording]

[2014-03-27, Library reflector vote]

The issue has been identified as Tentatively Ready based on six votes in favour.

Proposed resolution:

This wording is relative to N3936.

1. Add new new paragraph below 18.9 [support.initlist] p2:

   > -2- An object of type `initializer_list<E>` provides access to an array of objects of type `const E`. […]

   > -?- If an explicit specialization or partial specialization of `initializer_list` is declared, the program is ill-formed.

### 2130. Missing ordering constraints

Section: 29.3 [atomics.order] Status: C++14 Submitter: Mark Batty Opened: 2012-02-22 Last modified: 2015-04-08

View other active issues in [atomics.order].

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Discussion:

C11 issue 407

It seems that both C11 and C++11 are missing the following two derivatives of this rule:

> For atomic modifications `A` and `B` of an atomic object `M`, if there is a `memory_order_seq_cst` fence `X` such that `A` is sequenced before `X`, and `X` precedes `B` in `S`, then `B` occurs later than `A` in the modification order of `M`.

> For atomic modifications `A` and `B` of an atomic object `M`, if there is a `memory_order_seq_cst` fence `Y` such that `Y` is sequenced before `B`, and `A` precedes `Y` in `S`, then `B` occurs later than `A` in the modification order of `M`.

Above wording has been suggested for the Technical Corrigendum of C11 via issue 407, details can be found here.

[2012-03-19: Daniel proposes a slightly condensed form to reduce wording duplications]

[2012-03-20: Hans comments]

The usage of the term atomic operations in 29.3 [atomics.order] p7 is actually incorrect and should better be replaced by atomic modifications as used in the C11 407 wording.

There seems to be a similar wording incorrectness used in 1.10 [intro.multithread] p17 which should be corrected as well.

[2012, Portland: move to Review]

Olivier: does the fence really participate in the modifications?

Hans: S is the total set of all sequentially consistent operations, and sequentially consistent fences are in S.

Olivier: this sort of combination of a pair of half-open rules seems to imply the write must make it to main memory

But not all implementations treat a fence as a memory operation; cannot observe the half-open rule.

Hans: not sure this is actually prevented here. You could defer until the next load. What the wording doesn't quite show is that the third bullet in the new wording is already in the standard.

Hans: it is the interaction between fences on one side and other memory modifications on the other that is being defined here.

Pablo: S is not directly observable; it is a hypothetic ordering.

Moved to review

Hans: to alert C liaison

[2013-04-20, Bristol]

Accepted for the working paper

Proposed resolution:

This wording is relative to N3376.

1. [Drafting note: The project editor is kindly asked to consider to replace in 1.10 [intro.multithread] p17 the phrase "before an operation B on M" by "before a modification B of M".]

2. Change 29.3 [atomics.order] paragraph 7 as indicated: [Drafting note: Note that the wording change intentionally does also replace the term atomic operation by atomic modification]

   -7- For atomic operations A and B on an atomic object M, if there are `memory_order_seq_cst` fences X and Y such that A is sequenced before X, Y is sequenced before B, and X precedes Y in S, then B occurs later than A in the modification order of M. For atomic modifications A and B of an atomic object M, B occurs later than A in the modification order of M if:

   - there is a `memory_order_seq_cst` fence X such that A is sequenced before X, and X precedes B in S, or
   - there is a `memory_order_seq_cst` fence Y such that Y is sequenced before B, and A precedes Y in S, or
   - there are `memory_order_seq_cst` fences X and Y such that A is sequenced before X, Y is sequenced before B, and X precedes Y in S.

   -8- [ Note: `memory_order_seq_cst` ensures sequential consistency only for a program that is free of data races and uses exclusively `memory_order_seq_cst` operations. Any use of weaker ordering will invalidate this guarantee unless extreme care is used. In particular, `memory_order_seq_cst` fences ensure a total order only for the fences themselves. Fences cannot, in general, be used to restore sequential consistency for atomic operations with weaker ordering specifications. — end note ]

### 2132. `std::function` ambiguity

Section: 20.9.12.2.1 [func.wrap.func.con] Status: C++14 Submitter: Ville Voutilainen Opened: 2012-02-28 Last modified: 2015-04-08

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Discussion:

Consider the following:

>     #include <functional>

>     void f(std::function<void()>) {}
>     void f(std::function<void(int)>) {}

>     int main() {
>       f([]{});
>       f([](int){});
>     }

The calls to `f` in `main` are ambiguous. Apparently because the conversion sequences to `std::function` from the lambdas are identical. The standard specifies that the function object given to `std::function` "shall be Callable (20.8.11.2) for argument types `ArgTypes` and return type `R`." It doesn't say that if this is not the case, the constructor isn't part of the overload set.

Daniel: During the preparation of N3123 it turned out that there are no longer reasons to refer to INVOKE as a conceptually entity alone, its real implementation as a function template `invoke` is possible but was deferred for a later point in time. Defining a type trait for the Callable requirement would also be possible, so there seem to be no technical reasons why the template constructor of `std::function` should not be constrained. The below suggested wording does this without introducing a special trait for this. This corresponds to the way that has been used to specify the `result_of` trait. Note that the definition of the Callable requirement is perfectly suitable for this, because it is a pure syntactically based requirement and can be directly transformed into a constrained template.

The suggested resolution also applies such wording to the "perfectly forwarding" assignment operator

>     template<class F> function& operator=(F&&);

The positive side-effect of this is that it automatically implements a solution to a problem similar to that mentioned in issue 1234.

It would be possible to apply similar constraints to the member signatures

>     template<class F> function& operator=(reference_wrapper<F>);

>     template<class F, class A> void assign(F&&, const A&);

as well. At this point there does not seem to be a pestering reason to do so.

[2012-10 Portland: Move to Review]

STL: This is a real issue, but does not like a resolution relying on a SFINAEable metafunction that is not specified and available to the users.

`packaged_task` has the same issue.

STL strongly wants to see an `is_callable` type trait to clarify the proposed wording.

Jeremiah concerned about holding up what appears to be a correct resolution for a hypothetical better one later - the issue is real.

Why must `f` by CopyConstructible? Surely MoveConstructible would be sufficient?

Answer: because `function` is CopyConstructible, and the bound functor is type-erased so must support all the properties of `function` itself.

Replace various applications of `declval` in the proposed resolution with simply using the passed functor object, `f`.

Alisdair to apply similar changes to `packaged_task`.

[2012-11-09, Vicente J. Botet Escriba provides another example]

Consider the following:

>     class AThreadWrapper {
>     public:
>       explicit operator std::thread();
>       ...
>     };
>     std::thread th = std::thread(AThreadWrapper); // call to conversion operator intended

The call to the conversion operator is overloaded with the thread constructor. But thread constructor requirement makes it fail as `AThreadWrapper` is not a Callable and the compiler tries to instantiate the thread constructor and fails.

[2014-02-14 Issaquah meeting: Move to Immediate]

Proposed resolution:

This wording is relative to N3376.

1. Change the following paragraphs in 20.9.12.2.1 [func.wrap.func.con]: [Editorial comment: The removal of the seemingly additional no-throw requirements of copy constructor and destructor of `A` is recommended, because they are already part of the Allocator requirements. Similar clean-up has been suggested by 2070 — end comment]

   >     template<class F> function(F f);
   >     template<class F, class A> function(allocator_arg_t, const A& a, F f);

   > > -7- Requires: `F` shall be `CopyConstructible`. `f` shall be Callable (20.9.12.2 [func.wrap.func]) for argument types `ArgTypes` and return type `R`. The copy constructor and destructor of `A` shall not throw exceptions.

   > > -?- Remarks: These constructors shall not participate in overload resolution unless `f` is Callable (20.9.12.2 [func.wrap.func]) for argument types `ArgTypes...` and return type `R`.

   > […]

   >     template<class F> function& operator=(F&& f);

   > > -18- Effects: `function(std::forward<F>(f)).swap(*this);`

   > > -19- Returns: `*this`

   > > -?- Remarks: This assignment operator shall not participate in overload resolution unless `declval<typename decay<F>::type&>()` is Callable (20.9.12.2 [func.wrap.func]) for argument types `ArgTypes...` and return type `R`.

### 2135. Unclear requirement for exceptions thrown in `condition_variable::wait()`

Section: 30.5.1 [thread.condition.condvar], 30.5.2 [thread.condition.condvarany] Status: C++14 Submitter: Pete Becker Opened: 2012-03-06 Last modified: 2015-04-08

View all other issues in [thread.condition.condvar].

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Discussion:

`condition_varible::wait()` (and, presumably, `condition_variable_any::wait()`, although I haven't looked at it) says that it calls `lock.unlock()`, and if `condition_variable::wait()` exits by an exception it calls `lock.lock()` on the way out. But if the initial call to `lock.unlock()` threw an exception, does it make sense to call `lock.lock()`? We simply don't know the state of that lock object, and it's probably better not to touch it.

That aside, once the `wait()` call has been unblocked, it calls `lock.lock()`. If `lock.lock()` throws an exception, what happens? The requirement is:

> If the function exits via an exception, `lock.lock()` shall be called prior to exiting the function scope.

That can be read in two different ways. One way is as if it said "`lock.lock()` shall have been called …", i.e. the original, failed, call to `lock.lock()` is all that's required. But a more natural reading is that wait has to call `lock.lock()` again, even though it already failed.

I think this wording suffers from being too general. There are two possible exception sources: the initial call to `lock.unlock()` and the final call to `lock.lock()`. Each one should have its own requirement. Lumping them together muddles things.

[2012, Portland: move to Open]

Pablo: `unlock` failing is easy -- the call leaves it locked. The second case, trying to `lock` fails -- what can you do? This is an odd state as we had it locked before was called wait. Maybe we should call `terminate` as we cannot meet the post-conditions. We could throw a different exception.

Hans: calling `terminate` makes sense as we're likely to call it soon anyway and at least we have some context.

Detlef: what kind of locks might be being used?

Pablo: condition variables are 'our' locks so this is less of a problem. `condition_variable_any` might be more problematic.

The general direction is to call `terminate` if the lock cannot be reacquired.

Pablo: Can we change the wording to 'leaves the mutex locked' ?

Hans: so if the `unlock` throws we simply propagate the exception.

Move the issue to open and add some formal wording at a later time.

[2013-09 Chicago: Resolved]

Detlef improves wording. Daniel suggests to introduce a Remarks element for the special "If the function fails to meet the postcondition..." wording and applies this to the proposed wording.

Proposed resolution:

This wording is relative to N3691.

1. Edit 30.5.1 [thread.condition.condvar] as indicated:

   >     void wait(unique_lock<mutex>& lock);

   > > […]

   > > -10- Effects:

   > > - Atomically calls `lock.unlock()` and blocks on `*this`.

   > > - When unblocked, calls `lock.lock()` (possibly blocking on the lock), then returns.

   > > - The function will unblock when signaled by a call to `notify_one()` or a call to `notify_all()`, or spuriously.

   > > - If the function exits via an exception, `lock.lock()` shall be called prior to exiting the function scope.

   > > -?- Remarks: If the function fails to meet the postcondition, `std::terminate()` shall be called (15.5.1 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note]

   > > -11- Postcondition: `lock.owns_lock()` is true and `lock.mutex()` is locked by the calling thread.

   > > -12- Throws: Nothing `system_error` when an exception is required (30.2.2).

   > > -13- Error conditions:

   > > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

   > >     template <class Predicate>
   > >     void wait(unique_lock<mutex>& lock, Predicate pred);

   > > > […]

   > > > -?- Remarks: If the function fails to meet the postcondition, `std::terminate()` shall be called (15.5.1 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note]

   > > > -16- Postcondition: `lock.owns_lock()` is true and `lock.mutex()` is locked by the calling thread.

   > > > -17- Throws: `system_error` when an exception is required (30.2.2), timeout-related exceptions (30.2.4), or any exception thrown by `pred`.

   > > > -18- Error conditions:

   > > > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

   > >     template <class Clock, class Duration>
   > >       cv_status wait_until(unique_lock<mutex>& lock,
   > >         const chrono::time_point<Clock, Duration>& abs_time);

   > > > […]

   > > > -20- Effects:

   > > > - […]

   > > > - If the function exits via an exception, `lock.lock()` shall be called prior to exiting the function scope.

   > > > -?- Remarks: If the function fails to meet the postcondition, `std::terminate()` shall be called (15.5.1 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note]

   > > > -21- Postcondition: `lock.owns_lock()` is true and `lock.mutex()` is locked by the calling thread.

   > > > […]

   > > > -23- Throws: `system_error` when an exception is required (30.2.2) or timeout-related exceptions (30.2.4).

   > > > -24- Error conditions:

   > > > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

   > >     template <class Rep, class Period>
   > >       cv_status wait_for(unique_lock<mutex>& lock,
   > >         const chrono::duration<Rep, Period>& rel_time);

   > > > […]

   > > > -?- Remarks: If the function fails to meet the postcondition, `std::terminate()` shall be called (15.5.1 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note]

   > > > -28- Postcondition: `lock.owns_lock()` is true and `lock.mutex()` is locked by the calling thread.

   > > > […]

   > > > -29- Throws: `system_error` when an exception is required (30.2.2) or timeout-related exceptions (30.2.4).

   > > > -30- Error conditions:

   > > > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

   > >     template <class Clock, class Duration, class Predicate>
   > >       bool wait_until(unique_lock<mutex>& lock,
   > >         const chrono::time_point<Clock, Duration>& abs_time,
   > >     	Predicate pred);

   > > > […]

   > > > -?- Remarks: If the function fails to meet the postcondition, `std::terminate()` shall be called (15.5.1 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note]

   > > > -33- Postcondition: `lock.owns_lock()` is true and `lock.mutex()` is locked by the calling thread.

   > > > […]

   > > > -35- Throws: `system_error` when an exception is required (30.2.2), timeout-related exceptions (30.2.4), or any exception thrown by `pred`.

   > > > -36- Error conditions:

   > > > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

   > >     template <class Rep, class Period, class Predicate>
   > >       bool wait_for(unique_lock<mutex>& lock,
   > >         const chrono::duration<Rep, Period>& rel_time,
   > >     	Predicate pred);

   > > > […]

   > > > -?- Remarks: If the function fails to meet the postcondition, `std::terminate()` shall be called (15.5.1 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note]

   > > > -40- Postcondition: `lock.owns_lock()` is true and `lock.mutex()` is locked by the calling thread.

   > > > […]

   > > > -42- Throws: `system_error` when an exception is required (30.2.2), timeout-related exceptions (30.2.4), or any exception thrown by `pred`.

   > > > -43- Error conditions:

   > > > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

2. Edit 30.5.2 [thread.condition.condvarany] as indicated:

   >     template<class Lock>
   >     void wait(Lock& lock);

   > > […]

   > > -10- Effects:

   > > - Atomically calls `lock.unlock()` and blocks on `*this`.

   > > - When unblocked, calls `lock.lock()` (possibly blocking on the lock) and returns.

   > > - The function will unblock when signaled by a call to `notify_one()`, a call to `notify_all()`, or spuriously.

   > > - If the function exits via an exception, `lock.lock()` shall be called prior to exiting the function scope.

   > > -?- Remarks: If the function fails to meet the postcondition, `std::terminate()` shall be called (15.5.1 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note]

   > > -11- Postcondition: `lock` is locked by the calling thread.

   > > -12- Throws: Nothing `system_error` when an exception is required (30.2.2).

   > > -13- Error conditions:

   > > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

   > >     template <class Lock, class Clock, class Duration>
   > >       cv_status wait_until(Lock& lock, const chrono::time_point<Clock, Duration>& abs_time);

   > > > […]

   > > > -15- Effects:

   > > > - […]

   > > > - If the function exits via an exception, `lock.lock()` shall be called prior to exiting the function scope.

   > > > -?- Remarks: If the function fails to meet the postcondition, `std::terminate()` shall be called (15.5.1 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note]

   > > > -16- Postcondition: `lock` is locked by the calling thread.

   > > > […]

   > > > -18- Throws: `system_error` when an exception is required (30.2.2) or timeout-related exceptions (30.2.4).

   > > > -19- Error conditions:

   > > > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

   > >     template <class Lock, class Rep, class Period>
   > >       cv_status wait_for(Lock& lock, const chrono::duration<Rep, Period>& rel_time);

   > > > […]

   > > > -?- Remarks: If the function fails to meet the postcondition, `std::terminate()` shall be called (15.5.1 [except.terminate]). [Note: This can happen if the re-locking of the mutex throws an exception. — end note]

   > > > -22- Postcondition: `lock` is locked by the calling thread.

   > > > […]

   > > > -23- Throws: `system_error` when an exception is required (30.2.2) or timeout-related exceptions (30.2.4).

   > > > -24- Error conditions:

   > > > - equivalent error condition from `lock.lock()` or `lock.unlock()`.

### 2138. `atomic_flag::clear` should not accept `memory_order_consume`

Section: 29.7 [atomics.flag] Status: C++14 Submitter: Ben Viglietta Opened: 2012-03-08 Last modified: 2015-04-08

View all other issues in [atomics.flag].

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Discussion:

N3376 29.7 [atomics.flag]/7 says this about `atomic_flag::clear`:

> Requires: The `order` argument shall not be `memory_order_acquire` or `memory_order_acq_rel`.

In addition, `memory_order_consume` should be disallowed, since it doesn't meaningfully apply to store operations. It's already disallowed on the analogous `atomic<T>::store`. The proposed updated text would be:

> Requires: The `order` argument shall not be `memory_order_consume`, `memory_order_acquire`, or `memory_order_acq_rel`.

[2012, Portland: move to Review]

Hans: this is a clear oversight.

Moved to review

[2013-04-20, Bristol]

Accepted for the working paper

Proposed resolution:

[This wording is relative to N3376.]

>     void atomic_flag_clear(volatile atomic_flag *object) noexcept;
>     void atomic_flag_clear(atomic_flag *object) noexcept;
>     void atomic_flag_clear_explicit(volatile atomic_flag *object, memory_order order) noexcept;
>     void atomic_flag_clear_explicit(atomic_flag *object, memory_order order) noexcept;
>     void atomic_flag::clear(memory_order order = memory_order_seq_cst) volatile noexcept;
>     void atomic_flag::clear(memory_order order = memory_order_seq_cst) noexcept;

> > -7- Requires: The `order` argument shall not be `memory_order_consume`, `memory_order_acquire`, or `memory_order_acq_rel`.

> > -8- Effects: Atomically sets the value pointed to by `object` or by `this` to false. Memory is affected according to the value of `order`.

### 2140. Meaning of `notify_all_at_thread_exit` synchronization requirement?

Section: 30.5 [thread.condition] Status: C++14 Submitter: Pete Becker Opened: 2012-03-06 Last modified: 2015-04-08

View all other issues in [thread.condition].

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Discussion:

`notify_all_at_thread_exit` has the following synchronization requirement:

> Synchronization: The call to `notify_all_at_thread_exit` and the completion of the destructors for all the current thread's variables of thread storage duration synchronize with (1.10 [intro.multithread]) calls to functions waiting on `cond`.

The functions waiting on `cond` have already been called, otherwise they wouldn't be waiting. So how can a subsequent call to `notify_all_at_thread_exit` synchronize with them?

Also, "synchronizes with" is a relationship between library calls (1.10 [intro.multithread]/8), so it's not meaningful for completion of destructors for non-library objects. Presumably the intention wasn't so make library destructors special here.

[2012-03-09 Jeffrey Yasskin comments:]

I think the text should say that "`notify_all_at_thread_exit` and destructor calls are sequenced before the `lk.unlock()`", and leave it at that, unless there's a funny implementation I haven't thought of.

[2012-03-19 Hans Boehm comments:]

I think the synchronization clause should just be replaced with (modulo wording tweaks):

"The implied `lk.unlock()` call is sequenced after the destruction of all objects with thread storage duration associated with the current thread."

as Jeffrey suggested.

To use this correctly, the notifying thread has to essentially acquire the lock, set a variable indicating it's done, call `notify_all_at_thread_exit()`, while the waiting thread acquires the lock, and repeatedly waits on the cv until the variable is set, and then releases the lock. That ensures that we have the proper synchronizes with relationship as a result of the lock.

[2012, Portland: move to Review]

The `lk.unlock()` refers back to the wording the previous paragraph.

Moved to review

[2013-04-20, Bristol]

Accepted for the working paper

Proposed resolution:

This wording is relative to N3376.

1. Modify 30.5 [thread.condition] p8 as indicated:

   >     void notify_all_at_thread_exit(condition_variable& cond, unique_lock<mutex> lk);

   > > […]

   > > -8- Synchronization: The call to `notify_all_at_thread_exit` and the completion of the destructors for all the current thread's variables of thread storage duration synchronize with (1.10 [intro.multithread]) calls to functions waiting on `cond` The implied `lk.unlock()` call is sequenced after the destruction of all objects with thread storage duration associated with the current thread.

### 2141. `common_type` trait produces reference types

Section: 20.10.7.6 [meta.trans.other] Status: C++14 Submitter: Doug Gregor Opened: 2012-03-11 Last modified: 2015-04-08

View other active issues in [meta.trans.other].

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Discussion:

The type computation of the `common_type` type trait is defined as

>     template <class T, class U>
>      struct common_type<T, U> {
>        typedef decltype(true ? declval<T>() : declval<U>()) type;
>      };

This means that `common_type<int, int>::type` is `int&&`, because

- `declval<int>()` returns `int&&`
- The conditional operator returns an xvalue when its second and third operands have the same type and are both xvalues (5.16 [expr.cond] p4)
- `decltype` returns `T&&` when its expression is an xvalue (7.1.6.2 [dcl.type.simple] p4)

Users of `common_type` do not expect to get a reference type as the result; the expectation is that `common_type` will return a non-reference type to which all of the types can be converted.

Daniel: In addition to that it should be noted that without such a fix the definition of `std::unique_ptr`'s `operator<` in 20.8.1.5 [unique.ptr.special] (around p4) is also broken: In the most typical case (with default deleter), the determination of the common pointer type CT will instantiate `std::less<CT>` which can now be `std::less<T*&&>`, which will not be the specialization of pointer types that guarantess a total order.

Given the historic constext of `common_type` original specification, the proper resolution to me seems to be using `std::decay` instead of `std::remove_reference`:

>     template <class T, class U>
>     struct common_type<T, U> {
>       typedef typename decay<decltype(true ? declval<T>() : declval<U>())>::type type;
>     };

At that time rvalues had no identity in this construct and rvalues of non-class types have no cv-qualification. With this change we would ensure that

>     common_type<int, int>::type == common_type<const int, const int>::type == int

Note that this harmonizes with the corresponding heterogenous case, which has already the exact same effect:

>     common_type<int, long>::type == common_type<const int, const long>::type == long

[2012-10-11 Daniel comments]

While testing the effects of applying the proposed resolution I noticed that this will have the effect that the unary form of `common_type`, like

>     common_type<int>

is not symmetric to the n-ary form (n > 1). This is unfortunate, because this difference comes especially to effect when `common_type` is used with variadic templates. As an example consider the following `make_array` template:

>     #include <array>
>     #include <type_traits>
>     #include <utility>

>     template<class... Args>
>     std::array<typename std::common_type<Args...>::type, sizeof...(Args)>
>     make_array(Args&&... args)
>     {
>       typedef typename std::common_type<Args...>::type CT;
>       return std::array<CT, sizeof...(Args)>{static_cast<CT>(std::forward<Args>(args))...};
>     }

>     int main()
>     {
>       auto a1 = make_array(0); // OK: std::array<int, 1>
>       auto a2 = make_array(0, 1.2); // OK: std::array<double, 2>
>       auto a3 = make_array(5, true, 3.1415f, 'c'); // OK: std::array<float, 4>

>       int i = 0;
>       auto a1b = make_array(i); // Error, attempt to form std::array<int&, 1>

>       auto a2b = make_array(i, 1.2); // OK: std::array<double, 2>
>       auto a2c = make_array(i, 0); // OK: std::array<int, 2>
>     }

The error for `a1b` only happens in the unary case and it is easy that it remains unnoticed during tests. You cannot explain that reasonably to the user here.

Of-course it is possible to fix that in this example by applying `std::decay` to the result of the `std::common_type` deduction. But if this is necessary here, I wonder why it should also be applied to the binary case, where it gives the wrong illusion of a complete type decay? The other way around: Why is `std::decay` not also applied to the unary case as well?

This problem is not completely new and was already observed for the original `std::common_type` specification. At this time the `decltype` rules had a similar asymmetric effect when comparing

> `std::common_type<const int, const int>::type` (equal to '`int`' at this time)

with:

> `std::common_type<const int>::type` (equal to '`const int`')

and I wondered whether the unary form shouldn't also perform the same "decay" as the n-ary form.

This problem makes me think that the current resolution proposal might not be ideal and I expect differences in implementations (for those who consider to apply this proposed resolution already). I see at least three reasonable options:

1. Accept the current wording suggestion for LWG 2141 as it is and explain that to users.

2. Keep `std::common_type` as currently specified in the Standard and tell users to use `std::decay` where needed. Also fix other places in the library, e.g. the comparison functions of `std::unique_ptr` or a most of the time library functions.

3. Apply `std::decay` also in the unary specialization of `std::common_type` with the effect that `std::common_type<const int&>::type` returns `int`.

[2012-10-11 Marc Glisse comments]

If we are going with decay everywhere, I wonder whether we should also decay in the 2-argument version before and not only after. So if I specialize `common_type<mytype, double>`, `common_type<const mytype, volatile double&>` would automatically work.

[2012-10-11 Daniel provides wording for bullet 3 of his list:]

1. Change 20.10.7.6 [meta.trans.other] p3 as indicated:

   >     template <class T>
   >     struct common_type<T> {
   >       typedef typename decay<T>::type type;
   >     };

   >     template <class T, class U>
   >     struct common_type<T, U> {
   >       typedef typename decay<decltype(true ? declval<T>() : declval<U>())>::type type;
   >     };

[2013-03-15 Issues Teleconference]

Moved to Review.

Want to carefully consider the effect of `decay` vs. `remove_reference` with respect to constness before adopting, although this proposed resolution stands for review in Bristol.

[2013-04-18, Bristol meeting]

Previous wording:

> This wording is relative to N3376.

> 1. In 20.10.7.6 [meta.trans.other] p3, change the `common_type` definition to

>    >     template <class T, class U>
>    >     struct common_type<T, U> {
>    >       typedef typename decay<decltype(true ? declval<T>() : declval<U>())>::type type;
>    >     };

[2013-04-18, Bristol]

Move to Ready

[2013-09-29, Chicago]

Accepted for the working paper

Proposed resolution:

This wording is relative to N3485.

1. Change 20.10.7.6 [meta.trans.other] p3 as indicated:

   >     template <class T>
   >     struct common_type<T> {
   >       typedef typename decay<T>::type type;
   >     };

   >     template <class T, class U>
   >     struct common_type<T, U> {
   >       typedef typename decay<decltype(true ? declval<T>() : declval<U>())>::type type;
   >     };

### 2142. `packaged_task::operator()` synchronization too broad?

Section: 30.6.9.1 [futures.task.members] Status: C++14 Submitter: Pete Becker Opened: 2012-03-12 Last modified: 2015-04-08

View all other issues in [futures.task.members].

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Discussion:

According to 30.6.9.1 [futures.task.members] p.18:

> [A] successful call to [`packaged_task::`]`operator()` synchronizes with a call to any member function of a `future` or `shared_future` object that shares the shared state of `*this`.

This requires that the call to `operator()` synchronizes with calls to `future::wait_for`, `future::wait_until`, `shared_future::wait_for`, and `shared_future::wait_until`, even when these functions return because of a timeout.

[2012, Portland: move to Open]

If it said "a successful return from" (or "a return from" to cover exceptions) the problem would be more obvious.

Detlef: will ask Anthony Williams to draft some wording.

Moved to open (Anthony drafted to draft)

[2013-09, Chicago: move to Ready]

Anthony's conclusion is that the offending paragraph is not needed. Already included in the statement that the state is made ready.

Recommendation: Remove 30.6.9.1 [futures.task.members] p18 (the synchronization clause). Redundant because of 30.6.4 [futures.state] p9.

Moved to Ready

Proposed resolution:

This wording is relative to N3691.

1. Remove 30.6.9.1 [futures.task.members] p18 as indicated:

   >     void operator()(ArgTypes... args);

   > > […]

   > > -18- Synchronization: a successful call to `operator()` synchronizes with (1.10) a call to any member function of a `future` or `shared_future` object that shares the shared state of `*this`. The completion of the invocation of the stored task and the storage of the result (whether normal or exceptional) into the shared state synchronizes with (1.10) the successful return from any member function that detects that the state is set to ready. [Note: `operator()` synchronizes and serializes with other functions through the shared state. — end note]

### 2143. `ios_base::xalloc` should be thread-safe

Section: 27.5.3 [ios.base] Status: C++14 Submitter: Alberto Ganesh Barbati Opened: 2012-03-14 Last modified: 2015-04-08

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Discussion:

The static function `ios_base::xalloc()` could be called from multiple threads and is not covered by 17.6.4.10 [res.on.objects] and 17.6.5.9 [res.on.data.races]. Adding a thread-safety requirement should not impose a significant burden on implementations, as the function can be easily implemented with hopefully lock-free atomics.

[2013-04-20, Bristol]

Unanimous.

Resolution: move tentatively ready. (Inform Bill about this issue.)

[2013-09-29, Chicago]

Apply to Working Paper

Proposed resolution:

This wording is relative to N3376.

1. In 27.5.3.5 [ios.base.storage] add a new paragraph after paragraph 1:

   >     static int xalloc();

   > > -1- Returns: `index ++`.

   > > -?- Remarks: Concurrent access to this function by multiple threads shall not result in a data race (1.10 [intro.multithread]).

### 2144. Missing `noexcept` specification in `type_index`

Section: 20.14 [type.index] Status: C++14 Submitter: Daniel Krügler Opened: 2012-03-18 Last modified: 2015-04-08

View all other issues in [type.index].

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Discussion:

The class type `type_index` is a thin wrapper of `type_info` to adapt it as a valid associative container element. Similar to `type_info`, all member functions have an effective `noexcept(true)` specification, with the exception of `hash_code()` and `name()`. The actual effects of these functions is a direct call to `type_info`'s `hash_code()` and `name` function, but according to 18.7 [support.rtti] these are both `noexcept` functions, so there is no reason for not declaring them as `noexcept`, too. In fact, one of the suggested changes of the original proposing paper N2530 specifically was to ensure that `type_info` would get a `hash_code()` function that guarantees not to throw exceptions (during that time the `hash` requirements did not allow to exit with an exception). From this we can conclude that `type_index::hash_code()` was intended to be nothrow.

It seems both consistent and technically simply to require these functions to be `noexcept`.

[2013-03-15 Issues Teleconference]

Moved to Tentatively Ready.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3376.

1. Modify the class `type_index` synopsis, 20.14.2 [type.index.overview] as indicated:

   >     namespace std {
   >       class type_index {
   >       public:
   >         type_index(const type_info& rhs) noexcept;
   >         bool operator==(const type_index& rhs) const noexcept;
   >         bool operator!=(const type_index& rhs) const noexcept;
   >         bool operator< (const type_index& rhs) const noexcept;
   >         bool operator<= (const type_index& rhs) const noexcept;
   >         bool operator> (const type_index& rhs) const noexcept;
   >         bool operator>= (const type_index& rhs) const noexcept;
   >         size_t hash_code() const noexcept;
   >         const char* name() const noexcept;
   >       private:
   >         const type_info* target; // exposition only
   >         // Note that the use of a pointer here, rather than a reference,
   >         // means that the default copy/move constructor and assignment
   >         // operators will be provided and work as expected.
   >       };
   >     }

1. Modify the prototype definitions in 20.14.3 [type.index.members] as indicated:

   >     size_t hash_code() const noexcept;

   > > -8- Returns: `target->hash_code()`

   >     const char* name() const noexcept;

   > > -9- Returns: `target->name()`

### 2145. `error_category` default constructor

Section: 19.5.1 [syserr.errcat] Status: C++14 Submitter: Howard Hinnant Opened: 2012-03-21 Last modified: 2015-04-08

View all other issues in [syserr.errcat].

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Discussion:

Should `error_category` have a default constructor?

If you look at the synopsis in 19.5.1.1 [syserr.errcat.overview], it appears the answer is no. There is no default constructor declared and there is another constructor declared (which should inhibit a default constructor).

However in paragraph 1 of the same section, descriptive text says:

> Classes may be derived from `error_category` to support categories of errors in addition to those defined in this International Standard.

How shall classes derived from `error_category` construct their base?

Jonathan Wakely: In N2066 `error_category` was default-constructible. That is still the case in N2241, because no other constructor is declared. Then later N2422 (issue 6) declares the copy constructor as deleted, but doesn't add a default constructor, causing it to be no longer default-constructible. That looks like an oversight to me, and I think there should be a public default constructor.

Daniel: A default-constructor indeed should be provided to allow user-derived classes as described by the standard. I suggest this one to be both `noexcept` and `constexpr`. The latter allows user-derived non-abstract classes to take advantage of the special constant initialization rule of 3.6.2 [basic.start.init] p2 b2 for objects with static (or thread) storage duration in namespace scope. Note that a `constexpr` constructor is feasible here, even though there exists a non-trivial destructor and even though `error_category` is not a literal type (see `std::mutex` for a similar design choice).

In addition to that the proposed resolution fixes another minor glitch: According to 17.5.2.2 [functions.within.classes] virtual destructors require a semantics description.

Alberto Ganesh Barbati: I would suggest to remove `=default` from the constructor instead. Please consider that defaulting a constructor or destructor may actually define them as deleted under certain conditions (see 12.1 [class.ctor]/5 and 12.4 [class.dtor]/5). Removing `=default` is easier than providing wording to ensures that such conditions do not occur.

[2012-10 Portland: move to Ready]

The issue is real and the resolution looks good.

Are there similar issues elsewhere in this clause?

Potential to add `constexpr` to more constructors, but clearly a separable issue.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3376.

1. Modify the class `error_category` synopsis, 19.5.1.1 [syserr.errcat.overview] as indicated: [Drafting note: According to the general `noexcept` library guidelines destructors should not have any explicit exception specification. This destructor was overlooked during the paper analysis — end note]

   >     namespace std {
   >       class error_category {
   >       public:
   >         constexpr error_category() noexcept;
   >         virtual ~error_category() noexcept;
   >         error_category(const error_category&) = delete;
   >         error_category& operator=(const error_category&) = delete;
   >         virtual const char* name() const noexcept = 0;
   >         virtual error_condition default_error_condition(int ev) const noexcept;
   >         virtual bool equivalent(int code, const error_condition& condition) const noexcept;
   >         virtual bool equivalent(const error_code& code, int condition) const noexcept;
   >         virtual string message(int ev) const = 0;
   >         bool operator==(const error_category& rhs) const noexcept;
   >         bool operator!=(const error_category& rhs) const noexcept;
   >         bool operator<(const error_category& rhs) const noexcept;
   >       };
   >     }

2. Before 19.5.1.2 [syserr.errcat.virtuals] p1 insert a new prototype description as indicated:

   >     virtual ~error_category();

   > > -?- Effects: Destroys an object of class `error_category`.

3. Before 19.5.1.3 [syserr.errcat.nonvirtuals] p1 insert a new prototype description as indicated:

   >     constexpr error_category() noexcept;

   > > -?- Effects: Constructs an object of class `error_category`.

### 2147. Unclear hint type in `Allocator`'s `allocate` function

Section: 17.6.3.5 [allocator.requirements] Status: C++14 Submitter: Daniel Krügler Opened: 2012-03-05 Last modified: 2015-04-08

View other active issues in [allocator.requirements].

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Discussion:

According to Table 28 — "Allocator requirements", the expression

>     a.allocate(n, u)

expects as second argument a value `u` that is described in Table 27 as:

> a value of type `YY::const_pointer` obtained by calling `YY::allocate`, or else `nullptr`.

This description leaves it open, whether or whether not a value of type `YY::const_void_pointer` is valid or not. The corresponding wording in C++03 is nearly the same, but in C++03 there did not exist the concept of a general `void_pointer` for allocators. There is some evidence for support of void pointers because the general `allocator_traits` template declares

>     static pointer allocate(Alloc& a, size_type n, const_void_pointer hint);

and the corresponding function for `std::allocator<T>` is declared as:

>     pointer allocate(size_type, allocator<void>::const_pointer hint = 0);

As an additional minor wording glitch (especially when comparing with the `NullablePointer` requirements imposed on `const_pointer` and `const_void_pointer`), the wording seems to exclude lvalues of type `std::nullptr_t`, which looks like an unwanted artifact to me.

[ 2012-10 Portland: Move to Ready ]

No strong feeling that this is a big issue, but consensus that the proposed resolution is strictly better than the current wording, so move to Ready.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3376.

1. Change Table 27 — "Descriptive variable definitions" in 17.6.3.5 [allocator.requirements]:

   Table 27 — Descriptive variable definitions
   Variable | Definition
   --- | ---
   `u` | a value of type `YY::const_pointer` obtained by calling `YY::allocate`, or else `nullptr` `XX::const_void_pointer` obtained by conversion from a result value of `YY::allocate`, or else a value of type (possibly const) `std::nullptr_t`.

### 2148. Hashing enums should be supported directly by `std::hash`

Section: 20.9.13 [unord.hash] Status: C++14 Submitter: Ville Voutilainen Opened: 2012-04-10 Last modified: 2015-04-08

View other active issues in [unord.hash].

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Discussion:

The paper proposes various hashing improvements. What it doesn't mention is hashing of enums; enums are integral types, and users expect them to have built-in hashing support, rather than having to convert enums to ints for uses with unordered containers and other uses of hashes. Daniel Krügler explains in c++std-lib-32412 that this is not achievable with a SFINAEd hash specialization because it would require a partial specialization with a type parameter and a non-type parameter with a default argument, which is currently not allowed, and hence the fixes in N3333 should be adopted instead.

[2012-10 Portland: Move to Open]

We agree this is a real issue that should be resolved, by specifying such a hash.

It is not clear that we should specify this as calling hash on the `underlying_type`, or whether that is overspecification and we merely require that the hash be supplied.

STL already has shipped an implementation, and is keen to provide wording.

[ 2013-04-14 STL provides rationale and improved wording ]

Rationale:

This can be achieved by inserting a very small tweak to the Standardese. We merely have to require that `hash<Key>` be valid when `Key` is an "enumeration type" (which includes both scoped and unscoped enums). This permits, but does not require, `hash<Enum>` to behave identically to `hash<underlying_type<Enum>::type>`, following existing precedent — note that when `unsigned int` and `unsigned long` are the same size, `hash<unsigned int>` is permitted-but-not-required to behave identically to `hash<unsigned long>`.

This proposed resolution doesn't specify anything else about the primary template, allowing implementations to do whatever they want for non-enums: `static_assert` nicely, explode horribly at compiletime or runtime, etc.

While we're in the neighborhood, this proposed resolution contains an editorial fix. The 20.9 [function.objects] synopsis says "base template", which doesn't appear anywhere else in the Standard, and could confuse users into thinking that they need to derive from it. The proper phrase is "primary template".

[2013-04-18, Bristol]

Proposed resolution:

This wording is relative to N3485.

1. In 20.9 [function.objects], header functional synopsis, edit as indicated:

   >     namespace std {
   >       […]
   >       // 20.8.12, hash function baseprimary template:
   >       template <class T> struct hash;
   >       […]
   >     }

2. In 20.9.13 [unord.hash]/1 edit as indicated:

   > -1- The unordered associative containers defined in 23.5 [unord] use specializations of the class template `hash` as the default `hash` function. For all object types `Key` for which there exists a specialization `hash<Key>`, and for all enumeration types (7.2 [dcl.enum]) Key, the instantiation `hash<Key>` shall: […]

### 2149. Concerns about 20.8/5

Section: 20.9 [function.objects] Status: C++14 Submitter: Scott Meyers Opened: 2012-02-15 Last modified: 2015-04-08

View all other issues in [function.objects].

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Discussion:

20.9 [function.objects] p5 says:

> To enable adaptors and other components to manipulate function objects that take one or two arguments it is required that the function objects correspondingly provide typedefs `argument_type` and `result_type` for function objects that take one argument and `first_argument_type`, `second_argument_type`, and `result_type` for function objects that take two arguments.

I have two concerns about this paragraph. First, the wording appears to prescribe a requirement for all function objects in valid C++ programs, but it seems unlikely that that is the intent. As such, the scope of the requirement is unclear. For example, there is no mention of these typedefs in the specification for closures (5.1.2), and Daniel Krügler has explained in the thread at http://tinyurl.com/856plkn that conforming implementations can detect the difference between closures with and without these typedefs. (Neither gcc 4.6 nor VC10 appear to define typedefs such as `result_type` for closure types. I have not tested other compilers.)

Second, the requirement appears to be unimplementable in some cases, notably for function objects returned from `std::bind`, as Howard Hinnant explains in the thread at http://tinyurl.com/6q5bos4.

From what I can tell, the standard already defines which adaptability typedefs must be provided by various kinds of function objects in the specifications for those objects. Examples include the function objects specified in 20.9.4 [refwrap]-20.9.9 [negators]. I therefore suggest that 20.9 [function.objects]/5 simply be removed from the standard. I don't think it adds anything except opportunities for confusion.

[2012-10 Portland: Move to Open]

This wording caused confusion earlier in the week when reviewing Stefan's paper on `greater<>`.

This phrasing sounds normative, but is actually descriptive but uses unfortunate wording.

The main reason this wording exists is to document the protocol required to support the legacy binders in Annex D.

Stefan points out that `unary_negate` and `binary_negate` have not been deprecated and rely on this. He plans a paper to remove this dependency.

Consensus that this wording is inadequate, confusing, and probably should be removed. However, that leaves a big hole in the specification for the legacy binders, that needs filling.

While not opposed to striking this paragraph, we will need the additional wording to fix the openning hole before this issue can move forward.

[ 2013-04-14 STL provides rationale ]

Rationale:

I've concluded that Scott's original proposed resolution was correct and complete. There are two sides to this story: the producers and the consumers of these typedefs.

Producers: As Scott noted, the Standard clearly documents which function objects must provide these typedefs. Some function objects must provide them unconditionally (e.g. `plus<T>` (for `T != void`), 20.9.5 [arithmetic.operations]/1), some conditionally (e.g. `reference_wrapper<T>`, 20.9.4 [refwrap]/2-4), and some don't have to provide them at all (e.g. lambdas, 5.1.2 [expr.prim.lambda]). These requirements are clear, so we shouldn't change them or even add informative notes. Furthermore, because these typedefs aren't needed in the C++11 world with `decltype`/perfect forwarding/etc., we shouldn't add more requirements to provide them.

Consumers: This is what we were concerned about at Portland. However, the consumers also clearly document their requirements in the existing text. For example, `reference_wrapper<T>` is also a conditional consumer, and 20.9.4 [refwrap] explains what typedefs it's looking for. We were especially concerned about the old negators and the deprecated binders, but they're okay too. 20.9.9 [negators] clearly says that `unary_negate<Predicate>` requires `Predicate::argument_type` to be a type, and `binary_negate<Predicate>` requires `Predicate::first_argument_type` and `Predicate::second_argument_type` to be types. (`unary_negate`/`binary_negate` provide `result_type` but they don't consume it.) X [depr.lib.binders] behaves the same way with `Fn::first_argument_type`, `Fn::second_argument_type`, and `Fn::result_type`. No additional wording is necessary.

A careful reading of 20.9 [function.objects]/5 reveals that it wasn't talking about anything beyond the mere existence of the mentioned typedefs — for example, it didn't mention that the function object's return type should be `result_type`, or even convertible to `result_type`. As the producers and consumers are certainly talking about the existence of the typedefs (in addition to clearly implying semantic requirements), we lose nothing by deleting the unnecessary paragraph.

[2013-04-18, Bristol]

Previous wording:

> Remove 20.9 [function.objects] p5:

> > To enable adaptors and other components to manipulate function objects that take one or two arguments it is required that the function objects correspondingly provide typedefs `argument_type` and `result_type` for function objects that take one argument and `first_argument_type`, `second_argument_type`, and `result_type` for function objects that take two arguments.

Proposed resolution:

This wording is relative to N3485.

Edit 20.9 [function.objects] p5:

> [Note:To enable adaptors and other components to manipulate function objects that take one or two arguments it is required that the function objects many of the function objects in this clause correspondingly provide typedefs `argument_type` and `result_type` for function objects that take one argument and `first_argument_type`, `second_argument_type`, and `result_type` for function objects that take two arguments.— end note]

### 2150. Unclear specification of `find_end`

Section: 25.2.6 [alg.find.end] Status: C++14 Submitter: Andrew Koenig Opened: 2012-03-28 Last modified: 2015-04-08

View all issues with C++14 status.

Discussion:

25.2.6 [alg.find.end] describes the behavior of find_end as returning:

> The last iterator `i` in the range `[first1,last1 - (last2 - first2))` such that for any nonnegative integer `n < (last2 - first2)`, the following corresponding conditions hold: `*(i + n) == *(first2 + n), pred(*(i + n), *(first2 + n)) != false`.

Does "for any" here mean "for every" or "there exists a"? I think it means the former, but it could be interpreted either way.

Daniel: The same problem exists for the following specifications from Clause 25 [algorithms]:

1. 25.2.13 [alg.search] p2 and p6
2. 25.3.10 [alg.reverse] p4
3. 25.3.13 [alg.partitions] p5 and p9
4. 25.4 [alg.sorting] p5
5. 25.4.2 [alg.nth.element] p1
6. 25.4.3.1 [lower.bound] p2
7. 25.4.3.2 [upper.bound] p2
8. 25.4.7 [alg.min.max] p21 and p23

[2013-04-20, Bristol]

Unanimous agreement on the wording.

Resolution: move to tentatively ready

[2013-09-29, Chicago]

Apply to Working Paper

Proposed resolution:

This wording is relative to N3376.

1. Change 25.2.6 [alg.find.end] p2 as indicated:

   >     template<class ForwardIterator1, class ForwardIterator2>
   >     ForwardIterator1
   >     find_end(ForwardIterator1 first1, ForwardIterator1 last1,
   >              ForwardIterator2 first2, ForwardIterator2 last2);
   >     template<class ForwardIterator1, class ForwardIterator2,
   >              class BinaryPredicate>
   >     ForwardIterator1
   >     find_end(ForwardIterator1 first1, ForwardIterator1 last1,
   >              ForwardIterator2 first2, ForwardIterator2 last2,
   >              BinaryPredicate pred);

   > > […]

   > > -2- Returns: The last iterator `i` in the range `[first1,last1 - (last2 - first2))` such that for any every nonnegative integer `n < (last2 - first2)`, the following corresponding conditions hold: `*(i + n) == *(first2 + n), pred(*(i + n), *(first2 + n)) != false`. Returns `last1` if `[first2,last2)` is empty or if no such iterator is found.

2. Change 25.2.13 [alg.search] p2 and p6 as indicated:

   >     template<class ForwardIterator1, class ForwardIterator2>
   >     ForwardIterator1
   >     search(ForwardIterator1 first1, ForwardIterator1 last1,
   >            ForwardIterator2 first2, ForwardIterator2 last2);
   >     template<class ForwardIterator1, class ForwardIterator2,
   >              class BinaryPredicate>
   >     ForwardIterator1
   >     search(ForwardIterator1 first1, ForwardIterator1 last1,
   >            ForwardIterator2 first2, ForwardIterator2 last2,
   >            BinaryPredicate pred);

   > > […]

   > > -2- Returns: The first iterator `i` in the range `[first1,last1 - (last2-first2))` such that for any every nonnegative integer `n` less than `last2 - first2` the following corresponding conditions hold: `*(i + n) == *(first2 + n), pred(*(i + n), *(first2 + n)) != false`. Returns `first1` if `[first2,last2)` is empty, otherwise returns `last1` if no such iterator is found.

   […]

   >     template<class ForwardIterator, class Size, class T>
   >     ForwardIterator
   >     search_n(ForwardIterator first, ForwardIterator last, Size count,
   >              const T& value);
   >     template<class ForwardIterator, class Size, class T,
   >              class BinaryPredicate>
   >     ForwardIterator
   >     search_n(ForwardIterator first, ForwardIterator last, Size count,
   >              const T& value, BinaryPredicate pred);

   > > […]

   > > -6- Returns: The first iterator `i` in the range `[first,last-count)` such that for any every non-negative integer `n` less than `count` the following corresponding conditions hold: `*(i + n) == value, pred(*(i + n),value) != false`. Returns `last` if no such iterator is found.

3. Change 25.3.10 [alg.reverse] p4 as indicated:

   >     template<class BidirectionalIterator, class OutputIterator>
   >     OutputIterator
   >     reverse_copy(BidirectionalIterator first,
   >                  BidirectionalIterator last, OutputIterator result);

   > > […]

   > > -4- Effects: Copies the range `[first,last)` to the range `[result,result+(last-first))` such that for any every non-negative integer `i < (last - first)` the following assignment takes place: `*(result + (last - first) - i) = *(first + i)`.

4. Change 25.3.13 [alg.partitions] p5 and p9 as indicated:

   >     template<class ForwardIterator, class Predicate>
   >     ForwardIterator
   >     partition(ForwardIterator first,
   >               ForwardIterator last, Predicate pred);

   > > […]

   > > -5- Returns: An iterator `i` such that for any every iterator `j` in the range `[first,i) pred(*j) != false`, and for any every iterator `k` in the range `[i,last), pred(*k) == false`.

   […]

   >     template<class BidirectionalIterator, class Predicate>
   >     BidirectionalIterator
   >     stable_partition(BidirectionalIterator first,
   >                      BidirectionalIterator last, Predicate pred);

   > > […]

   > > -9- Returns: An iterator `i` such that for any every iterator `j` in the range `[first,i), pred(*j) != false`, and for any every iterator `k` in the range `[i,last), pred(*k) == false`. The relative order of the elements in both groups is preserved.

5. Change 25.4 [alg.sorting] p5 as indicated:

   > -5- A sequence is sorted with respect to a comparator `comp` if for any every iterator `i` pointing to the sequence and any every non-negative integer `n` such that `i + n` is a valid iterator pointing to an element of the sequence, `comp(*(i + n), *i) == false`.

6. Change 25.4.2 [alg.nth.element] p1 as indicated:

   >     template<class RandomAccessIterator>
   >     void nth_element(RandomAccessIterator first, RandomAccessIterator nth,
   >                      RandomAccessIterator last);
   >     template<class RandomAccessIterator, class Compare>
   >     void nth_element(RandomAccessIterator first, RandomAccessIterator nth,
   >                      RandomAccessIterator last, Compare comp);

   > > -1- After `nth_element` the element in the position pointed to by `nth` is the element that would be in that position if the whole range were sorted. Also for any every iterator `i` in the range `[first,nth)` and any every iterator `j` in the range `[nth,last)` it holds that: `!(*i > *j)` or `comp(*j, *i) == false`.

7. Change 25.4.3.1 [lower.bound] p2 as indicated:

   >     template<lass ForwardIterator, class T>
   >     ForwardIterator
   >     lower_bound(ForwardIterator first, ForwardIterator last,
   >                 const T& value);
   >     template<class ForwardIterator, class T, class Compare>
   >     ForwardIterator
   >     lower_bound(ForwardIterator first, ForwardIterator last,
   >                 const T& value, Compare comp);

   > > […]

   > > -2- Returns: The furthermost iterator `i` in the range `[first,last]` such that for any every iterator `j` in the range `[first,i)` the following corresponding conditions hold: `*j < value` or `comp(*j, value) != false`.

8. Change 25.4.3.2 [upper.bound] p2 as indicated:

   >     template<lass ForwardIterator, class T>
   >     ForwardIterator
   >     upper_bound(ForwardIterator first, ForwardIterator last,
   >                 const T& value);
   >     template<class ForwardIterator, class T, class Compare>
   >     ForwardIterator
   >     upper_bound(ForwardIterator first, ForwardIterator last,
   >                 const T& value, Compare comp);

   > > […]

   > > -2- Returns: The furthermost iterator `i` in the range `[first,last]` such that for any every iterator `j` in the range `[first,i)` the following corresponding conditions hold: `!(value < *j)` or `comp(value, *j) == false`.

9. Change 25.4.7 [alg.min.max] p21 and p23 as indicated:

   >     template<class ForwardIterator>
   >     ForwardIterator min_element(ForwardIterator first, ForwardIterator last);
   >     template<class ForwardIterator, class Compare>
   >     ForwardIterator min_element(ForwardIterator first, ForwardIterator last,
   >                                 Compare comp);

   > > -21- Returns: The first iterator `i` in the range `[first,last)` such that for any every iterator `j` in the range `[first,last)` the following corresponding conditions hold: `!(*j < *i)` or `comp(*j, *i) == false`. Returns `last` if `first == last`.

   […]

   >     template<class ForwardIterator>
   >     ForwardIterator max_element(ForwardIterator first, ForwardIterator last);
   >     template<class ForwardIterator, class Compare>
   >     ForwardIterator max_element(ForwardIterator first, ForwardIterator last,
   >                                 Compare comp);

   > > -23- Returns: The first iterator `i` in the range `[first,last)` such that for any every iterator `j` in the range `[first,last)` the following corresponding conditions hold: `!(*i < *j)` or `comp(*i, *j) == false`. Returns `last` if `first == last`.

### 2159. `atomic_flag` initialization

Section: 29.7 [atomics.flag] Status: C++14 Submitter: Alberto Ganesh Barbati Opened: 2012-05-24 Last modified: 2015-04-08

View all other issues in [atomics.flag].

View all issues with C++14 status.

Discussion:

29.7 [atomics.flag]/4 describes the `ATOMIC_FLAG_INIT`, but it's not quite clear about a couple of points:

1. it's said that `ATOMIC_FLAG_INIT` "can be used to initialize an object of type `atomic_flag`" and the following example:

   >     std::atomic_flag guard = ATOMIC_FLAG_INIT;

   is presented. It's not clear whether the macro can also be used in the other initialization contexts:

   >     std::atomic_flag guard ATOMIC_FLAG_INIT;
   >     std::atomic_flag guard {ATOMIC_FLAG_INIT};

   >     struct A { std::atomic_flag flag; A(); };
   >     A::A() : flag (ATOMIC_FLAG_INIT);
   >     A::A() : flag {ATOMIC_FLAG_INIT};

   Please also note that examples are non-normative, according to the ISO directives, meaning that the wording presents no normative way to use the macro.

2. it's said that "It is unspecified whether an uninitialized `atomic_flag` object has an initial state of set or clear.". I believe the use of "uninitialized" is inappropriate. First of all, if an object is uninitialized it is obvious that we cannot assert anything about its state. Secondly, it doesn't address the following cases:

   >     std::atomic_flag a; // object is "initialized" by trivial default constructor
   >     std::atomic_flag a {}; // object is value-initialized
   >     static std::atomic_flag a; // object is zero-initialized

   strictly speaking a trivial constructor "initializes" the object, although it doesn't actually initialize the sub-objects.

3. it's said that "For a static-duration object, that initialization shall be static.". Considering the following example:

   >     struct A
   >     {
   >       A(); // user-provided, not constexpr

   >       std::atomic_flag flag = ATOMIC_FLAG_INIT;
   >       // possibly other non-static data members
   >     };

   >     static A a;

   The object `a.flag` (as a sub-object of the object `a`) has static-duration, yet the initialization has to be dynamic because `A::A` is not `constexpr`.

[2012, Portland]

We would like to be able to allow more initialisation contexts for example:

1. C struct
2. C++ constructor initializer-list

However we need further input from experts with implementation specific knowledge to identify which additional contexts (if any) would be universally valid.

Moved to open

[2013, Chicago]

Move to Immediate, following review.

Some discussion over the explicit use of only copy initialization, and not direct initialization. This is necessary to allow the implementation of `atomic_flag` as an aggregate, and may be further reviewed in the future.

Accept for Working Paper

Proposed resolution:

[This wording is relative to N3376.]

Change 29.7 [atomics.flag]/4 as follows:

> The macro `ATOMIC_FLAG_INIT` shall be defined in such a way that it can be used to initialize an object of type `atomic_flag` to the clear state. The macro can be used in the form:

> >     atomic_flag guard = ATOMIC_FLAG_INIT;

> It is unspecified whether the macro can be used in other initialization contexts. For a complete static-duration object, that initialization shall be static. It is unspecified whether an uninitialized Unless initialized with `ATOMIC_FLAG_INIT`, it is unspecified whether an `atomic_flag` object has an initial state of set or clear. [ Example:

> >     atomic_flag guard = ATOMIC_FLAG_INIT;

> — end example ]

### 2160. Unintended destruction ordering-specification of `resize`

Section: 23.3.6.3 [vector.capacity] Status: WP Submitter: Daniel Krügler Opened: 2012-06-07 Last modified: 2015-05-22

View other active issues in [vector.capacity].

View all other issues in [vector.capacity].

View all issues with WP status.

Discussion:

As part of resolving LWG issue 2033 a wording change was done for `resize()` to respect the problem mentioned in the question:

> Does a call to 'void resize(size_type sz)' of `std::vector` require the element type to be `MoveAssignable` because the call `erase(begin() + sz, end())` mentioned in the Effects paragraph would require the element type to be `MoveAssignable`?

The wording change was to replace in 23.3.3.3 [deque.capacity] and 23.3.6.3 [vector.capacity]:

> -1- Effects: If `sz <= size()`, equivalent to `erase(begin() + sz, end())`; […]

by:

> -1- Effects: If `sz <= size()`, equivalent to calling `pop_back() size() - sz` times. […]

The overlooked side-effect of this wording change is that this implies a destruction order of the removed elements to be in reverse order of construction, but the previous version did not impose any specific destruction order due to the way how the semantics of `erase` is specified in Table 100.

Given the program:

>     #include <vector>
>     #include <iostream>

>     struct Probe {
>       int value;
>       Probe() : value(0) {}
>       Probe(int value) : value(value) {}
>       ~Probe() { std::cout << "~Probe() of " << value << std::endl; }
>     };

>     int main() {
>       std::vector<Probe> v;
>       v.push_back(Probe(1));
>       v.push_back(Probe(2));
>       v.push_back(Probe(3));
>       std::cout << "---" << std::endl;
>       v.resize(0);
>     }

the last three lines of the output for every compiler I tested was:

>     ~Probe() of 1
>     ~Probe() of 2
>     ~Probe() of 3

but a conforming implementation would now need to change this order to

>     ~Probe() of 3
>     ~Probe() of 2
>     ~Probe() of 1

This possible stringent interpretation makes sense, because one can argue that sequence containers (or at least `std::vector`) should have the same required destruction order of it's elements, as elements of a C array or controlled by memory deallocated with an array `delete` have. I also learned that libc++ does indeed implement `std::vector::resize` in a way that the second output form is observed.

While I agree that required reverse-destruction would better mimic the natural behaviour of `std::vector` this was not required in C++03 and this request may be too strong. My current suggestion would be to restore the effects of the previous wording in regard to the destruction order, because otherwise several currently existing implementations would be broken just because of this additional requirement.

[2013-03-15 Issues Teleconference]

Moved to Open.

Jonathan says that he believes this is a valid issue.

Walter wonders if this was intended when we made the previous change - if so, this would be NAD.

Jonathan said that Issue 2033 doesn't mention ordering.

Walter then asked if anyone is really unhappy that we're destroying items in reverse order of construction.

Jonathan points out that this conflicts with existing practice (libstc++, but not libc++).

Jonathan asked for clarification as to whether this change was intended by 2033.

[2014-06 Rapperswil]

Daniel points out that the ordering change was not intended.

General agreement that implementations should not be required to change.

[2014-06-28 Daniel provides alternative wording]

[Urbana 2014-11-07: Move to Ready]

Proposed resolution:

This wording is relative to N3936.

1. Change 23.3.3.3 [deque.capacity] as indicated: [Drafting note: The chosen wording form is similar to that for `forward_list`. Note that the existing Requires element already specifies the necessary operational requirements on the value type. — end drafting note]

   >     void resize(size_type sz);

   > > -1- Effects: If `sz <= size()`, erases the last `size() - sz` elements from the sequence equivalent to calling `pop_back() size() - sz` times. Otherwise If `size() <= sz`, appends `sz - size()` default-inserted elements to the sequence.

   > > […]

   >     void resize(size_type sz, const T& c);

   > > -3- Effects: If `sz <= size()`, erases the last `size() - sz` elements from the sequence equivalent to calling `pop_back() size() - sz` times. Otherwise If `size() < sz`, appends `sz - size()` copies of `c` to the sequence.

   > > […]

2. Change 23.3.6.3 [vector.capacity] as indicated: [Drafting note: See `deque` for the rationale of the used wording. — end drafting note]

   >     void resize(size_type sz);

   > > -12- Effects: If `sz <= size()`, erases the last `size() - sz` elements from the sequence equivalent to calling `pop_back() size() - sz` times. Otherwise If `size() < sz`, appends `sz - size()` default-inserted elements to the sequence.

   > > […]

   >     void resize(size_type sz, const T& c);

   > > -15- Effects: If `sz <= size()`, erases the last `size() - sz` elements from the sequence equivalent to calling `pop_back() size() - sz` times. Otherwise If `size() < sz`, appends `sz - size()` copies of `c` to the sequence.

   > > […]

### 2162. `allocator_traits::max_size` missing `noexcept`

Section: 17.6.3.5 [allocator.requirements], 20.7.8.2 [allocator.traits.members], 20.7.8 [allocator.traits] Status: C++14 Submitter: Bo Persson Opened: 2012-07-03 Last modified: 2015-04-08

View other active issues in [allocator.requirements].

View all other issues in [allocator.requirements].

View all issues with C++14 status.

Discussion:

N3376 describes in 20.7.8.2 [allocator.traits.members]/7

>     static size_type max_size(Alloc& a);

> Returns: `a.max_size()` if that expression is well-formed; otherwise, `numeric_limits<size_type>::max()`.

The `max_size` function is supposed to call one of two functions that are both `noexcept`. To make this intermediate function useful for containers, it should preserve the `noexcept` attribute.

Proposed changes:

In 20.7.8 [allocator.traits] and 20.7.8.2 [allocator.traits.members]/7, change the function signature to

>     static size_type max_size(Alloc& a) noexcept;

[2012-08-05 Daniel comments]

On the first sight this does not seem like a defect of the specification, because the Allocator requirements in 17.6.3.5 [allocator.requirements] (Table 28) do not impose a no-throw requirement onto `max_size()`; the table just describes the fall-back implementation for `max_size()` if a given allocator does not provide such a function.

`std::allocator` as a special model of this concept and is allowed to increase the exception-guarantees for `max_size()`, but this does not imply a corresponding rules for other allocators.

Furthermore, `max_size()` of Containers is not specified in terms of `Allocator::max_size()`, so again this is not a real contradiction.

Nonetheless I think that the following stronger decision should be considered:

1. Require that for all Allocators (as specified in 17.6.3.5 [allocator.requirements]) `max_size()` never throws an exception. This would it make much more useful to call this function in situations where no exception should leave the context.

2. Require that for all Allocators (as specified in 17.6.3.5 [allocator.requirements]) `max_size()` can be called on const allocator object. Together with the previous item this would allow an implementation of a container's `max_size()` function to delegate to the allocator's `max_size()` function.

In regard to the second statement it should be mentioned that there are two current specification deviations from that in the draft:

1. The synopsis of 20.7.8 [allocator.traits] uses a const allocator argument as part of the signature of the `max_size` function.

2. Both the synopsis of 20.13.1 [allocator.adaptor.syn] and the member specification in 20.13.4 [allocator.adaptor.members] p8 declare `scoped_allocator_adaptor::max_size` as const member function, but this function delegates to

   >     allocator_traits<OuterAlloc>::max_size(outer_allocator())

   where `outer_allocator()` resolves to the member function overload returning a `const outer_allocator_type&`.

The question arises whether these current defects actually point to a defect in the Allocator requirements and should be fixed there.

[ 2012-10 Portland: Move to Review ]

Consensus that the change seems reasonable, and that for any given type the template is intantiated with the contract should be 'wide' so this meets the guidelines we agreed in Madrid for C++11.

Some mild concern that while we don't imagine many allocator implementations throwing on this method, it is technically permited by current code that we would not be breaking, by turning `throw` expressions into disguised `terminate` calls. In this case, an example might be an instrumented 'logging' allocator that writes every function call to a log file or database, and might throw if that connection/file were no longer available.

Another option would be to make exception spefication a conditional no-except, much like we do for some `swap` functions and assignment operators. However, this goes against the intent of the Madrid adoption of `noexcept` which is that vendors are free to add such extensions, but we look for a clear line in the library specification, and do not want to introduce conditional-noexcept piecemeal. A change in our conventions here would require a paper addressing the library specification as a whole.

Consensus was to move forward, but move the issue only to Review rather than Ready to allow time for further comments. This issue should be considered 'Ready' next time it is reviewed unless we get such comments in the meantime.

[2013-04-18, Bristol]

Proposed resolution:

In 20.7.8 [allocator.traits] and 20.7.8.2 [allocator.traits.members]/7, change the function signature to

>     static size_type max_size(Alloc& a) noexcept;

### 2163. `nth_element` requires inconsistent post-conditions

Section: 25.4.2 [alg.nth.element] Status: C++14 Submitter: Peter Sommerlad Opened: 2012-07-06 Last modified: 2015-04-08

View all other issues in [alg.nth.element].

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Discussion:

The specification of nth_element refers to `operator<` whereas all sorting without a compare function is based on `operator<`. While it is true that for all regular types both operators should be defined accordingly, all other sorting algorithms only rely on existence of `operator<`. So I guess the paragraph p1

> After `nth_element` the element in the position pointed to by `nth` is the element that would be in that position if the whole range were sorted. Also for any iterator `i` in the range `[first,nth)` and any iterator `j` in the range `[nth,last)` it holds that: `!(*i > *j)` or `comp(*j, *i) == false`.

should read

> After `nth_element` the element in the position pointed to by `nth` is the element that would be in that position if the whole range were sorted. Also for any iterator `i` in the range `[first,nth)` and any iterator `j` in the range `[nth,last)` it holds that: `!(*j < *i)` or `comp(*j, *i) == false`.

Note only "`!(*i > *j)`" was changed to "`!(*j < *i)`" and it would be more symmetric with `comp(*j, *i)` as well.

In theory this might be a semantic change to the spec, but I believe the mistake is unintended.

[ 2012-10 Portland: Move to Ready ]

This is clearly correct by inspection, moved to Ready by unanimous consent.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3376.

1. Change 25.4.2 [alg.nth.element] p1 as indicated:

   >     template<class RandomAccessIterator>
   >     void nth_element(RandomAccessIterator first, RandomAccessIterator nth,
   >                      RandomAccessIterator last);
   >     template<class RandomAccessIterator, class Compare>
   >     void nth_element(RandomAccessIterator first, RandomAccessIterator nth,
   >                      RandomAccessIterator last, Compare comp);

   > > -1- After `nth_element` the element in the position pointed to by `nth` is the element that would be in that position if the whole range were sorted. Also for any iterator `i` in the range `[first,nth)` and any iterator `j` in the range `[nth,last)` it holds that: `!(*i > *j)!(*j < *i)` or `comp(*j, *i) == false`.

### 2168. Inconsistent specification of `uniform_real_distribution` constructor

Section: 26.5.8.2.2 [rand.dist.uni.real] Status: WP Submitter: Marshall Clow Opened: 2012-07-14 Last modified: 2015-05-22

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Discussion:

uniform_real says in 26.5.8.2.2 [rand.dist.uni.real] p1:

> A `uniform_real_distribution` random number distribution produces random numbers `x`, `a ≤ x < b`,

but also that (26.5.8.2.2 [rand.dist.uni.real] p2):

>     explicit uniform_real_distribution(RealType a = 0.0, RealType b = 1.0);

> > -2- Requires: `a ≤ b` and `b - a ≤ numeric_limits<RealType>::max()`.

If you construct a `uniform_real_distribution<RealType>(a, b)` where there are no representable numbers between 'a' and 'b' (using `RealType`'s representation) then you cannot satisfy 26.5.8.2.2 [rand.dist.uni.real].

An obvious example is when `a == b`.

[2014-11-04 Urbana]

Jonathan provides wording.

[2014-11-08 Urbana]

Moved to Ready with the note.

There remains concern that the constructors are permitting values that may (or may not) be strictly outside the domain of the function, but that is a concern that affects the design of the random number facility as a whole, and should be addressed by a paper reviewing and addressing the whole clause, not picked up in the issues list one distribution at a time. It is still not clear that such a paper would be uncontroversial.

Proposed resolution:

This wording is relative to N4140.

1. Add a note after paragraph 1 before the synopsis in 26.5.8.2.2 [rand.dist.uni.real]:

   > -1- A `uniform_real_distribution` random number distribution produces random numbers x , a≤x<b , distributed according to the constant probability density function

   > > p(x|a,b) = 1∕(b-a) .

   > [Note: This implies that p(x|a,b) is undefined when `a == b`. — end note]

   > Drafting note: p(x|a,b) should be in math font, and `a == b` should be in code font.

### 2169. Missing `reset()` requirements in `unique_ptr` specialization

Section: 20.8.1.3.4 [unique.ptr.runtime.modifiers] Status: C++14 Submitter: Geoffrey Romer Opened: 2012-07-16 Last modified: 2015-04-08

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Discussion:

In 20.8.1.3.4 [unique.ptr.runtime.modifiers]/p1-2 of N3376, the description of `reset()` in the array specialization of `unique_ptr` partially duplicates the description of the base template method (as specified in 20.8.1.2.5 [unique.ptr.single.modifiers]/p3-5), but lacks some significant requirements. Specifically, the text introduced in LWG 998, and item 13 of LWG 762, is present only in the base template, not the specialization.

This gives the appearance that these requirements specifically do not apply to the specialization, which I don't believe is correct or intended: the issue of `reset()` operation order addressed by LWG 998 applies just as much to the derived template as to the base template, and the derived template has just as much need to rely on `get_deleter()(get())` being well-defined, well-formed, and not throwing exceptions (arguably some of those properties follow from the fact that `T` is required to be a complete type, but not all).

Assuming the derived template's `reset()` semantics are intended to be identical to the base template's, there is no need to explicitly specify the semantics of `reset(pointer p)` at all (since 20.8.1.3 [unique.ptr.runtime]/3 specifies "Descriptions are provided below only for member functions that have behavior different from the primary template."), and `reset(nullptr_t p)` can be specified by reference to the 'pointer' overload. This is more concise, and eliminates any ambiguity about intentional vs. accidental discrepancies.

[2012-10 Portland: Move to Ready]

This resolution looks blatantly wrong, as it seems to do nothing but defer to primary template where we should describe the contract here.

Ongoing discussion points out that the primary template has a far more carefully worded semantic for `reset(p)` that we would want to copy here.

STL points out that we need the `nullptr` overload for this dynamic-array form, as there is a deleted member function template that exists to steal overloads of pointer-to-derived, avoiding undifined behavior, so we need the extra overload.

Finally notice that there is blanket wording further up the clause saying we describe only changes from the primary template, so the proposed wording is in fact exactly correct. Move to Ready.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3376.

Change 20.8.1.3.4 [unique.ptr.runtime.modifiers] as indicated:

>     void reset(pointer p = pointer()) noexcept;
>     void reset(nullptr_t p) noexcept;

> > -1- Effects: If `get() == nullptr` there are no effects. Otherwise `get_deleter()(get())` Equivalent to `reset(pointer())`.

> > -2- Postcondition: `get() == p`.

### 2170. Aggregates cannot be `DefaultConstructible`

Section: 17.6.3.1 [utility.arg.requirements] Status: WP Submitter: Daniel Krügler Opened: 2012-07-19 Last modified: 2015-04-08

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Discussion:

The lack of the definition of the `DefaultConstructible` requirements in C++03 was fixed by LWG 724 at a time where the core rules of list-initialization were slightly different than today, at that time value-initialization (shortly) was the primary rule for class types, i.e. just before applying CWG 1301, CWG 1324, and CWG 1368.

The order in 8.5.4 [dcl.init.list] p3 was changed to respect aggregate initialization, but that had the side-effect that formally aggregate types cannot satisfy the `DefaultConstructible` requirements anymore, because we require that

>     T u{};

value-initializes the object `u`.

Of-course exclusion of aggregates was not intended, therefore I suggest to extend the requirements in Table 19 (17.6.3.1 [utility.arg.requirements]) for empty aggregate-initialization as well.

[ 2012-10 Portland: Move to Core ]

We are not qualified to pick apart the Core rules quickly at this point, but the consensus is that if the core language has changed in this manner, then the fix should similarly be applied in Core - this is not something that we want users of the language to have to say every time they want to Value initialize (or aggregate initialize) an object.

More to Open until we get a clear response from Core, Alisdair to file an issue with Mike.

[2013-04 Bristol: Back to Library]

The Core Working group opened, discussed, and resolved CWG 1578 as NAD for this library-related problem: Empty aggregate initialization and value-initialization are different core language concepts, and this difference can be observed (e.g. for a type with a deleted default-constructor).

[2014-02-15 Issaquah: Move to Ready]

AM: core says still LWG issue, wording has been non-controversial, move to ready?

NJ: what about durations? think they are ok

Ville: `pair` and a few other have value initialize

AM: look at core 1578

AM: value initialize would require `()`, remove braces from third row?

STL: no

PH: core has new issue on aggregates and non-aggregates.

AM: right, they said does not affect this issue

NJ: why ok with `pair` and `tuple`?

STL: will use `()`, `tuple` of aggregates with deleted constructor is ill-formed

Ville: aggregate with reference can't have `()`

STL: `{}` would be an issue too

Ville: aggregate with reference will have `()` deleted implicitly

Move to Ready.

Proposed resolution:

This wording is relative to N3691.

Change Table 19 in 17.6.3.1 [utility.arg.requirements] as indicated:

Table 19 — `DefaultConstructible` requirements [defaultconstructible]
Expression | Post-condition
--- | ---
`T t;` | object `t` is default-initialized
`T u{};` | object `u` is value-initialized or aggregate-initialized
`T()` `T{}` | a temporary object of type `T` is value-initialized or aggregate-initialized

### 2172. Does `atomic_compare_exchange_*` accept `v == nullptr` arguments?

Section: 20.8.2.6 [util.smartptr.shared.atomic] Status: C++14 Submitter: Howard Hinnant Opened: 2012-07-28 Last modified: 2015-04-08

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Discussion:

Looking at 20.8.2.6 [util.smartptr.shared.atomic]/p31

>     template<class T>
>       bool
>       atomic_compare_exchange_strong_explicit(shared_ptr<T>* p,
>                                               shared_ptr<T>* v,
>                                               shared_ptr<T> w,
>                                               memory_order success, memory_order failure);

> > -31- Requires: `p` shall not be null.

What about `v`? Can it be null? And if so, what happens?

This looks closely related to C++11 issue LWG 1030, where we gave every signature in this section a:

> Requires: `p` shall not be null.

It looks like a simple oversight to me that we did not add for the `atomic_compare_exchange_*`:

> Requires: `p` shall not be null and `v` shall not be null.

[2012-10 Portland: Move to Ready]

This is clearly the right thing to do, and Lawrence concurs.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3376.

Change 20.8.2.6 [util.smartptr.shared.atomic] as indicated:

>     template<class T>
>       bool atomic_compare_exchange_weak(
>         shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w);

> > -27- Requires: `p` shall not be null and `v` shall not be null.

> > […]

>     template<class T>
>       bool atomic_compare_exchange_weak_explicit(
>         shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w,
>         memory_order success, memory_order failure);
>     template<class T>
>       bool atomic_compare_exchange_strong_explicit(
>         shared_ptr<T>* p, shared_ptr<T>* v, shared_ptr<T> w,
>         memory_order success, memory_order failure);

> > -31- Requires: `p` shall not be null and `v` shall not be null.

> > […]

### 2174. `wstring_convert::converted()` should be `noexcept`

Section: 22.3.3.2.2 [conversions.string] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-08-02 Last modified: 2015-04-08

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Discussion:

There is no reason `wstring_convert::converted()` shouldn't be `noexcept`.

It might be possible for `wstring_convert::state()` and `wbuffer_convert::state()` to be `noexcept` too, depending on the requirements on `mbstate_t`.

[2013-03-15 Issues Teleconference]

Moved to Tentatively Ready.

Defer the separate discsussion of `state()` to another issue, if anyone is ever motivated to file one.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3376.

1. Edit in the class template `wstring_convert` synopsis 22.3.3.2.2 [conversions.string] p2:

   >     size_t converted() const noexcept;

2. Edit the signature before 22.3.3.2.2 [conversions.string] p6:

   >     size_t converted() const noexcept;

### 2175. `wstring_convert` and `wbuffer_convert` validity

Section: 22.3.3.2.2 [conversions.string], 22.3.3.2.3 [conversions.buffer] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-08-02 Last modified: 2015-04-08

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Discussion:

See discussion following c++std-lib-32710.

It's not specified what happens if `wstring_convert` and `wbuffer_convert` objects are constructed with null `Codecvt` pointers.

Should the constructors have preconditions that the pointers are not null? If not, are conversions expected to fail, or is it undefined to attempt conversions if the pointers are null?

There are no observer functions to check whether objects were constructed with valid `Codecvt` pointers. If the types are made movable such observers would be necessary even if the constructors require non-null pointers (see also LWG 2176).

[2013-03-15 Issues Teleconference]

Moved to Tentatively Ready.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3376.

1. Insert a new paragraph before 22.3.3.2.2 [conversions.string] paragraph 16:

   >     wstring_convert(Codecvt *pcvt = new Codecvt);
   >     wstring_convert(Codecvt *pcvt, state_type state);
   >     wstring_convert(const byte_string& byte_err,
   >       const wide_string& wide_err = wide_string());

   > > -?- Requires: For the first and second constructors `pcvt != nullptr`.

   > > -16- Effects: The first constructor shall store `pcvt` in `cvtptr` and default values in `cvtstate`, `byte_err_string`, and `wide_err_string`. The second constructor shall store `pcvt` in `cvtptr`, `state` in `cvtstate`, and default values in `byte_err_string` and `wide_err_string`; moreover the stored state shall be retained between calls to `from_bytes` and `to_bytes`. The third constructor shall store `new Codecvt` in `cvtptr`, `state_type()` in cvtstate, `byte_err` in `byte_err_string`, and `wide_err` in `wide_err_string`.

2. Insert a new paragraph before 22.3.3.2.3 [conversions.buffer] paragraph 10:

   >     wbuffer_convert(std::streambuf *bytebuf = 0,
   >       Codecvt *pcvt = new Codecvt, state_type state = state_type());

   > > -?- Requires: `pcvt != nullptr`.

   > > -10- Effects: The constructor constructs a stream buffer object, initializes `bufptr` to `bytebuf`, initializes `cvtptr` to `pcvt`, and initializes `cvtstate` to `state`.

### 2176. Special members for `wstring_convert` and `wbuffer_convert`

Section: 22.3.3.2.2 [conversions.string], 22.3.3.2.3 [conversions.buffer] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-08-02 Last modified: 2015-04-08

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Discussion:

See discussion following c++std-lib-32699.

The constructors for `wstring_convert` and `wbuffer_convert` should be explicit, to avoid implicit conversions which take ownership of a `Codecvt` pointer and delete it unexpectedly.

Secondly, 22.3.3.2.3 [conversions.buffer] p11 describes a destructor which is not declared in the class synopsis in p2.

Finally, and most importantly, the definitions in 22.3.3.2.2 [conversions.string] and 22.3.3.2.3 [conversions.buffer] imply implicitly-defined copy constructors and assignment operators, which would do shallow copies of the owned `Codecvt` objects and result in undefined behaviour in the destructors.

`Codecvt` is not required to be `CopyConstructible`, so deep copies are not possible. The `wstring_convert` and `wstring_buffer` types could be made move-only, but the proposed resolution below doesn't do so because of the lack of preconditions regarding null `Codecvt` pointers and the absence of observer functions that would allow users to check preconditions (see also LWG 2175).

[2013-03-15 Issues Teleconference]

Moved to Review.

Jonathan pointed out that you can have an implicit constructor that takes ownership of a heap reference, which would result an unexpected deletion.

No-one really likes the 'naked new' in the interface here, either.

[2013-04-18, Bristol]

Proposed resolution:

This wording is relative to N3376.

1. Edit the class template `wstring_convert` synopsis in 22.3.3.2.2 [conversions.string] p2:

   >     explicit wstring_convert(Codecvt *pcvt = new Codecvt);
   >     wstring_convert(Codecvt *pcvt, state_type state);
   >     explicit wstring_convert(const byte_string& byte_err,
   >                              const wide_string& wide_err = wide_string());
   >     ~wstring_convert();

   >     wstring_convert(const wstring_convert&) = delete;
   >     wstring_convert& operator=(const wstring_convert&) = delete;

2. Edit the signatures before 22.3.3.2.2 [conversions.string] p16:

   >     explicit wstring_convert(Codecvt *pcvt = new Codecvt);
   >     wstring_convert(Codecvt *pcvt, state_type state);
   >     explicit wstring_convert(const byte_string& byte_err,
   >         const wide_string& wide_err = wide_string());

3. Edit the class template `wbuffer_convert` synopsis in 22.3.3.2.3 [conversions.buffer] p2:

   >     explicit wbuffer_convert(std::streambuf *bytebuf = 0,
   >                              Codecvt *pcvt = new Codecvt,
   >                              state_type state = state_type());

   >     ~wbuffer_convert();

   >     wbuffer_convert(const wbuffer_convert&) = delete;
   >     wbuffer_convert& operator=(const wbuffer_convert&) = delete;

4. Edit the signature before 22.3.3.2.3 [conversions.buffer] p10:

   >     explicit wbuffer_convert(std::streambuf *bytebuf = 0,
   >         Codecvt *pcvt = new Codecvt, state_type state = state_type());

### 2177. Requirements on `Copy/MoveInsertable`

Section: 23.2.1 [container.requirements.general] Status: C++14 Submitter: Loïc Joly Opened: 2012-08-10 Last modified: 2015-04-08

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Discussion:

See also discussion following c++std-lib-32883 and c++std-lib-32897.

The requirements on `CopyInsertable` and `MoveInsertable` are either incomplete, or complete but hard to figure out.

From e-mail c++std-lib-32897:

Pablo Halpern:

I agree that we need semantic requirements for all of the `*Insertable` concepts analogous to the requirements we have on similar concepts.

Howard Hinnant:

I've come to believe that the standard is actually correct as written in this area. But it is really hard to read. I would have no objection whatsoever to clarifications to `CopyInsertable` as you suggest (such as the post-conditions on `v`). And I do agree with you that the correct approach to the clarifications is to confirm that `CopyInsertable` implies `MoveInsertable`.

[2012, Portland: Move to Tentatively Ready]

Move to Tentatively Ready by unanimous consent.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3376.

1. Edit 23.2.1 [container.requirements.general] p13 as indicated:

   > -13- […] Given a container type `X` having an `allocator_type` identical to `A` and a `value_type` identical to `T` and given an lvalue `m` of type `A`, a pointer `p` of type `T*`, an expression `v` of type (possibly `const`) `T`, and an rvalue `rv` of type `T`, the following terms are defined. If `X` is not allocator-aware, the terms below are defined as if `A` were `std::allocator<T>` — no allocator object needs to be created and user specializations of `std::allocator<T>` are not instantiated:

   > - `T` is `DefaultInsertable` into `X` means that the following expression is well-formed:

   >   >     allocator_traits<A>::construct(m, p);

   > - An element of `X` is default-inserted if it is initialized by evaluation of the expression

   >   >     allocator_traits<A>::construct(m, p);

   >   where `p` is the address of the uninitialized storage for the element allocated within `X`.

   > - `T` is `CopyMoveInsertable` into `X` means that the following expression is well-formed:

   >   >     allocator_traits<A>::construct(m, p, rv);

   >   and when evaluated the following postconditions hold: The value of `*p` is equivalent to the value of `rv` before the evaluation. [Note: `rv` remains a valid object. Its state is unspecified — end note]

   > - `T` is `MoveCopyInsertable` into `X` means that, in addition to satisfying the `MoveInsertable` requirements, the following expression is well-formed:

   >   >     allocator_traits<A>::construct(m, p, rv);

   >   and when evaluated the following postconditions hold: The value of `v` is unchanged and is equivalent to `*p`.

   > - `T` is `EmplaceConstructible` into `X` from `args`, for zero or more arguments `args`, means that the following expression is well-formed:

   >   >     allocator_traits<A>::construct(m, p, args);

   > - `T` is `Erasable` from `X` means that the following expression is well-formed:

   >   >     allocator_traits<A>::destroy(m, p);

   > [Note: A container calls `allocator_traits<A>::construct(m, p, args)` to construct an element at `p` using `args`. The default construct in `std::allocator` will call `::new((void*)p) T(args)`, but specialized allocators may choose a different definition. — end note]

### 2180. Exceptions from `std::seed_seq` operations

Section: 26.5.7.1 [rand.util.seedseq] Status: C++14 Submitter: Daniel Krügler Opened: 2012-08-18 Last modified: 2015-04-08

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Discussion:

26.5.7.1 [rand.util.seedseq] p1 says upfront:

> No function described in this section 26.5.7.1 [rand.util.seedseq] throws an exception.

This constraint seems non-implementable to me when looking especially at the members

>     template<class T>
>     seed_seq(initializer_list<T> il);

>     template<class InputIterator>
>     seed_seq(InputIterator begin, InputIterator end);

which have the effect of invoking `v.push_back()` for the exposition-only member of type `std::vector` (or its equivalent) over all elements of the provided range, so out-of-memory exceptions are always possible and the `seed_seq` object doesn't seem to be constructible this way.

In addition to the potential lack-of-resources problem, the operations of `InputIterator` might also throw exceptions.

Aside to that it should me mentioned, that a default constructor of `vector<uint_least32_t>` in theory can also throw exceptions, even though this seems less of a problem to me in this context, because such an implementation could easily use a different internal container in `seed_seq` that can hold this no-throw exception guarantee.

Secondly, a slightly different problem category related to exceptions occurs for the member templates

>     template<class RandomAccessIterator>
>     void generate(RandomAccessIterator begin, RandomAccessIterator end);

>     template<class OutputIterator>
>     void param(OutputIterator dest) const;

where the actual operations performed by the implementation would never need to throw, but since they invoke operations of a user-provided customization point, the overall operation, like for example

>     copy(v.begin(), v.end(), dest);

could also throw exceptions. In this particular example we can just think of a `std::back_insert_iterator` applied to a container that needs to allocate its elements used as the type for `OutputIterator`.

Even though Clause 26 [numerics] has mostly stronger exception constraints than other parts of the library the here discussed are overrestrictive, especially since no operation of `std::seed_seq` except the template `generate` is actually needed within the library implementation, as mentioned in the discussion of LWG 2124.

I suggest to remove the general no-exception constraints for operations of `std::seed_seq` except for member `size()` and the default constructor and to provide specific wording for `generate()` and `param()` to ensure that the algorithm itself is a nothrow operation, which is especially for `generate()` important, because the templates specified in 26.5.3 [rand.eng] and 26.5.4 [rand.adapt] also depend on this property indirectly, which is further discussed in LWG 2181.

Howard:

I suggest to use a different form for the exception specification, something similar to 20.9.10.3 [func.bind.bind] p4:

> Throws: Nothing unless an operation on `RandomAccessIterator` throws an exception.

Daniel:

The currently suggested "what and when" form seems a bit more specific and harmonizes with the form used for function template `generate_canonical` from 26.5.7.2 [rand.util.canonical].

[2013-04-20, Bristol]

Open an editorial issue on the exception wording ("Throws: What and when").

Solution: move to tentatively ready.

[2013-09-29, Chicago]

Apply to Working Paper

Proposed resolution:

This wording is relative to N3376.

1. Edit 26.5.7.1 [rand.util.seedseq] p1 as indicated:

   > -1- No function described in this section 26.5.7.1 [rand.util.seedseq] throws an exception.

2. Edit 26.5.7.1 [rand.util.seedseq] around p2 as indicated:

   >     seed_seq();

   > > -2- Effects: Constructs a `seed_seq` object as if by default-constructing its member `v`.

   > > -?- Throws: Nothing.

3. Edit 26.5.7.1 [rand.util.seedseq] around p7 as indicated:

   >     template<class RandomAccessIterator>
   >       void generate(RandomAccessIterator begin, RandomAccessIterator end);

   > > -7- Requires: `RandomAccessIterator` shall meet the requirements of a mutable random access iterator (Table 111) type. Moreover, `iterator_traits<class RandomAccessIterator>::value_type` shall denote an unsigned integer type capable of accommodating 32-bit quantities.

   > > -8- Effects: Does nothing if `begin == end`. Otherwise, with `s = v.size()` and `n = end - begin`, fills the supplied range `[begin, end)` according to the following algorithm […]

   > > -?- Throws: What and when `RandomAccessIterator` operations of `begin` and `end` throw.

4. Edit 26.5.7.1 [rand.util.seedseq] around p9 as indicated:

   >     size_t size() const;

   > > -9- Returns: The number of 32-bit units that would be returned by a call to `param()`.

   > > -??- Throws: Nothing.

   > > -10- Complexity: constant time.

5. Edit 26.5.7.1 [rand.util.seedseq] around p11 as indicated:

   >     template<class OutputIterator>
   >       void param(OutputIterator dest) const;

   > > -11- Requires: `OutputIterator` shall satisfy the requirements of an output iterator (Table 108) type. Moreover, the expression `*dest = rt` shall be valid for a value `rt` of type `result_type`.

   > > -12- Effects: Copies the sequence of prepared 32-bit units to the given destination, as if by executing the following statement:

   > > >     copy(v.begin(), v.end(), dest);

   > > -??- Throws: What and when `OutputIterator` operations of `dest` throw.

### 2182. `Container::[const_]reference` types are misleadingly specified

Section: 23.2.1 [container.requirements.general] Status: C++14 Submitter: Daniel Krügler Opened: 2012-08-20 Last modified: 2015-04-08

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Discussion:

According to Table 96 (Container requirements) the return type of `X::reference` and `X::const_reference` is "lvalue of `T`" and "`const` lvalue of `T`", respectively. This does not make much sense, because an lvalue is an expression category, not a type. It could also refer to an expression that has a type, but this doesn't make sense either in this context, because obviously `X::[const_]reference` are intended to refer to types.

Given the fact that `vector<bool>` has no real reference type for `X::[const_]reference` and this definition presumably is intended to cover such situations as well, one might think that the wording is just a sloppy form of "type that represents a [const] lvalue of `T`". But this is also problematic, because basically all proxy reference expressions are rvalues.

It is unclear what the intention is. A straightward way of fixing this wording could make `X::[const_]reference` identical to `[const] T&`. This holds for all Library containers except for `vector<bool>`.

Another way of solving this definition problem would be to impose a requirement that holds for both references and reference-like proxies. Both `X::reference` and `X::const_reference` would need to be convertible to `const T&`. Additionally `X::reference` would need to support for a mutable container an assignment expression of the form `declval<X::reference>() = declval<T>()` (this presentation intentionally does not require `declval<X::reference&>() = declval<T>()`).

Further, the Table 96 does not impose any relations between `X::reference` and `X::const_reference`. It seems that at least `X::reference` needs to be convertible to `X::const_reference`.

A related question is whether `X::reference` is supposed to be a mutable reference-like type, irrespective of whether the container is an immutable container or not. The way, type `match_results` defines `reference` identical to `const_reference` indicates one specific interpretation (similarly, the `initializer_list` template also defines member type `reference` equal to `const value_type&`). Note that this can be a different decision as that for `iterator` and `const_iterator`, e.g. for sets the type `X::reference` still is a mutable reference, even though `iterator` is described as constant iterator.

The proposed resolution is incomplete in regard to the last question.

[2013-03-15 Issues Teleconference]

Moved to Review.

Alisdair notes that this looks like wording in the right direction. Wonders about congruence of these typedefs and the similar ones for iterators.

[2013-09 Chicago]

Moved to Ready.

Consensus that the requirements should require real references, just like iterators, as containers are required to support at least ForwardIterators, which have the same restriction on references.

Matt will file a new issue for some additional concerns with regex `match_results`.

[2014-02-10, Daniel comments]

The new issue opened by Matt is LWG 2306.

[Issaquah 20014-10-11: Move to Immediate]

Issue should have been Ready in pre-meeting mailing.

Proposed resolution:

This wording is relative to N3376.

1. Change Table 96 — "Container requirements" as indicated:

   Table p6 — Container requirements
   Expression | Return type | Operational Semantics | Assertion/note pre-/post-condition | Complexity
   --- | --- | --- | --- | ---
   `X::reference` | lvalue of `T&` |  |  | compile time
   `X::const_reference` | const lvalue of `const` `T&` |  |  | compile time

### 2185. Missing throws clause for `future/shared_future::wait_for/wait_until`

Section: 30.6.6 [futures.unique_future], 30.6.7 [futures.shared_future] Status: C++14 Submitter: Vicente J. Botet Escriba Opened: 2012-09-20 Last modified: 2015-04-08

View all other issues in [futures.unique_future].

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Discussion:

The functions `future::wait_for`, `future::wait_until`, `shared_future::wait_for`, and `shared_future::wait_for` can throw any timeout-related exceptions. It would be better if the wording could be more explicit. This is in line with the changes proposed in LWG 2093's Throws element of `condition_variable::wait` with predicate.

[2012, Portland: move to Review]

The phrase timeout-related exception does not exist.

2093 was put in review, and there is some dependency here with this issue.

If you provide a user-defined clock that throws, we need to put back an exception to allow that to be done.

We will put this in review and say that this cannot go in before 2093.

[2013-04-20, Bristol]

Accepted for the working paper

Proposed resolution:

[This resolution should not be adopted before resolving 2093]

[This wording is relative to N3376.]

1. Change 30.6.6 [futures.unique_future] as indicated:

   >     template <class Rep, class Period>
   >     future_status wait_for(const chrono::duration<Rep, Period>& rel_time) const;

   > > -21- Effects: none if the shared state contains a deferred function (30.6.8 [futures.async]), otherwise blocks until the shared state is ready or until the relative timeout (30.2.4 [thread.req.timing]) specified by `rel_time` has expired.

   > > -22- Returns: […]

   > > -??- Throws: timeout-related exceptions (30.2.4 [thread.req.timing]).

   >     template <class Clock, class Duration>
   >     future_status wait_until(const chrono::time_point<Clock, Duration>& abs_time) const;

   > > -23- Effects: none if the shared state contains a deferred function (30.6.8 [futures.async]), otherwise blocks until the shared state is ready or until the absolute timeout (30.2.4 [thread.req.timing]) specified by `abs_time` has expired.

   > > -24- Returns: […]

   > > -??- Throws: timeout-related exceptions (30.2.4 [thread.req.timing]).

2. Change 30.6.7 [futures.shared_future] as indicated:

   >     template <class Rep, class Period>
   >     future_status wait_for(const chrono::duration<Rep, Period>& rel_time) const;

   > > -23- Effects: none if the shared state contains a deferred function (30.6.8 [futures.async]), otherwise blocks until the shared state is ready or until the relative timeout (30.2.4 [thread.req.timing]) specified by `rel_time` has expired.

   > > -24- Returns: […]

   > > -??- Throws: timeout-related exceptions (30.2.4 [thread.req.timing]).

   >     template <class Clock, class Duration>
   >     future_status wait_until(const chrono::time_point<Clock, Duration>& abs_time) const;

   > > -25- Effects: none if the shared state contains a deferred function (30.6.8 [futures.async]), otherwise blocks until the shared state is ready or until the absolute timeout (30.2.4 [thread.req.timing]) specified by `abs_time` has expired.

   > > -26- Returns: […]

   > > -??- Throws: timeout-related exceptions (30.2.4 [thread.req.timing]).

### 2186. Incomplete action on `async/launch::deferred`

Section: 30.6.8 [futures.async] Status: C++14 Submitter: Vicente J. Botet Escriba Opened: 2012-09-20 Last modified: 2015-04-08

View all other issues in [futures.async].

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Discussion:

The description of the effects of `async` when the launch policy is `launch::deferred` doesn't state what is done with the result of the deferred function invocation and the possible exceptions as it is done for the asynchronous function when the policy is `launch::async`.

[2012, Portland: move to Open]

Detlef: agree with the problem but not with the resolution. The wording should be applied to all launch policies rather than having to be separately specified for each one.

Hans: we should redraft to factor out the proposed text outside the two bullets. Needs to be carefully worded to be compatible with the resolution of 2120 (see above).

Moved to open

[Issaquah 20014-10-11: Move to Immediate after SG1 review]

Proposed resolution:

[This wording is relative to N3376.]

1. Change 30.6.8 [futures.async] p3 bullet 2 as indicated:

   >     template <class F, class... Args>
   >     future<typename result_of<typename decay<F>::type(typename decay<Args>::type...)>::type>
   >     async(F&& f, Args&&... args);
   >     template <class F, class... Args>
   >     future<typename result_of<typename decay<F>::type(typename decay<Args>::type...)>::type>
   >     async(launch policy, F&& f, Args&&... args);

   > > -2- Requires: […]

   > > -3- Effects:: The first function behaves the same as a call to the second function with a `policy` argument of `launch::async | launch::deferred` and the same arguments for `F` and `Args`. […] The further behavior of the second function depends on the `policy` argument as follows (if more than one of these conditions applies, the implementation may choose any of the corresponding policies):

   > > - if `policy & launch::async` is non-zero […]

   > > - if `policy & launch::deferred` is non-zero — Stores `DECAY_COPY(std::forward<F>(f))` and `DECAY_COPY(std::forward<Args>(args))...` in the shared state. These copies of `f` and `args` constitute a deferred function. Invocation of the deferred function evaluates `INVOKE(std::move(g), std::move(xyz))` where `g` is the stored value of `DECAY_COPY(std::forward<F>(f))` and `xyz` is the stored copy of `DECAY_COPY(std::forward<Args>(args))...`. Any return value is stored as the result in the shared state. Any exception propagated from the execution of the deferred function is stored as the exceptional result in the shared state. The shared state is not made ready until the function has completed. The first call to a non-timed waiting function (30.6.4 [futures.state]) on an asynchronous return object referring to this shared state shall invoke the deferred function in the thread that called the waiting function. Once evaluation of `INVOKE(std::move(g), std::move(xyz))` begins, the function is no longer considered deferred. [Note: If this policy is specified together with other policies, such as when using a policy value of `launch::async | launch::deferred`, implementations should defer invocation or the selection of the `policy` when no more concurrency can be effectively exploited. — end note]

### 2187. `vector<bool>` is missing `emplace` and `emplace_back` member functions

Section: 23.3.7 [vector.bool] Status: C++14 Submitter: Nevin Liber Opened: 2012-09-21 Last modified: 2015-04-08

View all other issues in [vector.bool].

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Discussion:

It should have them so that it more closely matches the `vector<T>` interface, as this helps when writing generic code.

[2012, Portland: Move to Tentatively Ready]

Question on whether the variadic template is really needed, but it turns out to be needed to support `emplace` of no arguments.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3376.

1. Change the class template `vector<bool>` synopsis, 23.3.7 [vector.bool] p1, as indicated:

   >     namespace std {
   >       template <class Allocator> class vector<bool, Allocator> {
   >       public:
   >         […]
   >         // modifiers:
   >         template <class... Args> void emplace_back(Args&&... args);
   >         void push_back(const bool& x);
   >         void pop_back();
   >         template <class... Args> iterator emplace(const_iterator position, Args&&... args);
   >         iterator insert(const_iterator position, const bool& x);
   >         […]
   >       };
   >     }

### 2188. Reverse iterator does not fully support targets that overload `operator&`

Section: 24.5.1.3.5 [reverse.iter.opref] Status: C++14 Submitter: Alisdair Meredith Opened: 2012-09-23 Last modified: 2015-04-08

View all other issues in [reverse.iter.opref].

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Discussion:

The specification for `reverse_iterator::operator->` returns the address of the object yielded by dereferencing with `operator*`, but does not have the usual wording about returning the true address of the object. As `reverse_iterator` requires the adapted iterator have at least the bidirectional iterator category, we know that the returned reference is a true reference, and not a proxy, hence we can use `std::addressof` on the reference to get the right answer.

This will most likely show itself as an issue with a `list` or `vector` of a type with such an overloaded operator, where algorithms are likely to work with a forward iteration, but not with reverse iteration.

[2013-04-20, Bristol]

Resolution: Goes to open now and move to review as soon as Daniel proposes a new wording.

[2014-02-12 Issaquah meeting]

Use `std::addressof` as the library uses elsewhere, then move as Immediate.

Proposed resolution:

Revise 24.5.1.3.5 [reverse.iter.opref] p1, as indicated:

Returns: `addressof&(operator*())`.

### 2190. Condition variable specification

Section: 30.5 [thread.condition] Status: C++14 Submitter: Hans Boehm Opened: 2012-09-25 Last modified: 2015-04-08

View all other issues in [thread.condition].

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Discussion:

The condition variable specification possibly leaves it unclear whether the effect of a `notify_one()` call can effectively be delayed, so that a call unblocks a `wait()` call that happens after the `notify_one` call. (For `notify_all()` this is not detectable, since it only results in spurious wake-ups.) Although this may at first glance seem like a contrived interpretation, it gains relevance since glibc in fact allows the analogous behavior (see here) and it is currently controversial whether this is correct and the Posix specification allows it (see here).

The following proposed resolution disallows the glibc implementation, remaining consistent with the believed intent of C++11. To make that clear, we require that the "unspecified total order" O from 30.5 [thread.condition] p4 be consistent with happens-before. We also intend that the 3 components of a wait occur in order in O, but stating that explicitly seems too pedantic. Since they are numbered, it appears clear enough that they are sequenced one after the other.

Another uncertainty with the current phrasing is whether there is a single total order that includes all c.v. accesses, or one total order per c.v. We believe it actually doesn't matter, because there is no way to tell the difference, but this requires a bit more thought. We resolved it one way, just to remove the potential ambiguity.

[2012, Portland: Move to Review]

This is linked to a glibc issue, and a POSIX specification issue.

We believe the proposed wording fixes the ambiguity in C++ and is compatible with the proposed resolution for Posix (which confirms the glibc behaviour as illegal).

Moved to review (Detlef hopes to send some improved wording to the reflector).

[2013-04-20, Bristol]

Accepted for the working paper

Proposed resolution:

This wording is relative to N3376.

1. Change 30.5 [thread.condition] p4 as indicated:

   > -4- The implementation shall behave as if all executions of `notify_one`, `notify_all`, and each part of the `wait`, `wait_for`, and `wait_until` executions are executed in some a single unspecified total order consistent with the "happens before" order.

### 2193. Default constructors for standard library containers are explicit

Section: 23 [containers] Status: C++14 Submitter: Richard Smith Opened: 2012-10-04 Last modified: 2015-04-08

View other active issues in [containers].

View all other issues in [containers].

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Discussion:

Most (all?) of the standard library containers have explicit default constructors. Consequently:

>     std::set<int> s1 = { 1, 2 }; // ok
>     std::set<int> s2 = { 1 }; // ok
>     std::set<int> s3 = {}; // ill-formed, copy-list-initialization selected an explicit constructor

Note that Clang + libc++ rejects the declaration of `s3` for this reason. This cannot possibly match the intent.

Suggested fix: apply this transformation throughout the standard library:

>     set() : set(Compare()) {}
>     explicit set(const Compare& comp = Compare(),
>                  const Allocator& = Allocator());

[ 2012-10-06: Daniel adds concrete wording. ]

[2012, Portland: Move to Open]

This may be an issue better solved by a core language tweak. Throw the issue over to EWG and see whether they believe the issue is better resolved in Core or Library.

AJM suggest we spawn a new status of 'EWG' to handle such issues - and will move this issue appropriately when the software can record such resolutions.

[2013-08-27, Joaquín M López Muñoz comments:]

For the record, I'd like to point out that the resolution proposed by the submitter, namely replacing

>     explicit basic_string(const Allocator& a = Allocator());

by

>     basic_string() : basic_string(Allocator()) {}
>     explicit basic_string(const Allocator& a);

(and similarly for other container and container-like classes) might introduce a potential backwards-compatibility problem related with explicit instantiation. Consider for instance

>     struct my_allocator
>     {
>       my_allocator(...); // no default ctor
>       ...
>     };

>     template class std::basic_string<char, std::char_traits<char>, my_allocator<char>>;

This (which I understand is currently a valid explicit instantiation of `std::basic_string`) will break if `std::basic_string` ctors are modified as proposed by this issue, since `my_allocator` doesn't have a default ctor.

[2013-10-06, Daniel comments:]

Issue 2303 describes the more general problem related to explicit instantiation requests in the current library and may help to solve this problem here as well.

[2014-02-13, Issaquah, Jonathan revises wording]

Previous resolution from Daniel [SUPERSEDED]:

> This wording is relative to N3376.

> The more general criterion for performing the suggested transformation was: Any type with an initializer-list constructor that also has an explicit default constructor.

> 1. Change class template `basic_string` synopsis, 21.4 [basic.string] p5 as indicated:

>    >     basic_string() : basic_string(Allocator()) {}
>    >     explicit basic_string(const Allocator& a = Allocator());

> 2. Change 21.4.2 [string.cons] before p1 as indicated:

>    >     explicit basic_string(const Allocator& a = Allocator());

> 3. Change class template `deque` synopsis, 23.3.3.1 [deque.overview] p2 as indicated:

>    >     deque() : deque(Allocator()) {}
>    >     explicit deque(const Allocator& = Allocator());

> 4. Change 23.3.3.2 [deque.cons] before p1 as indicated:

>    >     explicit deque(const Allocator& = Allocator());

> 5. Change class template `forward_list` synopsis, 23.3.4.1 [forwardlist.overview] p3 as indicated:

>    >     forward_list() : forward_list(Allocator()) {}
>    >     explicit forward_list(const Allocator& = Allocator());

> 6. Change 23.3.4.2 [forwardlist.cons] before p1 as indicated:

>    >     explicit forward_list(const Allocator& = Allocator());

> 7. Change class template `list` synopsis, 23.3.5.1 [list.overview] p2 as indicated:

>    >     list() : list(Allocator()) {}
>    >     explicit list(const Allocator& = Allocator());

> 8. Change 23.3.5.2 [list.cons] before p1 as indicated:

>    >     explicit list(const Allocator& = Allocator());

> 9. Change class template `vector` synopsis, 23.3.6.1 [vector.overview] p2 as indicated:

>    >     vector() : vector(Allocator()) {}
>    >     explicit vector(const Allocator& = Allocator());

> 10. Change 23.3.6.2 [vector.cons] before p1 as indicated:

>     >     explicit vector(const Allocator& = Allocator());

> 11. Change class template specialization `vector<bool>` synopsis, 23.3.7 [vector.bool] p1 as indicated:

>     >     vector() : vector(Allocator()) {}
>     >     explicit vector(const Allocator& = Allocator());

> 12. Change class template `map` synopsis, 23.4.4.1 [map.overview] p2 as indicated:

>     >     map() : map(Compare()) {}
>     >     explicit map(const Compare& comp = Compare(),
>     >                  const Allocator& = Allocator());

> 13. Change 23.4.4.2 [map.cons] before p1 as indicated:

>     >     explicit map(const Compare& comp = Compare(),
>     >                  const Allocator& = Allocator());

> 14. Change class template `multimap` synopsis, 23.4.5.1 [multimap.overview] p2 as indicated:

>     >     multimap() : multimap(Compare()) {}
>     >     explicit multimap(const Compare& comp = Compare(),
>     >                       const Allocator& = Allocator());

> 15. Change 23.4.5.2 [multimap.cons] before p1 as indicated:

>     >     explicit multimap(const Compare& comp = Compare(),
>     >                       const Allocator& = Allocator());

> 16. Change class template `set` synopsis, 23.4.6.1 [set.overview] p2 as indicated:

>     >     set() : set(Compare()) {}
>     >     explicit set(const Compare& comp = Compare(),
>     >                  const Allocator& = Allocator());

> 17. Change 23.4.6.2 [set.cons] before p1 as indicated:

>     >     explicit set(const Compare& comp = Compare(),
>     >                  const Allocator& = Allocator());

> 18. Change class template `multiset` synopsis, 23.4.7.1 [multiset.overview] p2 as indicated:

>     >     multiset() : multiset(Compare()) {}
>     >     explicit multiset(const Compare& comp = Compare(),
>     >                       const Allocator& = Allocator());

> 19. Change 23.4.7.2 [multiset.cons] before p1 as indicated:

>     >     explicit multiset(const Compare& comp = Compare(),
>     >                       const Allocator& = Allocator());

> 20. Change class template `unordered_map` synopsis, 23.5.4.1 [unord.map.overview] p3 as indicated:

>     >     unordered_map() : unordered_map(see below) {}
>     >     explicit unordered_map(size_type n = see below,
>     >                            const hasher& hf = hasher(),
>     >                            const key_equal& eql = key_equal(),
>     >                            const allocator_type& a = allocator_type());

> 21. Change 23.5.4.2 [unord.map.cnstr] before p1 as indicated:

>     >     unordered_map() : unordered_map(see below) {}
>     >     explicit unordered_map(size_type n = see below,
>     >                            const hasher& hf = hasher(),
>     >                            const key_equal& eql = key_equal(),
>     >                            const allocator_type& a = allocator_type());

> 22. Change class template `unordered_multimap` synopsis, 23.5.5.1 [unord.multimap.overview] p3 as indicated:

>     >     unordered_multimap() : unordered_multimap(see below) {}
>     >     explicit unordered_multimap(size_type n = see below,
>     >                                 const hasher& hf = hasher(),
>     >                                 const key_equal& eql = key_equal(),
>     >                                 const allocator_type& a = allocator_type());

> 23. Change 23.5.5.2 [unord.multimap.cnstr] before p1 as indicated:

>     >     unordered_multimap() : unordered_multimap(see below) {}
>     >     explicit unordered_multimap(size_type n = see below,
>     >                                 const hasher& hf = hasher(),
>     >                                 const key_equal& eql = key_equal(),
>     >                                 const allocator_type& a = allocator_type());

> 24. Change class template `unordered_set` synopsis, 23.5.6.1 [unord.set.overview] p3 as indicated:

>     >     unordered_set() : unordered_set(see below) {}
>     >     explicit unordered_set(size_type n = see below,
>     >                            const hasher& hf = hasher(),
>     >                            const key_equal& eql = key_equal(),
>     >                            const allocator_type& a = allocator_type());

> 25. Change 23.5.6.2 [unord.set.cnstr] before p1 as indicated:

>     >     unordered_set() : unordered_set(see below) {}
>     >     explicit unordered_set(size_type n = see below,
>     >                            const hasher& hf = hasher(),
>     >                            const key_equal& eql = key_equal(),
>     >                            const allocator_type& a = allocator_type());

> 26. Change class template `unordered_multiset` synopsis, 23.5.7.1 [unord.multiset.overview] p3 as indicated:

>     >     unordered_multiset() : unordered_multiset(see below) {}
>     >     explicit unordered_multiset(size_type n = see below,
>     >                                 const hasher& hf = hasher(),
>     >                                 const key_equal& eql = key_equal(),
>     >                                 const allocator_type& a = allocator_type());

> 27. Change 23.5.7.2 [unord.multiset.cnstr] before p1 as indicated:

>     >     unordered_multiset() : unordered_multiset(see below) {}
>     >     explicit unordered_multiset(size_type n = see below,
>     >                                 const hasher& hf = hasher(),
>     >                                 const key_equal& eql = key_equal(),
>     >                                 const allocator_type& a = allocator_type());

[Issaquah 20014-10-11: Move to Immediate after final review]

Proposed resolution:

This wording is relative to N3376.

The more general criterion for performing the suggested transformation was: Any type with an initializer-list constructor that also has an explicit default constructor.

1. Change class template `basic_string` synopsis, 21.4 [basic.string] p5 as indicated:

   >     basic_string() : basic_string(Allocator()) { }
   >     explicit basic_string(const Allocator& a = Allocator());

2. Change 21.4.2 [string.cons] before p1 as indicated:

   >     explicit basic_string(const Allocator& a = Allocator());

3. Change class template `deque` synopsis, 23.3.3.1 [deque.overview] p2 as indicated:

   >     deque() : deque(Allocator()) { }
   >     explicit deque(const Allocator& = Allocator());

4. Change 23.3.3.2 [deque.cons] before p1 as indicated:

   >     explicit deque(const Allocator& = Allocator());

5. Change class template `forward_list` synopsis, 23.3.4.1 [forwardlist.overview] p3 as indicated:

   >     forward_list() : forward_list(Allocator()) { }
   >     explicit forward_list(const Allocator& = Allocator());

6. Change 23.3.4.2 [forwardlist.cons] before p1 as indicated:

   >     explicit forward_list(const Allocator& = Allocator());

7. Change class template `list` synopsis, 23.3.5.1 [list.overview] p2 as indicated:

   >     list() : list(Allocator()) { }
   >     explicit list(const Allocator& = Allocator());

8. Change 23.3.5.2 [list.cons] before p1 as indicated:

   >     explicit list(const Allocator& = Allocator());

9. Change class template `vector` synopsis, 23.3.6.1 [vector.overview] p2 as indicated:

   >     vector() : vector(Allocator()) { }
   >     explicit vector(const Allocator& = Allocator());

10. Change 23.3.6.2 [vector.cons] before p1 as indicated:

    >     explicit vector(const Allocator& = Allocator());

11. Change class template specialization `vector<bool>` synopsis, 23.3.7 [vector.bool] p1 as indicated:

    >     vector() : vector(Allocator()) { }
    >     explicit vector(const Allocator& = Allocator());

12. Change class template `map` synopsis, 23.4.4.1 [map.overview] p2 as indicated:

    >     map() : map(Compare()) { }
    >     explicit map(const Compare& comp = Compare(),
    >                  const Allocator& = Allocator());

13. Change 23.4.4.2 [map.cons] before p1 as indicated:

    >     explicit map(const Compare& comp = Compare(),
    >                  const Allocator& = Allocator());

14. Change class template `multimap` synopsis, 23.4.5.1 [multimap.overview] p2 as indicated:

    >     multimap() : multimap(Compare()) { }
    >     explicit multimap(const Compare& comp = Compare(),
    >                       const Allocator& = Allocator());

15. Change 23.4.5.2 [multimap.cons] before p1 as indicated:

    >     explicit multimap(const Compare& comp = Compare(),
    >                       const Allocator& = Allocator());

16. Change class template `set` synopsis, 23.4.6.1 [set.overview] p2 as indicated:

    >     set() : set(Compare()) { }
    >     explicit set(const Compare& comp = Compare(),
    >                  const Allocator& = Allocator());

17. Change 23.4.6.2 [set.cons] before p1 as indicated:

    >     explicit set(const Compare& comp = Compare(),
    >                  const Allocator& = Allocator());

18. Change class template `multiset` synopsis, 23.4.7.1 [multiset.overview] p2 as indicated:

    >     multiset() : multiset(Compare()) { }
    >     explicit multiset(const Compare& comp = Compare(),
    >                       const Allocator& = Allocator());

19. Change 23.4.7.2 [multiset.cons] before p1 as indicated:

    >     explicit multiset(const Compare& comp = Compare(),
    >                       const Allocator& = Allocator());

20. Change class template `unordered_map` synopsis, 23.5.4.1 [unord.map.overview] p3 as indicated:

    >     unordered_map();
    >     explicit unordered_map(size_type n = see below,
    >                            const hasher& hf = hasher(),
    >                            const key_equal& eql = key_equal(),
    >                            const allocator_type& a = allocator_type());

21. Change 23.5.4.2 [unord.map.cnstr] before p1 as indicated:

    >     unordered_map() : unordered_map(size_type(see below)) { }
    >     explicit unordered_map(size_type n = see below,
    >                            const hasher& hf = hasher(),
    >                            const key_equal& eql = key_equal(),
    >                            const allocator_type& a = allocator_type());

    > -1- Effects: Constructs an empty `unordered_map` using the specified hash function, key equality func-
    > tion, and allocator, and using at least n buckets. If n is not provided, For the default constructor
    > the number of buckets is implementation-defined. `max_load_factor()` returns 1.0.

22. Change class template `unordered_multimap` synopsis, 23.5.5.1 [unord.multimap.overview] p3 as indicated:

    >     unordered_multimap();
    >     explicit unordered_multimap(size_type n = see below,
    >                                 const hasher& hf = hasher(),
    >                                 const key_equal& eql = key_equal(),
    >                                 const allocator_type& a = allocator_type());

23. Change 23.5.5.2 [unord.multimap.cnstr] before p1 as indicated:

    >     unordered_multimap() : unordered_multimap(size_type(see below)) { }
    >     explicit unordered_multimap(size_type n = see below,
    >                                 const hasher& hf = hasher(),
    >                                 const key_equal& eql = key_equal(),
    >                                 const allocator_type& a = allocator_type());

    > -1- Effects: Constructs an empty `unordered_multimap` using the specified hash function, key equality
    > function, and allocator, and using at least n buckets. If n is not provided, For the default constructor
    > the number of buckets is implementation-defined. `max_load_factor()` returns 1.0.

24. Change class template `unordered_set` synopsis, 23.5.6.1 [unord.set.overview] p3 as indicated:

    >     unordered_set();
    >     explicit unordered_set(size_type n = see below,
    >                            const hasher& hf = hasher(),
    >                            const key_equal& eql = key_equal(),
    >                            const allocator_type& a = allocator_type());

25. Change 23.5.6.2 [unord.set.cnstr] before p1 as indicated:

    >     unordered_set() : unordered_set(size_type(see below)) { }
    >     explicit unordered_set(size_type n = see below,
    >                            const hasher& hf = hasher(),
    >                            const key_equal& eql = key_equal(),
    >                            const allocator_type& a = allocator_type());

    > -1- Effects: Constructs an empty `unordered_set` using the specified hash function, key equality func-
    > tion, and allocator, and using at least n buckets. If n is not provided, For the default constructor
    > the number of buckets is implementation-defined. `max_load_factor()` returns 1.0.

26. Change class template `unordered_multiset` synopsis, 23.5.7.1 [unord.multiset.overview] p3 as indicated:

    >     unordered_multiset();
    >     explicit unordered_multiset(size_type n = see below,
    >                                 const hasher& hf = hasher(),
    >                                 const key_equal& eql = key_equal(),
    >                                 const allocator_type& a = allocator_type());

27. Change 23.5.7.2 [unord.multiset.cnstr] before p1 as indicated:

    >     unordered_multiset() : unordered_multiset(size_type(see below)) { }
    >     explicit unordered_multiset(size_type n = see below,
    >                                 const hasher& hf = hasher(),
    >                                 const key_equal& eql = key_equal(),
    >                                 const allocator_type& a = allocator_type());

    > -1- Effects: Constructs an empty `unordered_multiset` using the specified hash function, key equality
    > function, and allocator, and using at least n buckets. If n is not provided, For the default constructor
    > the number of buckets is implementation-defined. `max_load_factor()` returns 1.0.

### 2194. Impossible container requirements for adaptor types

Section: 23.6 [container.adaptors] Status: C++14 Submitter: Sebastian Mach Opened: 2012-10-05 Last modified: 2015-04-08

View all other issues in [container.adaptors].

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Discussion:

The `stack` class template does not have an member type `iterator`, and therefore instantiations do not meet the general container requirements as described in 23.2.1 [container.requirements.general]. But 23.6.1 [container.adaptors.general] p1 says:

> The headers `<queue>` and `<stack>` define the container adaptors `queue`, `priority_queue`, and `stack`. These container adaptors meet the requirements for sequence containers.

Since sequence containers is a subset of general containers, this imposes requirements on the container adaptors that are not satisfied.

Daniel Krügler: The wording change was performed as an editorial reorganization as requested by GB 116 occuring first in N3242, as a side-effect it does now make the previous implicit C++03 classification to [lib.sequences]/1 more obvious. As the NB comment noticed, the adaptors really are not sequences nor containers, so this wording needs to be fixed. The most simple way to realize that is to strike the offending sentence.

[ Daniel adds concrete wording. ]

[2013-04-20, Bristol]

Unanimous consensus that `queue` and `stack` are not meant to be sequences.

Decision: move to tentatively ready

[2013-09-29, Chicago]

Apply to Working Paper

Proposed resolution:

This wording is relative to N3376.

1. Change 23.6.1 [container.adaptors.general] p1 as indicated:

   > -1- The headers `<queue>` and `<stack>` define the container adaptors `queue`, `priority_queue`, and `stack`. These container adaptors meet the requirements for sequence containers.

### 2196. Specification of `is_*[copy/move]_[constructible/assignable]` unclear for non-referencable types

Section: 20.10.4.3 [meta.unary.prop] Status: C++14 Submitter: Daniel Krügler Opened: 2012-10-06 Last modified: 2015-04-08

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Discussion:

The pre-conditions for the type `is_copy_constructible` allow for template argument types were the language forbids forming a reference, namely `void` types and function types that have cv-qualifiers or a ref-qualifier.

But the current wording in Table 49 defining the predicate condition,

> `is_constructible<T, const T&>::value` is `true`.

leaves it open whether such argument types would (a) create a well-formed instantiation of the trait template or if so (b) what the outcome of the trait evaluation would be, as an example consider `std::is_copy_constructible<void>`.

Current implementations differ, e.g. gcc accepts the instantiation and returns a false result, VS 2012 also accepts the instantiation but returns true.

I would suggest that the wording clarifies that the instantiation would be valid for these types and I also would strongly prefer the outcome that the trait would always return false for these types. The latter seems rather natural to me, because there is no way to define a variable of `void` type or of function type at all, so it would be surprising to return a positive result for copy or move construction if no other construction could succeed. It is also not possible to assign to a any of these values (because there is no way to form lvalues of them), so the same argumentation can be applied to the `is_copy/move_assignable` traits as well.

To reduce the amount of wording changes and repetitions, I suggest to define the term referenceable type in sub-clause 17.3 [definitions] or alternatively in the core language to describe types to which references can be created via a typedef name. This definition corresponds to what the support concept `ReferentType` intended to describe during concept time.

In addition, LWG issue 2101 can also take advantage of the definition of a referenceable type.

If the proposed resolution for LWG issue 2101 would be accepted, there is an alternative solution possible with the same effects. Now we would be able to use the now always well-formed instantiation of `std::add_lvalue_reference` to modify the current definition of `is_copy_constructible` to

> `is_constructible<T,
> typename add_lvalue_reference<
> typename add_const<T>::type>::type>::value` is true.

and similar changes for the other affected traits.

[2012-10 Portland: Move to Open]

Referencable-type should be defined as "something that can be bound into a reference" or similar, rather than a list of types where that is true today. We can then provide the list of known types that cannot be bound as examples that do not qualify in a note.

Otherwise we are happy with the wording. AJM to redraft the definition and move to Review.

[2013-04-18, Bristol]

Proposed resolution:

This wording is relative to N3376.

1. Add the following new definition to 17.3 [definitions] as indicated:

   referenceable type [defns.referenceable]

   > An object type, a function type that does not have cv-qualifiers or a ref-qualifier, or a reference type. [Note: The term describes a type to which a reference can be created, including reference types. — end note]

2. Change Table 49 as indicated:

   Table 49 — Type property predicates
   Template | Condition | Preconditions
   --- | --- | ---
   `template <class T>` `struct is_copy_constructible;` | For a referenceable type `T`, the same result as `is_constructible<T,` `const T&>::value` is true, otherwise false. | `T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown bound.
   `template <class T>` `struct is_move_constructible;` | For a referenceable type `T`, the same result as `is_constructible<T,` `T&&>::value` is true, otherwise false. | `T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown bound.
   `…`
   `template <class T>` `struct is_copy_assignable;` | For a referenceable type `T`, the same result as `is_assignable<T&,` `const T&>::value` is true, otherwise false. | `T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown bound.
   `template <class T>` `struct is_move_assignable;` | For a referenceable type `T`, the same result as `is_assignable<T&,` `T&&>::value` is true, otherwise false. | `T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown bound.
   `…`
   `template <class T>` `struct is_trivially_copy_constructible;` | For a referenceable type `T`, the same result as `is_trivially_constructible<T,` `const T&>::value` is true, otherwise false. | `T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown bound.
   `template <class T>` `struct is_trivially_move_constructible;` | For a referenceable type `T`, the same result as `is_trivially_constructible<T,` `T&&>::value` is true, otherwise false. | `T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown bound.
   `…`
   `template <class T>` `struct is_trivially_copy_assignable;` | For a referenceable type `T`, the same result as `is_trivially_assignable<T&,` `const T&>::value` is true, otherwise false. | `T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown bound.
   `template <class T>` `struct is_trivially_move_assignable;` | For a referenceable type `T`, the same result as `is_trivially_assignable<T&,` `T&&>::value` is true, otherwise false. | `T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown bound.
   `…`
   `template <class T>` `struct is_nothrow_copy_constructible;` | For a referenceable type `T`, the same result as `is_nothrow_constructible<T,` `const T&>::value` is true, otherwise false. | `T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown bound.
   `template <class T>` `struct is_nothrow_move_constructible;` | For a referenceable type `T`, the same result as `is_nothrow_constructible<T,` `T&&>::value` is true, otherwise false. | `T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown bound.
   `…`
   `template <class T>` `struct is_nothrow_copy_assignable;` | For a referenceable type `T`, the same result as `is_nothrow_assignable<T&,` `const T&>::value` is true, otherwise false. | `T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown bound.
   `template <class T>` `struct is_nothrow_move_assignable;` | For a referenceable type `T`, the same result as `is_nothrow_assignable<T&,` `T&&>::value` is true, otherwise false. | `T` shall be a complete type, (possibly cv-qualified) `void`, or an array of unknown bound.

### 2197. Specification of `is_[un]signed` unclear for non-arithmetic types

Section: 20.10.4.3 [meta.unary.prop] Status: C++14 Submitter: Daniel Krügler Opened: 2012-10-07 Last modified: 2015-04-08

View other active issues in [meta.unary.prop].

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Discussion:

The pre-conditions for the trait `is_signed` allow for any types as template arguments, including non-arithmetic ones.

But the current wording in Table 49 defining the predicate condition,

>     is_arithmetic<T>::value && T(-1) < T(0)

looks like real code and so leaves it open whether such argument types would create a well-formed instantiation of the trait template or not. As written this definition would lead to a hard instantiation error for a non-arithmetic type like e.g.

>     struct S {};

I would suggest that the wording clarifies that the instantiation would be valid for such types as well, by means of a specification that is not an exact code pattern. This also reflects how existing implementations behave.

[2013-03-15 Issues Teleconference]

Moved to Tentatively Ready.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3376.

1. Change Table 49 as indicated:

   Table 49 — Type property predicates
   Template | Condition | Preconditions
   --- | --- | ---
   `template <class T>` `struct is_signed;` | If `is_arithmetic<T>::value &&` is true, the same result as `integral_constant<bool, T(-1) < T(0)>::value`; otherwise, false. |
   `template <class T>` `struct is_unsigned;` | If `is_arithmetic<T>::value &&` is true, the same result as `integral_constant<bool, T(0) < T(-1)>::value`; otherwise, false. |

### 2200. Data race avoidance for all containers, not only for sequences

Section: 23.2.2 [container.requirements.dataraces] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-10-17 Last modified: 2015-04-08

View all other issues in [container.requirements.dataraces].

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Discussion:

23.2.2 [container.requirements.dataraces]/2 says "[…] implementations are required to avoid data races when the contents of the contained object in different elements in the same sequence, excepting `vector<bool>`, are modified concurrently."

This should say "same container" instead of "same sequence", to avoid the interpretation that it only applies to sequence containers.

[2013-03-15 Issues Teleconference]

Moved to Tentatively Ready.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3376.

1. Change 23.2.2 [container.requirements.dataraces]/2 as indicated:

   > -2- Notwithstanding (17.6.5.9 [res.on.data.races]), implementations are required to avoid data races when the contents of the contained object in different elements in the same sequence container, excepting `vector<bool>`, are modified concurrently.

   > -3- [Note: For a `vector<int> x` with a size greater than one, `x[1] = 5` and `*x.begin() = 10` can be executed concurrently without a data race, but `x[0] = 5` and `*x.begin() = 10` executed concurrently may result in a data race. As an exception to the general rule, for a `vector<bool> y, y[0] = true` may race with `y[1] = true`. — end note ]

### 2203. `scoped_allocator_adaptor` uses wrong argument types for piecewise construction

Section: 20.13.4 [allocator.adaptor.members] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-10-19 Last modified: 2015-04-08

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Discussion:

In 20.13.4 [allocator.adaptor.members] paragraph 11 the effects clause says a tuple should be constructed with `inner_allocator_type()`, but that creates an rvalue which cannot bind to `inner_allocator_type&`, and would also be wrong if `this->inner_allocator() != inner_allocator_type()`. This could be considered editorial, since the current wording doesn't even compile.

Secondly, in the same paragraph, the tuple objects `xprime` and `yprime` seem to be lvalues and might be constructed by copying `x` and `y`. This prevents using `scoped_allocator` to construct pairs from arguments of move-only types. I believe the `tuple_cast()` expressions should use `std::move(x)` and `std::move(y)` to move from the incoming arguments (which are passed by value to candidates for moving) and the final sentence of the paragraph should be:

then calls `OUTERMOST_ALLOC_TRAITS(*this)::construct(OUTERMOST (*this), p, piecewise_construct, std::move(xprime), std::move(yprime))`.

so that the objects are passed to `std::pair`'s piecewise constructor as rvalues and are eligible for moving into the constructor arguments. This could also be considered editorial, as the current wording prevents certain uses which were intended to be supported.

I've implemented these changes and can confirm they allow code to work that can't be compiled according to the current wording.

[2013-03-15 Issues Teleconference]

Moved to Review.

The resolution looks good, with wording provided by a recent implementer. However, it will take more time than the telecon allows to review with confidence, and we would like Pablo to at least take a look over the resolution and confirm that it matches the design intent.

[2013-04-18, Bristol]

Proposed resolution:

This wording is relative to N3376.

1. Change 20.13.4 [allocator.adaptor.members] paragraph 11 as indicated:

   > -11- Effects: Constructs a `tuple` object `xprime` from `x` by the following rules:

   > - If `uses_allocator<T1, inner_allocator_type>::value` is `false` and `is_constructible<T1, Args1...>::value` is `true`, then `xprime` is `x`.

   > - Otherwise, if `uses_allocator<T1, inner_allocator_type>::value` is `true` and `is_constructible<T1, allocator_arg_t, inner_allocator_type, Args1...>::value` is `true`, then `xprime` is `tuple_cat(tuple<allocator_arg_t, inner_allocator_type&>( allocator_arg, inner_allocator_type()), std::move(x))`.

   > - Otherwise, if `uses_allocator<T1, inner_allocator_type>::value` is `true` and `is_constructible<T1, Args1..., inner_allocator_type>::value` is `true`, then `xprime` is `tuple_cat(std::move(x), tuple<inner_allocator_type&>(inner_allocator_type()))`.

   > - Otherwise, the program is ill-formed.

   > and constructs a `tuple` object `yprime` from `y` by the following rules:

   > - If `uses_allocator<T2, inner_allocator_type>::value` is `false` and `is_constructible<T2, Args2...>::value` is `true`, then `yprime` is `y`.

   > - Otherwise, if `uses_allocator<T2, inner_allocator_type>::value` is `true` and `is_constructible<T2, allocator_arg_t, inner_allocator_type, Args2...>::value` is `true`, then `yprime` is `tuple_cat(tuple<allocator_arg_t, inner_allocator_type&>( allocator_arg, inner_allocator_type()), std::move(y))`.

   > - Otherwise, if `uses_allocator<T2, inner_allocator_type>::value` is `true` and `is_constructible<T2, Args2..., inner_allocator_type>::value` is `true`, then `yprime` is `tuple_cat(std::move(y), tuple<inner_allocator_type&>(inner_allocator_type()))`.

   > - Otherwise, the program is ill-formed.

   > then calls `OUTERMOST_ALLOC_TRAITS(*this)::construct(OUTERMOST(*this), p, piecewise_construct, std::move(xprime), std::move(yprime))`.

### 2205. Problematic postconditions of `regex_match` and `regex_search`

Section: 28.11.2 [re.alg.match], 28.11.3 [re.alg.search] Status: C++14 Submitter: Pete Becker Opened: 2012-10-24 Last modified: 2015-04-08

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Discussion:

Table 142 lists post-conditions on the `match_results` object when a call to `regex_match` succeeds. `regex_match` is required to match the entire target sequence. The post-condition for `m[0].matched` is "true if a full match was found." Since these are conditions for a successful search which is, by definition, a full match, the post-condition should be simply "true".

There's an analogous probem in Table 143: the condition for `m[0].matched` is "true if a match was found, false otherwise." But Table 143 gives post-conditions for a successful match, so the condition should be simply "true".

Furthermore, they have explicit requirements for `m[0].first`, `m[0].second`, and `m[0].matched`. They also have requirements for the other elements of `m`, described as `m[n].first`, `m[n].second`, and `m[n].matched`, in each case qualifying the value of `n` as "for `n < m.size()`". Since there is an explicit description for `n == 0`, this qualification should be "for `0 < n < m.size()`" in all 6 places.

[Issaquah 20014-10-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3376.

1. Change Table 142 as indicated:

   Table 142 — Effects of `regex_match` algorithm
   Element | Value
   --- | ---
   `…`
   `m[0].first` | `first`
   `m[0].second` | `last`
   `m[0].matched` | `true` if a full match was found.
   `m[n].first` | For all integers `0 < n < m.size()`, the start of the sequence that matched sub-expression `n`. Alternatively, if subexpression `n` did not participate in the match, then `last`.
   `m[n].second` | For all integers `0 < n < m.size()`, the end of the sequence that matched sub-expression `n`. Alternatively, if sub-expression `n` did not participate in the match, then `last`.
   `m[n].matched` | For all integers `0 < n < m.size()`, `true` if sub-expression `n` participated in the match, `false` otherwise.

2. Change Table 143 as indicated:

   Table 143 — Effects of `regex_search` algorithm
   Element | Value
   --- | ---
   `…`
   `m[0].first` | The start of the sequence of characters that matched the regular expression
   `m[0].second` | The end of the sequence of characters that matched the regular expression
   `m[0].matched` | `true` if a match was found, and false otherwise.
   `m[n].first` | For all integers `0 < n < m.size()`, the start of the sequence that matched sub-expression `n`. Alternatively, if subexpression `n` did not participate in the match, then `last`.
   `m[n].second` | For all integers `0 < n < m.size()`, the end of the sequence that matched sub-expression `n`. Alternatively, if sub-expression `n` did not participate in the match, then `last`.
   `m[n].matched` | For all integers `0 < n < m.size()`, `true` if sub-expression `n` participated in the match, `false` otherwise.

### 2207. `basic_string::at` should not have a Requires clause

Section: 21.4.5 [string.access] Status: C++14 Submitter: Nevin Liber Opened: 2012-10-26 Last modified: 2015-04-08

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Discussion:

`basic_string::at()` has a wide contract and should not have a "Requires" clause on it.

[2013-01-17, Juan Soulie comments]

This issue would also apply to every member function of `basic_string` that throws `out_of_range`, and to some cases where a `length_error` can be thrown.

[2013-03-15 Issues Teleconference]

Moved to Review.

While this could simply move to Ready on inspection, there is concern that this will not be the only such case. Alisdair volunteers to review clause 21/23 for more of such issues for Bristol, and update the proposed resolution as necessary.

[2013-04-18, Bristol]

Proposed resolution:

This wording is relative to N3376.

1. Remove 21.4.5 [string.access] p5:

   >     const_reference at(size_type pos) const;
   >     reference at(size_type pos);

   > > -5- Requires: `pos < size()`

   > > -6- Throws: `out_of_range` if `pos >= size()`.

   > > -7- Returns: `operator[](pos)`.

### 2209. `assign()` overspecified for sequence containers

Section: 23.3 [sequences] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-10-31 Last modified: 2015-04-08

View all other issues in [sequences].

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Discussion:

DR 704 ensures allocator-aware containers can reuse existing elements during copy/move assignment, and sequence containers can do the same for `assign()`.

But apart from `std::list` (which was changed by DR 320) the sequence containers define the Effects of `assign()` in terms of `clear()` followed by `insert`. A user-defined allocator can easily tell whether all old elements are cleared and then new elements inserted or whether existing elements are assigned to, so those Effects clauses cannot be ignored via the as-if rule.

The descriptions of the `assign()` members for `deque`, `forward_list` and `vector` should be removed. Their intended effects are entirely described by the sequence container requirements table, and the specific definitions of them are worse than redundant, they're contradictory (if the operations are defined in terms of `erase` and `insert` then there's no need for elements to be assignable.) The descriptions of `assign()` for `list` are correct but redundant, so should be removed too.

[2013-03-15 Issues Teleconference]

Moved to Tentatively Ready.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3376.

1. Edit 23.3.3.2 [deque.cons] to remove everything after paragraph 10:

   >     template <class InputIterator>
   >     void assign(InputIterator first, InputIterator last);

   > > -11- Effects:

   > > >     erase(begin(), end());
   > > >     insert(begin(), first, last);

   >     void assign(size_type n, const T& t);

   > > -12- Effects:

   > > >     erase(begin(), end());
   > > >     insert(begin(), n, t);

2. Edit 23.3.4.2 [forwardlist.cons] to remove everything after paragraph 10:

   >     template <class InputIterator>
   >     void assign(InputIterator first, InputIterator last);

   > > -11- Effects: `clear(); insert_after(before_begin(), first, last);`

   >     void assign(size_type n, const T& t);

   > > -12- Effects: `clear(); insert_after(before_begin(), n, t);`

3. Edit 23.3.5.2 [list.cons] to remove everything after paragraph 10:

   >     template <class InputIterator>
   >     void assign(InputIterator first, InputIterator last);

   > > -11- Effects: Replaces the contents of the list with the range `[first, last)`.

   >     void assign(size_type n, const T& t);

   > > -12- Effects: Replaces the contents of the list with `n` copies of `t`.

4. Edit 23.3.6.2 [vector.cons] to remove everything after paragraph 10:

   >     template <class InputIterator>
   >     void assign(InputIterator first, InputIterator last);

   > > -11- Effects:

   > > >     erase(begin(), end());
   > > >     insert(begin(), first, last);

   >     void assign(size_type n, const T& t);

   > > -12- Effects:

   > > >     erase(begin(), end());
   > > >     insert(begin(), n, t);

### 2210. Missing allocator-extended constructor for allocator-aware containers

Section: 23.3 [sequences], 23.4 [associative], 23.5 [unord] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-11-01 Last modified: 2015-04-08

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Discussion:

The `forward_list(size_type)` constructor has no allocator-extended equivalent, preventing the following code from compiling:

>     #include <forward_list>
>     #include <vector>
>     #include <scoped_allocator>

>     using namespace std;

>     int main()
>     {
>       using list = forward_list<int>;
>       vector<list, scoped_allocator_adaptor<list::allocator_type>> v;
>       v.emplace_back(1u);
>     }

The very same problem exists for all allocator-aware sequence containers.

In addition it exists for associative containers. For example, it's possible to construct `std::set<int>{0, 1, 2}` but not `std::set<int>{{0, 1, 2}, alloc}`, and possible to construct `std::set<int>{begin, end}` but not `std::set<int>{begin, end, alloc}`.

This makes the following program fail when `SCOPED` is defined:

>     #include <set>
>     #include <vector>
>     #include <scoped_allocator>

>     #if SCOPED
>     using A = std::scoped_allocator_adaptor<std::allocator<int>>;
>     #else
>     using A = std::allocator<int>;
>     #endif

>     int main()
>     {
>       int values[] = {0, 1, 2};
>       std::vector<std::set<int>, A> v;
>       v.emplace_back(std::begin(values), std::end(values));
>     }

[2013-03-15 Issues Teleconference]

Moved to Review.

Jonathan: There are lots of places where this is missing.

Howard: We should ping Pablo, this might be a deliberate design decision.

[2013-04-18, Bristol]

Proposed resolution:

This wording is relative to N3485.

1. Edit the synopsis in 23.3.3.1 [deque.overview]/2:

   >     namespace std {
   >       template <class T, class Allocator = allocator<T> >
   >       class deque {
   >       public:
   >         […]
   >         explicit deque(const Allocator& = Allocator());
   >         explicit deque(size_type n, const Allocator& = Allocator());
   >         […]
   >       };
   >     }

2. Edit 23.3.3.2 [deque.cons]/2:

   >     explicit deque(size_type n, const Allocator& = Allocator());

   > > -3- Effects: Constructs a `deque` with `n` default-inserted elements using the specified allocator.

3. Edit the synopsis in 23.3.4.1 [forwardlist.overview]/3:

   >     namespace std {
   >       template <class T, class Allocator = allocator<T> >
   >       class forward_list {
   >       public:
   >         […]
   >         explicit forward_list(const Allocator& = Allocator());
   >         explicit forward_list(size_type n, const Allocator& = Allocator());
   >         […]
   >       };
   >     }

4. Edit 23.3.4.2 [forwardlist.cons]/3:

   >     explicit forward_list(size_type n, const Allocator& = Allocator());

   > > -3- Effects: Constructs a `forward_list` object with `n` default-inserted elements using the specified allocator.

5. Edit the synopsis in 23.3.5.1 [list.overview]/2:

   >     namespace std {
   >       template <class T, class Allocator = allocator<T> >
   >       class list {
   >       public:
   >         […]
   >         explicit list(const Allocator& = Allocator());
   >         explicit list(size_type n, const Allocator& = Allocator());
   >         […]
   >       };
   >     }

6. Edit 23.3.5.2 [list.cons]/3:

   >     explicit list(size_type n, const Allocator& = Allocator());

   > > -3- Effects: Constructs a `list` with `n` default-inserted elements using the specified allocator.

7. Edit the synopsis in 23.3.6.1 [vector.overview]/2:

   >     namespace std {
   >       template <class T, class Allocator = allocator<T> >
   >       class vector {
   >       public:
   >         […]
   >         explicit vector(const Allocator& = Allocator());
   >         explicit vector(size_type n, const Allocator& = Allocator());
   >         […]
   >       };
   >     }

8. Edit 23.3.6.2 [vector.cons]/3:

   >     explicit vector(size_type n, const Allocator& = Allocator());

   > > -3- Effects: Constructs a `vector` with `n` default-inserted elements using the specified allocator.

9. Edit the synopsis in 23.3.7 [vector.bool]/1:

   >     namespace std {
   >       template <class Allocator> class vector<bool, Allocator> {
   >       class vector {
   >       public:
   >         […]
   >         explicit vector(const Allocator& = Allocator());
   >         explicit vector(size_type n, const Allocator& = Allocator());
   >         explicit vector(size_type n, const bool& value = bool(),
   >                         const Allocator& = Allocator());
   >         […]
   >       };
   >     }

10. Add to the synopsis in 23.4.4.1 [map.overview] p2:

    >     namespace std {
    >       template <class Key, class T, class Compare = less<Key>,
    >         class Allocator = allocator<pair<const Key, T> > > {
    >       class map {
    >       public:
    >         […]
    >         map(initializer_list<value_type>,
    >           const Compare& = Compare(),
    >           const Allocator& = Allocator());
    >         template <class InputIterator>
    >         map(InputIterator first, InputIterator last, const Allocator& a)
    >           : map(first, last, Compare(), a) { }
    >         map(initializer_list<value_type> il, const Allocator& a)
    >           : map(il, Compare(), a) { }
    >         ~map();
    >         […]
    >       };
    >     }

11. Add to the synopsis in 23.4.5.1 [multimap.overview] p2:

    >     namespace std {
    >       template <class Key, class T, class Compare = less<Key>,
    >         class Allocator = allocator<pair<const Key, T> > > {
    >       class multimap {
    >       public:
    >         […]
    >         multimap(initializer_list<value_type>,
    >           const Compare& = Compare(),
    >           const Allocator& = Allocator());
    >         template <class InputIterator>
    >         multimap(InputIterator first, InputIterator last, const Allocator& a)
    >           : multimap(first, last, Compare(), a) { }
    >         multimap(initializer_list<value_type> il, const Allocator& a)
    >           : multimap(il, Compare(), a) { }
    >         ~multimap();
    >         […]
    >       };
    >     }

12. Add to the synopsis in 23.4.6.1 [set.overview] p2:

    >     namespace std {
    >       template <class Key, class Compare = less<Key>,
    >         class Allocator = allocator<Key> > {
    >       class set {
    >       public:
    >         […]
    >         set(initializer_list<value_type>,
    >           const Compare& = Compare(),
    >           const Allocator& = Allocator());
    >         template <class InputIterator>
    >         set(InputIterator first, InputIterator last, const Allocator& a)
    >           : set(first, last, Compare(), a) { }
    >         set(initializer_list<value_type> il, const Allocator& a)
    >           : set(il, Compare(), a) { }
    >         ~set();
    >         […]
    >       };
    >     }

13. Add to the synopsis in 23.4.7.1 [multiset.overview] p2:

    >     namespace std {
    >       template <class Key, class Compare = less<Key>,
    >         class Allocator = allocator<Key> > {
    >       class multiset {
    >       public:
    >         […]
    >         multiset(initializer_list<value_type>,
    >           const Compare& = Compare(),
    >           const Allocator& = Allocator());
    >         template <class InputIterator>
    >         multiset(InputIterator first, InputIterator last, const Allocator& a)
    >           : multiset(first, last, Compare(), a) { }
    >         multiset(initializer_list<value_type> il, const Allocator& a)
    >           : multiset(il, Compare(), a) { }
    >         ~multiset();
    >         […]
    >       };
    >     }

14. Add to the synopsis in 23.5.4.1 [unord.map.overview] p3:

    >     namespace std {
    >       template <class Key, class T,
    >         class Hash = hash<Key>,
    >         class Pred = std::equal_to<Key>,
    >         class Allocator = std::allocator<std::pair<const Key, T> > > {
    >       class unordered_map {
    >       public:
    >         […]
    >         unordered_map(initializer_list<value_type>,
    >           size_type = see below,
    >           const hasher& hf = hasher(),
    >           const key_equal& eql = key_equal(),
    >           const allocator_type& a = allocator_type());
    >         unordered_map(size_type n, const allocator_type& a)
    >           : unordered_map(n, hasher(), key_equal(), a) { }
    >         unordered_map(size_type n, const hasher& hf, const allocator_type& a)
    >           : unordered_map(n, hf, key_equal(), a) { }
    >         template <class InputIterator>
    >           unordered_map(InputIterator f, InputIterator l, size_type n, const allocator_type& a)
    >           : unordered_map(f, l, n, hasher(), key_equal(), a) { }
    >         template <class InputIterator>
    >           unordered_map(InputIterator f, InputIterator l, size_type n, const hasher& hf,
    >     	    const allocator_type& a)
    >           : unordered_map(f, l, n, hf, key_equal(), a) { }
    >         unordered_map(initializer_list<value_type> il, size_type n, const allocator_type& a)
    >           : unordered_map(il, n, hasher(), key_equal(), a) { }
    >         unordered_map(initializer_list<value_type> il, size_type n, const hasher& hf,
    >     	  const allocator_type& a)
    >           : unordered_map(il, n, hf, key_equal(), a) { }
    >         ~unordered_map();
    >         […]
    >       };
    >     }

15. Add to the synopsis in 23.5.5.1 [unord.multimap.overview] p3:

    >     namespace std {
    >       template <class Key, class T,
    >         class Hash = hash<Key>,
    >         class Pred = std::equal_to<Key>,
    >         class Allocator = std::allocator<std::pair<const Key, T> > > {
    >       class unordered_multimap {
    >       public:
    >         […]
    >         unordered_multimap(initializer_list<value_type>,
    >           size_type = see below,
    >           const hasher& hf = hasher(),
    >           const key_equal& eql = key_equal(),
    >           const allocator_type& a = allocator_type());
    >         unordered_multimap(size_type n, const allocator_type& a)
    >           : unordered_multimap(n, hasher(), key_equal(), a) { }
    >         unordered_multimap(size_type n, const hasher& hf, const allocator_type& a)
    >           : unordered_multimap(n, hf, key_equal(), a) { }
    >         template <class InputIterator>
    >           unordered_multimap(InputIterator f, InputIterator l, size_type n, const allocator_type& a)
    >           : unordered_multimap(f, l, n, hasher(), key_equal(), a) { }
    >         template <class InputIterator>
    >           unordered_multimap(InputIterator f, InputIterator l, size_type n, const hasher& hf,
    >     	    const allocator_type& a)
    >           : unordered_multimap(f, l, n, hf, key_equal(), a) { }
    >         unordered_multimap(initializer_list<value_type> il, size_type n, const allocator_type& a)
    >           : unordered_multimap(il, n, hasher(), key_equal(), a) { }
    >         unordered_multimap(initializer_list<value_type> il, size_type n, const hasher& hf,
    >     	  const allocator_type& a)
    >           : unordered_multimap(il, n, hf, key_equal(), a) { }
    >         ~unordered_multimap();
    >         […]
    >       };
    >     }

16. Add to the synopsis in 23.5.6.1 [unord.set.overview] p3:

    >     namespace std {
    >       template <class Key,
    >         class Hash = hash<Key>,
    >         class Pred = std::equal_to<Key>,
    >         class Allocator = std::allocator<Key> > {
    >       class unordered_set {
    >       public:
    >         […]
    >         unordered_set(initializer_list<value_type>,
    >           size_type = see below,
    >           const hasher& hf = hasher(),
    >           const key_equal& eql = key_equal(),
    >           const allocator_type& a = allocator_type());
    >         unordered_set(size_type n, const allocator_type& a)
    >           : unordered_set(n, hasher(), key_equal(), a) { }
    >         unordered_set(size_type n, const hasher& hf, const allocator_type& a)
    >           : unordered_set(n, hf, key_equal(), a) { }
    >         template <class InputIterator>
    >           unordered_set(InputIterator f, InputIterator l, size_type n, const allocator_type& a)
    >           : unordered_set(f, l, n, hasher(), key_equal(), a) { }
    >         template <class InputIterator>
    >           unordered_set(InputIterator f, InputIterator l, size_type n, const hasher& hf,
    >     	    const allocator_type& a)
    >           : unordered_set(f, l, n, hf, key_equal(), a) { }
    >         unordered_set(initializer_list<value_type> il, size_type n, const allocator_type& a)
    >           : unordered_set(il, n, hasher(), key_equal(), a) { }
    >         unordered_set(initializer_list<value_type> il, size_type n, const hasher& hf,
    >     	  const allocator_type& a)
    >           : unordered_set(il, n, hf, key_equal(), a) { }
    >         ~unordered_set();
    >         […]
    >       };
    >     }

17. Add to the synopsis in 23.5.7.1 [unord.multiset.overview] p3:

    >     namespace std {
    >       template <class Key,
    >         class Hash = hash<Key>,
    >         class Pred = std::equal_to<Key>,
    >         class Allocator = std::allocator<Key> > {
    >       class unordered_multiset {
    >       public:
    >         […]
    >         unordered_multiset(initializer_list<value_type>,
    >           size_type = see below,
    >           const hasher& hf = hasher(),
    >           const key_equal& eql = key_equal(),
    >           const allocator_type& a = allocator_type());
    >         unordered_multiset(size_type n, const allocator_type& a)
    >           : unordered_multiset(n, hasher(), key_equal(), a) { }
    >         unordered_multiset(size_type n, const hasher& hf, const allocator_type& a)
    >           : unordered_multiset(n, hf, key_equal(), a) { }
    >         template <class InputIterator>
    >           unordered_multiset(InputIterator f, InputIterator l, size_type n, const allocator_type& a)
    >           : unordered_multiset(f, l, n, hasher(), key_equal(), a) { }
    >         template <class InputIterator>
    >           unordered_multiset(InputIterator f, InputIterator l, size_type n, const hasher& hf,
    >     	    const allocator_type& a)
    >           : unordered_multiset(f, l, n, hf, key_equal(), a) { }
    >         unordered_multiset(initializer_list<value_type> il, size_type n, const allocator_type& a)
    >           : unordered_multiset(il, n, hasher(), key_equal(), a) { }
    >         unordered_multiset(initializer_list<value_type> il, size_type n, const hasher& hf,
    >     	  const allocator_type& a)
    >           : unordered_multiset(il, n, hf, key_equal(), a) { }
    >         ~unordered_multiset();
    >         […]
    >       };
    >     }

### 2211. Replace ambiguous use of "Allocator" in container requirements

Section: 23.2.1 [container.requirements.general] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-11-07 Last modified: 2015-04-08

View other active issues in [container.requirements.general].

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Discussion:

23.2.1 [container.requirements.general]/7 says:

> All other constructors for these container types take an `Allocator&` argument (17.6.3.5 [allocator.requirements]), an allocator whose value type is the same as the container's value type.

This is a strange place to state the requirement on the allocator's `value_type`, because the allocator is a property (and template parameter) of the container type not of some of its constructors. It's also unclear whether "`Allocator&`" refers to the concept (as implied by the cross-reference to the allocator requirements in Clause 17) or to the container's template parameter (as implied by the fact it's shown as an lvalue-reference type.) I believe the latter is intended, because those constructors can't take any model of the allocator concept, they can only take the container's `allocator_type`.

I think it would be clearer to remove the value type requirement earlier in the paragraph (Table 99 already imposes that requirement) and to make it clear the constructor arguments are the container's `allocator_type`. There is already a cross-reference to the allocator requirements earlier in the paragraph, so it doesn't need to be repeated in another place where it causes confusion.

[2013-03-15 Issues Teleconference]

Moved to Tentatively Ready.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3485.

1. Edit 23.2.1 [container.requirements.general] paragraph 7:

   > Unless otherwise specified, all containers defined in this clause obtain memory using an allocator (see 17.6.3.5 [allocator.requirements]). Copy constructors for these container types obtain an allocator by calling `allocator_traits<allocator_type>::select_on_container_copy_construction` on their first parameters. Move constructors obtain an allocator by move construction from the allocator belonging to the container being moved. Such move construction of the allocator shall not exit via an exception. All other constructors for these container types take an `Allocator&` argument (17.6.3.5 [allocator.requirements]), an allocator whose value type is the same as the container's value type a `const allocator_type&` argument. [Note: If an invocation of a constructor uses the default value of an optional allocator argument, then the `Allocator` type must support value initialization. — end note] A copy of this allocator is used for any memory allocation performed, by these constructors and by all member functions, during the lifetime of each container object or until the allocator is replaced. […]

### 2212. `tuple_size` for `const pair` request `<tuple>` header

Section: 20.2 [utility] Status: WP Submitter: Alisdair Meredith Opened: 2012-11-09 Last modified: 2015-04-08

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Discussion:

The `<utility>` header declares sufficient of the tuple API to specialize the necessary templates for `pair`, notably `tuple_size` and `tuple_element`. However, it does not make available the partial specializations that support cv-qualified template arguments, so while I can write the following after including only `<utility>`:

>     #include <utility>

>     using TestType = std::pair<int, int>;
>     static_assert(2 == std::tuple_size<TestType>(), "Pairs have two elements");
>     std::tuple_element<0, TestType>::type var{1};

the following may fail to compile unless I also include `<tuple>`:

>     #include <utility>

>     using TestType = const std::pair<int, int>;
>     static_assert(2 == std::tuple_size<TestType>(), "Pairs have two elements");
>     std::tuple_element<0, TestType>::type var{1};

Note, however, that the latter may compile with some standard library implementations but not others, leading to subtle portability issues.

[2013-03-15 Issues Teleconference]

Moved to Open.

Howard notes that we have the same issue with array, so any resolution should apply to that header too.

[2013-10-18 Daniel provides wording]

The suggested wording uses a similar approach as we already have in 24.7 [iterator.range] to ensure that the range access templates are available when at least one of an enumerated list of header files is included.

I also think that the restricted focus on `tuple_size` of this issue is too narrow and should be extended to the similar partial template specializations of `tuple_element` as well. Therefore the suggested wording ensures this as well.

[2014-03-27 Library reflector vote]

The issue has been identified as Tentatively Ready based on eight votes in favour.

Proposed resolution:

This wording is relative to N3936.

1. Change 20.4.2.5 [tuple.helper] as indicated:

   >     template <class T> class tuple_size<const T>;
   >     template <class T> class tuple_size<volatile T>;
   >     template <class T> class tuple_size<const volatile T>;

   > > -3- Let `TS` denote `tuple_size<T>` of the cv-unqualified type `T`. Then each of the three templates shall meet the `UnaryTypeTrait` requirements (20.10.1) with a `BaseCharacteristic` of

   > > >     integral_constant<size_t, TS::value>

   > > -?- In addition to being available via inclusion of the `<tuple>` header, each of the three templates are available when any of the headers `<array>` or `<utility>` are included.

   >     template <size_t I, class T> class tuple_element<I, const T>;
   >     template <size_t I, class T> class tuple_element<I, volatile T>;
   >     template <size_t I, class T> class tuple_element<I, const volatile T>;

   > > -?- Let `TE` denote `tuple_element<I, T>` of the cv-unqualified type `T`. Then each of the three templates shall meet the `TransformationTrait` requirements (20.10.1) with a member typedef `type` that names the following type:

   > > - for the first specialization, `add_const<TE::type>::type`,

   > > - for the second specialization, `add_volatile<TE::type>::type`, and

   > > - for the third specialization, `add_cv<TE::type>::type`.

   > > -?- In addition to being available via inclusion of the `<tuple>` header, each of the three templates are available when any of the headers `<array>` or `<utility>` are included.

### 2213. Return value of `std::regex_replace`

Section: 28.11.4 [re.alg.replace] Status: C++14 Submitter: Pete Becker Opened: 2012-11-08 Last modified: 2015-04-08

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Discussion:

In 28.11.4 [re.alg.replace], the first two variants of `std::regex_replace` take an output iterator named "out" as their first argument. Paragraph 2 of that section says that the functions return "out". When I first implemented this, many years ago, I wrote it to return the value of the output iterator after all the insertions (cf. `std::copy`), which seems like the most useful behavior. But looking at the requirement now, it like the functions should return the original value of "out" (i.e. they have to keep a copy of the iterator for no reason except to return it). Is that really what was intended?

[Issaquah 20014-10-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3485.

1. Edit 28.11.4 [re.alg.replace] as indicated:

   >     template <class OutputIterator, class BidirectionalIterator,
   >       class traits, class charT, class ST, class SA>
   >     OutputIterator
   >     regex_replace(OutputIterator out, BidirectionalIterator first, BidirectionalIterator last,
   >       const basic_regex<charT, traits>& e, const basic_string<charT, ST, SA>& fmt,
   >       regex_constants::match_flag_type flags = regex_constants::match_default);
   >     template <class OutputIterator, class BidirectionalIterator,
   >       class traits, class charT>
   >     OutputIterator
   >     regex_replace(OutputIterator out, BidirectionalIterator first, BidirectionalIterator last,
   >       const basic_regex<charT, traits>& e, const charT* fmt,
   >       regex_constants::match_flag_type flags = regex_constants::match_default);

   > > -1- Effects: Constructs a `regex_iterator` object `i` as if by `regex_iterator<BidirectionalIterator, charT, traits> i(first, last, e, flags)`, and uses `i` to enumerate through all of the matches `m` of type `match_results<BidirectionalIterator>` that occur within the sequence `[first, last)`. If no such matches are found and `!(flags & regex_constants ::format_no_copy)` then calls `out = std::copy(first, last, out)`. If any matches are found then, for each such match, if `!(flags & regex_constants::format_no_copy)`, calls `out = std::copy(m.prefix().first, m.prefix().second, out)`, and then calls `out = m.format(out, fmt, flags)` for the first form of the function and `out = m.format(out, fmt, fmt + char_traits<charT>::length(fmt), flags)` for the second. Finally, if such a match is found and `!(flags & regex_constants ::format_no_copy)`, calls `out = std::copy(last_m.suffix().first, last_m.suffix().second, out)` where `last_m` is a copy of the last match found. If `flags & regex_constants::format_first_only` is non-zero then only the first match found is replaced.

   > > -2- Returns: `out`.

### 2217. `operator==(sub_match, string)` slices on embedded `'\0'`s

Section: 28.9.2 [re.submatch.op] Status: WP Submitter: Jeffrey Yasskin Opened: 2012-11-26 Last modified: 2015-04-08

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Discussion:

>     template <class BiIter, class ST, class SA>
>       bool operator==(
>         const basic_string<
>           typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
>         const sub_match<BiIter>& rhs);

is specified as:

> Returns: `rhs.compare(lhs.c_str()) == 0`.

This is odd because `sub_match::compare(basic_string)` is defined to honor embedded `'\0'` characters. This could allow a `sub_match` to `==` or `!=` a `std::string` unexpectedly.

[Daniel:]

This wording change was done intentionally as of LWG 1181, but the here mentioned slicing effect was not considered at that time. It seems best to use another overload of compare to fix this problem:

> Returns: `rhs.str().compare(0, rhs.length(), lhs.data(), lhs.size()) == 0`.

or

> Returns: `rhs.compare(sub_match<BiIter>::string_type(lhs.data(), lhs.size())) == 0`.

[2013-10-17: Daniel provides concrete wording]

The original wording was suggested to reduce the need to allocate memory during comparisons. The specification would be very much easier, if `sub_match` would provide an additional `compare` overload of the form:

>     int compare(const value_type* s, size_t n) const;

But given the fact that currently all of `basic_string`'s `compare` overloads are defined in terms of temporary string constructions, the following proposed wording does follow the same string-construction route as `basic_string` does (where needed to fix the embedded zeros issue) and to hope that existing implementations ignore to interpret this semantics in the literal sense.

I decided to use the second replacement form

>     Returns: rhs.compare(sub_match<BiIter>::string_type(lhs.data(), lhs.size())) == 0.

because it already reflects the existing style used in 28.9.2 [re.submatch.op] p31.

[2014-02-15 post-Issaquah session : move to Tentatively Ready]

Proposed resolution:

This wording is relative to N3691.

1. Change 28.9.2 [re.submatch.op] as indicated:

   >     template <class BiIter, class ST, class SA>
   >       bool operator==(
   >         const basic_string<
   >           typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
   >         const sub_match<BiIter>& rhs);

   > > -7- Returns: `rhs.compare(lhs.c_str()typename sub_match<BiIter>::string_type(lhs.data(), lhs.size())) == 0`.

   […]

   >     template <class BiIter, class ST, class SA>
   >       bool operator<(
   >         const basic_string<
   >           typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
   >         const sub_match<BiIter>& rhs);

   > > -9- Returns: `rhs.compare(lhs.c_str()typename sub_match<BiIter>::string_type(lhs.data(), lhs.size())) > 0`.

   […]

   >     template <class BiIter, class ST, class SA>
   >       bool operator==(const sub_match<BiIter>& lhs,
   >                       const basic_string<
   >                         typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);

   > > -13- Returns: `lhs.compare(rhs.c_str()typename sub_match<BiIter>::string_type(rhs.data(), rhs.size())) == 0`.

   […]

   >     template <class BiIter, class ST, class SA>
   >       bool operator<(const sub_match<BiIter>& lhs,
   >                      const basic_string<
   >                        typename iterator_traits<BiIter>::value_type, ST, SA>& rhs);

   > > -15- Returns: `lhs.compare(rhs.c_str()typename sub_match<BiIter>::string_type(rhs.data(), rhs.size())) < 0`.

### 2222. Inconsistency in description of `forward_list::splice_after` single-element overload

Section: 23.3.4.6 [forwardlist.ops] Status: C++14 Submitter: Edward Catmur Opened: 2012-12-11 Last modified: 2015-04-08

View all other issues in [forwardlist.ops].

View all issues with C++14 status.

Discussion:

23.3.4.6 [forwardlist.ops] p6 has

>     void splice_after(const_iterator position, forward_list& x, const_iterator i);
>     void splice_after(const_iterator position, forward_list&& x, const_iterator i);

> Effects: Inserts the element following `i` into `*this`, following `position`, and removes it from `x`. The result is unchanged if `position == i` or `position == ++i`. Pointers and references to `*i` continue to refer to the same element but as a member of `*this`. Iterators to `*i` (including `i` itself) continue to refer to the same element, but now behave as iterators into `*this`, not into `x`.

This overload splices the element following `i` from `x` to `*this`, so the language in the two latter sentences should refer to `++i`:

> Pointers and references to `*++i` continue to refer to the same element but as a member of `*this`. Iterators to `*++i` continue to refer to the same element, but now behave as iterators into `*this`, not into `x`.

[2013-03-15 Issues Teleconference]

Moved to Tentatively Ready.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3485.

1. Edit 23.3.4.6 [forwardlist.ops] p6 as indicated:

   >     void splice_after(const_iterator position, forward_list& x, const_iterator i);
   >     void splice_after(const_iterator position, forward_list&& x, const_iterator i);

   > -5- Requires: `position` is `before_begin()` or is a dereferenceable iterator in the range `[begin(),end())`. The iterator following `i` is a dereferenceable iterator in `x`. `get_allocator() == x.get_allocator()`.

   > -6- Effects: Inserts the element following `i` into `*this`, following `position`, and removes it from `x`. The result is unchanged if `position == i` or `position == ++i`. Pointers and references to `*++i` continue to refer to the same element but as a member of `*this`. Iterators to `*++i` (including `i` itself) continue to refer to the same element, but now behave as iterators into `*this`, not into `x`.

### 2225. Unrealistic header inclusion checks required

Section: 17.6.2.2 [using.headers] Status: C++14 Submitter: Richard Smith Opened: 2012-12-18 Last modified: 2015-04-08

View all other issues in [using.headers].

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Discussion:

17.6.2.2 [using.headers]/3 says:

A translation unit shall include a header only outside of any external declaration or definition, and shall include the header lexically before the first reference in that translation unit to any of the entities declared in that header.

Per 1.4 [intro.compliance]/1, programs which violate this rule are ill-formed, and a conforming implementation is required to produce a diagnostic. This does not seem to match reality. Presumably, this paragraph is missing a "no diagnostic is required".

[2013-03-15 Issues Teleconference]

Moved to Tentatively Ready.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3485.

1. Edit 17.6.2.2 [using.headers] p3 as indicated:

   -3- A translation unit shall include a header only outside of any external declaration or definition, and shall include the header lexically before the first reference in that translation unit to any of the entities declared in that header. No diagnostic is required.

### 2228. Missing SFINAE rule in `unique_ptr` templated assignment

Section: 20.8.1.2.3 [unique.ptr.single.asgn] Status: Resolved Submitter: Geoffrey Romer Opened: 2012-12-20 Last modified: 2015-05-06

View all other issues in [unique.ptr.single.asgn].

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Discussion:

20.8.1.2.3 [unique.ptr.single.asgn]/5 permits `unique_ptr`'s templated assignment operator to participate in overload resolution even when incompatibilities between `D` and `E` will render the result ill-formed, but the corresponding templated copy constructor is removed from the overload set in those situations (see the third bullet point of 20.8.1.2.1 [unique.ptr.single.ctor]/19). This asymmetry is confusing, and presumably unintended; it may lead to situations where constructing one `unique_ptr` from another is well-formed, but assigning from the same `unique_ptr` would be ill-formed.

There is a slight coupling between this and LWG 2118, in that my PR for LWG 2118 incorporates equivalent wording in the specification of the templated assignment operator for the array specialization; the two PRs are logically independent, but if my PR for 2118 is accepted but the above PR is not, the discrepancy between the base template and the specialization could be confusing.

Previous resolution [SUPERSEDED]:

> This wording is relative to N3485.

> 1. Revise 20.8.1.2.3 [unique.ptr.single.asgn] p5 as follows:

>    >     template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;

>    > > -4- Requires: If `E` is not a reference type, assignment of the deleter from an rvalue of type `E` shall be well-formed and shall not throw an exception. Otherwise, `E` is a reference type and assignment of the deleter from an lvalue of type `E` shall be well-formed and shall not throw an exception.

>    > > -5- Remarks: This operator shall not participate in overload resolution unless:

>    > > - `unique_ptr<U, E>::pointer` is implicitly convertible to `pointer` and

>    > > - `U` is not an array type., and

>    > > - either `D` is a reference type and `E` is the same type as `D`, or `D` is not a reference type and `E` is implicitly convertible to `D`.

>    > > -6- Effects: Transfers ownership from `u` to `*this` as if by calling `reset(u.release())` followed by an assignment from `std::forward<E>(u.get_deleter())`.

>    > > -7- Returns: `*this`.

[2013-03-15 Issues Teleconference]

Moved to Review.

The wording looks good, but we want a little more time than the telecon permits to be truly comfortable. We expect this issue to resolve fairly easily in Bristol.

[2015-05-18, Howard comments]

Updated proposed wording has been provided in N4366.

[2015-05, Lenexa]

Straw poll: send N4366 to full committee, with both fixes from the sections "What is the correct fix?" and "`unique_ptr<T[]>` needs the correct fix too"

Proposed resolution:

Resolved by accepting N4366.

### 2229. Standard code conversion facets underspecified

Section: 22.5 [locale.stdcvt] Status: C++14 Submitter: Beman Dawes Opened: 2012-12-30 Last modified: 2015-04-08

View all issues with C++14 status.

Discussion:

The only specification for the non-inherited members of classes `codecvt_utf8`, `codecvt_utf16`, and `codecvt_utf8_utf16` is a comment line in the synopsis that says `// unspecified`. There is no further indication of functionality, so a user does not know if one of these classes can be constructed or destroyed.

The proposed resolution adds a constructor that mimics the class `codecvt` constructor, and also adds a destructor. Following the practice of class `codecvt`, the semantics are not specified.

The only existing implementation I could find was libc++, and it does supply the proposed constructor and destructor for each of the three classes.

[2013-03-15 Issues Teleconference]

Moved to Review.

There was concern about the unspecified semantics - but that matches what is done in `codecvt`.

Jonathan: Should these constructor/destructors be public? Proposed wording is private. Base class constructor is public.

Howard noted that other facets do not have specified constructors.

Alisdair noted that this whole section was new in C++11.

Howard suggested looking at section 22.3.1.1.2 [locale.facet]p2/p3 for more info.

[2013-04-18, Bristol]

Proposed resolution:

In [locale.stdcvt] paragraph 2, Header codecvt synopsis:

>     template<class Elem, unsigned long Maxcode = 0x10ffff,
>       codecvt_mode Mode = (codecvt_mode)0>
>     class codecvt_utf8
>       : public codecvt<Elem, char, mbstate_t> {
>       // unspecified
>     public:
>       explicit codecvt_utf8(size_t refs = 0);
>       ~codecvt_utf8();
>       };

>     template<class Elem, unsigned long Maxcode = 0x10ffff,
>       codecvt_mode Mode = (codecvt_mode)0>
>     class codecvt_utf16
>       : public codecvt<Elem, char, mbstate_t> {
>       // unspecified
>     public:
>       explicit codecvt_utf16(size_t refs = 0);
>       ~codecvt_utf16();
>       };

>     template<class Elem, unsigned long Maxcode = 0x10ffff,
>       codecvt_mode Mode = (codecvt_mode)0>
>     class codecvt_utf8_utf16
>       : public codecvt<Elem, char, mbstate_t> {
>       // unspecified
>     public:
>       explicit codecvt_utf8_utf16(size_t refs = 0);
>       ~codecvt_utf8_utf16();
>       };

### 2230. "see below" for initializer-list constructors of unordered containers

Section: 23.5 [unord] Status: WP Submitter: Jonathan Wakely Opened: 2013-01-06 Last modified: 2015-04-08

View all other issues in [unord].

View all issues with WP status.

Discussion:

The `unordered_map` class definition in 23.5.4.1 [unord.map.overview] declares an initializer-list constructor that says "see below":

>     unordered_map(initializer_list<value_type>,
>         size_type = see below,
>         const hasher& hf = hasher(),
>         const key_equal& eql = key_equal(),
>         const allocator_type& a = allocator_type());

But that constructor isn't defined below. The same problem exists for the other unordered associative containers.

[2013-09 Chicago]

STL: ordered are also missing declarations, but issue is forthcoming

Walter: how does adding a signature address issue? — nevermind

Jayson: in his wording, isn't he just dropping the `size_type`?

Walter: partial fix is to introduce the name

Stefanus: explanation of requiring name because of n buckets

STL: solution for his issue satisfies both ordered and unordered and is simplier than provided wording

STL: patches general table instead

STL: proposes adding extra rows instead of extra declarations

Stefanus: clarify `n` in the synopsis

Walter: general rule, name is optional in declaration

Stefanus: how to proceed

Walter: significant overlap with forthcoming issue, suggestion to defer

[2014-02-20 Re-open Deferred issues as Priority 4]

[2014-03-27 Jonathan improves proposed wording]

[2014-05-20 STL and Jonathan communicate]

STL: With 2322 resolved, is there anything left for this issue to fix?

Jonathan: The synopsis still says "see below" and it's not immediately clear that "see below" means "see the definition of a different constructor, which defines the behaviour of this one due to a table defined much earlier".

[2014-05-23 Library reflector vote]

The issue has been identified as Tentatively Ready based on five votes in favour.

Proposed resolution:

This wording is relative to N3936.

1. Edit 23.5.4.1 [unord.map.overview], class template `unordered_map` synopsis, as follows:

   >     […]
   >     unordered_map(initializer_list<value_type> il,
   >       size_type n = see below,
   >       const hasher& hf = hasher(),
   >       const key_equal& eql = key_equal(),
   >       const allocator_type& a = allocator_type());
   >     […]

2. Edit 23.5.4.2 [unord.map.cnstr] as follows:

   >     template <class InputIterator>
   >     unordered_map(InputIterator f, InputIterator l,
   >       size_type n = see below,
   >       const hasher& hf = hasher(),
   >       const key_equal& eql = key_equal(),
   >       const allocator_type& a = allocator_type());
   >     unordered_map(initializer_list<value_type> il,
   >       size_type n = see below,
   >       const hasher& hf = hasher(),
   >       const key_equal& eql = key_equal(),
   >       const allocator_type& a = allocator_type());

   > > -3- Effects: Constructs an empty `unordered_map` using the specified hash function, key equality function, and allocator, and using at least `n` buckets. If `n` is not provided, the number of buckets is implementation-defined. Then inserts elements from the range `[f, l)` for the first form, or from the range `[il.begin(), il.end())` for the second form. `max_load_factor()` returns `1.0`.

3. Edit 23.5.5.1 [unord.multimap.overview], class template `unordered_multimap` synopsis, as follows:

   >     […]
   >     unordered_multimap(initializer_list<value_type> il,
   >       size_type n = see below,
   >       const hasher& hf = hasher(),
   >       const key_equal& eql = key_equal(),
   >       const allocator_type& a = allocator_type());
   >     […]

4. Edit 23.5.5.2 [unord.multimap.cnstr] as follows:

   >     template <class InputIterator>
   >     unordered_multimap(InputIterator f, InputIterator l,
   >       size_type n = see below,
   >       const hasher& hf = hasher(),
   >       const key_equal& eql = key_equal(),
   >       const allocator_type& a = allocator_type());
   >     unordered_multimap(initializer_list<value_type> il,
   >       size_type n = see below,
   >       const hasher& hf = hasher(),
   >       const key_equal& eql = key_equal(),
   >       const allocator_type& a = allocator_type());

   > > -3- Effects: Constructs an empty `unordered_multimap` using the specified hash function, key equality function, and allocator, and using at least `n` buckets. If `n` is not provided, the number of buckets is implementation-defined. Then inserts elements from the range `[f, l)` for the first form, or from the range `[il.begin(), il.end())` for the second form. `max_load_factor()` returns `1.0`.

5. Edit 23.5.6.1 [unord.set.overview], class template `unordered_set` synopsis, as follows:

   >     […]
   >     unordered_set(initializer_list<value_type> il,
   >       size_type n = see below,
   >       const hasher& hf = hasher(),
   >       const key_equal& eql = key_equal(),
   >       const allocator_type& a = allocator_type());
   >     […]

6. Edit 23.5.6.2 [unord.set.cnstr] as follows:

   >     template <class InputIterator>
   >     unordered_set(InputIterator f, InputIterator l,
   >       size_type n = see below,
   >       const hasher& hf = hasher(),
   >       const key_equal& eql = key_equal(),
   >       const allocator_type& a = allocator_type());
   >     unordered_set(initializer_list<value_type> il,
   >       size_type n = see below,
   >       const hasher& hf = hasher(),
   >       const key_equal& eql = key_equal(),
   >       const allocator_type& a = allocator_type());

   > > -3- Effects: Constructs an empty `unordered_set` using the specified hash function, key equality function, and allocator, and using at least `n` buckets. If `n` is not provided, the number of buckets is implementation-defined. Then inserts elements from the range `[f, l)` for the first form, or from the range `[il.begin(), il.end())` for the second form. `max_load_factor()` returns `1.0`.

7. Edit 23.5.7.1 [unord.multiset.overview], class template `unordered_multiset` synopsis, as follows:

   >     […]
   >     unordered_multiset(initializer_list<value_type> il,
   >       size_type n = see below,
   >       const hasher& hf = hasher(),
   >       const key_equal& eql = key_equal(),
   >       const allocator_type& a = allocator_type());
   >     […]

8. Edit 23.5.7.2 [unord.multiset.cnstr] as follows:

   >     template <class InputIterator>
   >     unordered_multiset(InputIterator f, InputIterator l,
   >       size_type n = see below,
   >       const hasher& hf = hasher(),
   >       const key_equal& eql = key_equal(),
   >       const allocator_type& a = allocator_type());
   >     unordered_multiset(initializer_list<value_type> il,
   >       size_type n = see below,
   >       const hasher& hf = hasher(),
   >       const key_equal& eql = key_equal(),
   >       const allocator_type& a = allocator_type());

   > > -3- Effects: Constructs an empty `unordered_multiset` using the specified hash function, key equality function, and allocator, and using at least `n` buckets. If `n` is not provided, the number of buckets is implementation-defined. Then inserts elements from the range `[f, l)` for the first form, or from the range `[il.begin(), il.end())` for the second form. `max_load_factor()` returns `1.0`.

### 2231. DR 704 removes complexity guarantee for `clear()`

Section: 23.2.3 [sequence.reqmts] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-12-30 Last modified: 2015-04-08

View all other issues in [sequence.reqmts].

View all issues with C++14 status.

Discussion:

From the question at stackoverflow.

Were we aware that the resolution to LWG 704 means there is no complexity guarantee for `clear()` on most sequence containers? Previously it was implied by defining it in terms of `erase(begin(), end())` but we no longer do that.

There are explicit complexity requirements for `std::list::clear()`, but not the other sequence containers.

Daniel:

The idea was that the notion of "destroys all elements in `a`" would imply a linear complexity, but the wording needs to be clearer, because this doesn't say that this step is the actual complexity bound.

[2013-03-15 Issues Teleconference]

Moved to Tentatively Ready.

[2013-04-20 Bristol]

Proposed resolution:

This wording is relative to N3485.

1. Change Table 100 as indicated:

   > Table 100 — Sequence container requirements (in addition to container) (continued)
   > Expression | Return type | Assertion/note pre-/post-condition
   > --- | --- | ---
   > `…`
   > `a.clear()` | `void` | Destroys all elements in `a`. Invalidates all references, pointers, and iterators referring to the elements of `a` and may invalidate the past-the-end iterator. post: `a.empty()` returns `true` complexity: linear
   > `…`

### 2233. `bad_function_call::what()` unhelpful

Section: 20.9.12.1 [func.wrap.badcall] Status: WP Submitter: Jonathan Wakely Opened: 2013-01-05 Last modified: 2015-04-08

View all issues with WP status.

Discussion:

A strict reading of the standard implies `std::bad_function_call{}.what()` returns the same string as `std::exception{}.what()` which doesn't help to know what happened if you catch an exception by reference to `std::exception`.

For consistency with `bad_weak_ptr::what()` it should return `"bad_function_call"`.

See c++std-lib-33515 for other details.

There was a considerable support on the reflector to instead change the specification of both `bad_weak_ptr::what()` and `bad_function_call::what()` to return an implementation-defined string instead.

[2013-03-15 Issues Teleconference]

Moved to Open.

Consensus that we want consistency in how this is treated. Less consensus on what the common direction should be.

Alisdair to provide wording proposing that all string literals held by standard exception objects are either unspecified, or implmentation defined.

[2014-02-15 Issauqah]

STL: I think it should be an implementation-defined NTBS, same on `bad_weak_ptr`. I will write a PR.

[2014-03-27, STL provides improved wording]

The new wording reflects better the general agreement of the committee, see also issue 2376 for similar wording.

[2014-03-28 Library reflector vote]

The issue has been identified as Tentatively Ready based on five votes in favour.

Proposed resolution:

This wording is relative to N3936.

1. Edit 20.9.12.1.1 [func.wrap.badcall.const]:

   >     bad_function_call() noexcept;

   > > -1- Effects: constructs a `bad_function_call object`.

   > > -?- Postconditions: `what()` returns an implementation-defined NTBS.

### 2235. Undefined behavior without proper requirements on `basic_string` constructors

Section: 21.4.2 [string.cons] Status: C++14 Submitter: Juan Soulie Opened: 2013-01-17 Last modified: 2015-04-08

View all other issues in [string.cons].

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Discussion:

In 21.4.2 [string.cons], I believe tighter requirements should be imposed on `basic_string`'s constructors taking an `s` argument (or, a behavior should be provided for the undefined cases). These requirements are properly stated in the other members functions taking `s` arguments (`append`, `assign`, `insert`,...).

>     basic_string(const charT* s, size_type n, const Allocator& a = Allocator());

Relative to N3485, 21.4.2 [string.cons]/6 says "Requires: `s` shall not be a null pointer and `n < npos`", where it should say: "Requires: `s` points to an array of at least `n` elements of `charT`"

>     basic_string(const charT* s, const Allocator& a = Allocator());

21.4.2 [string.cons]/8 says "Requires: `s` shall not be a null pointer.", where it should say: "Requires: `s` points to an array of at least `traits::length(s) + 1` elements of `charT`"

Daniel:

I think that 17.6.4.9 [res.on.arguments] p1 b2 basically requires this already, but the wording is indeed worth improving it.

[2013-03-15 Issues Teleconference]

Moved to Review.

The resolution could be worded more cleanly, and there is some concern about redundancy between Requirements and Effects clauses. Consensus that we do want to say something like this for the Requirements though.

[2013-04-18, Bristol]

Move to Ready

[2013-09-29, Bristol]

Apply to the Working Paper

Proposed resolution:

This wording is relative to N3485.

1. Change 21.4.2 [string.cons]/6 as indicated:

   >     basic_string(const charT* s, size_type n, const Allocator& a = Allocator());

   > > -6- Requires: `s` shall not be a null pointer and `n < npos` points to an array of at least `n` elements of `charT`.

2. Change 21.4.2 [string.cons]/8 as indicated:

   >     basic_string(const charT* s, const Allocator& a = Allocator());

   > > -8- Requires: `s` shall not be a null pointer points to an array of at least `traits::length(s) + 1` elements of `charT`.

### 2239. `min/max/minmax` requirements

Section: 25.4.7 [alg.min.max] Status: WP Submitter: Juan Soulie Opened: 2013-01-26 Last modified: 2015-05-22

View all other issues in [alg.min.max].

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Discussion:

25.4.7 [alg.min.max] requires type `T` in `min`, `max`, and `minmax` to be `LessThanComparable`, but I don't believe this should be required for the versions that take a `Compare` argument.

Paragraphs 1 to 4 of 25.4 [alg.sorting] should apply anyway, although I'm not sure about `Compare` being required to induce a strict weak ordering here.

Further, `min` and `max` also lack formal complexity guarantees.

[2014-06-07 Daniel comments and provides wording]

Certainly, the functions with `Compare` should not impose `LessThanComparable` requirements.

In regard to the question whether a strict weak ordering should be required as implied by the `Compare` requirements, I would like to point out that this is requirement is in fact needed, because the specification of the normative Remarks elements (e.g. "Returns the first argument when the arguments are equivalent.") do depend on the existence of a equivalence relation that can be relied on and this is also consistent with the same strict weak ordering requirement that is indirectly imposed by the `LessThanComparable` requirement set for functions referring to `operator<` (Let me note that the very same `StrictWeakOrder` language concept had intentionally been required for similar reasons during "concept-time" in N2914).

[2015-02 Cologne]

JY: We have library-wide requirements that `Comp` induce a strict weak ordering.

JY/MC: The un-marked-up "Complexity" (p16) is wrong. DK: I'll fix that.

DK will update the wording for Lenexa.

[2015-03-30 Daniel comments]

The Complexity element of p16 is correct, but some others involving `initializer_list` arguments are wrong.

[2015-04-02 Library reflector vote]

The issue has been identified as Tentatively Ready based on six votes in favour.

Proposed resolution:

This wording is relative to N4296.

1. Change 25.4.7 [alg.min.max] as indicated:

   >     template<class T> constexpr const T& min(const T& a, const T& b);
   >     template<class T, class Compare>
   >       constexpr const T& min(const T& a, const T& b, Compare comp);

   > > -1- Requires: For the first form, type `T` shall be Type `T` is `LessThanComparable` (Table 18).

   > > -2- Returns: The smaller value.

   > > -3- Remarks: Returns the first argument when the arguments are equivalent.

   > > -?- Complexity: Exactly one comparison.

   >     template<class T>
   >       constexpr T min(initializer_list<T> t);
   >     template<class T, class Compare>
   >       constexpr T min(initializer_list<T> t, Compare comp);

   > > -4- Requires: `T` is `LessThanComparable` and shall be `CopyConstructible` and `t.size() > 0`. For the first form, type `T` shall be `LessThanComparable`.

   > > -5- Returns: […]

   > > -6- Remarks: […]

   > > -?- Complexity: Exactly `t.size() - 1` comparisons.

   >     template<class T> constexpr const T& max(const T& a, const T& b);
   >     template<class T, class Compare>
   >       constexpr const T& max(const T& a, const T& b, Compare comp);

   > > -7- Requires: For the first form, type `T` shall be Type `T` is `LessThanComparable` (Table 18).

   > > -8- Returns: […]

   > > -9- Remarks: […]

   > > -?- Complexity: Exactly one comparison.

   >     template<class T>
   >       constexpr T max(initializer_list<T> t);
   >     template<class T, class Compare>
   >       constexpr T max(initializer_list<T> t, Compare comp);

   > > -10- Requires: `T` is `LessThanComparable` and shall be `CopyConstructible` and `t.size() > 0`. For the first form, type `T` shall be `LessThanComparable`.

   > > -11- Returns: […]

   > > -12- Remarks: […]

   > > -?- Complexity: Exactly `t.size() - 1` comparisons.

   >     template<class T> constexpr pair<const T&, const T&> minmax(const T& a, const T& b);
   >     template<class T, class Compare>
   >       constexpr pair<const T&, const T&> minmax(const T& a, const T& b, Compare comp);

   > > -13- Requires: For the first form, t Type `T` shall be `LessThanComparable` (Table 18).

   > > -14- Returns: […]

   > > -15- Remarks: […]

   > > -16- Complexity: Exactly one comparison.

   >     template<class T>
   >       constexpr pair<T, T> minmax(initializer_list<T> t);
   >     template<class T, class Compare>
   >       constexpr pair<T, T> minmax(initializer_list<T> t, Compare comp);

   > > -17- Requires: `T` is `LessThanComparable` and shall be `CopyConstructible` and `t.size() > 0`. For the first form, type `T` shall be `LessThanComparable`.

   > > -18- Returns: […]

   > > -19- Remarks: […]

   > > -20- Complexity: At most `(3/2) * t.size()` applications of the corresponding predicate.

### 2240. Probable misuse of term "function scope" in [thread.condition]

Section: 30.5.1 [thread.condition.condvar], 30.5.2 [thread.condition.condvarany] Status: Resolved Submitter: FrankHB1989 Opened: 2013-02-03 Last modified: 2015-04-08

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Discussion:

All usages of "function scope" in 30.5.1 [thread.condition.condvar] and 30.5.2 [thread.condition.condvarany], such as 30.5.1 [thread.condition.condvar] p10 b4:

> If the function exits via an exception, lock.lock() shall be called prior to exiting the function scope.

seem to be inappropriate compared to the actual core language definition of 3.3.5 [basic.funscope]:

> Labels (6.1) have function scope and may be used anywhere in the function in which they are declared. Only labels have function scope.

Probably the intended meaning is "outermost block scope of the function".

[2013-09 Chicago: Resolved by proposed resolution of LWG 2135]

Proposed resolution:

Resolved by proposed resolution of LWG 2135.

### 2246. `unique_ptr` assignment effects w.r.t. deleter

Section: 20.8.1.2.3 [unique.ptr.single.asgn] Status: C++14 Submitter: Jonathan Wakely Opened: 2013-03-13 Last modified: 2015-04-08

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Discussion:

The Effects clauses for `unique_ptr` assignment don't make sense, what is the target of "an assignment from `std::forward<D>(u.get_deleter())`"?

Obviously it's intended to be the deleter, but that isn't stated clearly.

[2013-04-20, Bristol]

Move to Ready

[2013-09-29, Chicago]

Apply to Working Paper

Proposed resolution:

This wording is relative to N3485.

1. Edit 20.8.1.2.3 [unique.ptr.single.asgn] paragraph 2:

   >     unique_ptr& operator=(unique_ptr&& u) noexcept;

   > > […]

   > > -2- Effects: Transfers ownership from `u` to `*this` as if by calling `reset(u.release())` followed by an assignment from `get_deleter() = std::forward<D>(u.get_deleter())`.

2. Edit 20.8.1.2.3 [unique.ptr.single.asgn] paragraph 6:

   >     template <class U, class E> unique_ptr& operator=(unique_ptr<U, E>&& u) noexcept;

   > > […]

   > > -6- Effects: Transfers ownership from `u` to `*this` as if by calling `reset(u.release())` followed by an assignment from `get_deleter() = std::forward<E>(u.get_deleter())`.

### 2247. Type traits and `std::nullptr_t`

Section: 20.10.4.1 [meta.unary.cat] Status: C++14 Submitter: Joe Gottman Opened: 2013-03-15 Last modified: 2015-05-22

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Discussion:

According to 20.10.4.1 [meta.unary.cat], for every type `T`, exactly one of the primary type traits is true. So which is true for the type `std::nullptr_t`? By 2.13.7 [lex.nullptr] `std::nullptr_t` is not a pointer type or a pointer-to-member type, so `is_pointer`, `is_member_object_pointer` and `is_member_function_pointer` can't be true for `std::nullptr_t`, and none of the other primary type traits seem to apply.

[2013-04-20, Bristol]

Rename to `is_null_pointer`, move to Ready

Previous wording:

> This wording is relative to N3485.

> 1. Edit 20.10.2 [meta.type.synop], header `<type_traits>` synopsis:

>    >     namespace std {
>    >       […]
>    >       // 20.9.4.1, primary type categories:
>    >       template <class T> struct is_void;
>    >       template <class T> struct is_nullptr;
>    >       template <class T> struct is_integral;
>    >       template <class T> struct is_floating_point;
>    >       […]
>    >     }

> 2. Edit Table 47 — "Primary type category predicates" as indicated:

>    > Table 47 — Primary type category predicates
>    > Template | Condition | Comments
>    > --- | --- | ---
>    > `…`
>    > `template <class T> struct is_nullptr;` | `T` is `std::nullptr_t` ([basic.fundamental]) |
>    > `…`

[2013-09-29, Chicago]

Apply to the Working Paper

Proposed resolution:

This wording is relative to N3485.

1. Edit 20.10.2 [meta.type.synop], header `<type_traits>` synopsis:

   >     namespace std {
   >       […]
   >       // 20.9.4.1, primary type categories:
   >       template <class T> struct is_void;
   >       template <class T> struct is_null_pointer;
   >       template <class T> struct is_integral;
   >       template <class T> struct is_floating_point;
   >       […]
   >     }

2. Edit Table 47 — "Primary type category predicates" as indicated:

   > Table 47 — Primary type category predicates
   > Template | Condition | Comments
   > --- | --- | ---
   > `…`
   > `template <class T> struct is_null_pointer;` | `T` is `std::nullptr_t` ([basic.fundamental]) |
   > `…`

### 2249. [CD] Remove `gets` from `<cstdio>`

Section: 27.9.2 [c.files] Status: Resolved Submitter: Jonathan Wakely Opened: 2013-04-17 Last modified: 2015-04-08

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Discussion:

Addresses GB 9

In 27.9.2 [c.files] the current C++ standard claims that `<cstdio>` defines a function called "gets" but it has no declaration or semantics, because it was removed from C11, having been deprecated since C99. We should remove it for C++14.

[2013-09 Chicago]

Will resolve with the wording in the NB comment.

Proposed resolution:

Resolved by resolution as suggested by NB comment GB 9

### 2252. Strong guarantee on `vector::push_back()` still broken with C++11?

Section: 23.3.6.5 [vector.modifiers] Status: C++14 Submitter: Nicolai Josuttis Opened: 2013-04-21 Last modified: 2015-04-08

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Discussion:

According to my understanding, the strong guarantee of `push_back()` led to the introduction of `noexcept` and to the typical implementation that vectors usually copy their elements on reallocation unless the move operations of their element type guarantees not to throw.

However, if I read the standard correctly, we still don't give the strong guarantee any more: Yes, 23.2.1 [container.requirements.general]/10 specifies:

> Unless otherwise specified (see 23.2.4.1, 23.2.5.1, 23.3.3.4, and 23.3.6.5) all container types defined in this Clause meet the following additional requirements:

> - […]
> - if an exception is thrown by a `push_back()` or `push_front()` function, that function has no effects.

However, 23.3.6.5 [vector.modifiers] specifies for vector modifiers, including `push_back()`:

> If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of `T` or by any `InputIterator` operation there are no effects. If an exception is thrown by the move constructor of a non-`CopyInsertable` `T`, the effects are unspecified.

I would interpret this as an "otherwise specified" behavior for `push_back()`, saying that the strong guarantee is only given if constructors and assignments do not throw.

That means, the strong guarantee of C++03 is broken with C++11.

In addition to all that 23.2.1 [container.requirements.general] p10 b2 doesn't mention the corresponding functions `emplace_back()` and `emplace_front()`. These are similar single-element additions and should provide the same strong guarantee.

Daniel adds:

It seems the error came in when N2350 and N2345 became accepted and where integrated into the working draft N2369. The merge resulted in a form that changed the previous meaning and as far as I understand it, this effect was not intended.

[2013-09-16, Nico provides concrete wording]

[2013-09-26, Nico improves wording]

The new proposed resolution is driven as follows:

- In the container requirements section, there shall be general statements that single element insertions and `push_back()`, `pop_back`, `emplace_front()`, and `emplace_back()` have no effect on any exception.

  That is: we extend the first two statements by adding `emplace_front()` and `emplace_back()`, which are missing.

- Formulate only the exceptions from that (or where other general statements might lead to the impression, that the blanket statement no longer applies):

  - remove the statement in `list::push_back()` saying again that exceptions have to effect.

  - Clarify that all single-element insertions at either end of a `deque` have the strong guarantee.

  - Clarify that all single-element insertions at the end of a `vector` have the strong guarantee.

Proposed resolution:

This wording is relative to N3691.

1. Edit 23.2.1 [container.requirements.general] p10 b2 as indicated:

   - if an exception is thrown by an `insert()` or `emplace()` function while inserting a single element, that function has no effects.

   - if an exception is thrown by a `push_back()` or, `push_front()`, `emplace_back()`, or `emplace_front()` function, that function has no effects.

2. Edit 23.3.3.4 [deque.modifiers] as indicated:

   >     iterator insert(const_iterator position, const T& x);
   >     iterator insert(const_iterator position, T&& x);
   >     iterator insert(const_iterator position, size_type n, const T& x);
   >     template <class InputIterator>
   >       iterator insert(const_iterator position,
   >                       InputIterator first, InputIterator last);
   >     iterator insert(const_iterator position, initializer_list<T>);
   >     template <class... Args> void emplace_front(Args&&... args);
   >     template <class... Args> void emplace_back(Args&&... args);
   >     template <class... Args> iterator emplace(const_iterator position, Args&&... args);
   >     void push_front(const T& x);
   >     void push_front(T&& x);
   >     void push_back(const T& x);
   >     void push_back(T&& x);

   > > -1- Effects: An insertion in the middle of the deque invalidates all the iterators and references to elements of the deque. An insertion at either end of the deque invalidates all the iterators to the deque, but has no effect on the validity of references to elements of the deque.

   > > -2- Remarks: If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of `T` there are no effects. If an exception is thrown while inserting a single element at either end, there are no effects. If Otherwise, if an exception is thrown by the move constructor of a non-`CopyInsertable` `T`, the effects are unspecified.

   > > -3- Complexity: The complexity is linear in the number of elements inserted plus the lesser of the distances to the beginning and end of the deque. Inserting a single element either at the beginning or end of a deque always takes constant time and causes a single call to a constructor of `T`.

3. Edit 23.3.6.5 [vector.modifiers] as indicated:

   >     iterator insert(const_iterator position, const T& x);
   >     iterator insert(const_iterator position, T&& x);
   >     iterator insert(const_iterator position, size_type n, const T& x);
   >     template <class InputIterator>
   >       iterator insert(const_iterator position, InputIterator first, InputIterator last);
   >     iterator insert(const_iterator position, initializer_list<T>);
   >     template <class... Args> void emplace_back(Args&&... args);
   >     template <class... Args> iterator emplace(const_iterator position, Args&&... args);
   >     void push_back(const T& x);
   >     void push_back(T&& x);

   > > -1- Remarks: Causes reallocation if the new size is greater than the old capacity. If no reallocation happens, all the iterators and references before the insertion point remain valid. If an exception is thrown other than by the copy constructor, move constructor, assignment operator, or move assignment operator of `T` or by any `InputIterator` operation there are no effects. If an exception is thrown while inserting a single element at the end and `T` is `CopyInsertable` or `is_nothrow_move_constructible<T>::value` is `true`, there are no effects. If Otherwise, if an exception is thrown by the move constructor of a non-`CopyInsertable` `T`, the effects are unspecified.

   > > -2- Complexity: The complexity is linear in the number of elements inserted plus the distance to the end of the vector.

### 2257. Simplify container requirements with the new algorithms

Section: 23.2.1 [container.requirements.general] Status: C++14 Submitter: Marshall Clow Opened: 2013-05-29 Last modified: 2015-04-08

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Discussion:

Currently (n3690) Table 96 says, in the row for "`a == b`", that the Operational semantics are:

> `==` is an equivalence relation.
> `distance(a.begin(), a.end()) == distance(b.begin(), b.end()) && equal(a.begin(), a.end(),b.begin())`

Given the extension of equal for C++14, this can be simplified to:

> `==` is an equivalence relation.
> `equal(a.begin(), a.end(), b.begin(), b.end())`

[ Alisdair notes that a similar edit would apply to the unordered containers requirements. ]

Previous resolution from Marshall Clow:

> Ammend the Operational Semantics for 23.2.1 [container.requirements.general], Table 96, row "`a == b`"

> > `==` is an equivalence relation.
> > `distance(a.begin(), a.end()) == distance(b.begin(), b.end()) && equal(a.begin(), a.end(), b.begin(), b.end())`

> Ammend 23.2.5 [unord.req] p12:

> > Two unordered containers `a` and `b` compare equal if `a.size() == b.size()` and, for every equivalent-key group `[Ea1,Ea2)` obtained from `a.equal_range(Ea1)`, there exists an equivalent-key group `[Eb1,Eb2)` obtained from `b.equal_range(Ea1)`, such that `distance(Ea1, Ea2) == distance(Eb1, Eb2)` and `is_permutation(Ea1, Ea2, Eb1, Eb2)` returns `true`. For ...

[2013-09 Chicago]

Marshall improves wording

[2013-09 Chicago (evening issues)]

Moved to ready, after confirming latest wording reflects the discussion earlier in the day.

Proposed resolution:

1. Ammend 23.2.1 [container.requirements.general], Table 96 as indicated:

   > Table 96 — Container requirements (continued)
   > Expression | Return type | Operational semantics | Assertion/note pre-/post-condition | Complexity
   > --- | --- | --- | --- | ---
   > `…`
   > `a == b` | convertible to `bool` | `==` is an equivalence relation. `distance(a.begin(), a.end()) == distance(b.begin(), b.end()) && equal(a.begin(), a.end(), b.begin(), b.end())` | Requires: `T` is `EqualityComparable` | Constant if `a.size() != b.size()`, linear otherwise
   > `…`

2. Ammend 23.2.5 [unord.req] p12:

   > Two unordered containers `a` and `b` compare equal if `a.size() == b.size()` and, for every equivalent-key group `[Ea1,Ea2)` obtained from `a.equal_range(Ea1)`, there exists an equivalent-key group `[Eb1,Eb2)` obtained from `b.equal_range(Ea1)`, such that `distance(Ea1, Ea2) == distance(Eb1, Eb2)` and `is_permutation(Ea1, Ea2, Eb1, Eb2)` returns `true`. For […]

3. Amend 23.3.4.1 [forwardlist.overview] p2:

   > -2- A `forward_list` satisfies all of the requirements of a container (Table 96), except that the `size()` member function is not provided and `operator==` has linear complexity. […]

### 2258. `a.erase(q1, q2)` unable to directly return `q2`

Section: 23.2.4 [associative.reqmts] Status: C++14 Submitter: Geoff Alexander Opened: 2013-05-11 Last modified: 2015-04-08

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Discussion:

Section 23.2.4 [associative.reqmts], Table 102, page 743 of the C++ 2011 Standard states that `a.erase(q1, q2)` returns `q2`. The problem is that `a.erase(q1, q2)` cannot directly return `q2` as the return type, `iterator`, differs from that of `q2`, `const_iterator`.

[2013-09 Chicago (evening issues group)]

The wording looks good, but is worded slightly differently to how we say the same for sequence containers, and for unordered associative containers. We should apply consistent wording in all three cases.

Alisdair to provide the wording.

[2014-02-12 Issaquah meeting]

Move a Immediate.

Proposed resolution:

1. In the specification of `a.erase(q1, q2)` in sub-clause 23.2.4 [associative.reqmts], Table 102 change as indicated:

   > Table 102 — Associative container requirements (in addition to container) (continued)
   > Expression | Return type | Assertion/note pre-/post-condition | Complexity
   > --- | --- | --- | ---
   > `…`
   > `a.erase(q1, q2)` | `iterator` | erases all the elements in the range `[q1,q2)`. Returns `q2` an iterator pointing to the element pointed to by `q2` prior to any elements being erased. If no such element exists, `a.end()` is returned. | `log(a.size()) + N` where `N` has the value `distance(q1, q2)`.
   > `…`

### 2263. Comparing iterators and allocator pointers with different const-character

Section: 17.6.3.5 [allocator.requirements], 23.2 [container.requirements] Status: C++14 Submitter: Howard Hinnant Opened: 2013-06-25 Last modified: 2015-04-08

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Discussion:

This ancient issue 179 says one ought to be able to compare iterators with `const_iterators` from any given container. I'm having trouble finding words that guarantee this in C++11. This impacts not only a container's iterators, but also the allocator requirements in allocator.requirements] surrounding `pointer`, `const_pointer`, `void_pointer` and `const_void_pointer`. E.g. can one compare a `pointer` with a `const_pointer`?

Since `allocator::pointer` and `const_pointer` are required to be random access iterators, one could expect that the 179 guarantees apply for them as well.

[ Daniel comments: ]

The wording for 179 was part of several working drafts (e.g. also in N3092) over some time and suddenly got lost in N3242, presumably by accident. Whatever we decide for allocator pointers, I expect that we need to restore the 179 wording as part of the overall resolution:

Reinsert after 23.2 [container.requirements] p6:

> -6- `begin()` returns an iterator referring to the first element in the container. `end()` returns an iterator which is the past-the-end value for the container. If the container is empty, then `begin() == end()`;

> -?- In the expressions

> >     i == j
> >     i != j
> >     i < j
> >     i <= j
> >     i >= j
> >     i > j
> >     i - j

> where `i` and `j` denote objects of a container's `iterator` type, either or both may be replaced by an object of the container's `const_iterator` type referring to the same element with no change in semantics.

[2014-02-13 Issaquah, Daniel comments and suggests wording]

First, I didn't originally move the seemingly lost wording to the resolution section because I wanted to ensure that the committee double-checks the reason of this loss.

Second, albeit restoring this wording will restore the comparability of `const_iterator` and `iterator` of containers specified in Clause 23, but this alone would not imply that this guarantee automatically extends to all other iterators, simply because there is no fundamental relation between a mutable iterator and a constant iterator by itself. This relation only exists under specific conditions, for example for containers which provide two such `typedefs` of that kind. Thus the wording restoration would not ensure that allocator `pointer` and `const_pointer` would be comparable with each other. To realize that, we would need additional guarantees added to the allocator requirements. In fact, it is crucial to separate these things, because allocators are not restricted to be used within containers, they have their own legitimate use for other places as well (albeit containers presumably belong to the most important use-cases), and this is also stated in the introduction of 17.6.3.5 [allocator.requirements], where it says:

> All of the string types (Clause 21), containers (Clause 23) (except array), string buffers and string streams (Clause 27), and `match_results` (Clause 28) are parameterized in terms of allocators.

[2014-02-12 Issaquah meeting]

Move a Immediate.

Proposed resolution:

1. Insert after 17.6.3.5 [allocator.requirements] p4 as indicated:

   > -4- An allocator type `X` shall satisfy the requirements of `CopyConstructible` (17.6.3.1). The `X::pointer`, `X::const_pointer`, `X::void_pointer`, and `X::const_void_pointer` types shall satisfy the requirements of `NullablePointer` (17.6.3.3). No constructor, comparison operator, copy operation, move operation, or swap operation on these types shall exit via an exception. `X::pointer` and `X::const_pointer` shall also satisfy the requirements for a random access iterator (24.2).

   > -?- Let `x1` and `x2` denote objects of (possibly different) types `X::void_pointer`, `X::const_void_pointer`, `X::pointer`, or `X::const_pointer`. Then, `x1` and `x2` are equivalently-valued pointer values, if and only if both `x1` and `x2` can be explicitly converted to the two corresponding objects `px1` and `px2` of type `X::const_pointer`, using a sequence of `static_cast`s using only these four types, and the expression `px1 == px2` evaluates to `true`.

   > > Drafting note: This wording uses the seemingly complicated route via `X::const_pointer`, because these are (contrary to `X::const_void_pointer`) random access iterators and we can rely here for dereferenceable values on the fundamental pointee equivalence of 24.2.5 [forward.iterators] p6:

   > > > If `a` and `b` are both dereferenceable, then `a == b` if and only if `*a` and `*b` are bound to the same object.

   > > while for null pointer values we can rely on the special equality relation induced by 17.6.3.3 [nullablepointer.requirements].

   > -?- Let `w1` and `w2` denote objects of type `X::void_pointer`. Then for the expressions

   > >     w1 == w2
   > >     w1 != w2

   > either or both objects may be replaced by an equivalently-valued object of type `X::const_void_pointer` with no change in semantics.

   > -?- Let `p1` and `p2` denote objects of type `X::pointer`. Then for the expressions

   > >     p1 == p2
   > >     p1 != p2
   > >     p1 < p2
   > >     p1 <= p2
   > >     p1 >= p2
   > >     p1 > p2
   > >     p1 - p2

   > either or both objects may be replaced by an equivalently-valued object of type `X::const_pointer` with no change in semantics.

2. Reinsert after 23.2 [container.requirements] p6:

   > -6- `begin()` returns an iterator referring to the first element in the container. `end()` returns an iterator which is the past-the-end value for the container. If the container is empty, then `begin() == end()`;

   > -?- In the expressions

   > >     i == j
   > >     i != j
   > >     i < j
   > >     i <= j
   > >     i >= j
   > >     i > j
   > >     i - j

   > where `i` and `j` denote objects of a container's `iterator` type, either or both may be replaced by an object of the container's `const_iterator` type referring to the same element with no change in semantics.

### 2266. `vector` and `deque` have incorrect `insert` requirements

Section: 23.2.3 [sequence.reqmts] Status: WP Submitter: Ahmed Charles Opened: 2013-05-17 Last modified: 2015-04-08

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Discussion:

According to Table 100 in n3485 23.2.3 [sequence.reqmts]/4 the notes for the expression `a.insert(p,i,j)` say:

> Requires: `T` shall be `EmplaceConstructible` into `X` from `*i`. For `vector`, if the iterator does not meet the forward iterator requirements (24.2.5), `T` shall also be `MoveInsertable` into `X` and `MoveAssignable`.

> Each iterator in the range `[i,j)` shall be dereferenced exactly once.

> pre: `i` and `j` are not iterators into `a`.

> Inserts copies of elements in `[i, j)` before `p`

There are two problems with that wording: First, the special constraints for `vector`, that are expressed to be valid for forward iterators only, are necessary for all iterator categories. Second, the same special constraints are needed for `deque`, too.

[2013-10-05, Stephan T. Lavavej comments and provides alternative wording]

In Chicago, we determined that the original proposed resolution was correct, except that it needed additional requirements. When `vector` `insert(p, i, j)` is called with input-only iterators, it can't know how many elements will be inserted, which is obviously problematic for insertion anywhere other than at the end. Therefore, implementations typically append elements (geometrically reallocating), followed by `rotate()`. Given forward+ iterators, some implementations append and `rotate()` when they determine that there is sufficient capacity. Additionally, `deque` `insert(p, i, j)` is typically implemented with prepending/appending, with a possible call to `reverse()`, followed by a call to `rotate()`. Note that `rotate()`'s requirements are strictly stronger than `reverse()`'s.

Therefore, when patching Table 100, we need to add `rotate()`'s requirements. Note that this does not physically affect code (implementations were already calling `rotate()` here), and even in Standardese terms it is barely noticeable — if an element is `MoveInsertable` and `MoveAssignable` then it is almost certainly `MoveConstructible` and swappable. However, this patch is necessary to be strictly correct.

Previous resolution from Ahmed Charles:

> 1. Change Table 100 as indicated:

>    > Table 100 — Sequence container requirements (in addition to container) (continued)
>    > Expression | Return type | Assertion/note pre-/post-condition
>    > --- | --- | ---
>    > `…`

>    > `a.insert(p,i,j)`

>    > `iterator`

>    > Requires: `T` shall be `EmplaceConstructible` into `X` from `*i`. For `vector` and `deque`, if the iterator does not meet the forward iterator requirements (24.2.5), `T` shall also be `MoveInsertable` into `X` and `MoveAssignable`.
>    > Each iterator in the range `[i,j)` shall be dereferenced exactly once.
>    > pre: `i` and `j` are not iterators into `a`.
>    > Inserts copies of elements in `[i, j)` before `p`

>    > `…`

[2014-02-15 post-Issaquah session : move to Tentatively Ready]

Pablo: We might have gone too far with the fine-grained requirements. Typically these things come in groups.

Alisdair: I think the concepts folks assumed we would take their guidance.

Move to Tentatively Ready.

Proposed resolution:

1. Change Table 100 as indicated:

   > Table 100 — Sequence container requirements (in addition to container) (continued)
   > Expression | Return type | Assertion/note pre-/post-condition
   > --- | --- | ---
   > `…`

   > `a.insert(p,i,j)`

   > `iterator`

   > Requires: `T` shall be `EmplaceConstructible` into `X`
   > from `*i`. For `vector` and `deque`, if the iterator
   > does not meet the forward iterator requirements (24.2.5), `T` shall also be
   > `MoveInsertable` into `X`, `MoveConstructible`,
   > and `MoveAssignable`, and swappable (17.6.3.2 [swappable.requirements]).
   > Each iterator in the range `[i,j)` shall be dereferenced exactly once.
   > pre: `i` and `j` are not iterators into `a`.
   > Inserts copies of elements in `[i, j)` before `p`

   > `…`

### 2268. Setting a default argument in the declaration of a member function `assign` of `std::basic_string`

Section: 21.4 [basic.string] Status: C++14 Submitter: Vladimir Grigoriev Opened: 2013-06-26 Last modified: 2015-04-08

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Discussion:

Constructors and member functions `assign` of class `std::basic_string` have one to one relation (except the explicit constructor that creates an empty string). The following list shows this relation:

>     explicit basic_string(const Allocator& a = Allocator());

>     basic_string(const basic_string& str);
>     basic_string& assign(const basic_string& str);

>     basic_string(basic_string&& str) noexcept;
>     basic_string& assign(basic_string&& str) noexcept;

>     basic_string(const basic_string& str, size_type pos, size_type n = npos,
>       const Allocator& a = Allocator());
>     basic_string& assign(const basic_string& str, size_type pos,
>       size_type n);

>     basic_string(const charT* s,
>       size_type n, const Allocator& a = Allocator());
>     basic_string& assign(const charT* s, size_type n);

>     basic_string(const charT* s, const Allocator& a = Allocator());
>     basic_string& assign(const charT* s);

>     basic_string(size_type n, charT c, const Allocator& a = Allocator());
>     basic_string& assign(size_type n, charT c);

>     template<class InputIterator>
>     basic_string(InputIterator begin, InputIterator end,
>       const Allocator& a = Allocator());
>     template<class InputIterator>
>     basic_string& assign(InputIterator first, InputIterator last);

>     basic_string(initializer_list<charT>, const Allocator& = Allocator());
>     basic_string& assign(initializer_list<charT>);

So in fact any creating of an object of type `std::basic_string` using any of the above constructors except the explicit constructor can be substituted for creating a (possibly non-empty) string and then applying to it the corresponding method assign.

For example these two code snippets give the same result:

>     std::string s("Hello World");

and

>     std::string s;
>     s.assign("Hello World");

However there is one exception that has no a logical support. It is the pair of the following constructor and member function `assign`

>     basic_string(const basic_string& str, size_type pos, size_type n = npos,
>                  const Allocator& a = Allocator());

>     basic_string& assign(const basic_string& str, size_type pos, size_type n);

The third parameter of the constructor has a default argument while in the `assign` function it is absent. So it is impossible one to one to substitute the following code snippet

>     std::string s("Hello World");
>     std::string t(s, 6);

by

>     std::string s("Hello World");
>     std::string t;
>     t.assign(s, 6); // error: no such function

To get an equivalent result using the `assign` function the programmer has to complicate the code that is error-prone

>     std::string s("Hello World");
>     std::string t;
>     t.assign(s, 6, s.size() - 6);

To fix that, the declaration of the member function `assign` should be changed in such a way that its declaration would be fully compatible with the declaration of the corresponding constructor, that is to specify the same default argument for the third parameter of the `assign`.

The `assign` function is not the only function that requires to be revised.

Now let include in the list of pairs constructor-assign with the modified method `assign` one more member function `append`. We will get:

>     explicit basic_string(const Allocator& a = Allocator());

>     basic_string(const basic_string& str);
>     basic_string& assign(const basic_string& str);
>     basic_string& append(const basic_string& str);

>     basic_string(basic_string&& str) noexcept;
>     basic_string& assign(basic_string&& str) noexcept;

>     basic_string(const basic_string& str, size_type pos, size_type n = npos,
>       const Allocator& a = Allocator());
>     basic_string& assign(const basic_string& str, size_type pos,
>       size_type n);
>     basic_string& append(const basic_string& str, size_type pos,
>       size_type n);

>     basic_string(const charT* s,
>       size_type n, const Allocator& a = Allocator());
>     basic_string& assign(const charT* s, size_type n);
>     basic_string& append(const charT* s, size_type n);

>     basic_string(const charT* s, const Allocator& a = Allocator());
>     basic_string& assign(const charT* s);
>     basic_string& append(const charT* s);

>     basic_string(size_type n, charT c, const Allocator& a = Allocator());
>     basic_string& assign(size_type n, charT c);
>     basic_string& append(size_type n, charT c);

>     template<class InputIterator>
>     basic_string(InputIterator begin, InputIterator end,
>       const Allocator& a = Allocator());
>     template<class InputIterator>
>     basic_string& assign(InputIterator first, InputIterator last);
>     template<class InputIterator>
>     basic_string& append(InputIterator first, InputIterator last);

>     basic_string(initializer_list<charT>, const Allocator& = Allocator());
>     basic_string& assign(initializer_list<charT>);
>     basic_string& append(initializer_list<charT>);

As it seen from this record:

>     basic_string(const basic_string& str, size_type pos, size_type n = npos,
>       const Allocator& a = Allocator());
>     basic_string& assign(const basic_string& str, size_type pos,
>       size_type n);
>     basic_string& append(const basic_string& str, size_type pos,
>       size_type n);

it is obvious that the function `append` also should have the default argument that is that it should be declared as:

>     basic_string& append(const basic_string& str, size_type pos,
>       size_type n = npos);

In fact there is no a great difference in using `assign` or `append` especially when the string is empty:

>     std::string s("Hello World");
>     std::string t;
>     t.assign(s, 6);

>     std::string s("Hello World");
>     std::string t;
>     t.append(s, 6);

In both cases the result will be the same. So the `assign` and `append` will be interchangeable from the point of view of used arguments.

There are another three member functions in class `std::basic_string` that could be brought in conformity with considered above functions. They are member functions `insert`, `replace`, and `compare`.

So it is suggested to substitute the following declarations of `insert`, `replace`, and `compare`:

>     basic_string& insert(size_type pos1, const basic_string& str,
>       size_type pos2, size_type n);

>     basic_string& replace(size_type pos1, size_type n1,
>       const basic_string& str, size_type pos2, size_type n2);

>     int compare(size_type pos1, size_type n1,
>       const basic_string& str, size_type pos2, size_type n2) const;

by the declarations:

>     basic_string& insert(size_type pos1, const basic_string& str,
>       size_type pos2, size_type n = npos);

>     basic_string& replace(size_type pos1, size_type n1,
>       const basic_string& str, size_type pos2, size_type n2 = npos);

>     int compare(size_type pos1, size_type n1,
>       const basic_string& str, size_type pos2, size_type n2 = npos) const;

[2013-09 Chicago]

Howard: Are we positive this won't conflict with any other overloads?

They all appear to be unambiguous.

Alisdair: Ok, move to Ready.

Proposed resolution:

1. Change class template `basic_string` synopsis, 21.4 [basic.string] p5, as indicated:

   >     namespace std {
   >       template<class charT, class traits = char_traits<charT>,
   >         class Allocator = allocator<charT> >
   >       class basic_string {
   >       public:
   >         […]
   >         basic_string& append(const basic_string& str, size_type pos, size_type n = npos);
   >         […]
   >         basic_string& assign(const basic_string& str, size_type pos, size_type n = npos);
   >         […]
   >         basic_string& insert(size_type pos1, const basic_string& str, size_type pos2, size_type n = npos);
   >         […]
   >         basic_string& replace(size_type pos1, size_type n1, const basic_string& str, size_type pos2, size_type n2 = npos);
   >         […]
   >         int compare(size_type pos1, size_type n1, const basic_string& str, size_type pos2, size_type n2 = npos) const;
   >         […]
   >       };
   >     }

2. Change 21.4.6.2 [string::append] before p3 as indicated:

   >     basic_string& append(const basic_string& str, size_type pos, size_type n = npos);

3. Change 21.4.6.3 [string::assign] before p4 as indicated:

   >     basic_string& assign(const basic_string& str, size_type pos, size_type n = npos);

4. Change 21.4.6.4 [string::insert] before p5 as indicated:

   >     basic_string& insert(size_type pos1, const basic_string& str, size_type pos2, size_type n = npos);

5. Change 21.4.6.6 [string::replace] before p5 as indicated:

   >     basic_string& replace(size_type pos1, size_type n1, const basic_string& str, size_type pos2, size_type n2 = npos);

6. Change 21.4.7.9 [string::compare] before p4 as indicated:

   >     int compare(size_type pos1, size_type n1, const basic_string& str, size_type pos2, size_type n2 = npos) const;

### 2271. `regex_traits::lookup_classname` specification unclear

Section: 28.7 [re.traits] Status: C++14 Submitter: Jonathan Wakely Opened: 2013-07-02 Last modified: 2015-04-08

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Discussion:

28.7 [re.traits] p9 says that `regex_traits::lookup_classname` should return a value that compares equal to 0, but there is no requirement that a bitmask type is equality comparable with 0, e.g. 17.5.2.1.3 [bitmask.types] says bitmask types can be implemented using `std::bitset`.

Either there should be an additional requirement on the type or the function definition should be fixed.

[2013-09 Chicago]

Stefanus: Resolution looks good, doesn't seem to need fixing anywhere else from a quick look through the draft.

Any objection to Ready?

No objection.

Action: Move to Ready.

Proposed resolution:

This wording is relative to N3691.

1. Edit 28.7 [re.traits] p9:

   >     template <class ForwardIterator>
   >       char_class_type lookup_classname(
   >         ForwardIterator first, ForwardIterator last, bool icase = false) const;

   > > -9- Returns: an unspecified value that represents the character classification named by the character sequence designated by the iterator range `[first,last)`. If the parameter `icase` is true then the returned mask identifies the character classification without regard to the case of the characters being matched, otherwise it does honor the case of the characters being matched. The value returned shall be independent of the case of the characters in the character sequence. If the name is not recognized then returns a value that compares equal to 0 `char_class_type()`.

### 2272. `quoted` should use `char_traits::eq` for character comparison

Section: 27.7.6 [quoted.manip] Status: C++14 Submitter: Marshall Clow Opened: 2013-07-12 Last modified: 2015-04-08

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Discussion:

In 27.7.6 [quoted.manip] p2 b2:

> — Each character in `s`. If the character to be output is equal to `escape` or `delim`, as determined by `operator==`, first output `escape`.

In 27.7.6 [quoted.manip] p3 b1 sb1:

> — If the first character extracted is equal to `delim`, as determined by `operator==`, […]

these should both use `traits::eq`.

Also, I believe that 27.7.6 [quoted.manip] p3 implies that:

>     std::ostream _stream;
>     std::string _string;
>     _stream << _string;
>     _stream << quoted(_string);

should both compile, or both fail to compile, based on whether or not their `char_traits` match. But I believe that the standard should say that explicitly.

[ 2013-09 Chicago ]

Marshall Clow improved the wording with support from Stefanus.

[ 2013-09 Chicago (late night issues) ]

Moved to Ready, after confirming wording correctly reflects discussion earlier in the day.

Proposed resolution:

This wording is relative to N3691.

1. Change 27.7.6 [quoted.manip] p2+3 as indicated:

   >     template <class charT>
   >       unspecified quoted(const charT* s, charT delim=charT('"'), charT escape=charT('\\'));
   >     template <class charT, class traits, class Allocator>
   >       unspecified quoted(const basic_string<charT, traits, Allocator>& s,
   >                          charT delim=charT('"'), charT escape=charT('\\'));

   > > -2- Returns: An object of unspecified type such that if `out` is an instance of `basic_ostream` with member type `char_type` the same as `charT` and with member type `traits_type`, which in the second form is the same as `traits`, then the expression `out << quoted(s, delim, escape)` behaves as if it inserts the following characters into `out` using character inserter function templates (27.7.3.6.4), which may throw `ios_base::failure` (27.5.3.1.1):

   > > - `delim`

   > > - Each character in `s`. If the character to be output is equal to `escape` or `delim`, as determined by `operator==``traits_type::eq`, first output `escape`.

   > > - `delim`

   >     template <class charT, class traits, class Allocator>
   >       unspecified quoted(basic_string<charT, traits, Allocator>& s,
   >                          charT delim=charT('"'), charT escape=charT('\\'));

   > > -3- Returns: An object of unspecified type such that:

   > > - If `in` is an instance of `basic_istream` with member types `char_type` and `traits_type` the same as `charT` and `traits`, respectively, then the expression `in >> quoted(s, delim, escape)` behaves as if it extracts the following characters from in using `basic_istream::operator>>` (27.7.2.2.3) which may throw `ios_base::failure` (27.5.3.1.1):

   > >   - If the first character extracted is equal to `delim`, as determined by `operator==``traits_type::eq`, then: […]

   > > - If `out` is an instance of `basic_ostream` with member types `char_type` and `traits_type` the same as `charT` and `traits`, respectively, then the expression `out << quoted(s, delim, escape)` behaves as specified for the `const basic_string<charT, traits, Allocator>&` overload of the quoted function.

### 2274. Does `map::operator[]` value-initialize or default-insert a missing element?

Section: 23.4.4.3 [map.access], 23.5.4.3 [unord.map.elem] Status: Resolved Submitter: Andrzej Krzemieński Opened: 2013-07-16 Last modified: 2015-05-22

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Discussion:

Suppose that I provide a custom allocator for type `int`, that renders value 1 rather than 0 in default-insertion:

>     struct Allocator1 : std::allocator<int>
>     {
>       using super = std::allocator<int>;

>       template<typename Up, typename... Args>
>       void construct(Up* p, Args&&... args)
>       { super::construct(p, std::forward<Args>(args)...); }

>       template<typename Up>
>       void construct(Up* p)
>       { ::new((void*)p) Up(1); }
>     };

Now, if I use this allocator with `std::map`, and I use `operator[]` to access a not-yet-existent value, what value of the `mapped_type` should be created? 0 (value-initialization) or 1 (default-insertion):

>     map<string, int, less<string>, Allocator1> map;
>     cout << map["cat"];

N3960 is not very clear. 23.4.4.3 [map.access] in para 1 says:

> "If there is no key equivalent to `x` in the map, inserts `value_type(x, T())` into the map."

So, it requires value-initialization.

But para 2 says:

> "`mapped_type` shall be `DefaultInsertable` into `*this`."

This implies default-insertion, because if not, why the requirement. Also similar functions like `vector::resize` already require default-insertion wherever they put `DefaultInsertable` requirements.

Not to mention that default-insertion is more useful, because it allows custom allocators to "override" the default value of `mapped_type`.

[2013-09 Chicago]

Alisdair: Matters only for POD or trivial types

Marshall: issue might show up elsewhere other than `map<>`

Alisdair: initialize elements in any containers — by calling construct on allocator traits

Marshall: existing wording is clear

Alisdair: main concern is difference in wording, discusses default initialization

Nico: different requirement needed

Alisdair: gut is issue is NAD, brings up `DefaultInsertable` definition — discusses definition

Nico: why do we have the requirement?

Alisdair: other containers have this requirement

Marshall: this applies to many other containers

Nico: `deque<>` in particular

Alisdair: discusses allocator construct

Alisdair: wording raises concerns that aren't said in existing standard

Nico: sees no benefit to change

Marshall: leery of change

Alisdair: can be made clearer; might need to add note to `DefaultInsertable`; borderline editorial, comfortable without note, willing to wait until other issues arise. close issue as NAD

[2015-01-20: Tomasz Kamiński comments]

With the addition of the `try_emplace` method the behavior of the `operator[]` for the maps, may be defined as follows:

>     T& operator[](const key_type& x);

> > Effects: Equivalent to: `try_emplace(x).first->second`;

>     T& operator[](key_type&& x);

> > Effects: Equivalent to `try_emplace(std::move(x)).first->second`;

This would simplify the wording and also after resolution of the issue 2464, this wording would also address this issue.

[2015-02 Cologne]

Wait until 2464 and 2469 are in, which solve this.

[2015-05-06 Lenexa: This is resolved by 2469.]

Proposed resolution:

This wording is relative to N3691.

1. Change 23.4.4.3 [map.access] p1+p5 as indicated:

   >     T& operator[](const key_type& x);

   > > -1- Effects: If there is no key equivalent to `x` in the map, inserts `value_type(x, T())` into the map into the map a value with `key_type` initialized using expression `x` and `mapped_type` initialized by default-insertion.

   > > -2- Requires: `key_type` shall be `CopyInsertable` and `mapped_type` shall be `DefaultInsertable` into `*this`.

   > > […]

   >     T& operator[](key_type&& x);

   > > -5- Effects: If there is no key equivalent to `x` in the map, inserts `value_type(std::move(x), T())` into the map into the map a value with `key_type` initialized using expression `std::move(x)` and `mapped_type` initialized by default-insertion.

   > > -6- Requires: `mapped_type` shall be `DefaultInsertable` into `*this`.

2. Change 23.5.4.3 [unord.map.elem] p2 as indicated:

   >     mapped_type& operator[](const key_type& k);
   >     mapped_type& operator[](key_type&& k);

   > > -1- Requires: `mapped_type` shall be `DefaultInsertable` into `*this`. For the first operator, `key_type` shall be `CopyInsertable` into `*this`. For the second operator, `key_type` shall be `MoveConstructible`.

   > > -2- Effects: If the `unordered_map` does not already contain an element whose key is equivalent to `k`, the first operator inserts the value `value_type(k, mapped_type())`a value with `key_type` initialized using expression `x` and `mapped_type` initialized by default-insertion and the second operator inserts the value `value_type(std::move(k), mapped_type())`a value with `key_type` initialized using expression `std::move(x)` and `mapped_type` initialized by default-insertion.

### 2275. [CD] Why is `forward_as_tuple` not `constexpr`?

Section: 20.4.2.4 [tuple.creation] Status: C++14 Submitter: Marshall Clow Opened: 2013-07-30 Last modified: 2015-04-08

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Discussion:

Addresses ES 11

In n3471, a bunch of routines from header `<tuple>` were made `constexpr`.

`make_tuple/tuple_cat/get<>(tuple)/`relational operators — all these were "constexpr-ified".

But not `forward_as_tuple`.

Why not?

This was discussed in Portland, and STL opined that this was "an omission" (along with `tuple_cat`, which was added)

In discussion on c++std-lib@accu.org list, Pablo agreed that `forward_as_tuple` should be `constexpr`.

[2013-09 Chicago]

Moved to Immediate, this directly addresses an NB comment and the wording is non-controversial.

Accept for Working Paper

Proposed resolution:

This wording is relative to N3691.

1. Change header `<tuple>` synopsis, 20.4.1 [tuple.general] as indicated:

   >     template <class... Types>
   >       constexpr tuple<Types&&...> forward_as_tuple(Types&&...) noexcept;

2. Change 20.4.2.4 [tuple.creation] before p5 as indicated:

   >     template <class... Types>
   >       constexpr tuple<Types&&...> forward_as_tuple(Types&&... t) noexcept;

### 2278. User-defined literals for Standard Library types

Section: 20.12.2 [time.syn], 21.3 [string.classes] Status: C++14 Submitter: Howard Hinnant Opened: 2013-07-22 Last modified: 2015-04-08

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Discussion:

This paper adds user-defined literals for `string`, `complex` and `chrono` types. It puts each new literal signature in an inline namespace inside of std. Section 3.1 of the paper gives the rationale for doing this:

> As a common schema this paper proposes to put all suffixes for user defined literals in separate inline namespaces that are below the inline namespace `std::literals`. [Note: This allows a user either to do a `using namespace std::literals;` to import all literal operators from the standard available through header file inclusion, or to use `using namespace std::string_literals;` to just obtain the literals operators for a specific type. — end note]

This isn't how inline namespaces work.

7.3.1 [namespace.def]/p8 says in part:

> Members of an inline namespace can be used in most respects as though they were members of the enclosing namespace. Specifically, the inline namespace and its enclosing namespace are both added to the set of associated namespaces used in argument-dependent lookup (3.4.2) whenever one of them is, and a using- directive (7.3.4) that names the inline namespace is implicitly inserted into the enclosing namespace as for an unnamed namespace (7.3.1.1). […]

I.e. these literals will appear to the client to already be imported into namespace `std`. The rationale in the paper appears to indicate that this is not the intended behavior, and that instead the intended behavior is to require the user to say:

>     using namespace std::literals;

or:

>     using namespace std::literals::string_literals;

prior to use. To get this behavior non-inlined (normal) namespaces must be used.

Originally proposed resolution:

Strike the use of "inline" from each use associated with `literals`, `string_literals`, `chrono_literals`.

My opinion is that this must be done prior to publishing C++14, otherwise we are stuck with this (apparently unwanted) decision forever.

Marshall Clow:

> The rationale that I recall was that:

> 1. Users could write "`using namespace std::literals;`" to get all the literal suffixes, or

> 2. Users could write "`using namespace std::literals::string_literals;`" or "`using namespace std::literals::chrono_literals;`" to get a subset of the suffixes.

> To accomplish that, I believe that:

> 1. Namespace "`std::literals`" should not be `inline`

> 2. Namespaces "`std::literals::string_literals`" and "`std::literals::chrono_literals`" should be `inline`

Further details see also reflector message c++std-lib-34256.

Previous resolution from Marshall Clow:

> 1. Modify header `<chrono>` synopsis, 20.12.2 [time.syn], as indicated:

>    >     namespace std {
>    >     namespace chrono {
>    >     […]
>    >     } // namespace chrono
>    >     inline namespace literals {
>    >     inline namespace chrono_literals {
>    >     […]
>    >     } // namespace chrono_literals
>    >     } // namespace literals
>    >     } // namespace std

> 2. Modify header `<string>` synopsis, 21.3 [string.classes] p1, as indicated:

>    >     #include <initializer_list>

>    >     namespace std {
>    >     […]
>    >     inline namespace literals {
>    >     inline namespace string_literals {
>    >     […]
>    >     }
>    >     }
>    >     }

[2013-09 Chicago]

After a discussion about intent, the conclusion was that if you hoist a type with a "using" directive, then you should also get the associated literal suffixes with the type.

This is accomplished by marking namespace `std::literals` as `inline`, but for types in their own namespace inside `std`, then they will need to do this as well. The only case in the current library is `chrono`.

Marshall Clow provides alternative wording.

[2013-09 Chicago (late night issues)]

Moved to Ready, after confirming wording reflects the intent of the earlier discussion.

Proposed resolution:

This wording is relative to N3691.

1. Modify header `<chrono>` synopsis, 20.12.2 [time.syn], as indicated:

   >     namespace std {
   >     […]
   >     inline namespace literals {
   >     inline namespace chrono_literals {
   >     […]
   >     constexpr chrono::duration<unspecified , nano> operator "" ns(long double);

   >     } // namespace chrono_literals
   >     } // namespace literals

   >     namespace chrono {
   >         using namespace literals::chrono_literals;
   >     } // namespace chrono

   >     } // namespace std

### 2280. `begin/end` for arrays should be `constexpr` and `noexcept`

Section: 24.7 [iterator.range] Status: C++14 Submitter: Andy Sawyer Opened: 2013-08-22 Last modified: 2015-04-08

View other active issues in [iterator.range].

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Discussion:

The array forms of `std::begin` and `std::end` should be `constexpr` and `noexcept`.

Previous resolution from Andy Sawyer:

> 1. Edit header `<iterator>` synopsis, 24.3 [iterator.synopsis] as indicated:

>    >     […]
>    >     template <class T, size_t N> constexpr T* begin(T (&array)[N]) noexcept;
>    >     template <class T, size_t N> constexpr T* end(T (&array)[N]) noexcept;
>    >     template <class C> constexpr auto cbegin(const C& c) -> decltype(std::begin(c));
>    >     template <class C> constexpr auto cend(const C& c) -> decltype(std::end(c));
>    >     […]

> 2. Edit 24.7 [iterator.range] before p4+5 as indicated:

>    >     template <class T, size_t N> constexpr T* begin(T (&array)[N]) noexcept;

>    > > -4- Returns: `array`.

>    >     template <class T, size_t N> constexpr T* end(T (&array)[N]) noexcept;

>    > > -5- Returns: `array + N`.

>    >     template <class C> constexpr auto cbegin(const C& c) -> decltype(std::begin(c));

>    > > -6- Returns: `std::begin(c)`.

>    >     template <class C> constexpr auto cend(const C& c) -> decltype(std::end(c));

>    > > -7- Returns: `std::end(c)`.

[2013-09 Chicago]

Add `noexcept(noexcept(std::begin/end(c)))` to `cbegin` and `cend`, move to ready

Proposed resolution:

This wording is relative to N3797.

1. Edit header `<iterator>` synopsis, 24.3 [iterator.synopsis] as indicated:

   >     […]
   >     template <class T, size_t N> constexpr T* begin(T (&array)[N]) noexcept;
   >     template <class T, size_t N> constexpr T* end(T (&array)[N]) noexcept;
   >     template <class C> constexpr auto cbegin(const C& c) noexcept(noexcept(std::begin(c))) -> decltype(std::begin(c));
   >     template <class C> constexpr auto cend(const C& c) noexcept(noexcept(std::end(c))) -> decltype(std::end(c));
   >     […]

2. Edit 24.7 [iterator.range] before p4+5 as indicated:

   >     template <class T, size_t N> constexpr T* begin(T (&array)[N]) noexcept;

   > > -4- Returns: `array`.

   >     template <class T, size_t N> constexpr T* end(T (&array)[N]) noexcept;

   > > -5- Returns: `array + N`.

   >     template <class C> constexpr auto cbegin(const C& c) noexcept(noexcept(std::begin(c))) -> decltype(std::begin(c));

   > > -6- Returns: `std::begin(c)`.

   >     template <class C> constexpr auto cend(const C& c) noexcept(noexcept(std::end(c))) -> decltype(std::end(c));

   > > -7- Returns: `std::end(c)`.

### 2282. [fund.ts] Incorrect `is_assignable` constraint in `optional::op=(U&&)`

Section: X [optional.object.assign] Status: Resolved Submitter: Howard Hinnant Opened: 2013-08-25 Last modified: 2015-04-08

View all other issues in [optional.object.assign].

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Discussion:

Addresses: fund.ts

Minor wording nit in X [optional.object.assign]/p15:

>     template <class U> optional<T>& operator=(U&& v);

> > -15- Requires: `is_constructible<T, U>::value` is true and `is_assignable<U, T>::value` is true.

Should be:

>     template <class U> optional<T>& operator=(U&& v);

> > -15- Requires: `is_constructible<T, U>::value` is true and `is_assignable<T&, U>::value` is true.

[2013-09 Chicago:]

Move to Deferred. This feature will ship after C++14 and should be revisited then.

[2014-06-06 pre-Rapperswill]

This issue has been reopened as fundamentals-ts.

[2014-06-07 Daniel comments]

This issue should be set to Resolved, because the wording fix is already applied in the last fundamentals working draft.

[2014-06-16 Rapperswill]

Confirmed that this issue is resolved in the current Library Fundamentals working paper.

Proposed resolution:

This wording is relative to N3691.

1. Edit X [optional.object.assign] p15 as indicated:

   >     template <class U> optional<T>& operator=(U&& v);

   > > -15- Requires: `is_constructible<T, U>::value` is true and `is_assignable<U, TT&, U>::value` is true.

### 2283. [fund.ts] `optional` declares and then does not define an `operator<()`

Section: X [optional.comp_with_t] Status: Resolved Submitter: Howard Hinnant Opened: 2013-08-26 Last modified: 2015-04-08

View all issues with Resolved status.

Discussion:

Addresses: fund.ts

In [optional.syn] there is:

>     template <class T> constexpr bool operator<(const T&, const optional<T>&);

But I can find no definition for this signature.

[2013-09 Chicago:]

Move to Deferred. This feature will ship after C++14 and should be revisited then.

[2014-06-06 pre-Rapperswill]

This issue has been reopened as fundamentals-ts.

[2014-06-07 Daniel comments]

This issue should be set to Resolved, because the wording fix is already applied in the last fundamentals working draft.

[2014-06-16 Rapperswill]

Confirmed that this issue is resolved in the current Library Fundamentals working paper.

Proposed resolution:

This wording is relative to N3691.

1. Add to X [optional.comp_with_t]:

   >     template <class T> constexpr bool operator<(const T& v, const optional<T>& x);

   > > -?- Returns: `bool(x) ? less<T>{}(v, *x) : false`.

### 2284. Inconsistency in `allocator_traits::max_size`

Section: 20.7.8 [allocator.traits] Status: C++14 Submitter: Marshall Clow Opened: 2013-08-27 Last modified: 2015-04-08

View all issues with C++14 status.

Discussion:

Section 20.7.8 [allocator.traits] says:

>     static size_type max_size(const Alloc& a) noexcept;

Section 20.7.8.2 [allocator.traits.members] says:

>     static size_type max_size(Alloc& a) noexcept;

These should be the same.

Discussion:

Pablo (who I believe wrote the `allocator_traits` proposal) says "The function should take a `const` reference."

[2013-09 Chicago]

No objections, so moved to Immediate.

Accept for Working Paper

Proposed resolution:

This wording is relative to N3691.

1. Change 20.7.8.2 [allocator.traits.members] as follows:

   >     static size_type max_size(const Alloc& a) noexcept;

### 2285. `make_reverse_iterator`

Section: 24.5.1 [reverse.iterators] Status: C++14 Submitter: Zhihao Yuan Opened: 2013-08-27 Last modified: 2015-04-08

View all other issues in [reverse.iterators].

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Discussion:

We have `make_move_iterator`, but not `make_reverse_iterator`, which is also useful when dealing with some types without an `rbegin`/`rend` support (like, C strings).

[2013-09 Chicago]

Billy: reviewed it last night STL: has suggested prior, but denied for complexity

Billy: Alisdair wanted to review for `reverse(reverse())`;

STL: likes the issue, was like him

Stefanus: likes definitions, places where things should be

STL: for consistency with `make_move_iterator`

Stefanus: minor editorial issue - subdivision in these 2 sections is different from [move.iter]. See 24.5.3.3.14 [move.iter.nonmember]

STL: motion to move to Ready

Move to Ready

Proposed resolution:

This wording is relative to N3691.

1. Change header `<iterator>` synopsis, 24.3 [iterator.synopsis] as indicated:

   >     namespace std {
   >       […]
   >       template <class Iterator>
   >         reverse_iterator<Iterator> operator+(
   >           typename reverse_iterator<Iterator>::difference_type n,
   >           const reverse_iterator<Iterator>& x);

   >       template <class Iterator>
   >         reverse_iterator<Iterator> make_reverse_iterator(Iterator i);

   >     }

2. Change class template `reverse_iterator` synopsis, 24.5.1.1 [reverse.iterator] as indicated:

   >     namespace std {
   >       […]
   >       template <class Iterator>
   >         reverse_iterator<Iterator> operator+(
   >           typename reverse_iterator<Iterator>::difference_type n,
   >           const reverse_iterator<Iterator>& x);

   >       template <class Iterator>
   >         reverse_iterator<Iterator> make_reverse_iterator(Iterator i);

   >     }

3. After 24.5.1.3.20 [reverse.iter.opsum] add the following new sub-clause to 24.5.1.3 [reverse.iter.ops]:

   >     template <class Iterator>
   >       reverse_iterator<Iterator> make_reverse_iterator(Iterator i);

   > > -?- Returns: `reverse_iterator<Iterator>(i)`.

### 2287. [fund.ts] Incorrect exception safety for `optional` copy assignment operator

Section: X [optional.object.assign] Status: Resolved Submitter: Howard Hinnant Opened: 2013-08-16 Last modified: 2015-04-08

View all other issues in [optional.object.assign].

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Discussion:

Addresses: fund.ts

The Exception safety paragraph of X [optional.object.assign] calls out `T`'s copy constructor when it should refer to `T`'s copy assignment operator.

[2013-09 Chicago:]

Move to Deferred. This feature will ship after C++14 and should be revisited then.

[2014-06-06 pre-Rapperswill]

This issue has been reopened as fundamentals-ts.

[2014-06-07 Daniel comments]

This issue should be set to Resolved, because the wording fix is already applied in the last fundamentals working draft.

[2014-06-16 Rapperswill]

Confirmed that this issue is resolved in the current Library Fundamentals working paper.

Proposed resolution:

This wording is relative to N3691.

1. Change X [optional.object.assign] as indicated:

   >     optional<T>& operator=(const optional<T>& rhs);

   > > […]

   > > -8- Exception safety: If any exception is thrown, the values of `init` and `rhs.init` remain unchanged. If an exception is thrown during the call to `T`'s copy constructor, no effect. If an exception is thrown during the call to `T`'s copy assignment, the state of its contained value is as defined by the exception safety guarantee of `T`'s copy constructor assignment.

### 2288. Inconsistent requirements for shared mutexes

Section: 30.4.1.4 [thread.sharedmutex.requirements] Status: C++14 Submitter: Daniel Krügler Opened: 2013-08-30 Last modified: 2015-04-08

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Discussion:

Albeit shared mutex types refine timed mutex types, the requirements imposed on the corresponding required member function expressions are inconsistent in several aspects, most probably because failing synchronisation with wording changes for timed mutexes applied by some issues:

1. Due to acceptance of N3568 a wording phrase came in 30.4.1.4 [thread.sharedmutex.requirements] p26,

   > Effects: If the tick period of `rel_time` is not exactly convertible to the native tick period, the duration shall be rounded up to the nearest native tick period. […]

   while a very similar one had been removed for 30.4.1.3 [thread.timedmutex.requirements] by LWG 2091.

   Having this guaranteed effect for `try_lock_shared_for` but not for `try_lock_for` seems inconsistent and astonishing.

   If the actual intended restriction imposed onto the implementation is to forbid early wakeups here, we should ensure that to hold for timed mutex's `try_lock_for` as well. Note that the rationale provided for LWG 2091 was a potential late wakeup situation, but it seems that there is no implementation restriction that prevents early wakeups.

2. The shared-lock requirements for any `*lock*()` functions don't provide the guarantee that "If an exception is thrown then a lock shall not have been acquired for the current execution agent.". For other mutex types this guarantee can be derived from the corresponding `TimedLockable` requirements, but there are no `SharedLockable` requirements.

3. The shared-lock requirements for `*lock_for/_until()` functions require "Throws: Nothing." instead of "Throws: Timeout-related exceptions (30.2.4)." which had been added by LWG 2093, because user-provided clocks, durations, or time points may throw exceptions.

4. With the addition of `std::shared_mutex`, the explicit lists of 30.4.1.2 [thread.mutex.requirements.mutex] p7+15,

   > Requires: If `m` is of type `std::mutex` or `std::timed_mutex`, the calling thread does not own the mutex.

   and of 30.4.1.3 [thread.timedmutex.requirements] p4+11,

   > Requires: If `m` is of type `std::timed_mutex`, the calling thread does not own the mutex.

   are incomplete and should add the non-recursive `std::shared_mutex` as well.

[2014-02-16: Moved as Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Change 30.4.1.2 [thread.mutex.requirements.mutex] p7+15 as indicated:

   > -6- The expression `m.lock()` shall be well-formed and have the following semantics:

   > > -7- Requires: If `m` is of type `std::mutex` or, `std::timed_mutex`, or `std::shared_mutex`, the calling thread does not own the mutex.

   > […]

   > -14- The expression `m.try_lock()` shall be well-formed and have the following semantics:

   > > -15- Requires: If `m` is of type `std::mutex` or, `std::timed_mutex`, or `std::shared_mutex`, the calling thread does not own the mutex.

2. Change 30.4.1.3 [thread.timedmutex.requirements] p4+11 as indicated:

   > -3- The expression `m.try_lock_for(rel_time)` shall be well-formed and have the following semantics:

   > > -4- Requires: If `m` is of type `std::timed_mutex` or `std::shared_mutex`, the calling thread does not own the mutex.

   > […]

   > -10- The expression `m.try_lock_until(abs_time)` shall be well-formed and have the following semantics:

   > > -11- Requires: If `m` is of type `std::timed_mutex` or `std::shared_mutex`, the calling thread does not own the mutex.

3. Change 30.4.1.4 [thread.sharedmutex.requirements] as indicated:

   > -3- The expression `m.lock_shared()` shall be well-formed and have the following semantics:

   > > -4- Requires: The calling thread has no ownership of the mutex.

   > > -5- Effects: Blocks the calling thread until shared ownership of the mutex can be obtained for the calling thread. If an exception is thrown then a shared lock shall not have been acquired for the current thread.

   > > […]

   > -24- The expression `m.try_lock_shared_for(rel_time)` shall be well-formed and have the following semantics:

   > > -25- Requires: The calling thread has no ownership of the mutex.

   > > -26- Effects: If the tick period of `rel_time` is not exactly convertible to the native tick period, the duration shall be rounded up to the nearest native tick period. Attempts to obtain shared lock ownership for the calling thread within the relative timeout (30.2.4) specified by `rel_time`. If the time specified by `rel_time` is less than or equal to `rel_time.zero()`, the function attempts to obtain ownership without blocking (as if by calling `try_lock_shared()`). The function shall return within the timeout specified by `rel_time` only if it has obtained shared ownership of the mutex object. [Note: As with `try_lock()`, there is no guarantee that ownership will be obtained if the lock is available, but implementations are expected to make a strong effort to do so. — end note] If an exception is thrown then a shared lock shall not have been acquired for the current thread.

   > > […]

   > > -30- Throws: Nothing Timeout-related exceptions (30.2.4 [thread.req.timing]).

   > -31- The expression `m.try_lock_shared_until(abs_time)` shall be well-formed and have the following semantics:

   > > -32- Requires: The calling thread has no ownership of the mutex.

   > > -33- Effects: The function attempts to obtain shared ownership of the mutex. If `abs_time` has already passed, the function attempts to obtain shared ownership without blocking (as if by calling `try_lock_shared()`). The function shall return before the absolute timeout (30.2.4) specified by `abs_time` only if it has obtained shared ownership of the mutex object. [Note: As with `try_lock()`, there is no guarantee that ownership will be obtained if the lock is available, but implementations are expected to make a strong effort to do so. — end note] If an exception is thrown then a shared lock shall not have been acquired for the current thread.

   > > […]

   > > -37- Throws: Nothing Timeout-related exceptions (30.2.4 [thread.req.timing]).

### 2291. `std::hash` is vulnerable to collision DoS attack

Section: 17.6.3.4 [hash.requirements] Status: C++14 Submitter: Zhihao Yuan Opened: 2013-09-02 Last modified: 2015-04-08

View all other issues in [hash.requirements].

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Discussion:

For a non-cryptographic hash function, it's possible to pre-calculate massive inputs with the same hashed value to algorithmically slow down the unordered containers, and results in a denial-of-service attack. Many languages with built-in hash table support have fixed this issue. For example, Perl has universal hashing, Python 3 uses salted hashes.

However, for C++, in 17.6.3.4 [hash.requirements] p2, Table 26:

> The value returned shall depend only on the argument `k`. [Note: Thus all evaluations of the expression `h(k)` with the same value for `k` yield the same result. — end note]

The wording is not clear here: does that mean all the standard library implementations must use the same hash function for a same type? Or it is not allowed for an implementation to change its hash function?

I suggest to explicitly allow the salted hash functions.

[2013-09 Chicago]

Moved to Ready.

There is some concern that the issue of better hashing, especially standardizing any kind of secure hashing, is a feature that deserves attention in LEWG

The proposed resolution is much simpler than the larger issue though, merely clarifying a permission that many implementers believe they already have, without mandating a change to more straight forward implementations.

Move to Ready, rather than Immediate, as even the permission has been contentious in reflector discussion, although the consensus in Chicago is to accept as written unless we hear a further strong objection.

Proposed resolution:

This wording is relative to N3691.

1. Edit 17.6.3.4 [hash.requirements] p2, Table 26, as indicated: [Editorial note: We can consider adding some additional guideline here. Unlike N3333, this proposed change makes the hashing per-execution instead of per-process. The standard does not discuss OS processes. And, practically, a per-process hashing makes a program unable to share an unordered container to a child process. — end note ]

   > Table 26 — Hash requirements [hash]
   > Expression | Return type | Requirement
   > --- | --- | ---
   > `h(k)` | `size_t` | The value returned shall depend only on the argument `k` for the duration of the program. [Note: Thus all evaluations of the expression `h(k)` with the same value for `k` yield the same result for a given execution of the program. — end note]
   > `…`

### 2293. Wrong facet used by `num_put::do_put`

Section: 22.4.2.2.2 [facet.num.put.virtuals] Status: C++14 Submitter: Juan Soulie Opened: 2013-09-04 Last modified: 2015-04-08

View all other issues in [facet.num.put.virtuals].

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Discussion:

At the end of 22.4.2.2.2 [facet.num.put.virtuals] (in p6), the return value is said to be obtained by calling `truename` or `falsename` on the wrong facet: `ctype` should be replaced by `numpunct`.

[Issaquah 20014-10-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Edit 22.4.2.2.2 [facet.num.put.virtuals] p6 as indicated:

   > -6- Returns: If `(str.flags() & ios_base::boolalpha) == 0` returns `do_put(out, str, fill, (int)val)`, otherwise obtains a string `s` as if by

   > >     string_type s =
   > >       val ? use_facet<ctypenumpunct<charT> >(loc).truename()
   > >           : use_facet<ctypenumpunct<charT> >(loc).falsename();

   > and then inserts each character c of s into out via *out++ = c and returns out.

### 2298. [CD] `is_nothrow_constructible` is always false because of `create<>`

Section: 20.10.4.3 [meta.unary.prop] Status: C++14 Submitter: Daniel Krügler Opened: 2013-09-24 Last modified: 2015-04-08

View other active issues in [meta.unary.prop].

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Discussion:

Addresses US 18

The trait `is_constructible<T, Args...>` is defined in terms of a helper template, `create<>`, that is identical to `std::declval<>` except for the latter's `noexcept` clause.

If the absence of `noexcept` is critical to this definition, insert a Note of explanation; otherwise, excise `create<>` and reformulate in terms of `declval<>` the definition of `is_constructible`.

[2013-09-24 Daniel comments and provides resolution suggestion]

Replacing `create<>` by `std::declval<>` would make the situation worse, because the definition of `is_constructible` is based on a well-formed variable definition and there is no way to specify a variable definition without odr-using its initializer arguments. It should also be added, that there is another problem with the specification of all existing `is_trivially_*` traits, because neither `create<>` nor `std::declval<>` are considered as trivial functions, but this should be solved by a different issue.

[2013-09-26 Nico improves wording]

The additional change is just to keep both places were `create()` is defined consistent.

[2013-09 Chicago]

No objections, so moved to Immediate.

Accept for Working Paper

Proposed resolution:

This wording is relative to N3691.

1. Change 20.10.4.3 [meta.unary.prop] around p6 as indicated:

   > -6- Given the following function prototype:

   > >     template <class T>
   > >       typename add_rvalue_reference<T>::type create() noexcept;

   > the predicate condition for a template specialization `is_constructible<T, Args...>` shall be satisfied if and only if the following variable definition would be well-formed for some invented variable `t`:

   > >     T t(create<Args>()...);

   > […]

2. Change 20.10.4.3 [meta.unary.prop] around p4 as indicated:

   > -4- Given the following function prototype:

   > >     template <class T>
   > >       typename add_rvalue_reference<T>::type create() noexcept;

   > the predicate condition for a template specialization `is_convertible<From, To>` shall be satisfied if and only if the return expression in the following code would be well-formed, including any implicit conversions to the return type of the function:

   > >     To test() {
   > >       return create<From>();
   > >     }

   > […]

### 2299. [CD] Effects of inaccessible `key_compare::is_transparent` type are not clear

Section: 23.2.4 [associative.reqmts] Status: C++14 Submitter: Daniel Krügler Opened: 2013-09-24 Last modified: 2015-04-08

View other active issues in [associative.reqmts].

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Discussion:

Addresses ES 16

The condition "`X::key_compare::is_transparent` exists" does not specify that the type be publicly accessible.

Consider the public accessibility of `X::key_compare::is_transparent` and whether its potential inaccessibility should be banned for a compliant `key_compare` type.

[2013-09-24 Daniel provides resolution suggestion]

[2013-09-25 Chicago]

Daniel's wording is good, advance to Immediate to respond to NB comment.

[2013-09-26 Chicago]

Moved back to Review as Daniel would like another look at the words, and to confirm implementability.

Previous resolution from Daniel [SUPERSEDED]:

> 1. Change 23.2.4 [associative.reqmts] p8 as indicated:

>    > -8- In Table 102, `X` denotes an associative container class, `a` denotes a value of `X`, `a_uniq` denotes a value of `X` when `X` supports unique keys, `a_eq` denotes a value of `X` when `X` supports multiple keys, `a_tran` denotes a value of `X` when the a publicly accessible type `X::key_compare::is_transparent` exists whose name is unambiguous and not hidden, […]

> 2. Change 23.2.4 [associative.reqmts] p13 as indicated:

>    > The member function templates `find`, `count`, `lower_bound`, `upper_bound`, and `equal_range` shall not participate in overload resolution unless the a publicly accessible type `Compare::is_transparent` exists whose name is unambiguous and not hidden.

[2014-02-10 Daniel comments provides alternative wording]

I could confirm that my previous concerns were unwarranted, because they turned out to be due to a compiler-bug. Nonetheless I would suggest to replace the previously suggested replication of core-wording situations (access, ambiguity, hidden) by a single more robust phrase based on "valid type".

[2014-02-12 Issaquah: Move to Immediate]

STL: This uses "valid type", which is a Phrase Of Power in Core, and Daniel has a citation for the term.

Jonathan: It's nice to rely on Core.

Proposed resolution:

This wording is relative to N3797.

1. Change 23.2.4 [associative.reqmts] p8 as indicated:

   > -8- In Table 102, `X` denotes an associative container class, `a` denotes a value of `X`, `a_uniq` denotes a value of `X` when `X` supports unique keys, `a_eq` denotes a value of `X` when `X` supports multiple keys, `a_tran` denotes a value of `X` when the type qualified-id `X::key_compare::is_transparent` exists is valid and denotes a type (14.8.2 [temp.deduct]), […]

2. Change 23.2.4 [associative.reqmts] p13 as indicated:

   > The member function templates `find`, `count`, `lower_bound`, `upper_bound`, and `equal_range` shall not participate in overload resolution unless the type qualified-id `Compare::is_transparent` exists is valid and denotes a type (14.8.2 [temp.deduct]).

### 2300. [CD] Redundant sections for `map` and `multimap` members should be removed

Section: X [map.ops], X [multimap.ops] Status: C++14 Submitter: Daniel Krügler Opened: 2013-09-25 Last modified: 2015-04-08

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Discussion:

Addresses ES 17

Sections are redundant with general associative container requirements at 23.2.4 [associative.reqmts], Table 102.

Suggested action:

Delete sections.

[2013-09-25 Daniel provides resolution suggestion]

[2013-09-25 Chicago]

Daniel's wording is good, move to Immediate to resolve NB comment.

[2013-09-29 Chicago]

Accept for Working Paper

Proposed resolution:

This wording is relative to N3691.

1. Change the header `<map>` synopsis, 23.4.4.1 [map.overview] p2 as indicated:

   >     // 23.4.4.5, map operations:
   >     iterator find(const key_type& x);
   >     const_iterator find(const key_type& x) const;
   >     template <class K> iterator find(const K& x);
   >     template <class K> const_iterator find(const K& x) const;

2. Delete the complete sub-clause X [map.ops]:

   23.4.4.5 map operations [map.ops]

   >     iterator find(const key_type& x);
   >     const_iterator find(const key_type& x) const;
   >     iterator lower_bound(const key_type& x);
   >     const_iterator lower_bound(const key_type& x) const;
   >     iterator upper_bound(const key_type& x);
   >     const_iterator upper_bound(const key_type &x) const;
   >     pair<iterator, iterator>
   >       equal_range(const key_type &x);
   >     pair<const_iterator, const_iterator>
   >       equal_range(const key_type& x) const;

   > > -1- The `find`, `lower_bound`, `upper_bound` and `equal_range` member functions each have two versions, one const and the other non-const. In each case the behavior of the two functions is identical except that the const version returns a `const_iterator` and the non-const version an `iterator` (23.2.4).

3. Delete the complete sub-clause X [multimap.ops]:

   23.4.5.4 multimap operations [multimap.ops]

   >     iterator find(const key_type &x);
   >     const_iterator find(const key_type& x) const;

   >     iterator lower_bound(const key_type& x);
   >     const_iterator lower_bound(const key_type& x) const;

   >     pair<iterator, iterator>
   >       equal_range(const key_type &x);
   >     pair<const_iterator, const_iterator>
   >       equal_range(const key_type& x) const;

   > > -1- The `find`, `lower_bound`, `upper_bound` and `equal_range` member functions each have two versions, one const and one non-const. In each case the behavior of the two versions is identical except that the const version returns a `const_iterator` and the non-const version an `iterator` (23.2.4).

### 2301. Why is `std::tie` not `constexpr`?

Section: 20.4.2.4 [tuple.creation] Status: C++14 Submitter: Rein Halbersma Opened: 2013-09-11 Last modified: 2015-04-08

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Discussion:

In N3471, a bunch of routines from header `<tuple>` were made `constexpr`.

`make_tuple/tuple_cat/get<>(tuple)/`relational operators — all these were "`constexpr`-ified".

But not `tie`. This is similar to Issue 2275, where the same observation was made about `forward_as_tuple`.

[2014-02-13 Issaquah: Move as Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Change the header `<tuple>` synopsis, 20.4.1 [tuple.general] p2 as indicated:

   >     template<class... Types>
   >       constexpr tuple<Types&...> tie(Types&...) noexcept;

2. Change 20.4.2.4 [tuple.creation] around p7 as indicated:

   >     template<class... Types>
   >       constexpr tuple<Types&...> tie(Types&... t) noexcept;

### 2304. Complexity of `count` in unordered associative containers

Section: 23.2.5 [unord.req] Status: C++14 Submitter: Joaquín M López Muñoz Opened: 2013-09-20 Last modified: 2015-04-08

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Discussion:

Table 103 in 23.2.5 [unord.req] states that the complexity of `b.count(k)` is average case 𝒪(`1`) rather than linear with the number of equivalent elements, which seems to be a typo as this requires holding an internal count of elements in each group of equivalent keys, something which hardly looks the intent of the standard and no (known by the submitter) stdlib implementation is currently doing.

[Issaquah 20014-10-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Change Table 103 as indicated:

   > Table 103 — Unordered associative container requirements (in addition to container)
   > Expression | Return type | Assertion/note pre-/post-condition | Complexity
   > --- | --- | --- | ---
   > `…`
   > `b.count(k)` | `size_type` | Returns the number of elements with key equivalent to `k`. | Average case 𝒪(`1b.count(k)`), worst case 𝒪(`b.size()`).
   > `…`

### 2306. `match_results::reference` should be `value_type&`, not `const value_type&`

Section: 28.10 [re.results] Status: C++14 Submitter: Matt Austern Opened: 2013-09-25 Last modified: 2015-04-08

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Discussion:

The match_results class synopsis has

>     typedef const value_type& const_reference;
>     typedef const_reference reference;

We're getting too enthusiastic about types here by insisting that `reference` is a const reference, even though `match_results` is a read-only container. In the container requirements table (Table 96, in section 23.2.1 [container.requirements.general] we say that `Container::reference` is "lvalue of `T`" and `Container::const_reference` is "const lvalue of `T`".

That phrasing in the container requirements table is admittedly a little fuzzy and ought to be clarified (as discussed in lwg issue 2182), but in context it's clear that `Container::reference` ought to be a `T&` even for constant containers. In the rest of Clause 23 we see that `Container::reference` is `T&`, not `const T&`, even for const-qualified containers and that it's `T&`, not `const T&`, even for containers like `set` and `unordered_set` that provide const iterators only.

The way we handle const containers is just that in the case of a const-qualified container (including `match_results`) there are no operations that return `Container::reference`. That's already the case, so this issue is complaining about an unused typedef.

[2013-10-17: Daniel comments]

The `std::initializer_list` synopsis, 18.9 [support.initlist] shows a similar problem:

>     template<class E> class initializer_list {
>     public:
>       typedef E value_type;
>       typedef const E& reference;
>       typedef const E& const_reference;
>       […]
>     }

Given the fact that `std::initializer_list` doesn't meet the container requirements anyway (and is such a core-language related type) I recommend to stick with the current state.

[Issaquah 20014-10-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Change the class template `match_results` header synopsis, 28.10 [re.results] p4 as indicated:

   >     typedef const value_type& const_reference;
   >     typedef const_referencevalue_type& reference;

### 2308. Clarify container destructor requirements w.r.t. `std::array`

Section: 23.2.1 [container.requirements.general] Status: C++14 Submitter: Jonathan Wakely Opened: 2013-09-26 Last modified: 2015-04-08

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Discussion:

It has been suggested that Table 96 — "Container requirements" makes confusing requirements for the destructor of `std::array`:

"note: the destructor is applied to every element of `a`; all the memory is deallocated."

Since `std::array` obtains no memory, there is none to deallocate, arguably making it unclear what the requirement means for `std::array::~array()`.

[Issaquah 20014-10-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Change in 23.2.1 [container.requirements.general], Table 96 — "Container requirements", the "Assertion/note/pre-/post-condition" for the expression "`(&a)->~X()`" as indicated:

   > note: the destructor is applied to every element of `a`; all the any memory obtained is deallocated.

### 2313. `tuple_size` should always derive from `integral_constant<size_t, N>`

Section: 20.4.2.5 [tuple.helper] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2015-04-08

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Discussion:

In 20.4.2.5 [tuple.helper], the "primary template" is depicted as:

>     template <class... Types>
>     class tuple_size<tuple<Types...> >
>       : public integral_constant<size_t, sizeof...(Types)> { };

However, 20.3.4 [pair.astuple]/1-2 and 23.3.2.9 [array.tuple]/1-2 are underspecified, saying:

>     tuple_size<pair<T1, T2> >::value

> > Returns: Integral constant expression.

> > Value: `2`.

>     tuple_size<array<T, N> >::value

> > Return type: integral constant expression.

> > Value: `N`

They should be required to behave like the "primary template". This is more than a stylistic decision — it allows `tuple_size` to be passed to a function taking `integral_constant`.

LWG 1118 noticed this underspecification, but instead of correcting it, the resolution changed 20.4.2.5 [tuple.helper]/3 to require `tuple_size<cv T>` to derive from `integral_constant<remove_cv<decltype(TS::value)>::type, TS::value>`. This is unnecessarily overgeneralized. `tuple_size` is primarily for tuples, where it is required to be `size_t`, and it has been extended to handle pairs and arrays, which (as explained above) should also be guaranteed to be `size_t`. `tuple_size<cv T>` works with cv-qualified tuples, pairs, arrays, and user-defined types that also want to participate in the `tuple_size` system. It would be far simpler and perfectly reasonable to expect that user-defined types supporting the "tuple-like protocol" should have `tuple_sizes` of `size_t`.

[Issaquah 20014-10-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Edit 20.3.4 [pair.astuple]/1-2 as indicated:

   >     tuple_size<pair<T1, T2> >::value
   >     template <class T1, class T2>
   >     struct tuple_size<pair<T1, T2>>
   >       : integral_constant<size_t, 2> { };

   > > -1- Returns: Integral constant expression.

   > > -2- Value: `2`.

2. Edit 23.3.2.9 [array.tuple]/1-2 as indicated:

   >     tuple_size<array<T, N> >::value
   >     template <class T, size_t N>
   >     struct tuple_size<array<T, N>>
   >       : integral_constant<size_t, N> { };

   > > -1- Returns: Integral constant expression.

   > > -2- Value: `N`.

3. Edit 20.4.2.5 [tuple.helper]/p1-p3 as indicated:

   >     template <class T> struct tuple_size;

   > > -?- Remarks: All specializations of `tuple_size<T>` shall meet the `UnaryTypeTrait` requirements (20.10.1 [meta.rqmts]) with a `BaseCharacteristic` of `integral_constant<size_t, N>` for some `N`.

   >     template <class... Types>
   >     struct tuple_size<tuple<Types...> >
   >       : integral_constant<size_t, sizeof...(Types)> { };

   >     template <size_t I, class... Types>
   >     class tuple_element<I, tuple<Types...> > {
   >     public:
   >       typedef TI type;
   >     };

   > > -1- Requires: `I < sizeof...(Types)`. The program is ill-formed if `I` is out of bounds.

   > > […]

   >     template <class T> class tuple_size<const T>;
   >     template <class T> class tuple_size<volatile T>;
   >     template <class T> class tuple_size<const volatile T>;

   > > -3- Let TS denote `tuple_size<T>` of the cv-unqualified type `T`. Then each of the three templates shall meet the `UnaryTypeTrait` requirements (20.10.1 [meta.rqmts]) with a `BaseCharacteristic` of

   > > >     integral_constant<remove_cv<decltype(TS::value)>::typesize_t, TS::value>

### 2314. `apply()` should return `decltype(auto)` and use `decay_t` before `tuple_size`

Section: 20.5.1 [intseq.general] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2015-04-08

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Discussion:

The example in 20.5.1 [intseq.general]/2 depicts `apply_impl()` and `apply()` as returning `auto`. This is incorrect because it will trigger decay and will not preserve `F`'s return type. For example, if invoking the functor returns `const int&`, `apply_impl()` and `apply()` will return `int`. `decltype(auto)` should be used for "perfect returning".

Additionally, this depicts `apply()` as taking `Tuple&&`, then saying "`std::tuple_size<Tuple>::value`". This is incorrect because when `apply()` is called with lvalue tuples, perfect forwarding will deduce `Tuple` to be cv `tuple&`, but 20.4.2.5 [tuple.helper] says that `tuple_size` handles only cv `tuple`, not references to tuples. Using `remove_reference_t` would avoid this problem, but so would `decay_t`, which has a significantly shorter name. (The additional transformations that `decay_t` does are neither beneficial nor harmful here.)

[Issaquah 20014-10-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Edit the example code in 20.5.1 [intseq.general]/2 as indicated:

   >     template<class F, class Tuple, std::size_t... I>
   >       autodecltype(auto) apply_impl(F&& f, Tuple&& t, index_sequence<I...>) {
   >         return std::forward<F>(f)(std::get<I>(std::forward<Tuple>(t))...);
   >       }
   >     template<class F, class Tuple>
   >       autodecltype(auto) apply(F&& f, Tuple&& t) {
   >         using Indices = make_index_sequence<std::tuple_size<std::decay_t<Tuple>>::value>;
   >         return apply_impl(std::forward<F>(f), std::forward<Tuple>(t), Indices());
   >       }

### 2315. `weak_ptr` should be movable

Section: 20.8.2.3 [util.smartptr.weak] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2015-04-08

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Discussion:

Like `shared_ptr`, `weak_ptr` should be movable to avoid unnecessary atomic increments/decrements of the weak refcount.

[2014-02-13 Issaquah: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Edit 20.8.2.3 [util.smartptr.weak]/1, class template `weak_ptr` synopsis, as indicated:

   >     namespace std {
   >       template<class T> class weak_ptr {
   >       public:
   >         typedef T element_type;

   >         // 20.9.2.3.1, constructors
   >         constexpr weak_ptr() noexcept;
   >         template<class Y> weak_ptr(shared_ptr<Y> const& r) noexcept;
   >         weak_ptr(weak_ptr const& r) noexcept;
   >         template<class Y> weak_ptr(weak_ptr<Y> const& r) noexcept;
   >         weak_ptr(weak_ptr&& r) noexcept;
   >         template<class Y> weak_ptr(weak_ptr<Y>&& r) noexcept;

   >         […]

   >         // 20.9.2.3.3, assignment
   >         weak_ptr& operator=(weak_ptr const& r) noexcept;
   >         template<class Y> weak_ptr& operator=(weak_ptr<Y> const& r) noexcept;
   >         template<class Y> weak_ptr& operator=(shared_ptr<Y> const& r) noexcept;
   >         weak_ptr& operator=(weak_ptr&& r) noexcept;
   >         template<class Y> weak_ptr& operator=(weak_ptr<Y>&& r) noexcept;
   >       };
   >     }

2. Add the following new paragraphs at the end of sub-clause 20.8.2.3.1 [util.smartptr.weak.const]:

   >     weak_ptr(weak_ptr&& r) noexcept;
   >     template<class Y> weak_ptr(weak_ptr<Y>&& r) noexcept;

   > > -?- Remark: The second constructor shall not participate in overload resolution unless `Y*` is implicitly convertible to `T*`.

   > > -?- Effects: Move-constructs a `weak_ptr` instance from `r`.

   > > -?- Postconditions: `*this` shall contain the old value of `r`. `r` shall be empty. `r.use_count() == 0`.

3. Edit 20.8.2.3.3 [util.smartptr.weak.assign] as indicated:

   >     weak_ptr& operator=(const weak_ptr& r) noexcept;
   >     template<class Y> weak_ptr& operator=(const weak_ptr<Y>& r) noexcept;
   >     template<class Y> weak_ptr& operator=(const shared_ptr<Y>& r) noexcept;

   > > -1- Effects: […]

   > > -2- Remarks: […]

   > > -?- Returns: `*this`.

   >     weak_ptr& operator=(weak_ptr&& r) noexcept;
   >     template<class Y> weak_ptr& operator=(weak_ptr<Y>&& r) noexcept;

   > > -?- Effects: Equivalent to `weak_ptr(std::move(r)).swap(*this)`.

   > > -?- Returns: `*this`.

### 2316. `weak_ptr::lock()` should be atomic

Section: 20.8.2.3.5 [util.smartptr.weak.obs] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2015-04-08

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Discussion:

20.8.2.2 [util.smartptr.shared]/4 says: "For purposes of determining the presence of a data race, member functions shall access and modify only the `shared_ptr` and `weak_ptr` objects themselves and not objects they refer to. Changes in `use_count()` do not reflect modifications that can introduce data races." This requires `shared_ptr/weak_ptr` implementations to protect their strong and weak refcounts with atomic operations, without the Standardese having to say this elsewhere. However, 20.8.2.3.5 [util.smartptr.weak.obs]/5 describes `weak_ptr::lock()` with "Returns: `expired() ? shared_ptr<T>() : shared_ptr<T>(*this)`." Even after considering the blanket wording about data races, this specification is insufficient. If this conditional expression were literally implemented, the `use_count()` could change from nonzero to zero after testing `expired()`, causing `shared_ptr<T>(*this)` to throw `bad_weak_ptr` when the intention is for `weak_ptr::lock()` to return empty or nonempty without throwing — indeed, `weak_ptr::lock()` is marked as `noexcept`.

We all know what `weak_ptr::lock()` should do, the Standardese just doesn't say it. `shared_ptr(const weak_ptr<Y>&)`'s specification is not really affected because 20.8.2.2.1 [util.smartptr.shared.const]/23-27 describes the behavior with English instead of code.

[Issaquah 20014-10-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Edit 20.8.2.3.5 [util.smartptr.weak.obs]/5 as indicated:

   >     shared_ptr<T> lock() const noexcept;

   > > -5- Returns: `expired() ? shared_ptr<T>() : shared_ptr<T>(*this)`, executed atomically.

### 2317. The type property queries should be `UnaryTypeTraits` returning `size_t`

Section: 20.10.5 [meta.unary.prop.query] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2015-04-08

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Discussion:

The sibling sections 20.10.4 [meta.unary], 20.10.6 [meta.rel], and 20.10.7 [meta.trans] respectively specify `UnaryTypeTraits`, `BinaryTypeTraits`, and `TransformationTraits`, as stated by each /2 paragraph. However, 20.10.5 [meta.unary.prop.query] is underspecified. `alignment_of`, `rank`, and `extent` are said to produce "Values", but the type of that Value is not specified, and the struct templates are not required to derive from `integral_constant`. Such derivation is more than stylistic — it allows the structs to be passed to functions taking `integral_constant`.

`alignment_of` returns `alignof(T)` which is `size_t` (5.3.6 [expr.alignof]/2). `extent` returns an array bound, which is clearly `size_t`. `rank` returns "the number of dimensions" of an array, so any type could be chosen, with `size_t` being a reasonable choice. (Another choice would be `unsigned int`, to match `extent`'s template parameter `I`.)

[Issaquah 20014-10-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Following 20.10.5 [meta.unary.prop.query]/1 add a new paragraph as indicated:

   > Each of these templates shall be a `UnaryTypeTrait` (20.10.1 [meta.rqmts]) with a `BaseCharacteristic` of `integral_constant<size_t, Value>`.

### 2319. `basic_string`'s move constructor should not be `noexcept`

Section: 21.4.2 [string.cons] Status: Resolved Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2015-04-08

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Discussion:

In debugging implementations, containers (including `basic_string`) may need to own dynamically allocated helper objects at all times, including in their default-constructed and moved-from states. This means that their default constructors and move constructors may throw exceptions. Therefore, the Standard should not mark them as `noexcept`. (Other implementations will still be permitted to add `noexcept`.)

[2014-02, Issaquah : move to Ready]

The issue discussion was highly controversial: The arguments in favour was that implementations exist that always need to allocate memory even for the move operations (similar as for some other containers) and that this cleans up an inconsistency between `std::string` and other container types. Counter arguments were that potentially throwing move operations reduce much of the advantages of move-support, e.g. in `vector<string>`.

straw poll: accept wording in the issue
SF 4 WF 4 N 0 WA 1 SA 1

straw poll: 14 or 17?
C++14: 4 C++17: 4

Move to Ready for C++17, as too close to 14 DIS without strong consensus.

It was suggested to introduce a special library vocabulary that specifies a "normative encouragement to not throw exceptions" for functions like these.

NJ: I offer to write a proposal to add encouragement for not throwing ... "`noexcept` in italics means should not throw"

[2014/11 Urbana]

Resolved by paper N4258

Proposed resolution:

This wording is relative to N3691.

1. In 21.4 [basic.string]/5, class template `basic_string` synopsis, and 21.4.2 [string.cons]/2 change as indicated:

   >     basic_string(basic_string&& str) noexcept;

2. Edit 21.4.2 [string.cons]/17 as indicated:

   >     basic_string(const basic_string& str, const Allocator& alloc);
   >     basic_string(basic_string&& str, const Allocator& alloc);

   > > […]

   > > -17- Throws: The second form throws nothing if `alloc == str.get_allocator()`..

### 2320. `select_on_container_copy_construction()` takes allocators, not containers

Section: 23.2.1 [container.requirements.general] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2015-04-08

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Discussion:

23.2.1 [container.requirements.general]/7 says "Copy constructors for these container types obtain an allocator by calling `allocator_traits<allocator_type>::select_on_container_copy_construction` on their first parameters." However, 20.7.8.2 [allocator.traits.members]/8 says that this takes `const Alloc&`, not a container. 23.2.1 [container.requirements.general]/7 goes on to say "Move constructors obtain an allocator by move construction from the allocator belonging to the container being moved." so we can follow that wording.

[Issaquah 20014-10-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. In 23.2.1 [container.requirements.general]/7 change as indicated:

   > -7- Unless otherwise specified, all containers defined in this clause obtain memory using an allocator (see 17.6.3.5). Copy constructors for these container types obtain an allocator by calling `allocator_traits<allocator_type>::select_on_container_copy_construction` on their first parameters the allocator belonging to the container being copied. Move constructors obtain an allocator by move construction from the allocator belonging to the container being moved. […]

### 2322. Associative(`initializer_list`, stuff) constructors are underspecified

Section: 23.2.4 [associative.reqmts], 23.2.5 [unord.req] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2015-04-08

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Discussion:

23.2.4 [associative.reqmts] specifies both `X(i,j)` and `X(i,j,c)`, but only `X(il)`. 23.4.4.1 [map.overview] declares "`map(initializer_list<value_type>, const Compare& = Compare(), const Allocator& = Allocator());`" but 23.4.4.2 [map.cons] intentionally doesn't explain it, relying on the big table's requirements. As a result, `map(il, c)`'s behavior is not actually specified by the Standard. (All of the other ordered associative containers also provide such constructors.)

The unordered associative containers are similarly affected, although they have more arguments. (Again, the actual containers are correctly depicted with the desired constructors, their behavior just isn't specified.)

[Issaquah 20014-10-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Edit 23.2.4 [associative.reqmts], Table 102 — "Associative container requirements", as indicated:

   > Table 102 — Associative container requirements (in addition to container) (continued)
   > Expression | Return type | Assertion/note pre-/post-condition | Complexity
   > --- | --- | --- | ---
   > `…`
   > `X(il);` | `` | Same as `X(il.begin(), il.end())`. | s Same as `X(il.begin(), il.end())`.
   > `X(il, c);` | `` | Same as `X(il.begin(), il.end(), c)`. | Same as `X(il.begin(), il.end(), c)`.
   > `…`

2. Edit 23.2.5 [unord.req], Table 103 "Unordered associative container requirements", as indicated:

   > Table 103 — Unordered associative container requirements (in addition to container)
   > Expression | Return type | Assertion/note pre-/post-condition | Complexity
   > --- | --- | --- | ---
   > `…`
   > `X(il)` | `X` | Same as `X(il.begin(), il.end())`. | Same as `X(il.begin(), il.end())`.
   > `X(il, n)` | `X` | Same as `X(il.begin(), il.end(), n)`. | Same as `X(il.begin(), il.end(), n)`.
   > `X(il, n, hf)` | `X` | Same as `X(il.begin(), il.end(), n, hf)`. | Same as `X(il.begin(), il.end(), n, hf)`.
   > `X(il, n, hf, eq)` | `X` | Same as `X(il.begin(), il.end(), n, hf, eq)`. | Same as `X(il.begin(), il.end(), n, hf, eq)`.
   > `…`

### 2323. `vector::resize(n, t)`'s specification should be simplified

Section: 23.3.6.3 [vector.capacity], 23.3.3.3 [deque.capacity] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2015-04-08

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Discussion:

First, 23.3.3.3 [deque.capacity]/4 and 23.3.6.3 [vector.capacity]/16 say that `resize(size_type sz, const T& c)` "Requires: `T` shall be `MoveInsertable` into `*this` and `CopyInsertable` into `*this`." The `CopyInsertable` requirement is correct (because `sz` might be `size() + 2` or more), but the `MoveInsertable` requirement is redundant due to 23.2.1 [container.requirements.general]/13: "`T` is `CopyInsertable` into `X` means that, in addition to `T` being `MoveInsertable` into `X`, the [...]". (LWG 2033's resolution said that this was "not redundant, because `CopyInsertable` is not necessarily a refinement of `MoveInsertable`" which was true at the time, but then LWG 2177's resolution made it a refinement.)

Second, 23.3.6.3 [vector.capacity]/17 says "Remarks: If an exception is thrown other than by the move constructor of a non-`CopyInsertable` `T` there are no effects." This is confusing because `T` is required to be `CopyInsertable`. (/14 says the same thing for `resize(size_type sz)`, where it is correct because that overload requires only `MoveInsertable` and `DefaultInsertable`.)

[Issaquah 20014-10-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Edit 23.3.3.3 [deque.capacity]/4 as indicated:

   >     void resize(size_type sz, const T& c);

   > > […]

   > > -4- Requires: `T` shall be `MoveInsertable` into `*this` and `CopyInsertable` into `*this`.

2. Edit 23.3.6.3 [vector.capacity]/16+17 as indicated:

   >     void resize(size_type sz, const T& c);

   > > […]

   > > -16- Requires: `T` shall be `MoveInsertable` into `*this` and `CopyInsertable` into `*this`.

   > > -17- Remarks: If an exception is thrown other than by the move constructor of a non-`CopyInsertable` `T` there are no effects.

### 2324. Insert iterator constructors should use `addressof()`

Section: 24.5.2.2.1 [back.insert.iter.cons], 24.5.2.4.1 [front.insert.iter.cons], 24.5.2.6.1 [insert.iter.cons] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2015-04-08

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Discussion:

24.5.2.2.1 [back.insert.iter.cons]/1, 24.5.2.4.1 [front.insert.iter.cons]/1, and 24.5.2.6.1 [insert.iter.cons]/1 say "Initializes container with `&x`", which doesn't defend against containers overloading `operator&()`. Containers are now required to have such defenses for their elements, so we may as well be consistent here.

[Issaquah 20014-10-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Edit 24.5.2.2.1 [back.insert.iter.cons]/1 as indicated:

   >     explicit back_insert_iterator(Container& x);

   > > -1- Effects: Initializes `container` with `&xstd::addressof(x)`.

2. Edit 24.5.2.4.1 [front.insert.iter.cons]/1 as indicated:

   >     explicit front_insert_iterator(Container& x);

   > > -1- Effects: Initializes `container` with `&xstd::addressof(x)`.

3. Edit 24.5.2.6.1 [insert.iter.cons]/1 as indicated:

   >     insert_iterator(Container& x, typename Container::iterator i);

   > > -1- Effects: Initializes `container` with `&xstd::addressof(x)` and `iter` with `i`.

### 2325. `minmax_element()`'s behavior differing from `max_element()`'s should be noted

Section: 25.4.7 [alg.min.max] Status: WP Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2015-04-08

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Discussion:

25.4.7 [alg.min.max]/23 says that `max_element()` finds the first biggest element, while /25 says that `minmax_element()` finds the last biggest element. This significant difference is unusual — it means that `minmax_element(args)` is not equivalent to `make_pair(min_element(args), max_element(args))`, whereas the other major "two for one" algorithm `equal_range(args)` is equivalent to `make_pair(lower_bound(args), upper_bound(args))`. `minmax_element()`'s behavior is intentional — it is a fundamental consequence of the `3N/2` algorithm — but the Standardese does not draw attention to this in any way. This wording came from LWG 715's resolution (which changed the semantics but didn't mention it), citing CLRS for the algorithm — but CLRS doesn't mention the behavior for equivalent elements! The wording here deeply confused me (as an STL maintainer fixing an incorrect implementation) until I walked through the algorithm by hand and figured out the fundamental reason. It would be really nice for the Standard to provide a hint that something magical is happening here.

[2014-06-06 Library reflector vote]

The issue has been identified as Tentatively Ready based on six votes in favour.

Proposed resolution:

This wording is relative to N3691.

1. Add a footnote to 25.4.7 [alg.min.max]/25 as indicated:

   >     template<class ForwardIterator>
   >       pair<ForwardIterator, ForwardIterator>
   >         minmax_element(ForwardIterator first, ForwardIterator last);
   >     template<class ForwardIterator, class Compare>
   >       pair<ForwardIterator, ForwardIterator>
   >         minmax_element(ForwardIterator first, ForwardIterator last, Compare comp);

   > > -25- Returns: `make_pair(first, first)` if `[first,last)` is empty, otherwise `make_pair(m, M)`, where `m` is the first iterator in `[first,last)` such that no iterator in the range refers to a smaller element, and where `M` is the last iterator [Footnote: This behavior intentionally differs from `max_element()`.] in `[first,last)` such that no iterator in the range refers to a larger element.

### 2329. `regex_match()/regex_search()` with `match_results` should forbid temporary strings

Section: 28.4 [re.syn] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2015-04-08

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Discussion:

Consider the following code:

>     const regex r(R"(meow(\d+)\.txt)");
>     smatch m;
>     if (regex_match(dir_iter->path().filename().string(), m, r)) {
>       DoSomethingWith(m[1]);
>     }

This occasionally crashes. The problem is that `dir_iter->path().filename().string()` returns a temporary string, so the `match_results` contains invalidated iterators into a destroyed temporary string.

It's fine for `regex_match/regex_search(str, reg)` to accept temporary strings, because they just return `bool`. However, the overloads taking `match_results` should forbid temporary strings.

[2014-02-13 Issaquah: Move as Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Edit 28.4 [re.syn], header `<regex>` synopsis, as indicated:

   >     #include <initializer_list>

   >     namespace std {

   >       […]

   >       // 28.11.2, function template regex_match:
   >       […]
   >       template <class ST, class SA, class Allocator, class charT, class traits>
   >       bool regex_match(const basic_string<charT, ST, SA>&&,
   >                        match_results<
   >                          typename basic_string<charT, ST, SA>::const_iterator,
   >                          Allocator>&,
   >                        const basic_regex<charT, traits>&,
   >                        regex_constants::match_flag_type =
   >                          regex_constants::match_default) = delete;

   >       // 28.11.3, function template regex_search:
   >       […]
   >       template <class ST, class SA, class Allocator, class charT, class traits>
   >       bool regex_search(const basic_string<charT, ST, SA>&&,
   >                         match_results<
   >                           typename basic_string<charT, ST, SA>::const_iterator,
   >                           Allocator>&,
   >                         const basic_regex<charT, traits>&,
   >                         regex_constants::match_flag_type =
   >                           regex_constants::match_default) = delete;
   >       […]
   >     }

### 2330. `regex("meow", regex::icase)` is technically forbidden but should be permitted

Section: 28.5.1 [re.synopt] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2015-04-08

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Discussion:

28.5.1 [re.synopt]/1 says "A valid value of type `syntax_option_type` shall have exactly one of the elements `ECMAScript`, `basic`, `extended`, `awk`, `grep`, `egrep`, set."

This "exactly one" wording technically forbids passing `icase` by itself! Users should not be required to pass `regex::ECMAScript | regex::icase`. (Note that the cost of an additional check for no grammar being explicitly requested is completely irrelevant, as regex construction is so much more expensive.)

[Issaquah 20014-10-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Edit 28.5.1 [re.synopt] as indicated:

   > -1- The type `syntax_option_type` is an implementation-defined bitmask type (17.5.2.1.3). Setting its elements has the effects listed in table 138. A valid value of type `syntax_option_type` shall have exactly at most one of the grammar elements `ECMAScript`, `basic`, `extended`, `awk`, `grep`, `egrep`, set. If no grammar element is set, the default grammar is `ECMAScript`.

### 2332. `regex_iterator/regex_token_iterator` should forbid temporary regexes

Section: 28.12 [re.iter] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2015-04-08

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Discussion:

Users can write "`for(sregex_iterator i(s.begin(), s.end(), regex("meow")), end; i != end; ++i)`", binding a temporary `regex` to `const regex&` and storing a pointer to it. This will compile silently, triggering undefined behavior at runtime. We now have the technology to prevent this from compiling, like how `reference_wrapper` refuses to bind to temporaries.

[2014-02-14 Issaquah meeting: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Change 28.12.1 [re.regiter]/1, class template `regex_iterator` synopsis, as indicated:

   >     regex_iterator();
   >     regex_iterator(BidirectionalIterator a, BidirectionalIterator b,
   >       const regex_type& re,
   >       regex_constants::match_flag_type m =
   >         regex_constants::match_default);
   >     regex_iterator(BidirectionalIterator a, BidirectionalIterator b,
   >       const regex_type&& re,
   >       regex_constants::match_flag_type m =
   >         regex_constants::match_default) = delete;

2. Change 28.12.2 [re.tokiter]/6, class template `regex_token_iterator` synopsis, as indicated:

   >     regex_token_iterator();
   >     regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
   >                          const regex_type& re,
   >                          int submatch = 0,
   >                          regex_constants::match_flag_type m =
   >                            regex_constants::match_default);
   >     regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
   >                          const regex_type& re,
   >                          const std::vector<int>& submatches,
   >                          regex_constants::match_flag_type m =
   >                            regex_constants::match_default);
   >     regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
   >                          const regex_type& re,
   >                          initializer_list<int> submatches,
   >                          regex_constants::match_flag_type m =
   >                            regex_constants::match_default);
   >     template <std::size_t N>
   >     regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
   >                          const regex_type& re,
   >                          const int (&submatches)[N],
   >                          regex_constants::match_flag_type m =
   >                            regex_constants::match_default);
   >     regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
   >                          const regex_type&& re,
   >                          int submatch = 0,
   >                          regex_constants::match_flag_type m =
   >                            regex_constants::match_default) = delete;
   >     regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
   >                          const regex_type&& re,
   >                          const std::vector<int>& submatches,
   >                          regex_constants::match_flag_type m =
   >                            regex_constants::match_default) = delete;
   >     regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
   >                          const regex_type&& re,
   >                          initializer_list<int> submatches,
   >                          regex_constants::match_flag_type m =
   >                            regex_constants::match_default) = delete;
   >     template <std::size_t N>
   >     regex_token_iterator(BidirectionalIterator a, BidirectionalIterator b,
   >                          const regex_type&& re,
   >                          const int (&submatches)[N],
   >                          regex_constants::match_flag_type m =
   >                            regex_constants::match_default) = delete;

### 2333. [fund.ts] Hashing disengaged `optional<T>` objects

Section: X [optional.hash] Status: Resolved Submitter: Jonathan Wakely Opened: 2013-10-03 Last modified: 2015-04-08

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Discussion:

Addresses: fund.ts

The spec for `hash<optional<T>>` doesn't say anything about disengaged objects, so 1.3 [defns.undefined] would imply it's undefined behaviour, but that's very unhelpful to users.

If hashing disengaged optional objects is undefined there should be a Requires, otherwise there should be some statement saying it's OK.

It would be possible to specify the value, e.g. saying it returns the same value as something like `std::hash<void*>()(nullptr)`, but leaving it unspecified would permit users to specialize `hash<optional<UserDefinedType>>` so that the hash value for a disengaged object is distinct from any value returned by `hash<UserDefinedType>`.

[2014-06-06 pre-Rapperswill]

This issue has been reopened as fundamentals-ts.

[2014-06-07 Daniel comments]

This issue should be set to Resolved, because the wording fix is already applied in the last fundamentals working draft.

[2014-06-16 Rapperswill]

Confirmed that this issue is resolved in the current Library Fundamentals working paper.

Proposed resolution:

This wording is relative to N3691.

1. Add to X [optional.hash]/3

   >     template <class T> struct hash<optional<T>>;

   > > […]

   > > -3- For an object `o` of type `optional<T>`, if `bool(o) == true`, `hash<optional<T>>()(o)` shall evaluate to the same value as `hash<T>()(*o)` otherwise it evaluates to an unspecified value.

### 2339. Wording issue in `nth_element`

Section: 25.4.2 [alg.nth.element] Status: C++14 Submitter: Christopher Jefferson Opened: 2013-10-19 Last modified: 2015-04-08

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Discussion:

The wording of `nth_element` says:

>     template<class RandomAccessIterator>
>       void nth_element(RandomAccessIterator first, RandomAccessIterator nth,
>                        RandomAccessIterator last);

> > After `nth_element` the element in the position pointed to by `nth` is the element that would be in that position if the whole range were sorted. Also for every iterator `i` in the range `[first,nth)` and every iterator `j` in the range `[nth,last)` it holds that: `!(*j < *i)` or `comp(*j, *i) == false`.

That wording, to me, implies that there must be an element at '`nth`'. However, gcc at least accepts `nth == last`, and returns without effect (which seems like the sensible option).

Is it intended to accept `nth == last`? If so, then I would suggest adding this to the wording explicitly, say:

> After `nth_element` the element in the position pointed to by `nth`, if any, is the element that would be in that position if the whole range were sorted. Also for every iterator `i` in the range `[first,nth)` and every iterator `j` in the range `[nth,last)` it holds that: `!(*j < *i)` or `comp(*j, *i) == false`.

[Issaquah 20014-10-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3797.

1. Modify 25.4.2 [alg.nth.element]/1 as indicated:

   >     template<class RandomAccessIterator>
   >       void nth_element(RandomAccessIterator first, RandomAccessIterator nth,
   >                        RandomAccessIterator last);
   >     template<class RandomAccessIterator, class Compare>
   >       void nth_element(RandomAccessIterator first, RandomAccessIterator nth,
   >                        RandomAccessIterator last, Compare comp);

   > > -1- After `nth_element` the element in the position pointed to by `nth` is the element that would be in that position if the whole range were sorted, unless `nth == last`. Also for every iterator `i` in the range `[first,nth)` and every iterator `j` in the range `[nth,last)` it holds that: `!(*j < *i)` or `comp(*j, *i) == false`.

### 2340. Replacement allocation functions declared as inline

Section: 17.6.4.6 [replacement.functions] Status: WP Submitter: David Majnemer Opened: 2013-10-20 Last modified: 2015-04-08

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Discussion:

N3290 17.6.4.6 [replacement.functions]/p3 says:

> The program's definitions shall not be specified as `inline`.

This seems to permit declarations of replacement allocation functions that are specified as `inline` so long as they aren't used. This behavior seems more like a bug than a feature, I propose that we do the following:

> The program's definitions declarations shall not be specified as `inline`.

[2014-02-15 Issaquah : Move to Ready]

Proposed resolution:

This wording is relative to N3797.

1. Modify 17.6.4.6 [replacement.functions]/3 as indicated:

   > -3- The program's definitions are used instead of the default versions supplied by the implementation (18.6). Such replacement occurs prior to program startup (3.2, 3.6). The program's definitions declarations shall not be specified as `inline`. No diagnostic is required.

### 2341. Inconsistency between `basic_ostream::seekp(pos)` and `basic_ostream::seekp(off, dir)`

Section: 27.7.3.5 [ostream.seeks] Status: C++14 Submitter: Marshall Clow Opened: 2013-10-21 Last modified: 2015-04-08

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Discussion:

In 27.7.3.5 [ostream.seeks], we have:

>     basic_ostream<charT,traits>& seekp(pos_type pos);

> > -3- Effects: If `fail() != true`, executes `rdbuf()->pubseekpos(pos, ios_base::out)`. In case of failure, the function calls `setstate(failbit)` (which may throw `ios_base::failure`).

> > -4- Returns: `*this`.

>     basic_ostream<charT,traits>& seekp(off_type off, ios_base::seekdir dir);

> > -5- Effects: If `fail() != true`, executes `rdbuf()->pubseekoff(off, dir, ios_base::out)`.

> > -6- Returns: `*this`.

The first call is required to set the `failbit` on failure, but the second is not

So (given two ostreams, `os1` and `os2`) the following code (confusingly) works:

>     os1.seekp(-1);
>     assert(os1.fail());

>     os2.seekp(-1, std::ios_base::beg);
>     assert(os2.good());

Note that the description of `basic_istream<charT,traits>& seekg(off_type off, ios_base::seekdir dir)` in 27.7.2.3 [istream.unformatted] p43 does require setting `failbit`.

[Issaquah 20014-10-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3797.

1. Modify 27.7.3.5 [ostream.seeks]p5 as indicated:

   >     basic_ostream<charT,traits>& seekp(off_type off, ios_base::seekdir dir);

   > > -5- Effects: If `fail() != true`, executes `rdbuf()->pubseekoff(off, dir, ios_base::out)`. In case of failure, the function calls `setstate(failbit)` (which may throw `ios_base::failure`).

   > > -6- Returns: `*this`.

### 2344. `quoted()`'s interaction with padding is unclear

Section: 27.7.6 [quoted.manip] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-11-01 Last modified: 2015-04-08

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Discussion:

Given this code:

>     cout << "[" << left << setfill('x') << setw(20) << R"("AB \"CD\" EF")" << "]" << endl;
>     cout << "[" << left << setfill('y') << setw(20) << quoted(R"(GH "IJ" KL)") << "]" << endl;

The first line prints `["AB \"CD\" EF"xxxxxx]`. The second line should probably print `["GH \"IJ\" KL"yyyyyy]`, but 27.7.6 [quoted.manip]/2 doesn't say whether or how `quoted()` should interact with padding. All it says is that

> "`out << quoted(s, delim, escape)` behaves as if it inserts the following characters into out using character inserter function templates (27.7.3.6.4)".

27.7.3.6.4 [ostream.inserters.character] specifies both single-character and null-terminated inserters, both referring to 27.7.3.6.1 [ostream.formatted.reqmts]/3 for padding. Literally implementing `quoted()` with single-character inserters would result in padding being emitted after the first character, with undesirable effects for `ios_base::left`.

It appears that 21.4.8.9 [string.io]/5 has the appropriate incantations to follow here. It says that `os << str`

> "Behaves as a formatted output function (27.7.3.6.1) of `os`. Forms a character sequence `seq`, initially consisting of the elements defined by the range `[str.begin(), str.end())`. Determines padding for `seq` as described in 27.7.3.6.1. Then inserts `seq` as if by calling `os.rdbuf()->sputn(seq, n)`, where `n` is the larger of `os.width()` and `str.size();` then calls `os.width(0)`."

Additionally, saying that it's a "formatted output function" activates 27.7.3.6.1 [ostream.formatted.reqmts]/1's wording for sentry objects.

[2014-02-14 Issaquah meeting: Move to Immediate]

Proposed resolution:

This wording is relative to N3797.

1. Edit 27.7.6 [quoted.manip] as follows:

   >     template <class charT>
   >       unspecified quoted(const charT* s, charT delim=charT('"'), charT escape=charT('\\'));
   >     template <class charT, class traits, class Allocator>
   >       unspecified quoted(const basic_string<charT, traits, Allocator>& s,
   >                          charT delim=charT('"'), charT escape=charT('\\'));

   > > -2- Returns: An object of unspecified type such that if `out` is an instance of `basic_ostream` with member type `char_type` the same as `charT`, then the expression `out << quoted(s, delim, escape)` behaves as if it inserts the following characters into `out` using character inserter function templates (27.7.3.6.4), which may throw `ios_base::failure` (27.5.3.1.1)a formatted output function (27.7.3.6.1 [ostream.formatted.reqmts]) of `out`. This forms a character sequence `seq`, initially consisting of the following elements:

   > > - `delim`.

   > > - Each character in `s`. If the character to be output is equal to escape or `delim`, as determined by `operator==`, first output `escape`.

   > > - `delim`.

   > > Let `x` be the number of elements initially in `seq`. Then padding is determined for `seq` as described in 27.7.3.6.1 [ostream.formatted.reqmts], `seq` is inserted as if by calling `out.rdbuf()->sputn(seq, n)`, where `n` is the larger of `out.width()` and `x`, and `out.width(0)` is called. The expression `out << quoted(s, delim, escape)` shall have type `basic_ostream<charT, traits>&` and value `out`.

### 2346. `integral_constant`'s member functions should be marked `noexcept`

Section: 20.10.3 [meta.help] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-11-05 Last modified: 2015-04-08

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Discussion:

Obvious.

[Issaquah 20014-10-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3797.

1. Edit 20.10.3 [meta.help] as indicated:

   >     namespace std {
   >       template<class T, T v>
   >       struct integral_constant {
   >         static constexpr T value = v;
   >         typedef T value_type;
   >         typedef integral_constant<T,v> type;
   >         constexpr operator value_type() const noexcept { return value; }
   >         constexpr value_type operator()() const noexcept { return value; }
   >       };
   >       […]
   >     }

### 2350. `min`, `max`, and `minmax` should be `constexpr`

Section: 25.4.7 [alg.min.max] Status: C++14 Submitter: Ville Voutilainen Opened: 2013-12-15 Last modified: 2015-04-08

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Discussion:

Having `min`, `max`, and `minmax` `constexpr` was a large part of the motivation to allow reference-to-const arguments for `constexpr` functions as per N3039. Furthermore, `initializer_lists` are immutable and not-movable-from for large part in order to allow using them in `constexpr` contexts and other hoisting-optimizations. In N3797 version of the draft none of these functions are `constexpr`, and they should be made `constexpr`.

Proposed resolution:

This wording is relative to N3797.

1. In 25.1 [algorithms.general], header `<algorithm>` synopsis, and 25.4.7 [alg.min.max], change as indicated (add `constexpr` to every signature before `min_element`):

   >     template<class T> constexpr const T& min(const T& a, const T& b);
   >     template<class T, class Compare>
   >     constexpr const T& min(const T& a, const T& b, Compare comp);
   >     […]
   >     template<class T>
   >     constexpr T min(initializer_list<T> t);
   >     template<class T, class Compare>
   >     constexpr T min(initializer_list<T> t, Compare comp);
   >     […]
   >     template<class T> constexpr const T& max(const T& a, const T& b);
   >     template<class T, class Compare>
   >     constexpr const T& max(const T& a, const T& b, Compare comp);
   >     […]
   >     template<class T>
   >     constexpr T max(initializer_list<T> t);
   >     template<class T, class Compare>
   >     constexpr T max(initializer_list<T> t, Compare comp);
   >     […]
   >     template<class T> constexpr pair<const T&, const T&> minmax(const T& a, const T& b);
   >     template<class T, class Compare>
   >     constexpr pair<const T&, const T&> minmax(const T& a, const T& b, Compare comp);
   >     […]
   >     template<class T>
   >     constexpr pair<T, T> minmax(initializer_list<T> t);
   >     template<class T, class Compare>
   >     constexpr pair<T, T> minmax(initializer_list<T> t, Compare comp);

### 2354. Unnecessary copying when inserting into maps with braced-init syntax

Section: 23.4.4.1 [map.overview], 23.4.5.1 [multimap.overview], 23.5.4.1 [unord.map.overview], 23.5.5.1 [unord.multimap.overview] Status: WP Submitter: Geoffrey Romer Opened: 2014-01-08 Last modified: 2015-04-08

View all issues with WP status.

Discussion:

The rvalue-reference `insert()` members of `map`, `multimap`, `unordered_map`, and `unordered_multimap` are specified as function templates, where the rvalue-reference parameter type depends on the template parameter. As a consequence, these overloads cannot be invoked via braced-initializer syntax (e.g. `my_map.insert({key, value})`), because the template argument cannot be deduced from a braced-init-list. Such calls instead resolve to the const lvalue reference overload, which forces a non-elidable copy of the argument, despite the fact that the argument is an rvalue, and so should be eligible for moving and copy elision.

This leads to sub-optimal performance for copyable values, and makes this syntax unusable with noncopyable values. This is particularly problematic because sources such as Josuttis's "C++ Standard Library" recommend this syntax as the preferred way to insert into a map in C++11.

I think this can be fixed by adding an equivalent non-template `value_type&&` overload for each affected member template. Simply declaring these members in the class synopses should be sufficient; their semantics are already dictated by the container concepts (c.f. the corresponding lvalue-reference overloads, which have no additional discussion beyond being listed in the synopsis).

[2012-02-13 Issaquah]

AJM: Is this not better solved by `emplace`?

Nico: `emplace` was a mistake, it breaks a uniform pattern designed into the STL. Hence, this fix is important, it should be the preferred way to do this.

JonW: `emplace` is still more efficient, as this form must make a non-elidable copy.

GeoffR: Also, cannot move from a `const` key, must always make a copy.

Poll for adopting the proposed wording:

SF: 1 WF: 4 N: 4 WA: 1 SA: 0

Move to Ready, pending implementation experience.

Proposed resolution:

This wording is relative to N3797.

1. Change 23.4.4.1 [map.overview], class template `map` synopsis, as indicated:

   >     […]
   >     pair<iterator, bool> insert(const value_type& x);
   >     pair<iterator, bool> insert(value_type&& x);
   >     template <class P> pair<iterator, bool> insert(P&& x);
   >     iterator insert(const_iterator position, const value_type& x);
   >     iterator insert(const_iterator position, value_type&& x);
   >     template <class P>
   >       iterator insert(const_iterator position, P&&);
   >     […]

2. Change 23.4.5.1 [multimap.overview], class template `multimap` synopsis, as indicated:

   >     […]
   >     iterator insert(const value_type& x);
   >     iterator insert(value_type&& x);
   >     template <class P> iterator insert(P&& x);
   >     iterator insert(const_iterator position, const value_type& x);
   >     iterator insert(const_iterator position, value_type&& x);
   >     template <class P> iterator insert(const_iterator position, P&& x);
   >     […]

3. Change 23.5.4.1 [unord.map.overview], class template `unordered_map` synopsis, as indicated:

   >     […]
   >     pair<iterator, bool> insert(const value_type& obj);
   >     pair<iterator, bool> insert(value_type&& obj);
   >     template <class P> pair<iterator, bool> insert(P&& obj);
   >     iterator insert(const_iterator hint, const value_type& obj);
   >     iterator insert(const_iterator hint, value_type&& obj);
   >     template <class P> iterator insert(const_iterator hint, P&& obj);
   >     […]

4. Change 23.5.5.1 [unord.multimap.overview], class template `unordered_multimap` synopsis, as indicated:

   >     […]
   >     iterator insert(const value_type& obj);
   >     iterator insert(value_type&& obj);
   >     template <class P> iterator insert(P&& obj);
   >     iterator insert(const_iterator hint, const value_type& obj);
   >     iterator insert(const_iterator hint, value_type&& obj);
   >     template <class P> iterator insert(const_iterator hint, P&& obj);
   >     […]

### 2356. Stability of erasure in unordered associative containers

Section: 23.2.5 [unord.req] Status: C++14 Submitter: Joaquín M López Muñoz Opened: 2014-01-21 Last modified: 2015-04-08

View other active issues in [unord.req].

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Discussion:

Issue 518 resolution for unordered associative containers, modelled after that of issue 371, which is related to associative containers, states that insertion, erasure and rehashing preserve the relative ordering of equivalent elements. Unfortunately, this is not sufficient to guarantee the validity of code such as this:

>     std::unordered_multimap<int, int> m;
>     auto i = m.begin();
>     while (i != m.end()) {
>       if (pred(i))
>         m.erase(i++);
>       else
>         ++i;
>     }

(which is a direct translation from `multimap` to `unordered_multimap` of the motivating example in 371), or even this:

>     std::unordered_multimap<int, int> m;
>     auto p = m.equal_range(k);
>     while (p.first != p.second) {
>       if (pred(p.first))
>         m.erase((p.first)++);
>       else
>         ++(p.first);
>     }

because the relative ordering of non-equivalent elements elements could potentially change after erasure (not that any actual implementation does that, anyway). Such an underspecification does not happen for regular associative containers, where the relative ordering of non-equivalent elements is kept by design.

[2014-02-13 Issaquah: Move to Immediate]

Proposed resolution:

This wording is relative to N3797.

1. Modify 23.2.5 [unord.req], p14 as indicated:

   > -14- The `insert` and `emplace` members shall not affect the validity of references to container elements, but may invalidate all iterators to the container. The `erase` members shall invalidate only iterators and references to the erased elements, and preserve the relative order of the elements that are not erased.

### 2357. Remaining "`Assignable`" requirement

Section: 25.3.13 [alg.partitions] Status: C++14 Submitter: Daniel Krügler Opened: 2014-02-01 Last modified: 2015-04-08

View all other issues in [alg.partitions].

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Discussion:

The Requires element of `partition_copy` says (emphasis mine):

> Requires: `InputIterator`'s value type shall be `Assignable`, and …

The C++03 term `Assignable` was replaced by `CopyAssignable`, remaining cleanups happened via LWG issue 972, but algorithm `partition_copy` was not affected at that time (during that time the requirements of `partition_copy` didn't mention writable nor assignable, but I cannot track down at the moment where these requirements had been added). Presumably this requirement should be corrected similarly to the approach used in 972.

Another question is whether a `CopyAssignable` is needed here, given the fact that we already require "writable to" an `OutputIterator` which is defined in 24.2.1 [iterator.requirements.general] and does already impose the necessary statement

>     *out = *in;

Given the fact that `partition_copy` never touches any input value twice, there is no reason why anything more than writable to should be necessary.

The below suggested primary resolution does not respond to the second part of this question.

[Issaquah 20014-10-11: Move to Immediate]

Proposed resolution:

This wording is relative to N3797.

1. Modify 25.3.13 [alg.partitions], p12 as indicated:

   > -12- Requires: `InputIterator`'s value type shall be `CopyAssignable`, and shall be writable to the `out_true` and `out_false` `OutputIterator`s, and shall be convertible to `Predicate`'s argument type. The input range shall not overlap with either of the output ranges.

### 2359. How does `regex_constants::nosubs` affect `basic_regex::mark_count()`?

Section: 28.5.1 [re.synopt] Status: C++14 Submitter: Jonathan Wakely Opened: 2014-02-01 Last modified: 2015-04-08

View other active issues in [re.synopt].

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Discussion:

As discussed in c++std-lib-35399 and its replies, I can see two possible interpretations of the effects of `regex_constants::nosubs`:

1. The effect of `nosubs` only applies during matching. Parentheses are still recognized as marking a sub-expression by the `basic_regex` compiler, and `basic_regex::mark_count()` still returns the number of marked sub-expressions, but anything they match is not stored in the results. This means it is not always true that `results.size() == r.mark_count() + 1` for a successful match.

2. `nosubs` affects how a regular expression is compiled, altering the state of the `std::basic_regex` object so that `mark_count()` returns zero. This also affects any subsequent matching.

The definition of `nosubs` should make this clear.

The wording in 28.5.1 [re.synopt]/1 seems to imply that `nosubs` only has effects during matching, which is (1), but all known implementations do (2). John Maddock confirmed that (2) was intended.

[Issaquah 2014-02-12: Move to Immediate]

Proposed resolution:

This wording is relative to N3797.

1. Apply the following edit to the table in 28.5.1 [re.synopt]/1

   > Specifies that no sub-expressions shall be considered to be marked, so that when a regular expression is matched against a character container sequence, no sub-expression matches shall be stored in the supplied `match_results` structure.

### 2360. `reverse_iterator::operator*()` is unimplementable

Section: 24.5.1.1 [reverse.iterator] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2014-02-07 Last modified: 2015-04-08

View all other issues in [reverse.iterator].

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Discussion:

Previously, C++03 24.4.1.3.3 [lib.reverse.iter.op.star] required:

>     reference operator*() const;

> > Effects:

> > >     Iterator tmp = current;
> > >     return *--tmp;

Now, N3797 24.5.1.1 [reverse.iterator] depicts:

>     private:
>       Iterator deref_tmp; // exposition only
>     };

And 24.5.1.3.4 [reverse.iter.op.star] requires:

>     reference operator*() const;

> > Effects:

> > >     deref_tmp = current;
> > >     --deref_tmp;
> > >     return *deref_tmp;

> > [Note: This operation must use an auxiliary member variable rather than a temporary variable to avoid returning a reference that persists beyond the lifetime of its associated iterator. (See 24.2.) — end note]

As written, this won't compile, because `operator*()` is `const` yet it's modifying (via assignment and decrement) the `deref_tmp` data member. So what happens if you say "`mutable Iterator deref_tmp;`"?

DANGER: WARP CORE BREACH IMMINENT.

The Standard requires `const` member functions to be callable from multiple threads simultaneously. This is 17.6.5.9 [res.on.data.races]/3: "A C++ standard library function shall not directly or indirectly modify objects (1.10) accessible by threads other than the current thread unless the objects are accessed directly or indirectly via the function's non-const arguments, including `this`."

Multiple threads simultaneously modifying `deref_tmp` will trigger data races, so both `mutable` and some form of synchronization (e.g. `mutex` or `atomic`) are actually necessary!

Here's what implementations currently do: Dinkumware/VC follows C++03 and doesn't use `deref_tmp` (attempting to implement it is what led me to file this issue). According to Jonathan Wakely, libstdc++ also follows C++03 (see PR51823 which is suspended until LWG 2204 is resolved). According to Marshall Clow, libc++ uses `deref_tmp` with `mutable` but without synchronization, so it can trigger data races.

This `deref_tmp` Standardese was added by LWG 198 "Validity of pointers and references unspecified after iterator destruction" and is present in Working Papers going back to N1638 on April 11, 2004, long before C++ recognized the existence of multithreading and developed the "`const` means simultaneously readable" convention.

A related issue is LWG 1052 "`reverse_iterator::operator->` should also support smart pointers" which mentioned the need to depict `mutable` in the Standardese, but it was resolved NAD Future and no change was made.

Finally, LWG 2204 "`reverse_iterator` should not require a second copy of the base iterator" talked about removing `deref_tmp`, but without considering multithreading.

I argue that `deref_tmp` must be removed. Its existence has highly undesirable consequences: either no synchronization is used, violating the Standard's usual multithreading guarantees, or synchronization is used, adding further costs for all users that benefit almost no iterators.

`deref_tmp` is attempting to handle iterators that return references to things "inside themselves", which I usually call "stashing iterators" (as they have a secret stash). Note that these are very unusual, and are different from proxy iterators like `vector<bool>::iterator`. While `vector<bool>::iterator`'s `operator*()` does not return a true reference, it refers to a bit that is unrelated to the iterator's lifetime.

[2014-02-14 Issaquah meeting: Move to Immediate]

Strike superfluous note to avoid potential confusion, and move to Immediate.

Proposed resolution:

This wording is relative to N3797.

1. Change class template `reverse_iterator` synopsis, 24.5.1.1 [reverse.iterator], as indicated:

   >     […]
   >     protected:
   >       Iterator current;
   >     private:
   >       Iterator deref_tmp; // exposition only
   >     };

2. Change 24.5.1.3.4 [reverse.iter.op.star] as indicated:

   >     reference operator*() const;

   > > -1- Effects:

   > > >     deref_tmp = current;
   > > >     --deref_tmp;
   > > >     return *deref_tmp;
   > > >     Iterator tmp = current;
   > > >     return *--tmp;

   > > -2- [Note: This operation must use an auxiliary member variable rather than a temporary variable to avoid returning a reference that persists beyond the lifetime of its associated iterator. (See 24.2.) — end note]

### 2361. Apply 2299 resolution throughout library

Section: 20.8.1.2 [unique.ptr.single], 20.7.3.1 [pointer.traits.types], 20.7.7.1 [allocator.uses.trait], 20.7.8.1 [allocator.traits.types], 23.2.3 [sequence.reqmts] Status: WP Submitter: Jonathan Wakely Opened: 2014-02-14 Last modified: 2015-04-08

View all other issues in [unique.ptr.single].

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Discussion:

LWG 2299 addressed a N.B. comment pointing out that recently added wording about a type existing was not clear what happens if the type exists but is inaccessible. There are 16 pre-existing uses of the same language in the library that should use the same wording used to resolve 2299.

The relevant paragraphs are:

20.8.1.2 [unique.ptr.single]

20.7.3.1 [pointer.traits.types]

20.7.7.1 [allocator.uses.trait]

20.7.8.1 [allocator.traits.types]

23.2.3 [sequence.reqmts]

[2014-05-16, Daniel provides wording]

[2014-05-18 Library reflector vote]

The issue has been identified as Tentatively Ready based on six votes in favour.

Proposed resolution:

This wording is relative to N3936.

1. Change 20.7.3.1 [pointer.traits.types] as indicated:

   >     typedef see below element_type;

   > > -1- Type: `Ptr::element_type` if such a type exists the qualified-id `Ptr::element_type` is valid and denotes a type (14.8.2 [temp.deduct]); otherwise, `T` if `Ptr` is a class template instantiation of the form `SomePointer<T, Args>`, where `Args` is zero or more type arguments; otherwise, the specialization is ill-formed.

   >     typedef see below difference_type;

   > > -2- Type: `Ptr::difference_type` if such a type exists the qualified-id `Ptr::difference_type` is valid and denotes a type (14.8.2 [temp.deduct]); otherwise, `std::ptrdiff_t`.

   >     template <class U> using rebind = see below;

   > > -3- Alias template: `Ptr::rebind<U>` if such a type exists the qualified-id `Ptr::rebind<U>` is valid and denotes a type (14.8.2 [temp.deduct]); otherwise, `SomePointer<U, Args>` if `Ptr` is a class template instantiation of the form `SomePointer<T, Args>`, where `Args` is zero or more type arguments; otherwise, the instantiation of `rebind` is ill-formed.

2. Change 20.7.7.1 [allocator.uses.trait] p1 as indicated:

   >     template <class T, class Alloc> struct uses_allocator;

   > > -1- Remarks: automatically detects whether `T` has a nested `allocator_type` that is convertible from `Alloc`. Meets the `BinaryTypeTrait` requirements (20.10.1). The implementation shall provide a definition that is derived from `true_type` if a type the qualified-id `T::allocator_type` exists is valid and denotes a type (14.8.2 [temp.deduct]) and `is_convertible<Alloc, T::allocator_type>::value != false`, otherwise it shall be derived from `false_type`. […]

3. Change 20.7.8.1 [allocator.traits.types] as indicated:

   >     typedef see below pointer;

   > > -1- Type: `Alloc::pointer` if such a type exists the qualified-id `Alloc::pointer` is valid and denotes a type (14.8.2 [temp.deduct]); otherwise, `value_type*`.

   >     typedef see below const_pointer;

   > > -2- Type: `Alloc::const_pointer` if such a type exists the qualified-id `Alloc::const_pointer` is valid and denotes a type (14.8.2 [temp.deduct]); otherwise, `pointer_traits<pointer>::rebind<const value_type>`.

   >     typedef see below void_pointer;

   > > -3- Type: `Alloc::void_pointer` if such a type exists the qualified-id `Alloc::void_pointer` is valid and denotes a type (14.8.2 [temp.deduct]); otherwise, `pointer_traits<pointer>::rebind<void>`.

   >     typedef see below const_void_pointer;

   > > -4- Type: `Alloc::const_void_pointer` if such a type exists the qualified-id `Alloc::const_void_pointer` is valid and denotes a type (14.8.2 [temp.deduct]); otherwise, `pointer_traits<pointer>::rebind<const void>`.

   >     typedef see below difference_type;

   > > -5- Type: `Alloc::difference_type` if such a type exists the qualified-id `Alloc::difference_type` is valid and denotes a type (14.8.2 [temp.deduct]); otherwise, `pointer_traits<pointer>::difference_type`.

   >     typedef see below size_type;

   > > -6- Type: `Alloc::size_type` if such a type exists the qualified-id `Alloc::size_type` is valid and denotes a type (14.8.2 [temp.deduct]); otherwise, `make_unsigned_t<difference_type>`.

   >     typedef see below propagate_on_container_copy_assignment;

   > > -7- Type: `Alloc::propagate_on_container_copy_assignment` if such a type exists the qualified-id `Alloc::propagate_on_container_copy_assignment` is valid and denotes a type (14.8.2 [temp.deduct]); otherwise, `false_type`.

   >     typedef see below propagate_on_container_move_assignment;

   > > -8- Type: `Alloc::propagate_on_container_move_assignment` if such a type exists the qualified-id `Alloc::propagate_on_container_move_assignment` is valid and denotes a type (14.8.2 [temp.deduct]); otherwise, `false_type`.

   >     typedef see below propagate_on_container_swap;

   > > -9- Type: `Alloc::propagate_on_container_swap` if such a type exists the qualified-id `Alloc::propagate_on_container_swap` is valid and denotes a type (14.8.2 [temp.deduct]); otherwise, `false_type`.

   >     template <class T> using rebind_alloc = see below;

   > > -10- Alias template: `Alloc::rebind<T>::other` if such a type exists the qualified-id `Alloc::rebind<T>::other` is valid and denotes a type (14.8.2 [temp.deduct]); otherwise, `Alloc<T, Args>` if `Alloc` is a class template instantiation of the form `Alloc<U, Args>`, where `Args` is zero or more type arguments; otherwise, the instantiation of `rebind_alloc` is ill-formed.

4. Change 20.8.1.2 [unique.ptr.single] p3 as indicated:

   > -3- If the type qualified-id `remove_reference_t<D>::pointer` exists is valid and denotes a type (14.8.2 [temp.deduct]), then `unique_ptr<T, D>::pointer` shall be a synonym for `remove_reference_t<D>::pointer`. […]

5. Change 23.2.3 [sequence.reqmts] p3 as indicated:

   > -3- In Tables 100 and 101, `X` denotes a sequence container class, `a` denotes a value of `X` containing elements of type `T`, `A` denotes `X::allocator_type` if it exists the qualified-id `X::allocator_type` is valid and denotes a type (14.8.2 [temp.deduct]) and `std::allocator<T>` if it doesn't, […]

### 2364. `deque` and `vector` `pop_back` don't specify iterator invalidation requirements

Section: 23.3.3.4 [deque.modifiers], 23.3.6.5 [vector.modifiers] Status: WP Submitter: Deskin Miller Opened: 2014-02-17 Last modified: 2015-05-22

View all other issues in [deque.modifiers].

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Discussion:

I think it's obvious that `vector::pop_back` invalidates the path-the-end iterator, but I cannot find language that says so to my satisfaction in the Standard. N3797 23.2.3 [sequence.reqmts] Table 101 lists `a.pop_back()` semantics as "Destroys the last element", but nowhere do I see this required to invalidate the end iterator (or iterators previously referring to the last element). [container.reqmts.general]/11 states "Unless otherwise specified (either explicitly or by defining a function in terms of other functions), invoking a container member function or passing a container as an argument to a library function shall not invalidate iterators to, or change the values of, objects within that container." 23.3.6.5 [vector.modifiers]/3 says that each flavor of `vector::erase` "Invalidates iterators and references at or after the point of the erase", but `pop_back` isn't discussed, and it wasn't specified in terms of `erase`.

Similarly for `std::deque`, 23.2.3 [sequence.reqmts] Table 101 and [container.reqmts.general]/11 both apply. Yet 23.3.3.4 [deque.modifiers] likewise doesn't discuss `pop_back` nor `pop_front`. Furthermore paragraph 4 fails to specify the iterator-invalidation guarantees when erasing the first element but not the last.

Both `std::vector` and `std::deque` are in contrast to `std::list`, which says in 23.3.5.4 [list.modifiers]/3 regarding `pop_back` (as well as all forms of `erase`, `pop_front`, and `clear`) "Effects: Invalidates only the iterators and references to the erased elements."

[2014-06-16 Jonathan comments and improves wording]

I believe this reflects our preferred form discussed earlier, specifically putting the signatures with the `erase` signatures, so that the full specification of `erase()` applies to the `pop_xxx()` functions. This covers the case for `deque` where `pop_front()` erases the only element (which is both the first and last element).

Open question: the "erase" wording talks about "An erase operation" — are `pop_front` and `pop_back` clearly covered by "erase operations"? I believe so, as 23.3.3.1 [deque.overview]/1 and other places talk about "insert and erase operations" which covers `push`/`pop` functions too. I've added a note which could be used to clarify that if desired. Previous resolution [SUPERSEDED]:

> This wording is relative to N3936.

> 1. Change 23.3.3.4 [deque.modifiers] as indicated:

>    >     iterator erase(const_iterator position);
>    >     iterator erase(const_iterator first, const_iterator last);

>    > > -4- Effects: An erase operation that erases the last element of a deque invalidates only the past-the-end iterator and all iterators and references to the erased elements. An erase operation that erases the first element of a deque but not the last element invalidates only iterators and references to the erased elements. An erase operation that erases neither the first element nor the last element of a deque invalidates the past-the-end iterator and all iterators and references to all the elements of the deque.

>    > > -5- […]

>    > > -6- […]

>    >     void pop_front();
>    >     void pop_back();

>    > > -?- Effects: `pop_front` invalidates iterators and references to the first element of the `deque`. `pop_back` invalidates the past-the-end iterator, and all iterators and references to the last element of the `deque`.

> 2. Change 23.3.6.5 [vector.modifiers] as indicated:

>    > > -5- […]

>    >     void pop_back();

>    > > -?- Effects: Invalidates the past-the-end iterator, and iterators and references to the last element of the vector.

[2014-06-21 Rapperswil]

Tony van Eerd: Would be good to define "an erase operation is ..." somewhere.

AM: The containers clause is known to be suboptimal in many ways.

Looks good

[Urbana 2014-11-07: Move to Ready]

Proposed resolution:

This wording is relative to N3936.

1. Change 23.3.3.4 [deque.modifiers] as indicated:

   >     iterator erase(const_iterator position);
   >     iterator erase(const_iterator first, const_iterator last);
   >     void pop_front();
   >     void pop_back();

   > > -4- Effects: An erase operation that erases the last element of a deque invalidates only the past-the-end iterator and all iterators and references to the erased elements. An erase operation that erases the first element of a deque but not the last element invalidates only iterators and references to the erased elements. An erase operation that erases neither the first element nor the last element of a deque invalidates the past-the-end iterator and all iterators and references to all the elements of the deque. [Note: `pop_front` and `pop_back` are erase operations — end note]

2. Change 23.3.6.5 [vector.modifiers] as indicated:

   >     iterator erase(const_iterator position);
   >     iterator erase(const_iterator first, const_iterator last);
   >     void pop_back();

   > > -3- Effects: Invalidates iterators and references at or after the point of the erase.

### 2365. Missing `noexcept` in `shared_ptr::shared_ptr(nullptr_t)`

Section: 20.8.2.2 [util.smartptr.shared] Status: WP Submitter: Cassio Neri Opened: 2014-02-13 Last modified: 2015-04-08

View other active issues in [util.smartptr.shared].

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Discussion:

The declaration and definition of `shared_ptr::shared_ptr(nullptr_t)`, given in 20.8.2.2 [util.smartptr.shared], is

>     constexpr shared_ptr(nullptr_t) : shared_ptr() { }

The intention seems clear: this constructor should have the same semantics of the default constructor. However, contrarily to the default constructor, this one is not `noexcept`. In contrast, `unique_ptr::unique_ptr(nullptr_t)` is `noexcept`, as per 20.8.1.2 [unique.ptr.single]:

>     constexpr unique_ptr(nullptr_t) noexcept : unique_ptr() { }

Both libstdc++ and libc++ have added `noexcept` to `shared_ptr::shared_ptr(nullptr_t)`. Microsoft's STL has not.

[2014-03-26 Library reflector vote]

The issue has been identified as Tentatively Ready based on six votes in favour.

Proposed resolution:

This wording is relative to N3936.

1. Change class template `shared_ptr` synopsis, 20.8.2.2 [util.smartptr.shared], as indicated:

   >     constexpr shared_ptr(nullptr_t) noexcept : shared_ptr() { }

### 2369. `constexpr max(initializer_list)` vs `max_element`

Section: 25.4.7 [alg.min.max] Status: WP Submitter: Marc Glisse Opened: 2014-02-21 Last modified: 2015-05-22

View all other issues in [alg.min.max].

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Discussion:

As part of the resolution for LWG issue 2350, `max(initializer_list)` was marked as `constexpr`. Looking at two implementations of this function (libstdc++ and libc++), both implement it in terms of `max_element`, which is not marked as `constexpr`. This is inconsistent and forces some small amount of code duplication in the implementation. Unless we remove `constexpr` from this overload of `max`, I believe we should add `constexpr` to `max_element`.

[2015-02 Cologne]

AM: Can we implement this with the C++14 `constexpr` rules? JM: Yes. AM: Ready? [Yes]

Accepted.

Proposed resolution:

This wording is relative to N3936.

1. In 25.1 [algorithms.general], header `<algorithm>` synopsis, and 25.4.7 [alg.min.max], change as indicated (add `constexpr` to every signature from the first `min_element` to the second `minmax_element`)::

   >     template<class ForwardIterator>
   >     constexpr ForwardIterator min_element(ForwardIterator first, ForwardIterator last);
   >     template<class ForwardIterator, class Compare>
   >     constexpr ForwardIterator min_element(ForwardIterator first, ForwardIterator last,
   >                                           Compare comp);
   >     […]
   >     template<class ForwardIterator>
   >     constexpr ForwardIterator max_element(ForwardIterator first, ForwardIterator last);
   >     template<class ForwardIterator, class Compare>
   >     constexpr ForwardIterator max_element(ForwardIterator first, ForwardIterator last,
   >                                           Compare comp);
   >     […]
   >     template<class ForwardIterator>
   >     constexpr pair<ForwardIterator, ForwardIterator>
   >     minmax_element(ForwardIterator first, ForwardIterator last);
   >     template<class ForwardIterator, class Compare>
   >     constexpr pair<ForwardIterator, ForwardIterator>
   >     minmax_element(ForwardIterator first, ForwardIterator last, Compare comp);

### 2371. [fund.ts] No template aliases defined for new type traits

Section: 20.10.2 [meta.type.synop] Status: WP Submitter: Joe Gottman Opened: 2014-03-07 Last modified: 2015-04-08

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Discussion:

Addresses: fund.ts

The library fundamentals specification defines two new type trait template classes: `invocation_type` and `raw_invocation_type`. But it does not define the corresponding template aliases. Note that both of these classes define a member typedef `type` and no other public members, so according to the argument in N3887 the template aliases should be defined.

[2013-06-21 Rapperswil]

Accept for Library Fundamentals TS Working Paper

Proposed resolution:

This wording is relative to N3908.

1. Add the following to section 3.3.1[meta.type.synop] of the Library Fundamentals specification as indicated:

   >     namespace std {
   >     namespace experimental {
   >     inline namespace fundamentals_v1 {
   >       […]
   >       // 3.3.2, Other type transformations
   >       template <class> class invocation_type; // not defined
   >       template <class F, class... ArgTypes> class invocation_type<F(ArgTypes...)>;
   >       template <class> class raw_invocation_type; // not defined
   >       template <class F, class... ArgTypes> class raw_invocation_type<F(ArgTypes...)>;

   >       template <class T>
   >         using invocation_type_t = typename invocation_type<T>::type;
   >       template <class T>
   >         using raw_invocation_type_t = typename raw_invocation_type<T>::type;

   >     } // namespace fundamentals_v1
   >     } // namespace experimental
   >     } // namespace std

### 2374. [fund.ts] Remarks for `optional::to_value` are too restrictive

Section: X [optional.object.observe] Status: WP Submitter: Jonathan Wakely Opened: 2014-03-25 Last modified: 2015-04-08

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Discussion:

Addresses: fund.ts

In Bristol I think I claimed that the remarks for `optional::to_value()` were unimplementable and the function could only be `constexpr` if both constructors that could be called were `constexpr`, but I was wrong. The remarks should be reverted to the original pre-n3793 form.

[2013-06-21 Rapperswil]

Accept for Library Fundamentals TS Working Paper

Proposed resolution:

This wording is relative to N3908.

1. Change [optional.object.observe] p23 of the Library Fundamentals specification as indicated:

   >     template <class U> constexpr T value_or(U&& v) const &;

   > > […]

   > > -23- Remarks: If both constructors of `T` which could be selected are `constexpr` constructors the selected constructor of `T` is a `constexpr` constructor, this function shall be a `constexpr` function.

### 2376. `bad_weak_ptr::what()` overspecified

Section: 20.8.2.1 [util.smartptr.weakptr] Status: WP Submitter: Jonathan Wakely Opened: 2014-03-27 Last modified: 2015-04-08

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Discussion:

20.8.2.1 [util.smartptr.weakptr] p2 requires `bad_weak_ptr` to return precisely the string `"bad_weak_ptr"`.

There was general agreement on the reflector and at the Issaquah meeting that this is over-constrained and implementation should be free to return something more descriptive if desired.

The proposed resolution makes `bad_weak_ptr` consistent with other exception types such as `bad_alloc` and `bad_cast`.

If accepted, the P/R for issue 2233, which currently uses similar wording to `bad_weak_ptr`, could be updated appropriately.

[2014-03-27 Library reflector vote]

The issue has been identified as Tentatively Ready based on six votes in favour.

Proposed resolution:

This wording is relative to N3936.

1. Edit 20.8.2.1 [util.smartptr.weakptr]:

   >     bad_weak_ptr() noexcept;

   > > -2- Postconditions: `what()` returns `"bad_weak_ptr"`an implementation-defined NTBS.

### 2377. `std::align` requirements overly strict

Section: 20.7.5 [ptr.align] Status: WP Submitter: Peter Dimov Opened: 2014-03-30 Last modified: 2015-04-08

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Discussion:

`std::align` requires that its `alignment` argument shall be "a fundamental alignment value or an extended alignment value supported by the implementation in this context".

This requirement is overly strict. There are use cases that require a buffer aligned at values that are not tied to the C++ implementation, such as page size, cache line size, sector size. These come from the hardware or the OS and are generally not known until run time. The implementation of `std::align` does not depend on the requirement that `alignment` be a fundamental or an extended alignment value; any power of two would be handled the same way.

In addition, it is not possible for the user to even determine whether a value is "a fundamental alignment value or an extended alignment value supported by the implementation in this context". One would expect values coming from `alignof` to be fine, but I'm not sure whether even that is guaranteed in the presence of `alignas`.

Therefore, I propose that

> Requires:

> - `alignment` shall be a fundamental alignment value or an extended alignment value supported by the implementation in this context

be changed to

> Requires:

> - `alignment` shall be a power of two

[2014-06-16 Rapperswil]

Move to Ready

Proposed resolution:

This wording is relative to N3936.

1. Edit 20.7.5 [ptr.align] p2 as indicated:

   >     void* align(std::size_t alignment, std::size_t size,
   >       void*& ptr, std::size_t& space);

   > > -1- […]

   > > -2- Requires:

   > > - `alignment` shall be a fundamental alignment value or an extended alignment value supported by the implementation in this context power of two

### 2378. Behaviour of standard exception types

Section: 18.6.2.1 [bad.alloc], 18.6.2.2 [new.badlength], 18.7.2 [bad.cast], 18.7.3 [bad.typeid], 18.8.2 [bad.exception] Status: WP Submitter: Andy Sawyer Opened: 2014-03-31 Last modified: 2015-05-22

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Discussion:

I think we have an issue with the specification of some of the standard exception types. In particular, several of them have default constructors with remarks to the effect that "The result of calling `what()` on the newly constructed object is implementation-defined". (In some cases this is contradictory to a further specification of `what()`, which is specified to return an implementation-defined NTBS.)

Previous resolution from Andy [SUPERSEDED]:

> This wording is relative to N3936.

> 1. Edit 18.6.2.1 [bad.alloc] p3 as indicated:

>    >     bad_alloc() noexcept;

>    > > […]

>    > > -3- Remarks: The result of calling `what()` on the newly constructed object is implementation-defined `what()` returns an implementation-defined NTBS.

> 2. Edit 18.6.2.2 [new.badlength] p3 as indicated: [Drafting note: Added the Postcondition, since we don't say anything else about `bad_array_new_length::what()` — end of note]

>    >     bad_array_new_length() noexcept;

>    > > […]

>    > > -3- Remarks Postcondition: The result of calling `what()` on the newly constructed object is implementation-defined `what()` returns an implementation-defined NTBS.

> 3. Edit 18.7.2 [bad.cast] p3 as indicated:

>    >     bad_cast() noexcept;

>    > > […]

>    > > -3- Remarks: The result of calling `what()` on the newly constructed object is implementation-defined..

> 4. Edit 18.7.3 [bad.typeid] p3 as indicated:

>    >     bad_typeid() noexcept;

>    > > […]

>    > > -3- Remarks: The result of calling `what()` on the newly constructed object is implementation-defined..

> 5. Edit 18.8.2 [bad.exception] p3 as indicated:

>    >     bad_exception() noexcept;

>    > > […]

>    > > -3- Remarks: The result of calling `what()` on the newly constructed object is implementation-defined..

[2014-06-17, Rapperswil]

Jonathan provides alternative wording.

[2015-02, Cologne]

NJ: I don't know why we need the explict statement about `what()` here, since `bad_array_new_length` already derives.
AM: yes.
NJ: Then "what()" is missing from the synopsis.
AM: Yes, that's an error and it needs to be added.

Conclusion: Update the wording to add the missing entry in the synopsis.

AM: The issue needs another update; we need to add missing "Remarks". DK updates the paper.
AM: Any objections to "tentatively ready"? No objections.

Proposed resolution:

This wording is relative to N4296.

1. Edit 18.6.2.1 [bad.alloc] p3 as indicated:

   >     bad_alloc() noexcept;

   > > […]

   > > -3- Remarks: The result of calling `what()` on the newly constructed object is implementation-defined.

2. Edit 18.6.2.1 [bad.alloc] p5 as indicated:

   >     virtual const char* what() const noexcept;

   > > -5- Returns: An implementation-defined NTBS.

   > > -?- Remarks: The message may be a null-terminated multibyte string (17.5.2.1.4.2), suitable for conversion and display as a `wstring` (21.3, 22.4.1.4).

3. Edit class `bad_array_new_length` synopsis 18.6.2.2 [new.badlength] as indicated:

   >     namespace std {
   >       class bad_array_new_length : public bad_alloc {
   >       public:
   >         bad_array_new_length() noexcept;
   >         virtual const char* what() const noexcept;
   >       };
   >     }

4. Edit 18.6.2.2 [new.badlength] as indicated:

   >     bad_array_new_length() noexcept;

   > > […]

   > > -3- Remarks: The result of calling `what()` on the newly constructed object is implementation-defined.

   >     virtual const char* what() const noexcept;

   > > -?- Returns: An implementation-defined NTBS.

   > > -?- Remarks: The message may be a null-terminated multibyte string (17.5.2.1.4.2), suitable for conversion and display as a `wstring` (21.3, 22.4.1.4).

5. Edit 18.7.2 [bad.cast] p3 as indicated:

   >     bad_cast() noexcept;

   > > […]

   > > -3- Remarks: The result of calling `what()` on the newly constructed object is implementation-defined..

6. Edit 18.7.3 [bad.typeid] p3 as indicated:

   >     bad_typeid() noexcept;

   > > […]

   > > -3- Remarks: The result of calling `what()` on the newly constructed object is implementation-defined..

7. Edit 18.8.2 [bad.exception] p3 as indicated:

   >     bad_exception() noexcept;

   > > […]

   > > -3- Remarks: The result of calling `what()` on the newly constructed object is implementation-defined..

### 2387. More nested types that must be accessible and unambiguous

Section: 20.9.2 [func.require], 20.9.4 [refwrap] Status: WP Submitter: Jonathan Wakely Opened: 2014-05-23 Last modified: 2015-04-08

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Discussion:

Further to 2299 and 2361, 20.9.2 [func.require] p3 and 20.9.4 [refwrap] p3 and p4 talk about member types without any mention of being accessible and unambiguous.

[2014-06-05 Daniel provides wording]

[2014-06-06 Library reflector vote]

The issue has been identified as Tentatively Ready based on six votes in favour.

Proposed resolution:

This wording is relative to N3936.

1. Change 20.9.2 [func.require] p3 as indicated:

   > -3- If a call wrapper (20.9.1) has a weak result type the type of its member type `result_type` is based on the type `T` of the wrapper's target object (20.9.1):

   > - if `T` is a pointer to function type, `result_type` shall be a synonym for the return type of `T`;

   > - if `T` is a pointer to member function, `result_type` shall be a synonym for the return type of `T`;

   > - if `T` is a class type and the qualified-id `T::result_type` is valid and denotes a type (14.8.2 [temp.deduct])with a member type `result_type`, then `result_type` shall be a synonym for `T::result_type`;

   > - otherwise `result_type` shall not be defined.

2. Change 20.9.4 [refwrap] p3+p4 as indicated:

   > -3- The template instantiation `reference_wrapper<T>` shall define a nested type named `argument_type` as a synonym for `T1` only if the type `T` is any of the following:

   > - a function type or a pointer to function type taking one argument of type `T1`

   > - a pointer to member function `R T0::f` cv (where cv represents the member function_s cv-qualifiers); the type `T1` is cv `T0*`

   > - a class type where the qualified-id `T::argument_type` is valid and denotes a type (14.8.2 [temp.deduct])with a member type `argument_type`; the type `T1` is `T::argument_type`.

   > -4- The template instantiation `reference_wrapper<T>` shall define two nested types named `first_argument_type` and `second_argument_type` as synonyms for `T1` and `T2`, respectively, only if the type `T` is any of the following:

   > - a function type or a pointer to function type taking two arguments of types `T1` and `T2`

   > - a pointer to member function `R T0::f(T2)` cv (where cv represents the member function's cv-qualifiers); the type `T1` is cv `T0*`

   > - a class type where the qualified-ids `T::first_argument_type` and `T::second_argument_type` are both valid and both denote types (14.8.2 [temp.deduct])with member types `first_argument_type` and `second_argument_type`; the type `T1` is `T::first_argument_type`. and the type `T2` is `T::second_argument_type`.

### 2389. [fund.ts] `function::operator=` is over-specified and handles allocators incorrectly

Section: X [mods.func.wrap] Status: WP Submitter: Pablo Halpern Opened: 2014-05-23 Last modified: 2015-04-08

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Discussion:

Addresses: fund.ts

This issue against the TS is similar to LWG 2386, which is against the standard. The Effects clauses for the assignment operator for class template `function` are written as code that constructs a temporary `function` and then swaps it with `*this`. The intention appears to be that assignment should have the strong exception guarantee, i.e., `*this` is not modified if an exception is thrown. The description in the standard is incorrect when `*this` was originally constructed using an allocator. The TS attempts to correct the problem, but the correction is incomplete.

The wording in the TS uses `get_memory_resource()` to construct a temporary `function` object with the same allocator as the left-hand size (lhs) of the assignment. The intended result of using this pattern was that the allocator for `*this` would be unchanged, but it doesn't quite work. The problem is that the allocator returned by `get_memory_resource()` is not the same type as the type-erased allocator used to construct the `function` object, but rather a type-erased distillation of that type that is insufficient for making a true copy of the allocator. The rules for type-erased allocators in the TS ([memory.type.erased.allocator]) specify that the lifetime of the object returned by `get_memory_resource()` is sometimes tied to the lifetime of `*this`, which might cause the (single copy of) the allocator to be destroyed if the `swap` operation destroys and reconstructs `*this`, as some implementations do (and are allowed to do).

The desired behavior is that assignment would leave the allocator of the lhs unchanged. The way to achieve this behavior is to construct the temporary `function` using the original allocator. Unfortunately, we cannot describe the desired behavior in pure code, because `get_memory_resource()` does not really name the type-erased allocator, as mentioned above. The PR below, therefore, uses pseudo-code, inventing a fictitious `ALLOCATOR_OF(f)` expression that evaluates to the actual allocator type, even if that allocator was type erased. I have implemented this PR successfully.

[2014-06-21, Rapperswil]

Apply to Library Fundamentals TS (after removing the previous "Throws: Nothing" element to prevent an editorial conflict with 2401).

Proposed resolution:

This wording is relative to N3908.

1. Change in [mods.func.wrap] in the Library TS as indicated:

   > In the following descriptions, let `ALLOCATOR_OF(f)` be the allocator specified in the construction of `function` `f`, or `allocator<char>()` if no allocator was specified.

   >     function& operator=(const function& f);

   > > -5- Effects: `function(allocator_arg, get_memory_resource()ALLOCATOR_OF(*this), f).swap(*this);`

   > > […]

   >     function& operator=(function&& f);

   > > -8- Effects: `function(allocator_arg, get_memory_resource()ALLOCATOR_OF(*this), std::move(f)).swap(*this);`

   > > […]

   >     function& operator=(nullptr_t);

   > > -11- Effects: If `*this != NULL`, destroys the target of `this`.

   > > -12- Postconditions: `!(*this)`. The memory resource returned by `get_memory_resource()` after the assignment is equivalent to the memory resource before the assignment. [Note: the address returned by `get_memory_resource()` might change — end note]

   > > -13- Returns: `*this`

   >     template<class F> function& operator=(F&& f);

   > > -15- Effects: `function(allocator_arg, get_memory_resource()ALLOCATOR_OF(*this), std::forward<F>(f)).swap(*this);`

   > > […]

   >     template<class F> function& operator=(reference_wrapper<F> f);

   > > -18- Effects: `function(allocator_arg, get_memory_resource()ALLOCATOR_OF(*this), f).swap(*this);`

   > > […]

### 2390. [fund.ts] Invocation types and rvalues

Section: X [mods.func.wrap] Status: WP Submitter: Michael Spertus Opened: 2014-05-26 Last modified: 2015-04-08

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Discussion:

Addresses: fund.ts

`invocation_type` falls short of its stated goals in the following case. Using the notation of Invocation type traits, consider

>     void f(int const& i);
>     more_perfect_forwarding_async(f, int(7)); // Oops. Dangling reference because rvalue gone when async runs

This was always the advertised intent of the proposal, but while the language reflects this for the first parameter in the case of a member pointer, it failed to include corresponding language for other parameters.

[2014-06-18, Rapperswil]

Mike Spertus, Richard Smith, Jonathan Wakely, and Jeffrey Yasskin suggest improved wording.

Previous resolution [SUPERSEDED]:

> This wording is relative to N3908.

> 1. Change Table 3, [meta.trans.other] in the Library TS as indicated:

>    > Table 3 — Other type transformations
>    > Template | Condition | Comments
>    > --- | --- | ---
>    > `…`

>    > `template <class Fn, class... ArgTypes>
>    > struct invocation_type<Fn(ArgTypes...)>;`

>    > `Fn` and all types in the parameter pack `ArgTypes`
>    > shall be complete types, (possibly cv-qualified) `void`,
>    > or arrays of unknown bound.

>    > If `A1, A2,...` denotes `ArgTypes...` and
>    > `raw_invocation_type<Fn(ArgTypes...)>::type`
>    > is the function type `R(T1, T2, ...)` then let `Ui` be
>    > `decay<Ai>::type` if `Ai` is an rvalue otherwise `Ti`.
>    > If and `Fn` is a pointer to member type and `T1` is
>    > an rvalue reference, then let `U1` be `R(decay<T1>::type,
>    > T2, ...)`.
>    > Otherwise, `raw_invocation_type<Fn(ArgTypes...)>::type`
>    > The member typedef `type` shall equal `R(U1, U2, ...)`.

[2013-06-21 Rapperswil]

Accept for Fundamentals TS Working Paper

Proposed resolution:

This wording is relative to N4023.

1. Change Table 3, [meta.trans.other] in the Library TS as indicated:

   > Table 3 — Other type transformations
   > Template | Condition | Comments
   > --- | --- | ---
   > `…`

   > `template <class Fn, class... ArgTypes>
   > struct invocation_type<Fn(ArgTypes...)>;`

   > `Fn` and all types in the parameter pack `ArgTypes`
   > shall be complete types, (possibly cv-qualified) `void`,
   > or arrays of unknown bound.

   > The nested typedef `invocation_type<Fn(ArgTypes...)>::type`
   > shall be defined as follows. If
   > `raw_invocation_type<Fn(ArgTypes...)>::type`
   > does not exist, there shall be no member typedef `type`. Otherwise:

   > - Let `A1, A2,` … denote `ArgTypes...`

   > - Let `R(T1, T2, …)` denote
   >   `raw_invocation_type_t<Fn(ArgTypes...)>`

   > - Then the member typedef `type` shall name the function
   >   type `R(U1, U2, …)` where `Ui` is `decay_t<Ai>`
   >   if `declval<Ai>()` is an rvalue otherwise `Ti`.

   > If `raw_invocation_type<Fn(ArgTypes...)>::type`
   > is the function type `R(T1, T2, …)`
   > and `Fn` is a pointer to member type and `T1` is
   > an rvalue reference, then `R(decay<T1>::type,
   > T2, …)`.
   > Otherwise, `raw_invocation_type<Fn(ArgTypes...)>::type`.

### 2395. [fund.ts] Preconditions: is defined nowhere

Section: X [mods.func.wrap], X [memory.resource.priv], X [memory.polymorphic.allocator.ctor], X [memory.polymorphic.allocator.mem], X [memory.resource.pool.ctor], X [memory.resource.monotonic.buffer.ctor] Status: WP Submitter: Zhihao Yuan Opened: 2014-06-09 Last modified: 2015-04-08

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Discussion:

Addresses: fund.ts

This element has been introduced by N3916, but the standard does not define it. The standard defines Requires: to indicate a precondition (17.5.1.4 [structure.specifications] p3).

Proposed wording:

Substitute all Preconditions: with Requires:.

[2013-06-21 Rapperswil]

Accept for Fundamentals TS Working Paper

Proposed resolution:

This wording is relative to N4023.

1. Substitute all Preconditions: with Requires:.

### 2396. `underlying_type` doesn't say what to do for an incomplete enumeration type

Section: 20.10.7.6 [meta.trans.other] Status: WP Submitter: Richard Smith Opened: 2014-06-12 Last modified: 2015-04-08

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Discussion:

Consider:

>     enum E {
>       e = std::underlying_type<E>::type(1)
>     };

Clearly this should be ill-formed, but the library section doesn't appear to ban it. Suggestion:

Change in 20.10.7.6 [meta.trans.other] Table 57:

> Template: `template<class T> struct underlying_type;`

> Condition: `T` shall be a complete an enumeration type (7.2)

> Comments: […]

[2014-06-16 Rapperswil]

Move to Ready

Proposed resolution:

This wording is relative to N3936.

1. Change Table 57 — "Other transformations" as indicated:

> Table 3 — Other type transformations
> Template | Condition | Comments
> --- | --- | ---
> `…`
> `template <class T> struct underlying_type;` | T shall be a complete an enumeration type (7.2) | […]

### 2397. `map<K, V>::emplace` and explicit `V` constructors

Section: 20.10.7.6 [meta.trans.other] Status: Resolved Submitter: Peter Dimov Opened: 2014-06-12 Last modified: 2015-05-05

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Discussion:

Please consider the following example:

>     #include <map>
>     #include <atomic>

>     int main()
>     {
>        std::map<int, std::atomic<int>> map_;

>        map_.emplace(1, 0);  // fail
>        map_.emplace(1);     // fail
>        map_.emplace(1, {}); // fail

>        map_.emplace(std::piecewise_construct,
>            std::tuple<int>(1), std::tuple<>()); // OK
>     }

The first three calls represent attempts by an ordinary programmer (in which role I appear today) to construct a map element. Since `std::atomic<int>` is non-copyable and immovable, I was naturally drawn to `emplace()` because it constructs in-place and hence doesn't need to copy or move. The logic behind the attempts was that `K=int` would be constructed from '`1`', and `V=std::atomic<int>` would be (directly) constructed by '`0`', default constructed, or constructed by '`{}`'.

Yet none of the obvious attempts worked.

I submit that at least two of the three ought to have worked, and that we have therefore a defect in either `map::emplace` or `pair`.

Ville:

There exists a related EWG issue for this.

Daniel:

If the proposal N4387 would be accepted, it would solve the first problem mentioned above.

[2015-02, Cologne]

AM: I think Peter's expectation is misguided that the second and third "//fail" cases should work.
DK: Howard's paper [note: which hasn't been written yet] will make the second case work... AM: ...but the third one will never work without core changes.

Case 1 is solved by DK's paper, cases 2 and 3 are not defects; at best they are extensions.

[2015-05, Lenexa]

STL: think this is covered with N4387
MC: this was accepted in Cologne
STL: only want to fix the first emplace
MC: leave alone and mark as closed by N4387

Proposed resolution:

Resolved by acceptance of N4387.

### 2399. `shared_ptr`'s constructor from `unique_ptr` should be constrained

Section: 20.8.2.2.1 [util.smartptr.shared.const] Status: WP Submitter: Stephan T. Lavavej Opened: 2014-06-14 Last modified: 2015-04-08

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Discussion:

Consider the following code:

>     #include <iostream>
>     #include <memory>
>     #include <string>

>     using namespace std;

>     void meow(const shared_ptr<int>& sp) {
>       cout << "int: " << *sp << endl;
>     }

>     void meow(const shared_ptr<string>& sp) {
>       cout << "string: " << *sp << endl;
>     }

>     int main() {
>       meow(make_unique<int>(1729));
>       meow(make_unique<string>("kitty"));
>     }

This fails to compile due to ambiguous overload resolution, but we can easily make this work. (Note: `shared_ptr`'s constructor from `auto_ptr` is also affected, but I believe that it's time to remove `auto_ptr` completely.)

[2014-06-16 Rapperswil]

Move to Ready

Proposed resolution:

This wording is relative to N3936.

1. Change 20.8.2.2.1 [util.smartptr.shared.const] around p33 as indicated:

   >     template <class Y, class D> shared_ptr(unique_ptr<Y, D>&& r);

   > -?- Remark: This constructor shall not participate in overload resolution unless `unique_ptr<Y, D>::pointer` is convertible to `T*`.

   > -33- Effects: Equivalent to `shared_ptr(r.release(), r.get_deleter())` when `D` is not a reference type, otherwise `shared_ptr(r.release(), ref(r.get_deleter()))`.

### 2400. `shared_ptr`'s `get_deleter()` should use `addressof()`

Section: 20.8.2.2.10 [util.smartptr.getdeleter] Status: WP Submitter: Stephan T. Lavavej Opened: 2014-06-14 Last modified: 2015-04-08

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Discussion:

The Standard Library should consistently use `addressof()` to defend itself against overloaded `operator&()`.

While we're in the neighbourhood, we should editorially change `0` to `nullptr`.

[2014-06-16 Rapperswil]

Move to Ready

Proposed resolution:

This wording is relative to N3936.

1. Change 20.8.2.2.10 [util.smartptr.getdeleter] as indicated:

   >     template <class D, class T> get_deleter(const shared_ptr<T>& p) noexcept;

   > -1- Returns: If `p` owns a deleter `d` of type cv-unqualified `D`, returns `&std::addressof(d)`; otherwise returns `0nullptr`. […]

### 2401. `std::function` needs more `noexcept`

Section: 20.9.12.2 [func.wrap.func] Status: WP Submitter: Stephan T. Lavavej Opened: 2014-06-14 Last modified: 2015-04-08

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Discussion:

There are two issues here:

1. `std::function`'s constructor from `nullptr_t` is marked as `noexcept`, but its assignment operator from `nullptr_t` isn't. This assignment can and should be marked as `noexcept`.

2. `std::function`'s comparisons with `nullptr_t` are marked as `noexcept` in two out of three places.

[2014-06-16 Rapperswil]

Move to Ready

Proposed resolution:

This wording is relative to N3936.

1. Change 20.9 [function.objects] p2, header `<functional>` synopsis, as indicated:

   >     namespace std {
   >       […]
   >       // 20.9.11 polymorphic function wrappers:
   >       […]
   >       template<class R, class... ArgTypes>
   >       bool operator==(const function<R(ArgTypes...)>&, nullptr_t) noexcept;
   >       template<class R, class... ArgTypes>
   >       bool operator==(nullptr_t, const function<R(ArgTypes...)>&) noexcept;
   >       template<class R, class... ArgTypes>
   >       bool operator!=(const function<R(ArgTypes...)>&, nullptr_t) noexcept;
   >       template<class R, class... ArgTypes>
   >       bool operator!=(nullptr_t, const function<R(ArgTypes...)>&) noexcept;
   >       […]
   >     }

2. Change 20.9.12.2 [func.wrap.func], class template `function` synopsis, as indicated:

   >     […]
   >     // 20.9.11.2.1, construct/copy/destroy:
   >     […]
   >     function& operator=(nullptr_t) noexcept;
   >     […]

3. Change 20.9.12.2.1 [func.wrap.func.con] before p16 as indicated:

   >     function& operator=(nullptr_t) noexcept;

### 2403. `stof()` should call `strtof()` and `wcstof()`

Section: 21.5 [string.conversions] Status: WP Submitter: Stephan T. Lavavej Opened: 2014-06-14 Last modified: 2015-05-22

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Discussion:

`stof()` is currently specified to call `strtod()`/`wcstod()` (which converts the given string to `double`) and then it's specified to convert that `double` to `float`. This performs rounding twice, which introduces error. Here's an example written up by James McNellis:

Consider the following number `X`:

>     1.999999821186065729339276231257827021181583404541015625 (X)

This number is exactly representable in binary as:

>     1.111111111111111111111101000000000000000000000000000001
>     * ^1st                  ^23rd                        ^52nd

I've marked the 23rd and 52nd fractional bits. These are the least significant bits for `float` and `double`, respectively.

If we convert this number directly to `float`, we take the 23 most significant bits:

>     1.11111111111111111111110

The next bit is a one and the tail is nonzero (the 54th fractional bit is a one), so we round up. This gives us the correctly rounded result:

>     1.11111111111111111111111

So far so good. But... If we convert `X` to `double`, we take the 52 most significant bits:

>     1.1111111111111111111111010000000000000000000000000000 (Y)

The next bit is a zero, so we round down (truncating the value). If we then convert `Y` to `float`, we take its 23 most significant bits:

>     1.11111111111111111111110

The next bit is a one and the tail is zero, so we round to even (leaving the value unchanged). This is off by 1ulp from the correctly rounded result.

[2014-06 Rapperswil]

Marshall Clow will look at this.

[Urbana 2014-11-07: Move to Ready]

Proposed resolution:

This wording is relative to N3936.

1. Change 21.5 [string.conversions] p4+p6 as indicated:

   >     float stof(const string& str, size_t* idx = 0);
   >     double stod(const string& str, size_t* idx = 0);
   >     long double stold(const string& str, size_t* idx = 0);

   > > -4- Effects: the first two These functions call `strtof(str.c_str(), ptr)`, `strtod(str.c_str(), ptr)`, and the third function calls `strtold(str.c_str(), ptr)`, respectively. Each function returns the converted result, if any. […]

   > > […]

   > > -6- Throws: `invalid_argument` if `strtof`, `strtod`, or `strtold` reports that no conversion could be performed. Throws `out_of_range` if `strtof`, `strtod`, or `strtold` sets `errno` to `ERANGE` or if the converted value is outside the range of representable values for the return type.

2. Change 21.5 [string.conversions] p11+p13 as indicated:

   >     float stof(const wstring& str, size_t* idx = 0);
   >     double stod(const wstring& str, size_t* idx = 0);
   >     long double stold(const wstring& str, size_t* idx = 0);

   > > -11- Effects: the first two These functions call `wcstof(str.c_str(), ptr)`, `wcstod(str.c_str(), ptr)`, and the third function calls `wcstold(str.c_str(), ptr)`, respectively. Each function returns the converted result, if any. […]

   > > […]

   > > -13- Throws: `invalid_argument` if `wcstof`, `wcstod`, or `wcstold` reports that no conversion could be performed. Throws `out_of_range` if `wcstof`, `wcstod`, or `wcstold` sets `errno` to `ERANGE`.

### 2404. `mismatch()`'s complexity needs to be updated

Section: 25.2.10 [mismatch] Status: WP Submitter: Stephan T. Lavavej Opened: 2014-06-14 Last modified: 2015-04-08

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Discussion:

N3671 updated the complexities of `equal()` and `is_permutation()`, but not `mismatch()`.

[2014-06-16 Rapperswil]

Move to Ready

Proposed resolution:

This wording is relative to N3936.

1. Change 25.2.10 [mismatch] p3 as indicated:

   > -3- Complexity: At most `min(last1 - first1, last2 - first2)` applications of the corresponding predicate.

### 2406. `negative_binomial_distribution` should reject `p == 1`

Section: 26.5.8.3.4 [rand.dist.bern.negbin] Status: WP Submitter: Stephan T. Lavavej Opened: 2014-06-14 Last modified: 2015-05-22

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Discussion:

26.5.8.3.4 [rand.dist.bern.negbin] p2 requires "`0 < p <= 1`". Consider what happens when `p == 1`. The discrete probability function specified by p1 involves "`* p^k * (1 - p)^i`". For `p == 1`, this is "`* 1^k * 0^i`", so every integer `i >= 0` is produced with zero probability. (Let's avoid thinking about `0^0`.)

Wikipedia states that `p` must be within `(0, 1)`, exclusive on both sides. Previous resolution [SUPERSEDED]:

> 1. Change 26.5.8.3.4 [rand.dist.bern.negbin] p2 as indicated: [Drafting note: This should be read as: Replace the symbol "`≤`" by "`<`" — end drafting note]

>    >     explicit negative_binomial_distribution(IntType k = 1, double p = 0.5);

>    > > -2- Requires: `0 < p ≤< 1` and `0 < k`.

[2014-11 Urbana]

SG6 suggests better wording.

[2014-11-08 Urbana]

Moved to Ready with the node.

There remains concern that the constructors are permitting values that may (or may not) be strictly outside the domain of the function, but that is a concern that affects the design of the random number facility as a whole, and should be addressed by a paper reviewing and addressing the whole clause, not picked up in the issues list one distribution at a time. It is still not clear that such a paper would be uncontroversial.

Proposed resolution:

This wording is relative to N4140.

1. Add a note after paragraph 1 before the synopsis in 26.5.8.3.4 [rand.dist.bern.negbin]:

   > A `negative_binomial_distribution` random number distribution produces random integers i≥0 distributed according to the discrete probability function

   > > P(i|k,p) = k + i - 1 i · p k · (1-p) i .

   > [Note: This implies that P(i|k,p) is undefined when `p == 1`. — end note]

   > Drafting note: P(i|k,p) should be in math font, and `p == 1` should be in code font.

### 2407. `packaged_task(allocator_arg_t, const Allocator&, F&&)` should neither be constrained nor `explicit`

Section: 30.6.9.1 [futures.task.members] Status: WP Submitter: Stephan T. Lavavej Opened: 2014-06-14 Last modified: 2015-05-22

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Discussion:

LWG 2097's resolution was slightly too aggressive. It constrained `packaged_task(allocator_arg_t, const Allocator&, F&&)`, but that's unnecessary because `packaged_task` doesn't have any other three-argument constructors. Additionally, it's marked as `explicit` (going back to WP N2798 when `packaged_task` first appeared) which is unnecessary.

[2015-02 Cologne]

Handed over to SG1.

[2015-05 Lenexa, SG1 response]

Back to LWG; not an SG1 issue.

[2015-05 Lenexa]

STL improves proposed wording by restoring the constraint again.

Proposed resolution:

This wording is relative to N3936.

1. Change 30.6.9 [futures.task] p2, class template `packaged_task` as indicated:

   >     template <class F>
   >     explicit packaged_task(F&& f);
   >     template <class F, class Allocator>
   >     explicit packaged_task(allocator_arg_t, const Allocator& a, F&& f);

2. Change 30.6.9.1 [futures.task.members] as indicated:

   >     template <class F>
   >     packaged_task(F&& f);
   >     template <class F, class Allocator>
   >     explicit packaged_task(allocator_arg_t, const Allocator& a, F&& f);

   > > […]

   > > -3- Remarks: These constructors shall not participate in overload resolution if `decay_t<F>` is the same type as `std::packaged_task<R(ArgTypes...)>`.

### 2408. SFINAE-friendly `common_type/iterator_traits` is missing in C++14

Section: 20.10.7.6 [meta.trans.other], 24.4.1 [iterator.traits] Status: WP Submitter: Daniel Krügler Opened: 2014-06-19 Last modified: 2015-04-08

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Discussion:

During the Rapperswil meeting the proposal N4041 had been discussed and there seemed to be strong consensus to apply the SFINAE-friendly definitions that currently exist within the fundamentals-ts to the C++ Standard working draft. This issue requests this change to happen.

Proposed resolution:

This wording is relative to N3936.

1. Change 20.10.7.6 [meta.trans.other] p3 as indicated:

   > -3- For the `common_type` trait applied to a parameter pack `T` of types, the member `type` shall be either defined or not present as follows:

   > - If `sizeof...(T)` is zero, there shall be no member `type`.

   > - If `sizeof...(T)` is one, let `T0` denote the sole type comprising `T`. The member typedef `type` shall denote the same type as `decay_t<T0>`.

   > - If `sizeof...(T)` is greater than one, let `T1`, `T2`, and `R`, respectively, denote the first, second, and (pack of) remaining types comprising `T`. [Note: `sizeof...(R)` may be zero. — end note] Finally, let `C` denote the type, if any, of an unevaluated conditional expression (5.16 [expr.cond]) whose first operand is an arbitrary value of type `bool`, whose second operand is an xvalue of type `T1`, and whose third operand is an xvalue of type `T2`. If there is such a type `C`, the member typedef `type` shall denote the same type, if any, as `common_type_t<C,R...>`. Otherwise, there shall be no member `type`.

   > The nested typedef `common_type::type` shall be defined as follows:

   > >     template <class ...T> struct common_type;

   > >     template <class T>
   > >     struct common_type<T> {
   > >       typedef decay_t<T> type;
   > >     };

   > >     template <class T, class U>
   > >     struct common_type<T, U> {
   > >       typedef decay_t<decltype(true ? declval<T>() : declval<U>())> type;
   > >     };

   > >     template <class T, class U, class... V>
   > >     struct common_type<T, U, V...> {
   > >       typedef common_type_t<common_type_t<T, U>, V...> type;
   > >     };

2. Change 24.4.1 [iterator.traits] p2 as indicated:

   > -2- The template `iterator_traits<Iterator>` is defined as shall have the following as publicly accessible members, and have no other members, if and only if `Iterator` has valid (14.8.2 [temp.deduct]) member types `difference_type`, `value_type`, `pointer`, `reference`, and `iterator_category`; otherwise, the template shall have no members:

   > >     namespace std {
   > >       template<class Iterator> struct iterator_traits {
   > >         typedef typename Iterator::difference_type difference_type;
   > >         typedef typename Iterator::value_type value_type;
   > >         typedef typename Iterator::pointer pointer;
   > >         typedef typename Iterator::reference reference;
   > >         typedef typename Iterator::iterator_category iterator_category;
   > >       };
   > >     }

### 2409. [fund.ts] SFINAE-friendly `common_type/iterator_traits` should be removed from the fundamental-ts

Section: X [mods.meta.trans.other], X [mods.iterator.traits] Status: WP Submitter: Daniel Krügler Opened: 2014-06-19 Last modified: 2015-04-08

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Discussion:

Addresses: fund.ts

During the Rapperswil meeting the proposal N4041 had been discussed and there seemed to be strong consensus to apply the SFINAE-friendly definitions that currently exist within the fundamentals-ts to the C++17 working draft. If this happens, the fundamentals-ts needs to remove its own specification regarding these templates. This issue requests this change to happen.

[2013-06-21 Rapperswil]

Accept for Fundamentals TS Working Paper

Proposed resolution:

This wording is relative to N4023 in regard to fundamental-ts changes.

1. In fundamental-ts, change Table 2 — "Significant features in this technical specification" as indicated:

   > Table 2 — Significant features in this technical specification
   > Doc. No. | Title | Primary Section | Macro Name Suffix | Value | Header
   > --- | --- | --- | --- | --- | ---
   > `…`
   > N3843 | A SFINAE- Friendly `common_type` | 2.4 [mods.meta.trans.other] | `common_type_sfinae` | `201402` | `<type_traits>`
   > N3843 | A SFINAE- Friendly `iterator_traits` | 2.5 [mods.iterator.traits] | `iterator_traits_sfinae` | `201402` | `<iterator>`
   > `…`

2. In fundamental-ts, remove the existing sub-clause 2.4 [mods.meta.trans.other] in its entirety:

   2.4 Changes to `std::common_type` [mods.meta.trans.other]

   > -1- […]

3. In fundamental-ts, remove the existing sub-clause 2.5 [mods.iterator.traits] in its entirety:

   2.5 Changes to `std::iterator_traits` [mods.iterator.traits]

   > -1- […]

### 2410. [fund.ts] `shared_ptr<array>`'s constructor from `unique_ptr` should be constrained

Section: X [mods.util.smartptr.shared.const] Status: WP Submitter: Jeffrey Yasskin Opened: 2014-06-16 Last modified: 2015-05-22

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Discussion:

Addresses: fund.ts

The proposed resolution for LWG 2399 doesn't apply cleanly to the Fundamentals TS, but the issue is still present.

[2015-02, Cologne]

Unanimous consent.

Proposed resolution:

This wording is relative to N4023 in regard to fundamental-ts changes.

1. In fundamental-ts, change [mods.util.smartptr.shared.const] p34 as indicated:

   >     template <class Y, class D> shared_ptr(unique_ptr<Y, D>&& r);

   > > -34- Requires Remarks: This constructor shall not participate in overload resolution unless `Y*` shall be is compatible with `T*`.

   > > -35- Effects: Equivalent to `shared_ptr(r.release(), r.get_deleter())` when `D` is not a reference type, otherwise `shared_ptr(r.release(), ref(r.get_deleter()))`.

   > > -36- Exception safety: If an exception is thrown, the constructor has no effect.

### 2411. `shared_ptr` is only contextually convertible to `bool`

Section: 20.8.2.2 [util.smartptr.shared] Status: WP Submitter: Jonathan Wakely Opened: 2014-06-21 Last modified: 2015-05-22

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Discussion:

N3920 made this edit, which is correct but unrelated to the support for arrays:

> Change 20.7.2.2 [util.smartptr.shared] p2 as follows:

> > Specializations of `shared_ptr` shall be `CopyConstructible`, `CopyAssignable`, and `LessThanComparable`, allowing their use in standard containers. Specializations of `shared_ptr` shall be contextually convertible to `bool`, allowing their use in boolean expressions and declarations in conditions. […]

That change is actually fixing a defect in the current wording and should be applied directly to the working paper, not just to the Library Fundamentals TS. The declarations of the conversion operator in 20.8.2.2 [util.smartptr.shared] and 20.8.2.2.5 [util.smartptr.shared.obs] are `explicit` which contradicts the "convertible to `bool`" statement. The intention is definitely for `shared_ptr` to only be contextually convertible to `bool`.

[Urbana 2014-11-07: Move to Ready]

Proposed resolution:

This wording is relative to N3936.

1. Change 20.8.2.2 [util.smartptr.shared] p2 as indicated:

   > -2- Specializations of `shared_ptr` shall be `CopyConstructible`, `CopyAssignable`, and `LessThanComparable`, allowing their use in standard containers. Specializations of `shared_ptr` shall be contextually convertible to `bool`, allowing their use in boolean expressions and declarations in conditions. The template parameter `T` of `shared_ptr` may be an incomplete type.

### 2415. Inconsistency between `unique_ptr` and `shared_ptr`

Section: 20.8.2.2.1 [util.smartptr.shared.const] Status: WP Submitter: Jonathan Wakely Opened: 2014-07-03 Last modified: 2015-05-22

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Discussion:

`unique_ptr` guarantees that it will not invoke its deleter if it stores a null pointer, which is useful for deleters that must not be called with a null pointer e.g.

>     unique_ptr<FILE, int(*)(FILE*)> fptr(file, &::fclose);

However, `shared_ptr` does invoke the deleter if it owns a null pointer, which is a silent change in behaviour when transferring ownership from `unique_ptr` to `shared_ptr`. That means the following leads to undefined behaviour:

>     std:shared_ptr<FILE> fp = std::move(fptr);

Peter Dimov's suggested fix is to construct an empty `shared_ptr` from a `unique_ptr` that contains a null pointer.

[2015-01-18 Library reflector vote]

The issue has been identified as Tentatively Ready based on eight votes in favour.

Proposed resolution:

This wording is relative to N4296.

1. Change 20.8.2.2.1 [util.smartptr.shared.const] p29 as indicated:

   >     template <class Y, class D> shared_ptr(unique_ptr<Y, D>&& r);

   > > […]

   > > -29- Effects: If `r.get() == nullptr`, equivalent to `shared_ptr()`. Otherwise, if `D` is not a reference type, equivalent to `shared_ptr(r.release(), r.get_deleter())`. Otherwise, equivalent to `shared_ptr(r.release(), ref(r.get_deleter()))`Equivalent to `shared_ptr(r.release(), r.get_deleter())` when `D` is not a reference type, otherwise `shared_ptr(r.release(), ref(r.get_deleter()))`.

### 2416. [fund.ts] `std::experimental::any allocator` support is unimplementable

Section: X [any.class] Status: Resolved Submitter: Jonathan Wakely Opened: 2014-06-16 Last modified: 2015-04-08

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Discussion:

Addresses: fund.ts

The allocator-extended copy constructor for `any` requires an arbitrary template parameter to be available in a type-erased context where the dynamic type of the contained object is known. This is not believed to be possible in C++.

If the allocator-extended copy constructor cannot be defined it questions the usefulness of the other allocator-extended constructors.

[Urbana, 2014-11]

Resolved by paper N4270.

Proposed resolution:

### 2418. [fund.ts] `apply` does not work with member pointers

Section: X [tuple.apply] Status: WP Submitter: Zhihao Yuan Opened: 2014-07-08 Last modified: 2015-05-22

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Discussion:

Addresses: fund.ts

The definition of `apply` present in §3.2.2 [tuple.apply] prevents this function template to be used with pointer to members type passed as the first argument.

> Effects:

> […]

> `return std::forward<F>(f)(std::get<I>(std::forward<Tuple>(t))...);`

This makes this utility inconsistent with other standard library components and limits its usability.

We propose to define its functionally in terms of `INVOKE`.

[2015-02, Cologne]

DK: We should use the new `std::invoke`.
TK: Is this a defect?
AM: `std::invoke` goes into C++17, and this is a defect against a TS based on C++14. We can change this later, but now leave it as INVOKE.
GR: The TS lets you have Editor's Notes, so leave a note to make that change for C++17.

[…]

GR: I can't see how we can assume this is part of the design. I cannot believe it was ever intended for this design to exclude function pointers.
AM: I can give you the exact evolution: We had "apply" as an example explaining the usefulness of `index_sequence`. Then someone looked at it and said, "why isn't this in the Standard". NJ to VV: Why are you against useful steps? We are trying to converge on a consistent standard across multiple documents. The alternative is to reopen this in a later discussion.
VV: All I said is that this is not defect, whether or not people like it.
AM: So you'd be fine with the issue, but not as a DR?
Straw poll: Who's happy to make this tentatively ready as a DR against the Fundamentals TS? Lots of agreement, no opposition, 3 neutrals

Proposed resolution:

This wording is relative to N4081 in regard to fundamental-ts changes.

1. Edit §3.2.2 [tuple.apply] paragraph 2:

   >     template <class F, class Tuple>
   >     constexpr decltype(auto) apply(F&& f, Tuple&& t);

   > > -2- Effects: Given the exposition only function

   > > >     template <class F, class Tuple, size_t... I>
   > > >     constexpr decltype(auto) apply_impl(  // exposition only
   > > >         F&& f, Tuple&& t, index_sequence<I...>) {
   > > >       return INVOKE(std::forward<F>(f)(, std::get<I>(std::forward<Tuple>(t))...);
   > > >     }

   > > > […]

### 2420. `function<void(ArgTypes...)>` does not discard the return value of the target object

Section: 20.9.12.2 [func.wrap.func] Status: WP Submitter: Agustín Bergé Opened: 2014-07-12 Last modified: 2015-05-22

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Discussion:

`function<void(ArgTypes...)>` should discard the return value of the target object. This behavior was in the original proposal, and it was removed (accidentally?) by the resolution of LWG 870.

Previous resolution [SUPERSEDED]:

> 1. Edit 20.9.12.2 [func.wrap.func] paragraph 2:

>    > A callable object `f` of type `F` is Callable for argument types `ArgTypes` and return type `R` if the expression `INVOKE(f, declval<ArgTypes>()..., R)`, considered as an unevaluated operand (Clause 5), is well formed (20.9.2 [func.require]) and, if `R` is not `void`, implicitly convertible to `R`.

[2014-10-05 Daniel comments]

This side-effect was indeed not intended by 870.

[2015-05, Lenexa]

STL provides improved wording. It replaces the current PR, and intentionally leaves 20.9.12.2 [func.wrap.func] unchanged.

Due to 5 [expr]/6, `static_cast<void>` is correct even when `R` is `const void`.

Proposed resolution:

This wording is relative to N4431.

1. Edit 20.9.2 [func.require] as depicted:

   > -2- Define `INVOKE(f, t1, t2, ..., tN, R)` as `static_cast<void>(INVOKE(f, t1, t2, ..., tN))` if `R` is cv `void`, otherwise `INVOKE(f, t1, t2, ..., tN)` implicitly converted to `R`.

2. Change 20.9.12.2.4 [func.wrap.func.inv] as depicted:

   >     R operator()(ArgTypes... args) const;

   > > -1- Effects Returns: `INVOKE(f, std::forward<ArgTypes>(args)..., R)` (20.9.2), where `f` is the target object (20.9.1) of `*this`.

   > > -2- Returns: Nothing if `R` is `void`, otherwise the return value of `INVOKE(f, std::forward<ArgTypes>(args)..., R)`.

### 2425. `operator delete(void*, size_t)` doesn't invalidate pointers sufficiently

Section: 18.6.1 [new.delete] Status: WP Submitter: Richard Smith Opened: 2014-08-29 Last modified: 2015-05-22

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Discussion:

18.6.1.1 [new.delete.single]/12 says:

> Requires: `ptr` shall be a null pointer or its value shall be a value returned by an earlier call to the (possibly replaced) `operator new(std::size_t)` or `operator new(std::size_t,const std::nothrow_t&)` which has not been invalidated by an intervening call to `operator delete(void*)`.

This should say:

> […] by an intervening call to `operator delete(void*)` or `operator delete(void*, std::size_t)`.

Likewise at the end of 18.6.1.2 [new.delete.array]/11, `operator delete[](void*, std::size_t)`.

[Urbana 2014-11-07: Move to Ready]

Proposed resolution:

1. Change 18.6.1.1 [new.delete.single]p12 as indicated:

   > -12- Requires: `ptr` shall be a null pointer or its value shall be a value returned by an earlier call to the (possibly replaced) `operator new(std::size_t)` or `operator new(std::size_t,const std::nothrow_t&)` which has not been invalidated by an intervening call to `operator delete(void*)` or `operator delete(void*, std::size_t)`.

2. Change 18.6.1.2 [new.delete.array]p11 as indicated:

   > -11- Requires: `ptr` shall be a null pointer or its value shall be the value returned by an earlier call to `operator new[](std::size_t)` or `operator new[](std::size_t,const std::nothrow_t&)` which has not been invalidated by an intervening call to `operator delete[](void*)` or `operator delete[](void*, std::size_t)`.

### 2427. Container adaptors as sequence containers, redux

Section: 23.3.1 [sequences.general] Status: WP Submitter: Tim Song Opened: 2014-08-29 Last modified: 2015-05-22

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Discussion:

LWG 2194 removed "These container adaptors meet the requirements for sequence containers." from 23.6.1 [container.adaptors.general].

However, N3936 23.3.1 [sequences.general]/p2 still says "The headers `<queue>` and `<stack>` define container adaptors (23.6) that also meet the requirements for sequence containers." I assume this is just an oversight.

[Urbana 2014-11-07: Move to Ready]

Proposed resolution:

1. Delete paragraph 2 of 23.3.1 [sequences.general] as indicated:

   > -2- The headers `<queue>` and `<stack>` define container adaptors (23.6) that also meet the requirements for sequence containers.

### 2428. "External declaration" used without being defined

Section: 17.6.2.2 [using.headers] Status: WP Submitter: Tim Song Opened: 2014-09-03 Last modified: 2015-05-22

View all other issues in [using.headers].

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Discussion:

17.6.2.2 [using.headers]/3 says

> A translation unit shall include a header only outside of any external declaration or definition […]

This wording appears to be borrowed from the C standard. However, the term "external declaration" is not defined in the C++ standard, and in fact is only used here as far as I can tell, so it is unclear what it means. The C standard does define external declarations as (WG14 N1570 6.9 External definitions/4-5):

> As discussed in 5.1.1.1, the unit of program text after preprocessing is a translation unit, which consists of a sequence of external declarations. These are described as "external" because they appear outside any function (and hence have file scope). [...] An external definition is an external declaration that is also a definition of a function (other than an inline definition) or an object.

The corresponding description of a translation unit in C++ is "A translation unit consists of a sequence of declarations." (3.5 [basic.link]/3).

So it appears that the C++ counterpart of "external declaration" in C is simply a "declaration" at file scope. There is no need to specifically limit the statement in 17.6.2.2 [using.headers]/3 to file-scope declarations, however, since every non-file-scope declaration is necessarily inside a file-scope declaration, so banning including a header inside file-scope declarations necessarily bans including one inside non-file-scope declarations as well.

[Urbana 2014-11-07: Move to Ready]

Proposed resolution:

This wording is relative to N3936.

1. Edit 17.6.2.2 [using.headers] as indicated:

   > A translation unit shall include a header only outside of any external declaration or definition, and shall include the header lexically before the first reference in that translation unit to any of the entities declared in that header. No diagnostic is required.

### 2433. `uninitialized_copy()`/etc. should tolerate overloaded `operator&`

Section: 20.7.12 [specialized.algorithms] Status: WP Submitter: Stephan T. Lavavej Opened: 2014-10-01 Last modified: 2015-05-22

View all other issues in [specialized.algorithms].

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Discussion:

This restriction isn't necessary anymore. In fact, this is the section that defines `addressof()`.

(Editorial note: We can depict these algorithms as calling `addressof()` instead of `std::addressof()` thanks to 17.6.1.1 [contents]/3 "Whenever a name `x` defined in the standard library is mentioned, the name `x` is assumed to be fully qualified as `::std::x`, unless explicitly described otherwise.")

[Urbana 2014-11-07: Move to Ready]

Proposed resolution:

This wording is relative to N3936.

1. Change 20.7.12 [specialized.algorithms] p1 as depicted:

   > -1- All the iterators that are used as formal template parameters in the following algorithms are required to have their `operator*` return an object for which `operator&` is defined and returns a pointer to `T`. In the algorithm `uninitialized_copy`, the formal template parameter `InputIterator` is required to satisfy the requirements of an input iterator (24.2.3). In all of the following algorithms, the formal template parameter `ForwardIterator` is required to satisfy the requirements of a forward iterator (24.2.5), and is required to have the property that no exceptions are thrown from increment, assignment, comparison, or indirection through valid iterators. In the following algorithms, if an exception is thrown there are no effects.

2. Change 20.7.12.2 [uninitialized.copy] p1 as depicted:

   > -1- Effects:

   > >     for (; first != last; ++result, ++first)
   > >       ::new (static_cast<void*>(addressof(&*result)))
   > >         typename iterator_traits<ForwardIterator>::value_type(*first);

3. Change 20.7.12.2 [uninitialized.copy] p3 as depicted:

   > -3- Effects:

   > >     for (; n > 0; ++result, ++first, --n) {
   > >       ::new (static_cast<void*>(addressof(&*result)))
   > >         typename iterator_traits<ForwardIterator>::value_type(*first);
   > >     }

4. Change 20.7.12.3 [uninitialized.fill] p1 as depicted:

   > -1- Effects:

   > >     for (; first != last; ++first)
   > >       ::new (static_cast<void*>(addressof(&*first)))
   > >         typename iterator_traits<ForwardIterator>::value_type(x);

5. Change 20.7.12.4 [uninitialized.fill.n] p1 as depicted:

   > -1- Effects:

   > >     for (; n--; ++first)
   > >       ::new (static_cast<void*>(addressof(&*first)))
   > >         typename iterator_traits<ForwardIterator>::value_type(x);
   > >     return first;

### 2434. `shared_ptr::use_count()` is efficient

Section: 20.8.2.2.5 [util.smartptr.shared.obs] Status: WP Submitter: Stephan T. Lavavej Opened: 2014-10-01 Last modified: 2015-05-22

View all other issues in [util.smartptr.shared.obs].

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Discussion:

`shared_ptr` and `weak_ptr` have Notes that their `use_count()` might be inefficient. This is an attempt to acknowledge reflinked implementations (which can be used by Loki smart pointers, for example). However, there aren't any `shared_ptr` implementations that use reflinking, especially after C++11 recognized the existence of multithreading. Everyone uses atomic refcounts, so `use_count()` is just an atomic load.

[Urbana 2014-11-07: Move to Ready]

Proposed resolution:

This wording is relative to N3936.

1. Change 20.8.2.2.5 [util.smartptr.shared.obs] p7-p10 as depicted:

   >     long use_count() const noexcept;

   > > -7- Returns: the number of `shared_ptr` objects, `*this` included, that share ownership with `*this`, or 0 when `*this` is empty.

   > > -8- [Note: `use_count()` is not necessarily efficient. — end note]

   >     bool unique() const noexcept;

   > > -9- Returns: `use_count() == 1`.

   > > -10- [Note: `unique()` may be faster than `use_count()`. If you are using `unique()` to implement copy on write, do not rely on a specific value when `get() == 0`. — end note]

2. Change 20.8.2.3.5 [util.smartptr.weak.obs] p1-p4 as depicted:

   >     long use_count() const noexcept;

   > > -1- Returns: 0 if `*this` is empty; otherwise, the number of `shared_ptr` instances that share ownership with `*this`.

   > > -2- [Note: `use_count()` is not necessarily efficient. — end note]

   >     bool expired() const noexcept;

   > > -3- Returns: `use_count() == 0`.

   > > -4- [Note: `expired()` may be faster than `use_count()`. — end note]

### 2437. `iterator_traits<OutIt>::reference` can and can't be `void`

Section: 24.2.2 [iterator.iterators] Status: WP Submitter: Stephan T. Lavavej Opened: 2014-10-01 Last modified: 2015-05-22

View all other issues in [iterator.iterators].

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Discussion:

24.2.2 [iterator.iterators]/2 requires an `Iterator`'s `*r` to return `reference`, i.e. `iterator_traits<X>::reference` according to 24.2.1 [iterator.requirements.general]/11.

24.2.4 [output.iterators]/1 requires an `OutputIterator`'s `*r = o` to do its job, so `*r` clearly can't return `void`.

24.4.1 [iterator.traits]/1 says: "In the case of an output iterator, the types

>     iterator_traits<Iterator>::difference_type
>     iterator_traits<Iterator>::value_type
>     iterator_traits<Iterator>::reference
>     iterator_traits<Iterator>::pointer

may be defined as `void`."

This is contradictory. I suggest fixing this by moving the offending requirement down from `Iterator` to `InputIterator`, and making `Iterator` say that `*r` returns an unspecified type. This will have the following effects:

- Output-only iterators will inherit Iterator's "`*r` returns unspecified" requirement, while 24.4.1 [iterator.traits]/1 clearly permits reference/etc. to be `void`.

- Input-or-stronger iterators (whether constant or mutable) are unaffected — they still have to satisfy "`*r` returns reference", they're just getting that requirement from `InputIterator` instead of `Iterator`.

[2015-02 Cologne]

EF: This is related to 2438. MC: I'd like to take up 2438 right after this.

AM: Does anyone think this is wrong?

GR: Why do we give output iterators to have reference type void? AM: we've mandated that certain output iterators define it as void since 1998. GR: Oh OK, I'm satisfied.

Accepted. And 2438 is already Ready.

Proposed resolution:

This wording is relative to N3936.

1. In 24.2.2 [iterator.iterators] Table 106 "Iterator requirements" change as indicated:

   > Table 106 — Iterator requirements
   > Expression | Return type | Operational semantics | Assertion/note pre-/post-condition
   > --- | --- | --- | ---
   > `*r` | `reference` unspecified | `` | pre: `r` is dereferenceable.
   > `…`

2. In 24.2.3 [input.iterators] Table 107 "Input iterator requirements" change as indicated:

   > Table 107 — Input iterator requirements (in addition to Iterator)
   > Expression | Return type | Operational semantics | Assertion/note pre-/post-condition
   > --- | --- | --- | ---
   > `…`
   > `*a` | `reference`, convertible to `T` | `` | […]
   > `…`

### 2438. `std::iterator` inheritance shouldn't be mandated

Section: 24.4.2 [iterator.basic] Status: WP Submitter: Stephan T. Lavavej Opened: 2014-10-01 Last modified: 2015-05-22

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Discussion:

For LWG convenience, nine STL iterators are depicted as deriving from `std::iterator` to get their `iterator_category`/etc. typedefs. Unfortunately (and unintentionally), this also mandates the inheritance, which is observable (not just through `is_base_of`, but also overload resolution). This is unfortunate because it confuses users, who can be misled into thinking that their own iterators must derive from `std::iterator`, or that overloading functions to take `std::iterator` is somehow meaningful. This is also unintentional because the STL's most important iterators, the container iterators, aren't required to derive from `std::iterator`. (Some are even allowed to be raw pointers.) Finally, this unnecessarily constrains implementers, who may not want to derive from `std::iterator`. (For example, to simplify debugger views.)

We could add wording to 24.4.2 [iterator.basic] saying that any depicted inheritance is for exposition only, but that wouldn't really solve reader confusion. Replacing the depicted inheritance with direct typedefs will prevent confusion. Note that implementers won't be required to change their code — they are free to continue deriving from `std::iterator` if they want.

(Editorial note: The order of the typedefs follows the order of `std::iterator`'s template parameters.)

[Urbana 2014-11-07: Move to Ready]

Proposed resolution:

This wording is relative to N3936.

1. Change 20.7.10 [storage.iterator], class template `raw_storage_iterator` synopsis, as depicted:

   >     template <class OutputIterator, class T>
   >     class raw_storage_iterator
   >       : public iterator<output_iterator_tag,void,void,void,void> {
   >     public:
   >       typedef output_iterator_tag iterator_category;
   >       typedef void value_type;
   >       typedef void difference_type;
   >       typedef void pointer;
   >       typedef void reference;

   >       explicit raw_storage_iterator(OutputIterator x);
   >       […]
   >     };

2. Change 24.5.1.1 [reverse.iterator], class template `reverse_iterator` synopsis, as depicted (editorial note: this reorders "`reference`, `pointer`" to "`pointer`, `reference`" and aligns whitespace):

   >     template <class Iterator>
   >     class reverse_iterator : public
   >       iterator<typename iterator_traits<Iterator>::iterator_category,
   >       typename iterator_traits<Iterator>::value_type,
   >       typename iterator_traits<Iterator>::difference_type,
   >       typename iterator_traits<Iterator>::pointer,
   >       typename iterator_traits<Iterator>::reference> {
   >     public:
   >       typedef Iterator iterator_type;
   >       typedef typename iterator_traits<Iterator>::difference_type difference_type;
   >       typedef typename iterator_traits<Iterator>::reference reference;
   >       typedef typename iterator_traits<Iterator>::pointer pointer;
   >       typedef Iterator                                              iterator_type;
   >       typedef typename iterator_traits<Iterator>::iterator_category iterator_category;
   >       typedef typename iterator_traits<Iterator>::value_type        value_type;
   >       typedef typename iterator_traits<Iterator>::difference_type   difference_type;
   >       typedef typename iterator_traits<Iterator>::pointer           pointer;
   >       typedef typename iterator_traits<Iterator>::reference         reference;

   >       reverse_iterator();
   >       […]
   >     };

3. Change 24.5.2.1 [back.insert.iterator], class template `back_insert_iterator` synopsis, as depicted:

   >     template <class Container>
   >     class back_insert_iterator :
   >       public iterator<output_iterator_tag,void,void,void,void> {
   >     protected:
   >       Container* container;

   >     public:
   >       typedef output_iterator_tag iterator_category;
   >       typedef void value_type;
   >       typedef void difference_type;
   >       typedef void pointer;
   >       typedef void reference;
   >       typedef Container container_type;
   >       explicit back_insert_iterator(Container& x);
   >       […]
   >     };

4. Change 24.5.2.3 [front.insert.iterator], class template `front_insert_iterator` synopsis, as depicted:

   >     template <class Container>
   >     class front_insert_iterator :
   >       public iterator<output_iterator_tag,void,void,void,void> {
   >     protected:
   >       Container* container;

   >     public:
   >       typedef output_iterator_tag iterator_category;
   >       typedef void value_type;
   >       typedef void difference_type;
   >       typedef void pointer;
   >       typedef void reference;
   >       typedef Container container_type;
   >       explicit front_insert_iterator(Container& x);
   >       […]
   >     };

5. Change 24.5.2.5 [insert.iterator], class template `insert_iterator` synopsis, as depicted:

   >     template <class Container>
   >     class insert_iterator :
   >       public iterator<output_iterator_tag,void,void,void,void> {
   >     protected:
   >       Container* container;
   >       typename Container::iterator iter;

   >     public:
   >       typedef output_iterator_tag iterator_category;
   >       typedef void value_type;
   >       typedef void difference_type;
   >       typedef void pointer;
   >       typedef void reference;
   >       typedef Container container_type;
   >       insert_iterator(Container& x, typename Container::iterator i);
   >       […]
   >     };

6. Change 24.6.1 [istream.iterator], class template `istream_iterator` synopsis, as depicted:

   >     template <class T, class charT = char, class traits = char_traits<charT>,
   >       class Distance = ptrdiff_t>
   >     class istream_iterator :
   >       public iterator<input_iterator_tag, T, Distance, const T*, const T&> {
   >     public:
   >       typedef input_iterator_tag iterator_category;
   >       typedef T value_type;
   >       typedef Distance difference_type;
   >       typedef const T* pointer;
   >       typedef const T& reference;
   >       […]
   >     };

7. Change 24.6.2 [ostream.iterator], class template `ostream_iterator` synopsis, as depicted:

   >     template <class T, class charT = char, class traits = char_traits<charT>>
   >     class ostream_iterator :
   >       public iterator<output_iterator_tag, void, void, void, void> {
   >     public:
   >       typedef output_iterator_tag iterator_category;
   >       typedef void value_type;
   >       typedef void difference_type;
   >       typedef void pointer;
   >       typedef void reference;
   >       […]
   >     };

8. Change 24.6.3 [istreambuf.iterator], class template `istreambuf_iterator` synopsis, as depicted:

   >     template <class charT = char, class traits = char_traits<charT> >
   >     class istreambuf_iterator :
   >       public iterator<input_iterator_tag, charT,
   >                       typename traits::off_type, unspecified, charT> {
   >     public:
   >       typedef input_iterator_tag iterator_category;
   >       typedef charT value_type;
   >       typedef typename traits::off_type difference_type;
   >       typedef unspecified pointer;
   >       typedef charT reference;
   >       […]
   >     };

9. Change 24.6.4 [ostreambuf.iterator], class template `ostreambuf_iterator` synopsis, as depicted (editorial note: this removes a redundant "public:"):

   >     template <class charT = char, class traits = char_traits<charT>>
   >     class ostreambuf_iterator :
   >       public iterator<output_iterator_tag, void, void, void, void> {
   >     public:
   >       typedef output_iterator_tag iterator_category;
   >       typedef void value_type;
   >       typedef void difference_type;
   >       typedef void pointer;
   >       typedef void reference;
   >       […]
   >     public:
   >       […]
   >     };

### 2439. `unique_copy()` sometimes can't fall back to reading its output

Section: 25.3.9 [alg.unique] Status: WP Submitter: Stephan T. Lavavej Opened: 2014-10-01 Last modified: 2015-05-22

View all other issues in [alg.unique].

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Discussion:

`unique_copy()`'s wording says that if it's given input-only and output-only iterators, it needs the input's value type to be copyable. This is correct, because in this case the algorithm must have a local element copy in order to detect duplicates.

The wording also says that if it's given an `InputIterator` that's forward or stronger, the input's value type doesn't have to be copyable. This is also correct, because in this case the algorithm can reread the input in order to detect duplicates.

Finally, the wording says that if it's given an input-only iterator with an `OutputIterator` that's forward or stronger, the input's value type doesn't have to be copyable. This is telling the algorithm to compare its input to its output in order to detect duplicates, but that isn't always possible! If the input and output have the same value type, then they can be compared (as long as `*result = *first` behaves sanely; see below). If they have different value types, then we can't compare them.

This could be resolved by requiring heterogeneous value types to be comparable in this situation, but that would be extremely tricky to wordsmith (as it would challenge the concept of "group of equal elements" used by the Effects). It will be vastly simpler and more effective to extend the "local element copy" requirement to this scenario.

Note that the input-only, output forward-or-stronger, identical value types scenario needs a bit of work too. We always require *result = *first to be "valid", but in this case we need to additionally require that the assignment actually transfers the value. (Otherwise, we'd be allowing an `op=()` that ignores `*first` and always sets `*result` to zero, or other unacceptable behavior.) This is just `CopyAssignable`.

(What happens when `unique_copy()` is given a `move_iterator` is a separate issue.)

To summarize:

> input forward+: no additional requirements

> input-only, output forward+, same value types: needs `CopyAssignable`

> input-only, output forward+, different value types: needs `CopyConstructible` and `CopyAssignable`

> input-only, output-only: needs `CopyConstructible` and `CopyAssignable`

[Urbana 2014-11-07: Move to Ready]

Proposed resolution:

This wording is relative to N3936.

1. Change 25.3.9 [alg.unique] p5, as depicted:

   >     template<class InputIterator, class OutputIterator>
   >     OutputIterator
   >     unique_copy(InputIterator first, InputIterator last,
   >                 OutputIterator result);
   >     template<class InputIterator, class OutputIterator,
   >              class BinaryPredicate>
   >     OutputIterator
   >     unique_copy(InputIterator first, InputIterator last,
   >                 OutputIterator result, BinaryPredicate pred);

   > > -5- Requires: The comparison function shall be an equivalence relation. The ranges [`first,last`) and [`result,result+(last-first)`) shall not overlap. The expression `*result = *first` shall be valid. If neither `InputIterator` nor `OutputIterator` meets the requirements of forward iterator then the value type of `InputIterator` shall be `CopyConstructible` (Table 21) and `CopyAssignable` (Table 23). Otherwise `CopyConstructible` is not required.Let `T` be the value type of `InputIterator`. If `InputIterator` meets the forward iterator requirements, then there are no additional requirements for `T`. Otherwise, if `OutputIterator` meets the forward iterator requirements and its value type is the same as `T`, then `T` shall be `CopyAssignable` (Table 23). Otherwise, `T` shall be both `CopyConstructible` (Table 21) and `CopyAssignable`.

### 2440. `seed_seq::size()` should be `noexcept`

Section: 26.5.7.1 [rand.util.seedseq] Status: WP Submitter: Stephan T. Lavavej Opened: 2014-10-01 Last modified: 2015-05-22

View all other issues in [rand.util.seedseq].

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Discussion:

Obvious.

[Urbana 2014-11-07: Move to Ready]

Proposed resolution:

This wording is relative to N3936.

1. Change 26.5.7.1 [rand.util.seedseq], class `seed_seq` synopsis, as depicted:

   >     class seed_seq
   >     {
   >     public:
   >       […]
   >       size_t size() const noexcept;
   >       […]
   >     };

2. Change 26.5.7.1 [rand.util.seedseq] around p10, as depicted:

   >     size_t size() const noexcept;

   > > -10- Returns: The number of 32-bit units that would be returned by a call to `param()`.

   > > -11- Throws: Nothing.

   > > -12- Complexity: Constant time.

### 2442. `call_once()` shouldn't `DECAY_COPY()`

Section: 30.4.4.2 [thread.once.callonce] Status: WP Submitter: Stephan T. Lavavej Opened: 2014-10-01 Last modified: 2015-05-22

View all other issues in [thread.once.callonce].

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Discussion:

When LWG 891 overhauled `call_once()`'s specification, it used `decay_copy()`, following LWG 929's overhaul of `thread`'s constructor.

In `thread`'s constructor, this is necessary and critically important. 30.3.1.2 [thread.thread.constr]/5 "The new thread of execution executes `INVOKE(DECAY_COPY(std::forward<F>(f)), DECAY_COPY(std::forward<Args>(args))...)` with the calls to `DECAY_COPY` being evaluated in the constructing thread." requires the parent thread to copy arguments for the child thread to access.

In `call_once()`, this is unnecessary and harmful. It's unnecessary because `call_once()` doesn't transfer arguments between threads. It's harmful because:

- `decay_copy()` returns a prvalue. Given `meow(int&)`, `meow(i)` can be called directly, but `call_once(flag, meow, i)` won't compile.

- `decay_copy()` moves from modifiable rvalues. Given `purr(const unique_ptr<int>&)`, `purr(move(up))` won't modify `up`. (This is observable, because moved-from `unique_ptr`s are guaranteed empty.) However, `call_once(flag, purr, move(up))` will leave `up` empty after the first active execution. Observe the behavioral difference — if `purr()` is directly called like this repeatedly until it doesn't throw an exception, each call will observe `up` unchanged. With `call_once()`, the second active execution will observe `up` to be empty.

`call_once()` should use perfect forwarding without `decay_copy()`, in order to avoid interfering with the call like this.

[2015-02 Cologne]

Handed over to SG1.

[2015-05 Lenexa, SG1 response]

Looks good to us, but this is really an LWG issue.

[2015-05-07 Lenexa: Move Immediate]

LWG 2442 call_once shouldn't decay_copy

STL summarizes the SG1 minutes.

Marshall: Jonathan updated all the issues with SG1 status last night. Except this one.

STL summarizes the issue.

Dietmar: Of course, call_once has become useless.

STL: With magic statics.

Jonathan: Magic statics can't be per object, which I use in future.

Marshall: I see why you are removing the MoveConstructible on the arguments, but what about Callable?

STL: That's a type named Callable, which we will no longer decay_copy. We're still requiring the INVOKE expression to be valid.

Marshall: Okay. Basically, ripping the decay_copy out of here.

STL: I recall searching the Standard for other occurrences and I believe this is the only inappropriate use of decay_copy.

Marshall: We do the decay_copy.

Jonathan: Us too.

Marshall: What do people think?

Jonathan: I think STL's right. In the use I was mentioning inside futures, I actually pass them by reference_wrapper and pointers, to avoid the decay causing problems. Inside the call_once, I then extract the args. So I've had to work around this and didn't realize it was a defect.

Marshall: What do people think is the right resolution?

STL: I would like to see Immediate.

Hwrd: No objections to Immediate.

Marshall: Bill is nodding.

PJP: He said it. Everything STL says applies to our other customers.

Marshall: Any objections to Immediate?

Jonathan: I can't see any funky implementations where a decay_copy would be necessary?

Marshall: 6 votes for Immediate, 0 opposed, 0 abstaining.

Proposed resolution:

This wording is relative to N3936.

1. Change 30.4.4.2 [thread.once.callonce] p1+p2 as depicted:

   >     template<class Callable, class ...Args>
   >       void call_once(once_flag& flag, Callable&& func, Args&&... args);

   > > -1- Requires: `Callable` and each `Ti` in `Args` shall satisfy the `MoveConstructible` requirements. `INVOKE(DECAY_COPY(std::forward<Callable>(func)), DECAY_COPY(std::forward<Args>(args))...)` (20.9.2) shall be a valid expression.

   > > -2- Effects; […] An active execution shall call `INVOKE(DECAY_COPY(std::forward<Callable>(func)), DECAY_COPY(std::forward<Args>(args))...)`. […]

### 2448. Non-normative Container destructor specification

Section: 23.2.1 [container.requirements.general] Status: WP Submitter: Daniel Krügler Opened: 2014-10-18 Last modified: 2015-05-22

View other active issues in [container.requirements.general].

View all other issues in [container.requirements.general].

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Discussion:

According to Table 96 — "Container requirements" the specification:

> note: the destructor is applied to every element of `a`; any memory obtained is deallocated.

The initial "note:" can be read as if that part of the specification would not be normative (This note form differs from footnotes in tables, which have normative meaning).

It seems that this initial part of the specification exists since C++98. But comparing with the similar SGI Container specification there is no evidence for that being intended to be non-normative.

[2015-02, Cologne]

NJ: If we fix this, we should also fix it elsewhere. Oh, this is the only place?
GR: If this is intended to be different from elsewhere, we should make sure.
AM: `valarray` specifies this without the "note:".
DK: `valarray` requires trivially destructible types!
GR: That's good enough for me.
NJ: First time `valarray` has been useful for something!

Proposed resolution:

This wording is relative to N4140.

1. Change 23.2.1 [container.requirements.general], Table 96 — "Container requirements", as indicated:

   > Table 96 — Container requirements
   > Expression | Return type | Operational semantics | Assertion/note pre-/post-condition | Complexity
   > --- | --- | --- | --- | ---
   > `…`
   > `(&a)->~X()` | `void` |  | note: the destructor is applied to every element of `a`; any memory obtained is deallocated. | linear
   > `…`

### 2454. Add `raw_storage_iterator::base()` member

Section: 20.7.10 [storage.iterator] Status: WP Submitter: Jonathan Wakely Opened: 2014-11-11 Last modified: 2015-05-22

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Discussion:

Eric Niebler pointed out that `raw_storage_iterator` should give access to the `OutputIterator` it wraps.

This helps alleviate the exception-safety issue pointed out in the discussion of LWG 2127, as an exception can be caught and then destructors can be run for the constructed elements in the range `[begin, raw.base())`

[2015-02 Cologne]

NJ: Is this "const" correct [in "base()"]? DK: Yes, we always do that. NJ: And the output iterator is not qualifying in any way? AM/DK: That wouldn't make sense. NJ: OK.

VV: What did LEWG say about this feature request? In other words, why is this a library issue? AM: LEWG/JY thought this wouldn't be a contentious issue.

NJ: I really hope the split of LEWG and LWG will be fixed soon, since it's only wasting time. VV: So you want to spend even more of your time on discussions that LEWG has?

AM: I think this specified correctly. I'm not wild about it. But no longer bothered to stand in its way.

GR: Why do we need to repeat the type in "Returns" even though it's part of the synopsis? AM: Good point, but not worth fixing.

NJ: Why is "`base()`" for `reverse_iterator` commented with "// explicit"? AM: I guess in 1998 that was the only way to say this.

AM: So, it's tentatively ready.

Proposed resolution:

This wording is relative to N4140.

1. Add a new function to the synopsis in 20.7.10 [storage.iterator] p1:

   >     namespace std {
   >       template <class OutputIterator, class T>
   >       class raw_storage_iterator
   >         : public iterator<output_iterator_tag,void,void,void,void> {
   >       public:
   >         explicit raw_storage_iterator(OutputIterator x);

   >         raw_storage_iterator<OutputIterator,T>& operator*();
   >         raw_storage_iterator<OutputIterator,T>& operator=(const T& element);
   >         raw_storage_iterator<OutputIterator,T>& operator++();
   >         raw_storage_iterator<OutputIterator,T> operator++(int);
   >         OutputIterator base() const;
   >     };
   >     }

2. Insert the new function and a new paragraph series after p7:

   >     OutputIterator base() const;

   > > -?- Returns: An iterator of type `OutputIterator` that points to the same value as `*this` points to.

### 2455. Allocator default construction should be allowed to throw

Section: 17.6.3.5 [allocator.requirements] Status: WP Submitter: Pablo Halpern Opened: 2014-11-11 Last modified: 2015-05-22

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Discussion:

17.6.3.5 [allocator.requirements]/4 in the 2014-10 WP (N4140), says:

> An allocator type `X` shall satisfy the requirements of `CopyConstructible` (17.6.3.1). The `X::pointer`, `X::const_pointer`, `X::void_pointer`, and `X::const_void_pointer` types shall satisfy the requirements of `NullablePointer` (17.6.3.3). No constructor, comparison operator, copy operation, move operation, or swap operation on these types shall exit via an exception. `X::pointer` and `X::const_pointer` shall also satisfy the requirements for a random access iterator (24.2).

The words "these types" would normally apply only to the previous sentence only, i.e., only to the pointer types. However, an alternative reading would be that the allocator constructors themselves cannot throw. The change to the `vector` and `string` default constructors, making them unconditionally `noexcept` depends on this alternative reading.

I believe that the wording in the standard is not intended to forbid throwing default constructors for allocators. Indeed, I believe that allocators do not require default constructors and that if they provide a default constructor they should be allowed to throw.

In addition, the `noexcept` specifications for the `string` and `vector` default constructors should be changed to make them conditional.

[2015-01-18 Library reflector vote]

The issue has been identified as Tentatively Ready based on six votes in favour.

Proposed resolution:

1. Change 17.6.3.5 [allocator.requirements] p4 as indicated:

   > An allocator type `X` shall satisfy the requirements of `CopyConstructible` (17.6.3.1). The `X::pointer`, `X::const_pointer`, `X::void_pointer`, and `X::const_void_pointer` types shall satisfy the requirements of `NullablePointer` (17.6.3.3). No constructor, comparison operator, copy operation, move operation, or swap operation on these pointer types shall exit via an exception. `X::pointer` and `X::const_pointer` shall also satisfy the requirements for a random access iterator (24.2).

2. Change 21.4 [basic.string] following p5, class template `basic_string` synopsis, as indicated: (This change assumes that N4258 has been applied, as voted on in Urbana on 2014-11-08)

   >     // 21.4.2, construct/copy/destroy:
   >     basic_string() noexcept(noexcept(Allocator())) : basic_string(Allocator()) { }

   > An alternative formulation of the above would be:

   >     // 21.4.2, construct/copy/destroy:
   >     basic_string() noexcept(is_nothrow_default_constructible<Allocator>{}) : basic_string(Allocator()) { }

3. Change 23.3.6.1 [vector.overview] following p2, class template `vector` synopsis, as indicated: (This change assumes that N4258 has been applied, as voted on in Urbana on 2014-11-08)

   >     // 23.3.6.2, construct/copy/destroy:
   >     vector() noexcept(noexcept(Allocator())) : vector(Allocator()) { }

   > An alternative formulation of the above would be:

   >     // 23.3.6.2, construct/copy/destroy:
   >     vector() noexcept(is_nothrow_default_constructible<Allocator>{}) : vector(Allocator()) { }

### 2458. N3778 and new library deallocation signatures

Section: 18.6 [support.dynamic], 18.6.1.1 [new.delete.single], 18.6.1.2 [new.delete.array] Status: WP Submitter: Richard Smith Opened: 2014-11-23 Last modified: 2015-05-22

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Discussion:

N3778 added the following sized deallocation signatures to the library:

>     void operator delete(void* ptr, std::size_t size) noexcept;
>     void operator delete[](void* ptr, std::size_t size) noexcept;

>     void operator delete(void* ptr, std::size_t size, const std::nothrow_t&) noexcept;
>     void operator delete[](void* ptr, std::size_t size, const std::nothrow_t&) noexcept;

The former two are an essential part of the proposal. The latter two seem spurious — they are not called when `new (std::nothrow) X` fails due to `X::X()` throwing, because the core language rules for selecting a placement deallocation function do not consider passing a `size` argument. Instead, the above would be the matching deallocation functions for:

>     void *operator new(std::size_t size, std::size_t size_again, const std::nothrow_t&) noexcept;
>     void *operator new[](std::size_t size, std::size_t size_again, const std::nothrow_t&) noexcept;

... which don't exist.

Since they're not implicitly called, the only other possible use for those functions would be to perform an explicitly non-throwing deallocation. But... the first two overloads are already explicitly non-throwing and are required to be semantically identical to the second two. So there's no point in making an explicit call to the second pair of functions either.

It seems to me that we should remove the `(void*, size_t, nothrow_t)` overloads, because the core working group decided during the Urbana 2014 meeting, that no change to the core language was warranted.

[2014-11-23, Daniel suggests concrete wording changes]

[2015-02 Cologne]

Nobody can call those overloads, since the nothrow allocation functions cannot throw. JY: Ship it. GR: Should we do due diligence and make sure we're deleting what we mean to be deleting? [Some checking, everything looks good.]

Accepted.

Proposed resolution:

This wording is relative to N4140.

1. Change 18.6 [support.dynamic], header `<new>` synopsis, as indicated:

   >     […]
   >     void operator delete(void* ptr, std::size_t size) noexcept;
   >     void operator delete(void* ptr, std::size_t size, const std::nothrow_t&) noexcept;
   >     […]
   >     void operator delete[](void* ptr, std::size_t size) noexcept;
   >     void operator delete[](void* ptr, std::size_t size, const std::nothrow_t&) noexcept;
   >     […]

2. Change 18.6.1.1 [new.delete.single], starting before p19, as indicated:

   >     void operator delete(void* ptr, const std::nothrow_t&) noexcept;
   >     void operator delete(void* ptr, std::size_t size, const std::nothrow_t&) noexcept;

   > > […]

   > > -20- Replaceable: a C++ program may define a function with signature `void operator delete(void* ptr, const std::nothrow_t&) noexcept` that displaces the default version defined by the C++ standard library. If this function (without `size` parameter) is defined, the program should also define `void operator delete(void* ptr, std::size_t size, const std::nothrow_t&) noexcept`. If this function with `size` parameter is defined, the program shall also define the version without the `size` parameter. [Note: The default behavior below may change in the future, which will require replacing both deallocation functions when replacing the allocation function. — end note]

   > > […]

   > > -22- Requires: If present, the `std::size_t size` argument must equal the `size` argument passed to the allocation function that returned `ptr`.

   > > -23- Required behavior: Calls to `operator delete(void* ptr, std::size_t size, const std::nothrow_t&)` may be changed to calls to `operator delete(void* ptr, const std::nothrow_t&)` without affecting memory allocation. [Note: A conforming implementation is for `operator delete(void* ptr, std::size_t size, const std::nothrow_t&)` to simply call `operator delete(void* ptr, const std::nothrow_t&)`. — end note]

   > > -24- Default behavior: `operator delete(void* ptr, std::size_t size, const std::nothrow_t&)` calls `operator delete(ptr, std::nothrow)`, and `operator delete(void* ptr, const std::nothrow_t&)` calls `operator delete(ptr)`.

3. Change 18.6.1.2 [new.delete.array], starting before p16, as indicated:

   >     void operator delete[](void* ptr, const std::nothrow_t&) noexcept;
   >     void operator delete[](void* ptr, std::size_t size, const std::nothrow_t&) noexcept;

   > > […]

   > > -17- Replaceable: a C++ program may define a function with signature `void operator delete[](void* ptr, const std::nothrow_t&) noexcept` that displaces the default version defined by the C++ standard library. If this function (without `size` parameter) is defined, the program should also define `void operator delete[](void* ptr, std::size_t size, const std::nothrow_t&) noexcept`. If this function with `size` parameter is defined, the program shall also define the version without the `size` parameter. [Note: The default behavior below may change in the future, which will require replacing both deallocation functions when replacing the allocation function. — end note]

   > > […]

   > > -19- Requires: If present, the `std::size_t size` argument must equal the `size` argument passed to the allocation function that returned `ptr`.

   > > -20- Required behavior: Calls to `operator delete[](void* ptr, std::size_t size, const std::nothrow_t&)` may be changed to calls to `operator delete[](void* ptr, const std::nothrow_t&)` without affecting memory allocation. [Note: A conforming implementation is for `operator delete[](void* ptr, std::size_t size, const std::nothrow_t&)` to simply call `operator delete[](void* ptr, const std::nothrow_t&)`. — end note]

   > > -21- Default behavior: `operator delete[](void* ptr, std::size_t size, const std::nothrow_t&)` calls `operator delete[](ptr, std::nothrow)`, and `operator delete[](void* ptr, const std::nothrow_t&)` calls `operator delete[](ptr)`.

### 2459. `std::polar` should require a non-negative rho

Section: 26.4.7 [complex.value.ops] Status: WP Submitter: Marshall Clow Opened: 2014-10-22 Last modified: 2015-05-22

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Discussion:

Different implementations give different answers for the following code:

>     #include <iostream>
>     #include <complex>

>     int main ()
>     {
>       std::cout << std::polar(-1.0, -1.0) << '\n';
>       return 0;
>     }

One implementation prints:

>     (nan, nan)

Another:

>     (-0.243068, 0.243068)

Which is correct? Or neither?

In this list, Howard Hinnant wrote:

> I've read this over several times. I've consulted C++11, C11, and IEC 10967-3. [snip]

> I'm finding:

> 1. The magnitude of a complex number `== abs(c) == hypot(c.real(), c.imag())` and is always non-negative (by all three of the above standards).

> 2. Therefore no complex number exists for which `abs(c) < 0`.

> 3. Therefore when the first argument to `std::polar` (which is called `rho`) is negative, no complex number can be formed which meets the post-conidtion that `abs(c) == rho`.

> One could argue that this is already covered in 26.4 [complex.numbers]/3, but I think it's worth making explicit.

[2015-02, Cologne]

Discussion on whether theta should also be constrained.
TK: infinite theta doesn't make sense, whereas infinite rho does (theta is on a compact domain, rho is on a non-compact domain).
AM: We already have a narrow contract, so I don't mind adding further requirements. Any objections to requiring that theta be finite?
Some more discussion, but general consensus. Agreement that if someone finds the restrictions problematic, they should write a proper paper to address how `std::polar` should behave. For now, we allow infinite rho (but not NaN and not negative), and require finite theta.

No objections to tentatively ready.

Proposed resolution:

This wording is relative to N4296.

1. Change 26.4.7 [complex.value.ops] around p9 as indicated

   >     template<class T> complex<T> polar(const T& rho, const T& theta = 0);

   > > -?- Requires: `rho` shall be non-negative and non-NaN. `theta` shall be finite.

   > > -9- Returns: The complex value corresponding to a complex number whose magnitude is `rho` and whose phase angle is `theta`.

### 2463. [fund.ts] Incorrect complexity for `sample()` algorithm

Section: X [alg.random.sample] Status: WP Submitter: Joe Gottman Opened: 2014-12-17 Last modified: 2015-05-22

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Discussion:

Addresses: fund.ts

According to paragraph 10.1 of the Library Fundamentals 1 draft, the complexity of the new `std::experimental::sample` template function is O(`n`). Note that `n` is actually a parameter of this function, corresponding to the sample size. But both common algorithms for sampling, the selection algorithm and the reservoir algorithm, are linear with respect to the population size, which is often many orders of magnitude bigger than the sample size.

[2015-02, Cologne]

AM: I suggest we make this a DR against the Fundamentals TS.
GR: Agreed, this is a no-brainer.

Proposed resolution:

This wording is relative to N4335 in regard to fundamental-ts changes.

1. Change X [alg.random.sample] p5 to read:

   > -5- Complexity: O(`nlast - first`).

### 2464. `try_emplace` and `insert_or_assign` misspecified

Section: 23.4.4.4 [map.modifiers], 23.5.4.4 [unord.map.modifiers] Status: WP Submitter: Thomas Koeppe Opened: 2014-12-17 Last modified: 2015-05-22

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Discussion:

The specification of the `try_emplace` and `insert_or_assign` member functions in N4279 contains the following errors and omissions:

1. In `insert_or_assign`, each occurrence of `std::forward<Args>(args)...` should be `std::forward<M>(obj)`; this is was a mistake introduced in editing.

2. In `try_emplace`, the construction of the `value_type` is misspecified, which is a mistake that was introduced during the evolution from a one-parameter to a variadic form. As written, `value_type(k, std::forward<Args>(args)...)` does not do the right thing; it can only be used with a single argument, which moreover must be convertible to a `mapped_type`. The intention is to allow direct-initialization from an argument pack, and the correct constructor should be `value_type(piecewise_construct, forward_as_tuple(k), forward_as_tuple(std::forward<Args>(args)...)`.

3. Both `try_emplace` and `insert_or_assign` are missing requirements on the argument types. Since the semantics of these functions are specified independent of other functions, they need to include their requirements.

[2015-02, Cologne]

This issue is related to 2469.

AM: The repeated references to "first and third forms" and "second and fourth forms" is a bit cumbersome. Maybe split the four functions?
GR: We don't have precendent for "EmplaceConstructible from a, b, c". I don't like the ambiguity between code commas and text commas.
TK: What's the danger?
GR: It's difficult to follow standardese.
AM: It seems fine with code commas. What's the problem?
GR: It will lead to difficulties when we use a similar construction that's not at the end of a sentence.
AM: That's premature generalization. DK: When that happens, let's look at this again.
AM: Clean up "if the map does contain"
TK: Can we call both containers "map"? DK/GR: yes.
TK will send updated wording to DK.

Conclusion: Update wording, then poll for tentatively ready

[2015-03-26, Thomas provides improved wording]

The approach is to split the descriptions of the various blocks of four functions into two blocks each so as to make the wording easier to follow.

Previous resolution [SUPERSEDED]:

> This wording is relative to N4296.

> 1. Apply the following changes to section 23.4.4.4 [map.modifiers] p3:

>    >     template <class... Args> pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);
>    >     template <class... Args> pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
>    >     template <class... Args> iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);
>    >     template <class... Args> iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);

>    > > -?- Requires: For the first and third forms, `value_type` shall be `EmplaceConstructible` into map from `piecewise_construct`, `forward_as_tuple(k)`, `forward_as_tuple(forward<Args>(args)...)`. For the second and fourth forms, `value_type` shall be `EmplaceConstructible` into map from `piecewise_construct`, `forward_as_tuple(move(k))`, `forward_as_tuple(forward<Args>(args)...)`.

>    > > -3- Effects: If the key `k` already exists in the map, there is no effect. Otherwise, inserts an element into the map. In the first and third forms, the element is constructed from the arguments as `value_type(k, std::forward<Args>(args)...)`. In the second and fourth forms, the element is constructed from the arguments as `value_type(std::move(k), std::forward<Args>(args)...)`. In the first two overloads, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the element of the map whose key is equivalent to `k` If the map does already contain an element whose key is equivalent to `k`, there is no effect. Otherwise for the first and third forms inserts a `value_type` object `t` constructed with `piecewise_construct`, `forward_as_tuple(k)`, `forward_as_tuple(forward<Args>(args)...)`, for the second and fourth forms inserts a `value_type` object `t` constructed with `piecewise_construct`, `forward_as_tuple(move(k))`, `forward_as_tuple(forward<Args>(args)...)`.

>    > > -?- Returns: In the first two overloads, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the map element whose key is equivalent to `k`.

> 2. Apply the following changes to section 23.4.4.4 [map.modifiers] p5:

>    >     template <class M> pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);
>    >     template <class M> pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
>    >     template <class M> iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);
>    >     template <class M> iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);

>    > > -?- Requires: `is_assignable<mapped_type&, M&&>::value` shall be true. For the first and third forms, `value_type` shall be `EmplaceConstructible` into map from `k`, `forward<M>(obj)`. For the second and fourth forms, `value_type` shall be `EmplaceConstructible` into map from `move(k), forward<M>(obj)`.

>    > > -5- Effects: If the key `k` does not exist in the map, inserts an element into the map. In the first and third forms, the element is constructed from the arguments as `value_type(k, std::forward<Args>(args)...)`. In the second and fourth forms, the element is constructed from the arguments as `value_type(std::move(k), std::forward<Args>(args)...)`. If the key already exists, `std::forward<M>(obj)` is assigned to the `mapped_type` corresponding to the key. In the first two overloads, the `bool` component of the returned value is true if and only if the insertion took place. The returned iterator points to the element that was inserted or updated If the map does already contain an element whose key is equivalent to `k`, `forward<M>(obj)` is assigned to the `mapped_type` corresponding to the key. Otherwise the first and third forms inserts a `value_type` object `t` constructed with `k`, `forward<M>(obj)`, the second and fourth forms inserts a `value_type` object `t` constructed with `move(k), forward<M>(obj)`.

>    > > -?- Returns: In the first two overloads, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the element of the map whose key is equivalent to `k`.

> 3. Apply the following changes to section 23.5.4.4 [unord.map.modifiers] p5:

>    >     template <class... Args> pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);
>    >     template <class... Args> pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
>    >     template <class... Args> iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);
>    >     template <class... Args> iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);

>    > > -?- Requires: For the first and third forms, `value_type` shall be `EmplaceConstructible` into unordered_map from `piecewise_construct`, `forward_as_tuple(k)`, `forward_as_tuple(forward<Args>(args)...)`. For the second and fourth forms, `value_type` shall be `EmplaceConstructible` into unordered_map from `piecewise_construct`, `forward_as_tuple(move(k))`, `forward_as_tuple(forward<Args>(args)...)`.

>    > > -5- Effects: If the key `k` already exists in the map, there is no effect. Otherwise, inserts an element into the map. In the first and third forms, the element is constructed from the arguments as `value_type(k, std::forward<Args>(args)...)`. In the second and fourth forms, the element is constructed from the arguments as `value_type(std::move(k), std::forward<Args>(args)...)`. In the first two overloads, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the element of the map whose key is equivalent to `k` If the unordered_map does already contain an element whose key is equivalent to `k`, there is no effect. Otherwise for the first and third forms inserts a `value_type` object `t` constructed with `piecewise_construct`, `forward_as_tuple(k)`, `forward_as_tuple(forward<Args>(args)...)`, for the second and fourth forms inserts a `value_type` object `t` constructed with `piecewise_construct`, `forward_as_tuple(move(k))`, `forward_as_tuple(forward<Args>(args)...)`.

>    > > -?- Returns: In the first two overloads, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the element of the unordered_map whose key is equivalent to `k`.

> 4. Apply the following changes to section 23.5.4.4 [unord.map.modifiers] p7:

>    >     template <class M> pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);
>    >     template <class M> pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
>    >     template <class M> iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);
>    >     template <class M> iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);

>    > > -?- Requires: `is_assignable<mapped_type&, M&&>::value` shall be true. For the first and third forms, `value_type` shall be `EmplaceConstructible` into unordered_map from `k`, `forward<M>(obj)`. For the second and fourth forms, `value_type` shall be `EmplaceConstructible` into unordered_map from `move(k), forward<M>(obj)`.

>    > > -7- Effects: If the key `k` does not exist in the map, inserts an element into the map. In the first and third forms, the element is constructed from the arguments as `value_type(k, std::forward<Args>(args)...)`. In the second and fourth forms, the element is constructed from the arguments as `value_type(std::move(k), std::forward<Args>(args)...)`. If the key already exists, `std::forward<M>(obj)` is assigned to the `mapped_type` corresponding to the key. In the first two overloads, the `bool` component of the returned value is true if and only if the insertion took place. The returned iterator points to the element that was inserted or updated If the unordered_map does already contain an element whose key is equivalent to `k`, `forward<M>(obj)` is assigned to the `mapped_type` corresponding to the key. Otherwise the first and third forms inserts a `value_type` object `t` constructed with `k`, `forward<M>(obj)`, the second and fourth forms inserts a `value_type` object `t` constructed with `move(k), forward<M>(obj)`.

>    > > -?- Returns: In the first two overloads, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the element of the unordered_map whose key is equivalent to `k`.

[2015-05, Lenexa]

STL: existing wording is horrible, this is Thomas' wording and his issue
STL: already implemented the piecewise part
MC: ok with changes
STL: changes are mechanical
STL: believe this is P1, it must be fixed, we have wording
PJP: functions are sensible
STL: has been implemented
MC: consensus is to move to ready

Proposed resolution:

This wording is relative to N4296.

1. Apply the following changes to 23.4.4.4 [map.modifiers] p3+p4:

   >     template <class... Args> pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);
   >     template <class... Args> pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
   >     template <class... Args> iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);
   >     template <class... Args> iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);

   > > -?- Requires: `value_type` shall be `EmplaceConstructible` into `map` from `piecewise_construct, forward_as_tuple(k), forward_as_tuple(forward<Args>(args)...)`.

   > > -3- Effects: If the key `k` already exists in the map, there is no effect. Otherwise, inserts an element into the map. In the first and third forms, the element is constructed from the arguments as `value_type(k, std::forward<Args>(args)...)`. In the second and fourth forms, the element is constructed from the arguments as `value_type(std::move(k), std::forward<Args>(args)...)`. In the first two overloads, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the element of the map whose key is equivalent to `k` If the map already contains an element whose key is equivalent to `k`, there is no effect. Otherwise inserts an object of type `value_type` constructed with `piecewise_construct, forward_as_tuple(k), forward_as_tuple(forward<Args>(args)...)`.

   > > -?- Returns: In the first overload, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the map element whose key is equivalent to `k`.

   > > -4- Complexity: The same as `emplace` and `emplace_hint`, respectively.

   >     template <class... Args> pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
   >     template <class... Args> iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);

   > > -?- Requires: `value_type` shall be `EmplaceConstructible` into `map` from `piecewise_construct, forward_as_tuple(move(k)), forward_as_tuple(forward<Args>(args)...)`.

   > > -?- Effects: If the map already contains an element whose key is equivalent to `k`, there is no effect. Otherwise inserts an object of type `value_type` constructed with `piecewise_construct, forward_as_tuple(move(k)), forward_as_tuple(forward<Args>(args)...)`.

   > > -?- Returns: In the first overload, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the map element whose key is equivalent to `k`.

   > > -?- Complexity: The same as `emplace` and `emplace_hint`, respectively.

2. Apply the following changes to 23.4.4.4 [map.modifiers] p5+p6:

   >     template <class M> pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);
   >     template <class M> pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
   >     template <class M> iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);
   >     template <class M> iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);

   > > -?- Requires: `is_assignable<mapped_type&, M&&>::value` shall be `true`. `value_type` shall be `EmplaceConstructible` into `map` from `k, forward<M>(obj)`.

   > > -5- Effects: If the key `k` does not exist in the map, inserts an element into the map. In the first and third forms, the element is constructed from the arguments as `value_type(k, std::forward<Args>(args)...)`. In the second and fourth forms, the element is constructed from the arguments as `value_type(std::move(k), std::forward<Args>(args)...)`. If the key already exists, `std::forward<M>(obj)` is assigned to the `mapped_type` corresponding to the key. In the first two overloads, the `bool` component of the returned value is true if and only if the insertion took place. The returned iterator points to the element that was inserted or updated If the map already contains an element `e` whose key is equivalent to `k`, assigns `forward<M>(obj)` to `e.second`. Otherwise inserts an object of type `value_type` constructed with `k, forward<M>(obj)`.

   > > -?- Returns: In the first overload, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the map element whose key is equivalent to `k`.

   > > -6- Complexity: The same as `emplace` and `emplace_hint`, respectively.

   >     template <class M> pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
   >     template <class M> iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);

   > > -?- Requires: `is_assignable<mapped_type&, M&&>::value` shall be `true`. `value_type` shall be `EmplaceConstructible` into `map` from `move(k), forward<M>(obj)`.

   > > -?- Effects: If the map already contains an element `e` whose key is equivalent to `k`, assigns `forward<M>(obj)` to `e.second`. Otherwise inserts an object of type `value_type` constructed with `move(k), forward<M>(obj)`.

   > > -?- Returns: In the first overload, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the map element whose key is equivalent to `k`.

   > > -?- Complexity: The same as `emplace` and `emplace_hint`, respectively.

3. Apply the following changes to 23.5.4.4 [unord.map.modifiers] p5+p6:

   >     template <class... Args> pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);
   >     template <class... Args> pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
   >     template <class... Args> iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);
   >     template <class... Args> iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);

   > > -?- Requires: `value_type` shall be `EmplaceConstructible` into `unordered_map` from `piecewise_construct, forward_as_tuple(k), forward_as_tuple(forward<Args>(args)...)`.

   > > -5- Effects: If the key `k` already exists in the map, there is no effect. Otherwise, inserts an element into the map. In the first and third forms, the element is constructed from the arguments as `value_type(k, std::forward<Args>(args)...)`. In the second and fourth forms, the element is constructed from the arguments as `value_type(std::move(k), std::forward<Args>(args)...)`. In the first two overloads, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the element of the map whose key is equivalent to `k` If the map already contains an element whose key is equivalent to `k`, there is no effect. Otherwise inserts an object of type `value_type` constructed with `piecewise_construct, forward_as_tuple(k), forward_as_tuple(forward<Args>(args)...)`.

   > > -?- Returns: In the first overload, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the map element whose key is equivalent to `k`.

   > > -6- Complexity: The same as `emplace` and `emplace_hint`, respectively.

   >     template <class... Args> pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
   >     template <class... Args> iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);

   > > -?- Requires: `value_type` shall be `EmplaceConstructible` into `unordered_map` from `piecewise_construct, forward_as_tuple(move(k)), forward_as_tuple(forward<Args>(args)...)`.

   > > -?- Effects: If the map already contains an element whose key is equivalent to `k`, there is no effect. Otherwise inserts an object of type `value_type` constructed with `piecewise_construct, forward_as_tuple(move(k)), forward_as_tuple(forward<Args>(args)...)`.

   > > -?- Returns: In the first overload, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the map element whose key is equivalent to `k`.

   > > -?- Complexity: The same as `emplace` and `emplace_hint`, respectively.

4. Apply the following changes to 23.5.4.4 [unord.map.modifiers] p7+p8:

   >     template <class M> pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);
   >     template <class M> pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
   >     template <class M> iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);
   >     template <class M> iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);

   > > -?- Requires: `is_assignable<mapped_type&, M&&>::value` shall be `true`. `value_type` shall be `EmplaceConstructible` into `unordered_map` from `k, forward<M>(obj)`.

   > > -7- Effects: If the key `k` does not exist in the map, inserts an element into the map. In the first and third forms, the element is constructed from the arguments as `value_type(k, std::forward<Args>(args)...)`. In the second and fourth forms, the element is constructed from the arguments as `value_type(std::move(k), std::forward<Args>(args)...)`. If the key already exists, `std::forward<M>(obj)` is assigned to the `mapped_type` corresponding to the key. In the first two overloads, the `bool` component of the returned value is true if and only if the insertion took place. The returned iterator points to the element that was inserted or updated If the map already contains an element `e` whose key is equivalent to `k`, assigns `forward<M>(obj)` to `e.second`. Otherwise inserts an object of type `value_type` constructed with `k, forward<M>(obj)`.

   > > -?- Returns: In the first overload, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the map element whose key is equivalent to `k`.

   > > -8- Complexity: The same as `emplace` and `emplace_hint`, respectively.

   >     template <class M> pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
   >     template <class M> iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);

   > > -?- Requires: `is_assignable<mapped_type&, M&&>::value` shall be `true`. `value_type` shall be `EmplaceConstructible` into `unordered_map` from `move(k), forward<M>(obj)`.

   > > -?- Effects: If the map already contains an element `e` whose key is equivalent to `k`, assigns `forward<M>(obj)` to `e.second`. Otherwise inserts an object of type `value_type` constructed with `move(k), forward<M>(obj)`.

   > > -?- Returns: In the first overload, the `bool` component of the returned pair is `true` if and only if the insertion took place. The returned iterator points to the map element whose key is equivalent to `k`.

   > > -?- Complexity: The same as `emplace` and `emplace_hint`, respectively.

### 2467. `is_always_equal` has slightly inconsistent default

Section: 17.6.3.5 [allocator.requirements], 20.7.8.1 [allocator.traits.types] Status: WP Submitter: Howard Hinnant Opened: 2015-01-18 Last modified: 2015-05-22

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Discussion:

Table 28 — "Allocator requirements" says that `X::is_always_equal` has a default value of `is_empty<X>`, and this is consistent with the return type description:

> Identical to or derived from `true_type` or `false_type`

`is_empty<X>` is guaranteed to be derived from either `true_type` or `false_type`. So far so good.

20.7.8.1 [allocator.traits.types]/p10 says:

>     typedef see below is_always_equal;

> > Type: `Alloc::is_always_equal` if the qualified-id `Alloc::is_always_equal` is valid and denotes a type (14.8.2); otherwise `is_empty<Alloc>::type`.

This is subtly different than what Table 28 says is the default: `is_empty<Alloc>::type` is not `is_empty<Alloc>`, but is rather one of `true_type` or `false_type`.

There are two ways to fix this:

1. Change Table 28 to say: `is_empty<X>::type`.

   or

2. Change 20.7.8.1 [allocator.traits.types]/p10:

> Type: `Alloc::is_always_equal` if the qualified-id `Alloc::is_always_equal` is valid and denotes a type (14.8.2); otherwise `is_empty<Alloc>::type`.

Both options are correct, and I see no reason to prefer one fix over the other. But Table 28 and 20.7.8.1 [allocator.traits.types]/p10 should be consistent with one another.

[2015-02 Cologne]

DK: We should accept the first bullet. GR: Why does `is_empty` even have a type member? AM: All type traits have a type member. I agree with DK's preference for the first type.

Proposed resolution:

This wording is relative to N4296.

1. Change 17.6.3.5 [allocator.requirements], Table 28 — "Allocator requirements" as presented:

   > Table 28 — Allocator requirements
   > Expression | Return type | Assertion/note pre-/post-condition | Default
   > --- | --- | --- | ---
   > `…`
   > `X::is_always_equal` | Identical to or derived from `true_type` or `false_type` | […] | `is_empty<X>::type`
   > `…`

### 2470. Allocator's `destroy` function should be allowed to fail to instantiate

Section: 17.6.3.5 [allocator.requirements] Status: WP Submitter: Daniel Krügler Opened: 2015-03-22 Last modified: 2015-05-22

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Discussion:

This issue is a spin-off of issue LWG 2447: It focuses on the observation that 17.6.3.5 [allocator.requirements] p9 (based on the numbering of working draft N4296) gives the template member `construct` more relaxations than the template member `destroy`:

> An allocator may constrain the types on which it can be instantiated and the arguments for which its `construct` member may be called. If a type cannot be used with a particular allocator, the allocator class or the call to `construct` may fail to instantiate.

Construction and destruction of a type `T` are usually intimately related to each other, so it seems similarly useful to allow the `destroy` member to fail to instantiate for a possible sub-set of instantiation types.

[2015-04-01 Library reflector vote]

The issue has been identified as Tentatively Ready based on six votes in favour.

Proposed resolution:

This wording is relative to N4296.

1. Change 17.6.3.5 [allocator.requirements] p9 as indicated:

   > -8- An allocator may constrain the types on which it can be instantiated and the arguments for which its `construct` or `destroy` members may be called. If a type cannot be used with a particular allocator, the allocator class or the call to `construct` or `destroy` may fail to instantiate.

### 2482. [c.strings] Table 73 mentions nonexistent functions

Section: 21.8 [c.strings] Status: WP Submitter: S. B.Tam Opened: 2015-01-18 Last modified: 2015-05-22

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Discussion:

N4296 Table 73 mentions the functions `mbsrtowc` and `wcsrtomb`, which are not defined in ISO C or ISO C++. Presumably they should be `mbsrtowcs` and `wcsrtombs` instead.

[2015-04-02 Library reflector vote]

The issue has been identified as Tentatively Ready based on six votes in favour.

Proposed resolution:

This wording is relative to N4296.

1. > Table 33 — Potential `mbstate_t` data races
   > `mbrlen` | `mbrtowc` | `mbsrtowcs` | `mbtowc` | `wcrtomb`
   > `wcsrtombs` | `wctomb`

### 2488. Placeholders should be allowed and encouraged to be `constexpr`

Section: 20.9.10.4 [func.bind.place] Status: WP Submitter: Stephan T. Lavavej Opened: 2015-03-27 Last modified: 2015-05-22

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Discussion:

`piecewise_construct` (20.3.5 [pair.piecewise]), `allocator_arg` (20.7.6 [allocator.tag]), and `adopt_lock`/`defer_lock`/`try_to_lock` (30.4.2 [thread.lock]) are all required to be `constexpr` with internal linkage. `bind()`'s placeholders should be allowed to follow this modern practice, for increased consistency and reduced implementer headaches (header-only is easier than separately-compiled).

[2015-05-07 Lenexa: Move to Immediate]

STL: I'd like this one immediate.

Jonathan: I want to think about forcing constexpr, but the current issue, I have no objections. I'd say ready, but I won't object to immediate if STL wants it.

Marshall: You should report in Kona how it worked out.

STL: We went around a bit on the reflector about how to phrase the encouragement.

Jonathan: I think the shall may be not quite right.

Marshall: I see, you can change your implementation, but you don't.

Jonathan: There's precedent for the shall, but it's wrong, see editorial issue 493.

STL: I would prefer not to ask Daniel to reword, and 493 can fix it later.

Marshall: I remove my objection to immediate because it doesn't force implementations to change.

Marshall: Any other discussion?

Marshall: Immediate vote: 6. Opposed, 0. Abstain, 1.

Proposed resolution:

This wording is relative to N4296.

1. Change 20.9 [function.objects] p2 "Header `<functional>` synopsis" as depicted:

   >     namespace placeholders {
   >       // M is the implementation-defined number of placeholders
   >       see belowextern unspecified _1;
   >       see belowextern unspecified _2;
   >       ...
   >       see belowextern unspecified _M;
   >     }

2. Change 20.9.10.4 [func.bind.place] p2 as depicted:

   >     namespace std::placeholders {
   >       // M is the implementation-defined number of placeholders
   >       see belowextern unspecified _1;
   >       see belowextern unspecified _2;
   >                  .
   >                  .
   >                  .
   >       see belowextern unspecified _M;
   >     }

   > -1- All placeholder types shall be `DefaultConstructible` and `CopyConstructible`, and their default constructors and copy/move constructors shall not throw exceptions. It is implementation-defined whether placeholder types are `CopyAssignable`. `CopyAssignable` placeholders' copy assignment operators shall not throw exceptions.

   > -?- Placeholders should be defined as:

   > >     constexpr unspecified _1{};

   > If they are not, they shall be declared as:

   > >     extern unspecified _1;