# Utilities

ISO C++
library

## Functors

If you don't know what functors are, you're not alone. Many people get slightly the wrong idea. In the interest of not reinventing the wheel, we will refer you to the introduction to the functor concept written by SGI as part of their STL, in their http://www.sgi.com/tech/stl/functors.html.

## Pairs

The `pair<T1,T2>` is a simple and handy way to carry around a pair of objects. One is of type T1, and another of type T2; they may be the same type, but you don't get anything extra if they are. The two members can be accessed directly, as `.first` and `.second`.

Construction is simple. The default ctor initializes each member with its respective default ctor. The other simple ctor,

        pair (const T1& x, const T2& y);

does what you think it does, `first` getting `x` and `second` getting `y`.

There is a constructor template for copying pairs of other types:

        template <class U, class V> pair (const pair<U,V>& p);

The compiler will convert as necessary from U to T1 and from V to T2 in order to perform the respective initializations.

The comparison operators are done for you. Equality of two `pair<T1,T2>`s is defined as both `first` members comparing equal and both `second` members comparing equal; this simply delegates responsibility to the respective `operator==` functions (for types like MyClass) or builtin comparisons (for types like int, char, etc).

The less-than operator is a bit odd the first time you see it. It is defined as evaluating to:

        x.first  <  y.first  ||
    	( !(y.first  <  x.first)  &&  x.second  <  y.second )

The other operators are not defined using the `rel_ops` functions above, but their semantics are the same.

Finally, there is a template function called `make_pair` that takes two references-to-const objects and returns an instance of a pair instantiated on their respective types:

        pair<int,MyClass> p = make_pair(4,myobject);

## Memory

Memory contains three general areas. First, function and operator calls via `new` and `delete` operator or member function calls. Second, allocation via `allocator`. And finally, smart pointer and intelligent pointer abstractions.

## Traits