std::iterator_traits

template <class Iterator> class iterator_traits;template <class T> class iterator_traits<T*>;template <class T> class iterator_traits<const T*>;

Iterator traits

Traits class defining properties of iterators.

Standard algorithms determine certain properties of the iterators passed to them and the range they represent by using the members of the corresponding iterator_traits instantiation.

For every iterator type, a corresponding specialization of iterator_traits class template shall be defined, with at least the following member types defined:

member	description
`difference_type`	Type to express the result of subtracting one iterator from another.
`value_type`	The type of the element the iterator can point to.
`pointer`	The type of a pointer to an element the iterator can point to.
`reference`	The type of a reference to an element the iterator can point to.
`iterator_category`	The iterator category. It can be one of these: `input_iterator_tag` `output_iterator_tag` `forward_iterator_tag` `bidirectional_iterator_tag` `random_access_iterator_tag`

Note: For output iterators that are not at least forward iterators , any of these member types (except for iterator_category) may be defined as void.

The iterator_traits class template comes with a default definition that obtains these types from the iterator type itself (see below). It is also specialized for pointers (T*) and pointers to const (const T*).

Note that any custom class will have a valid instantiation of iterator_traits if it publicly inherits the base class std::iterator .

Member types

member	generic definition	`T*` specialization	`const T*` specialization
`difference_type`	`Iterator::difference_type`	`ptrdiff_t`	`ptrdiff_t`
`value_type`	`Iterator::value_type`	`T`	`T`
`pointer`	`Iterator::pointer`	`T*`	`const T*`
`reference`	`Iterator::reference`	`T&`	`const T&`
`iterator_category`	`Iterator::iterator_category`	`random_access_iterator_tag`	`random_access_iterator_tag`

Example

// iterator_traits example
#include <iostream> // std::cout
#include <iterator> // std::iterator_traits
#include <typeinfo> // typeid
int main() {
 typedef std::iterator_traits<int*> traits;
 if (typeid(traits::iterator_category)==typeid(std::random_access_iterator_tag))
 std::cout << "int* is a random-access iterator";
 return 0;
}

Output:


int* is a random-access iterator

functions

iterator categories

predefined iterators

std::iterator_traits

Member types

Example

See also