std::iterator_traits

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

 

Iterator library

Iterator primitives
iterator_traits
input_iterator_tag output_iterator_tag forward_iterator_tag bidirectional_iterator_tag random_access_iterator_tag
iterator
Iterator adaptors
reverse_iterator
move_iterator (C++11) back_insert_iterator front_insert_iterator insert_iterator	make_move_iterator (C++11) back_inserter front_inserter inserter
Stream iterators
istream_iterator
ostream_iterator
istreambuf_iterator
ostreambuf_iterator
Iterator operations
advance
distance
prev (C++11)
next (C++11)
Range access
begin (C++11)
end (C++11)

 

Defined in header <iterator>
template< class Iterator> struct iterator_traits;
template< class T > struct iterator_traits<T*>;
template< class T > struct iterator_traits<const T*>;

std::iterator_traits is the type trait class that provides uniform interface to the properties of iterator types. This makes it possible to implement algorithms only in terms of iterators.

Member types

Member type	Definition
difference_type	Iterator::difference_type
value_type	Iterator::value_type
pointer	Iterator::pointer
reference	Iterator::reference
iterator_category	Iterator::iterator_category

Specializations

This type trait may be specialized for user-provided types that may be used as iterators. The standard library provides two partial specializations for pointer types T*, which makes it possible to use all iterator-based algorithms with raw pointers.

T* specialization member types

Member type	Definition
difference_type	std::ptrdiff_t
value_type	T
pointer	T*
reference	T&
iterator_category	std::random_access_iterator_tag

const T* specialization member types

Member type	Definition
difference_type	std::ptrdiff_t
value_type	T
pointer	const T*
reference	const T&
iterator_category	std::random_access_iterator_tag

Example

general-purpose reverse() implementation for bidirectional iterators

run this code

#include <iostream>
#include <iterator>
#include <vector>
#include <list>
 
template<class BDIter>
void my_reverse(BDIter first, BDIter last)
{
    typename std::iterator_traits<BDIter>::difference_type n = std::distance(first, last);
    --n;
    while(n > 0) {
        typename std::iterator_traits<BDIter>::value_type tmp = *first;
        *first++ = *--last;
        *last = tmp;
        n -= 2;
    }
}
 
int main()
{
    std::vector<int> v{1, 2, 3, 4, 5};
    my_reverse(v.begin(), v.end());
    for (int n : v)
        std::cout << n << ' ';
    std::cout << '\n';
 
    std::list<int> l{1, 2, 3, 4, 5};
    my_reverse(l.begin(), l.end());
    for (auto n : l)
        std::cout << n << ' ';
    std::cout << '\n';
 
//    std::istreambuf_iterator<char> i1(std::cin), i2;
//    my_reverse(i1, i2); // compilation error
 
}

Output:

5 4 3 2 1
5 4 3 2 1

See also

iterator	the basic iterator (class template)
input_iterator_tag output_iterator_tag forward_iterator_tag bidirectional_iterator_tag random_access_iterator_tag	empty class types used to indicate iterator categories (class)