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std::iterator_traits

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.

The class defines the following types:

difference_type - a type that can be used to identify distance between iterators
value_type - the type of the values that can be obtained by dereferencing the iterator. This type is void for output iterators.
pointer - defines a pointer to the type iterated over (value_type)
reference - defines a reference to the type iterated over (value_type)
iterator_category - the category of the iterator. Must be one of iterator category tags.

The template can be specialized for user-defined iterators so that the information about the iterator can be retrieved even if the type does not provide the usual typedefs.

Template parameters

Iterator

the iterator type to retrieve properties for

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`

If Iterator does not have the five member types difference_type, value_type, pointer, reference, and iterator_category, then this template has no member types (std::iterator_traits is SFINAE-friendly).

(since C++17)

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

The following example shows a general-purpose reverse() implementation for bidirectional iterators.

#include <iostream>
#include <iterator>
#include <vector>
#include <list>
 
template<class BidirIt>
void my_reverse(BidirIt first, BidirIt last)
{
    typename std::iterator_traits<BidirIt>::difference_type n = std::distance(first, last);
    --n;
    while(n > 0) {
        typename std::iterator_traits<BidirIt>::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';
 
    int a[] = {1, 2, 3, 4, 5};
    my_reverse(a, a+5);
    for (int i=0; i<5; ++i) {
        std::cout << a[i] << ' ';
    }
    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
5 4 3 2 1

iterator (deprecated in C++17)	base class to ease the definition of required types for simple iterators (class template)
input_iterator_tagoutput_iterator_tagforward_iterator_tagbidirectional_iterator_tagrandom_access_iterator_tag	empty class types used to indicate iterator categories (class)