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std::iterator_traits

Defined in header `<iterator>`
template< class Iter > struct iterator_traits;
template< class T > struct iterator_traits<T*>;
template< class T > struct iterator_traits<const T*>;		(removed in C++20)

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

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.

User specializations may define the member type iterator_concept to one of iterator category tags, to indicate conformance to the iterator concepts.

(since C++20)

Template parameters

Iter

the iterator type to retrieve properties for

Member types

Member type	Definition
`difference_type`	`Iter::difference_type`
`value_type`	`Iter::value_type`
`pointer`	`Iter::pointer`
`reference`	`Iter::reference`
`iterator_category`	`Iter::iterator_category`

If Iter does not have all five member types difference_type, value_type, pointer, reference, and iterator_category, then this template has no members by any of those names (std::iterator_traits is SFINAE-friendly).

(since C++17)
(until C++20)

If Iter does not have pointer, but has all four remaining member types, then the member types are declared as follows:

Member type	Definition
`difference_type`	`Iter::difference_type`
`value_type`	`Iter::value_type`
`pointer`	`void`
`reference`	`Iter::reference`
`iterator_category`	`Iter::iterator_category`

Otherwise, if Iter satisfies the exposition-only concept __LegacyInputIterator, the member types are declared as follows:

Member type	Definition
`difference_type`	`std::incrementable_traits<Iter>::difference_type`
`value_type`	`std::indirectly_readable_traits<Iter>::value_type`
`pointer`	`Iter::pointer` if valid, otherwise `decltype(std::declval<Iter&>().operator->())` if valid, otherwise `void`
`reference`	`Iter::reference` if valid, otherwise `std::iter_reference_t<Iter>`
`iterator_category`	`Iter::iterator_category` if valid, otherwise, `std::random_access_iterator_tag` if `Iter` satisfies `__LegacyRandomAccessIterator`, otherwise, `std::bidirectional_iterator_tag` if `Iter` satisfies `__LegacyBidirectionalIterator`, otherwise, `std::forward_iterator_tag` if `Iter` satisfies `__LegacyForwardIterator`, otherwise, `std::input_iterator_tag`.

Otherwise, if Iter satisfies the exposition-only concept __LegacyIterator, the member types are declared as follows:

Member type	Definition
`difference_type`	`std::incrementable_traits<Iter>::difference_type` if valid, otherwise `void`
`value_type`	`void`
`pointer`	`void`
`reference`	`void`
`iterator_category`	`std::output_iterator_tag`

Otherwise, this template has no members by any of those names (std::iterator_traits is SFINAE-friendly).

(since C++20)

Specializations

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

The standard library also provides partial specializations for some standard iterator adaptors.

(since C++20)

`T*` specialization member types

Only specialized if std::is_object_v<T> is true.

(since C++20)

Member type	Definition
`difference_type`	`std::ptrdiff_t`
`value_type`	`T` (until C++20)`std::remove_cv_t<T>` (since C++20)
`pointer`	`T*`
`reference`	`T&`
`iterator_category`	`std::random_access_iterator_tag`
`iterator_concept`(C++20)	`std::contiguous_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`

(until C++20)

Specializations for library types

std::iterator_traits<std::common_iterator> (C++20)	provides uniform interface to the properties of the `std::common_iterator` type (class template specialization)
std::iterator_traits<std::counted_iterator> (C++20)	provides uniform interface to the properties of the `std::counted_iterator` type (class template specialization)

Example

Shows a general-purpose std::reverse() implementation for bidirectional iterators.

#include <iostream>
#include <iterator>
#include <list>
#include <vector>
 
template<class BidirIt>
void my_reverse(BidirIt first, BidirIt last)
{
    typename std::iterator_traits<BidirIt>::difference_type n = std::distance(first, last);
    for (--n; n > 0; n -= 2)
    {
        typename std::iterator_traits<BidirIt>::value_type tmp = *first;
        *first++ = *--last;
        *last = tmp;
    }
}
 
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 (int n : l)
        std::cout << n << ' ';
    std::cout << '\n';
 
    int a[] {1, 2, 3, 4, 5};
    my_reverse(a, a + std::size(a));
    for (int n : a)
        std::cout << n << ' ';
    std::cout << '\n';
 
//  std::istreambuf_iterator<char> i1(std::cin), i2;
//  my_reverse(i1, i2); // compilation error: i1, i2 are input iterators
 
}

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_tagcontiguous_iterator_tag (C++20)	empty class types used to indicate iterator categories (class)
iter_value_titer_reference_titer_const_reference_titer_difference_titer_rvalue_reference_titer_common_reference_t (C++20)(C++20)(C++23)(C++20)(C++20)(C++20)	computes the associated types of an iterator (alias template)

std::iterator_traits

Template parameters

Member types

Specializations

T* specialization member types

const T* specialization member types

Specializations for library types

Example

See also

`T*` specialization member types

`const T*` specialization member types