std::iterator_traits

 

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Iterator library

Iterator concepts
indirectly_readable (C++20) indirectly_writable (C++20) weakly_incrementable (C++20) incrementable (C++20) is-integer-like is-signed-integer-like (C++20)(C++20)	sentinel_for (C++20) sized_sentinel_for (C++20) input_iterator (C++20) output_iterator (C++20) input_or_output_iterator (C++20)	forward_iterator (C++20) bidirectional_iterator (C++20) random_access_iterator (C++20) contiguous_iterator (C++20)
Iterator primitives
input_iterator_tagoutput_iterator_tagforward_iterator_tagbidirectional_iterator_tagrandom_access_iterator_tagcontiguous_iterator_tag (C++20)	iter_value_titer_difference_titer_reference_titer_const_reference_titer_rvalue_reference_titer_common_reference_t (C++20)(C++20)(C++20)(C++23)(C++20)(C++20)	iterator (deprecated in C++17) iterator_traits incrementable_traits (C++20) indirectly_readable_traits (C++20)
Algorithm concepts and utilities
Indirect callable concepts
indirectly_unary_invocableindirectly_regular_unary_invocable (C++20)(C++20)	indirect_unary_predicate (C++20) indirect_binary_predicate (C++20)	indirect_equivalence_relation (C++20) indirect_strict_weak_order (C++20)
Common algorithm requirements
indirectly_movable (C++20) indirectly_movable_storable (C++20) indirectly_copyable (C++20)	indirectly_copyable_storable (C++20)   indirectly_swappable (C++20) indirectly_comparable (C++20)	permutable (C++20) mergeable (C++20) sortable (C++20)
Utilities
indirect_result_t (C++20)	projected (C++20)	projected_value_t (C++26)
Iterator adaptors
reverse_iterator make_reverse_iterator (C++14) move_iterator (C++11) make_move_iterator (C++11) default_sentinel_tdefault_sentinel (C++20)(C++20) unreachable_sentinel_tunreachable_sentinel (C++20)(C++20)	front_insert_iterator back_insert_iterator inserter insert_iterator front_inserter back_inserter move_sentinel (C++20)	common_iterator (C++20) counted_iterator (C++20) basic_const_iterator (C++23) const_iterator (C++23) const_sentinel (C++23) make_const_iterator (C++23) make_const_sentinel (C++23)
Stream iterators istream_iterator    ostream_iterator istreambuf_iterator ostreambuf_iterator	Iterator customization points ranges::iter_move (C++20) ranges::iter_swap (C++20)
Iterator operations advance distance prev (C++11)   next (C++11) ranges::advance (C++20) ranges::distance (C++20) ranges::prev (C++20) ranges::next (C++20)	Range access begincbegin (C++11)(C++14) rbegincrbegin (C++14)(C++14)   endcend (C++11)(C++14) rendcrend (C++14)(C++14)   sizessize (C++17)(C++20) empty (C++17) data (C++17)

 

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 nested 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

Nested 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 any of the five nested types above, 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 nested types, then these four nested types are declared as follows: Nested 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 nested types are declared as follows: Nested 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 nested types are declared as follows: Nested 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 nested types

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

(since C++20)

Nested 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 (since C++20)	std::contiguous_iterator_tag

const T* specialization nested types Nested 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.

Run this code

#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

See also

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)