std::iter_value_t, std::iter_reference_t, std::iter_const_reference_t, std::iter_difference_t, std::iter_rvalue_reference_t, std::iter_common_reference_t

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Diagnostics library

Memory management library

Metaprogramming library (C++11)

General utilities library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library (C++17)

Concurrency support library (C++11)

Execution control library (C++26)

Technical specifications

Symbols index

External libraries

 

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 T > using iter_value_t = /* see below */;	(1)	(since C++20)
template< /*dereferenceable*/ T > using iter_reference_t = decltype(*std::declval<T&>());	(2)	(since C++20)
template< std::indirectly_readable T > using iter_const_reference_t = std::common_reference_t<const std::iter_value_t<T>&&, std::iter_reference_t<T>>;	(3)	(since C++23)
template< class T > using iter_difference_t = /* see below */;	(4)	(since C++20)
template< /*dereferenceable*/ T> requires /* see below */ using iter_rvalue_reference_t = decltype(ranges::iter_move(std::declval<T&>()));	(5)	(since C++20)
template< std::indirectly_readable T > using iter_common_reference_t = std::common_reference_t<std::iter_reference_t<T>, /*indirect-value-t*/<T>>;	(6)	(since C++20)
Helper templates
template< class T > concept /*dereferenceable*/ = /* see below */;	(7)	(exposition only*)
template< std::indirectly_readable T > using /*indirect-value-t*/ = /* see below */;	(8)	(exposition only*)

Compute the associated types of an iterator.

1) Computes the value type of T.

• If std::iterator_traits<std::remove_cvref_t<T>> is not specialized, then std::iter_value_t<T> is std::indirectly_readable_traits<std::remove_cvref_t<T>>::value_type.
• Otherwise, it is std::iterator_traits<std::remove_cvref_t<T>>::value_type.

2) Computes the reference type of T.

3) Computes the const reference type of T.

4) Computes the difference type of T.

• If std::iterator_traits<std::remove_cvref_t<T>> is not specialized, then std::iter_difference_t<T> is std::incrementable_traits<std::remove_cvref_t<T>>::difference_type.
• Otherwise, it is std::iterator_traits<std::remove_cvref_t<T>>::difference_type.

5) Computes the rvalue reference type of T. The constraint on this alias template is satisfied if and only if the expression ranges::iter_move(std::declval<T&>()) is valid and its type is not (possibly cv-qualified) void.

6) Computes the common reference type of T. This is the common reference type between its reference type and an lvalue reference to its value type.

7) The exposition-only concept dereferenceable is satisfied if and only if the expression *std::declval<T&>() is valid and its type is not (possibly cv-qualified) void.

8) The exposition-only alias template indirect-value-t denotes the following:

• std::invoke_result_t<Proj&, /*indirect-value-t*/<I>> if T is the same as std::projected<I, Proj> for some types I and Proj.
• Otherwise, std::iter_value_t<T>&.

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
P2609R3	C++20	std::iter_common_reference_t was defined in terms of std::iter_value_t<T>& which incorrectly handled std::projected types that project into rvalue reference types	defined in terms of /*indirect-value-t*/<T> to handle such cases

See also

indirectly_readable (C++20)	specifies that a type is indirectly readable by applying operator * (concept)
weakly_incrementable (C++20)	specifies that a semiregular type can be incremented with pre- and post-increment operators (concept)
indirectly_readable_traits (C++20)	computes the value type of an indirectly_readable type (class template)
incrementable_traits (C++20)	computes the difference type of a weakly_incrementable type (class template)
iterator_traits	provides uniform interface to the properties of an iterator (class template)
ranges::iterator_tranges::const_iterator_tranges::sentinel_tranges::const_sentinel_t (C++20)(C++23)(C++20)(C++23)	obtains iterator and sentinel types of a range (alias template)
ranges::range_reference_tranges::range_const_reference_tranges::range_rvalue_reference_tranges::range_common_reference_t (C++20)(C++23)(C++20)(C++20)	obtains reference types of a range (alias template)
ranges::range_difference_tranges::range_size_tranges::range_value_t (C++20)(C++20)(C++20)	obtains size, difference, and value types of a range (alias template)