std::indirectly_readable_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 I > struct indirectly_readable_traits {};	(1)	(since C++20)
template< class T > struct indirectly_readable_traits<T*> :     /* cond-value-type */<T> {};	(2)	(since C++20)
template< class I >     requires std::is_array_v<I> struct indirectly_readable_traits<I>; { using value_type = std::remove_cv_t<std::remove_extent_t<I>>; }	(3)	(since C++20)
template< class T > struct indirectly_readable_traits<const T> :     indirectly_readable_traits<T> {};	(4)	(since C++20)
template< /* has-member-value-type */ T > struct indirectly_readable_traits<T> :     /* cond-value-type */<typename T::value_type> {};	(5)	(since C++20)
template< /* has-member-element-type */ T > struct indirectly_readable_traits<T> :     /* cond-value-type */<typename T::element_type> {};	(6)	(since C++20)
template< /* has-member-value-type */ T >     requires /* has-member-element-type */<T> struct indirectly_readable_traits<T> {};	(7)	(since C++20)
template< /* has-member-value-type */ T >     requires /* has-member-element-type */<T> &&              std::same_as<std::remove_cv_t<typename T::element_type>,                           std::remove_cv_t<typename T::value_type>> struct indirectly_readable_traits<T> :     /* cond-value-type */<typename T::value_type> {};	(8)	(since C++20)
Helper classes and concepts
template< class > struct /* cond-value-type */ {};	(1)	(exposition only*)
template< class T >     requires std::is_object_v<T> struct /* cond-value-type */ <T> { using value_type = std::remove_cv_t<T>; };	(2)	(exposition only*)
template< class T > concept /* has-member-value-type */ =     requires { typename T::value_type; };	(3)	(exposition only*)
template< class T > concept /* has-member-element-type */ =     requires { typename T::element_type; };	(4)	(exposition only*)

Computes the associated value type of the template argument. If the associated value type exists, it is represented by the nested type value_type, otherwise value_type is not defined. A program may specialize indirectly_readable_traits for a program-defined type.

Explanation

The specializations above can be informally described as below.

Given a type T, its associated value type V is determined as follows:

• If T is const-qualified, V is the associated value type of const-unqualified T.
• Otherwise, if T is an array type, V is the cv-unqualified array element type.
• Otherwise, a conditional value type C is determined first:

• If T is a pointer type, C is the pointed-to type.
• Otherwise, if T has nested types value_type and element_type:

• If these types are the same (not considering cv-qualification), C is typename T::value_type.
• Otherwise, C is undefined.

• Otherwise, if T has the nested type value_type but not element_type, C is typename T::value_type.
• Otherwise, if T has the nested type element_type but not value_type, C is typename T::element_type.
• Otherwise, C is undefined.

Then V is determined from C as follows:

• If C is undefined, or C is not an object type, V is undefined.
• Otherwise, V is cv-unqualified C.

Notes

value_type is intended for use with indirectly_readable types such as iterators. It is not intended for use with ranges.

Defect reports

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

DR	Applied to	Behavior as published	Correct behavior
LWG 3446	C++20	specializations (5,6) were ambiguous for types having both value_type and element_type nested types	added specialization (8)
LWG 3541	C++20	LWG 3446 introduced hard error for ambiguous cases that value_type and element_type are different	added specialization (7)

See also

indirectly_readable (C++20)	specifies that a type is indirectly readable by applying operator * (concept)
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)
iterator_traits	provides uniform interface to the properties of an iterator (class template)