std::indirectly_readable_traits

 

C++

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Feature test macros (C++20)

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Iterator library

Iterator concepts
indirectly_readable (C++20) indirectly_writable (C++20) weakly_incrementable (C++20) incrementable (C++20) input_or_output_iterator (C++20) sentinel_for (C++20) sized_sentinel_for (C++20)	input_iterator (C++20) output_iterator (C++20) forward_iterator (C++20) bidirectional_iterator (C++20) random_access_iterator (C++20) contiguous_iterator (C++20)
Iterator primitives
iterator_traits input_iterator_tagoutput_iterator_tagforward_iterator_tagbidirectional_iterator_tagrandom_access_iterator_tagcontiguous_iterator_tag (C++20) iterator (deprecated in C++17)	incrementable_traits (C++20) indirectly_readable_traits (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)
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)
Iterator adaptors
reverse_iterator move_iterator (C++11) 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) back_insert_iterator front_insert_iterator insert_iterator	make_reverse_iterator (C++14) make_move_iterator (C++11) default_sentinel_tdefault_sentinel (C++20)(C++20) unreachable_sentinel_tunreachable_sentinel (C++20)(C++20) make_const_iterator (C++23) make_const_sentinel (C++23) back_inserter front_inserter inserter
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) endcend (C++11)(C++14) sizessize (C++17)(C++20)	rbegincrbegin (C++14)(C++14) rendcrend (C++14)(C++14) 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*>;	(2)	(since C++20)
template< class I >   requires std::is_array_v<I> struct indirectly_readable_traits<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>;	(5)	(since C++20)
template </*has-member-element-type*/ T> struct indirectly_readable_traits<T>;	(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>;	(8)	(since C++20)

Computes the associated value type of the type I, if any. Users may specialize indirectly_readable_traits for a program-defined type.

1) Primary template has no member value_type.

2) Specialization for pointers. If T is an object type, provides a member type value_type equal to std::remove_cv_t<T>. Otherwise, there is no member value_type.

3) Specialization for array types. Provides a member type value_type equal to std::remove_cv_t<std::remove_extent_t<I>>.

4) Specialization for const-qualified types.

5) Specialization for types that define a public and accessible member type value_type (e.g., std::reverse_iterator). If T::value_type is an object type, provides a member type value_type equal to std::remove_cv_t<typename T::value_type>. Otherwise, there is no member value_type.

6) Specialization for types that define a public and accessible member type element_type (e.g., std::shared_ptr). If T::element_type is an object type, provides a member type value_type equal to std::remove_cv_t<typename T::element_type>. Otherwise, there is no member value_type.

7-8) Specialization for types that define public and accessible member types value_type and element_type (e.g., std::span). If both T::value_type and T::element_type are object types and they become the same type after removing cv-qualifiers on the top-level, provides a member type value_type equal to std::remove_cv_t<typename T::value_type>. Otherwise, there is no member value_type.

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 were ambiguous for types containing both value_type and element_type member types	a disambiguating specialization added
LWG 3541	C++20	LWG 3446 introduced hard error for ambiguous cases that value_type and element_type are different	made resulting substitution failure

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)