std::indirectly_readable_traits

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Feature test macros(C++20)
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Metaprogramming library(C++11)
General utilities library
Containers library
Iterators library
Ranges library(C++20)
Algorithms library
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Date and time library
Input/output library
Filesystem library(C++17)
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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)

(C++20)

(C++20)

(C++26)

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)

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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> requiresstd::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> requiresstd::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 typevalue_type, otherwisevalue_type is not defined. A program may specializeindirectly_readable_traits for aprogram-defined type.

[edit]Explanation

The specializations above can be informally described as below.

Given a typeT, its associated value typeV is determined as follows:

IfT is const-qualified,V is the associated value type of const-unqualifiedT.
Otherwise, ifT is an array type,V is the cv-unqualified array element type.
Otherwise, a conditional value typeC is determined first:

IfT is a pointer type,C is the pointed-to type.
Otherwise, ifT has nested typesvalue_type andelement_type:

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

Otherwise, ifT has the nested typevalue_type but notelement_type,C istypename T::value_type.
Otherwise, ifT has the nested typeelement_type but notvalue_type,C istypename T::element_type.
Otherwise,C is undefined.

ThenV is determined fromC as follows:

IfC is undefined, orC is not anobject type,V is undefined.
Otherwise,V is cv-unqualifiedC.

[edit]Notes

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

[edit]Example

This section is incomplete
Reason: no example

[edit]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)

[edit]See also

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