Difference between revisions of "cpp/iterator/indirectly readable traits"
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indirectly_readable_traits<T> {}; | indirectly_readable_traits<T> {}; | ||
}} | }} | ||
| − | {{dcl|num=5|since=c++20 | + | {{dcl|num=5|since=c++20| |
template</*has-member-value-type*/ T> | template</*has-member-value-type*/ T> | ||
struct indirectly_readable_traits<T>; | struct indirectly_readable_traits<T>; | ||
}} | }} | ||
| − | {{dcl|num=6|since=c++20 | + | {{dcl|num=6|since=c++20| |
template</*has-member-element-type*/ T> | template</*has-member-element-type*/ T> | ||
struct indirectly_readable_traits<T>; | struct indirectly_readable_traits<T>; | ||
}} | }} | ||
| − | {{dcl|num=7|since=c++20 | + | {{dcl|num=7|since=c++20| |
template</*has-member-value-type*/ T> | template</*has-member-value-type*/ T> | ||
requires /*has-member-element-type*/<T> | requires /*has-member-element-type*/<T> | ||
struct indirectly_readable_traits<T> {}; | struct indirectly_readable_traits<T> {}; | ||
}} | }} | ||
| − | {{dcl|num=8|since=c++20 | + | {{dcl|num=8|since=c++20| |
template</*has-member-value-type*/ T> | template</*has-member-value-type*/ T> | ||
requires /*has-member-element-type*/<T> && | requires /*has-member-element-type*/<T> && | ||
Revision as of 16:08, 3 October 2023
| 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> |
(3) | (since C++20) |
| template< class T > struct indirectly_readable_traits<const 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> |
(7) | (since C++20) |
| template</*has-member-value-type*/ T> requires /*has-member-element-type*/<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.Contents |
Notes
value_type is intended for use with indirectly_readable types such as iterators. It is not intended for use with ranges.
Example
| This section is incomplete Reason: no example |
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 thatvalue_type and element_type are different
|
made resulting substitution failure |
See also
| (C++20) |
specifies that a type is indirectly readable by applying operator * (concept) |
| (C++20)(C++20)(C++23)(C++20)(C++20)(C++20) |
computes the associated types of an iterator (alias template) |
| provides uniform interface to the properties of an iterator (class template) |
