std::indire...dable_traits(3) C++ Standard Libary std::indire...dable_traits(3) NAME std::indirectly_readable_traits - std::indirectly_readable_traits Synopsis Defined in header <iterator> template< class I > (1) (since C++20) struct indirectly_readable_traits {}; template< class T > struct indirectly_readable_traits<T*> : (2) (since C++20) /* cond-value-type */<T> {}; template< class I > requires std::is_array_v<I> struct indirectly_readable_traits<I>; (3) (since C++20) { using value_type = std::remove_cv_t<std::remove_extent_t<I>>; } template< class T > struct indirectly_readable_traits<const T> : (4) (since C++20) indirectly_readable_traits<T> {}; template< /* has-member-value-type */ T > struct indirectly_readable_traits<T> : (5) (since C++20) /* cond-value-type */<typename T::value_type> {}; template< /* has-member-element-type */ T > struct indirectly_readable_traits<T> : (6) (since C++20) /* cond-value-type */<typename T::element_type> {}; template< /* has-member-value-type */ T > requires /* has-member-element-type */<T> (7) (since C++20) struct indirectly_readable_traits<T> {}; template< /* has-member-value-type */ T > requires /* has-member-element-type */<T> && std::same_as<std::remove_cv_t<typename T::element_type>, (8) (since C++20) std::remove_cv_t<typename T::value_type>> struct indirectly_readable_traits<T> : /* cond-value-type */<typename T::value_type> {}; Helper classes and concepts template< class > (1) (expo- sition only*) struct /* cond-value-type */ {}; template< class T > requires std::is_object_v<T> (2) (expo- sition only*) struct /* cond-value-type */ <T> { using value_type = std::remove_cv_t<T>; }; template< class T > concept /* has-member-value-type */ = (3) (expo- sition only*) requires { typename T::value_type; }; template< class T > concept /* has-member-element-type */ = (4) (expo- sition only*) requires { typename T::element_type; }; Computes the associated value type of the template argument. If the as- sociated 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-un- qualified T. * Otherwise, if T is an array type, V is the cv-unqualified array ele- ment 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. Example This section is incomplete Reason: no example Defect reports The following behavior-changing defect reports were applied retroac- tively to previously published C++ standards. DR Applied to Behavior as published Correct behavior specializations (5,6) were ambiguous LWG 3446 C++20 for types having added spe- cialization (8) both value_type and element_type nested types LWG 3446 introduced hard error for LWG 3541 C++20 ambiguous cases added spe- cialization (7) that value_type and element_type are different See also indirectly_readable specifies that a type is indirectly readable by applying (C++20) operator * (concept) iter_value_t iter_reference_t iter_const_reference_t iter_difference_t iter_rvalue_reference_t iter_common_reference_t computes the associated types of an iterator (C++20) (alias template) (C++20) (C++23) (C++20) (C++20) (C++20) iterator_traits provides uniform interface to the properties of an iterator (class template) Category: * Todo no example http://cppreference.com 2024.06.10 std::indire...dable_traits(3)
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