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std::indirectly_readable_traits

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.

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 that
value_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)

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