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Showing content from https://en.cppreference.com/w/cpp/language/../algorithm/../header/../iterator/readable_traits.html below:

std::indirectly_readable_traits - cppreference.com

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) (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*) (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 type value_type, otherwise value_type is not defined. A program may specialize indirectly_readable_traits for a program-defined type.

[edit] 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-unqualified T.
Otherwise, if T is an array type, V is the cv-unqualified array element 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.

[edit] Notes

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

[edit] 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

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