std::indirectly_readable_traits - cppreference.com (original) (raw)

Defined in header
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 > requires std::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 > requires std::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 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 havingboth value_type and element_type nested types	added specialization (8)
LWG 3541	C++20	LWG 3446 introduced hard error for ambiguous casesthat value_type and element_type are different	added specialization (7)

std::indirectly_readable_traits - cppreference.com (original) (raw)

[edit] Explanation

[edit] Notes

[edit] Example

[edit] Defect reports

[edit] See also