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 * > : /* 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 .

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 .

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 (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)

See also

indirectly_readable (C++20)	specifies that a type is indirectly readable by applying operator * (concept)
iter_value_t iter_reference_t iter_const_reference_t iter_difference_t iter_rvalue_reference_t iter_common_reference_t (C++20) (C++20) (C++23) (C++20) (C++20) (C++20)	computes the associated types of an iterator (alias template)
iterator_traits	provides uniform interface to the properties of an iterator (class template)