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) (exposition only*)
struct /* cond-value-type */ {};
template< class T >

requires std::is_object_v<T> (2) (exposition only*)
struct /* cond-value-type */ <T>

{ using value_type = std::remove_cv_t<T>; };
template< class T >

concept /* has-member-value-type */ = (3) (exposition only*)

requires { typename T::value_type; };
template< class T >

concept /* has-member-element-type */ = (4) (exposition only*)

requires { typename T::element_type; };

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
specializations (5,6) were ambiguous
LWG 3446 C++20 for types having added specialization (8)
both value_type and element_type nested
types
LWG 3446 introduced hard error for
LWG 3541 C++20 ambiguous cases added specialization (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