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std::indirectly_readable_traits

From cppreference.com
< cpp‎ | iterator
 
 
Iterator library
Iterator concepts
Iterator primitives
indirectly_readable_traits
(C++20)

Algorithm concepts and utilities
Indirect callable concepts
Common algorithm requirements
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(C++20)
Iterator adaptors
Stream iterators
Iterator customization points
Iterator operations
(C++11)
(C++11)
Range access
(C++11)(C++14)
(C++11)(C++14)
(C++17)(C++20)
(C++14)(C++14)
(C++14)(C++14)
(C++17)
(C++17)
 
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.

Contents

[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 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

[edit] See also

specifies that a type is indirectly readable by applying operator *
(concept) [edit]
computes the associated types of an iterator
(alias template) [edit]
provides uniform interface to the properties of an iterator
(class template) [edit]