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std::indirectly_readable_traits

From cppreference.com
< cpp‎ | iterator
 
 
Iterator library
Iterator concepts
Iterator primitives
(deprecated in C++17)
indirectly_readable_traits
(C++20)


Algorithm concepts and utilities
Indirect callable concepts
Common algorithm requirements
(C++20)
(C++20)
(C++20)
Utilities
(C++20)

Iterator adaptors
Iterator customization points
Iterator operations
(C++11)    
(C++11)
Range access
(C++11)(C++14)
(C++14)(C++14)    
(C++11)(C++14)
(C++14)(C++14)    
(C++17)(C++20)
(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*> :

    /* 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.

Contents

[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

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]