std::basic_common_reference<std::reference_wrapper>

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basic_common_reference<std::reference_wrapper>
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Defined in header <functional>
template < class R, class T,

template < class > RQual, template < class > TQual >
    requires ( /*ref-wrap-common-reference-exists-with*/ <R, T, RQual<R>, TQual<T>> &&
! /*ref-wrap-common-reference-exists-with*/ <T, R, TQual<T>, RQual<R>> )

struct basic_common_reference<R, T, RQual, TQual> ;
(1) (since C++23)
template < class T, class R,

template < class > TQual, template < class > RQual >
    requires ( /*ref-wrap-common-reference-exists-with*/ <R, T, RQual<R>, TQual<T>> &&
! /*ref-wrap-common-reference-exists-with*/ <T, R, TQual<T>, RQual<R>> )

struct basic_common_reference<T, R, TQual, RQual> ;
(2) (since C++23)
Helper concepts
template < class R, class T, class RQ, class TQ >

concept /*ref-wrap-common-reference-exists-with*/ =
/*is-ref-wrapper*/ <R> &&
    requires { typename std::common_reference_t < typename R:: type &, TQ> ; } &&

    std:: convertible <RQ, std::common_reference_t < typename R:: type &, TQ>> ;
(3) (exposition only*)

The common reference type of a reference_wrapper (denoted as R) and a type T, correspondingly applied with cv and reference qualifiers (denoted as RQ and TQ respectively), is equivalent to a common reference type of underlying type of R applied with lvalue reference and the TQ

The common reference type is defined only if R is the only reference_wrapper, the underlying type of R applied with lvalue reference and the TQ have a common reference type to which RQ

The exposition-only constant /*is-ref-wrapper*/<R> is true if and only if R is a specialization of std::reference_wrapper

Member types

Member type Definition
type std::common_reference_t < typename R:: type &, TQual<T>> (1,2)

Notes

Feature-test macro Value Std Feature
__cpp_lib_common_reference_wrapper 202302L (C++23) Make std::common_reference_t of std::reference_wrapper a reference type

Example

Run this code
#include <concepts>
#include <functional>
 
static_assert(std::same_as<std::common_reference_t<int&,
                                                   std::reference_wrapper<int>>,
                                                   int&>);
static_assert(std::same_as<std::common_reference_t<std::reference_wrapper<int>&,
                                                   int&>,
                                                   int&>);
static_assert(std::same_as<std::common_reference_t<int&,
                                                   const std::reference_wrapper<int>&>,
                                                   int&>);
int main() {}

See also

(C++20)
 determines the common reference type of a group of types
(class template)
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