std:: basic_common_reference <std::reference_wrapper>

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std::reference_wrapper
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 must be convertible.

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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