std::basic_common_reference<std::reference_wrapper>

Since C++23

Header: <functional>

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.

# Declarations

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>;

(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>;

(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>>;

(exposition only*)

# 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

#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