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std::common_reference(3) C++ Standard Libary std::common_reference(3) NAME std::common_reference - std::common_reference Synopsis Defined in header <type_traits> template< class... T > (since C++20) struct common_reference; Determines the common reference type of the types T..., that is, the type to which all the types in T... can be converted or bound. If such a type ex- ists (as determined according to the rules below), the member type names that type. Otherwise, there is no member type. The behavior is undefined if any of the types in T... is an incomplete type other than (possibly cv-qualified) void. When given reference types, common_reference attempts to find a ref- erence type to which the supplied reference types can all be bound, but may return a non-reference type if it cannot find such a reference type. * If sizeof...(T) is zero, there is no member type. * If sizeof...(T) is one (i.e., T... contains only one type T0), the member type names the same type as T0. * If sizeof...(T) is two (i.e., T... contains two types T1 and T2): * Let type S be the simple common reference type of T1 and T2 (as defined below). The member type type names S if all of the condi- tions below are satisfied: * T1 and T2 are both reference types * S is well-formed * std::is_convertible_v<std::add_pointer_t<T1>, std::add_pointer_t<S>> and (since C++23) std::is_convertible_v<std::add_pointer_t<T2>, std::add_pointer_t<S>> are true; * Otherwise, if std::basic_common_reference<std::re- move_cvref_t<T1>, std::remove_cvref_t<T2>, T1Q, T2Q>::type exists, where TiQ is a unary alias template such that TiQ<U> is U with the addition of Ti's cv- and reference qualifiers, then the member type type names that type; * * Otherwise, if decltype(false? val<T1>() : val<T2>()), where val is a function template template<class T> T val();, is a valid type, then the member type type names that type; * Otherwise, if std::common_type_t<T1, T2> is a valid type, then the member type type names that type; * Otherwise, there is no member type. * If sizeof...(T) is greater than two (i.e., T... consists of the types T1, T2, R...), then if std::common_reference_t<T1, T2> exists, the mem- ber type denotes std::common_reference_t<std::common_reference_t<T1, T2>, R...> if such a type exists. In all other cases, there is no member type. The simple common reference type of two reference types T1 and T2 is defined as follows: * If T1 is cv1 X& and T2 is cv2 Y& (i.e., both are lvalue refer- ence types): their simple common reference type is decltype(false? std::de- clval<cv12 X&>() : std::declval<cv12 Y&>()), where cv12 is the union of cv1 and cv2, if that type exists and is a reference type; * If T1 and T2 are both rvalue reference types: if the simple com- mon reference type of T1& and T2& (determined according to the previous bul- let) exists, then let C denote that type's corresponding rvalue reference type. If std::is_convertible_v<T1, C> and std::is_convertible_v<T2, C> are both true, then the simple common reference type of T1 and T2 is C; * Otherwise, one of the two types must be an lvalue reference type A& and the other must be an rvalue reference type B&& (A and B might be cv- qualified). Let D denote the simple common reference type of A& and B const&, if any. If D exists and std::is_convertible_v<B&&, D> is true, then the sim- ple common reference type is D; * Otherwise, there's no simple common reference type. See Conditional operator for the definition of the type of expres- sion false ? X : Y like the ones used above. Member types Name Definition type the common reference type for all T... Helper types template< class... T > using common_reference_t = std::common_reference<T...>::type; template< class T, class U, template<class> class TQual, tem- plate<class> class UQual > struct basic_common_reference {}; The class template basic_common_reference is a customization point that allows users to influence the result of common_reference for user-defined types (typically proxy references). The primary template is empty. Specializations A program may specialize std::basic_common_reference<T, U, TQual, UQual> on the first two parameters T and U if std::is_same_v<T, std::decay_t<T>> and std::is_same_v<U, std::decay_t<U>> are both true and at least one of them depends on a program-defined type. If such a specialization has a member named type, it must be a pub- lic and unambiguous member that names a type to which both TQual<T> and UQual<U> are convertible. Additionally, std::basic_common_reference<T, U, TQual, UQual>::type and std::basic_common_reference<U, T, UQual, TQual>::type must denote the same type. A program may not specialize basic_common_reference on the third or fourth parameters, nor may it specialize common_reference itself. A program that adds specializations in violation of these rules has undefined behavior. The standard library provides following specializations of ba- sic_common_reference: determines the common reference std::basic_common_reference<std::pair> type of two pairs (C++23) (class template specialization) determines the common reference std::basic_common_reference<tuple-like> type of a tuple and a tuple-like (C++23) type (class template specialization) determines the common reference std::basic_common_reference<std::reference_wrapper> type of refer- ence_wrapper and (C++23) non-refer- ence_wrapper (class template specialization) Notes Feature-test macro Value Std Feature __cpp_lib_common_reference 202302L (C++23) Make std::common_refer- ence_t of std::reference_wrapper a reference type Examples // Run this code #include <concepts> #include <type_traits> static_assert( std::same_as< int&, std::common_reference_t< std::add_lvalue_reference_t<int>, std::add_lvalue_reference_t<int>&, std::add_lvalue_reference_t<int>&&, std::add_lvalue_reference_t<int>const, std::add_lvalue_reference_t<int>const& > > ); int main() {} See also common_type determines the common type of a group of types (C++11) (class template) common_reference_with specifies that two types share a common refer- ence type (C++20) (concept) http://cppreference.com 2024.06.10 std::common_reference(3)
NAME | Synopsis | Member types | Helper types | Specializations | Notes | Examples | See also
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