std::reference_constructs_from_temporary

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Defined in header <type_traits>
template < class T, class U >
struct reference_constructs_from_temporary;
(since C++23)

Let V be std::remove_cv_t<U> if U is a scalar type or cv void, or U otherwise. If T is a reference type, and given a hypothetic expression e such that decltype(e) is V, the variable definition T ref(e); is well-formed and binds a temporary object to ref, then provides the member constant value equal to true. Otherwise, value is false

If T is an lvalue reference type to a const- but not volatile-qualified object type or an rvalue reference type, both std::remove_reference_t<T> and std::remove_reference_t<U> shall be complete types, cv void, or an arrays of unknown bound

If an instantiation of a template above depends, directly or indirectly, on an incomplete type, and that instantiation could yield a different result if that type were hypothetically completed, the behavior is undefined.

If the program adds specializations for std::reference_constructs_from_temporary or std::reference_constructs_from_temporary_v, the behavior is undefined.

Helper variable template

template < class T, class U >

inline constexpr bool reference_constructs_from_temporary_v =

    std:: reference_constructs_from_temporary <T, U> :: value ;
(since C++23)

Inherited from std::integral_constant

Member constants

value
[static]
 true if T is a reference type, a U value can be bound to T in direct-initialization, and a temporary object would be bound to the reference, false otherwise
(public static member constant)

Member functions

operator bool
 converts the object to bool, returns value
(public member function)
operator()
(C++14)
 returns value
(public member function)

Member types

Type Definition
value_type bool
type std::integral_constant < bool, value>

Notes

std::reference_constructs_from_temporary can be used for rejecting some cases that always produce dangling references.

It is also possible to use member initializer list to reject binding a temporary object to a reference if the compiler has implemented CWG1696

Example

Run this code
#include <type_traits>
 
static_assert(std::reference_constructs_from_temporary_v<int&&, int> == true);
static_assert(std::reference_constructs_from_temporary_v<const int&, int> == true);
static_assert(std::reference_constructs_from_temporary_v<int&&, int&&> == false);
static_assert(std::reference_constructs_from_temporary_v<const int&, int&&> == false);
static_assert(std::reference_constructs_from_temporary_v<int&&, long&&> == true);
static_assert(std::reference_constructs_from_temporary_v<int&&, long> == true);
 
int main() {}

See also

(C++11)(C++11)(C++11)
 checks if a type has a constructor for specific arguments
(class template)
constructs a new tuple
(public member function of std::tuple<Types...>)
constructs new pair
(public member function of std::pair<T1,T2>)
(C++17)
 construct an object with a tuple of arguments
(function template)
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