std::reference_constructs_from_temporary

std::reference_constructs_from_temporary - cppreference.com

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`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; otherwise the behavior is undefined.

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

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

	converts the object to `bool`, returns `value` (public member function)
	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

#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() {}