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; 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.

The behavior of a program that adds specializations for reference_constructs_from_temporary or reference_constructs_from_temporary_v is undefined.

Helper variable template

template< class T, class U > inline constexpr 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

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>
#include <iostream>
 
int main()
{
    std::cout << std::boolalpha
        << std::reference_constructs_from_temporary_v<int&&, int> << '\n'
        << std::reference_constructs_from_temporary_v<const int&, int> << '\n'
        << std::reference_constructs_from_temporary_v<int&&, int&&> << '\n'
        << std::reference_constructs_from_temporary_v<const int&, int&&> << '\n'
        << std::reference_constructs_from_temporary_v<int&&, long&&> << '\n';
        << std::reference_constructs_from_temporary_v<int&&, long> << '\n';
}

Output:

true
true
false
false
true
true

See also

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