std::reference_constructs_from_temporary

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 std::reference_constructs_from_temporary or std::reference_constructs_from_temporary_v is undefined.

Helper variable template

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 <iostream>
#include <type_traits>
 
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';
}

true
true
false
false
true
true

See also