std::reference_constructs_from_temporary

template< class T, class U >
struct reference_constructs_from_temporary;

If.

• T is a reference type, and
• given the hypothetic function declared as U makeval();, the variable definition T ref(makeval()); is well-formed and binds a temporary object to ref,

provides the member constant value equal to true. Otherwise value is false.

If T is a reference type, both std::remove_reference_t<T> and 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;

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

#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';
}

true
true
false
false
true
true

See also