std::reference_converts_from_temporary

Defined in header `<type_traits>`
template< class T, class U > struct reference_converts_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_converts_from_temporary or reference_converts_from_temporary_v is undefined.

Helper variable template

template< class T, class U > inline constexpr reference_converts_from_temporary_v = std::reference_converts_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 copy-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_converts_from_temporary can be used for rejecting some cases that always produce dangling references.

Example

Run this code

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

Output:

true
true
false
false
true
true

is_convertibleis_nothrow_convertible (C++11)(C++20)	checks if a type can be converted to the other type (class template)
invokeinvoke_r (C++17)(C++23)	invokes any Callable object with given arguments and possibility to specify return type (since C++23) (function template)
bind (C++11)	binds one or more arguments to a function object (function template)
visit (C++17)	calls the provided functor with the arguments held by one or more variants (function template)
(constructor)	constructs a new `std::function` instance (public member function of `std::function<R(Args...)>`)
(constructor) (C++23)	constructs a new `std::move_only_function` object (public member function of `std::move_only_function`)
(constructor)	constructs the task object (public member function of `std::packaged_task<R(Args...)>`)

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