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std::unwrap_reference, std::unwrap_ref_decay

Defined in header `<type_traits>`
Defined in header `<functional>`
template< class T > struct unwrap_reference;	(1)	(since C++20)
template< class T > struct unwrap_ref_decay;	(2)	(since C++20)

1) If T is std::reference_wrapper<U> for some type U, provides a member type alias type that names U&; otherwise, provides a member type alias type that names T.

2) If T is std::reference_wrapper<U> for some type U, ignoring cv-qualification and referenceness, provides a member type alias type that names U&; otherwise, provides a member type alias type that names std::decay_t<T>.

The behavior of a program that adds specializations for any of the templates described on this page is undefined.

Member types

Name	Definition
`type`	(1) `U&` if `T` is `std::reference_wrapper<U>`; `T` otherwise (2) `U&` if `std::decay_t<T>` is `std::reference_wrapper<U>`; `std::decay_t<T>` otherwise.

Helper types

template<class T> using unwrap_reference_t = typename unwrap_reference<T>::type;	(1)	(since C++20)
template<class T> using unwrap_ref_decay_t = typename unwrap_ref_decay<T>::type;	(2)	(since C++20)

Possible implementation

template<class T>
struct unwrap_reference { using type = T; };
template<class U>
struct unwrap_reference<std::reference_wrapper<U>> { using type = U&; };
 
template<class T>
struct unwrap_ref_decay : std::unwrap_reference<std::decay_t<T>> {};

Notes

std::unwrap_ref_decay performs the same transformation as used by std::make_pair and std::make_tuple.

Feature-test macro	Value	Std	Comment
`__cpp_lib_unwrap_ref`	201811L	(C++20)	`std::unwrap_ref_decay` and `std::unwrap_reference`

Example

#include <cassert>
#include <functional>
#include <iostream>
#include <type_traits>
 
int main()
{
    static_assert(std::is_same_v<std::unwrap_reference_t<int>, int>);
    static_assert(std::is_same_v<std::unwrap_reference_t<const int>, const int>);
    static_assert(std::is_same_v<std::unwrap_reference_t<int&>, int&>);
    static_assert(std::is_same_v<std::unwrap_reference_t<int&&>, int&&>);
    static_assert(std::is_same_v<std::unwrap_reference_t<int*>, int*>);
 
    {
        using T = std::reference_wrapper<int>;
        using X = std::unwrap_reference_t<T>;
        static_assert(std::is_same_v<X, int&>);
    }
    {
        using T = std::reference_wrapper<int&>;
        using X = std::unwrap_reference_t<T>;
        static_assert(std::is_same_v<X, int&>);
    }
 
    static_assert(std::is_same_v<std::unwrap_ref_decay_t<int>, int>);
    static_assert(std::is_same_v<std::unwrap_ref_decay_t<const int>, int>);
    static_assert(std::is_same_v<std::unwrap_ref_decay_t<const int&>, int>);
 
    {
        using T = std::reference_wrapper<int&&>;
        using X = std::unwrap_ref_decay_t<T>;
        static_assert(std::is_same_v<X, int&>);
    }
 
    {
        auto reset = []<typename T>(T&& z)
        {
        //  x = 0; // Error: does not work if T is reference_wrapper<>
            // converts T&& into T& for ordinary types
            // converts T&& into U& for reference_wrapper<U>
            decltype(auto) r = std::unwrap_reference_t<T>(z);
            std::cout << "r: " << r << '\n';
            r = 0; // OK, r has reference type
        };
 
        int x = 1;
        reset(x);
        assert(x == 0);
 
        int y = 2;
        reset(std::ref(y));
        assert(y == 0);
    }
}

Output:

r: 1
r: 2