std::unwrap_reference, std::unwrap_ref_decay

 

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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)

Unwraps any std::reference_wrapper: changing std::reference_wrapper<U> to U&.

1) If T is a specialization of std::reference_wrapper, unwraps it; otherwise, T remains the same.

2) If the decayed T is a specialization of std::reference_wrapper, unwraps it; otherwise, T is decayed.

If the program adds specializations for any of the templates described on this page, the behavior is undefined.

Nested types

Type	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 = unwrap_reference<T>::type;	(1)	(since C++20)
template<class T> using unwrap_ref_decay_t = 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	Feature
__cpp_lib_unwrap_ref	201811L	(C++20)	std::unwrap_ref_decay and std::unwrap_reference

Example

Run this code

#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

See also

reference_wrapper (C++11)	CopyConstructible and CopyAssignable reference wrapper (class template)
make_pair	creates a pair object of type, determined by the argument types (function template)
make_tuple (C++11)	creates a tuple object of the type defined by the argument types (function template)