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std::unwrap_reference, std::unwrap_ref_decay

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unwrap_referenceunwrap_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)
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.

Contents

[edit] 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

[edit] 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)

[edit] 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>> {};

[edit] 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

[edit] Example

#include <cassert>
#include <iostream>
#include <functional>
#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

[edit] See also

CopyConstructible and CopyAssignable reference wrapper
(class template) [edit]
creates a pair object of type, defined by the argument types
(function template) [edit]
creates a tuple object of the type defined by the argument types
(function template) [edit]