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<constint>, constint>); 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<constint>, int>); static_assert(std::is_same_v<std::unwrap_ref_decay_t<constint&>, 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)