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std::reference_wrapper<T>::reference_wrapper

(1) 
template< class U >
reference_wrapper( U&& x ) noexcept(/*see below*/) ;
 (since C++11)
(until C++20)		 
template< class U >
constexpr reference_wrapper( U&& x ) noexcept(/*see below*/) ;
 (since C++20)
(2) 
reference_wrapper( const reference_wrapper& other ) noexcept;
 (since C++11)
(until C++20)		 
constexpr reference_wrapper( const reference_wrapper& other ) noexcept;
 (since C++20)

Constructs a new reference wrapper.

1) Converts x to T& as if by T& t = std::forward<U>(x);, then stores a reference to t. This overload participates in overload resolution only if typename std::decay<U>::type is not the same type as reference_wrapper and the expression FUN(std::declval<U>()) is well-formed, where FUN names the set of imaginary functions 
void FUN(T&) noexcept;
void FUN(T&&) = delete;
2) Copy constructor. Stores a reference to other.get().

Parameters

x - an object to wrap
other - another reference wrapper

Exceptions

1)
noexcept specification:
noexcept(noexcept(FUN(std::declval<U>())))
where FUN is the set of imaginary functions described in the description above.

Example

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 2993 C++11 deleted reference_wrapper(T&&) constructor interferes
with overload resolution in some cases		 replaced with a constructor template

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