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std::reference_wrapper(3)      C++ Standard Libary     std::reference_wrapper(3)

NAME
     std::reference_wrapper - std::reference_wrapper

Synopsis
	Defined in header <functional>
	template< class T >		(since C++11)
	class reference_wrapper;

	std::reference_wrapper	is  a class template that wraps a reference in a
     copyable,
	assignable object.

	Specifically, std::reference_wrapper is a CopyConstructible and  CopyAs-
     signable
	wrapper around a reference to object or reference to function of type T.
     Instances
	of  std::reference_wrapper  are objects (they can be copied or stored in
     containers)
	but they are implicitly convertible to T&, so that they can be	used  as
     arguments
	with the functions that take the underlying type by reference.

	If  the stored reference is Callable, std::reference_wrapper is callable
     with the
	same arguments.

	Helper functions std::ref and std::cref are often used to generate
	std::reference_wrapper objects.

	std::reference_wrapper	is  used  to  pass  objects  by   reference   to
     std::bind, the
	constructor of std::thread, or the helper functions std::make_pair and
	std::make_tuple.  It can also be used as a mechanism to store references
     inside
	standard containers (like std::vector) that cannot normally hold  refer-
     ences.

	std::reference_wrapper	is  guaranteed	to  be TriviallyCopyable. (since
     C++17)

	T may be an incomplete type. (since C++20)

Member types
	type				 definition
	type				 T
	result_type			 The return type of T if T  is	a  func-
     tion. Otherwise,
	(deprecated in C++17)(removed in not defined
	C++20)
					   *  if  T  is a function or pointer to
     function that
					     takes one argument of type A1, then
					     argument_type is A1
	argument_type			   * if T is a pointer to  member  func-
     tion of class
	(deprecated  in  C++17)(removed  in	T0 that takes no arguments, then
     argument_type
	C++20)				     is T0*, possibly cv-qualified
					   * if T is a class type with a  member
     type
					     T::argument_type,	   then    argu-
     ment_type is an
					     alias of that
					   * if T is a function  or  pointer  to
     function that
					     takes two arguments of types A1 and
     A2, then
					     first_argument_type is A1
	first_argument_type		    *  if T is a pointer to member func-
     tion of class
	(deprecated in C++17)(removed in     T0 that takes one argument, then
	C++20)				     first_argument_type is T0*,  possi-
     bly
					     cv-qualified
					   *  if T is a class type with a member
     type
					     T::first_argument_type, then
					     first_argument_type is an alias  of
     that
					   *  if  T  is a function or pointer to
     function that
					     takes two arguments of  type  s  A1
     and A2, then
					     second_argument_type is A2
	second_argument_type		    *  if T is a pointer to member func-
     tion of class
	(deprecated in C++17)(removed in     T0 that takes one argument A1, then
	C++20)				     second_argument_type is A1,  possi-
     bly
					     cv-qualified
					   *  if T is a class type with a member
     type
					     T::second_argument_type, then
					     second_argument_type is an alias of
     that

Member functions
	constructor   stores a reference in a new std::reference_wrapper object
		      (public member function)
	operator=     rebinds a std::reference_wrapper
		      (public member function)
	get	      accesses the stored reference
	operator T&   (public member function)
	operator()    calls the stored function
		      (public member function)

Non-member functions
	operator==  compares reference_wrapper objects as  their  stored  refer-
     ences
	operator<=> (function)
	(C++26)

	Deduction guides(since C++17)

Helper classes
							    determines	the com-
     mon reference
	std::basic_common_reference<std::reference_wrapper>   type   of   refer-
     ence_wrapper and
	(C++23) 					     non-reference_wrap-
     per
							    (class template spe-
     cialization)

Possible implementation
	namespace detail
	{
	    template<class T> constexpr T& FUN(T& t) noexcept { return t; }
	    template<class T> void FUN(T&&) = delete;
	}

	template<class T>
	class reference_wrapper
	{
	public:
	    // types
	    using type = T;

	    // construct/copy/destroy
	    template<class U, class = decltype(
		detail::FUN<T>(std::declval<U>()),
		std::enable_if_t<!std::is_same_v<reference_wrapper,	std::re-
     move_cvref_t<U>>>()
	    )>
	    constexpr reference_wrapper(U&& u)
		noexcept(noexcept(detail::FUN<T>(std::forward<U>(u))))
		: _ptr(std::addressof(detail::FUN<T>(std::forward<U>(u)))) {}

	    reference_wrapper(const reference_wrapper&) noexcept = default;

	    // assignment
	    reference_wrapper&	operator=(const reference_wrapper& x) noexcept =
     default;

	    // access
	    constexpr operator T& () const noexcept { return *_ptr; }
	    constexpr T& get() const noexcept { return *_ptr; }

	    template<class... ArgTypes>
	    constexpr std::invoke_result_t<T&, ArgTypes...>
		operator() (ArgTypes&&... args ) const
		    noexcept(std::is_nothrow_invocable_v<T&, ArgTypes...>)
	    {
		return std::invoke(get(), std::forward<ArgTypes>(args)...);
	    }

	private:
	    T* _ptr;
	};

	// deduction guides
	template<class T>
	reference_wrapper(T&) -> reference_wrapper<T>;

Example
	Demonstrates the use of std::reference_wrapper as a container of  refer-
     ences, which
	makes it possible to access the same container using multiple indices.

     // Run this code

      #include <algorithm>
      #include <functional>
      #include <iostream>
      #include <list>
      #include <numeric>
      #include <random>
      #include <vector>

      void println(auto const rem, std::ranges::range auto const& v)
      {
	  for (std::cout << rem; auto const& e : v)
	      std::cout << e << ' ';
	  std::cout << '\n';
      }

      int main()
      {
	  std::list<int> l(10);
	  std::iota(l.begin(), l.end(), -4);

	  //  can't  use shuffle on a list (requires random access), but can use
     it on a vector
	  std::vector<std::reference_wrapper<int>> v(l.begin(), l.end());

	  std::ranges::shuffle(v, std::mt19937{std::random_device{}()});

	  println("Contents of the list: ", l);
	  println("Contents of the list, as seen through a shuffled  vector:  ",
     v);

	  std::cout << "Doubling the values in the initial list...\n";
	  std::ranges::for_each(l, [](int& i) { i *= 2; });

	  println("Contents  of  the list, as seen through a shuffled vector: ",
     v);
      }

Possible output:
      Contents of the list: -4 -3 -2 -1 0 1 2 3 4 5
      Contents of the list, as seen through a shuffled vector: -1 2 -2 1 5  0  3
     -3 -4 4
      Doubling the values in the initial list...
      Contents	of the list, as seen through a shuffled vector: -2 4 -4 2 10 0 6
     -6 -8 8

See also
	ref		 creates a std::reference_wrapper with	a  type  deduced
     from its
	cref		 argument
	(C++11) 	 (function template)
	(C++11)
	bind		 binds one or more arguments to a function object
	(C++11) 	 (function template)
	unwrap_reference
	unwrap_ref_decay  get the reference type wrapped in std::reference_wrap-
     per
	(C++20) 	 (class template)
	(C++20)

http://cppreference.com 	   2024.06.10	       std::reference_wrapper(3)
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