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 CopyAssignable
   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 references.
   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 function. 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 function 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 argument_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 function of class
   (deprecated in C++17)(removed in T0 that takes one argument, then
   C++20) first_argument_type is T0*, possibly
                        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 function of class
   (deprecated in C++17)(removed in T0 that takes one argument A1, then
   C++20) second_argument_type is A1, possibly
                        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 references
   operator<=> (function)
   (C++26)
   Deduction guides(since C++17)

Helper classes
                        determines the common reference
   std::basic_common_reference<std::reference_wrapper> type of reference_wrapper and
   (C++23) non-reference_wrapper
                        (class template specialization)

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::remove_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 references, 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_wrapper
   (C++20) (class template)
   (C++20)