std::reference_wrapper (3) - Linux Manuals

std::reference_wrapper: std::reference_wrapper

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. It is frequently used as a mechanism to store references inside standard containers (like std::vector) which cannot normally hold references.
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 also 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.

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(deprecated in C++17)(removed in C++20) The return type of T if T is a function. Otherwise, not defined
                                                            1) if T is a function or pointer to function that takes one argument of type A1, then argument_type is A1.
argument_type(deprecated in C++17)(removed in C++20) 2) if T is a pointer to member function of class T0 that takes no arguments, then argument_type is T0*, possibly cv-qualified
                                                            3) if T is a class type with a member type T::argument_type, then argument_type is an alias of that
                                                            1) 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(deprecated in C++17)(removed in C++20) 2) if T is a pointer to member function of class T0 that takes one argument, then first_argument_type is T0*, possibly cv-qualified
                                                            3) if T is a class type with a member type T::first_argument_type, then first_argument_type is an alias of that
                                                            1) 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(deprecated in C++17)(removed in C++20) 2) if T is a pointer to member function of class T0 that takes one argument A1, then second_argument_type is A1, possibly cv-qualified
                                                            3) 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

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

Deduction_guides(since C++17)

Possible implementation

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

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

    // construct/copy/destroy
    template <class U, class = decltype(
      detail::FUN<T>(std::declval<U>()),
      std::enable_if_t<!std::is_same_v<reference_wrapper, remove_cvref_t<U>>>()
    )>
    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
    operator T& () const noexcept { return *_ptr; }
    T& get() const noexcept { return *_ptr; }

    template< class... ArgTypes >
    std::invoke_result_t<T&, ArgTypes...>
      operator() ( ArgTypes&&... args ) const {
      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 reference_wrapper as a container of references, which makes it possible to access the same container using multiple indexes
// Run this code

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

  int main()
  {
      std::list<int> l(10);

      std::iota(l.begin(), l.end(), -4);
      std::vector<std::reference_wrapper<int>> v(l.begin(), l.end());

      // can't use shuffle on a list (requires random access), but can use it on a vector
      std::shuffle(v.begin(), v.end(), std::mt19937{std::random_device{}()});

      std::cout << "Contents of the list: ";
      for (int n : l){
          std::cout << n << ' ';
      }

      std::cout << "\nContents of the list, as seen through a shuffled vector: ";
      for (int i : v){
          std::cout << i << ' ';
      }

      std::cout << "\n\nDoubling the values in the initial list...\n\n";
      for (int& i : l) {
          i *= 2;
      }

      std::cout << "Contents of the list, as seen through a shuffled vector: ";
      for (int i : v){
         std::cout << i << ' ';
      }
  }

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