# std::reference_wrapper

From cppreference.com
< cpp‎ | utility‎ | functional

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

`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

`argument_type`(deprecated in C++17)(removed in C++20)

1) if `T` is a function or pointer to function that takes one argument of type `A1`, then `argument_type` is `A1`.

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

`first_argument_type`(deprecated in C++17)(removed in C++20)

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`.

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

`second_argument_type`(deprecated in C++17)(removed in C++20)

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`.

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

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

### See also

ref cref (C++11)(C++11) | creates a std::reference_wrapper with a type deduced from its argument (function template)
bind (C++11) | binds one or more arguments to a function object (function template)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/utility/functional/reference_wrapper&oldid=109607"