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# 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.

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 to std::bind or to the constructor of std::thread by reference.

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

### [edit] Member types

type | definition
`type` | `T`
`result_type`(deprecated in C++17) | The return type of `T` if `T` is a function. Otherwise, not defined
`argument_type`(deprecated in C++17) | 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) | 1) if `T` is a function or pointer to function that takes two arguments of type s `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) | 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

### [edit] Member functions

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

### [edit] Possible implementation

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

      // construct/copy/destroy
      reference_wrapper(T& ref) noexcept : _ptr(std::addressof(ref)) {}
      reference_wrapper(T&&) = delete;
      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 >
      typename std::result_of<T&(ArgTypes&&...)>::type
        operator() ( ArgTypes&&... args ) const {
        return std::invoke(get(), std::forward<ArgTypes>(args)...);
      }

    private:
      T* _ptr;
    };

### [edit] 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 << '\n';

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

        std::cout << "Doubling the values in the initial list...\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 << ' '; std::cout << '\n';
    }

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

### [edit] See also

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

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