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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 or std::pair) which can not 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.

### [edit] Member types

type | definition
`type` | `T`
`result_type` | The return type of `T` if `T` is a function. Otherwise, not defined
`argument_type` | 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` | 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` | 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] 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 <functional>

    int main()
    {
        std::list<int> l = {-4, -3, -2, -1, 0, 1, 2, 3, 4};

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

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

        std::cout << "Contents of the list, shuffled: ";
        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 << "Shuffled vector actually contains references: ";
        for (int i : v) {
            std::cout << i << ' ';
        }
        std::cout << '\n';
    }

Output:

    Contents of the list: -4 -3 -2 -1 0 1 2 3 4
    Contents of the list, shuffled: 0 -1 3 4 -4 1 -2 -3 2
    Doubling the values in the initial list...
    Shuffled vector actually contains references: 0 -2 6 8 -8 2 -4 -6 4

### [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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