std::reference_wrapper

template< class T >
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 Template:concept and Template:concept 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.

Member types

Member functions

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; }
 
private:
  T* _ptr;
};

Example

#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';
}

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

See also