std::reference_wrapper

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)
getoperator 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.

#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

refcref (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)