std::reference_wrapper

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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

 stores a reference in a new std::reference_wrapper object
(public member function)
rebinds a std::reference_wrapper
(public member function)
accesses the stored reference
(public member function)
calls the stored function
(public member function)

Deduction guides(since C++17)

Possible implementation

namespace detail {
template <class T> constexpr 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, std::remove_cvref_t<U>>>()
  )>
  constexpr 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
  constexpr operator T& () const noexcept { return *_ptr; }
  constexpr T& get() const noexcept { return *_ptr; }
 
  template< class... ArgTypes >
  constexpr 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 std::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>
 
void print(auto const rem, std::ranges::range auto const& v) {
    for (std::cout << rem; auto const& e : v)
        std::cout << e << ' ';
    std::cout << '\n';
}
 
int main()
{
    std::list<int> l(10);
    std::iota(l.begin(), l.end(), -4);
 
    // can't use shuffle on a list (requires random access), but can use it on a vector
    std::vector<std::reference_wrapper<int>> v(l.begin(), l.end());
 
    std::ranges::shuffle(v, std::mt19937{std::random_device{}()});
 
    print("Contents of the list: ", l);
    print("Contents of the list, as seen through a shuffled vector: ", v);
 
    std::cout << "Doubling the values in the initial list...\n";
    std::ranges::for_each(l, [](int& i) { i *= 2; });
 
    print("Contents of the list, as seen through a shuffled vector: ", v);
}

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

(C++11)(C++11)
 creates a std::reference_wrapper with a type deduced from its argument
(function template)
(C++11)
 binds one or more arguments to a function object
(function template)
(C++20)(C++20)
 get the reference type wrapped in std::reference_wrapper
(class template)
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