std::reference_wrapper

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 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 . It can also be used as a mechanism to store references inside standard containers (like std::vector ) that cannot normally hold references.

std::reference_wrapper is guaranteed to be TriviallyCopyable.

(since C++17)

T may be an incomplete type.

(since C++20)

if T is a function or pointer to function that takes one argument of type A1, then argument_type is A1
if T is a pointer to member function of class T0 that takes no arguments, then argument_type is T0*, possibly cv-qualified
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)

if T is a function or pointer to function that takes two arguments of types A1 and A2, then first_argument_type is A1
if T is a pointer to member function of class T0 that takes one argument, then first_argument_type is T0*, possibly cv-qualified
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)

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
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
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]

getoperator T&

accesses the stored reference
(public member function) [edit]

operator()

calls the stored function
(public member function) [edit]

[edit] Non-member functions

operator==operator<=>

(C++26)

compares reference_wrapper objects as their stored references
(function) [edit]

[edit] Deduction guides (since C++17)

[edit] Helper classes

std::basic_common_reference<std::reference_wrapper>

(C++23)

determines the common reference type of reference_wrapper and non-reference_wrapper
(class template specialization) [edit]

[edit] 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
 using type = T;
 
 // 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
 noexcept(std::is_nothrow_invocable_v <T&, ArgTypes...>)
 {
 return std::invoke (get(), std::forward <ArgTypes>(args)...);
 }
 
private:
 T* _ptr;
};
 
// deduction guides
template<class T>
reference_wrapper(T&) -> reference_wrapper<T>;

[edit] Example

Demonstrates the use of std::reference_wrapper as a container of references, which makes it possible to access the same container using multiple indices.

Run this code

#include <algorithm>
#include <functional>
#include <iostream>
#include <list>
#include <numeric>
#include <random>
#include <vector>
 
void println(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 {}()});
 
 println("Contents of the list: ", l);
 println("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; });
 
 println("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

[edit] See also

refcref

(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]

unwrap_referenceunwrap_ref_decay

(C++20)(C++20)

get the reference type wrapped in std::reference_wrapper
(class template) [edit]

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[edit] Member types