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)	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

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)

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)

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>;

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.

#include <algorithm>
#include <functional>
#include <iostream>
#include <list>
#include <numeric>
#include <random>
#include <vector>
 
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

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)
unwrap_referenceunwrap_ref_decay (C++20)(C++20)	get the reference type wrapped in `std::reference_wrapper` (class template)

Docs

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