NAME

std::reference_wrapper - std::reference_wrapper

Synopsis

Defined in header <functional>
template< class T > (since C++11)
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 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 The return type of T if T is a function.
in C++20) Otherwise, not defined
1) if T is a function or pointer to
function that takes one argument of type
A1, then argument_type is A1.

argument_type(deprecated in C++17)(removed 2) if T is a pointer to member function
in C++20) 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
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.

first_argument_type(deprecated in 2) if T is a pointer to member function
C++17)(removed in C++20) 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
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.

second_argument_type(deprecated in 2) if T is a pointer to member function
C++17)(removed in C++20) 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)
get accesses the stored reference
operator T& (public member function)
operator() 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