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.

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)

Member types¶

type definition
type T
result_type The return type of T if T is a function. Otherwise,
(deprecated in C++17)(removed in not defined
C++20)
* if T is a function or pointer to function that
takes one argument of type A1, then
argument_type is A1
argument_type * if T is a pointer to member function of class
(deprecated in C++17)(removed in T0 that takes no arguments, then argument_type
C++20) 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
* 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 * if T is a pointer to member function of class
(deprecated in C++17)(removed in T0 that takes one argument, then
C++20) 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
* 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 * if T is a pointer to member function of class
(deprecated in C++17)(removed in T0 that takes one argument A1, then
C++20) 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)
get accesses the stored reference
operator T& (public member function)
operator() calls the stored function
(public member function)

Non-member functions¶

operator== compares reference_wrapper objects as their stored references
operator<=> (function)
(C++26)

Deduction guides(since C++17)

Helper classes¶

determines the common reference
std::basic_common_reference<std::reference_wrapper> type of reference_wrapper and
(C++23) 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 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

See also¶

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