std::reference_wrapper
| Defined in header <functional>
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template< class T >
class reference_wrapper;
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(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.
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(since C++17) |
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(since C++20) |
Member types
| type | definition |
type
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T
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result_type(deprecated in C++17)(removed in C++20)
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The return type of T if T is a function. Otherwise, not defined
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argument_type(deprecated in C++17)(removed in C++20)
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1) if T is a function or pointer to function that takes one argument of type A1, then argument_type is A1.2) if |
first_argument_type(deprecated in C++17)(removed in C++20)
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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 |
second_argument_type(deprecated in C++17)(removed in C++20)
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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 |
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>;
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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 <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) |