Difference between revisions of "cpp/utility/functional/reference wrapper"
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template< class... ArgTypes > | template< class... ArgTypes > | ||
constexpr std::invoke_result_t<T&, ArgTypes...> | constexpr std::invoke_result_t<T&, ArgTypes...> | ||
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| + | noexcept(std::is_nothrow_invocable_v<T&, ArgTypes...>) | ||
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return std::invoke(get(), std::forward<ArgTypes>(args)...); | return std::invoke(get(), std::forward<ArgTypes>(args)...); | ||
} | } | ||
Revision as of 03:31, 15 February 2023
| Defined in header <functional>
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| 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.
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(since C++17) |
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(since C++20) |
Contents |
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 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 <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) |
| (C++20)(C++20) |
get the reference type wrapped in std::reference_wrapper (class template) |
