std::reference_wrapper
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) |
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> 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, remove_cvref_t<U>>>() )> 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 operator T& () const noexcept { return *_ptr; } T& get() const noexcept { return *_ptr; } template< class... ArgTypes > 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 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> int main() { std::list<int> l(10); std::iota(l.begin(), l.end(), -4); std::vector<std::reference_wrapper<int>> v(l.begin(), l.end()); // can't use shuffle on a list (requires random access), but can use it on a vector std::shuffle(v.begin(), v.end(), std::mt19937{std::random_device{}()}); std::cout << "Contents of the list: "; for (int n : l){ std::cout << n << ' '; } std::cout << "\nContents of the list, as seen through a shuffled vector: "; for (int i : v){ std::cout << i << ' '; } std::cout << "\n\nDoubling the values in the initial list...\n\n"; for (int& i : l) { i *= 2; } std::cout << "Contents of the list, as seen through a shuffled vector: "; for (int i : v){ std::cout << i << ' '; } }
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) |