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(C++26)(C++26) | ||||
| Deduction guides(C++17) | ||||
| Helper classes | ||||
Defined in header <functional> | ||
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.
Specifically,std::reference_wrapper is aCopyConstructible andCopyAssignable wrapper around a reference to object or reference to function of typeT. Instances ofstd::reference_wrapper are objects (they can be copied or stored in containers) but they are implicitly convertible toT&, so that they can be used as arguments with the functions that take the underlying type by reference.
If the stored reference isCallable,std::reference_wrapper is callable with the same arguments.
Helper functionsstd::ref andstd::cref are often used to generatestd::reference_wrapper objects.
std::reference_wrapper is used to pass objects by reference tostd::bind, the constructor ofstd::thread, or the helper functionsstd::make_pair andstd::make_tuple. It can also be used as a mechanism to store references inside standard containers (likestd::vector) that cannot normally hold references.
| (since C++17) |
| (since C++20) |
Contents |
| type | definition |
type | T |
result_type(deprecated in C++17) (removed in C++20) | The return type ofT ifT is a function. Otherwise, not defined. |
argument_type(deprecated in C++17) (removed in C++20) |
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first_argument_type(deprecated in C++17) (removed in C++20) |
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second_argument_type(deprecated in C++17) (removed in C++20) |
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| stores a reference in a newstd::reference_wrapper object (public member function)[edit] | |
| rebinds astd::reference_wrapper (public member function)[edit] | |
| accesses the stored reference (public member function)[edit] | |
| calls the stored function (public member function)[edit] |
(C++26) | comparesreference_wrapper objects as their stored references(function)[edit] |
determines the common reference type ofreference_wrapper and non-reference_wrapper(class template specialization)[edit] |
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:// typesusing type= T; // construct/copy/destroytemplate<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; // accessconstexpr operator T&()constnoexcept{return*_ptr;}constexpr T& get()constnoexcept{return*_ptr;} template<class...ArgTypes>constexprstd::invoke_result_t<T&, ArgTypes...> operator()(ArgTypes&&...args)constnoexcept(std::is_nothrow_invocable_v<T&, ArgTypes...>){returnstd::invoke(get(),std::forward<ArgTypes>(args)...);} private: T* _ptr;}; // deduction guidestemplate<class T>reference_wrapper(T&)-> reference_wrapper<T>; |
Demonstrates the use ofstd::reference_wrapper as a container of references, which makes it possible to access the same container using multiple indices.
#include <algorithm>#include <functional>#include <iostream>#include <list>#include <numeric>#include <random>#include <vector> void println(autoconst rem, std::ranges::rangeautoconst& v){for(std::cout<< rem;autoconst& 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 vectorstd::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 5Contents of the list, as seen through a shuffled vector: -1 2 -2 1 5 0 3 -3 -4 4Doubling 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
(C++11)(C++11) | creates astd::reference_wrapper with a type deduced from its argument (function template)[edit] |
(C++11) | binds one or more arguments to a function object (function template)[edit] |
(C++20)(C++20) | get the reference type wrapped instd::reference_wrapper (class template)[edit] |
