std::reference_wrapper
| 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.  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.
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 to std::bind or to the constructor of std::thread by reference.
| 
 | (since C++17) | 
| Contents | 
[edit] Member types
| type | definition | 
| type | T | 
| result_type(deprecated in C++17) | The return type of TifTis a function. Otherwise, not defined | 
| argument_type(deprecated in C++17) | 1) if Tis a function or pointer to function that takes one argument of typeA1,  thenargument_typeisA1.2) if  | 
| first_argument_type(deprecated in C++17) | 1) if Tis a function or pointer to function that takes two arguments of type sA1andA2, thenfirst_argument_typeisA1.2) if  | 
| second_argument_type(deprecated in C++17) | 1) if Tis a function or pointer to function that takes two arguments of type sA1andA2, thensecond_argument_typeisA2.2) if  | 
[edit] 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) | 
[edit] Possible implementation
| template <class T> class reference_wrapper { public: // types typedef T type; // construct/copy/destroy reference_wrapper(T& ref) noexcept : _ptr(std::addressof(ref)) {} reference_wrapper(T&&) = delete; 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 > typename std::result_of<T&(ArgTypes&&...)>::type operator() ( ArgTypes&&... args ) const { return std::invoke(get(), std::forward<ArgTypes>(args)...); } private: T* _ptr; }; | 
[edit] 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 << '\n'; std::cout << "Contents of the list, as seen through a shuffled vector: "; for (int i : v) std::cout << i << ' '; std::cout << '\n'; std::cout << "Doubling the values in the initial list...\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 << ' '; std::cout << '\n'; }
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
[edit] 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) | 


