| 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) |
Member types
| type | definition |
|---|---|
type | T |
result_type | The return type of T if T is a function. Otherwise, not defined |
argument_type | 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 | 1) if T is a function or pointer to function that takes two arguments of type s A1 and A2, then first_argument_type is A1.2) if |
second_argument_type | 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) | |
| operator=
| rebinds a std::reference_wrapper (public member function) |
| accesses the stored reference (public member function) | |
| operator()
| calls the stored function (public member function) |
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;
}; |
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
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) |
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