std::reference_wrapper

 

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mem_fun_t mem_fun1_t const_mem_fun_t const_mem_fun1_t (deprecated) (deprecated) (deprecated) (deprecated)
mem_fun_ref (deprecated)
mem_fun_ref_t mem_fun1_ref_t const_mem_fun_ref_t const_mem_fun1_ref_t (deprecated) (deprecated) (deprecated) (deprecated)
binder1st binder2nd (deprecated) (deprecated)
bind1st bind2nd (deprecated) (deprecated)

 

std::reference_wrapper

Member functions

reference_wrapper::reference_wrapper

reference_wrapper::operator=

reference_wrapper::get reference_wrapper::operator T&

reference_wrapper::operator()

 

Defined in header <functional>
template< class T > class reference_wrapper;		(since C++11)

Class template 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 (can be copied or stored in containers), but have reference semantics: calling a member function on a reference_wrapper invokes the member function of the underlying object.

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.

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 T is a pointer to member function of class T0 that takes no arguments, then argument_type is T0*, possibly cv-qualified 3) if T is a class type with a member type T::argument_type, then argument_type is an alias of that
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 T is a pointer to member function of class T0 that takes one argument, then first_argument_type is T0*, possibly cv-qualified 3) if T is a class type with a member type T::first_argument_type, then first_argument_type is an alias of that
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 T is a pointer to member function of class T0 that takes one argument A1, then second_argument_type is A1, possibly cv-qualified 3) if T is a class type with a member type T::second_argument_type, then first_argument_type is an alias of that

Member functions

(constructor)	stores a reference in a new std::reference_wrapper object (public member function)
operator=	rebinds a std::reference_wrapper (public member function)
get operator T&	accesses the stored reference (public member function)
operator()	calls the stored function (public member function)

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 <functional>
 
int main()
{
    std::list<int> l = {-4, -3, -2, -1, 0, 1, 2, 3, 4};
 
    std::vector<std::reference_wrapper<int>> v(l.begin(), l.end());
    std::random_shuffle(v.begin(), v.end());
 
    std::vector<std::reference_wrapper<int>> v2(v.begin(), v.end());
    std::partition(v2.begin(), v2.end(), [](int n){return n<0;});
 
    std::cout << "Contents of the list: ";
    for(int n: l) {
        std::cout << n << ' ';
    }
    std::cout << '\n';
 
    std::cout << "Contents of the list, shuffled: ";
    for(int i: v) {
        std::cout << i << ' ';
    }
    std::cout << '\n';
 
    std::cout << "Shuffled elements, partitioned: ";
    for(int i: v2) {
        std::cout << i << ' ';
    }
    std::cout << '\n';
}

Output:

Contents of the list: -4 -3 -2 -1 0 1 2 3 4 
Contents of the list, shuffled: 0 -1 3 4 -4 1 -2 -3 2 
Shuffled elements, partitioned: -3 -1 -2 -4 4 1 3 0 2

See also

ref cref (C++11) (C++11)	creates a reference_wrapper of type, deduced from its argument (function template)
bind (C++11)	binds one or more arguments to a function object (function template)