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std::reference_wrapper

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Template:ddcl list begin <tr class="t-dsc-header">

<td>
Defined in header <functional>
</td>

<td></td> <td></td> <tr class="t-dcl ">

<td >
template< class T >
class reference_wrapper;
</td>

<td class="t-dcl-nopad"> </td> <td > (since C++11) </td> </tr> Template:ddcl list end

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 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.

Contents

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

stores a reference in a new std::reference_wrapper object
(public member function) [edit]
rebinds a std::reference_wrapper
(public member function) [edit]
accesses the stored reference
(public member function) [edit]
calls the stored function
(public member function) [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 <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

(C++11)(C++11)
creates a std::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]