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    cpp/utility/functional/reference wrapper: Difference between revisions

    From cppreference.com
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    Printing the unshuffled list must be done before the call to random_shuffle().
    Undo revision 73784 by 94.246.4.144 (talk) Appears I was wrong.
    Line 62: Line 62:
         std::list<int> l = {-4, -3, -2, -1, 0, 1, 2, 3, 4};
         std::list<int> l = {-4, -3, -2, -1, 0, 1, 2, 3, 4};
       
       
         std::cout << "Contents of the list: ";
         std::cout << "Contents of the list: ";
         for (int n : l) {
         for (int n : l) {
    Line 67: Line 70:
         }
         }
         std::cout << '\n';
         std::cout << '\n';
        std::vector<std::reference_wrapper<int>> v(l.begin(), l.end());
        std::random_shuffle(v.begin(), v.end());


         std::cout << "Contents of the list, shuffled: ";
         std::cout << "Contents of the list, shuffled: ";

    Revision as of 11:52, 16 November 2014

     
    C++
     
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    Old binders and adaptors
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    (until C++17*)  
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    (until C++17*)(until C++17*)(until C++17*)(until C++17*)
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    (until C++17*)(until C++17*)
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    (until C++17*)(until C++17*)(until C++17*)(until C++17*)
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    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 Template:concept and Template:concept 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.

    std::reference_wrapper is Template:concept

    		(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 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 second_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

    Run this code
    #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::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 << "Doubling the values in the initial list...\n";
    for (int &i : l) {
        i *= 2;
    }

    std::cout << "Shuffled vector actually contains references: ";
    for (int i : v) {
        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 
Doubling the values in the initial list...
Shuffled vector actually contains references: 0 -2 6 8 -8 2 -4 -6 4

    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]