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    {{cpp/utility/functional/reference_wrapper/navbar}}
    {{cpp/utility/functional/reference_wrapper/navbar}}
    {{dcl begin}}
    {{dcl begin}}
    {{dcl header | functional }}
    {{dcl header|functional}}
    {{dcl | since=c++11 |
    {{dcl|since=c++11|
    template< class T >
    template< class T >
    class reference_wrapper;
    class reference_wrapper;
    Line 9: Line 9:
    {{dcl end}}
    {{dcl end}}


    {{tt|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 {{lc|std::vector}}) which cannot normally hold references.
    {{tt|std::reference_wrapper}} is a class template that wraps a reference in a copyable, assignable object.


    Specifically, {{tt|std::reference_wrapper}} is a {{named req|CopyConstructible}} and {{named req|CopyAssignable}} wrapper around a reference to object or reference to function of type {{tt|T}}. Instances of {{tt|std::reference_wrapper}} are objects (they can be copied or stored in containers) but they are implicitly convertible to {{c|T&}}, so that they can be used as arguments with the functions that take the underlying type by reference.
    Specifically, {{tt|std::reference_wrapper}} is a {{named req|CopyConstructible}} and {{named req|CopyAssignable}} wrapper around a reference to object or reference to function of type {{tt|T}}. Instances of {{tt|std::reference_wrapper}} are objects (they can be copied or stored in containers) but they are implicitly convertible to {{c|T&}}, so that they can be used as arguments with the functions that take the underlying type by reference.
    Line 15: Line 15:
    If the stored reference is {{named req|Callable}}, {{tt|std::reference_wrapper}} is callable with the same arguments.
    If the stored reference is {{named req|Callable}}, {{tt|std::reference_wrapper}} is callable with the same arguments.


    Helper functions {{lc|std::ref}} and {{lc|std::cref}} are often used to generate {{tt|std::reference_wrapper}} objects.  
    Helper functions {{lc|std::ref}} and {{lc|std::cref}} are often used to generate {{tt|std::reference_wrapper}} objects.


    {{tt|std::reference_wrapper}} is also used to pass objects by reference to {{lc|std::bind}}, the constructor of {{lc|std::thread}}, or the helper functions {{lc|std::make_pair}} and {{lc|std::make_tuple}}.
    {{tt|std::reference_wrapper}} is used to pass objects by reference to {{lc|std::bind}}, the constructor of {{lc|std::thread}}, or the helper functions {{lc|std::make_pair}} and {{lc|std::make_tuple}}.


    {{rrev|since=c++17|
    {{rrev|since=c++17|
    Line 29: Line 29:
    ===Member types===
    ===Member types===
    {{dsc begin}}
    {{dsc begin}}
    {{dsc hitem | type | definition }}
    {{dsc hitem|type|definition}}
    {{dsc | {{tt|type}} | {{tt|T}}}}
    {{dsc|{{tt|type}}|{{tt|T}}}}
    {{dsc | {{tt|result_type}}{{mark life|deprecated=c++17|removed=c++20}} | The return type of {{tt|T}} if {{tt|T}} is a function. Otherwise, not defined }}
    {{dsc|{{tt|result_type}}{{mark life|deprecated=c++17|removed=c++20}}|The return type of {{tt|T}} if {{tt|T}} is a function. Otherwise, not defined}}
    {{dsc | {{tt|argument_type}}{{mark life|deprecated=c++17|removed=c++20}} | 1) if {{tt|T}} is a function or pointer to function that takes one argument of type {{tt|A1}}, then {{tt|argument_type}} is {{tt|A1}}.<br>
    {{dsc|{{tt|argument_type}}{{mark life|deprecated=c++17|removed=c++20}}|
    2) if {{tt|T}} is a pointer to member function of class {{tt|T0}} that takes no arguments, then {{tt|argument_type}} is {{tt|T0*}}, possibly cv-qualified<br>
    if {{tt|T}} is a function or pointer to function that takes one argument of type {{tt|A1}}, then {{tt|argument_type}} is {{tt|A1}}<br>
    3) if {{tt|T}} is a class type with a member type {{tt|T::argument_type}}, then {{tt|argument_type}} is an alias of that}}
    if {{tt|T}} is a pointer to member function of class {{tt|T0}} that takes no arguments, then {{tt|argument_type}} is {{|T0*}}, possibly cv-qualified<br>
    {{dsc | {{tt|first_argument_type}}{{mark life|deprecated=c++17|removed=c++20}} | 1) if {{tt|T}} is a function or pointer to function that takes two arguments of types {{tt|A1}} and {{tt|A2}}, then {{tt|first_argument_type}} is {{tt|A1}}.<br>
    if {{tt|T}} is a class type with a member type {{|T::argument_type}}, then {{tt|argument_type}} is an alias of that}}
    2) if {{tt|T}} is a pointer to member function of class {{tt|T0}} that takes one argument, then {{tt|first_argument_type}} is {{tt|T0*}}, possibly cv-qualified<br>
    {{dsc|{{tt|first_argument_type}}{{mark life|deprecated=c++17|removed=c++20}}|
    3) if {{tt|T}} is a class type with a member type {{tt|T::first_argument_type}}, then {{tt|first_argument_type}} is an alias of that
    if {{tt|T}} is a function or pointer to function that takes two arguments of types {{tt|A1}} and {{tt|A2}}, then {{tt|first_argument_type}} is {{tt|A1}}<br>
    if {{tt|T}} is a pointer to member function of class {{tt|T0}} that takes one argument, then {{tt|first_argument_type}} is {{|T0*}}, possibly cv-qualified<br>
    if {{tt|T}} is a class type with a member type {{|T::first_argument_type}}, then {{tt|first_argument_type}} is an alias of that
    }}
    }}
    {{dsc | {{tt|second_argument_type}}{{mark life|deprecated=c++17|removed=c++20}} |1) if {{tt|T}} is a function or pointer to function that takes two arguments of type s {{tt|A1}} and {{tt|A2}}, then {{tt|second_argument_type}} is {{tt|A2}}.<br>
    {{dsc|{{tt|second_argument_type}}{{mark life|deprecated=c++17|removed=c++20}}|
    2) if {{tt|T}} is a pointer to member function of class {{tt|T0}} that takes one argument {{tt|A1}}, then {{tt|second_argument_type}} is {{tt|A1}}, possibly cv-qualified<br>
    if {{tt|T}} is a function or pointer to function that takes two arguments of type s {{tt|A1}} and {{tt|A2}}, then {{tt|second_argument_type}} is {{tt|A2}}<br>
    3) if {{tt|T}} is a class type with a member type {{tt|T::second_argument_type}}, then {{tt|second_argument_type}} is an alias of that}}
    if {{tt|T}} is a pointer to member function of class {{tt|T0}} that takes one argument {{tt|A1}}, then {{tt|second_argument_type}} is {{tt|A1}}, possibly cv-qualified<br>
    if {{tt|T}} is a class type with a member type {{|T::second_argument_type}}, then {{tt|second_argument_type}} is an alias of that}}
    {{dsc end}}
    {{dsc end}}


    ===Member functions===
    ===Member functions===
    {{dsc begin}}
    {{dsc begin}}
    {{dsc inc | cpp/utility/functional/reference_wrapper/dsc constructor}}
    {{dsc inc|cpp/utility/functional/reference_wrapper/dsc constructor}}
    {{dsc inc | cpp/utility/functional/reference_wrapper/dsc operator{{=}} }}
    {{dsc inc|cpp/utility/functional/reference_wrapper/dsc operator{{=}}}}
    {{dsc inc | cpp/utility/functional/reference_wrapper/dsc get}}
    {{dsc inc|cpp/utility/functional/reference_wrapper/dsc get}}
    {{dsc inc | cpp/utility/functional/reference_wrapper/dsc operator()}}
    {{dsc inc|cpp/utility/functional/reference_wrapper/dsc operator()
     
    }}
    {{dsc end}}
    {{dsc end}}


    ==={{rl|deduction guides|Deduction guides}}{{mark since c++17}}===
    ==={{rl|deduction guides|Deduction guides}}{{mark since c++17}}===


    ===Possible implementation===
    ===Possible implementation===
    {{eq fun
    {{eq fun
    | 1=
    |1=
    namespace detail {
    namespace detail
    template <class T> constexpr T& FUN(T& t) noexcept { return t; }
    {
    template <class T> void FUN(T&&) = delete;
    template<class T> constexpr T& FUN(T& t) noexcept { return t; }
    template<class T> void FUN(T&&) = delete;
    }
    }


    template <class T>
    template<class T>
    class reference_wrapper {
    class reference_wrapper
    {
    public:
    public:
      // types
    // types
      using type = T;
    using type = T
     
     
    ;
       
       
      // construct/copy/destroy
    //
      template <class U, class = decltype(
         reference_wrapper& (const reference_wrapper& ) noexcept = default;
         detail::FUN<T>(std::declval<U>()),
        std::enable_if_t<!std::is_same_v<reference_wrapper, std::remove_cvref_t<U>>>()
      )>
      constexpr reference_wrapper(U&& u) noexcept(noexcept(detail::FUN<T>(std::forward<U>(u))))
        : _ptr(std::addressof(detail::FUN<T>(std::forward<U>(u)))) {}
      reference_wrapper(const reference_wrapper&) noexcept = default;
       
       
      // assignment
    // access
      reference_wrapper& operator=(const reference_wrapper& x) noexcept = default;
    constexpr operator T& () const noexcept { return *_ptr; }
    constexpr T& get() const noexcept { return *_ptr; }
      // access
      constexpr operator T& () const noexcept { return *_ptr; }
      constexpr T& get() const noexcept { return *_ptr; }


      template< class... ArgTypes >
    template<class... ArgTypes>
      constexpr std::invoke_result_t<T&, ArgTypes...>
    constexpr std::invoke_result_t<T&, ArgTypes...>
        operator() ( ArgTypes&&... args ) const  
    operator() (ArgTypes&&... args ) const
        noexcept(std::is_nothrow_invocable_v<T&, ArgTypes...>)  
    noexcept(std::is_nothrow_invocable_v<T&, ArgTypes...>)
      {
    {
        return std::invoke(get(), std::forward<ArgTypes>(args)...);
    return std::invoke(get(), std::forward<ArgTypes>(args)...);
      }
    }
       
       
    private:
    private:
      T* _ptr;
    T* _ptr;
    };
    };


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    ===Example===
    ===Example===
    {{example
    {{example
    | Demonstrates the use of {{tt|std::reference_wrapper}} as a container of references, which makes it possible to access the same container using multiple indexes.
    |Demonstrates the use of {{tt|std::reference_wrapper}} as a container of references, which makes it possible to access the same container using multiple .
    | code=
    |code=
    #include <algorithm>
    #include <algorithm>
    #include <list>
    #include <list>
    #include <vector>
    #include <iostream>
    #include <numeric>
    #include <numeric>
    #include <random>
    #include <random>
    #include <functional>
    #include <>


    void print(auto const rem, std::ranges::range auto const& v) {
    void (auto const rem, std::ranges::range auto const& v)
    {
         for (std::cout << rem; auto const& e : v)
         for (std::cout << rem; auto const& e : v)
             std::cout << e << ' ';
             std::cout << e << ' ';
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         std::ranges::shuffle(v, std::mt19937{std::random_device{}()});
         std::ranges::shuffle(v, std::mt19937{std::random_device{}()});


         print("Contents of the list: ", l);
         ("Contents of the list: ", l);
         print("Contents of the list, as seen through a shuffled vector: ", v);
         ("Contents of the list, as seen through a shuffled vector: ", v);


         std::cout << "Doubling the values in the initial list...\n";
         std::cout << "Doubling the values in the initial list...\n";
         std::ranges::for_each(l, [](int& i) { i *= 2; });
         std::ranges::for_each(l, [](int& i) { i *= 2; });


         print("Contents of the list, as seen through a shuffled vector: ", v);
         ("Contents of the list, as seen through a shuffled vector: ", v);
    }
    }
    |p=true
    |p=true
    | output=
    |output=
    Contents of the list: -4 -3 -2 -1 0 1 2 3 4 5  
    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  
    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...
    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  
    Contents of the list, as seen through a shuffled vector: -2 4 -4 2 10 0 6 -6 -8 8
    }}
    }}


    ===See also===
    ===See also===
    {{dsc begin}}
    {{dsc begin}}
    {{dsc inc | cpp/utility/functional/dsc ref}}
    {{dsc inc|cpp/utility/functional/dsc ref}}
    {{dsc inc | cpp/utility/functional/dsc bind}}
    {{dsc inc|cpp/utility/functional/dsc bind}}
    {{dsc inc | cpp/utility/functional/dsc unwrap_reference}}
    {{dsc inc|cpp/utility/functional/dsc unwrap_reference}}
    {{dsc end}}
    {{dsc end}}


    {{langlinks|de|es|fr|it|ja|pt|ru|zh}}
    {{langlinks|de|es|fr|it|ja|pt|ru|zh}}

    Latest revision as of 10:04, 8 July 2024

     
    C++
     
    Utilities library
    General utilities
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    Function objects
    Function invocation
    (C++17)(C++23)
    Identity function object
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    Transparent operator wrappers
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    Old binders and adaptors
    (until C++17*)
    (until C++17*)
    (until C++17*)
    (until C++17*)
    (until C++17*)  
    (until C++17*)
    (until C++17*)(until C++17*)(until C++17*)(until C++17*)
    (until C++20*)
    (until C++20*)
    (until C++17*)(until C++17*)
    (until C++17*)(until C++17*)

    (until C++17*)
    (until C++17*)(until C++17*)(until C++17*)(until C++17*)
    (until C++20*)
    (until C++20*)
     
    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.

    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.

    If the stored reference is Callable, std::reference_wrapper is callable with the same arguments.

    Helper functions std::ref and std::cref are often used to generate std::reference_wrapper objects.

    std::reference_wrapper is used to pass objects by reference to std::bind, the constructor of std::thread, or the helper functions std::make_pair and std::make_tuple. It can also be used as a mechanism to store references inside standard containers (like std::vector) that cannot normally hold references.

    std::reference_wrapper is guaranteed to be TriviallyCopyable.

    		(since C++17)

    T may be an incomplete type.

    		(since C++20)