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Difference between revisions of "cpp/utility/functional/reference wrapper"

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< cpp‎ | utility‎ | functional
m (fmt)
m (→‎Member types: fmt: +br=yes)
 
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{{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.
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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}}. It can also be used as a mechanism to store references inside standard containers (like {{lc|std::vector}}) that cannot normally hold references.
  
 
{{rrev|since=c++17|
 
{{rrev|since=c++17|
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{{dsc hitem|type|definition}}
 
{{dsc hitem|type|definition}}
 
{{dsc|{{tt|type}}|{{tt|T}}}}
 
{{dsc|{{tt|type}}|{{tt|T}}}}
−
{{dsc|{{tt|result_type}}<br>{{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}}<br>{{mark life|deprecated=c++17|removed=c++20|br=yes}}|The return type of {{tt|T}} if {{tt|T}} is a function. Otherwise, not defined.}}
−
{{dsc|{{tt|argument_type}}<br>{{mark life|deprecated=c++17|removed=c++20}}|
+
{{dsc|{{tt|argument_type}}<br>{{mark life|deprecated=c++17|removed=c++20|br=yes}}|
−
* 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>
+
* 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>
−
* 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 pointer to member function of class {{tt|T0}} that takes no arguments, then {{tt|argument_type}} is {{c/core|T0*}}, possibly cv-qualified<br>
−
* 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 class type with a member type {{c/core|T::argument_type}}, then {{tt|argument_type}} is an alias of that}}
−
{{dsc|{{tt|first_argument_type}}<br>{{mark life|deprecated=c++17|removed=c++20}}|
+
{{dsc|{{tt|first_argument_type}}<br>{{mark life|deprecated=c++17|removed=c++20|br=yes}}|
−
* 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 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 {{tt|T0*}}, possibly cv-qualified<br>
+
* if {{tt|T}} is a pointer to member function of class {{tt|T0}} that takes one argument, then {{tt|first_argument_type}} is {{c/core|T0*}}, possibly cv-qualified<br>
−
* 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 class type with a member type {{c/core|T::first_argument_type}}, then {{tt|first_argument_type}} is an alias of that
 
}}
 
}}
−
{{dsc|{{tt|second_argument_type}}<br>{{mark life|deprecated=c++17|removed=c++20}}|
+
{{dsc|{{tt|second_argument_type}}<br>{{mark life|deprecated=c++17|removed=c++20|br=yes}}|
 
* 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>
 
* 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>
 
* 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 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 {{tt|T::second_argument_type}}, then {{tt|second_argument_type}} is an alias of that}}
+
* if {{tt|T}} is a class type with a member type {{c/core|T::second_argument_type}}, then {{tt|second_argument_type}} is an alias of that}}
 
{{dsc end}}
 
{{dsc end}}
  
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{{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}}
 +
 +
===Non-member functions===
 +
{{dsc begin}}
 +
{{dsc inc|cpp/utility/functional/reference_wrapper/dsc operator_cmp}}
 
{{dsc end}}
 
{{dsc end}}
  
 
==={{rl|deduction guides|Deduction guides}}{{mark since c++17}}===
 
==={{rl|deduction guides|Deduction guides}}{{mark since c++17}}===
 +
 +
===Helper classes===
 +
{{dsc begin}}
 +
{{dsc inc|cpp/utility/functional/reference_wrapper/dsc basic_common_reference}}
 +
{{dsc end}}
  
 
===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
 
{
 
{
Line 73: Line 84:
  
 
     // construct/copy/destroy
 
     // construct/copy/destroy
−
     template <class U, class = decltype(
+
     template<class U, class = decltype(
 
         detail::FUN<T>(std::declval<U>()),
 
         detail::FUN<T>(std::declval<U>()),
 
         std::enable_if_t<!std::is_same_v<reference_wrapper, std::remove_cvref_t<U>>>()
 
         std::enable_if_t<!std::is_same_v<reference_wrapper, std::remove_cvref_t<U>>>()
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     constexpr T& get() 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...>)
 
     {
 
     {
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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 indices.
 
|code=
 
|code=
 
#include <algorithm>
 
#include <algorithm>
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#include <vector>
 
#include <vector>
  
−
void print(auto const rem, std::ranges::range auto const& v)
+
void println(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)
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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);
+
     println("Contents of the list: ", l);
−
     print("Contents of the list, as seen through a shuffled vector: ", v);
+
     println("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);
+
     println("Contents of the list, as seen through a shuffled vector: ", v);
 
}
 
}
 
|p=true
 
|p=true

Latest revision as of 02:04, 8 July 2024

 
C++
 
Utilities library
General utilities
Relational operators (deprecated in C++20)
 
Function objects
Function invocation
(C++17)(C++23)
Identity function object
(C++20)
Reference wrappers
reference_wrapper
(C++11)
(C++11)(C++11)
(C++20)(C++20)
Transparent operator wrappers
(C++14)
(C++14)
(C++14)
(C++14)  
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)

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)

Contents

[edit] Member types

type definition
type T
result_type
(deprecated in C++17)
(removed in C++20) 		The return type of T if T is a function. Otherwise, not defined.
argument_type
(deprecated in C++17)
(removed in C++20)
  • if T is a function or pointer to function that takes one argument of type A1, then argument_type is A1
  • if T is a pointer to member function of class T0 that takes no arguments, then argument_type is T0*, possibly cv-qualified
  • if T is a class type with a member type T::argument_type, then argument_type is an alias of that
first_argument_type
(deprecated in C++17)
(removed in C++20)
  • if T is a function or pointer to function that takes two arguments of types A1 and A2, then first_argument_type is A1
  • if T is a pointer to member function of class T0 that takes one argument, then first_argument_type is T0*, possibly cv-qualified
  • 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
(deprecated in C++17)
(removed in C++20)
  • 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
  • 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
  • if T is a class type with a member type T::second_argument_type, then second_argument_type is an alias of that

[edit] 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]

[edit] Non-member functions

(C++26)
 compares reference_wrapper objects as their stored references
(function) [edit]

[edit] Deduction guides(since C++17)

[edit] Helper classes

(C++23)
 determines the common reference type of reference_wrapper and non-reference_wrapper
(class template specialization) [edit]

[edit] Possible implementation

namespace detail
{
    template<class T> constexpr T& FUN(T& t) noexcept { return t; }
    template<class T> void FUN(T&&) = delete;
}
 
template<class T>
class reference_wrapper
{
public:
    // types
    using type = T;
 
    // construct/copy/destroy
    template<class U, class = decltype(
        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
    reference_wrapper& operator=(const reference_wrapper& x) noexcept = default;
 
    // access
    constexpr operator T& () const noexcept { return *_ptr; }
    constexpr T& get() const noexcept { return *_ptr; }
 
    template<class... ArgTypes>
    constexpr std::invoke_result_t<T&, ArgTypes...>
        operator() (ArgTypes&&... args ) const
            noexcept(std::is_nothrow_invocable_v<T&, ArgTypes...>)
    {
        return std::invoke(get(), std::forward<ArgTypes>(args)...);
    }
 
private:
    T* _ptr;
};
 
// deduction guides
template<class T>
reference_wrapper(T&) -> reference_wrapper<T>;

[edit] Example

Demonstrates the use of std::reference_wrapper as a container of references, which makes it possible to access the same container using multiple indices.

Run this code
#include <algorithm>
#include <functional>
#include <iostream>
#include <list>
#include <numeric>
#include <random>
#include <vector>
 
void println(auto const rem, std::ranges::range auto const& v)
{
    for (std::cout << rem; auto const& e : v)
        std::cout << e << ' ';
    std::cout << '\n';
}
 
int main()
{
    std::list<int> l(10);
    std::iota(l.begin(), l.end(), -4);
 
    // can't use shuffle on a list (requires random access), but can use it on a vector
    std::vector<std::reference_wrapper<int>> v(l.begin(), l.end());
 
    std::ranges::shuffle(v, std::mt19937{std::random_device{}()});
 
    println("Contents of the list: ", l);
    println("Contents of the list, as seen through a shuffled vector: ", v);
 
    std::cout << "Doubling the values in the initial list...\n";
    std::ranges::for_each(l, [](int& i) { i *= 2; });
 
    println("Contents of the list, as seen through a shuffled vector: ", v);
}

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

[edit] 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]
(C++20)(C++20)
 get the reference type wrapped in std::reference_wrapper
(class template) [edit]
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