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

Latest revision as of 02:04, 8 July 2024

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)

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

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

[edit] Non-member functions

operator==operator<=>

(C++26)

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

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

[edit] Helper classes

std::basic_common_reference<std::reference_wrapper>

(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

refcref (C++11)(C++11)	creates a std::reference_wrapper with a type deduced from its argument (function template) [edit]
bind (C++11)	binds one or more arguments to a function object (function template) [edit]
unwrap_referenceunwrap_ref_decay (C++20)(C++20)	get the reference type wrapped in std::reference_wrapper (class template) [edit]

@@ Line 2: / Line 2: @@
 {{cpp/utility/functional/reference_wrapper/navbar}}
 {{dcl begin}}
-{{dcl header | functional }}
+{{dcl header|functional}}
-{{dcl | since=c++11 |
+{{dcl|since=c++11|
 template< class T >
 class reference_wrapper;
@@ Line 9: / Line 9: @@
 {{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 {{concept|CopyConstructible}} and {{concept|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.
-Helper functions {{lc|std::ref}} and {{lc|std::cref}} are often used to generate {{tt|std::reference_wrapper}} objects.
+If the stored reference is {{named req|Callable}}, {{tt|std::reference_wrapper}} is callable with the same arguments.
-{{tt|std::reference_wrapper}} is also used to pass objects to {{lc|std::bind}} or to the constructor of {{lc|std::thread}} by reference.
+Helper functions {{lc|std::ref}} and {{lc|std::cref}} are often used to generate {{tt|std::reference_wrapper}} objects.
-{{rev begin}}
+{{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.
-{{rev|since=c++17|
-{{tt|std::reference_wrapper}} is {{concept|TriviallyCopyable}}
+{{rrev|since=c++17|
+{{tt|std::reference_wrapper}} is guaranteed to be {{named req|TriviallyCopyable}}.
+}}
+{{rrev|since=c++20|
+{{tt|T}} may be an incomplete type.
 }}
-{{rev end}}
 ===Member types===
 {{dsc begin}}
-{{dsc hitem | type | definition }}
+{{dsc hitem|type|definition}}
-{{dsc | {{tt|type}} | {{tt|T}}}}
+{{dsc|{{tt|type}}|{{tt|T}}}}
-{{dsc | {{tt|result_type}} | 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}} | 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}}<br>{{mark life|deprecated=c++17|removed=c++20|br=yes}}|
-) 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>
-) 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 {{c/core|T0*}}, possibly cv-qualified<br>
-{{dsc | {{tt|first_argument_type}} | 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|first_argument_type}} is {{tt|A1}}.<br>
+* 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}}
-) 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}}<br>{{mark life|deprecated=c++17|removed=c++20|br=yes}}|
-) 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 {{c/core|T0*}}, possibly cv-qualified<br>
+* 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}} |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}}<br>{{mark life|deprecated=c++17|removed=c++20|br=yes}}|
-) 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>
-) 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 {{c/core|T::second_argument_type}}, then {{tt|second_argument_type}} is an alias of that}}
 {{dsc end}}
 ===Member functions===
 {{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}}
+===Non-member functions===
+{{dsc begin}}
+{{dsc inc|cpp/utility/functional/reference_wrapper/dsc operator_cmp}}
+{{dsc end}}
+==={{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===
+{{eq fun
+|1=
+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>;
+}}
 ===Example===
 {{example
- | Demonstrates the use of 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 <functional>
+#include <iostream>
 #include <list>
+#include <numeric>
+#include <random>
 #include <vector>
-#include <iostream>
-#include <functional>
+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 = {-4, -3, -2, -1, 0, 1, 2, 3, 4};
+     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::random_shuffle(v.begin(), v.end());
-     std::cout << "Contents of the list: ";
+     std::ranges::shuffle(v, std::mt19937{std::random_device{}()});
-    for (int n : l) {
-        std::cout << n << ' ';
-    }
-    std::cout << '\n';
-     std::cout << "Contents of the list, shuffled: ";
+     println("Contents of the list: ", l);
-     for (int i : v) {
+     println("Contents of the list, as seen through a shuffled vector: ", v);
-        std::cout << i << ' ';
-    }
-    std::cout << '\n';
      std::cout << "Doubling the values in the initial list...\n";
-     for (int &i : l) {
+     std::ranges::for_each(l, [](int& i) { i *= 2; });
-        i *= 2;
-    }
-     std::cout << "Shuffled vector actually contains references: ";
+     println("Contents of the list, as seen through a shuffled vector: ", v);
-    for (int i : v) {
-        std::cout << i << ' ';
-    }
-    std::cout << '\n';
 }
- | output=
+|p=true
-Contents of the list: -4 -3 -2 -1 0 1 2 3 4
+|output=
-Contents of the list, shuffled: 0 -1 3 4 -4 1 -2 -3 2
+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...
-Shuffled vector actually contains references: 0 -2 6 8 -8 2 -4 -6 4
+Contents of the list, as seen through a shuffled vector: -2 4 -4 2 10 0 6 -6 -8 8
 }}
 ===See also===
 {{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 end}}
-[[de:cpp/utility/functional/reference wrapper]]
+{{langlinks|de|es|fr|it|ja|pt|ru|zh}}
-[[es:cpp/utility/functional/reference wrapper]]
-[[fr:cpp/utility/functional/reference wrapper]]
-[[it:cpp/utility/functional/reference wrapper]]
-[[ja:cpp/utility/functional/reference wrapper]]
-[[pt:cpp/utility/functional/reference wrapper]]
-[[ru:cpp/utility/functional/reference wrapper]]
-[[zh:cpp/utility/functional/reference wrapper]]

Compiler support
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Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

Member functions
reference_wrapper::reference_wrapper
reference_wrapper::operator=
reference_wrapper::getreference_wrapper::operator T&
reference_wrapper::operator()
Non-member functions
operator==operator<=> (C++26)(C++26)
Deduction guides (C++17)
Helper classes
basic_common_reference<std::reference_wrapper> (C++23)

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

Latest revision as of 02:04, 8 July 2024

Contents

[edit] Member types