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< cpp‎ | utility‎ | functional
m (Shorten template names. Use {{lc}} where appropriate.)
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{{cpp/utility/functional/navbar}}
 
{{cpp/utility/functional/navbar}}
 
{{dcl begin}}
 
{{dcl begin}}
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{{dcl header | functional }}
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{{dcl header|functional}}
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{{dcl | notes={{mark since c++11}} | num=1|
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{{dcla|num=1|since=c++11|constexpr=c++20|1=
 
template< class T >
 
template< class T >
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std::reference_wrapper<T> ref(T& t);
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std::reference_wrapper<T> ref( T& t ) noexcept;
 
}}
 
}}
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{{dcl | notes={{mark since c++11}} | num=2|
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{{dcla|num=2|since=c++11|constexpr=c++20|1=
 
template< class T >
 
template< class T >
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std::reference_wrapper<T> ref( std::reference_wrapper<T> t );
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std::reference_wrapper<T>
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    ref( std::reference_wrapper<T> t ) noexcept;
 
}}
 
}}
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{{dcl | notes={{mark since c++11}} | num=3|
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{{dcla|num=3|since=c++11|1=
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template <class T>
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template< class T >
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void ref(const T&&) {{=}} delete;
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void ref( const T&& ) = delete;
 
}}
 
}}
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{{dcl | notes={{mark since c++11}} | num=4|
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{{dcla|num=4|since=c++11|constexpr=c++20|1=
 
template< class T >
 
template< class T >
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std::reference_wrapper<const T> cref( const T& t );
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std::reference_wrapper<const T> cref( const T& t ) noexcept;
 
}}
 
}}
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{{dcla|num=5|since=c++11|constexpr=c++20|1=
 
template< class T >
 
template< class T >
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std::reference_wrapper<const T> cref( std::reference_wrapper<T> t )
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std::reference_wrapper<const T>
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    cref( std::reference_wrapper<T> t ) noexcept;
 
}}
 
}}
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{{dcl | notes={{mark since c++11}} | num=6|
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{{dcla|num=6|since=c++11|1=
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template <class T>
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template< class T >
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void cref(const T&&) {{=}} delete;
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void cref( const T&& ) = delete;
 
}}
 
}}
 
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{{dcl end}}
  
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Function templates {{tt|ref}} and {{tt|cref}} are helper functions that generate an object of type {{lc|std::reference_wrapper}}, using template argument deduction to determine the template argument of the result.
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Function templates {{tt|ref}} and {{tt|cref}} are helper functions that generate an object of type {{lc|std::reference_wrapper}}, using [[cpp/language/template_argument_deduction|template argument deduction]] to determine the template argument of the result.
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{{rrev|since=c++20|{{tt|T}} may be an incomplete type.}}
  
 
===Parameters===
 
===Parameters===
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===Return value===
 
===Return value===
 
@1@ {{c|std::reference_wrapper<T>(t)}}
 
@1@ {{c|std::reference_wrapper<T>(t)}}
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@2@ {{c|ref(t.get())}}
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@2@ {{c|t}}
 
@4@ {{c|std::reference_wrapper<const T>(t)}}
 
@4@ {{c|std::reference_wrapper<const T>(t)}}
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@5@ {{c|cref(t.get())}}
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@5@ {{c|t}}
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@3,6@ rvalue reference wrapper is deleted.
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===Exeptions===
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{{noexcept}}
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===Example===
 
===Example===
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After function: 10 12 12
 
After function: 10 12 12
 
}}
 
}}
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===Defect reports===
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{{dr list begin}}
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{{dr list item|wg=lwg|dr=3146|std=C++11|before=unwrapping overloads sometimes led to error|after=made always valid}}
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{{dr list end}}
  
 
===See also===
 
===See also===
 
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{{dsc inc | cpp/utility/functional/dcl list reference_wrapper}}
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{{dsc inc | cpp/utility/functional/dsc reference_wrapper}}
 
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Latest revision as of 21:48, 13 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
(C++11)
refcref
(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*)
 
Defined in header <functional>
template< class T >
std::reference_wrapper<T> ref( T& t ) noexcept;
 (1) (since C++11)
(constexpr since C++20)
template< class T >

std::reference_wrapper<T>

    ref( std::reference_wrapper<T> t ) noexcept;
 (2) (since C++11)
(constexpr since C++20)
template< class T >
void ref( const T&& ) = delete;
 (3) (since C++11)
template< class T >
std::reference_wrapper<const T> cref( const T& t ) noexcept;
 (4) (since C++11)
(constexpr since C++20)
template< class T >

std::reference_wrapper<const T>

    cref( std::reference_wrapper<T> t ) noexcept;
 (5) (since C++11)
(constexpr since C++20)
template< class T >
void cref( const T&& ) = delete;
 (6) (since C++11)

Function templates ref and cref are helper functions that generate an object of type std::reference_wrapper, using template argument deduction to determine the template argument of the result.

T may be an incomplete type.

(since C++20)

Contents

[edit] Parameters

t - lvalue reference to object that needs to be wrapped or an instance of std::reference_wrapper

[edit] Return value

1) std::reference_wrapper<T>(t)
2) t
4) std::reference_wrapper<const T>(t)
5) t
3,6) rvalue reference wrapper is deleted.

[edit] Example

Run this code
#include <functional>
#include <iostream>
 
void f(int& n1, int& n2, const int& n3)
{
    std::cout << "In function: " << n1 << ' ' << n2 << ' ' << n3 << '\n';
    ++n1; // increments the copy of n1 stored in the function object
    ++n2; // increments the main()'s n2
    // ++n3; // compile error
}
 
int main()
{
    int n1 = 1, n2 = 2, n3 = 3;
    std::function<void()> bound_f = std::bind(f, n1, std::ref(n2), std::cref(n3));
    n1 = 10;
    n2 = 11;
    n3 = 12;
    std::cout << "Before function: " << n1 << ' ' << n2 << ' ' << n3 << '\n';
    bound_f();
    std::cout << "After function: " << n1 << ' ' << n2 << ' ' << n3 << '\n';
}

Output: 

Before function: 10 11 12
In function: 1 11 12
After function: 10 12 12

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 3146 C++11 unwrapping overloads sometimes led to error made always valid

[edit] See also

(C++11)
 CopyConstructible and CopyAssignable reference wrapper
(class template) [edit]
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