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std::ref, std::cref

Defined in header <functional>
(1) 
template< class T >
std::reference_wrapper<T> ref( T& t ) noexcept;
 (since C++11)
(until C++20) 
template< class T >
constexpr std::reference_wrapper<T> ref( T& t ) noexcept;
 (since C++20)
(2) 
template< class T >
std::reference_wrapper<T>
    ref( std::reference_wrapper<T> t ) noexcept;
 (since C++11)
(until C++20) 
template< class T >
constexpr std::reference_wrapper<T>
    ref( std::reference_wrapper<T> t ) noexcept;
 (since C++20) 
template< class T >
void ref( const T&& ) = delete;
 (3) (since C++11)
(4) 
template< class T >
std::reference_wrapper<const T> cref( const T& t ) noexcept;
 (since C++11)
(until C++20) 
template< class T >
constexpr std::reference_wrapper<const T> cref( const T& t ) noexcept;
 (since C++20)
(5) 
template< class T >
std::reference_wrapper<const T>
    cref( std::reference_wrapper<T> t ) noexcept;
 (since C++11)
(until C++20) 
template< class T >
constexpr std::reference_wrapper<const T>
    cref( std::reference_wrapper<T> t ) noexcept;
 (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)

Parameters

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

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.

Example

#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

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

See also

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

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