std::ref, std::cref
From cppreference.com
| 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.
|
|
(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)
t4)
std::reference_wrapper<const T>(t)5)
t3,6) rvalue reference wrapper is deleted.
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
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) |
