std::ref, std::cref
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Defined in header
<functional>
template< class T >
std::reference_wrapper <T> ref( T& t ) noexcept;
(1)
(since C++11) std::reference_wrapper <T> ref( T& t ) noexcept;
(constexpr since C++20)
template< class T >
ref( std::reference_wrapper <T> t ) noexcept;
(2)
(since C++11) ref( std::reference_wrapper <T> t ) noexcept;
(constexpr since C++20)
template< class T >
void ref( const T&& ) = delete;
(3)
(since C++11)
void ref( const T&& ) = delete;
template< class T >
std::reference_wrapper <const T> cref( const T& t ) noexcept;
(4)
(since C++11) std::reference_wrapper <const T> cref( const T& t ) noexcept;
(constexpr since C++20)
template< class T >
(5)
(since C++11) std::reference_wrapper <const T>
(constexpr since C++20)
template< class T >
void cref( const T&& ) = delete;
(6)
(since C++11)
void cref( const T&& ) = delete;
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.
[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 |