std::ref, std::cref - cppreference.com (original) (raw)

Defined in header
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)

[edit] Parameters

[edit] Return value

  1. t

  2. t

3,6) rvalue reference wrapper is deleted.

[edit] Example

#include #include   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