std::ref,std::cref (3) - Linux Manuals

std::ref,std::cref: std::ref,std::cref

NAME

std::ref,std::cref - std::ref,std::cref

Synopsis

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