std::ref,std::cref(3)

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

Synopsis
   Defined in header <functional>
   template< class T > (since
   std::reference_wrapper<T> ref( T& t ) C++11)
   noexcept; (until
                        C++20)
   template< class T > (since
   constexpr std::reference_wrapper<T> ref( T& C++20)
   t ) noexcept;
   template< class T >
                        (since
   std::reference_wrapper<T> C++11)
                        (until
       ref( std::reference_wrapper<T> t ) C++20)
   noexcept;
   template< class T >
   constexpr std::reference_wrapper<T> (since
                        C++20)
       ref( std::reference_wrapper<T> t )
   noexcept;
   template< class T > (3) (since
   void ref( const T&& ) = delete; (1) C++11)
   template< class T > (since
   std::reference_wrapper<const T> cref( const C++11)
   T& t ) noexcept; (until
                        (2) C++20)
   template< class T > (since
   constexpr std::reference_wrapper<const T> C++20)
   cref( const T& t ) noexcept;
   template< class T >
                        (since
   std::reference_wrapper<const T> C++11)
                        (4) (until
       cref( std::reference_wrapper<T> t ) C++20)
   noexcept;
   template< class T > (5)
   constexpr std::reference_wrapper<const T> (since
                        C++20)
       cref( std::reference_wrapper<T> t )
   noexcept;
   template< class T > (6) (since
   void cref( const T&& ) = delete; 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
// 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
   reference_wrapper CopyConstructible and CopyAssignable reference wrapper
   (C++11) (class template)