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#include <iostream>
using namespace std;
 
void Func(int& x, const int& y);    // x CAN be changed.  y cannot
 
 
int main()
{
   int a1 = 5, a2 = 8;
   cout << "a1 = " << a1 << "\ta2 = " << a2 << '\n';
 
   // legal call   
   Func(a1, a2);
   cout << "a1 = " << a1 << "\ta2 = " << a2 << '\n';
 
   // legal call
   Func(a1, 20);
   cout << "a1 = " << a1 << "\ta2 = " << a2 << '\n';
 
 
 
   // ILLEGAL call
   Func(20, a2);
   cout << "a1 = " << a1 << "\ta2 = " << a2 << '\n';
 
 
   return 0;
}
 
 
void Func(int& x, const int& y)
// x CAN be changed.  y cannot
{
   x = x * 2;
   y = y * 2;		// this is illegal
 
   cout << "x = " << x << '\n';
   cout << "y = " << y << '\n';
}

prog.cpp: In function 'int main()':
prog.cpp:23:15: error: invalid initialization of non-const reference of type 'int&' from an rvalue of type 'int'
    Func(20, a2);
               ^
prog.cpp:4:6: note: in passing argument 1 of 'void Func(int&, const int&)'
 void Func(int& x, const int& y);    // x CAN be changed.  y cannot
      ^
prog.cpp: In function 'void Func(int&, const int&)':
prog.cpp:35:6: error: assignment of read-only reference 'y'
    y = y * 2;  // this is illegal
      ^

Standard output is empty