Virtual methods

     void  activate(Alarm&  a)  {
             a.turn_on();
     }
     BuzzerAlarm  b;
     activate(b);
If  turn_on()  is not virtual:
Calls  turn_on()  in  Alarm
If  turn_on()  is virtual:
Calls  turn_on()  in  BuzzerAlarm
Virtual methods require virtual destructor