fork download
copy 
  1. #include <iostream>
  2.  
  3. class Event {
  4.   public: 
  5.     virtual void Run(int Param) = 0;
  6. };
  7.  
  8. // This is a user event and I have no idea what the class name is,
  9. // but I still have to call it's method Run() that is common to the interface "Event"
  10. class UserEvent: public Event {
  11.   public:
  12.     virtual void Run(int Param)
  13.     {
  14.       std::cout << "Derived Event Dispatched " << Param << std::endl;
  15.     }
  16. };
  17.  
  18. // This parameter is of pure abstract base class Event because
  19. // I have no idea what my user class is called.
  20. void CallEvent(Event *WhatEvent)
  21. {
  22.   std::cout << "in CallEvent(Event *WhatEvent):" << std::endl;
  23.   // Huh? WhatEvent = new Event();
  24.   // wrong: WhatEvent.Run(123);
  25.   // Instead, use ->.
  26.   // For pointers, check for non-nullptr is very reasonable:
  27.   if (WhatEvent) WhatEvent->Run(123); 
  28.   // obsolete: delete WhatEvent;
  29. }
  30.  
  31. // second approach using a reference (as recommended in comments):
  32. void CallEvent(Event &WhatEvent)
  33. {
  34.   std::cout << "in CallEvent(Event &WhatEvent):" << std::endl;
  35.   WhatEvent.Run(123); // for references - select operator . is fine
  36. }
  37.  
  38. int main()
  39. {
  40.   std::cout << "Hello World!" << std::endl;
  41.   /* nullptr does not make sense:
  42.    * UserEvent *mE = nullptr;
  43.    * Go back to original approach:
  44.    */
  45.   UserEvent mE;
  46.   CallEvent(&mE); // calling the first (with Event*)
  47.   CallEvent(mE); // calling the second (with Event&)
  48.   return 0;
  49. }
Success #stdin #stdout 0s 15240KB
comments (?)
stdin
copy 
Standard input is empty
stdout
copy 
Hello World!
in CallEvent(Event *WhatEvent):
Derived Event Dispatched 123
in CallEvent(Event &WhatEvent):
Derived Event Dispatched 123
https://ideone.com/dXQdrh
language:
C++ (gcc 8.3)
created:
7 years ago
visibility:
public

Share or Embed source code

Discover > Sphere Engine API

The brand new service which powers Ideone!

Discover > IDE Widget

Widget for compiling and running the source code in a web browser!