Here is an example of the a double dispatch. Let's say it's a game. You have a wildanimal that attacks types of humans, and a warrior which derives from human. Let us consider a wildanimal attacking a warrior. I strongly suggest running this code for a better understanding.
The problem is on line 51 you have w->attack(h). This leads to static compile time function overloading and you end up selecting the 'Human Attack' at line 35. This leads to a wild animal attacking a human despite the fact that it should be attacking a warrior.
The solution is to have a DD that allows the proper dynamic cast by using the 'this' pointer (line 28). Now when you call h->attackedbywildanimal(w); at line 54 you will get the correct wild animal attacking a warrior instead of a human.
Note that a simpler but far less flexible solution would have been to simply statically cast the pointer at line 51 to be w->attack((warrior*)h);.
Note that to have all the code in a single file I needed class definitions at the top and the wildanimal implementation is actuall at the bottom. Remind me to use java next time!
1. #include <iostream>
2.
3. using namespace std;
4.
5.
6. class human;
7. class warrior;
8. class wildanimal{
9. public:
10. void attack(human*);
11. void attack(warrior*);
12. };
13.
14.
15. class human{
16. public:
17. virtual void attackedbywildanimal(wildanimal *animal){
18. animal->attack(this);
19. }
20. void GetHealth(){
21. cout<<"Human health\n";
22. }
23. };
24.
25. class warrior : public human{
26. public:
27. virtual void attackedbywildanimal(wildanimal *animal){
28. animal->attack(this);
29. }
30. void GetHealth(){
31. cout<<"Warrior health\n";
32. }
33. };
34.
35. void wildanimal::attack(human* h){
36. cout<<"Animal attacks human ";
37. h->GetHealth();
38. }
39. void wildanimal::attack(warrior* w){
40. cout<<"Animal attacks warrior ";
41. w->GetHealth();
42. }
43.
44. int main(int argv, char** argc){
45.
46.
47. human * h = new warrior();
48. wildanimal * w = new wildanimal();
49.
50. //Problem
51. w->attack(h);
52.
53. //Double Dispatch solution
54. h->attackedbywildanimal(w);
55. }Hide line numbers |
