2. Motivation
void f(String o) { ... }
void f(Object o) { ... }
Object o = new String("foo");
f(o);
Michal P´ıˇse (CTU in Prague) Object Programming L. 6: Multiple Dispatch November 2, 2010 2 / 8

3. Motivation
void f(String o) { ... }
void f(Object o) { ... }
Object o = new String("foo");
f(o);
Invoked function may change with the static type of o.
Michal P´ıˇse (CTU in Prague) Object Programming L. 6: Multiple Dispatch November 2, 2010 2 / 8

4. Types of Dispatch
Static dispatch.
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5. Types of Dispatch
Static dispatch.
Single dispatch.
Michal P´ıˇse (CTU in Prague) Object Programming L. 6: Multiple Dispatch November 2, 2010 3 / 8

6. Types of Dispatch
Static dispatch.
Single dispatch.
Multiple dispatch.
Michal P´ıˇse (CTU in Prague) Object Programming L. 6: Multiple Dispatch November 2, 2010 3 / 8

7. Types of Dispatch
Static dispatch.
Single dispatch.
Multiple dispatch.
. . .
Michal P´ıˇse (CTU in Prague) Object Programming L. 6: Multiple Dispatch November 2, 2010 3 / 8

8. Multiple Dispatch
Method lookup is based on runtime classes of parameters.
Does not depend on static types of parameters.
Michal P´ıˇse (CTU in Prague) Object Programming L. 6: Multiple Dispatch November 2, 2010 4 / 8

9. Multiple Dispatch
Method lookup is based on runtime classes of parameters.
Does not depend on static types of parameters.
Can be emulated (albeit in a restricted fashion) by the visitor pattern.
Michal P´ıˇse (CTU in Prague) Object Programming L. 6: Multiple Dispatch November 2, 2010 4 / 8

10. Multiple Dispatch
Method lookup is based on runtime classes of parameters.
Does not depend on static types of parameters.
Can be emulated (albeit in a restricted fashion) by the visitor pattern.
Resembles (in a way) pattern matching.
Michal P´ıˇse (CTU in Prague) Object Programming L. 6: Multiple Dispatch November 2, 2010 4 / 8

11. Advantages
More natural interaction with overloading.
Michal P´ıˇse (CTU in Prague) Object Programming L. 6: Multiple Dispatch November 2, 2010 5 / 8

12. Advantages
More natural interaction with overloading.
More equal parameters.
Michal P´ıˇse (CTU in Prague) Object Programming L. 6: Multiple Dispatch November 2, 2010 5 / 8

13. Advantages
More natural interaction with overloading.
More equal parameters.
Better composability.
Michal P´ıˇse (CTU in Prague) Object Programming L. 6: Multiple Dispatch November 2, 2010 5 / 8

14. Disadvantages
May be slightly confusing.
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15. Disadvantages
May be slightly confusing.
Blurred notion of method containment.
Michal P´ıˇse (CTU in Prague) Object Programming L. 6: Multiple Dispatch November 2, 2010 6 / 8

16. Disadvantages
May be slightly confusing.
Blurred notion of method containment.
The diamond problem may cause ambiguity.
Michal P´ıˇse (CTU in Prague) Object Programming L. 6: Multiple Dispatch November 2, 2010 6 / 8

17. CZ
Multiple dispatch only works across inheritance hierarchy (not across
requires hierarchy).
Michal P´ıˇse (CTU in Prague) Object Programming L. 6: Multiple Dispatch November 2, 2010 7 / 8

18. CZ
Multiple dispatch only works across inheritance hierarchy (not across
requires hierarchy).
No diamonds in inheritance hierarchy.
Michal P´ıˇse (CTU in Prague) Object Programming L. 6: Multiple Dispatch November 2, 2010 7 / 8

19. See
Malayeri, D. and Aldrich, J. CZ: Multiple Inheritance Without
Diamonds. Proceeding of the 24th ACM SIGPLAN Conference on Object
Oriented Programming Systems Languages and Applications. 21–40.
http://doi.acm.org/10.1145/1640089.1640092
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