This document discusses different types of dispatch in object-oriented programming. It explains static dispatch, single dispatch, and multiple dispatch. Multiple dispatch allows method lookup to be based on the runtime classes of parameters rather than their static types. It can be emulated using the visitor pattern and resembles pattern matching. The document also lists advantages of multiple dispatch like natural interaction with overloading, but notes disadvantages like potential confusion and ambiguity due to the "diamond problem". It discusses how the CZ language implements multiple dispatch across inheritance hierarchies without diamonds.

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.
. . .
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8. Multiple Dispatch
Method lookup is based on runtime classes of parameters.
Does not depend on static types of parameters.
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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.
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11. Advantages
More natural interaction with overloading.
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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.
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16. Disadvantages
May be slightly confusing.
Blurred notion of method containment.
The diamond problem may cause ambiguity.
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17. CZ
Multiple dispatch only works across inheritance hierarchy (not across
requires hierarchy).
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18. CZ
Multiple dispatch only works across inheritance hierarchy (not across
requires hierarchy).
No diamonds in inheritance hierarchy.
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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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