Re: Double Dispatch Problem: Mobile Creatures and Projectiles in a Game World

Responding to M....

I'm designing a game, and I've come across a problem that's similar
to the one listed here (Google Groups): http://groups.google.ca/group/comp.object/browse_thread/thread/4a5c70e3560388c8


I have a World, and in this world are Creatures and Projectiles.
Both of these have a location, are able to move around somehow, and
can collide with each other. Creatures also create Projectiles which
get added to the World.

For that reason I thought about having both of them be subclasses of
a class Mobile. In addition to sharing behavior, Mobile is
convenient because the World only has to keep one big list of Mobiles
instead of a list of Creatures and a list of Projectiles.
1 N 1
World --------> Mobile <----\
has A 1| | collides with
| \------/
|
/----------^----------\
| |
| 1 N |
Creature -----------> Projectile
shoots

(Did Google Groups take away the fixed width font option?)

I don't use Google Groups, but probably so. Most such services use HTML now and they default to variable font for rendering. You can get around that by inserting <pre> ... </pre> around the text drawing.

The problem is when Mobiles try to collide with each other. Two Projectiles colliding doesn't do anything (they pass over each other)
and neither does two Creatures colliding (they don't move). It's
only when a Creature and a Projectile collide that something
interesting happens (the Creature is hit). I couldn't really do that
with Mobile without either double dispatch or the Visitor pattern.

The only "interesting" collision collaboration seems to be Projectiles hitting Creatures since all other combinations seem to the no-ops. IOW, the "collides with" relationship is symmetric with the "shoots" relationship:

populates 1 1 populates
+-------------- [World] ------------------+
| R1 R2 |
| |
| * 1 R3 fired * | *
[Creature] ----------------------------- [Projectile]
| 0..1 | 0..*
| impacts |
| R4 |
+-----------------------------------------+

To avoid that problem I can just have World hold two lists: one of Creatures and one of Projectile. But...then I'd need two lists.
That might not be a bad thing, but it is something extra the World
has to take into account and I'm also lazy.

I think you would need that anyway. What is the problem space commonality of Mobile that justifies generalization? It only seems to exist to support the reflexive "collides with" relationship, but that relationship doesn't seem to capture what you are really interested in the problem solution (Projectiles hitting Creatures).

There is also a semantic inconsistency since you said that Creature/Creature collisions don't count because they don't move. That makes it kind of tough to rationalize them as a subset of things that move. B-)

As far as your concerns with lists are concerned, I don't see that as very relevant. We're talking about collection classes to implement * relationships during OOP. The relationships and their multiplicity are supposed to be abstracted from the problem space, so if that is the way the problem space works, that's how you have to abstract it in the OOA/D.

Note that you already had two * multiplicities in your model. If one regards R3 and R4 relationships above as instantiated on a "current" basis like your "collides with", then they will be 1:1 and you would have the same number of lists.

In addition, for the "collides with" relationship to be 1:1 it needs to be dynamically instantiated and de-instantitated as different objects are involved. That is probably going to be significant trouble (e.g., what happens when multiple Projectiles are in the air between several Creatures?) and it will need to be conditional when no Projectiles are in the air. Managing that for all combinations is likely to be a lot more trouble than managing the R4 relationship in the model above.


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H. S. Lahman
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