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Issue 1.6

COLUMN

Double Dispatch

Better Design Through Indirection

Issue: 1.6 (June/July 2003)
Author: Charles Yeomans
Author Bio: Charles Yeomans is a software developer in Lexington, Kentucky.
Article Description: No description available.
Article Length (in bytes): 5,479
Starting Page Number: 36
Article Number: 1618
Resource File(s):

Download Icon 1618.zip Updated: 2013-03-11 19:07:56

Related Link(s): None

Excerpt of article text...

You're writing a spaceship game in which ships of various types collide. Let's suppose that the game currently has three types of ships: ShipA, ShipB, and ShipC, all of which inherit from a common abstract class SpaceShip. SpaceShip has an abstract function CollideWith(ship as SpaceShip) as CollisionOutcome. How do you implement ShipA, ShipB, and ShipC?

It's immediately clear that there are six possible collisions, and it's certainly possible that the code in each situation will be quite different. Our first step will be to move collision resolution into its own class, SpaceshipCollisionResolver, and give it a single method, Resolve, that we'll overload to handle all possible cases.

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