Help on implementing how creatures and items interact in a computer role playing game

Question

I am programming a simple role playing game (to learn and for fun) and I'm at the point where I'm trying to come up with a way for game objects to interact with each other. There are two things I am trying to avoid.

1. Creating a gigantic game object that can be anything and do everything
2. Complexity - so I am staying away from a component based design like you see here

So with those parameters in mind I need advice on a good way for game objects to perform actions on each other.

For example

• Creatures (Characters, Monsters, NPCs) can perform actions on Creatures or Items (weapons, potions, traps, doors)
• Items can perform actions on Creatures or Items as well. An example would be a trap going off when a character tries to open a chest

What I've come up with is a PerformAction method that can take Creatures or Items as parameters. Like this

PerformAction(Creature sourceC, Item sourceI, Creature targetC, Item targetI)
// this will usually end up with 2 null params since
// only 1 source and 1 target will be valid

Or should I do this instead?

PerformAction(Object source, Object target)
// cast to correct types and continue

Or is there a completely different way I should be thinking about this?

Answer 1

This is a "double dispatch" problem. In regular OO programming, you "dispatch" the operation of a virtual method call to the concrete type of the class implementing the object instance you call against. A client doesn't need to know the actual implementation type, it is simply making a method call against an abstract type description. That's "single dispatch".

Most OO languages don't implement anything but single-dispatch. Double-dispatch is when the operation that needs to be called depends on two different objects. The standard mechanism for implementing double dispatch in OO languages without direct double-dispatch support is the "Visitor" design pattern. See the link for how to use this pattern.