Pair collision detection on quadtree

Question

I'm planning to use a quadtree for optimizing collision detection. But this quadtree can handle only one type of objects (so I used a base class for my objects).

If I use double dispatch after two object pointers are returned by the quadtree in a collision (derived from the same base class that the quadtree can handle) knowing their ID that identifies their type (an int in the base class) , how am I able to use these pointers with their respective type in order to double dispatch?

A LIMITED SOLUTION:

Before double dispatching, I could make a resolver function that combines both IDs as they were flags into a single parameter, and make a switch that decides which type are obj1 and obj2.

However, its still not very elegant. Any solution or better way to do this? thanks.

Answer 1

It depends how you implement double dispatching how elegant it is. You are effectively trying to dispatch to a function that's able to handle collision between types X and Y and execute function for that type pair. This is basically a matrix lookup and you could implement it by registering collision functions to this matrix.

Let say your type-ID is 0-based number. You can allocate a 2D array of function pointers of type bool(const void*, const void*) and register functions to the array look-up based on the type-ID's. Because this matrix/array is symmetric about the diagonal (e.g. testing OOBB vs Capsule is the same as Capsule vs OOBB with the parameters swapped), you can register only the lower-triangle of this matrix. At the dispatch you check which type-ID is smaller and set that as your first argument to cut the collision test functions in half. Then fetch the collision test function from the matrix using the type ID's as indices to the matrix.