Question

I am making a physics engine in c# / XNA, I have three basic objects…

Sphere
Cube
Plane

that are all derived from

GameObject

I store all my objects in a list of GameObjects and I would like to loop through this list and be able to call a CheckCollision function that will go to the correct function for each pair of objects

eg

go is a Sphere,

go2 is a Sphere

if(CheckCollision(go, go2))
{
  //do stuff
}

bool CheckCollision(Sphere one, Sphere two)
{
  //Check Sphere to Sphere
}

bool CheckCollision(Sphere sphere, Plane plane)
{
  //Check Sphere to Plane
}

and I would like it to just go to the correct function withour having to use if checks.

Thank you.

Answer 1

You could use virtual dispatch:

abstract class GameObject
{
    abstract bool CheckCollision (GameObject other);
    abstract bool CheckCollision (Sphere other);
    abstract bool CheckCollision (Cube other);
    abstract bool CheckCollision (Plane other);
}

class Sphere : GameObject
{
    override bool CheckCollision (GameObject other) { return other.CheckCollision(this); }
    override bool CheckCollision (Sphere other) { /* ... implementation ... */ }
    override bool CheckCollision (Cube other) { /* ... implementation ... */ }
    override bool CheckCollision (Plane other) { /* ... implementation ... */ }
}

class Cube : GameObject
{
    override bool CheckCollision (GameObject other) { return other.CheckCollision(this); }
    override bool CheckCollision (Sphere other) { /* ... implementation ... */ }
    override bool CheckCollision (Cube other) { /* ... implementation ... */ }
    override bool CheckCollision (Plane other) { /* ... implementation ... */ }
}

class Plane : GameObject
{
    override bool CheckCollision (GameObject other) { return other.CheckCollision(this); }
    override bool CheckCollision (Sphere other) { /* ... implementation ... */ }
    override bool CheckCollision (Cube other) { /* ... implementation ... */ }
    override bool CheckCollision (Plane other) { /* ... implementation ... */ }
}

EDIT

Consider what happens when you have this:

GameObject go1 = new Sphere();
GameObject go2 = new Cube();
bool collision = go1.CheckCollision(go2);

• The call goes to the abstract GameObject.CheckCollision(GameObject) method on the sphere.
• Virtual dispatch means that we go to Sphere.CheckCollision(GameObject)
• Sphere.CheckCollision(GameObject) calls the abstract GameObject.CheckCollision(Sphere) method on the cube.
• Virtual dispatch means that we go to Cube.CheckCollision(Sphere)!

Therefore, no type-checking if statements are necessary.

EDIT 2

See Eric Lippert’s answer at https://stackoverflow.com/a/2367981/385844; the first option — the visitor pattern — is essentially the approach outlined above. Eric’s other answer at https://stackoverflow.com/a/9069976/385844 also discusses this issue.