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Asked: 2026-05-23T14:22:01+00:00

Here is my problem. I’m using a physics library called Box2D. I’m able to

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Here is my problem.

I’m using a physics library called Box2D. I’m able to hook up a listener that will tell me when 2 fixtures collide.

Essentially the way Box2D works is by creating b2Bodies. Therefore, I only know which b2Body the fixture that collided belongs to. In my game, I have an Entity, and from that I have a PhisicsEntity. A PhysicsEntity holds a pointer to a b2Body. It also has a sendMessage method that comes from Entity. The problem is, from the b2Body, how do I send the PhysicsEntity a collision message. What I thought of doing was to set the userData void* of the b2Body to its corresponding PhysicsEntity. It seems very wrong to have to cast a void* to do this though.

Is there a better way that I could very quickly know the Physics Entity associated with the b2Body without casting or lookup?

Thanks

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1 Answer

  1. Editorial Team

    This is exactly the purpose that fields like that userData field are for — to refer to your application’s data related to the library’s object. This is an entirely appropriate use of void *.

    If you were asking about C, I would say that you should also not have to use any casts, as converting to and from void * can be done without any casts and needing to use a cast is a sign of a possible problem, but if I recall correctly C++ requires those casts (I do not use C++ enough to be sure).

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