b2Body A rigid body.
 b2BodyDef A body definition holds all the data needed to construct a rigid body.
 b2BoundaryListener This is called when a body's shape passes outside of the world boundary.
 b2ContactFilter Implement this class to provide collision filtering.
 b2ContactListener Implement this class to get collision results.
 b2DebugDraw Implement and register this class with a b2World to provide debug drawing of physics entities in your game.
 b2DestructionListener Joints and shapes are destroyed when their associated body is destroyed.
 b2World The world class manages all physics entities, dynamic simulation, and asynchronous queries.