Getting Physical – Box2D - Cocos2d-x by Example Beginner's Guide - page 286

Sprite plus b2Body equal to b2Sprite
The most common way to work with b2Body objects in Cocos2d-x is to combine them
with sprites. In the games I'll show you, I created a class called b2Sprite that extends
sprite with the addition of a _body member property that points to its very own b2Body .
I also add a few helper methods to deal with our pesky PTM_RATIO . Feel free to add as
many of these as you think necessary.
b2Body objects have an incredibly helpful property called userData . You can store any-
thing you wish inside it and the bodies will carry it with them throughout the simulation.
So, what most developers do is that they store inside the body's userData property a ref-
erence to the instance of sprite wrapping it. So b2Sprite knows about its body, and the
body knows about its b2Sprite .
Tip
As a matter of fact, composition is key when working with Box2D. So, when designing
your games, make sure every object knows of every other object or can get to them quickly.
This will help immensely.