Thanks guys! I already thought about creating an array but I see two problems with that. Correct me if I'm wrong.
1. It would have to be an array of pointers, not an array of actual entities
2. If I delete a ball, the slot it took in the array will still be there. So when I create another ball later, the total number of balls may not have changed but I won't be able to access ball 6 by checking index 4 of the array, since index 4 was originally for Ball 5 which got deleted.