Question

In C++, in what cases must pointers be used over references?

Answer 1

Whenever you want to change the address. Internally, a reference is no different from a pointer. The differences are mostly semantic - it needs to be assigned on creation of the reference, can't be assigned to a null value, and it can't be re-seated (you can't change the reference to refer to a different object than what it was assigned to on its creation).
So, if you want to change the object a variable refers to, or use null values to indicate a condition you safely handle, use a pointer. If that stays constant, use a reference. For example for most cases of passing an object to a function as a parameter, a reference should be used.