What does "can be compared" mean? To me it means that "<=>" [1] is well-defined. It doesn't say anything about numerical values.
[1] Spaceship operator. Think "return value of memcmp" but generalized to any operand types.
[1] Spaceship operator. Think "return value of memcmp" but generalized to any operand types.
(Edit: and for any pointers that you can compare for ordering, you may also calculate their offset from one another. However, like GP said, in practice you can do that for any pointers on common platforms.)
One is trying to learn how your current target architecture actually works. This is not production code. It's a "where in memory did the compiler decide to put that" question. You may get the results you expect or you may get someting totally bizzare. But since the purpose is discovering how the compiler works there is no wrong result.
Another is if you are writing your own memory manager. Here there are a lot of pointer comparisons to not the same allocated objects. But a memory manager is very OS and architecture specific so you will probably have to rewrite it when porting the code.
Having said all that, I wouldn't subtract the two void pointers like the example did. I would have converted them both to integers first and then subtract. Again, since this is for debugging and/or learning purposes then it's OK to bend the language rules a bit.