Well, you just might get rid of the extra variable.

Yes. I actually mentioned this in an earlier draft, but I took it out. If you have some regular attribute stored in a hash, and you're guaranteed that it always has a value (and typically that's true), you can use that to tell when you're the last object to be destroyed.

What I don't like about that method is that it ties proper destruction to something that's not otherwise related to it. Say there are many attributes, and you pick one to be your surrogate counter. A programmer who comes to look later might wonder why that one? If the attribute you choose changes (its name, or the fact that it always gets a value), the destructor has to change too. You can (and should) make all that explicit in comments to avoid confusion, but having a separate counter is self documenting.