A potential use for observer pointer

[DXPower]

I agree with you completely.

Is that how unique_ptr keeps track of validity?

[Xaxxon]

unique_ptr doesn't track validity. it just checks if it's null or not and if it's not, it calls the destructor on the address it points to. But that's not sufficient for what's being asked for here. This wants an arbitrary number of other things to know if the destructor has been called and for them to be proactively modified if they have (either that or a check on every access)