Deriving your tracked class T from LLInstanceTracker<T> gives you
T::getInstance() et al. But what about a subclass S derived from T?
S::getInstance() still delivers a pointer to T, requiring explicit downcast.
And so on for other LLInstanceTracker methods.
Instead, derive S from LLInstanceTrackerSubclass<S, T>. This implies that S is
a grandchild class of T, but it also recasts the LLInstanceTracker methods to
deliver results for S rather than for T.
Nat Goodspeed