Is there a way to find all classes (In fact, all instances) which implement a given Interface?

I are thinking of force the interface to add a reference of himself to a global array.
Each .new() constructor of each class should run the interface routine to add the instance being initialized to a shared global array.

But it looks like an interface cannot be defined with a shared variable (is not a class), so I gonna need to add a new data definition, which can be trouble to track is related to the interface, and worst, it would clutter my code.

Probably exist a better way.

Some tips?