[vtkusers] why are destructors protected?

[Volpe, Christopher R (CRD)]

> Sorry if this is a dummy question, but... 

No, not at all...

> 
> I wonder why constructor, destructor, operator=, copy constructor, ...
> are *protected* in vtk classes?
> It is just to know.

To prevent people from using them to create static (file/global scope) and automatic (on the stack)
instances of objects. There are aspects of object/memory management (e.g. reference counting) that
depend on all instances existing on the heap only. Therefore, you have to use a class's New() method
to instantiate one of them, and the instance's Delete() method to destroy that instance.

Chris