Common base class + abstract interface class

[nuzzle]

I'd say your design is bad.

Using a top base class is quite common in OO designs actually. In both Java and C# it's even built into the language. The best known C++ example is maybe Qt where every class inherits a common top class called QObject.

I'm using the top class design strategy to uniformly handle a global memory management scheme based on the Boost intrusive smartpointers. It works very well, almost like full GC. In fact it's only since I started with this I felt really at home with C++. It's simple to use. It's efficient, close to bald pointer usage. Object ownership is a no-brainer. No risk for memory leaks. It's threadsafe. It's not in the way of multiple inheritance. No problems with upcasting. No problems handling this-pointers.

I wouldn't call the use of top classes bad design. But resorting to downcasting definately is.