Virtual destructors

[jml]

This is an issue that has been bugging me for some time since starting to use Qt. Why do so many of the classes not have virtual destructors, since it seems clear that they may be used as base classes for derived classes?

An obvious example is QWidget. Clearly this is a base class for many other classes, but according the documentation, the destructor is not virtual. My understanding has always been is that if a class can be a base class for other classes, the destructor should always be virtual. Is there some essential gap in my knowledge of C++ programming (surely not!)?

Thanks for any input.