HP Integrity NonStop J-Series - Manual (Page 282)

or make A a virtual base class.
The first approach is error-prone because the user must know the details of the inheritance tree in
order to make the proper cast.
The second approach, making A a virtual base class, solves this problem, but introduces another:
it becomes nearly impossible to make a cast back to the derived class! This is because there are
now two or more paths back through the inheritance hierarchy or, if you prefer a more physical
reason, the compiler implements virtual base classes as pointers to the base class and you can't
follow a pointer backwards.
We could exhaustively search all possible paths in the object's inheritance hierarchy, looking for
a match. (This is the approach of the NIH Classes.) However, this search is slow, even if speeded
up by "memorizing" the resulting addresses, since it must be done for every cast. Since it is also
bulky and always complicated, we decided that it was unacceptable.
Hence, we went back to the first route. This can be made acceptable if we keep the inheritance
trees simple by not making everything derive from the same base class. Hence, rather than using
a large secular base class with lots of functionality, we have chosen to tease out the separate bits
of functionality into separate, smaller base classes.
The idea is to first build your object, then tack on the base class that will supply the functionality
you need, such as collectability. You thus avoid multiple base classes of the same type and the
resulting ambiguous calls.