idioms for abstract base classes

[John J. Lee]

On Tue, 24 Apr 2001 zooko at zooko.com wrote:
[...]
> It turned out that in addition to wasting my time implementing a
> feature that we never needed, I also made the resulting code hard for
> my fellow hackers to follow.  Due to the polymorphism and due to the
> common "parallel inheritance hierarchy" problem, it was very difficult
> even for *me* to understand how control flowed through the classes,
> which became a serious problem when I came back to them six months
> later in order to evolve and debug them.
[...]

What do you mean by "parallel inheritance hierarchy" problem?


John