trouble subclassing str

[Kent Johnson]

Donn Cave wrote:
> Left unexplained is ``true "is-a" relationships''.  Sounds
> like an implicit contradiction -- you can't implement
> something that truly is something else.  Without that, and
> maybe a more nuanced replacement for "is-implemented-using-a",
> I don't see how you could really be sure of the point.

Try this article for an explanation of is-a:
http://www.objectmentor.com/resources/articles/lsp.pdf

IMO Robert Martin explains what good OO design is better than anyone else. His book "Agile Software Development" is excellent.

Kent