25 Using inheritance well
We have now studied all the technical details of inheritance
and related mechanisms. But this does not necessarily mean that we completely
understand inheritance. Of all the issues in object orientation, none causes as
much discussion as the question of when and how to use this relation. Sweeping
opinions abound, for example on Internet discussion groups, but the literature
is surprisingly poor in precise and useful advice. In this chapter we will
probe further into the meaning of inheritance, not for the sake of theory, but
to make sure we use it best to benefit our software development projects. We
will in particular try to understand how inheritance differs from the
other inter-module relation in object-oriented system structures, its
sister and rival, the client relation: when to use one, when to use the other,
when both choices are acceptable. Once we have set the basic criteria for using
inheritance --- identifying along the way the typical cases in which it is
wrong to use it --- we will be able to devise a classification of the various
legitimate uses, some widely accepted (subtype inheritance), others, such as
implementation or facility inheritance, more controversial. Along the way we
will try to learn a little from the experience in taxonomy, or
systematics, gained from older scientific disciplines.
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