Re: OOD, Ada, and Inheritance

[cis.ohio-state.edu!zaphod.mps.ohio-state.edu!darwin.sura.net!paladin.amer]

>>>>: Can someone give me a good
>>>>: example of a case where one wants to have multiple implementations for
>>>>: SIMILAR but NOT IDENTICAL abstract behaviors, AND for which this
>>>>: abstract behavior is specified formally and in implementation-neutral
>>>>: terms (or at least could be in principle)? ...
>>>>: As part of the example I would, of
>>>>: course, expect to see the abstract specification of that part of their
>>>>: behavior that these different implementations would all have in
>>>>: common.

I'm not sure what is meant by `similar but not identical' in this
context, so I'll talk a little more generally.

The classic example is of course the graphic drawing library, where all
the drawable classes must have common behaviour such as being
drawable, rotatable, expandable, stretchable etc, but some classes
have additional behaviour, such as a drawable text class which also
has features that allow the text to be altered.  In this case the
drawable features for the various classes are similar (they draw the
shapes) but not identical (each shape is different).

Or if you want the same logical behaviour with different
implementations, consider the single and double linked list.  A
developer may not know when coding starts which implementation to
choose.  If both single and double linked lists share a common
interface then the developer can pick one safe in the knowledge that
switching to the other is a trivial change.

If your data structure library has tables of objects and lists of
objects then you could also have a class which inherits from both
table and list.  This allows you to pass this to a routine that expects a
list and to one that expects a table.

It is important to remember that inheritance is not about reusing the
implementation of your class (you can do that by delegation).  It is
about reusing the *interface*, because that way you get to reuse all
the other code out there that uses the interface.

Paul.

-- 
Paul Johnson (paj@gec-mrc.co.uk).	    | Tel: +44 245 73331 ext 3245
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