Re: Does Ada really need MULTIPLE inheritance?

[baker@IDA.ORG (Paul Baker)]

>From article <8910161521.AA27039@ajpo.sei.cmu.edu>, 
>   by NCOHEN@IBM.COM ("Norman H. Cohen"):
> Since nobody at either workshop was a strenuous advocate of multiple
> inheritance, I would like to see the argument for multiple inheritance

I attended one of these workshops and didn't advocate multiple
inheritance.  The response to Norm's remark on this network seems to be
based on the notion that the power of multiple inheritance was
overlooked at these workshops.  If my memory serves, the issue on the
table was the value of this feature in the context of its dangers and
the fact that it is not essential for OOP. Its power was recognized.

The danger is that there is currently no rigorous notion of how we shall
interpret the fusion of methods from alternative lineages.  CLOS offers
a fixed and reasonable interpretation of the order of delegation
but the order is not based on any convincing metaphor.  In the face of
this problem, it will be difficult to teach design for multiple
inheritance and hard for one developer to maintain another's system.

Multiple inheritance is not essential because its effects can be
achieved by defining a new class whose slots include components from
various classes.  The developer must then define all the new methods -
none are inherited automatically.  Naturally, that new definition can
be a delegation to one of the ancestor classes. Thus, composition
achieves the same functional effect as inheritance but requires an
explicit explanation of the intended fusion of methods.

----------
Paul L. Baker
CTA INCORPORATED
E-MAIL: bakerp@ajpo.sei.cmu.edu  PHONE: 301-816-1242