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From: casseres@apple.com (David Casseres)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Multiple Inheritance -- Is It A Luxury?
Message-ID: <8967@goofy.Apple.COM>
Date: 3 Jul 90 17:55:59 GMT
Sender: usenet@Apple.COM
Organization: Apple Computer, Inc.
Lines: 36
References:<15132@reed.UUCP> <268BA8DC.4CD4@intercon.com> <8937@goofy.Apple.COM> <268C032E.5137@intercon.com> <1990Jul2.181147.1672@efi.com>

In article <1990Jul2.181147.1672@efi.com> tim@efi.com (Tim Maroney) writes:
> The people doing the MPW compilers seem to think that multiple
> inheritance is not important.  It's not possible at all in Object
> Pascal, and apparently is impossible to use in real (handle-based)
> applications in C++.  I think this is a cavalier attitude.

One clarification and two comments.  The clarification:  C++ on the Mac 
does not allow multiple inheritance for subclasses of THandleObject; but 
it can be used with all other classes.  It doesn't matter whether the 
application uses THandleObject subclasses in addition.  Note that it is 
not necessary for everything to be handle-based.

First comment:  The restriction on THandleObject subclasses and multiple 
inheritance is due to serious implementation difficulties that have been 
explained in this group, not to a cavalier attitude.

Second comment:  There is some justification for a cavalier attitude about 
multiple inheritance, just as there is justification for wanting it very 
badly in some cases.  Brian Bechtel sums it up very well in saying 
"Multiple inheritance is the GOTO of the 1990's."  It is tremendously 
powerful, lets you do any damn thing you like, elegantly solves certain 
problems, leads to utterly obscure code, and invites massive abuse that 
can cause enormous damage to the integrity of code.

In my own limited experience with C++, I have found that a redesign of the 
hierarchy always leads to cleaner, more elegant code than multiple 
inheritance.  But then, I'm not using a hierarchy containing a lot of 
libraries produced by other people.  If I were, I might want to use a lot 
of multiple inheritance; but on the other hand, I think that in that 
situation what I would *really* want is source code for all the libraries 
so I could re-implement them to suit my purposes!  That, not any lack of 
features, is the problem with any OOPS that gives you only compiled 
libraries instead of source code.

David Casseres
     Exclaimer:  Hey!