Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!mit-eddie!bloom-beacon!tut.cis.ohio-state.edu!ucbvax!ulysses!hector!jss
From: jss@hector.UUCP (Jerry Schwarz)
Newsgroups: comp.lang.c++
Subject: Re: Use of inheritance for classification?
Message-ID: <11557@ulysses.homer.nj.att.com>
Date: 19 May 89 14:34:47 GMT
References: <30582@apple.Apple.COM> <10248@riks.csl.sony.JUNET> <329@odi.ODI.COM> <327@calmasd.Prime.COM>
Sender: netnews@ulysses.homer.nj.att.com
Reply-To: jss@hector.UUCP (Jerry Schwarz)
Organization: AT&T Bell Laboratories
Lines: 49

In article <327@calmasd.Prime.COM> wlp@calmasd.Prime.COM (Walter L. Peterson, Jr.) writes:
>Part of the problem with multiple inheritance is finding good cases
>for its proper use.  Most of the examples that I have seen, including
>the original posting above, have the common problem of being too
>contrived and more appropriately solved by other means.  The only
>example that I have seen that is both simple and from a familiar
>problem domain to be used as a general example is from biological
>taxonomy.  

Here is an example that I think satisfies the above request. It isn't
contrived, its simple, and its a familiar "problem domain".

The iostream library that is scheduled to be part of 2.0 uses
mulitple inheritance in the following way:

class ios { ... }

class istream : virtual public ios { ... } ;

class ostream : virtual public ios { ... } ;

class iostream : public istream, public ostream { ... } ;

Or in pictures

                         ios
			 / \
                        /   \
                       /     \
                  istream   ostream
		       \     /
			\   /
			 \ /
                       iostream


Input operations are allowed on istreams.  Output operations on
ostreams.  iostreams allow both operations (as when updating disk
files)

The abstract model of a stream is a sequence of characters with a
pointer.  istream, ostream, iostream operate on the same abstraction
but allow different operations.  This improves compile time type.
checking.  (E.g. attempts to input from cout are caught at compile
time.)


Jerry Schwarz
AT&T Bell Labs, Murray Hill