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From: mikem@otc.OZ (Mike Mowbray)
Newsgroups: comp.lang.c++
Subject: Re: Questions about C++
Message-ID: <129@otc.OZ>
Date: Tue, 16-Jun-87 23:07:12 EDT
Article-I.D.: otc.129
Posted: Tue Jun 16 23:07:12 1987
Date-Received: Sun, 21-Jun-87 09:23:09 EDT
References: <964@vaxb.calgary.UUCP>
Distribution: comp
Organization: O.T.C. Systems Development, Australia
Lines: 39

In article <964@vaxb.calgary.UUCP>, west@calgary.UUCP (Darrin West) says:

>> > o	Associated operators like 'in' and 'qua' [ see below ] aren't available
>> 
>> These are not really needed. You achieve the same result in other ways. See
>> below.
> 
> I don't think you can make this blanket statement.  There are a lot of
> simulationists who would not like to live without it. :-) ?

Earlier in my C++ "career", I wondered about this very thing, thinking
it would be nice to know at run-time if an instance of class x was
actually a class something_else. The thing that leads me to make the above
"blanket statement" is that operators like in and qua are basically only
needed is to enable you to *do* things based on what they return.

I.e:  the crucial thing (I believe) is to shift one's design focus from
"what is xyz?" to "what do I want to do with xyz?" Then it becomes
clear that virtual functions are perfect for the job. Someone else
mentioned that it's not clear yet exactly what techniques are good
techniques. "Run-time explicit type info" versus "virtual functions"
are a classic case of this and I'm sure we haven't heard the last of it.

By the way, if you haven't read Bjarne's paper on multiple-inheritance
(delivered at EUUG in May I understand), get a copy somehow. It's
fascinating. Not only can you have multiple base classes in the obvious
sense, you can have virtual base classes (where the grandparent base
part of two separate parent base classes is a single entity in the
derived child class rather than two separate lots of grandparent).  You
can also derive (actually name-extend) from a pointer within the class
or outside it (great for getting rid of all those dummy little member
functions we all know and hate). I for one will be very interested to
play with it if/when it becomes part of commercial C++.

			Mike Mowbray
			Systems Development
			Overseas Telecommunications Commission (Australia)

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