NetNews Usenet Archive 1992 #26/NN_1992_26.iso/spool/comp/lang/cplus/16007

Text File | 1992-11-09 | 4.2 KB | 89 lines

Newsgroups: comp.lang.c++
Path: sparky!uunet!tessi!eaglet!slipknot!robert
From: [e-mail] (Robert Reed)
Subject: Re: Const Inheritance
Message-ID: <[e-mail]>
Reply-To: [e-mail] (Robert Reed)
Organization: Home Animation Ltd.
References: <[e-mail]> <[e-mail]> <[e-mail]>
Date: Tue, 10 Nov 1992 02:41:01 GMT
Lines: 77

In article <[e-mail]> [e-mail] (Frank Adams) writes:
|>I see nothing in my counter-proposal which would deny the operation of this
|>function.
|
|But I can do the following:
|
|    Square s;
|    setDimensions(s, 2, 3);
|
|This does not work; after the call, the width will not be 2.  (Of course,
|presented this baldly, I would not expect it to work; but I could be passing
|s to some other function which took a rectangle argument, and called
|setDimensions on it; the resulting bug is quite subtle.)

My point remains that we can design conventions to deal with overspecification
problems like this under the auspices of the current language.  Your
alternative would be a bludgeon that would deny the access to ANY object
modification functions.

|>|There is another problem with this, if I decide to generalize a bit: I may
|>|want to define:
|>|
|>|    template<class WidthT, class HeightT>
|>|    class Rectangle {...}
|>|
|>|    template<class SideT>
|>|    class Square : public Rectangle<SideT, SideT> {...}
|>|
|>|In this case, Rectangle cannot inherit from Square; it would have to change
|>|the types of the instance variables.  This is not an altogether artificial
|>|example; I worked on a word processor (not written in an O.O. language) in
|>|which horizontal and vertical distances were expressed in different units.
|>|(This program had no use for squares, however.)
|>
|>If your intent is to use Squares on a system with different horizontal and
|>vertical dimensions, then even your scheme of Const Inheritance doesn't solve
|>the problem.  You still have to deal with how to specify a square.  Is it
|>expressed in horizontal or vertical units?  Or both?  If you're going to
|>constrain the problem so as to exclude alternate solutions, make sure your own
|>solution still works.
|
|Sigh.  Do I have to explain every little detail?  If I were simply using
|different dimensions for width and height, I wouldn't have bothered making a
|template.

Duh!  You're the one who proposed this example, and of course I understand that
the use of class variables implies multiple class instantiations with varying
measure types.  You're also the one who insists that conventions are not enough
to solve the problem of overspecification.  I still don't understand how your
const inheritance proposal makes this example workable.  When you say:

|I would not be able to use Square in the first context [lines, chars], but it
|would be quite usable in the other [points].

You seem to imply that you would allow the creation of template instantiations
that would violate your strictures, or else somehow limit the set of classes
that could be substituted in template variables.  Somehow I don't think this is
what you have in mind, but you don't make it very clear what you mean, and you
seem to be getting further away from your const inheritance proposal.  How does
that all fit in to this?  By preventing all of it?

|Selective inheritance might be a useful feature to add to the language.  But
|I see const inheritance as a completion of a feature already in the
|language, not as an entirely new feature.

Const inheritance is a form of selective inheritance!  As such, it is a subset
which provides one crude criterion for passing members down.  It's as much an
entirely new feature as selective inheritance, and I'm arguing for the
inclusion of neither.
________________________________________________________________________________
Robert Reed            Home Animation Ltd.        503-656-8414
[e-mail]    5686 First Court, West Linn, OR 97068

I have a 3 year old dog I named, "Stay."  It was great when he was a puppy.
I'd call him, "Come here, Stay, Come here, Stay."
--Steve Wright
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