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From: johnson@p.cs.uiuc.edu
Newsgroups: comp.lang.c++
Subject: Re: Design question
Message-ID: <77300051@p.cs.uiuc.edu>
Date: 26 May 90 12:33:00 GMT
References: <1338@calmasd.Prime.COM>
Lines: 39
Nf-ID: #R:calmasd.Prime.COM:1338:p.cs.uiuc.edu:77300051:000:1611
Nf-From: p.cs.uiuc.edu!johnson    May 26 07:33:00 1990


Reid Spencer wrote:
> Initially, we might think of a hierarchy containing an extremely abstract
> type "geo_obj" as the base of the hierarchy:
>		 geo_obj
>			point
>			line
>				ray
>					segment
>			region

Marshall Cline didn't like this, suggesting that ray and line were actually
subtypes of segment.  Rays are just segments with one end point infinitely
far away, while both of the endpoints of a line are infinitely far away.
Walt Peterson argued the opposite, that a line has a slope and a zero 
intercept, while a ray and a segment each add a constraint of an endpoint.
Both points of view are valid in certain circumstances, which leads to
my claim:

	line and segment are not subtypes of each other.

Taking either point of view is too restrictive.  Either class hierarchy
will not be general purpose.  The solution is an abstract class.

A good rule of thumb is that most superclasses in a library should be
abstract.  Subclassing a nonabstract class is usually a little messy.
It is fine for application programmers, who just want to get their job
done and are not so concerned about other people having to reuse and
learn their code, but it is usually a bad idea for class library designers
to use it.

I would have a class "AbstractLine" that defines abstract operations
to return the slope, a point on the line (if any) that intersects with
another line, etc.  Each subclass will implement these operations
differently.  This will allow each subclass to have the most efficient
implementation, rather than using an over general one.


Ralph Johnson -- University of Illinois at Urbana-Champaign