Xref: utzoo comp.lang.eiffel:1366 comp.object:2493
Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!cs.utexas.edu!wuarchive!zaphod.mps.ohio-state.edu!rpi!uupsi!sunic!dkuug!iesd!iesd.auc.dk!amanda
From: amanda@iesd.auc.dk (Per Abrahamsen)
Newsgroups: comp.lang.eiffel,comp.object
Subject: Re: Inheritance and Information Hiding
Message-ID: <AMANDA.91Feb2184801@ra.iesd.auc.dk>
Date: 2 Feb 91 18:48:01 GMT
References: <1991Jan23.224203.3206@runx.oz.au> <120021@gore.com>
Sender: news@iesd.auc.dk (UseNet News)
Organization: Games Research, University of Aalborg, Denmark
Lines: 33
In-reply-to: jacob@gore.com's message of 1 Feb 91 16:30:48 GMT


Sorry to interrupt, but why does nobody accept Bertrand Meyers first
two solutions?

>>>>> On 1 Feb 91 16:30:48 GMT, jacob@gore.com (Jacob Gore) said:

Jacob> However, if POLYGON's 'add_vertex' contains in its contract the
Jacob> postcondition "num_vertices = num_vertices + 1", there is no
Jacob> way RECTANGLE can uphold the contract.

But in that case a RECTANGLE is not, conceptually, a POLYGON.  There
are then two possibilities (as Bertrand Meyer mentioned in
<488@eiffel.UUCP>: 

1) The postcondition in correct: You can always add a vertex to a
   polygon. 

   In this case, a rectangle is not a polygon, and it is misleading to
   use inheritance. 

2) The postcondition is wrong.  You can only add a vertex to some
   kinds of polygons.

   If the postcondition is wrong, it should be fixed, like any other
   bug! 

In a program which should be read and understood by other people
later, these are the only long term solutions.  However, when you are
creating a throw away prototype, inheritance might be useful
nonetheless.  In this case, it does not matter much whether the
language provides "implementation inheritance (solution 3), "extended
type check" (solution 4), or "run time check" (solution 5), except
that (5) is the most flexible, and (3) the most safe.