Xref: utzoo comp.lang.eiffel:1349 comp.object:2473
Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!thunder.mcrcim.mcgill.edu!snorkelwacker.mit.edu!think.com!spool.mu.edu!uunet!mcsun!ukc!pyrltd!tetrauk!rick
From: rick@tetrauk.UUCP (Rick Jones)
Newsgroups: comp.lang.eiffel,comp.object
Subject: Re: Inheritance and Information Hiding
Message-ID: <1080@tetrauk.UUCP>
Date: 31 Jan 91 09:49:14 GMT
References: <1991Jan23.224203.3206@runx.oz.au> <1991Jan24.214652.18515@Think.COM> <808@puck.mrcu> <10612@pasteur.Berkeley.EDU>
Reply-To: rick@tetrauk.UUCP (Rick Jones)
Organization: Tetra Ltd., Maidenhead, UK
Lines: 35

Given that this thread is starting to look like an Eiffel v C++ debate, there
is a significant aspect of Eiffel which has been overlooked - assertions.

One of the concepts of Eiffel is that the properties of a class may (and
should) be specified in terms of the class invariant and pre- and
post-conditions of routines.  When using inheritance to write a new class, you
can access and/or redefine anything in the parent class, but you cannot
override the assertions.  (In the current version you can override the pre- and
post-conditions, but this is a recognised compiler problem which is going to be
rectified.)  You certainly can't override the invariant, but you can strengthen
it by adding additional invariant properties.

The correct way to use Eiffel is to be rigorous with the use of assertions, and
run all your tests with all assertion checking on.  The assertion checking may
then be turned off for a production version once you are sure that the
assertions never get violated.

This is a sometimes subtle but significantly different approach to the problem
of protecting the semantics of classes, and avoids the need to hide the
implementation details from descendant classes.  You can look at it by saying
that the interface of a class is public, the implementation is protected, and
the assertions are private (it's not an exact analogy, but it gives the right
feel).

Of course you have to get your assertions correct, but I don't believe that is
any harder than getting the public/private/protected bit right.

I have already had several occasions to be glad of Eiffel's assertion system,
since it has been able to nip in the bud a number of errors which would
otherwise have gone unnoticed and become obscure time-bomb bugs.
-- 
Rick Jones
Tetra Ltd.  Maidenhead, 	Was it something important?  Maybe not
Berks, UK			What was it you wanted?  Tell me again I forgot
rick@tetrauk.uucp					-- Bob Dylan