Xref: utzoo comp.lang.eiffel:1368 comp.object:2495
Path: utzoo!censor!geac!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!uunet!pdn!tscs!tct!chip
From: chip@tct.uucp (Chip Salzenberg)
Newsgroups: comp.lang.eiffel,comp.object
Subject: Re: Inheritance and Information Hiding
Message-ID: <27A86309.281A@tct.uucp>
Date: 31 Jan 91 18:33:45 GMT
References: <10612@pasteur.Berkeley.EDU> <485@eiffel.UUCP> <488@eiffel.UUCP>
Organization: Teltronics/TCT, Sarasota, FL
Lines: 51

According to bertrand@eiffel.UUCP (Bertrand Meyer):
>A class POLYGON exists and has a procedure
>``add_vertex'', which it exports. You want
>to add a class RECTANGLE. What do you do?
>
>1. Decide not to use inheritance.
>2. Redo the existing inheritance hierarchy.
>3. Accept that RECTANGLE inherits from POLYGON and does not export
>   ``add_vertex''. ... and disallow polymorphic assignments of the form
>   p := r where p is of type POLYGON and r of type RECTANGLE.
>4. Accept that RECTANGLE inherits from POLYGON and does not export 
>   ``add_vertex''. Permit polymorphic assignments of the above form;
>   but have the type checker reject as invalid any system which would
>   contain both such an assignment and a call p.add_vertex (...), since
>   it may lead to a run-time inconsistency.

In C++, the add_vertex() member function would almost certainly be
virtual; that is, it would act correctly in cases where a |POLYGON*|
actually points to an object of type |RECTANGLE|.  (If it were not
virtual already, it could be made so by the insertion of the word
"virtual" at one place in the code.)

Given this language feature, add_vertex becomes a non-problem.  The
implementor of RECTANGLE simply provides an appropriate definition of
the RECTANGLE::add_vertex(): a do-nothing function body that returns a
failure indication (if appropriate).

Analyzing this use of C++ results in a fifth item for Bertrand's list:

5.  Accept that RECTANGLE inherits from POLYGON and exports add_vertex,
    and require the programmer to provide an implementation of add_vertex
    appropriate for a RECTANGLE.

In the real world, how is this solution deficient?

>As an aside (meant to be thought- rather than flame-provoking)
>I have come to realize, although not early enough, that
>information hiding is a greatly overrated goal.

Which reminds me of a nice phrase from Leo Brodie's _Thinking_Forth_,
that goes something like this:

    Some programmers use information hiding as if programs are
    prone to attack from roving bands of hostile programmers.

Of course, Forth isn't exactly my favorite programming environment;
I prefer driving with seatbelts (but not a padded cell).  :-)
-- 
Chip Salzenberg at Teltronics/TCT     <chip@tct.uucp>, <uunet!pdn!tct!chip>
 "I want to mention that my opinions whether real or not are MY opinions."
             -- the inevitable William "Billy" Steinmetz