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From: Matt Austern <austern@isolde.mti.sgi.com>
Subject: Re: Interface/Implementation (was Re: Design by Contract)
Date: 1997/09/11
Message-ID: <fxtiuw7ert4.fsf@isolde.mti.sgi.com>#1/1
X-Deja-AN: 271625270
References: <EG0nDv.EJD@ecf.toronto.edu> <EG7zJn.366@syd.csa.com.au>
 <EG9wrJ.F3t@ecf.toronto.edu> <34167245.0@news.uni-ulm.de>
 <EGBGD3.C5G@ecf.toronto.edu>
Organization: SGI
Newsgroups: comp.object,comp.software-eng,comp.lang.ada,comp.lang.eiffel
Date: 1997-09-11T00:00:00+00:00
List-Id: <comp.lang.ada>


doylep@ecf.toronto.edu (Patrick Doyle) writes:

> Close, but IMO no cigar.  If we are faithfully to compare Ada and
> Eiffel ways of doing this, then just as an Ada package is accessed
> through its interface, so would the Eiffel class be accessed only
> through the abstract superclass.  Thus, even if the subclass
> added public features, the client couldn't use them.

Except that that's impossible.  You have to know the name of the
derived class, and the signatures of its creation functions, in order
to create objects of that class.  So the interface of the derived
class cannot be completely hidden.

(Or, alternatively, you could have a third class, a "factory" class,
that exists only to create instance of the derived implementation
class.  In that case, you could hide the derived class completely.
Even so, though, you would have to know the interfaces of at least two
classes; the abstract base class does not suffice.  And note that
Eiffel doesn't let you use this method for expanded classes or
objects.)

The bottom line: inheritance is useful for some purposes, but it is
not a panacea.  Base-derived semantics is not at all the same as
interface-implementation semantics.