NetNews Usenet Archive 1992 #31/NN_1992_31.iso/spool/comp/lang/cplus/18302

Internet Message Format | 1992-12-22 | 1.8 KB

Path: sparky!uunet!spool.mu.edu!olivea!apple!voder!genie!roger
From: [e-mail] (Roger H. Scott)
Newsgroups: comp.lang.c++
Subject: Re: Virtual base classes
Message-ID: <[e-mail]>
Date: 22 Dec 92 05:29:13 GMT
References: <[e-mail]>
Reply-To: [e-mail] (Roger H. Scott)
Organization: proCASE Corporation, Santa Clara, CA
Lines: 23

In article <[e-mail]> [e-mail] writes:
>In article <[e-mail]> [e-mail] (Roger H. Scott) writes:
>> If Kludge is a non-public subclass of WellFormed then there
>> isn't much point in talking about Kludge redefining WellFormed's virtual
>> functions since you can't use a Kludge in a place where a WellFormed is
>> expected (i.e., who would ever have hold of a Kludge and send it one of
>> WellFormed'd messages, other than a member function of Kludge?).
>
>Imagine that WellFormed's constructor adds all WellFormeds to some
>Collection of WellFormeds. Notice that all Kludges will also be added to
>this collection. Now imagine some calling some virtual function for all
>WellFormeds in the Collection.

This seems to raise an interesting question: does there exist an Is-A
relationship between Kludge and WellFormed, and if not, what does it mean to
have a collection of objects that *claim* to be WellFormeds but are in fact
Kludges?  Do we see a small whole in the type system here, where within
member functions inherited from a non-public base class an object which
is in fact a Kludge can be used interchangably with an object that is a
WellFormed (the example cited by Nikki uses the base class's constructor, but
any member function of the base class will do)?  I'd like to see some sort
of comprehensive statement about the (intended? actual?) semantics of non-
public inheritance in C++.