Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!nrl-cmf!ames!pasteur!agate!violet.berkeley.edu!pete
From: pete@violet.berkeley.edu (Pete Goodeve)
Newsgroups: comp.lang.c++
Subject: Re: destructors, derived classes, and pointers
Summary: Thanks (but I'm not totally happy..)
Message-ID: <16250@agate.BERKELEY.EDU>
Date: 31 Oct 88 04:46:50 GMT
References: <16219@agate.BERKELEY.EDU> <1285@cod.NOSC.MIL>
Sender: usenet@agate.BERKELEY.EDU
Organization: University of California, Berkeley
Lines: 36


Mike Ball [in <1285@cod.NOSC.MIL>] answers my screams of anguish:

> >The problem is that all the managing procedure has is a pointer to the
> >object (of the base type naturally).  Now I would expect that a delete call
> >on this pointer would detect the correct destructor to invoke before it
> >actually disposed of the memory, but NOOO!  It calls ONLY the parent type
> >destructor!!!  So much for nice neat cleanup...
> >
> Make your destructor virtual and all will work as you wish......

Hmmm. Yup, that works.  Thanks.

Actually I thought I'd tried that in the course of my rushing in five
directions at once...  I was probably fooled by the fact that virtual
CONstructors are illegal. Also my only reference at the moment is Wiener
and Pearson, who don't mention that possibility.  [It seems that our local
bookstores are out of Stroustrup's book at the moment.  Is that maybe a
good sign?  On the other hand, the UC Berkeley library doesn't have a
SINGLE copy!]

I'm still not entirely happy, because this solution seems particularly ad
hoc.  I don't like Ifs Ands and Buts in a language.  I can't see that you
would EVER want ONLY the parent class destructor to be invoked, so that
there should be some protection against this.  I realize that this is a
case where efficiency has taken precedence over security, but I'm not sure
this was a wise choice.  Could the compiler raise an error flag if you ever
assigned a derived object with a destructor to a pointer of the base class
WITHOUT a virtual destructor?  I think this might solve the dilemma.

As a final carp, there is still the other inconsistency I noted [section
(1) of my last message] in that a 'this->' reference in a parent
constructor picks up the PARENT's virtual function, rather than the derived
class function that would be invoked in any other case.  This surely is
wrong?
                                            -- Pete --