From 3188096413690495068 X-Google-Language: ENGLISH,ASCII X-Google-Thread: f78e5,3cdb654b702f7476 X-Google-Attributes: gidf78e5,public X-Google-ArrivalTime: 2000-08-26 19:35:19 PST Path: supernews.google.com!sn-xit-02!sn-east!supernews.com!newspeer.monmouth.com!newsfeed.icl.net!dispose.news.demon.net!demon!news.demon.co.uk!demon!mail2news.demon.co.uk!not-for-mail From: kanze@gabi-soft.de Newsgroups: comp.std.c++ Subject: Re: virtual functions Date: Sun, 27 Aug 2000 02:34:24 GMT Organization: Customer of UUNET Deutschland GmbH Approved: Fergus Henderson , moderator of comp.std.c++ Message-ID: <86vgwowaoq.fsf@gabi-soft.de> References: <399A78E4.8345D7B@gmx.at> <399B8FA6.540D5B7F@gmx.at> <86itsr207l.fsf@gabi-soft.de> X-Trace: mail2news.demon.co.uk 967343673 mail2news:28926 mail2news mail2news.demon.co.uk X-Complaints-To: abuse@demon.net X-Mail2News-Path: news.demon.net!mulga.cs.mu.oz.au X-Authentication-Warning: mulga.cs.mu.OZ.AU: fjh set sender to devnull@stump.algebra.com using -f X-Robomod: STUMP, ichudov@algebra.com (Igor Chudov) User-Agent: Gnus/5.0805 (Gnus v5.8.5) Emacs/20.4 MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: quoted-printable X-MIME-Autoconverted: from 8bit to quoted-printable by mulga.cs.mu.OZ.AU id NAA06737 Lines: 50 Xref: supernews.google.com comp.std.c++:383 "michael" writes: |> > > > It is true only inside constructors *and* destructors. In all |> > > > other cases, the overriding method of the object's dynamic |> > > > type is called. |> > > It is true during construction and destruction. You can't |> > > defeat the behaviour by having the ctor forward to a helper. |> > In all cases, the overriding method of the object's dynamic type |> > is called. There are no special rules for constructors or |> > destructors. (Of course, in the constructor or destructor for X, |> > the dynamic type of the object is X.) |> Yes, a fine unification. |> (but there _is_ a special rule: |> 12.7/3&4 _define_ the dynamic type of the object during |> contruction/destruction in this special way. Without this rule, I |> might think the dynamic type was the type used in the new expresion |> or declaration of the object.) I wouldn't call it a "special rule". Without the rule, we wouldn't know what the dynamic type was during construction. A language must define it. Java defines it differently. I've had to track down a couple of NullPointerException because of it, and add some special code to handle the case where a function is called on an unconstructed object. Admittedly, I get a NullPointerException, rather than undefined behavior, and since Java always 0 initializes an object before calling constructors, it is possible to test. But having used both rules, I can only say that C++ is a lot safer in this regard. (The safest rule would probably be that you cannot call a virtual function, period, until the object is fully constructed. However, I don't see any way the compiler could enforce this, and the C++ rule is definitly preferrable to yet another case of undefined behavior.) --=20 James Kanze mailto:kanze@gabi-soft.de Conseils en informatique orient=E9e objet/ Beratung in objektorientierter Datenverarbeitung Ziegelh=FCttenweg 17a, 60598 Frankfurt, Germany Tel. +49(069)63198627 --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://reality.sgi.com/austern_mti/std-c++/faq.html ]