Can I have Pure Virtual Destructor?
in [C++]
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From: red floyd on 6 Mar 2007 07:59 red floyd wrote: > Darn, forgot one critical element. [redacted] struct ok_to_instantiate : public i_cant_be_instantiated [redacted] -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Andrew Koenig on 6 Mar 2007 08:12 "Ron Natalie" <ron(a)spamcop.net> wrote in message news:45ed8ca7$0$7450$9a6e19ea(a)news.newshosting.com... > sk.smawsk(a)gmail.com wrote: >> Can there be any use of "Pure Virtual Destructor"? >> If yes, then in what scenario do we use it? > Certainly, you can. It's not tremendously useful > other than to force a class to be abstract in the > absence of any other pure methods. > Note that the pure virtual destructor is still called > when the derived class is destroyed so you need to have > an implementation of it. But it's not really useful for that purpose either, because using it that way forces you to define a destructor that is almost surely useless becasue it has nothing to destroy. I don't like being required to write dead code. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Francis Glassborow on 6 Mar 2007 08:13 In article <1173196295.237848.323320(a)s48g2000cws.googlegroups.com>, Val <valosmith(a)gmail.com> writes >You can't have a pure virtual destructor if the compiler is following >the language spec. If you were to derive a concrete class from the >abstract one, then create and destroy an object, the base class's >destructor would be invoked immediately before the derived >destructor. If the base destructor isn't implemented, then this is a >problem. No, making a function a pure virtual does not prevent you from implementing it. All it does is to require that ever derived concrete class has an implementation (possibly inherited from an intermediate derived class) -- Francis Glassborow ACCU Author of 'You Can Do It!' and "You Can Program in C++" see http://www.spellen.org/youcandoit For project ideas and contributions: http://www.spellen.org/youcandoit/projects [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: James Kanze on 6 Mar 2007 20:43 On Mar 7, 2:12 am, "Andrew Koenig" <a...(a)acm.org> wrote: > "Ron Natalie" <r...(a)spamcop.net> wrote in message > news:45ed8ca7$0$7450$9a6e19ea(a)news.newshosting.com... > > sk.sma...(a)gmail.com wrote: > >> Can there be any use of "Pure Virtual Destructor"? > >> If yes, then in what scenario do we use it? > > Certainly, you can. It's not tremendously useful > > other than to force a class to be abstract in the > > absence of any other pure methods. > > Note that the pure virtual destructor is still called > > when the derived class is destroyed so you need to have > > an implementation of it. > But it's not really useful for that purpose either, because using it > that way forces you to define a destructor that is almost surely useless > becasue it has nothing to destroy. > I don't like being required to write dead code. But as soon as you need a virtual destructor, you're in that case anyway. I'll admit that the virtual destructor of a class in which all other functions are pure virtual is the one case I'll use a function definition directly in the class---which I couldn't do if the destructor were pure virtual. (It leads to the somewhat curious situation that the only function I have which is inline is virtual. But I'm not going to create a separate .cc file just for a single, empty function.) -- James Kanze (GABI Software) email:james.kanze(a)gmail.com Conseils en informatique orient�e objet/ Beratung in objektorientierter Datenverarbeitung 9 place S�mard, 78210 St.-Cyr-l'�cole, France, +33 (0)1 30 23 00 34 -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: James Kanze on 6 Mar 2007 20:45
On Mar 7, 2:13 am, Francis Glassborow <fran...(a)robinton.demon.co.uk> wrote: > In article <1173196295.237848.323...(a)s48g2000cws.googlegroups.com>, Val > <valosm...(a)gmail.com> writes > >You can't have a pure virtual destructor if the compiler is following > >the language spec. If you were to derive a concrete class from the > >abstract one, then create and destroy an object, the base class's > >destructor would be invoked immediately before the derived > >destructor. If the base destructor isn't implemented, then this is a > >problem. > No, making a function a pure virtual does not prevent you from > implementing it. All it does is to require that ever derived concrete > class has an implementation (possibly inherited from an intermediate > derived class) That's right, but maybe a bit confusing. What it means is that until at least one class on a direct path from the most derived class to the base class has a user declared (and defined) destructor, the class is abstract, and cannot be instantiated. What declaring the destructor pure virtual does, in practice, is to require every derived class to provide a user defined destructor, even when it wouldn't need one otherwise. IMHO, this is not a good idea. -- James Kanze (GABI Software) email:james.kanze(a)gmail.com Conseils en informatique orient�e objet/ Beratung in objektorientierter Datenverarbeitung 9 place S�mard, 78210 St.-Cyr-l'�cole, France, +33 (0)1 30 23 00 34 -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] |