From 3940770712758158891 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: fc772,ef0c226459c03bdf X-Google-Attributes: gidfc772,public X-Google-Thread: f78e5,edd477ec92e5348f X-Google-Attributes: gidf78e5,public From: smeyers@aristeia.com (Scott Meyers) Subject: Re: Protected Inheritance Date: 1999/02/17 Message-ID: #1/1 X-Deja-AN: 446350363 Approved: daveed@vandevoorde.com Sender: cppmods@netlab.cs.rpi.edu References: <79aesq$qlr$1@uuneo.neosoft.com> <36b9fd49.0@10.1.1.65> <36C046BE.4233@clipper.ens.fr> X-Submission-Address: c++-submit@netlab.cs.rpi.edu X-Approved-For-Group: stephen.clamage@sun.com comp.std.c++ X-Original-Date: Tue, 16 Feb 1999 17:26:52 -0800 Organization: unknown X-Auth: PGPMoose V1.1 PGP comp.lang.c++.moderated iQBVAwUANspc8UHMCo9UcraBAQHSsQH/fAnKayha7tVbza/cHFE8HnSmu5PPDqhf LVH/DHaRgX2LkqxU4ZPgToa1M4jFdVH04dQmbkmHRYmzyVp1GODbRg== =OQFK Newsgroups: comp.std.c++,comp.lang.c++.moderated In article <36C046BE.4233@clipper.ens.fr>, bonnard@clipper.ens.fr says... > Scott Meyers wrote: > > > True, but don't forget the copy constructor: > > > > class MoreDerived: public Something { > > public: > > MoreDerived() > > : Something(*new Something) {} // initialize DontDeriveFromSomething > > }; // via copy ctor > > Could you explain why you think that it should work ? It should work because the virtual base class has an implicit public copy constructor, so a more derived class could initialize DontDeriveFromSomething via this constructor. Alas, that's not what the code snippet above does. This is the correct code: class MoreDerived: public Something { public: MoreDerived() : DontDeriveFromSomething(*new Something) {} }; To prevent this, DontDerivedFromSomething needs to declare its copy constructor private. > > >By the way, I tried to templatize the idiom, and I thought it should work. > > > > I think it should, too. > > No, sadly it won't, because of some strange [moderators, I can say stupid > ?] rule in the standard, which prohibits making template parameters > friends. Can somebody summarize for me the motivation behind this rule? If, as I suspect, this has already been hashed to death in this newsgroup, feel free to just point me to a subject thread I can look up at Deja News. Thanks, Scott -- Scott Meyers, Ph.D. smeyers@aristeia.com Software Development Consultant http://www.aristeia.com/ Visit http://meyerscd.awl.com/ to demo the Effective C++ CD [ 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 ] [ Send an empty e-mail to c++-help@netlab.cs.rpi.edu for info ] [ about comp.lang.c++.moderated. First time posters: do this! ]