From -3713250171294921841 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: f78e5,4441c5f6d7843223 X-Google-Attributes: gidf78e5,public X-Google-ArrivalTime: 2002-08-16 05:36:03 PST Path: archiver1.google.com!news1.google.com!newsfeed.stanford.edu!canoe.uoregon.edu!logbridge.uoregon.edu!kibo.news.demon.net!news.demon.co.uk!demon!mail2news.demon.co.uk!not-for-mail From: rmaddox@isicns.com (Randy Maddox) Newsgroups: comp.std.c++ Subject: Re: C++0x Wish List (explicit class) Date: Fri, 16 Aug 2002 12:35:08 GMT Organization: http://groups.google.com/ Approved: Fergus Henderson , moderator of comp.std.c++ Message-ID: <8c8b368d.0208150528.2bc34601@posting.google.com> References: <3D48C729.40503@spamnot.codeweavers.com> <8c8b368d.0208060921.43e9e9d5@posting.google.com> <3D589892.3050008@spamnot.codeweavers.com> X-Trace: mail2news.demon.co.uk 1029501314 mail2news:3353 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) Delivered-To: std-c++@ncar.ucar.edu Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8bit NNTP-Posting-Date: 15 Aug 2002 13:28:14 GMT Lines: 83 Xref: archiver1.google.com comp.std.c++:13264 Ken Thomases wrote in message news:<3D589892.3050008@spamnot.codeweavers.com>... > > Is it true that providing a constructor causes suppression of > compiler-supplied destructor? I don't think it is. I just tried this > in CodeWarrior 8.1: > > #include > #include > > class MyTest > { > public: > MyTest() : m_string("blat"), m_vector(10) { } > MyTest(const MyTest& in_other) : > m_string(in_other.m_string), m_vector() > { m_vector.push_back(m_string); } > > private: > const std::string m_string; > std::vector m_vector; > }; > > int main() > { > MyTest foo; > MyTest bar(foo); > // MyTest gub; > // gub = foo; // error > } > > > I figure those constructors have enough meat to be clearly non-trivial. > The string is const to test suppression of compiler-supplied > assignment operator. It compiles quite nicely. Now, clearly, for this > code to be legal the compiler must be getting a destructor from > somewhere. Right? Neither object's m_string or m_vector are allowed to > leak, are they? > > For kicks, I tried making MyTest privately inherit boost::noncopyable. > Still compiled, although the compiler error if I uncommented the gub > lines changed from "using implicit copy assignment for class with const > or reference member" to "illegal access to private/protected member". > > With explicit class, no destructor would be provided. The compiler > would produce errors for almost any instantiation of the class (except, > dynamically allocated on the heap and never deleted). What would be the > value of this? It would remind a programmer who suffered a lapse in > concentration that their complex class needed a programmer-supplied > destructor up to the task. Better a compiler error than a > compiler-supplied destructor which masks the problem. > > Ken > Speaking of dtors, I would also point out that boost::noncopyable has a non-virtual dtor, which is pretty unusual for a class intended as a base class. Now, I know that the usual idiom is to use private derivation so that no reference or pointer to a class derived from boost::noncopyable will be automatically converted to a reference or pointer to its base class, but there is nothing to enforce this. It's just a convention. If somehow, somewhere, someone does delete an object of a class derived from boost::noncopyable using a pointer to its base class, the correct dtor will not be used, and problems are likely. Of course, this could easily be fixed by adding a virtual dtor, but I would presume the reason it was left non-virtual in the first place is to allow the "empty base class" optimization, which a vtable would prevent. Oops! Maybe this solution isn't quite as perfect as we might assume. So tell me again how boost::noncopyable is better than extending the use of an existing keyword in a sensible way? Randy. --- [ 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://www.jamesd.demon.co.uk/csc/faq.html ]