From 4803161280099104803 X-Google-Thread: f78e5,8983dbb7c598f774 X-Google-Attributes: gidf78e5,public X-Google-Language: ENGLISH,ASCII-7-bit Path: g2news2.google.com!news1.google.com!news3.google.com!border1.nntp.dca.giganews.com!nntp.giganews.com!local02.nntp.dca.giganews.com!nntp.speakeasy.net!news.speakeasy.net.POSTED!not-for-mail NNTP-Posting-Date: Sun, 30 Jul 2006 19:10:04 -0500 Return-Path: X-Authentication-Warning: mulga.csse.unimelb.edu.au: fjh set sender to devnull@stump.algebra.com using -f X-Robomod: STUMP, ichudov@algebra.com (Igor Chudov) X-Original-To: std-c++@mailman.ucar.edu Delivered-To: std-c++@mailman.ucar.edu From: "Greg Herlihy" Newsgroups: comp.std.c++ Subject: Re: Ambiguous conversion to base? Organization: http://groups.google.com Message-ID: <1154303691.576895.287710@s13g2000cwa.googlegroups.com> References: Mime-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" X-Complaints-To: groups-abuse@google.com User-Agent: G2/0.2 X-HTTP-UserAgent: Mozilla/5.0 (Macintosh; U; PPC Mac OS X; en) AppleWebKit/418.8 (KHTML, like Gecko) Safari/419.3,gzip(gfe),gzip(gfe) Complaints-To: groups-abuse@google.com Injection-Info: s13g2000cwa.googlegroups.com; posting-host=71.141.225.205; posting-account=JdllFQ0AAAC-QghphnHMZz5q0GHnzGUJ X-Virus-Scanned: amavisd-new at ucar.edu X-Virus-Scanned: amavisd-new at csse.unimelb.edu.au Approved: Fergus Henderson , moderator of comp.std.c++ X-Virus-Scanned: amavisd-new at csse.unimelb.edu.au Date: Sun, 30 Jul 2006 19:08:32 CST Lines: 65 NNTP-Posting-Host: 65.182.171.162 X-Trace: sv3-lJ08VdoSCX+5GNxVfdL7Ek1FyQn57zG3xw63ZOS4f7Voy5npERFomhijUWjnoBiuEOpoDVGZARSqx25!jw6CvCBSoXaYCRs6LBsXhiNvQGalFlV1tmjVtyBv5mtmhFXpX47CMDAdKy7KK8PHTrxaT22fu4w8!eboGyZFrAEMlt+VpyT3wo9Qdtba4GYEuV2J/nnX+x6I= X-Complaints-To: abuse@speakeasy.net X-DMCA-Complaints-To: abuse@speakeasy.net X-Abuse-and-DMCA-Info: Please be sure to forward a copy of ALL headers X-Abuse-and-DMCA-Info: Otherwise we will be unable to process your complaint properly X-Postfilter: 1.3.32 Xref: g2news2.google.com comp.std.c++:3148 "Roger Orr" wrote: > Should this fragment compile? > > class Base > { > }; > > class Derived : public Base > { > public: > Derived( Derived const & ); > Derived( Base const ); // Note: no '&' > }; > > class Final : public Derived > { > Final( Final const & f ) > : Derived( f ) {} > }; > > MSVC 2003 and Comeau say 'Yes' > g++ 3.4.4 and 4.0.2 say 'No': > > copyctor.cpp: In copy constructor `Final::Final(const Final&)': > copyctor.cpp:15: error: call of overloaded `Derived(const Final&)' is > ambiguous > copyctor.cpp:9: note: candidates are: Derived::Derived(Base) > copyctor.cpp:8: note: Derived::Derived(const Derived&) > > As far as I understand it conversion from Final const & -> Derived const & > should be a better conversion than any conversion to Base. > Or have I missed something? I would vote with gcc - the call Derived( f ) does indeed appear to be ambiguous. The two eligible functions Derived(Base) and Derived(const Derived&) both require a derived-to-base conversion for the argument. Therefore both functions are awarded Conversion rank. Now, the "distance" of a derived-to-base conversion can often serve a tiebreaker when deciding between two derived-to-base conversions. The compiler will favor more "immediate" conversions (such as the one-hop Final to Derived conversion) over those farther apart in the class hierarchy (such as the two-hop Final to Base conversion). But the compiler can apply this tiebreaker only when the parameters requiring the conversions are both references, both pointers or both values. In this case, one parameter is a reference and the other is a value. Since the parameters do not agree in form, the tiebreaker never gets to be excercised. The call to Derived remains ambiguous. (Note that changing the parameters to be both references, both values or both pointers resolves the ambiguity). Moreover, an error seems the most appropriate outcome in this situation. After all, whether to call a function with a value parameter or with a reference parameter is a more important decision (and one which the compiler wisely refrains from making itself) than which derived-to-base class conversion is preferable. Greg --- [ 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.comeaucomputing.com/csc/faq.html ]