From 5671039125210202544 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: f78e5,e7ce30bc16218953 X-Google-Attributes: gidf78e5,public X-Google-ArrivalTime: 2003-01-07 20:27:11 PST Path: archiver1.google.com!news1.google.com!newsfeed.stanford.edu!news-spur1.maxwell.syr.edu!news.maxwell.syr.edu!newsfeed.icl.net!newsfeed.fjserv.net!kibo.news.demon.net!mutlu.news.demon.net!demon!mail2news.demon.co.uk!devnull From: gp1@paradise.net.nz (Graeme Prentice) Newsgroups: comp.std.c++ Subject: Re: function template specialization deduction Date: Wed, 8 Jan 2003 04:27:06 +0000 (UTC) Lines: 69 Approved: fjh@cs.mu.oz.au (Fergus Henderson , moderator of comp.std.c++) Message-ID: <5t8n1v8134h48gcmf2rir4s2po7alth29m@4ax.com> References: Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Trace: mail2news.demon.co.uk 1042000026 14643 10.0.0.1 (8 Jan 2003 04:27:06 GMT) X-Complaints-To: abuse@demon.net NNTP-Posting-Date: Wed, 8 Jan 2003 04:27:06 +0000 (UTC) X-Received: from mulga.cs.mu.oz.au ([128.250.1.22]) by news.demon.co.uk with esmtp (Exim 4.05) id 18W7nz-0003o2-00 for mail2news@news.news.demon.net; Wed, 08 Jan 2003 04:27:04 +0000 X-Received: from localhost (localhost [[UNIX: localhost]]) by mulga.cs.mu.OZ.AU id PAA18075; Wed, 8 Jan 2003 15:26:57 +1100 (EST) X-Authentication-Warning: mulga.cs.mu.OZ.AU: fjh set sender to devnull@stump.algebra.com using -f X-Path: comp-std-cpp-robomod!not-for-mail X-Robomod: STUMP, ichudov@algebra.com (Igor Chudov) X-Delivered-To: std-c++@ncar.ucar.edu X-Newsgroups: comp.std.c++ X-Orig-NNTP-Posting-Host: 203-96-149-214.apx1.paradise.net.nz (203.96.149.214) X-Orig-X-Trace: fu-berlin.de 1041998843 16166200 203.96.149.214 (16 [98036]) X-Newsreader: Forte Agent 1.92/32.572 X-MailScanner: PASSED (v1.2.7 62724 h0847OfP076833 mailbox2.ucsd.edu) X-Spam-Status: No, hits=-6.4 required=5.0 tests=EMAIL_ATTRIBUTION,QUOTED_EMAIL_TEXT,REFERENCES, SPAM_PHRASE_00_01 version=2.41 Xref: archiver1.google.com comp.std.c++:16882 On Tue, 7 Jan 2003 18:55:11 +0000 (UTC), nesotto@cs.auc.dk ("THORSTEN OTTOSEN") wrote: [snip] I think I'll have a third go at answering this since my first two attempts were slightly wrong. > My problem is then simply that cannot explain why >the latter version is a better >match than the first. I have tried looking at paragraph 13.3.3, 14.5.52 and >14.8.3 but could not find anything >that says that > >template< typename C, typename V > > inline void insert( C& c, const V& ); > >is a worse match than > > template< typename A > > inline void > insert( A& c, const typename A::container_type::value_type& v ); > >Is there some rule that "fewer template parameters is better" ? When the deduction process tries to call the second function with arguments made up from the first, it has argument types of C& and const V& - C and V here are "made up"/synthesized types that are unique - the std doesn't define precisely what is meant by unique but the most obvious interpretation is that C and V are types that appear nowhere else in the template parameter list or function parameter list of either function. Having made up the types C and V, the compiler tries to "call" the other function - for the first parameter, it can deduce the template parameter A in the called function to be C - for the second parameter it can "plug in" C where A is and get const typename C::container_type::value_type & from this, it is unable to determine the actual type of value_type to do a comparison with V - so deduction fails because it cannot determine that value_type is the same type as V - but even if it could work out the actual type of value_type, it is guaranteed that it is not the same type as V because V is a unique type that doesn't appear anywhere else. Section 14.5.4.2 gives this example of "uniqueness" applying to non-type parameters template void f(X); // #A template void f(X); // #B 14.5.4.2 says B is more specialized than A because when you make up unique values for I and J in the first function, deduction into B fails because of the template parameter I being deduced as two different values. Graeme --- [ 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 ]