From 4453684935260099539 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: f78e5,18c6d861cef54158 X-Google-Attributes: gidf78e5,public X-Google-ArrivalTime: 2003-01-02 12:24:51 PST Path: archiver1.google.com!news1.google.com!newsfeed.stanford.edu!newsmi-us.news.garr.it!NewsITBone-GARR!news.mailgate.org!newsfeed.icl.net!newsfeed.fjserv.net!newsfeed.icl.net!newsfeed.fjserv.net!kibo.news.demon.net!mutlu.news.demon.net!demon!mail2news.demon.co.uk!devnull From: danielgutson@hotmail.com (danielgutson@hotmail.com) Newsgroups: comp.std.c++ Subject: Re: automatic downcast support through function parameters Date: Thu, 2 Jan 2003 20:24:44 +0000 (UTC) Organization: http://groups.google.com/ Lines: 91 Approved: fjh@cs.mu.oz.au (Fergus Henderson , moderator of comp.std.c++) Message-ID: <23478c42.0301020923.79359e66@posting.google.com> References: <23478c42.0212310830.23dbc580@posting.google.com> <3E120608.6060608@tellabs.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8bit X-Trace: mail2news.demon.co.uk 1041539084 16848 10.0.0.1 (2 Jan 2003 20:24:44 GMT) X-Complaints-To: abuse@demon.net NNTP-Posting-Date: Thu, 2 Jan 2003 20:24:44 +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 18UBtS-0004Nb-00 for mail2news@news.news.demon.net; Thu, 02 Jan 2003 20:24:42 +0000 X-Received: from localhost (localhost [[UNIX: localhost]]) by mulga.cs.mu.OZ.AU id HAA07975; Fri, 3 Jan 2003 07:24:38 +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-NNTP-Posting-Date: 2 Jan 2003 17:23:47 GMT X-Spam-Status: No, hits=-8.4 required=5.0 tests=FORGED_HOTMAIL_RCVD,NOSPAM_INC,QUOTED_EMAIL_TEXT, REFERENCES,SPAM_PHRASE_03_05 version=2.41 Xref: archiver1.google.com comp.std.c++:16658 Daniel.Miller@tellabs.com ("Dan'l Miller") wrote in message news:<3E120608.6060608@tellabs.com>... > danielgutson@hotmail.com wrote: > > People: I'd like to ask about the feasibility of this: > > automatic downcast during function calls. > > let's have: > > struct Base{}; > > struct Der : Base{}; > > > > void g(Base* b); > > void g(Der* d); > > > > void f(upcasted Base* b) // see below > > { > > g(b); > > } > > > > void main(void) > > { > > Der d; > > f(&d); > > } > > > > - The idea here is that f ignores the function that will be finally > > invoked, but knows that the pointer was upcasted ('upcasted' > > keyword). > > Then, the function accepting the most derived class (closest to the > > original type) will be invoked, in this case: g(Der*). > > > > The 'spirit' of this is to recover specificity: in the process flow: > > main -> f -> g > > specificity(main) = specificity(g) > specificity(f) > > and this would be an automatic way of recoviring specificity at g, > > without loosing generality at f. > > > > What you think? > > I think that this works well at compile-time only 1) if one has a single > compilation unit or 2) if one has global analysis among separate compilation > units. In practice "compilation unit" normally corresponds to a .c/.cpp/.cxx/.C > file which begets a .o/.obj file. > > Because C's (and thus C++'s) compilation model is predicated on separate > compilation, this proposed feature is in turn predicated on the hypothetical > removal of separate compilation from C++/C. Such removal of the > separate-compilation axiom appears quite unlikely to me in the foreseeable future. > > I think that human-authored code can be written to > overtly/manually/nonautomatically accomplish this via RTTI at run-time. > > --- > [ 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 ] First of all, thanks for your answers. Two things: 1) [To Kalle]: Yes, I'm talking about the second: a kind of condition using RTTI. The advantage -and I'm not the best for asserting this as far as I'm not in quality of implementations- is that -I think- this can be achieved using a reduced set of RTTI primitives. I think, that in fact RTTI is no needed (as a whole), despite any run time information is needed in some kind. I think that this can be achieved just with some additional entry in the v-table. What if a null pointer is passed: the null pointer contains no information, what means that it wasn't upcasted, it's just an 'original null pointer to base', then the g(Base*) should be called (btw, your 'else' works). 2) About separated compilation units: I think that there is no need of a whole program analysis (am I getting your point?), as far as, as I think above, this can be achieved with an additional entry in the vtables, and any indirection mechanism using that entry. Do you think it is possible in this way? Before I mess in the implementation of such mechanism, please provide me some feedback about the usage. What do you think about the concept I describe? Thanks! Daniel. --- [ 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 ]