From -1166474090811759940 X-Google-Thread: f78e5,6978327f896d5d5b X-Google-Attributes: gidf78e5,public X-Google-Language: ENGLISH,ASCII-7-bit Path: g2news1.google.com!news2.google.com!proxad.net!proxad.net!news-hub.cableinet.net!blueyonder!news-out.ntli.net!newsrout1.ntli.net!news-in.ntli.net!ntli.net!kibo.news.demon.net!mutlu.news.demon.net!demon!mail2news.demon.co.uk!devnull From: jdennett@acm.org (James Dennett) Newsgroups: comp.std.c++ Subject: Re: What is the use of the null directive # ? Date: Mon, 28 Jun 2004 05:36:13 +0000 (UTC) Organization: Cox Communications Lines: 81 Approved: fjh@cs.mu.oz.au (Fergus Henderson , moderator of comp.std.c++) Message-ID: References: <607f883e.0406251028.5412fc80@posting.google.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit X-Trace: mail2news.demon.co.uk 1088400973 21892 10.0.0.1 (28 Jun 2004 05:36:13 GMT) X-Complaints-To: abuse@demon.net NNTP-Posting-Date: Mon, 28 Jun 2004 05:36:13 +0000 (UTC) X-Received: from mulga.cs.mu.oz.au ([128.250.1.22]) by news.demon.co.uk with esmtp (Exim 4.12) id 1BeooO-0005gx-00 for mail2news@news.news.demon.net; Mon, 28 Jun 2004 05:36:12 +0000 X-Received: from mulga.cs.mu.OZ.AU (localhost [127.0.0.1]) by mulga.cs.mu.OZ.AU with ESMTP id i5S5aAb9026737; Mon, 28 Jun 2004 15:36:10 +1000 (EST) X-Received: (from fjh@localhost) by mulga.cs.mu.OZ.AU (8.12.10+Sun/8.12.9/Submit) id i5S5aAYO026728; Mon, 28 Jun 2004 15:36:10 +1000 (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++@ucar.edu X-Authentication-Warning: fed1read02.mgt.cox.net: news set sender to newsmaster@cox.net using -f X-User-Agent: Mozilla/5.0 (Macintosh; U; PPC Mac OS X Mach-O; en-US; rv:1.6) Gecko/20040113 X-Accept-Language: en-us, en X-Newsgroups: comp.std.c++ X-NNTP-Posting-Date: Sun, 27 Jun 2004 20:33:56 EDT X-Spam-Checker-Version: SpamAssassin 2.60-mulga_r1 (1.212-2003-09-23-exp) on mulga.cs.mu.OZ.AU X-Spam-Status: No, hits=-4.8 required=5.2 tests=AWL,BAYES_00, MAILTO_TO_SPAM_ADDR autolearn=no version=2.60-mulga_r1 Xref: g2news1.google.com comp.std.c++:1051 Francis Glassborow wrote: > In article , > Barry Margolin writes > >> In article , >> francis@robinton.demon.co.uk (Francis Glassborow) wrote: >> >>> In article <607f883e.0406251028.5412fc80@posting.google.com>, Prateek R >>> Karandikar writes >>> >What is the use of the null-directive # ? >>> > >>> > >>> > 16.7 Null directive [cpp.null] >>> >1 A preprocessing directive of the form >>> > # new-line >>> > has no effect. >>> > >>> >>> It allows implementors to add directives as extensions without having >>> impact on the portability of code using them. >> >> >> All it says is that the line: >> >> # >> >> is ignored. How does that allow an implementor to add something like: >> >> #foobar > > > No it does not say that the implementor is _required_ to ignore such a > directive. In fact, it says *nothing* about how an implementation can or must interpret something such as #foobar but rather says only that # has no effect in a conforming compiler > What it does say is that such a directive is not cause to > abort compilation, nor does its existence require a diagnostic. You're thinking, perhaps, of the requirement that #praga foo, where foo is not a recognized pragma name, should be a no-op. > It generalises the #pragma directive so that I can write: > > #packed > > happy in the knowledge that any compiler that does not understand that > directive will simply ignore it. That generalization is explicit in the standard, and independent of how # is interpreted -- an isolated # portably does nothing, but #pragma packed might do anything at all, if the compiler so chooses to "recognize" the "packed" pragma. For example, on one compiler #pragma packed might control alignment within objects, while another could quite legally implement it to mean that the generated *code* should be as small as possible. To avoid the risk of such name clashes, some considerate implementors put their own pragmas effectively into another namespace, and allow them to be used as #pragma __VENDOR_X__ packed which are unlikely to be "recognized" and interpreted differently by another system. -- James --- [ 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 ]