From -6765025701996471364 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: f78e5,8372ae6de42be399 X-Google-Attributes: gidf78e5,public X-Google-ArrivalTime: 2004-02-20 11:54:03 PST Path: archiver1.google.com!news2.google.com!news.maxwell.syr.edu!easynet-monga!easynet.net!kibo.news.demon.net!mutlu.news.demon.net!demon!mail2news.demon.co.uk!devnull From: stephen.clamage@sun.com (Steve Clamage) Newsgroups: comp.std.c++ Subject: Re: Array of size 0 Date: Fri, 20 Feb 2004 19:54:01 +0000 (UTC) Organization: Sun Microsystems, Inc. Lines: 54 Approved: fjh@cs.mu.oz.au (Fergus Henderson , moderator of comp.std.c++) Message-ID: References: <403601F6.8010706@ircad.u-strasbg.fr> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit X-Trace: mail2news.demon.co.uk 1077306841 27480 10.0.0.1 (20 Feb 2004 19:54:01 GMT) X-Complaints-To: abuse@demon.net NNTP-Posting-Date: Fri, 20 Feb 2004 19:54:01 +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 1AuGim-000795-00 for mail2news@news.news.demon.net; Fri, 20 Feb 2004 19:54:00 +0000 X-Received: from mulga.cs.mu.OZ.AU (localhost [127.0.0.1]) by mulga.cs.mu.OZ.AU with ESMTP id i1KJrvi2002435; Sat, 21 Feb 2004 06:53:57 +1100 (EST) X-Received: (from fjh@localhost) by mulga.cs.mu.OZ.AU (8.12.10+Sun/8.12.9/Submit) id i1KJrujb002426; Sat, 21 Feb 2004 06:53:56 +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++@ucar.edu X-Newsgroups: comp.std.c++ X-NNTP-Posting-Date: Fri, 20 Feb 2004 19:20:16 +0000 (UTC) X-User-Agent: Mozilla/5.0 (X11; U; SunOS sun4u; en-US; rv:1.4) Gecko/20031128 X-Accept-Language: en-US, en X-Spam-Level: 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=0.0 required=5.2 tests=none autolearn=no version=2.60-mulga_r1 Xref: archiver1.google.com comp.std.c++:1183 Julien Lamy wrote: > Hello, > According to paragraph 6 of item 5.3.4 in the Standard, it is legal to > dynamically allocate an array of size 0, i.e. > new int[0] > is legal. > However, as I understand paragraph 1 of item 8.3.4 ("if the cosntant > expression is present, [...] its value shall be greater than zero"), it > is not legal to have > int myArray[0] > What is the rationale behind this difference ? > Thanks in advance, If you could write int myArray[0]; it would have no use. You could not access any elements. In addition, we say that C++ has no zero-sized complete objects. We'd have to make a special case for myArray[0]. C++ currently allows int myArray[n]; only when n is an integer constant expression. So you know at compile time whether the array has any elements, and you can take reasonable steps to avoid trying to create a fixed array with no elements. If nothing else, you could write int myArray[ n<=0 ? 1 : n ]; Now consider int* myArray = new int[n]; If n were not allowed to be zero, the runtime system would have to check for zero elements and generate some kind of runtime error, probably an exception. The program would have to check specially for zero size or be prepared to deal with the exception -- perhaps in many places. If instead the results of the program were undefined, the situation would be worse. The program would run on some systems, not on others. A defensive programmer would have to treat n==0 as a disallowed case. Allowing n to be zero is useful. Functions and algorithms become simpler. You can process all the elements (if any) of myArray using for( int i = 0; i < n; ++n ) { ... myArray[i] ... } You usually won't need special runtime checks for zero size. Finally, "new int[0]" is not a special case. It is analagous to malloc(0). In both cases you get a valid pointer that you cannot dereference. --- Steve Clamage, stephen.clamage@sun.com --- [ 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 ]