From 1255969866566680117 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: f78e5,2847ea2daacb5072 X-Google-Attributes: gidf78e5,public X-Google-ArrivalTime: 2004-04-17 10:36:21 PST Path: archiver1.google.com!news2.google.com!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: qg4h9ykc5m@yahoo.com (David Olsen) Newsgroups: comp.std.c++ Subject: Re: Proposal: free() and delete/delete[] compatibility Date: Sat, 17 Apr 2004 17:36:19 +0000 (UTC) Lines: 43 Approved: fjh@cs.mu.oz.au (Fergus Henderson , moderator of comp.std.c++) Message-ID: References: Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit X-Trace: mail2news.demon.co.uk 1082223379 15774 10.0.0.1 (17 Apr 2004 17:36:19 GMT) X-Complaints-To: abuse@demon.net NNTP-Posting-Date: Sat, 17 Apr 2004 17:36:19 +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 1BEtjm-00046H-00 for mail2news@news.news.demon.net; Sat, 17 Apr 2004 17:36:18 +0000 X-Received: from mulga.cs.mu.OZ.AU (localhost [127.0.0.1]) by mulga.cs.mu.OZ.AU with ESMTP id i3HHaGi2014292; Sun, 18 Apr 2004 03:36:16 +1000 (EST) X-Received: (from fjh@localhost) by mulga.cs.mu.OZ.AU (8.12.10+Sun/8.12.9/Submit) id i3HHaFTq014278; Sun, 18 Apr 2004 03:36:15 +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-Newsgroups: comp.std.c++ X-Orig-NNTP-Posting-Host: bi01p1.co.us.ibm.com (32.97.110.142) X-Orig-X-Trace: news.uni-berlin.de 1082172494 4823686 I 32.97.110.142 ([214800]) X-User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.5) Gecko/20031007 X-Accept-Language: en-us, en X-Spamscanner: mailbox4.ucsd.edu (v1.4 Mar 10 2004 15:18:19, 5.6/5.0 2.63) X-MailScanner: PASSED (v1.2.8 35237 i3H3SFv6041655 mailbox4.ucsd.edu) 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=2.3 required=5.2 tests=FORGED_YAHOO_RCVD, FROM_HAS_MIXED_NUMS,FROM_HAS_MIXED_NUMS3 autolearn=no version=2.60-mulga_r1 Xref: archiver1.google.com comp.std.c++:1724 Ioannis Vranos wrote: > Proposal: A guarantee in the standard that if you call free() upon > objects created by new/new[] ,and a call of delete/delete[] on > objects created by malloc()/calloc()/realloc() is safe. > > I am not talking about the constructors issue where they are not > called in the case of malloc() family, just the interchange ability > between free() and the delete family. I immediately see two problems, apart from the constructor/destructor problem: 1. The program may have its own user-defined versions of operator new and operator delete, and these don't have to use malloc/free. So the proposed guarantee can't apply when user-defined versions of the operators are used. But when writing code that uses new and delete, there is no way to force the standard versions of operator new and operator delete to be used, so there is no way to know for sure that the guarantee applies. 2. A "new" expression that allocates an array is allowed to allocate some extra space at the front of the array. This is usually done to store the number of items in the array so delete[] will call the destructor the correct number of times. A delete[] expression knows about this and knows how to correctly adjust the pointer that it passes to operator delete[]. So, assuming that operator new[] calls operator new, which calls malloc, and given the following code: T *x = new T[10]; free((void*)x); The address returned by malloc (within operator new) might not be the address that is passed to free. So therefore the guarantee can't apply to new[] and delete[]. -- David Olsen qg4h9ykc5m@yahoo.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 ]