From 5501594677066145397 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: f78e5,53d56717e82ab663 X-Google-Attributes: gidf78e5,public From: Jack Klein Subject: Re: cout << null pointers Date: 2000/07/09 Message-ID: #1/1 X-Deja-AN: 643937700 Content-Transfer-Encoding: 7bit Approved: Fergus Henderson , moderator of comp.std.c++ References: <39656D42.E049FCAA@ix.netcom.com> X-Authentication-Warning: mulga.cs.mu.OZ.AU: fjh set sender to devnull@stump.algebra.com using -f Content-Type: text/plain; charset=us-ascii X-Complaints-To: abuse@demon.net X-Mail2News-Path: news.demon.net!mulga.cs.mu.oz.au X-Trace: mail2news.demon.co.uk 963065525 mail2news:1722 mail2news mail2news.demon.co.uk Organization: AT&T Worldnet X-Robomod: STUMP, ichudov@algebra.com (Igor Chudov) MIME-Version: 1.0 NNTP-Posting-Date: Sat, 08 Jul 2000 03:27:00 GMT Newsgroups: comp.std.c++ On Sat, 8 Jul 2000 03:08:48 CST, Paul Meyerholtz wrote in comp.std.c++: > I have found an inconsistent output by various compilers of the following: > > char * p = 0; This initialization makes p a null pointer. It does not point to anything and dereferencing it is undefined behavior. > cout * q = ""; This creates an unnamed array of one character, containing the character value '\0', and points q at that character. It is in a legal memory location and represents a C string of zero length. > cout << p << "\n\n"; Passing a pointer to char to cout causes it to dereference the pointer to access all of the characters in sequence up to, and excluding, the terminating '\0'. Since p is a null pointer, dereferencing it produces undefined behavior. > cout << q; This causes q to be dereferenced, resulting in the char '\0'. Since that is the terminating character, nothing is done. > > Most compilers seem to handle "cout << q" without a problem. (But I > have heard that Visual C++ 6 fails this test). I seriously doubt that it does. It should merely output nothing. > However, the handling of "cout << p" seems to cause problems for most > computers, with results from segment faults to garbage output. > (CodeWarrior for the Mac, which I use, outputs a couple of garbage > characters, but always the same ones). > > Why is the output of p and q different? They are not the same thing. p points to nothing, q points to a 0 length C string, which is a perfectly legal object. > Is there anything in the standard about this? Should cout check for the > null pointer? No, that's up to the programmer. Passing a null pointer to any function member or otherwise that is not specifically defined as accepting a null pointer is a bug, and produces undefined behavior. > Paul Jack Klein -- Home: http://jackklein.home.att.net --- [ 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://reality.sgi.com/austern_mti/std-c++/faq.html ]