From -6196153726978869447 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: f78e5,f1dec9ce20a9c98f X-Google-Attributes: gidf78e5,public From: David R Tribble Subject: Re: null pointers (was: Why's of C++ -- Part 1) Date: 1999/08/14 Message-ID: <37B48FBA.2DE77379@tribble.com>#1/1 X-Deja-AN: 512555421 Content-Transfer-Encoding: 7bit Approved: Fergus Henderson References: <7opqvm$vpa$1@news.hal-pc.org> <7p0mu0$36f$1@mulga.cs.mu.OZ.AU> X-Original-Date: Fri, 13 Aug 1999 16:35:54 -0500 X-Accept-Language: en X-Authentication-Warning: backdraft.briar.org: smap set sender to using -f Content-Type: text/plain; charset=us-ascii X-Complaints-To: news@news.unimelb.edu.au X-Trace: izvestia.its.unimelb.edu.au 934618929 1695 128.250.29.17 (14 Aug 1999 08:22:09 GMT) Organization: The Vast Right-Wing Conspiracy X-Auth: PGPMoose V1.1 PGP comp.std.c++ iQBFAgUAN7UnGeEDnX0m9pzZAQEH0AF8CfteyEH5NHV5zBpX2KFOXq8JG75qvHk5 sBd0E19h0DezHGndksfJMxRDYhjlhDXL =jbdO Mime-Version: 1.0 NNTP-Posting-Date: 14 Aug 1999 08:22:09 GMT Newsgroups: comp.std.c++ Fergus Henderson wrote: >> IMHO the change to disallow `(void *)0' as a null pointer >> constant in C++ was just a bad decision. Pete Becker wrote: > There was no such decision. It is a consequence of the more general > decision (made back in the dawn of time) that pointers to void are not > implicitly convertible to other pointer types. Bjarne Stroustrup wrote: > Please note that I did not make a decision to change C++ in a way that > was incompatible with C; nor did the ISO C++ committee. There simply > was no change. The null pointer in K&R C was defined exactly as in is > now in C++. At some point in its work, the ANSI C committee gave > void* a very special semantics (primarily) to allow things like > > struct S *p = malloc(sizeof(struct C); > > rather than requiring a cast as in K&R C (and C++): > > struct S *p = (struct S *)malloc(sizeof(struct C); > > This relaxation of the type system wasn't necessary in C++ (given > new) and opens a type hole, so I didn't follow suit. By the time > ANSI C was approved (with the void* type hole), C++ had been defined > and used without that hole and compatible with C for about five > years. > > At some point, it was discovered that given ANSI C type rules, > benefits could be had by using (void*)0 as the null pointer. > > I think a good argument can be made for introducing a null pointer > (with some suitable notation) into C++, but I don't see a good > argument for simply adopting the void* hole in the type system from > C. > > The following should be rejected by a compiler: > > char c; > char* pc = &c; > void* pv = pc; // legal C, not legal C++ > int* pi = pv; > int i = *pi; But '(void*)0' could have been deemed a special case in C++, so that it would be treated like a true generic null pointer constant, and not simply as a pointer-to-void. This special case could have been made without opening up the while void* type hole. Given this special handling of '(void*)0', all of the statements above would still be illegal, but the following statements would now be legal in both C and C++: char * pc = (void*)0; void * pv = (void*)0; int * pi = (void*)0; The benefit of this approach over other proposal (such as adding a 'null' or '__null__' keyword, or inventing new syntax like '&0') is that it doesn't affect any existing C++ or C code. It's also a pretty trivial change to make to the standard. -- David R. Tribble, david@tribble.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://reality.sgi.com/austern_mti/std-c++/faq.html ]