From -5076173971530212351 X-Google-Language: ENGLISH,ASCII X-Google-Thread: f78e5,f1dec9ce20a9c98f X-Google-Attributes: gidf78e5,public From: James.Kanze@dresdner-bank.com Subject: Re: null pointers (was: Why's of C++ -- Part 1) Date: 1999/08/24 Message-ID: <7puhhs$5ng$1@nnrp1.deja.com>#1/1 X-Deja-AN: 516719508 Approved: Fergus Henderson References: <7opqvm$vpa$1@news.hal-pc.org> <37B7C714.44E4EFB4@lucent.com> <7pcoem$105@abyss.West.Sun.COM> <7pdh80$qaf$1@engnews1.eng.sun.com> <7phdj7$4oj@abyss.West.Sun.COM> <2yG4gOAhsRv3EwJN@robinton.demon.co.uk> <37C16C7E.3C05BBA2@lucent.com> X-Original-Date: Tue, 24 Aug 1999 16:34:04 GMT X-Http-User-Agent: Mozilla/4.06 [de] (WinNT; I) X-Complaints-To: news@news.unimelb.edu.au X-Http-Proxy: 1.0 x24.deja.com:80 (Squid/1.1.22) for client 193.194.7.84 X-Trace: izvestia.its.unimelb.edu.au 935525839 29074 128.250.29.17 (24 Aug 1999 20:17:19 GMT) Organization: Deja.com - Share what you know. Learn what you don't. X-Article-Creation-Date: Tue Aug 24 16:34:04 1999 GMT X-Auth: PGPMoose V1.1 PGP comp.std.c++ iQBFAgUAN8L9tuEDnX0m9pzZAQETzwF+NvYEJvPxvqEcu3Cm2smj54bso73p0Ut+ CwJxV/HGVyVJfwkIJ8AoTaIOAA8z9v+0 =NmLu X-MyDeja-Info: XMYDJUIDkanze NNTP-Posting-Date: 24 Aug 1999 20:17:19 GMT Newsgroups: comp.std.c++ In article <37C16C7E.3C05BBA2@lucent.com>, Salters wrote: > > Francis Glassborow wrote: > > A further problem has just occurred to me. Given an overload set > > including f(int) and f(char *) how do I pass the value of the null > > pointer constant to the second one? > > f(0) resolves in a call to f(int) > > f((void *)0) falls foul of there being no conversion > > f((char *)0) may seem correct but it is not the null pointer constant > > and can be coded differently (and, I believe, actually is on some > > debugging implementations). > > It seems to me that C++ provides no standard conforming mechanism to > > pass a null pointer constant to f(char *) in the presence of any version > > of f that can take an integer type argument. > > I know it usually works in practice, but that is not the point. If I am > > right, there is a defect in the Standard. > Brute force - f( (char *)( (void *)0 ) )? Not necessary. f( (char*)0 ) works perfectly well (guaranteed by the standard). 0 is a null pointer constant, and converting it to a pointer must result in a null pointer. > This must be a pointer to a char, initialized by a null pointer. > Parsing: > char * ( ) // create temporary of type char * > (void *) 0 ) // with this null pointer. > > The problem I fear is the "value of the null pointer constant". > I'm not sure if the standard actually defines such a thing, the > only requirement on null pointers being that they compare equal. Correct. The "null pointer constant" is a syntactical construct. There is, for example, no requirement that the representation of a char* null pointer be the same as that of an int* null pointer. -- James Kanze mailto: James.Kanze@dresdner-bank.com Conseils en informatique orient�e objet/ Beratung in objekt orientierter Datenverarbeitung Ziegelh�ttenweg 17a, 60598 Frankfurt, Germany Tel. +49(069)63198627 Sent via Deja.com http://www.deja.com/ Share what you know. Learn what you don't. --- [ 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 ]