C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Offseting in Structures          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Sekhar Guest Posted: Thu Jun 29, 2006 9:10 am    Post subject: Offseting in Structures While going through one discussion forum i come across one interesting query. Without creating an object how to find the offset of a structure member variable. Something like struct A { double i; int j; } OFFSETOF(A, j) should return offset of member variable j. I come across one standard function in stddef.h where this is already defined as #define offsetof(s,m) (size_t)&(((s *)0)->m) and is working fine and the above snippet is giving me 8 on a VC++ 6.0 compiler Can anyone shed some light of how this is working. Thanks and Regards Sujil C Back to top Jim Langston Guest Posted: Thu Jun 29, 2006 9:10 am    Post subject: Re: Offseting in Structures "Alf P. Steinbach" <alfps (AT) start (DOT) no> wrote in message news:4ghhd9F1kkov1U1 (AT) individual (DOT) net... Quote: * Sekhar: While going through one discussion forum i come across one interesting query. Without creating an object how to find the offset of a structure member variable. Something like struct A { double i; int j; } OFFSETOF(A, j) should return offset of member variable j. I come across one standard function in stddef.h where this is already defined as #define offsetof(s,m) (size_t)&(((s *)0)->m) and is working fine and the above snippet is giving me 8 on a VC++ 6.0 compiler Can anyone shed some light of how this is working. It's compiler specific, relying on knowledge that with this compiler a nullpointer corresponds to address 0, and that with this compiler it can be dereferenced. The standard library /implementation/ can use such compiler-specific features, and this implementation does. It won't necessarily work with other compilers. I don't neccessarily see where it counts a null pointer. It seems the same result would be concluded by: A* ap = 0; size_t offset = (size_t) &ap->m; Back to top Alf P. Steinbach Guest Posted: Thu Jun 29, 2006 9:10 am    Post subject: Re: Offseting in Structures * Sekhar: Quote: While going through one discussion forum i come across one interesting query. Without creating an object how to find the offset of a structure member variable. Something like struct A { double i; int j; } OFFSETOF(A, j) should return offset of member variable j. I come across one standard function in stddef.h where this is already defined as #define offsetof(s,m) (size_t)&(((s *)0)->m) and is working fine and the above snippet is giving me 8 on a VC++ 6.0 compiler Can anyone shed some light of how this is working. It's compiler specific, relying on knowledge that with this compiler a nullpointer corresponds to address 0, and that with this compiler it can be dereferenced. The standard library /implementation/ can use such compiler-specific features, and this implementation does. It won't necessarily work with other compilers. -- A: Because it messes up the order in which people normally read text. Q: Why is it such a bad thing? A: Top-posting. Q: What is the most annoying thing on usenet and in e-mail? Back to top Stuart Redmann Guest Posted: Fri Jun 30, 2006 9:10 am    Post subject: Re: Offseting in Structures BigBrian wrote: Quote: Stuart Redmann wrote: Alf is supposedly a mathematician, because his answer is (a) completely right, and (b) utterly useless. IMHO, Alf's answer wasn't "utterly useless". This is a statement of your opinion, not absolute fact. -Brian Well, I thought people would spot that this comment was meant sarcastically. As an ex-student of theoretical computer science I have deep respect of all mathematicians (including my wife), so I hope Alf won't hold a grudge against me. The original joke goes like this: A man climbs a mountain and gets lost. By chance there is a balloon flying by, so he shouts 'Do you know where you are?' After some time he gets the reply 'In a balloon.' How do we know that the balloon driver is a mathematician? Well, there are three indicators: (a) his answer is well thought about, (b) his answer is completey right, and (c) his answer is utterly useless. Regards, Stuart Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         C++Talk.NET Forum Index -> C++ language (comp.lang.c++) All times are GMT Page 1 of 1   Jump to: Select a forum C/C++----------------C LanguageC Language (Moderated)C++ Language (Moderated)C++ language (comp.lang.c++)C++ language, library and standards Non-English C/C++ Newsgroups----------------C++ (German)C++ (French)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.