From 3989623712584827855 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: f78e5,28671cb074853115 X-Google-Attributes: gidf78e5,public From: phalpern@truffle.ma.ultranet.com (Pablo Halpern) Subject: Re: offsetof macro, can it be used in non-trivial classes? Date: 1996/12/27 Message-ID: <32c50648.4321059@news.ma.ultranet.com>#1/1 X-Deja-AN: 206252322 x-nntp-posting-host: truffle.ultranet.com references: <01bbead3$6fbf55f0$c07797cd@hacknowledge> content-length: 1076 content-type: text/plain; charset=us-ascii organization: UltraNet Communications, Inc. mime-version: 1.0 newsgroups: comp.std.c++ originator: clamage@taumet Jean-Louis Leroy <100611.1330@compuserve.com> wrote: ><< How reliable is the offsetof macro when used on a complex class under MI? >> > >As the others say, it's a bad idea. In most situations, pointers to data members >will do the job better. The problem with pointers to data members is that they only work one way. As far as I know, there is no portable solution to the following problem: // Assume bar is not a POD, so foo is not a POD struct foo : public bar { int x, y, z; }; int f(int *p) { // Problem: If we know that p is the y member of a foo object, // get the address of the entire foo object } The above problem would be solvable if &foo::y could be applied backwards. It would also be solvable using offsetof() if foo were a POD. Fortunately, there are very few situations like this that couldn't be fixed by using a better design. ------------------------------------------------------------- Pablo Halpern phalpern@truffle.ultranet.com I am self-employed. Therefore, my opinions *do* represent those of my employer. [ 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 ] [ FAQ: http://reality.sgi.com/employees/austern_mti/std-c++/faq.html ] [ Policy: http://reality.sgi.com/employees/austern_mti/std-c++/policy.html ] [ Comments? mailto:std-c++-request@ncar.ucar.edu ]