From 4734985271937811505 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: f78e5,855073145fb9fdb7 X-Google-Attributes: gidf78e5,public From: ncm@nospam.cantrip.org (Nathan Myers) Subject: Re: Minimum Size of Array in Standard (solution) Date: 1998/11/05 Message-ID: <71p6m5$4m1$1@shell7.ba.best.com>#1/1 X-Deja-AN: 408605702 Approved: Fergus Henderson References: <199811032027.NAA25799@ncar.ucar.EDU> X-Original-Date: 4 Nov 1998 01:27:33 -0800 X-Complaints-To: news@news.unimelb.edu.au X-Trace: izvestia.its.unimelb.edu.au 910257179 16275 128.250.29.17 (5 Nov 1998 09:12:59 GMT) Organization: http://www.cantrip.org/ X-Auth: PGPMoose V1.1 PGP comp.std.c++ iQBFAgUANkFsC+EDnX0m9pzZAQG7nQF/Xc1etDNobxz3udqtuJgd5QpxQ+8lMPPq SPfuH90gzuH53S6jOktPcd9kM9qyRic4 =Fdfe NNTP-Posting-Date: 5 Nov 1998 09:12:59 GMT Newsgroups: comp.std.c++ >> "Jim Cobban" writes: >> ... >> |> This is used in a lot of existing code to >> |> represent an array whose size is known by some implementation specific >> |> method, for example in a structure such as the following: >> |> >> |> struct varstruct >> |> { >> |> unsigned int num_elements; >> |> int elements[0]; >> |> }; >> >> This code is illegal (in C). There is a standard-conforming way to get a similar effect. template struct varstruct { union { unsigned int num_elements; T dummy[(sizeof(unsigned int)+sizeof(T)-1)/sizeof(T)]; }; T* elements() { return (T*)(char*)(this+1); } }; The dummy array ensures that (this+1) is aligned properly for a T. If you have more data members than just num_elements, you can put them in a struct and use that in the sizeof expression in place of unsigned int. Of course you have to make sure there is usable storage at this[1]. -- Nathan Myers ncm@nospam.cantrip.org http://www.cantrip.org/ --- [ 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 ]