From 5210990963254891896 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: f78e5,10e355266b6fd117 X-Google-Attributes: gidf78e5,public X-Google-Thread: fc772,10e355266b6fd117 X-Google-Attributes: gidfc772,public X-Google-ArrivalTime: 2002-05-28 19:52:11 PST Path: archiver1.google.com!news1.google.com!newsfeed.stanford.edu!news.uchicago.edu!newsfeed.cs.wisc.edu!nnxp1.twtelecom.net!209.98.3.200.MISMATCH!priapus.visi.com!news-out.visi.com!hermes.visi.com!feed.news.qwest.net!namche.sun.com!news2me.EBay.Sun.COM!engnews1.eng.sun.com!taumet!clamage From: Attila Feher Newsgroups: comp.std.c++,comp.lang.c++.moderated Subject: Re: A humble attempt to suggest alignment support for C++ Date: 29 May 2002 02:42:07 GMT Organization: Oy L M Ericsson Ab Lines: 83 Approved: stephen.clamage@sun.com (comp.std.c++) Message-ID: <3CF3B6F6.5DE49C64@lmf.ericsson.se> References: <3CEA1A08.C6D95CC7@lmf.ericsson.se> <53e3f3b2.0205270557.bdd703d@posting.google.com> NNTP-Posting-Host: taumet.eng.sun.com X-NNTP-Posting-Host: netlab.cs.rpi.edu X-Original-Date: Tue, 28 May 2002 19:57:26 +0300 X-Submission-Address: c++-submit@netlab.cs.rpi.edu X-Auth: PGPMoose V1.1 PGP comp.lang.c++.moderated iQBVAwUAPPPxSUHMCo9UcraBAQGocAH/fGw9Igpa/RgZrsbkDb3IBatl/ftWqk6E N6kABryJQTd41I9hg1J3eFw9MCEo1IoLZs2ANyG2q+bNmAVgsYPOyw== =NUVV X-Approved-For-Group: hsutter@acm.org comp.lang.c++.moderated X-Scanned-By: MIMEDefang 2.3 (www dot roaringpenguin dot com slash mimedefang) Content-Length: 3378 Originator: clamage@taumet Xref: archiver1.google.com comp.std.c++:11646 comp.lang.c++.moderated:44097 McNepp wrote: [SNIP] > Yes, portable support for alignment is long overdue. > But why not keep it as simple as possible? I guess what I have suggested is simple. > We want this declaration: > > char buffer[ count * sizeof(T) ]; > > to be a suitably aligned buffer for any type T. Nope. I wan't it to be suitably aligned to the _specific_ type T used there! Not _any_, because this means wasting of memory! > So what is really needed is just a simple specifier (e.g. "max_align") > that does the trick: > > char max_align buffer[ count * sizeof(T) ]; One point of my proposal was to avoid using maximum alignment everywhere. > Now we can safely cast this buffer to a T-array, using two > static_casts... which immediately triggers the question: Why not skip > the new keyword altogether and even get rid of one of the > static_casts? What about allowing this syntax: > > void buffer[ count * sizeof(T) ]; > > This generic buffer can be safely cast (with one simple static_cast) > to any type T. No new keywords needed, simple and self-explanatory, > isn't it? It is simple, but it isn't obvious (self explanatory) at all. And what if I have already (for some reason) the size given to me? So the type T will not even be mentioned in this line, unless there is a separate aligment specification? Also your proposal does not support if I need to align any arbitrary pointer. Like I have a buffer in shared memory and I would like to place there an object, which needs alignment. I have a pointer to the first free byte. So I guess why being simple is A OK with me, being ambiguously implicit ends up being "too simple". I believe that this aligment issue needs two kind of support: an alignment specifier and an "aligment calculator" taking a void * plus a length and returning void * to the first suitable address, where type T can be constructed _or_ throws std::ollala if it cannot be done within the given buffer size. Or you can give "one past end iterator" as well (instead of length), I guess it even would be better. When I say we need these two I mean that we need if we want to have a complete solution and forget about the problem. And of course: we might need to allow user defined extension (that's the reason why for the template syntax) of these two functions (well, I have no idea how could we actually do sth for the aligment specifier part) so that people can create their own special aligment requirements for their own special types. But I guess once we enter this zone, the implementation will _always_ be platform specific, so we can leave it out as well - since the whole idea was to remain platform independent. And whoever needs any more wonders can just use the platform specific address arithmetics to create a good enough pointer. Since these special cases of more strict aligment requirements (I guess) can only come up if some special HW requires it in low level code. Attila [ Send an empty e-mail to c++-help@netlab.cs.rpi.edu for info ] [ about comp.lang.c++.moderated. First time posters: do this! ] [ 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://www.jamesd.demon.co.uk/csc/faq.html ]