From -3946379657406234215 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: fc772,10e355266b6fd117 X-Google-Attributes: gidfc772,public X-Google-Thread: f78e5,10e355266b6fd117 X-Google-Attributes: gidf78e5,public X-Google-ArrivalTime: 2002-05-30 09:22:11 PST Path: archiver1.google.com!news1.google.com!newsfeed.stanford.edu!bloom-beacon.mit.edu!nycmny1-snh1.gtei.net!washdc3-snh1.gtei.net!denver-snf1.gtei.net!news.gtei.net!namche.sun.com!sunnews1.Eng.Sun.COM!engnews1.eng.sun.com!taumet!clamage From: Christopher Eltschka Newsgroups: comp.std.c++,comp.lang.c++.moderated Subject: Re: A humble attempt to suggest alignment support for C++ Date: 30 May 2002 16:17:20 GMT Organization: Vienna University of Technology, Austria Lines: 129 Approved: stephen.clamage@sun.com (comp.std.c++) Message-ID: References: <3CEA1A08.C6D95CC7@lmf.ericsson.se> NNTP-Posting-Host: taumet.eng.sun.com X-NNTP-Posting-Host: netlab.cs.rpi.edu X-Original-Date: 29 May 2002 20:08:10 +0200 X-Submission-Address: c++-submit@netlab.cs.rpi.edu X-Auth: PGPMoose V1.1 PGP comp.lang.c++.moderated iQBVAwUAPPZD7UHMCo9UcraBAQGSLAH9FLveA3pHbtxYkva0pzeQYWKHgjCvK6aP XG1eRL4E4JyF2Wp9P58F+GXoHfalAo1rrm3v+VTwnX8aomzzJKjehA== =/rFm 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: 3990 Originator: clamage@taumet Xref: archiver1.google.com comp.std.c++:11707 comp.lang.c++.moderated:44251 Attila Feher writes: [...] > Solution #1: > ------------ > > This solution is more work for the users of the language, the way to > solve the problem is not so elegant, but it is probably easy to > implement. > > Two additions to the language are required here: > > 1.) max_align(T) compile time operator(?) > > 2.) T *std::align( void *ptr[, std::size_t size]) runtime function > > The max_align works like sizeof. It gives the maximum > "alignment loss" in bytes one can get with the type T. So if type T > needs to be aligned on 8 bytes boundary, max_align(T) would give 7, > because we can loose max. 7 bytes, if we align this type. > > The align(ptr[, size]) template like function would give return an > address _after_ ptr, which is properly aligned for type T. The caller > can then create a type T at that address using replacement new. > > The second overload of align would check if it aligns ptr to a T is > there any space left in those size bytes to create a T. If not, it > throws some std::whatever_we_call_it. > > The max_align would help in allocation: > > template > class fsdc_array { > > T *arr_; > char buff_[sizeof(T)+max_align(T)]; > }; > > The align would help in figuring out the address of the first element: > > template blabla > fsdc_array::fsdc_array() { > arr_ = align( buff_); > } > > The code of the array template could then address its elements via > arr_. > > > Solution #2: > ------------ > > This solution requires a new, special keyword in the declarations, > hides all the know-how, and therefore a bit more complicated for the > implementors but much easier for the users. And it is less flexible > than #1, since with #1 I can even create a T *tp with align, then say > Y *yp=align( tp+1); and position a Y after the T in the memory. > > This solution would enable the coder to change alignment of any type > or just char arrays in declarations. As it seems it is enough for > just char arrays, but you may see a good reason for open it up to > arrays or any other type. > > It could look something like this: > > align_as char T_arr[sizeof(T)*S]; > > So align_as would be a sort of "storage specifier like thing", > possibly behaving the same as static from syntax point of view. > Except that it would not "collide" with any storage specifiers, since > it is a different > thing: alignment specifier. > > > Solution #3: > ------------ > > Provide both #1 and #2, allowing the nice "tricks" described about #1 > under #2 (opening up a great flexibility) as well as give an easy way > to do the easy thing. Solution #4: Provide a built-in "class template" align with the following properties: a) align has the samme alignment restrictions as T b) align has the smallest size compatrible with those alignment restrictions (i.e. sizeof(align) gives the alignment of T) c) align is POD As a special case, make align be suitable allignd for Now you can get an aligned memory area f.ex. with union { align int_alignment; // forces alignment unsigned char intarr[100*sizeof(int)]; // aligned for int }; union { align void_alignment; unsigned char arr[10000]; // aligned for arbitrary type } char* p = arr + sizeof(align); // p is the second position in arr correctly aligned for double char* q = arr + sizeof(align); // q is the second position in arr correctly aligned for any type [...] [ 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. 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