From 4404890062892277276 X-Google-Thread: f78e5,3c7d746be594880e X-Google-Attributes: gidf78e5,public X-Google-Language: ENGLISH,ASCII Path: g2news1.google.com!news2.google.com!news.germany.com!news.osn.de!diablo2.news.osn.de!news.tele.dk!news.tele.dk!small.news.tele.dk!lnewsinpeer00.lnd.ops.eu.uu.net!emea.uu.net!peer-uk.news.demon.net!kibo.news.demon.net!mutlu.news.demon.net!news.demon.co.uk!demon!stump.algebra.com!devnull From: howard.hinnant@gmail.com (Howard Hinnant) Newsgroups: comp.std.c++ Subject: Re: Concerning n2157 and is_empty specification Date: Mon, 12 Mar 2007 19:21:47 GMT Organization: Road Runner High Speed Online http://www.rr.com Lines: 72 Sender: mail2news@demon.net Approved: fjh@cs.mu.oz.au (Fergus Henderson , moderator of comp.std.c++) Message-ID: References: <1173648852.230192.11800@p10g2000cwp.googlegroups.com> NNTP-Posting-Host: news.news.demon.net Mime-Version: 1.0 Content-Type: text/plain; charset=unknown-8bit Content-Transfer-Encoding: quoted-printable X-Trace: news.demon.co.uk 1173727318 20174 158.152.254.254 (12 Mar 2007 19:21:58 GMT) X-Complaints-To: abuse@demon.net NNTP-Posting-Date: Mon, 12 Mar 2007 19:21:58 +0000 (UTC) X-Original-To: std-c++@mailman.ucar.edu X-Robomod: STUMP, ichudov@algebra.com (Igor Chudov) X-User-Agent: MT-NewsWatcher/3.5.2 (Intel Mac OS X) X-MIME-Autoconverted: from 8bit to quoted-printable by mulga.csse.unimelb.edu.au id l2CJLsRr011882 X-Source: X-Virus-Scanned: amavisd-new at csse.unimelb.edu.au X-Source-Dir: X-Path: comp-std-cpp-robomod!not-for-mail X-Received: (from fjh@localhost) by mulga.csse.unimelb.edu.au (8.13.8+Sun/8.13.8/Submit) id l2CJLl6k011810; Tue, 13 Mar 2007 06:21:47 +1100 (EST) X-Delivered-To: std-c++@mailman.ucar.edu X-Authentication-Warning: mulga.csse.unimelb.edu.au: fjh set sender to devnull@stump.algebra.com using -f X-Source-Args: X-AntiAbuse: Sender Address Domain - gmail.com X-Newsgroups: comp.std.c++ Xref: g2news1.google.com comp.std.c++:7973 In article <1173648852.230192.11800@p10g2000cwp.googlegroups.com>, "Daniel Kr=FCgler" wrote: > Just recently I stumbled once again since a longer > time across chapter 16.2.1 of the first edition of > "C++ Templates" from Vandevoorde/Josuttis, which > explains the Empty Base Class Optimization (EBCO) > principle as well as its limits. > One interesting example is given, which must necessarily > forbid EBCO: >=20 > class Empty { > }; >=20 > class EmptyToo : public Empty { > }; >=20 > class NonEmpty : public Empty, public EmptyToo { > }; >=20 > The reasoning is that NonEmpty cannot be an empty > class, because this would have the effect that two > subobjects of the same type would end up at the same > offset, which is not permitted by C++ object layout > rules. >=20 > The revised wording in n2157, found at >=20 > http://www2.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2157.html >=20 > does *not* exclude the above NonEmpty class from > is_empty categorization, because it says: >=20 > " template struct is_empty : public > integral_constant {}; >=20 > If is_class::value is false then b is false. Otherwise T is > considered empty if and only if: >=20 > * T has no non-static data members, or all non-static data > members, if any, are bit-fields of length 0, and > * T has no virtual members, and > * T has no virtual base classes, and > * T has no base classes B for which is_empty::value is false. >=20 > Requires: T shall be a complete type, an array type of unknown bounds, > or is a void type." >=20 > Now I wonder whether this is (a) an oversight in the is_empty > specification or (b) a deliberate design decision? It is an oversight. This was an attempt to give meaning to "empty=20 class". Note the (marked out) definition it is replacing. =20 Unfortunately "empty class" isn't defined elsewhere in the standard,=20 though the phrase is occasionally used. Perhaps the best thing to do would be to go back to the old definition=20 for is_empty, but insist that it be defined somewhere in the language=20 spec. And then is_empty can refer to that spec. That makes it somebody=20 else's problem. :-) Although, that definition may not be compatible=20 with our intent for is_empty... Suggestions for better wording welcome. -Howard --- [ 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.comeaucomputing.com/csc/faq.html ]