From -4703527173574563673 X-Google-Thread: f78e5,3c7d746be594880e,start X-Google-Attributes: gidf78e5,public X-Google-Language: ENGLISH,ASCII Path: g2news1.google.com!news3.google.com!border1.nntp.dca.giganews.com!nntp.giganews.com!local01.nntp.dca.giganews.com!nntp.speakeasy.net!news.speakeasy.net.POSTED!not-for-mail NNTP-Posting-Date: Mon, 12 Mar 2007 13:40:05 -0500 Return-Path: X-Authentication-Warning: mulga.csse.unimelb.edu.au: fjh set sender to devnull@stump.algebra.com using -f X-Robomod: STUMP, ichudov@algebra.com (Igor Chudov) X-Original-To: std-c++@mailman.ucar.edu Delivered-To: std-c++@mailman.ucar.edu From: "=?iso-8859-1?q?Daniel_Kr=FCgler?=" Newsgroups: comp.std.c++ Subject: Concerning n2157 and is_empty specification Organization: http://groups.google.com Message-ID: <1173648852.230192.11800@p10g2000cwp.googlegroups.com> Mime-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" X-Complaints-To: groups-abuse@google.com User-Agent: G2/1.0 X-HTTP-UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; de; rv:1.8.1.2) Gecko/20070219 Firefox/2.0.0.2,gzip(gfe),gzip(gfe) Complaints-To: groups-abuse@google.com Injection-Info: p10g2000cwp.googlegroups.com; posting-host=89.48.199.115; posting-account=0YhmiQ0AAABRDjD_6coNmBVB3rgPlaOq X-Virus-Scanned: amavisd-new at ucar.edu X-Virus-Scanned: amavisd-new at csse.unimelb.edu.au Content-Transfer-Encoding: 8bit X-MIME-Autoconverted: from quoted-printable to 8bit by mulga.csse.unimelb.edu.au id l2BLYdcG021120 Approved: Fergus Henderson , moderator of comp.std.c++ X-Virus-Scanned: amavisd-new at csse.unimelb.edu.au Date: Mon, 12 Mar 2007 13:36:42 CST Lines: 59 NNTP-Posting-Host: 65.182.171.162 X-Trace: sv3-eraWippM1btKIOcARXasaX5iVs12qzwepjGFa5cF+dE1GGf97h5RaU5R9+XE3CiWqPvRqTcNP1V7c+n!fzsPidfHgI5wMmbbXoxF2fn4+1c0JS3twBthgVhMMVl26tN+QbBeHVd+8B3upN3ejQyxL9qY+AwG!I4+F1WofdsfNl8l1z0IkNr15pUm9H6wOvqBMUW5CSA== X-Complaints-To: abuse@speakeasy.net X-DMCA-Complaints-To: abuse@speakeasy.net X-Abuse-and-DMCA-Info: Please be sure to forward a copy of ALL headers X-Abuse-and-DMCA-Info: Otherwise we will be unable to process your complaint properly X-Postfilter: 1.3.34 Xref: g2news1.google.com comp.std.c++:7971 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: class Empty { }; class EmptyToo : public Empty { }; class NonEmpty : public Empty, public EmptyToo { }; 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. The revised wording in n2157, found at http://www2.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2157.html does *not* exclude the above NonEmpty class from is_empty categorization, because it says: " template struct is_empty : public integral_constant {}; If is_class::value is false then b is false. Otherwise T is considered empty if and only if: * 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. Requires: T shall be a complete type, an array type of unknown bounds, or is a void type." Now I wonder whether this is (a) an oversight in the is_empty specification or (b) a deliberate design decision? Greetings from Bremen, Daniel Kr�gler --- [ 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 ]