220 31702 <5741111.lt9RR1yvSQ@tjmaciei-mobl1> article Path: news.gmane.org!.POSTED!not-for-mail From: Thiago Macieira Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Simplify virtual inheritance rules Date: Sun, 19 Mar 2017 12:10:31 -0700 Lines: 60 Approved: news@gmane.org Message-ID: <5741111.lt9RR1yvSQ@tjmaciei-mobl1> References: <6aa2e439-345e-4f04-8843-f38bb7b825b2@isocpp.org> <2697868.vdYFrBLFDC@tjmaciei-mobl1> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-Trace: blaine.gmane.org 1489950634 31560 195.159.176.226 (19 Mar 2017 19:10:34 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sun, 19 Mar 2017 19:10:34 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCB4TK757YBRBKVPXPDAKGQE5X5PSJI@isocpp.org Sun Mar 19 20:10:30 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yw0-f198.google.com ([209.85.161.198]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1cpgDl-0007g9-Fb for gclcip-std-proposals@m.gmane.org; Sun, 19 Mar 2017 20:10:29 +0100 Original-Received: by mail-yw0-f198.google.com with SMTP id p77sf385172306ywg.3 for ; Sun, 19 Mar 2017 12:10:35 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=from:to:subject:date:message-id:in-reply-to:references:mime-version :content-transfer-encoding:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=WEzOWsxwGxuxtCUE10HmrT5pUeJRWfovaE/NOs+k9OM=; b=Y+oJkXcmHHljG//DYYu54oCy29xhrEHzexnucsb1syx1aEFMHKcjT5cxbPDiCiJ2jS D6zF0U+8QZNK5vlB39G4ifuaSMfmQfH2fCstAcULMkydux+wbtsstB/JyaxbNFpy4p3b rSexkAjq6pMuFEFW0ThraX/FeleSv2ooqxjziBuQRzi9CA3s3vnWGw23frqtzEfycmPL QKwtS+WTFCuW5LuwkQwsRWPsIDbqXp+MHeY1AkNfG6QgyRVJLJ7obayEUgykfD/n/rYC 34NwKM3Dneo7LUO2RRqlOoG74+cKLPRVqNtqi1Zx7z1nbq/KT/YVKkm5YO6x78IL8eHg nFGA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:subject:date:message-id:in-reply-to :references:mime-version:content-transfer-encoding:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=WEzOWsxwGxuxtCUE10HmrT5pUeJRWfovaE/NOs+k9OM=; b=KBMgDtbBlfdLJtvSVME6Wu6av0jC61/xWKKlMKLYtTVmc9MR6Gy6s5bRhE9og8sYf7 VlEve0d8erZaBx2c2O0uyo5NHkxa+TRQq+nOE6GeyA1R0L0wDrX1NPlcniD+8C4iE45r +3kNVXcoKcGKwSBTW6vRkiiD1U8fmbq+q0KtnlrsjHhIK0akhMViO97hX5VuDI27g0bd rs5U25qfpPSnhiPHDPfh2K6MJHvxvDsUzBlWEmlPjc25pMfzRPuJh/djzMDV1HbxwkCB 8g0QGjblw4YQWRjgoywMycLnHeLia7PEVZZTHwEFN/XkaNudRL0N9oXWSR2HrAYXzg/B 5IK X-Gm-Message-State: AFeK/H2cnUo7A4C9Xo+7nofVBWkOlnZ2xb1zVevnDtVAoBbStj+rvTPrEEIGrI7HBSdS0Q== X-Received: by 10.129.101.133 with SMTP id z127mr8574417ywb.148.1489950635239; Sun, 19 Mar 2017 12:10:35 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.25.33 with SMTP id j33ls2349996ota.21.gmail; Sun, 19 Mar 2017 12:10:34 -0700 (PDT) X-Received: by 10.13.200.3 with SMTP id k3mr12416831ywd.157.1489950634259; Sun, 19 Mar 2017 12:10:34 -0700 (PDT) Original-Received: from gondolin.macieira.info (gondolin.macieira.info. [78.47.120.188]) by mx.google.com with ESMTP id j186si4750506ywd.318.2017.03.19.12.10.34 for ; Sun, 19 Mar 2017 12:10:34 -0700 (PDT) Received-SPF: pass (google.com: domain of thiago@macieira.org designates 78.47.120.188 as permitted sender) client-ip=78.47.120.188; Original-Received: from tjmaciei-mobl1.localnet (unknown [IPv6:2601:1c0:4501:5f9f:ad30:6dbd:5d7:7a4d]) by gondolin.macieira.info (Postfix) with ESMTPSA id 3F1BA11B62E for ; Sun, 19 Mar 2017 12:10:33 -0700 (PDT) In-Reply-To: X-Original-Sender: thiago@macieira.org X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of thiago@macieira.org designates 78.47.120.188 as permitted sender) smtp.mailfrom=thiago@macieira.org Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:31702 Archived-At: On domingo, 19 de mar=C3=A7o de 2017 11:32:04 PDT Ville Voutilainen wrote: > On 19 March 2017 at 20:20, Thiago Macieira wrote: > >> Right, so this proposal doesn't produce UB. That leaves the other > >> compatibility issues. > >=20 > > Which ones? >=20 > See a couple of emails ago. The cases where the most-derived class > doesn't have access to the constructor > of the virtual base; that makes intentionally ill-formed cases well-forme= d. Right. This may have been used in existing code to limit inheritance in a t= ree=20 to only classes that are explicitly white-listed as friends at some point. I'm wondering if now allowing this inheritance is a problem. > > Does it make a difference in behaviour which base class actually called > > the > > virtual base's construtor? Is there any situation in which it changes w= ith > > this proposal? >=20 > There may well be cases where the designer of the class hierarchy > intends to force the authors > of derived classes to provide a value to be used for initializing the > virtual base.=20 And in this case, the author of the new class is simply saying "I'll have w= hat=20 he's having" (the direct base class that includes the virtual base). I=20 understand that this technique would have been very useful to force a uniqu= e=20 identifier per class in the hierarchy, but now we're allowing a derived cla= ss=20 not to override the identifier (or silently forget to). Do we need a keyword to specifically allow this new functionality? > It might also be the case > that that value is an invariant for the hierarchy, although I'm not > sure whether anyone writes such hierarchies. > Practically, that means passing 'this' up the hierarchy into the virtual > base. --=20 Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org Software Architect - Intel Open Source Technology Center --=20 You received this message because you are subscribed to the Google Groups "= ISO C++ Standard - Future Proposals" group. To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposals+unsubscribe@isocpp.org. To post to this group, send email to std-proposals@isocpp.org. To view this discussion on the web visit https://groups.google.com/a/isocpp= ..org/d/msgid/std-proposals/5741111.lt9RR1yvSQ%40tjmaciei-mobl1. .