220 31696 article Path: news.gmane.org!.POSTED!not-for-mail From: Ville Voutilainen Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Simplify virtual inheritance rules Date: Sun, 19 Mar 2017 15:24:33 +0200 Lines: 58 Approved: news@gmane.org Message-ID: References: <6aa2e439-345e-4f04-8843-f38bb7b825b2@isocpp.org> <2340e1b0-09bd-4f5b-bf15-7605eaed858b@isocpp.org> <25e054ca-aac4-4bcc-805a-4e1357de4f37@isocpp.org> <633f2640-aef5-46a2-9ae2-6db626bc47fb@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 X-Trace: blaine.gmane.org 1489929875 2431 195.159.176.226 (19 Mar 2017 13:24:35 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sun, 19 Mar 2017 13:24:35 +0000 (UTC) To: "ISO C++ Standard - Future Proposals" Original-X-From: std-proposals+bncBC5JHI7A7ALRBEUNXLDAKGQEMSBRTAI@isocpp.org Sun Mar 19 14:24:31 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yw0-f197.google.com ([209.85.161.197]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1cpaov-0008QB-1z for gclcip-std-proposals@m.gmane.org; Sun, 19 Mar 2017 14:24:29 +0100 Original-Received: by mail-yw0-f197.google.com with SMTP id k13sf371225371ywk.2 for ; Sun, 19 Mar 2017 06:24:35 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=mime-version:in-reply-to:references:from:date:message-id:subject:to :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=AMPolLMGbHNOkIxPb7fi+skAnfM0zLbG3QtriFNond0=; b=fBse8T+jSf7NnxKqWN1Hgv5ZHSG1n0QzuT7Qj6dxevyRcoB+SFEznzzKew86e08L+8 0by9TRBc9PWXEqK5Ze7KkDJ2WDRCctXvWnJKwjJfQi+5SUENSPBbs4t4Mo2kG64/wrQx WSbUiPXSiqdO7FD6I5ukeMbQb0GPt2vHVEzB2ea6a2lzzlrsgHI4L+75VQ7PgVaBhUrT DwJQEYCm+pCsf1f++g07Xy6GgnYQaTmct0j0uUKV6ot/FUl4v0x5WhIOH8EiuY8gigd1 OEJ5bs0Ac/yHkFGclxjBFpemqhHLSsIF9qrCKONRcMTn+hNfx7Ona9gf+2cgqfiSB2jG g4Gg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:in-reply-to:references:from:date :message-id:subject:to: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=AMPolLMGbHNOkIxPb7fi+skAnfM0zLbG3QtriFNond0=; b=ETXCjotqE7jVo4X/D+ZjM8MfYIharDC4OuI5+v05UtOOJD2NyrgyPe3LqU2/VonWjY 0Gf2b0P1dP7MiH5zAuyXFVyW9D5Kb2NE92A96/LLHWKriqOeDHzOTCI7aB7f3la24fAH 0ZYv20zVWAyV8WQKaB0vbamxQ51ZefM30wIe2GcEi7fHZoZINVf/oG6mWZ1QqhIX8jVb L/RsL1P6cgUOq/2CSMKz4fSAMdBdGEluZE388sohdvhiaVmmrT2yL0qKRSBZL2Sl3YQg fPeq5OzY5lcb4rBzAugKvJoQ5wgCabj8IDGzOwnHpYlYX9yCPRrctMazjsaumPAV/5PK sl/w== X-Gm-Message-State: AFeK/H0KSwejq9pvWLzTvz3d7Roke0fNz/pjaCnRk+A7acAAFf9ZgtYv1qd5qIiwLj0p1w== X-Received: by 10.13.200.3 with SMTP id k3mr11889194ywd.136.1489929874880; Sun, 19 Mar 2017 06:24:34 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.11.3 with SMTP id a3ls10732519ota.23.gmail; Sun, 19 Mar 2017 06:24:34 -0700 (PDT) X-Received: by 10.202.235.214 with SMTP id j205mr2179474oih.209.1489929874306; Sun, 19 Mar 2017 06:24:34 -0700 (PDT) Original-Received: from mail-oi0-x230.google.com (mail-oi0-x230.google.com. [2607:f8b0:4003:c06::230]) by mx.google.com with ESMTPS id e140si5737415oih.71.2017.03.19.06.24.34 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 19 Mar 2017 06:24:34 -0700 (PDT) Received-SPF: pass (google.com: domain of ville.voutilainen@gmail.com designates 2607:f8b0:4003:c06::230 as permitted sender) client-ip=2607:f8b0:4003:c06::230; Original-Received: by mail-oi0-x230.google.com with SMTP id a94so12466249oic.2 for ; Sun, 19 Mar 2017 06:24:34 -0700 (PDT) X-Received: by 10.202.58.69 with SMTP id h66mr11514186oia.30.1489929873994; Sun, 19 Mar 2017 06:24:33 -0700 (PDT) Original-Received: by 10.157.13.22 with HTTP; Sun, 19 Mar 2017 06:24:33 -0700 (PDT) In-Reply-To: X-Original-Sender: ville.voutilainen@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com; spf=pass (google.com: domain of ville.voutilainen@gmail.com designates 2607:f8b0:4003:c06::230 as permitted sender) smtp.mailfrom=ville.voutilainen@gmail.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=gmail.com 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:31696 Archived-At: On 19 March 2017 at 15:07, Ville Voutilainen wrote: > On 19 March 2017 at 14:55, Denis Kotov wrote: >> Ville Voutilainen, >> >> That is the point: We will anyway know that VirtualBase is performed, but by >> parent class where two VirtualBase (VC class) class inherites in class C3. > > There are situations where the bases do not need to initialize the virtual base: > http://open-std.org/JTC1/SC22/WG21/docs/cwg_defects.html#1611 However, thanks for the clarification, so what we have here is 1) if the intermediate bases are non-abstract, they will have compiled the virtual base initialization into them 2) therefore the more-derived class wouldn't need to do it So we don't have a case of UB. We have cases where code that wasn't previously valid will now be valid, though: struct B { private: B(int) {} friend class D; }; struct D : virtual B // thou shalt not initialize me from a derived class { D() : B(42) {} }; struct E : D { E() : B(42) {} // this attempt to initialize D is ill-formed }; int main() { D d; // ok E e; // we can't even get this far } With your proposal, E could skip the initialization of B and be well-formed. Another problem with the proposal is that today it's possible to write class hierarchies that rely on the most-derived class being the one to initialize the virtual base. This proposal changes how such hierarchies behave. -- 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 email 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/CAFk2RUam2MkSDw_6-%3DZ%2BJppmnJ0q%3DuCc99q0RZ5--U5AqwqBbQ%40mail.gmail.com. .