220 35269 <1540337.mpxU4uUWLP@tjmaciei-mobl1> article Path: news.gmane.org!.POSTED!not-for-mail From: Thiago Macieira Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: Overriding virtual functions of member class Date: Sat, 04 Nov 2017 12:25:53 -0700 Lines: 60 Approved: news@gmane.org Message-ID: <1540337.mpxU4uUWLP@tjmaciei-mobl1> References: <26216c34-9ff6-435b-8da5-9b21460fee1c@isocpp.org> <389229bd-7f50-4efa-b6c9-177df26bcd0a@isocpp.org> <5b06f168-cde4-45a9-8614-6025c8b16e85@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" Content-Transfer-Encoding: quoted-printable X-Trace: blaine.gmane.org 1509823557 5999 195.159.176.226 (4 Nov 2017 19:25:57 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sat, 4 Nov 2017 19:25:57 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCB4TK757YBRBQ5I7DHQKGQE4RD5UGY@isocpp.org Sat Nov 04 20:25:51 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-wr0-f200.google.com ([209.85.128.200]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1eB44j-0000ya-Dy for gclcip-std-proposals@m.gmane.org; Sat, 04 Nov 2017 20:25:49 +0100 Original-Received: by mail-wr0-f200.google.com with SMTP id o44sf3586827wrf.0 for ; Sat, 04 Nov 2017 12:25:57 -0700 (PDT) ARC-Seal: i=2; a=rsa-sha256; t=1509823557; cv=pass; d=google.com; s=arc-20160816; b=QdbyziHf6t1fGFhbgmNw5SwzkceKGDohPGAH4hWOqsGXiNZ6pg/VoVspY5R2JUnzzw 3WxJ2mxBkWcihukPPt7yxdL2/QBgKtXwxi92RLPbXim6iSBzaS2QMIyygblH4BVwwGNE rFSywPgzvUGZKihOrUrGyL/85MiFfndkkh+bqT+S2/5CfXdSr6NdtBzXjDVO3Ah93sQu MLZDH73vqnvwPlsk2VeHnr2QrI7aV/DD8HIT4GyZ7P9WIukPDJXzk8aGU1TTfBhwxwpp MrpzzYEAjR2DkHtkW207QkICgUQ161e77dg5vyn4Hn4j5Ni5uG0VLgCFgt1BCx+mTVYK JUPw== ARC-Message-Signature: i=2; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=list-unsubscribe:list-subscribe:list-archive:list-help:list-post :list-id:mailing-list:precedence:reply-to:content-transfer-encoding :mime-version:references:in-reply-to:message-id:date:subject:to:from :arc-authentication-results:arc-message-signature:dkim-signature :arc-authentication-results; bh=qbEiRr3SAVOt+QmtweO9SF5r6ARqqVRBFqSTqlwffGE=; b=Nwx2p2fMjjj8o7ioJmT6beIuMBJV43KYpOyAj4BBDezAaCkWwaH/nyrtUoSJUFsoKQ 5FsHyDBFma8erly67G7CtrFI10Bz8SuP/bhiIMBCQoKwGneQCdpH+Gz0JjXXJvlujDgz SfaB+1qFPZwuTTL3pcITBfyto0deLqrOYk1QRR3TDJe4bNHQmJq4KasdJ2maTitMeWgf nYHNMnPXV1+2B4sCNnSoOL24CEJaOprZc35VjvfoJmdsNwaInKAb4ek9SZCqzRoOui8Q tk0PgVncMP5/Cpw4b2Ka8MtDNhFlAC3/1K3/SWYjyfjvhcMC+iX+0Lyy2X29TrA5hnA2 iHKw== ARC-Authentication-Results: i=2; mx.google.com; spf=pass (google.com: domain of thiago@macieira.org designates 2a01:4f8:d13:f81:21c:14ff:fe01:12a3 as permitted sender) smtp.mailfrom=thiago@macieira.org 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:list-post:list-help:list-archive:list-subscribe :list-unsubscribe; bh=qbEiRr3SAVOt+QmtweO9SF5r6ARqqVRBFqSTqlwffGE=; b=MzDErwPAuA2laTq3OqSatvbGSu2m+eHF7gSrEoiviq0iBzEJr7q/wD1L3w28O6odSg cwffyZbZgze4z3IJLOoO/jdjA8i9je+p1jwGRYtODM7byP4AUIpzOCLDgiN2yU1bVJG6 p823oi4BKPj9Jq7eAUzT/HdW0Of9ysiTgujavXRhKXBnRj0Fm4DeULyCPxwLHK5GDL1l b9UrAaiQc6PD364cvUC+1OsW1r9VptiaYqyNSxgQhzarBPNlx1x44pU5Yc/2bs56e3Ag 8yploxEs9iPpsJBDF5L4C/xfHxbZUKyzycVdn+WKqPra4KQDS2N5yycKI1WX9lNN7vnw Q+Fw== 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=qbEiRr3SAVOt+QmtweO9SF5r6ARqqVRBFqSTqlwffGE=; b=HBUNUQspfU7Hf0vJb6wEYJvmMEwU/7Sh09E4EUaJHicufM1C7Y5AezRQJByWuYG9rP xQO6neZjhkrVSFPgMzuFBxPlfbwdSA3mDPHAbzuGAAGgjNL3xsiMzlWg0GCVN4SUQy4S gRym0/O7NdPRWMB57BUC41sNh0pf6xFFe6C2AbH6w8C5QWtEJxZJuCgbjVAwuiEs49Lp CgczLiW2qPWrDxzySYCr5NUUtrsUk8/p8dy/YpdsYravbhiKuOiZkUCJIW8ukGaCvTx7 tR87tw9Fn4l0M6Y09HZzwosuvAPF0BgPEPxi7PHZSd8B4pUH6eE+U+ePZovywTVyt/+Y ayW X-Gm-Message-State: AJaThX4UxWrn3QvuoKLG5UQh2io1jB1dgv0vS+7OC05NsTIs30zTqak8 lPq4++zkHab+Q91ctyaFKwhsMg== X-Google-Smtp-Source: ABhQp+RlC0hFhjj7gesk8nsAzVGnJth+SsmlSyO5dI1yOiF0OsQlrnw7/vdf/4nmtaoear2vp8T7rQ== X-Received: by 10.28.7.75 with SMTP id 72mr425717wmh.23.1509823556968; Sat, 04 Nov 2017 12:25:56 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.28.103.131 with SMTP id b125ls775731wmc.12.gmail; Sat, 04 Nov 2017 12:25:55 -0700 (PDT) X-Received: by 10.223.148.38 with SMTP id 35mr9624081wrq.49.1509823555602; Sat, 04 Nov 2017 12:25:55 -0700 (PDT) ARC-Seal: i=1; a=rsa-sha256; t=1509823555; cv=none; d=google.com; s=arc-20160816; b=WAxsu9ZCc/UFiCk1N2zqRSryUkiU3cGcEnklIjFQuRSSn2WOHKDE8cXpwJFcTLie4x rgq2tIsC+TC8mJnD/2pd0fCfuuHNwMRPxOTqGQgOrI4iugHDhhwcVAOBovj2SICihUsM rw9+1yzfZNr+bERuT5Y9el8zrCPPJ+O77vORSuDkVVzybySJN7dOsqOvrsZBf0rsCA+E zyca3JmYl6VHI3nAUAiZAWn24pDjsfDjJGM7vk3g4ha431Tq7aAufs4MxgET9c+/Ywye nW+P+aADujpSxggjORuH3SMMVO4eugASGo3800aEyaUxbM4WOoqfQUlWGy+6Fk+GTQWM fiUw== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:to:from:arc-authentication-results; bh=SmGUn25z1NKL6hg3G6VnmFhtLCqf9jUaSMonhc42WrE=; b=LyXAj0+DqQfnWGxLs7kurbz9P5vztkQrx3BG+M/j+hAeW0Iet0B/PSJhtQ+YkD/jQH UQ9IQjqEtLZJe8yi5HC/CdSsPH2tRGMN4WfiPlSfUJmsiZd8EB4Ixqin7Ml7ulwOl2Hq +ODCvIjJoIXjpU0SeYFCMXV6oFJqneMwkV1pN9gf7L/sHgumZRrmCOy5NipsPIw8hCtr n3jgOFX3g53kxXbD85WhWMTpk7GEWpl+TmeQgFYIPa2rYExRryp6z6ehEyMuNcuYnmQ8 VI7pNxs2oVhg10bNDCTsQwA868BK93jJt9GQrSeRAhlSF/fq0QKSlnWFzl/CDO3/h6Jd 9rPw== ARC-Authentication-Results: i=1; mx.google.com; spf=pass (google.com: domain of thiago@macieira.org designates 2a01:4f8:d13:f81:21c:14ff:fe01:12a3 as permitted sender) smtp.mailfrom=thiago@macieira.org Original-Received: from gondolin.macieira.info (gondolin.macieira.info. [2a01:4f8:d13:f81:21c:14ff:fe01:12a3]) by mx.google.com with ESMTP id o81si8522134wrb.226.2017.11.04.12.25.55 for ; Sat, 04 Nov 2017 12:25:55 -0700 (PDT) Received-SPF: pass (google.com: domain of thiago@macieira.org designates 2a01:4f8:d13:f81:21c:14ff:fe01:12a3 as permitted sender) client-ip=2a01:4f8:d13:f81:21c:14ff:fe01:12a3; Original-Received: from tjmaciei-mobl1.localnet (unknown [IPv6:2601:1c0:4501:5f9f:c4cb:f753:d6a5:e233]) by gondolin.macieira.info (Postfix) with ESMTPSA id 1597C11B6FE for ; Sat, 4 Nov 2017 12:25:55 -0700 (PDT) In-Reply-To: <5b06f168-cde4-45a9-8614-6025c8b16e85@isocpp.org> X-Original-Sender: thiago@macieira.org X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of thiago@macieira.org designates 2a01:4f8:d13:f81:21c:14ff:fe01:12a3 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:35269 Archived-At: On s=C3=A1bado, 4 de novembro de 2017 04:07:46 PDT ambrop7@gmail.com wrote: > > struct A { > > virtual void virtualFunction() =3D 0; > > > >}; > > > >struct B { > > > > struct A1 : A { > > void virtualFunction() override; > > } m_a1; > > struct A2 : A { > > void virtualFunction() override; > > } m_a2; > > > >}; >=20 > The issue with this approach is that the implementations of > virtualFunctions() will need to get B* from a pointer to A1*/A2*. That > cannot be done in general (only when B is a standard-layout class by usin= g > offsetof). So to be portable code, A1 and A2 would each need to have a "B > *parent" member. That adds size overhead and requires boilerplate. Correct. And remember that offsetof is restricted for a reason. If you're not allowe= d=20 to use it, what makes you think the compiler could resolve the pointer from= A=20 back to B without storing a pointer somewhere? Since B is not standard layo= ut,=20 a subclass C of B could change the offsets from m_a1 and m_a2 to the top of= B=20 (it doesn't happen with the major ABIs, but it is theoretically possible). Now, I'll grant you that there are advantages to leaving this to the compil= er: 1) if the distance is fixed by ABI, then the adjustment can be in code 2) even if it isn't, the adjustment may be stored in the vtable itself, as= =20 opposed to storing it in each object. I'm just wondering if this is worth the complexity. I see no other advantag= e=20 to your feature suggestion, except that of saving sizeof(void*) per object. --=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/1540337.mpxU4uUWLP%40tjmaciei-mobl1. .