220 16286 article Path: news.gmane.org!not-for-mail From: Jean-Marc Bourguet Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Pointers to members of unknown class Date: Mon, 16 Feb 2015 07:58:17 -0800 (PST) Lines: 141 Approved: news@gmane.org Message-ID: References: <9b03a820-0ac5-4931-9825-b2fc9ef893f9@isocpp.org> <885c1fbd-8701-4f70-b7ad-89ee9104f42d@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_593_1451923908.1424102297524" X-Trace: ger.gmane.org 1424102305 21391 80.91.229.3 (16 Feb 2015 15:58:25 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Mon, 16 Feb 2015 15:58:25 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBC56XVMTUYPRBGNHRCTQKGQEAUKDIFQ@isocpp.org Mon Feb 16 16:58:25 2015 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yh0-f71.google.com ([209.85.213.71]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1YNO40-0005O3-Du for gclcip-std-proposals@m.gmane.org; Mon, 16 Feb 2015 16:58:24 +0100 Original-Received: by mail-yh0-f71.google.com with SMTP id b6sf126104816yha.2 for ; Mon, 16 Feb 2015 07:58:18 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :content-type:x-original-sender:reply-to:precedence:mailing-list :list-id:list-post:list-help:list-archive:list-subscribe :list-unsubscribe; bh=eq4uFQ4PgGMiEEhUtKykaGePNyV5RxFmkU+m448vJ9c=; b=wPbT9id6W3wfr7mTwk6R/MpH+pbKEDcQl4xBi6/jwbViOCgoRZuOGEYChvAnOv8/Zu 1qbFifSFzb11ln3fMn3Sw3t5lew2F8gk4IKf8EuHQmUK/GZ4+wSnoF2qOV43dSVcqwqN GvAYk9CIbzMS2EzR59xmn/ZZ6o8CO3jUuLcnEq2tL4cYkgkoVOz4K2RpibXs0djxjDNK U5vwr0NRZ4Wdrx8yV4F7Nv89/7FerPHpScAUA8aWTLofC5gvtIwScEBkenA2saRseGIj J/l3MK1BsrVvDHLdsq0gMNWeM1k814CvQSTiM8xboFFBcBqx6GXeOinWW+2XnYIHvB9b we+Q== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:date:from:to:message-id:in-reply-to:references :subject:mime-version:content-type:x-original-sender:reply-to :precedence:mailing-list:list-id:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=eq4uFQ4PgGMiEEhUtKykaGePNyV5RxFmkU+m448vJ9c=; b=bMNfv3OsSCIlrJ9upIWy5RP99lVos75bHP8unkiieLmATn8MzofGwEoS4dC2MybLn1 4GTTLwCN4QjuUqqyEmTbPtIsaYoJvsEYT18y9242+Qp0p8Yi7fcC+iiSdSMFip1+kFzx Lo2YyClcC+mii3BlvO/ZlFzd/rMz/Rkrr0SEnuG7uoMbRmAjyEXxmYHlWX/SYT2ew4Hi CiK08OeXIIMAJx+4EM2rjcmZFwkYoZH1DcjxTkAg5NE0lqXYUGluuxNBeEuzCThPncRw rHeds2p9Zrg9fwhZI83n636UUiZjZD57sE9lWlbQnK+pCRW/YRN+xQ8QyL4Q3cyOog5T 5dXQ== X-Gm-Message-State: ALoCoQmyASizyJMDsddtrQMpWYiizC/AncXMiyM5oYggnRI4TEqJiF9az1ivQMTR5FMnARL0G25X X-Received: by 10.182.142.102 with SMTP id rv6mr5067997obb.47.1424102298500; Mon, 16 Feb 2015 07:58:18 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.140.91.2 with SMTP id y2ls2561554qgd.53.gmail; Mon, 16 Feb 2015 07:58:17 -0800 (PST) X-Received: by 10.140.34.56 with SMTP id k53mr233337qgk.19.1424102297821; Mon, 16 Feb 2015 07:58:17 -0800 (PST) In-Reply-To: X-Original-Sender: JM.Bourguet@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:16286 Archived-At: ------=_Part_593_1451923908.1424102297524 Content-Type: multipart/alternative; boundary="----=_Part_594_628440895.1424102297524" ------=_Part_594_628440895.1424102297524 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Le lundi 16 f=C3=A9vrier 2015 15:58:57 UTC+1, Scott Prager a =C3=A9crit : > > > > On Monday, February 16, 2015 at 5:58:52 AM UTC-5, Markus Grech wrote: >> >> I have updated the proposal, feedback would be very appreciated. >> >> https://docs.google.com/document/d/1B4zWiym1twufigKA4RSXYR5xU4g32EJ8XNl_= bzFtX08/edit?usp=3Dsharing >> > > One possible use-case that I don't see in this proposal: > > struct X { int mem; }; > struct Y { int mem; }; > > > int void::* p =3D X::mem; > auto xp =3D static_cast(p); // (1) > auto yp =3D static_cast(p); // (2) > > > Obviously, *(1) *should be valid, and you describe such a round trip in= =20 > the proposal, but what about *(2)*? > Certainly not valid, but I don't see anything prohibiting it. > As usual, if there is nothing which give it a meaning, it is undefined. And= =20 UB -- explicit or just by lack of definition -- looks like the only reasonable thing in general. In the presented case X and Y are layout-compatible, there is an obvious=20 meaning which we may want to mandate. --=20 Jean-Marc --=20 ---=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. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposa= ls/. ------=_Part_594_628440895.1424102297524 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
Le lundi 16 f=C3=A9vrier 2015 15:58:57 UTC+1, Scott Prager= a =C3=A9crit :


On Monday, February 16, 2015 at 5:58:52 AM UTC-5, Markus G= rech wrote:

<= /div>
One possible use-case that I don't see in this proposal:

struct X { int mem; };
struc= t Y { int mem;= };


int void::* p =3D X<= span style=3D"color:#660">::mem;
auto xp =3D static_cast<int X<= span style=3D"color:#660">::*>(p);  // (1)
<= /span>auto yp = =3D static_cast<int Y::*>(p);=  // (2)


Obviously, (1) should be valid, and= you describe such a round trip in the proposal, but what about (2)?=
Certainly not valid, but I don't see anything prohibiting it.

As usual, if there is nothing whi= ch give it a meaning, it is undefined. And UB -- explicit or just by lack o= f
definition -- looks like the only reasonable thing in general.<= /div>

In the presented case X and Y are layout-compatibl= e, there is an obvious meaning which we may want
to mandate.

-- 
Jean-Marc

--

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