220 37481 article Path: news.gmane.org!.POSTED!not-for-mail From: Hyman Rosen Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: Allowing std::complex's magic permission for other types Date: Mon, 26 Mar 2018 18:13:06 -0400 Lines: 66 Approved: news@gmane.org Message-ID: References: <6ec944b3-16c3-4f00-b0c9-a449a522d4ac@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="001a114b332c4e46fc05685815bf" X-Trace: blaine.gmane.org 1522102290 5866 195.159.176.226 (26 Mar 2018 22:11:30 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Mon, 26 Mar 2018 22:11:30 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDKJJPVBQMKRBCHB4XKQKGQEFL6QQTQ@isocpp.org Tue Mar 27 00:11:26 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-wr0-f199.google.com ([209.85.128.199]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1f0aKn-0001Qn-Dx for gclcip-std-proposals@m.gmane.org; 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[209.85.220.41]) by mx.google.com with SMTPS id f14sor10701wmi.51.2018.03.26.15.13.27 for (Google Transport Security); Mon, 26 Mar 2018 15:13:27 -0700 (PDT) Received-SPF: pass (google.com: domain of hyman.rosen@gmail.com designates 209.85.220.41 as permitted sender) client-ip=209.85.220.41; X-Received: by 10.28.101.6 with SMTP id z6mr15178141wmb.110.1522102407413; Mon, 26 Mar 2018 15:13:27 -0700 (PDT) Original-Received: by 10.28.55.19 with HTTP; Mon, 26 Mar 2018 15:13:06 -0700 (PDT) In-Reply-To: <6ec944b3-16c3-4f00-b0c9-a449a522d4ac@isocpp.org> X-Original-Sender: hyman.rosen@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=QMbta4az; spf=pass (google.com: domain of hyman.rosen@gmail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=hyman.rosen@gmail.com; dmarc=pass (p=NONE sp=QUARANTINE dis=NONE) header.from=gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Spam-Checked-In-Group: std-proposals@isocpp.org 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:37481 Archived-At: --001a114b332c4e46fc05685815bf Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Mon, Mar 26, 2018 at 4:39 PM, Nicol Bolas wrote: > > What isn't OK is the pointer arithmetic that follows, or the assumption > that there is no padding between the elements. > [class.mem] says [ *Note:* There might therefore be unnamed padding within a standard-layout struct object, but not at its beginning, as necessary to achieve appropriate alignment. *=E2=80=94end note* ] Granted that notes are non-normative, the assumption that padding is present only to achieve appropriate alignment allows for reasoning about pointer arithmetic between class members. --=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/CAHSYqdba84TLvQ8HRCcae-nCQ-5Skyj%2Bi5RA7Z1wfTwcC= 4u2bQ%40mail.gmail.com. --001a114b332c4e46fc05685815bf Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
On M= on, Mar 26, 2018 at 4:39 PM, Nicol Bolas <jmckesson@gmail.com> wrote:
What isn't OK is the pointer arithmetic that follows, or the as= sumption that there is no padding between the elements.

[class.mem] says
[ Note: There might therefore= be unnamed padding within a standard-layout struct object, but not at its = beginning, as necessary to achieve appropriate alignment. =E2=80=94end n= ote ]

Granted that notes are non-normative, the assumptio= n that padding is present only to achieve appropriate alignment allows for = reasoning about pointer arithmetic between class members.
=

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