220 12644 <507AB10F-66B5-4E72-AF78-98B4AECBE6B3@gmail.com> article Path: news.gmane.org!not-for-mail From: David Krauss Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Usability extensions to offsetof Date: Wed, 3 Sep 2014 12:46:39 +0800 Lines: 81 Approved: news@gmane.org Message-ID: <507AB10F-66B5-4E72-AF78-98B4AECBE6B3@gmail.com> References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 (Mac OS X Mail 7.3 \(1878.6\)) Content-Type: multipart/alternative; boundary="Apple-Mail=_7A7F992C-532A-4032-8EF8-97F739D2CC24" X-Trace: ger.gmane.org 1409719613 3603 80.91.229.3 (3 Sep 2014 04:46:53 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 3 Sep 2014 04:46:53 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCW25A7E3QCRBNF2TKQAKGQEZO3HDNY@isocpp.org Wed Sep 03 06:46:47 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yk0-f200.google.com ([209.85.160.200]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1XP2Sz-000358-O5 for gclcip-std-proposals@m.gmane.org; Wed, 03 Sep 2014 06:46:46 +0200 Original-Received: by mail-yk0-f200.google.com with SMTP id 9sf25654052ykp.7 for ; Tue, 02 Sep 2014 21:46:44 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:from:message-id:mime-version:subject:date :references:to:in-reply-to:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:list-post:list-help:list-archive:list-subscribe :list-unsubscribe:content-type; bh=wyRurOJ1STuLuIT0Y+xvzf3Ga8t5e4M/F4eSUbAiQK4=; b=ltsCPikOZTfP+YSi5uUx/s0Jq6xpy2IPK1zNf9a+iqmzJnhfjbAT4+TMdKR/Uw4vv7 vLeslxG2BuPuEDZeUCHqhIt4b/9S+Ep4kZ7OW7fKsMMRG5rZ/3CTxcxCJTC9aI7nt/Hw JRuTb6GQTF0lDjtx3BgNzXiMylgYOSNEjotDgggW+wRNTNvzeeX7HHCr5KRRDyj625Fd YhcYv3yZf+HGQGlCtmHj4Jvhm+rxT3s1mHIWwlPjTpaSRrXulkFK20El7o5cAcN71BqE MaHoU/x5Ov7YoDGnNl3owrZUqSNa7NMtUW0MEyRLVN7EPOb8YtUcb7SZSyKgVSPIjtE0 FFZg== X-Gm-Message-State: ALoCoQlUGn1v96vXzeUSu/YlysiLSBWSV+xi/8hKHfeLM0a1qEOD/Nt6CieePUE11RA/QdI/MY7F X-Received: by 10.236.118.37 with SMTP id k25mr19435289yhh.1.1409719604649; Tue, 02 Sep 2014 21:46:44 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.50.43.135 with SMTP id w7ls2869582igl.21.canary; Tue, 02 Sep 2014 21:46:44 -0700 (PDT) X-Received: by 10.66.197.132 with SMTP id iu4mr18336011pac.132.1409719604020; Tue, 02 Sep 2014 21:46:44 -0700 (PDT) Original-Received: from mail-pd0-x22b.google.com (mail-pd0-x22b.google.com [2607:f8b0:400e:c02::22b]) by mx.google.com with ESMTPS id s2si9079178pdj.41.2014.09.02.21.46.44 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Tue, 02 Sep 2014 21:46:44 -0700 (PDT) Received-SPF: pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:400e:c02::22b as permitted sender) client-ip=2607:f8b0:400e:c02::22b; Original-Received: by mail-pd0-f171.google.com with SMTP id y13so10196317pdi.30 for ; Tue, 02 Sep 2014 21:46:43 -0700 (PDT) X-Received: by 10.70.61.162 with SMTP id q2mr53397975pdr.50.1409719603856; Tue, 02 Sep 2014 21:46:43 -0700 (PDT) Original-Received: from [172.20.10.2] ([121.54.44.91]) by mx.google.com with ESMTPSA id zf5sm5574253pbc.44.2014.09.02.21.46.42 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Tue, 02 Sep 2014 21:46:43 -0700 (PDT) In-Reply-To: X-Mailer: Apple Mail (2.1878.6) X-Original-Sender: potswa@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:400e:c02::22b as permitted sender) smtp.mail=potswa@gmail.com; dkim=pass header.i=@gmail.com; dmarc=pass (p=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:12644 Archived-At: --Apple-Mail=_7A7F992C-532A-4032-8EF8-97F739D2CC24 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=ISO-8859-1 On 2014-09-03, at 12:37 PM, Myriachan wrote: > As a generalization of std::is_standard_layout, how about an std::is_offs= etof_compatible? Requiring standard-layout in order to use offsetof is too= restrictive; almost all implementations could support offsetof in more sit= uations than the Standard's "standard-layout" definition allows. offsetof is a vestige of C compatibility, and standard layout is specifical= ly tailored to C ABI compatibility. So the restriction is probably exactly = what its designers intended. Where would you prefer to use offsetof, instead of a pointer-to-member? > With this, there could also be member_pointer_to_offset and offset_to_mem= ber_pointer library functions that work with compatible classes. There always could have been, but why? --=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/. --Apple-Mail=_7A7F992C-532A-4032-8EF8-97F739D2CC24 Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=ISO-8859-1
On 2014&= ndash;09–03, at 12:37 PM, Myriachan <myriachan@gmail.com> wrote:

As a generalizati= on of std::is_standard_= layout, how about an std::is_offsetof_compatible?  Requiring standard-layout in= order to use offsetof<= /span> is too restrictive; almost all implementations could support offsetof in more situat= ions than the Standard's "standard-layout" definition allows.

offsetof is a ve= stige of C compatibility, and standard layout is specifically tailored to C= ABI compatibility. So the restriction is probably exactly what its designe= rs intended.

Where would you prefer to use offsetof, instead of a pointer-to-member?

<= blockquote type=3D"cite">
With this, there could also be member_pointer_to_offs= et and offset= _to_member_pointer library functions that work with compatible class= es.

There always could have been, but why?=

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