220 37537 article Path: news.gmane.org!.POSTED!not-for-mail From: Viacheslav Usov Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: Allowing std::complex's magic permission for other types Date: Wed, 28 Mar 2018 17:41:04 +0200 Lines: 102 Approved: news@gmane.org Message-ID: References: <018c02d9-7c65-4451-acc1-c49196fc7622@isocpp.org> <1e25a8ae-4752-4fc0-8c89-639cef53121c@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="000000000000c1678605687ad548" X-Trace: blaine.gmane.org 1522251544 10874 195.159.176.226 (28 Mar 2018 15:39:04 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Wed, 28 Mar 2018 15:39:04 +0000 (UTC) To: "ISO C++ Standard - Future Proposals" Original-X-From: std-proposals+bncBDU6VRPKW4BBBEXP53KQKGQEEFK3EDA@isocpp.org Wed Mar 28 17:39:00 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pf0-f199.google.com ([209.85.192.199]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1f1DAB-0002j6-NR for gclcip-std-proposals@m.gmane.org; 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[209.85.220.41]) by mx.google.com with SMTPS id t10-v6sor1860611plh.91.2018.03.28.08.41.05 for (Google Transport Security); Wed, 28 Mar 2018 08:41:05 -0700 (PDT) Received-SPF: pass (google.com: domain of via.usov@gmail.com designates 209.85.220.41 as permitted sender) client-ip=209.85.220.41; X-Received: by 2002:a17:902:8481:: with SMTP id c1-v6mr4248568plo.304.1522251665072; Wed, 28 Mar 2018 08:41:05 -0700 (PDT) Original-Received: by 10.236.175.138 with HTTP; Wed, 28 Mar 2018 08:41:04 -0700 (PDT) In-Reply-To: <1e25a8ae-4752-4fc0-8c89-639cef53121c@isocpp.org> X-Original-Sender: via.usov@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=VFZNamvY; spf=pass (google.com: domain of via.usov@gmail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=via.usov@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:37537 Archived-At: --000000000000c1678605687ad548 Content-Type: text/plain; charset="UTF-8" On Wed, Mar 28, 2018 at 4:42 PM, Edward Catmur wrote: > bar[2] is well-defined, but it refers to the past-the-end element of foo[0], which cannot be accessed. It does not refer to foo[1][0], although it has the same address. What does "not refer to" mean, exactly, given that "it has the same address", and, obviously, the same type, to which [basic.compound]/3.4 "Pointers to layout-compatible types shall have the same value representation and alignment requirements" trivially applies? Are you saying that by doing auto baz = reinterpret_cast(&foo[1][0]); and then by doing (bar + 2) == baz you might encounter undefined behaviour or get that evaluated as false, or, if not, that bar[2] is undefined behaviour while baz[0] is not? Cheers, V. -- 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/CAA7YVg1ZHes2s7mMGqGAgh0JTrNOq0gHmSNFgdRJo6yOb78Q-Q%40mail.gmail.com. --000000000000c1678605687ad548 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
On W= ed, Mar 28, 2018 at 4:42 PM, Edward Catmur <ed@catmur.co.uk> w= rote:

> bar[2] is well-defined, but it refers to = the past-the-end element of foo[0], which cannot be accessed. It does not r= efer to foo[1][0], although it has the same address.

What does "not refer to" mean, exactly, given that "it has the same address", and, obviously, the sam= e type, to which=C2=A0=C2=A0[basic.compound]/3.4=C2=A0 "Pointer= s to layout-compatible types shall=C2=A0have the same value representation = and alignment requirements" trivially applies?

Are you saying that by doing

auto baz =3D reinterpret_cast<int*>(&foo[1]<= span style=3D"color:rgb(34,34,34);font-family:arial,sans-serif;font-size:12= ..8px;font-style:normal;font-variant-ligatures:normal;font-variant-caps:norm= al;font-weight:400;letter-spacing:normal;text-align:start;text-indent:0px;t= ext-transform:none;white-space:normal;word-spacing:0px;background-color:rgb= (255,255,255);text-decoration-style:initial;text-decoration-color:initial;f= loat:none;display:inline">[0]);

and then by doing (bar + 2) =3D=3D baz you might encounte= r=C2=A0undefined=C2=A0behaviour or get that evaluated as false, or, if not, that bar[2] is undefined=C2=A0= behaviour while baz[0] is not?

Cheers,
V.

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