220 37531 <1e25a8ae-4752-4fc0-8c89-639cef53121c@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: Edward Catmur Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: Allowing std::complex's magic permission for other types Date: Wed, 28 Mar 2018 07:42:54 -0700 (PDT) Lines: 97 Approved: news@gmane.org Message-ID: <1e25a8ae-4752-4fc0-8c89-639cef53121c@isocpp.org> References: <018c02d9-7c65-4451-acc1-c49196fc7622@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_22308_2114014568.1522248174798" X-Trace: blaine.gmane.org 1522248056 21439 195.159.176.226 (28 Mar 2018 14:40:56 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Wed, 28 Mar 2018 14:40:56 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBDZLZTXF7UJBB36T53KQKGQE7PKUPMI@isocpp.org Wed Mar 28 16:40:52 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-vk0-f72.google.com ([209.85.213.72]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1f1CFt-0005OS-Dp for gclcip-std-proposals@m.gmane.org; Wed, 28 Mar 2018 16:40:49 +0200 Original-Received: by mail-vk0-f72.google.com with SMTP id d67sf1743218vka.12 for ; Wed, 28 Mar 2018 07:42:57 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :list-post:list-help:list-archive:list-subscribe:list-unsubscribe; bh=lZnDImU9QH8ZDzpH2JP1V40zIL3iDwoy7QsoqT6JQNQ=; b=DX3kFIBKNtuE9m6UvKLmH8S2FgrKIfTgJt+2GYPNAyIanYynX5qln7zIkknepNnQp+ XMzS4yp74HfGIu6niOPhzTtYXreHG8ypUWf5GuS+kTlziK5TV4lhElWU7TUDFDo2iUKI d+MGz+/RsmPjV9CNSsT0eDcHW6YsHGo401ZKrdJ1/RJAhNPdGamoWiVpKQFkbA21/Ilc 9ks23LnouMR/XiXJs2s7f+PsBZWvdjRGQ/jAsiAYKUrjlgtdkNtdAZSmczksdXdSXIX5 kVa5YIBSD8eSw5diRPikAta5HANolkDDT6wmtNkJeXl+P6gdtKZ8YfTI5Rtl4bJa7dG2 L1gQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:message-id:in-reply-to:references :subject:mime-version:x-original-sender:reply-to:precedence :mailing-list:list-id:x-spam-checked-in-group:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=lZnDImU9QH8ZDzpH2JP1V40zIL3iDwoy7QsoqT6JQNQ=; b=crQyl4ri2D+6D0TOnJ12D4wgoxN9Bqlbp9bkOvUsfz1TRjtHJNRVe3nZrWGSJNez19 m9K+t9BOycYEN4yC9Pl2qaVRbzEG+k5t0soxCmF7l3S4wj+8mud8/zwco+lTdgOE7jG0 8Uj56xYX3Svd5oBfxYlpkKm5bwf9k0QY9AmvTTTpYKlYgOybfQ0oTOe+MIXew7Y/etD5 guKBeNE7ICXqI4e/Bea4WX2wjeET0d75T2A0ggM0a9PH2faOzWos88v1aJT1B76mzunp EUjlOteCiMHREGlPBH0+9dtEwTRzSCyECPuaQVk/3G5YtyZC3+40drJmDRz9P3GD4MdD E4iw== X-Gm-Message-State: AElRT7HljRyOwLREp9A75Fuh+1Ba1oSzFNboGLBKuDO4rU75SFIuE93s q8GLy+qHkbT+tbLOlcwxnKd0nA== X-Google-Smtp-Source: AIpwx4+JSYNMG80ghiXvkODNizgvC1ZpXcaQKK0oT/sZLPOMwiUuf2Va7AcEbS13ZLC0eMKEusXkFw== X-Received: by 10.31.216.69 with SMTP id p66mr2672632vkg.68.1522248176799; Wed, 28 Mar 2018 07:42:56 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.31.157.199 with SMTP id g190ls2434218vke.10.gmail; Wed, 28 Mar 2018 07:42:55 -0700 (PDT) X-Received: by 10.31.193.140 with SMTP id r134mr5982054vkf.6.1522248175222; Wed, 28 Mar 2018 07:42:55 -0700 (PDT) In-Reply-To: X-Original-Sender: ed@catmur.co.uk 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:37531 Archived-At: ------=_Part_22308_2114014568.1522248174798 Content-Type: multipart/alternative; boundary="----=_Part_22309_1419773551.1522248174798" ------=_Part_22309_1419773551.1522248174798 Content-Type: text/plain; charset="UTF-8" On Wednesday, 28 March 2018 15:19:49 UTC+1, Viacheslav Usov wrote: > > On Wed, Mar 28, 2018 at 3:24 PM, Edward Catmur > wrote: > > > No, you need magic to perform aliasing between any kind of array types. > [expr.add]/4 does not permit aliasing, so any array arithmetic must remain > within the bounds of an array that actually exists. > > Let's tackle a simpler question first. Suppose: > > int foo[5][2]; > auto bar = reinterpret_cast(&foo[0][0]); > > Are you saying that bar[2] is not well-defined? > 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. This is because there is only one array of type int[N] containing foo[0][0], and that is the array foo[0], which has only 2 elements. bar[3] is not well-defined, and likewise the expression (bar + 3) has undefined behavior. 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/1e25a8ae-4752-4fc0-8c89-639cef53121c%40isocpp.org. ------=_Part_22309_1419773551.1522248174798 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable


On Wednesday, 28 March 2018 15:19:49 UTC+1, Viache= slav Usov wrote:
On Wed, Mar 28, 2018 at 3:24 PM, Edward= Catmur <e...@catmur.co.uk> wrote:
> No, you need magic to perform aliasing between any kind o= f array types. [expr.add]/4 does not permit aliasing, so any array arithmet= ic must remain within the bounds of an array that actually exists.

Let's tackle a simpler question first. Suppose:
<= div>
int foo[5][2];
auto bar =3D reinterpret_cast&l= t;int*>(&foo[0][0]);

Are you saying th= at bar[2] is not well-defined?
bar[2] is well-defined, but it refers to the past-the-end eleme= nt of foo[0], which cannot be accessed. It does not refer to foo[1][0], alt= hough it has the same address.

This is because the= re is only one array of type int[N] containing foo[0][0], and that is the a= rray foo[0], which has only 2 elements.

bar[3] is = not well-defined, and likewise the expression (bar + 3) has undefined behav= ior.

Cheers,
V.

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