220 37545 article Path: news.gmane.org!.POSTED!not-for-mail From: "'Edward Catmur' via ISO C++ Standard - Future Proposals" Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: Allowing std::complex's magic permission for other types Date: Wed, 28 Mar 2018 17:21:51 +0100 Lines: 188 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="f403045f1c6a9d772f05687b6785" X-Trace: blaine.gmane.org 1522253996 20599 195.159.176.226 (28 Mar 2018 16:19:56 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Wed, 28 Mar 2018 16:19:56 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDZLZTXF7UJBBIMC57KQKGQET5RVGJI@isocpp.org Wed Mar 28 18:19:52 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-wm0-f70.google.com ([74.125.82.70]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1f1Dne-0005BY-NQ for gclcip-std-proposals@m.gmane.org; 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[209.85.220.41]) by mx.google.com with SMTPS id v13sor2009529wrg.44.2018.03.28.09.21.52 for (Google Transport Security); Wed, 28 Mar 2018 09:21:52 -0700 (PDT) Received-SPF: pass (google.com: domain of ecatmur@googlemail.com designates 209.85.220.41 as permitted sender) client-ip=209.85.220.41; X-Received: by 10.223.161.145 with SMTP id u17mr3716102wru.61.1522254112190; Wed, 28 Mar 2018 09:21:52 -0700 (PDT) Original-Received: by 10.28.174.72 with HTTP; Wed, 28 Mar 2018 09:21:51 -0700 (PDT) In-Reply-To: X-Original-Sender: ecatmur@googlemail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@googlemail.com header.s=20161025 header.b=GStNKjgU; spf=pass (google.com: domain of ecatmur@googlemail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=ecatmur@googlemail.com; dmarc=pass (p=QUARANTINE sp=QUARANTINE dis=NONE) header.from=googlemail.com X-Original-From: Edward Catmur 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:37545 Archived-At: --f403045f1c6a9d772f05687b6785 Content-Type: text/plain; charset="UTF-8" On Wed, Mar 28, 2018 at 4:41 PM, Viacheslav Usov wrote: > 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? > It's unspecified per [expr.eq]/2.1, so (bar + 2) == baz must have the value true or false, but can be either and does not need to be consistently one or the other; ((bar + 2) == baz) == ((bar + 2) == baz) can also be false. At least it isn't undefined. I think I was incorrect about the validity of bar[2] - looking at [expr.unary.op]/1, I think that *(bar + 2) is undefined although (bar + 2) is OK. Cheers, > V. > > -- > You received this message because you are subscribed to a topic in the > Google Groups "ISO C++ Standard - Future Proposals" group. > To unsubscribe from this topic, visit https://groups.google.com/a/ > isocpp.org/d/topic/std-proposals/HpRRF7RWzTY/unsubscribe. > To unsubscribe from this group and all its topics, 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/CAA7YVg1ZHes2s7mMGqGAgh0JTrNOq > 0gHmSNFgdRJo6yOb78Q-Q%40mail.gmail.com > > . > -- 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/CAJnLdObRm%3DN%2BcYEaAV%2BK6_Nnvoc4ef9Efimw-YNrkFYMQothFg%40mail.gmail.com. --f403045f1c6a9d772f05687b6785 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable


On Wed, Mar 28, 2018 at 4:41 PM, Viacheslav Usov <= via.usov@gmail.com<= /a>> wrote:
On Wed, Mar 28, 2018 at= 4:42 PM, Edward Catmur <ed@catmur.co.uk> wrote:

<= /div>
> bar[2] is well-defined, but it refers to the past-the-end el= ement 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 "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?
<= div>
It's unspecified per [expr.eq]/2.1, so (bar + 2) =3D= =3D baz must have the value true or false, but can be either and does not n= eed to be consistently one or the other; ((bar + = 2) =3D=3D baz) =3D=3D=C2=A0((bar + 2) =3D=3D baz) can also be false. At least it isn't = undefined.

I think I was incorrect a= bout the validity of=C2=A0bar[2] - looking at [e= xpr.unary.op]/1, I think that *(bar + 2) is undefined although (bar + 2) is= OK.=C2=A0

Cheers,
V.

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