220 37521 <018c02d9-7c65-4451-acc1-c49196fc7622@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 06:24:19 -0700 (PDT) Lines: 93 Approved: news@gmane.org Message-ID: <018c02d9-7c65-4451-acc1-c49196fc7622@isocpp.org> References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_22928_1271282971.1522243459364" X-Trace: blaine.gmane.org 1522243338 16971 195.159.176.226 (28 Mar 2018 13:22:18 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Wed, 28 Mar 2018 13:22:18 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBDZLZTXF7UJBBBFP53KQKGQEU3XZSWA@isocpp.org Wed Mar 28 15:22:14 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ua0-f197.google.com ([209.85.217.197]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1f1B1q-0004Ju-5F for gclcip-std-proposals@m.gmane.org; Wed, 28 Mar 2018 15:22:14 +0200 Original-Received: by mail-ua0-f197.google.com with SMTP id s19sf1582215uad.11 for ; Wed, 28 Mar 2018 06:24:21 -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=eoL9Tco7MNoFmf3MYtyUmiCZLltI2Lr1s7oDEm47JwM=; b=zWFEqNCjP2oNFUJ4R+T4ezFJKTOghKOa3TvSxigNASJs7U9ASx3Etdrnjx/anP8fo/ x4ELAF+qB5wtyK249NGb8ySrbAMbj2aPFCuUb3jusPI0BEnFRrbyPHbE3z5f4PkuUHKt /3lJXB3TP0voNLG7pB5SzVLQmzgoZFg7kz7QnK7wICVCXZT0SqGZY1EDo0Bbh1pgnh5U i861pdUF1OqLdnTSlqJYUIkf726JrlFSjo9rBLphEpoAWvBr2jbaORNjqgxQ3AEfCiV7 uWd80iHLOq2P/R11jOMDObJCRzRBHzHBkcTePNhPlbg97xjyHsv4iWhO/wfJAVp5BWYb lZgg== 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=eoL9Tco7MNoFmf3MYtyUmiCZLltI2Lr1s7oDEm47JwM=; b=tpVFH4gBz0LUfGKgmOcBR86sTcjfCaiOaLcMPCsBEFCw3WP5Xhwz7+v50mk4Fn20ex SEIvKMvIrOpS3wRmqOlDTpj9kQBqrOZi3M9NBqMznJ+2UAZ4dIgQROlqtucwNFKoZTX0 Ly6h78TlwpuUykRmVbgeClo5TLOXHNrLtnI2M1hAfxZY2ZMuq8VrMyZTMaNChM5BwrIh s6G8szROf+O/SeSobb15zWYWzj2kOQoLkQrwsrLEfGxXiaTE7ZfnVoyTE09BccRJORRU DC4GRfgHbxvrsTxM0X5ovzqvojAQpvGNNwQNT0oeGmKGgTXNGOJ1cyX0iY0DzPueXa3J f6Og== X-Gm-Message-State: AElRT7HSBbxX5BFnb7tfqQtUPXlX0zRUdrMGYyFXAbzJ91NH8kQBcFli Z51OISZh+7PQA7ThVfu9B13t9w== X-Google-Smtp-Source: AG47ELtqJFI/CMKvgndKIvVUVjl/xFgBorDWO1QgKS2DKshpFRlAN59tXGcUcU9STwZyeBFQSGahsg== X-Received: by 10.176.27.79 with SMTP id n15mr13213915uai.8.1522243461207; Wed, 28 Mar 2018 06:24:21 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.31.208.1 with SMTP id h1ls2195930vkg.19.gmail; Wed, 28 Mar 2018 06:24:20 -0700 (PDT) X-Received: by 10.31.157.18 with SMTP id g18mr5944119vke.2.1522243459790; Wed, 28 Mar 2018 06:24:19 -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:37521 Archived-At: ------=_Part_22928_1271282971.1522243459364 Content-Type: multipart/alternative; boundary="----=_Part_22929_983829046.1522243459364" ------=_Part_22929_983829046.1522243459364 Content-Type: text/plain; charset="UTF-8" On Wednesday, 28 March 2018 11:35:02 UTC+1, Viacheslav Usov wrote: > > On Wed, Mar 28, 2018 at 1:17 AM, Myriachan > wrote: > > > The C++ Standard rules state that you can treat an array of > std::complex as an array of float of double the number of elements. > That is, you can do pointer arithmetic on a float * that crosses > complex objects. That cannot be done other than with compiler magic. > > Only "if a is an expression of type cv complex* and the expression a[i] > is well-defined for an integer expression i", which has to mean that we > have an array of complex objects. Given that each is required to be layout > compatible with T[2], reinterpreting the bigger array as T[2 * (i + 1)] > does not need any more magic. > 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. > 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/018c02d9-7c65-4451-acc1-c49196fc7622%40isocpp.org. ------=_Part_22929_983829046.1522243459364 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable


On Wednesday, 28 March 2018 11:35:02 UTC+1, Viache= slav Usov wrote:
On Wed, Mar 28, 2018 at 1:17 AM, Myriac= han <myri...@gmail.com> wrote:

=
> The C++ Standard rules state that you can treat an array of= std::complex<float> as an array of float of double the number of ele= ments.=C2=A0 That is, you can do pointer arithmetic on a float * that cross= es complex<float> objects.=C2=A0 That cannot be done other than with = compiler magic.

Only "if a is an expression o= f type cv complex<T>* and the expression a[i] is well-defined for an = integer expression i", which has to mean that we have an array of comp= lex objects. Given that each is required to be layout compatible with T[2],= reinterpreting the bigger array as T[2 * (i + 1)] does not need any more m= agic.

No, you need = magic to perform aliasing between any kind of array types. [expr.add]/4 doe= s not permit aliasing, so any array arithmetic must remain within the bound= s of an array that actually exists.
=C2=A0
Cheers,
V.

=

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