220 29566 <14dc989b-7a2f-478b-b16e-b9297fb1782f@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: Edward Catmur Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: "A Proposal to Relax Constexpr Restrictions for some Reinterpret Casts" ? Date: Mon, 28 Nov 2016 14:35:37 -0800 (PST) Lines: 99 Approved: news@gmane.org Message-ID: <14dc989b-7a2f-478b-b16e-b9297fb1782f@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_11810_967191564.1480372537100" X-Trace: blaine.gmane.org 1480372543 4056 195.159.176.226 (28 Nov 2016 22:35:43 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Mon, 28 Nov 2016 22:35:43 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBDZLZTXF7UJBBOPC6LAQKGQE4RGZRXY@isocpp.org Mon Nov 28 23:35:39 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pf0-f197.google.com ([209.85.192.197]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1cBUWN-00088I-Gn for gclcip-std-proposals@m.gmane.org; Mon, 28 Nov 2016 23:35:35 +0100 Original-Received: by mail-pf0-f197.google.com with SMTP id 83sf233583440pfx.1 for ; Mon, 28 Nov 2016 14:35:39 -0800 (PST) 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 :x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=QuU/fiCnJX8zCI5D1zJgwHtxJx6mRlxbGByL+pZWeyQ=; b=R9YVizoNDvtUdJKnJfZfNzKQkB7wKv2AWACGP8KZZp8OONELPBTqCJnP8aIMIVc71L r0VHYjBQjqjdZtGB/cTqK9Peu9Si+Dog90ly/KWA/NVSRl/NtrKb/agDw/fRCkLQqqBV IgTj1/uF7DMroJkEAnIfRRGBdiLpzNn9pLuSlnveFhXgmUfryopCplnB+iIajJQntocZ osu3/UL/wun26gw17lht6eaFNMoMcNf1hXSb9vp6UM4qKDoez3Rxo7iZavgT5Vsp1K2A pjgh0y73GSV32Q9E006XEJs0QxEnbcqj4Y8MWF8uyhHkzZhK/XxHGhswSmD/EZzwIxih Nrug== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; 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=QuU/fiCnJX8zCI5D1zJgwHtxJx6mRlxbGByL+pZWeyQ=; b=RVf81oyyreZQCVH5EhXpqkME/xLN1t2+iM+c6U2MMgK8BvICAz0m3m5M9w/HGH/ZnZ fxCdjb/9F7Rirp0zt82C3dZ3ElkgBrG0aF9i+xnePVS346Xp8XfigJC++mfx0VP2Zn3g OrE+ZOcMKyPXS7r12Wn+Cbq73ZQrgF4Vxm6Ic58GP4DWsu7qeOydbevrwiI1Kq9Y6Xz9 Ek8gcHByBKzshuDdpeutUSJnPWMux9kFKFOUzD4VIb84Dhs2DxcufbX6GE+6nD2f4br9 ocyaTEtdr1NIY7Ocw9ZmBoxzZ7VgGSkf/BEmU80fxA411vteUdH4RFxBTt9DG3v5IlPj pcTQ== X-Gm-Message-State: AKaTC03GQMMVCarxLQskn5ksv5hbgWOmxtQxY9V/Iv6VdJpMEd5LavKjKdrG9tTdA6ec2Q== X-Received: by 10.98.82.208 with SMTP id g199mr4015141pfb.7.1480372538756; Mon, 28 Nov 2016 14:35:38 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.21.102 with SMTP id z35ls1347664otz.38.gmail; Mon, 28 Nov 2016 14:35:37 -0800 (PST) X-Received: by 10.157.37.125 with SMTP id j58mr739346otd.18.1480372537510; Mon, 28 Nov 2016 14:35:37 -0800 (PST) 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-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:29566 Archived-At: ------=_Part_11810_967191564.1480372537100 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Monday, 28 November 2016 22:04:42 UTC, Peet Nick wrote: > I am not sure how that would help? The destination type must have the sam= e size as the source type, how would that help me getting the "raw byte vie= w" of an scalar/pod? You would bit_cast to an array (or std::array) of unsigned char of the same= size as your pod.=20 > On Fri, Nov 25, 2016 at 4:32 PM, Mathias Gaunard wro= te: >=20 > I think you would have better luck with the new function bit_cast which m= ight end up being constexpr. >=20 >=20 >=20 >=20 > On 25 November 2016 at 11:32, Peet wrote: >=20 > Hello, >=20 >=20 > I was basically trying to implement a hash function in C++ using constant= expressions (constexpr). > Sometimes it is necessary to cast a scalar type (or arrays) to cv unsigne= d/signed char* to simply get the bytes and do some calculation. > Although it seems that reinterpret_casts aren't allowed in constexpr (5.2= 0/2.14). > Quickly googling this issue got me to this proposal:=C2=A0"A Proposal to = Relax Constexpr Restrictions for some Reinterpret Casts" by=C2=A0Antony Pol= ukhin [1]. > This proposal relaxes the rule for reinterpret_cast in constexpr so that = we can actually cast cv void* to cv unsigned/signed char*. > Although I couldn't find any discussions about this online, also it seems= that the proposal hasn't been sent to the openstd mailing-list. > Does anyone know if this proposal is still "alive" and if this is being c= onsidered proposing? >=20 >=20 > Regards, > Nick >=20 >=20 > [1]=C2=A0https://apolukhin.github.io/constexpr_algorithms/reinterpret.htm= l >=20 >=20 >=20 >=20 > --=20 >=20 > You received this message because you are subscribed to the Google Groups= "ISO C++ Standard - Future Proposals" group. >=20 > To unsubscribe from this group and stop receiving emails from it, send an= email to std-proposal...@isocpp.org. >=20 > To post to this group, send email to std-pr...@isocpp.org. >=20 > To view this discussion on the web visit https://groups.google.com/a/isoc= pp.org/d/msgid/std-proposals/afbaf79e-305d-4408-98ab-5b35cfb11f27%40isocpp.= org. >=20 >=20 >=20 >=20 >=20 >=20 >=20 > --=20 >=20 > You received this message because you are subscribed to the Google Groups= "ISO C++ Standard - Future Proposals" group. >=20 > To unsubscribe from this group and stop receiving emails from it, send an= email to std-proposal...@isocpp.org. >=20 > To post to this group, send email to std-pr...@isocpp.org. >=20 > To view this discussion on the web visit https://groups.google.com/a/isoc= pp.org/d/msgid/std-proposals/CALnjya_VWQUMYGGd9tYy_cJWiwvsyUF2joMuCMPsPy%3D= T6e15XA%40mail.gmail.com. --=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. To view this discussion on the web visit https://groups.google.com/a/isocpp= ..org/d/msgid/std-proposals/14dc989b-7a2f-478b-b16e-b9297fb1782f%40isocpp.or= g. ------=_Part_11810_967191564.1480372537100-- .