220 29565 article Path: news.gmane.org!.POSTED!not-for-mail From: Peet Nick Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: "A Proposal to Relax Constexpr Restrictions for some Reinterpret Casts" ? 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[2a00:1450:400c:c01::241]) by mx.google.com with ESMTPS id i2si56435638wjw.89.2016.11.28.14.04.40 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 28 Nov 2016 14:04:40 -0800 (PST) Received-SPF: pass (google.com: domain of peetnick2@gmail.com designates 2a00:1450:400c:c01::241 as permitted sender) client-ip=2a00:1450:400c:c01::241; Original-Received: by mail-wj0-x241.google.com with SMTP id jb2so15852216wjb.3 for ; Mon, 28 Nov 2016 14:04:40 -0800 (PST) X-Received: by 10.194.103.100 with SMTP id fv4mr20873280wjb.102.1480370679565; Mon, 28 Nov 2016 14:04:39 -0800 (PST) Original-Received: by 10.80.159.78 with HTTP; Mon, 28 Nov 2016 14:04:39 -0800 (PST) In-Reply-To: X-Original-Sender: peetnick2@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com; spf=pass (google.com: domain of peetnick2@gmail.com designates 2a00:1450:400c:c01::241 as permitted sender) smtp.mailfrom=peetnick2@gmail.com; dmarc=pass (p=NONE dis=NONE) header.from=gmail.com 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:29565 Archived-At: --089e0102e15e7dd374054263a876 Content-Type: text/plain; charset=UTF-8 I am not sure how that would help? The destination type must have the same size as the source type, how would that help me getting the "raw byte view" of an scalar/pod? On Fri, Nov 25, 2016 at 4:32 PM, Mathias Gaunard wrote: > I think you would have better luck with the new function bit_cast which > might end up being constexpr. > > On 25 November 2016 at 11:32, Peet wrote: > >> Hello, >> >> 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 >> unsigned/signed char* to simply get the bytes and do some calculation. >> Although it seems that reinterpret_casts aren't allowed in constexpr >> (5.20/2.14). >> Quickly googling this issue got me to this proposal: "A Proposal to Relax >> Constexpr Restrictions for some Reinterpret Casts" by Antony Polukhin [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 >> considered proposing? >> >> Regards, >> Nick >> >> [1] https://apolukhin.github.io/constexpr_algorithms/reinterpret.html >> >> -- >> 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/is >> ocpp.org/d/msgid/std-proposals/afbaf79e-305d-4408-98ab- >> 5b35cfb11f27%40isocpp.org >> >> . >> > > -- > 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/CALnjya_VWQUMYGGd9tYy_ > cJWiwvsyUF2joMuCMPsPy%3DT6e15XA%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/CAJwqTS9XiyDK%2B7wWCRTCTjcnFnKQk6WudsrRTidgf4Ygoc0yng%40mail.gmail.com. --089e0102e15e7dd374054263a876 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
I am not sure how that would help? The destination type mu= st have the same size as the source type, how would that help me getting th= e "raw byte view" of an scalar/pod?

On Fri, Nov 25, 2016 at 4:32 PM, Mathias = Gaunard <mathias@gaunard.com> wrote:
I think you would have better luck with the = new function bit_cast which might end up being constexpr.

On 25 = November 2016 at 11:32, Peet <peetnick2@gmail.com> wrote:<= br>
Hello,

I was basically trying to implement a hash function in C++ using constant= expressions (constexpr).
Sometimes it is necessary to cast a sca= lar type (or arrays) to cv unsigned/signed char* to simply get the bytes an= d do some calculation.
Although it seems that reinterpret_casts a= ren't allowed in constexpr (5.20/2.14).
Quickly googling this= issue got me to this proposal:=C2=A0"A Proposal to Relax Constexpr Re= strictions for some Reinterpret Casts" by=C2=A0Antony Polukhin [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 see= ms that the proposal hasn't been sent to the openstd mailing-list.
Does anyone know if this proposal is still "alive" and if t= his is being considered proposing?

Regards,
<= div>Nick

<= /div>

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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/afba= f79e-305d-4408-98ab-5b35cfb11f27%40isocpp.org.

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