220 29567 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:c09::241]) by mx.google.com with ESMTPS id eh3si56545241wjd.247.2016.11.28.14.50.34 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 28 Nov 2016 14:50:34 -0800 (PST) Received-SPF: pass (google.com: domain of peetnick2@gmail.com designates 2a00:1450:400c:c09::241 as permitted sender) client-ip=2a00:1450:400c:c09::241; Original-Received: by mail-wm0-x241.google.com with SMTP id a20so21545430wme.2 for ; Mon, 28 Nov 2016 14:50:34 -0800 (PST) X-Received: by 10.28.141.143 with SMTP id p137mr20370554wmd.5.1480373433575; Mon, 28 Nov 2016 14:50:33 -0800 (PST) Original-Received: by 10.80.159.78 with HTTP; Mon, 28 Nov 2016 14:50:33 -0800 (PST) In-Reply-To: <14dc989b-7a2f-478b-b16e-b9297fb1782f@isocpp.org> 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:c09::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:29567 Archived-At: --001a114717b6a4b0f60542644c2a Content-Type: text/plain; charset=UTF-8 Ah, I understand. Although I understand this now, I am not entirely sure how it will be actually implemented. How would someone implement bit_cast as a constexpr without having std::memcpy constexpr? On Mon, Nov 28, 2016 at 11:35 PM, Edward Catmur wrote: > 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 > same size as the source type, how would that help me getting the "raw byte > view" of an scalar/pod? > > You would bit_cast to an array (or std::array) of unsigned char of the > same size as your 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-proposal...@isocpp.org. > > > > To post to this group, send email to std-pr...@isocpp.org. > > > > To view this discussion on the web visit https://groups.google.com/a/ > isocpp.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-proposal...@isocpp.org. > > > > To post to this group, send email to std-pr...@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/14dc989b-7a2f-478b- > b16e-b9297fb1782f%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/CAJwqTS_OGEOUiUE6rSRrREV6Yx9%2B4Sj5gA8CDHeYgXnPwErW6Q%40mail.gmail.com. --001a114717b6a4b0f60542644c2a Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
Ah, I understand. Although I understand this now, I am not= entirely sure how it will be actually implemented. How would someone imple= ment bit_cast as a constexpr without having std::memcpy constexpr?

On Mon, Nov 28, 2016= at 11:35 PM, Edward Catmur <ed@catmur.co.uk> wrote:
On Monday, 28 November 2016 22:04= :42 UTC, Peet Nick=C2=A0 wrote:
> 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?

You would bit_cast to an array (or std::array) of unsigned char of t= he same size as your pod.

> On Fri, Nov 25, 2016 at 4:32 PM, Mathias Gaunard <mat...@gaunard.com> wrote:
>
> I think you would have better luck with the new function bit_cast whic= h might end up being constexpr.
>
>
>
>
> On 25 November 2016 at 11:32, Peet <peet...@gmail.com> wrote:
>
> Hello,
>
>
> I was basically trying to implement a hash function in C++ using const= ant expressions (constexpr).
> Sometimes it is necessary to cast a scalar type (or arrays) to cv unsi= gned/signed char* to simply get the bytes and do some calculation.
> Although it seems that reinterpret_casts aren't allowed in constex= pr (5.20/2.14).
> Quickly googling this issue got me to this proposal:=C2=A0"A Prop= osal to Relax Constexpr Restrictions for some Reinterpret Casts" by=C2= =A0Antony Polukhin [1].
> This proposal relaxes the rule for reinterpret_cast in constexpr so th= at we can actually cast cv void* to cv unsigned/signed char*.
> Although I couldn't find any discussions about this online, also i= t seems that the proposal hasn't been sent to the openstd mailing-list.=
> Does anyone know if this proposal is still "alive" and if th= is is being considered proposing?
>
>
> Regards,
> Nick
>
>
> [1]=C2=A0https://apolukhin.git= hub.io/constexpr_algorithms/reinterpret.html
>
>
>
>
> --
>
> You received this message because you are subscribed to the Google Gro= ups "ISO C++ Standard - Future Proposals" group.
>
> To unsubscribe from this group and stop receiving emails from i= t, send an email to std-propo= sal...@isocpp.org.
>
> To post to this group, send email to std-pr...@isocpp.org.
>
> To view this discussion on the web visit https://groups.goog= le.com/a/isocpp.org/d/msgid/std-proposals/afbaf79e-305d-4408-98ab-5b35cfb11f27%40isocpp.org.
>
>
>
>
>
>
>
> --
>
> You received this message because you are subscribed to the Google Gro= ups "ISO C++ Standard - Future Proposals" group.
>
> To unsubscribe from this group and stop receiving emails from i= t, send an email to std-propo= sal...@isocpp.org.
>
> To post to this group, send email to std-pr...@isocpp.org.
>
> To view this discussion on the web visit https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/CA= Lnjya_VWQUMYGGd9tYy_cJWiwvsyUF2joMuCMPsPy%3DT6e15XA%40mail.g= mail.com.

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