220 29568 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: "A Proposal to Relax Constexpr Restrictions for some Reinterpret Casts" ? 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[2607:f8b0:400d:c09::244]) by mx.google.com with ESMTPS id g2si25868800qte.28.2016.11.28.15.00.56 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 28 Nov 2016 15:00:56 -0800 (PST) Received-SPF: pass (google.com: domain of ecatmur@googlemail.com designates 2607:f8b0:400d:c09::244 as permitted sender) client-ip=2607:f8b0:400d:c09::244; Original-Received: by mail-qk0-x244.google.com with SMTP id n204so15327790qke.2 for ; Mon, 28 Nov 2016 15:00:56 -0800 (PST) X-Received: by 10.55.174.135 with SMTP id x129mr23235066qke.133.1480374056400; Mon, 28 Nov 2016 15:00:56 -0800 (PST) Original-Received: by 10.200.41.176 with HTTP; Mon, 28 Nov 2016 15:00:55 -0800 (PST) Original-Received: by 10.200.41.176 with HTTP; Mon, 28 Nov 2016 15:00:55 -0800 (PST) In-Reply-To: X-Original-Sender: ecatmur@googlemail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@googlemail.com; spf=pass (google.com: domain of ecatmur@googlemail.com designates 2607:f8b0:400d:c09::244 as permitted sender) smtp.mailfrom=ecatmur@googlemail.com; dmarc=pass (p=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-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:29568 Archived-At: --94eb2c070d62c45ee705426471e3 Content-Type: text/plain; charset=UTF-8 On 28 Nov 2016 22:50, "Peet Nick" wrote: > > 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? They'd have to use a compiler intrinsic; according to [1] llvm already has a suitable opcode, so the other compilers would have to do the same, if they don't have an appropriate intrinsic available already. Remember, it's fine for library functions to need compiler magic to implement; part of the job of the standard library is to provide standardized interfaces to such magic. 1. http://open-std.org/JTC1/SC22/WG21/docs/papers/2016/p0476r0.html > 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 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/6tkocdbwVGU/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/CAJwqTS_OGEOUiUE6rSRrREV6Yx9%2B4Sj5gA8CDHeYgXnPwErW6Q%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/CAJnLdOaJRPtdnHPX0%3DL89Pip2GC90aXrEYtJdUYxaHw1vwNJ%3Dw%40mail.gmail.com. --94eb2c070d62c45ee705426471e3 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable

On 28 Nov 2016 22:50, "Peet Nick" <peetnick2@gmail.com> wrote:
>
> Ah, I understand. Although I understand this now, I am not entirely su= re how it will be actually implemented. How would someone implement bit_cas= t as a constexpr without having std::memcpy constexpr?

They'd have to use a compiler intrinsic; according to [1= ] llvm already has a suitable opcode, so the other compilers would have to = do the same, if they don't have an appropriate intrinsic available alre= ady.

Remember, it's fine for library functions to need compil= er magic to implement; part of the job of the standard library is to provid= e standardized interfaces to such magic.

1. http://open-std.org/JTC1/SC22/WG21/docs/papers/2016/p04= 76r0.html

> 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 &= quot;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 <mat...@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 <peet...@gmail.com> wrote:
>> >
>> > Hello,
>> >
>> >
>> > I was basically trying to implement a hash function in C++ us= ing constant expressions (constexpr).
>> > Sometimes it is necessary to cast a scalar type (or arrays) t= o cv unsigned/signed char* to simply get the bytes and do some calculation.=
>> > Although it seems that reinterpret_casts aren't allowed i= n constexpr (5.20/2.14).
>> > Quickly googling this issue got me to this proposal:=C2=A0&qu= ot;A Proposal to Relax Constexpr Restrictions for some Reinterpret Casts&qu= ot; by=C2=A0Antony Polukhin [1].
>> > This proposal relaxes the rule for reinterpret_cast in conste= xpr so that we can actually cast cv void* to cv unsigned/signed char*.
>> > Although I couldn't find any discussions about this onlin= e, also it seems that the proposal hasn't been sent to the openstd mail= ing-list.
>> > Does anyone know if this proposal is still "alive" = and if this is being considered proposing?
>> >
>> >
>> > Regards,
>> > Nick
>> >
>> >
>> > [1]=C2=A0https://apolukhin.github.io/constexpr_algorithms/= reinterpret.html
>> >
>> >
>> >
>> >
>> > --
>> >
>> > You received this message because you are subscribed to the G= oogle Groups "ISO C++ Standard - Future Proposals" group.
>> >
>> > To unsubscribe from this group and stop receiving emails from= it, send an email to std-pro= posal...@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 G= oogle Groups "ISO C++ Standard - Future Proposals" group.
>> >
>> > To unsubscribe from this group and stop receiving emails from= it, send an email to std-pro= posal...@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_cJWiwvsyUF2joMuCMP= sPy%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-p= roposals/14dc989b-7a2f-478b-b16e-b9297fb1782f%40isocpp.org.
>
>
> --
> 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://gr= oups.google.com/a/isocpp.org/d/topic/std-proposals/6tkocdbwVGU/unsubscribe<= /a>.
> To unsubscribe from this group and all its topics, send an email to std-proposals+unsub= scribe@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%2B4Sj5gA8CDHeYgXnPwE= rW6Q%40mail.gmail.com.

--
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