220 30293 article Path: news.gmane.org!.POSTED!not-for-mail From: "D. B." Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: P0298: A byte type definition: with undefined pointer arithmetic? 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[2a00:1450:400c:c01::230]) by mx.google.com with ESMTPS id k7si714749wjz.57.2017.01.05.04.08.32 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 05 Jan 2017 04:08:32 -0800 (PST) Received-SPF: pass (google.com: domain of db0451@gmail.com designates 2a00:1450:400c:c01::230 as permitted sender) client-ip=2a00:1450:400c:c01::230; Original-Received: by mail-wj0-x230.google.com with SMTP id ew7so19710922wjc.3 for ; Thu, 05 Jan 2017 04:08:32 -0800 (PST) X-Received: by 10.194.60.195 with SMTP id j3mr63616945wjr.149.1483618112443; Thu, 05 Jan 2017 04:08:32 -0800 (PST) Original-Received: by 10.28.144.66 with HTTP; Thu, 5 Jan 2017 04:08:32 -0800 (PST) In-Reply-To: X-Original-Sender: db0451@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com; spf=pass (google.com: domain of db0451@gmail.com designates 2a00:1450:400c:c01::230 as permitted sender) smtp.mailfrom=db0451@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:30293 Archived-At: --047d7b86e2f893198b054557c27d Content-Type: text/plain; charset=UTF-8 On Thu, Jan 5, 2017 at 10:45 AM, Bengt Gustafsson < bengt.gustafsson@beamways.com> wrote: > I can't say that I have checked but isn't an array of enum values > equivalent to its underlying type when it comes to size and alignment? > > As byte is defined as an enum with a unsigned char as underlying type > pointer arithmetic should be defined as expected: to be bytewise. I don't > think this requires specific mention in the standard text. > It's worse than that! The problem is that char* itself does not seem to be allowed to have the special properties it supposedly has, for reading other objects as a "sequence" o char*, because it is formally undefined to increment that pointer to the next byte in the "sequence". The linked issue is an attempt to clarify what this nebulous "sequence" really is and whether (or why not) it is equivalent to an *array* for the purposes of indexing/arithmetic. So if char* isn't allowed to do these things, std::byte won't be, regardless of any transitive properties it gets from other facets of the Standard. -- 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/CACGiwhE9c%2Bcgwrp0mVutjoxC7kP5OvH_zoLtOj7fLcNq_h3xHg%40mail.gmail.com. --047d7b86e2f893198b054557c27d Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
On T= hu, Jan 5, 2017 at 10:45 AM, Bengt Gustafsson <bengt.gustafsso= n@beamways.com> wrote:
I can't say that I have checked but isn't an array of = enum values equivalent to its underlying type when it comes to size and ali= gnment?

As byte is defined as an enum with a unsigned ch= ar as underlying type pointer arithmetic should be defined as expected: to = be bytewise. I don't think this requires specific mention in the standa= rd text.

It's worse than that! The problem is that= char* itself does not seem to be allowed to have the special properties it= supposedly has, for reading other objects as a "sequence" o char= *, because it is formally undefined to increment that pointer to the next b= yte in the "sequence". The linked issue is an attempt to clarify = what this nebulous "sequence" really is and whether (or why not) = it is equivalent to an array for the purposes of indexing/arithmetic= ..

So if char* isn't allowed to = do these things, std::byte won't be, regardless of any transitive prope= rties it gets from other facets of the Standard.

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