220 37338 article Path: news.gmane.org!.POSTED!not-for-mail From: Nicolas Lesser Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: `offsetof` in a macro-less, modular world Date: Thu, 15 Mar 2018 17:21:05 +0000 Lines: 124 Approved: news@gmane.org Message-ID: References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="001a1136e056204cc3056776b895" X-Trace: blaine.gmane.org 1521134358 11015 195.159.176.226 (15 Mar 2018 17:19:18 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Thu, 15 Mar 2018 17:19:18 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDXKNLEUTADRBDWXVLKQKGQEDOJZZQA@isocpp.org Thu Mar 15 18:19:14 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ot0-f197.google.com ([74.125.82.197]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1ewWX2-0002iK-Bl for gclcip-std-proposals@m.gmane.org; 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[209.85.220.41]) by mx.google.com with SMTPS id v9sor898756otv.268.2018.03.15.10.21.16 for (Google Transport Security); Thu, 15 Mar 2018 10:21:16 -0700 (PDT) Received-SPF: pass (google.com: domain of blitzrakete@gmail.com designates 209.85.220.41 as permitted sender) client-ip=209.85.220.41; X-Received: by 10.157.19.92 with SMTP id q28mr5871600otq.69.1521134476478; Thu, 15 Mar 2018 10:21:16 -0700 (PDT) In-Reply-To: X-Original-Sender: blitzrakete@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=T1IzJj7b; spf=pass (google.com: domain of blitzrakete@gmail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=blitzrakete@gmail.com; dmarc=pass (p=NONE sp=QUARANTINE dis=NONE) header.from=gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Spam-Checked-In-Group: std-proposals@isocpp.org 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:37338 Archived-At: --001a1136e056204cc3056776b895 Content-Type: text/plain; charset="UTF-8" Not that I know of. This was mentioned in passing while discussing P0908, where it was suggested to make it an operator. EWG then went back and forth a bit, as making offsetof an operator would mean incompatibilities with POSIX headers but better for modules. You might want to write a proposal for this. There is the Atom proposal which proposes to have a "module header" which can also solve this issue in a relatively clean way (P0947). On Thu, Mar 15, 2018, 12:12 PM Nicol Bolas wrote: > The C standard library defines a large number of things that are macros. > In the vast majority of cases, these are constants or functions, which can > in C++ code be converted into `constexpr` variables or actual functions. > For all intents and purposes, such constructs can be drop-in replacements > for the C macros, which allows us to still work with them in a modular > standard library that doesn't allow macros. > > `offsetof` is a special case because it's not a function. Or at least, > it's not a function that C++ at present allows you to write. Without > reflection, there is no way to pass a typename or member variable as a > function parameter. And while there are ways we can work around it, none of > those ways would be drop-in replacements for `offsetof`; the user would > have to change their code. > > And even ignoring the non-drop-in part, there is currently no C++ > replacement for `offsetof`. As modules moves forward, are there plans to > resolve this issue and similar ones (beside the "let's get modules to > export macros" plan)? > > -- > 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/c5f5ccc4-3267-442d-8cb2-de1ae103a38e%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/CALmDwq38-kBPK2QnEPLkzM9z7r_jXif7oBoO%2BA7CMm9XBUC96g%40mail.gmail.com. --001a1136e056204cc3056776b895 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
Not that I know of. This was mentioned in passing while d= iscussing P0908, where it was suggested to make it an operator.

EWG then went back and forth a bit, as ma= king offsetof an operator would mean incompatibilities with POSIX headers b= ut better for modules.

Y= ou might want to write a proposal for this.

There is the Atom proposal which proposes to have a &qu= ot;module header" which can also solve this issue in a relatively clea= n way (P0947).

O= n Thu, Mar 15, 2018, 12:12 PM Nicol Bolas <jmckesson@gmail.c= om> wrote:
= The C standard library defines a large number of things that are macros. In= the vast majority of cases, these are constants or functions, which can in= C++ code be converted into `constexpr` variables or actual functions. For = all intents and purposes, such constructs can be drop-in replacements for t= he C macros, which allows us to still work with them in a modular standard = library that doesn't allow macros.

`offsetof` is a special case = because it's not a function. Or at least, it's not a function that = C++ at present allows you to write. Without reflection, there is no way to = pass a typename or member variable as a function parameter. And while there= are ways we can work around it, none of those ways would be drop-in replac= ements for `offsetof`; the user would have to change their code.

And= even ignoring the non-drop-in part, there is currently no C++ replacement = for `offsetof`. As modules moves forward, are there plans to resolve this i= ssue and similar ones (beside the "let's get modules to export mac= ros" plan)?

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