220 37337 article Path: news.gmane.org!.POSTED!not-for-mail From: Hyman Rosen Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: `offsetof` in a macro-less, modular world Date: Thu, 15 Mar 2018 13:07:48 -0400 Lines: 101 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="f4f5e80aede42a0db10567768908" X-Trace: blaine.gmane.org 1521133568 19746 195.159.176.226 (15 Mar 2018 17:06:08 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Thu, 15 Mar 2018 17:06:08 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDKJJPVBQMKRB6OQVLKQKGQEVVQU25Q@isocpp.org Thu Mar 15 18:06:04 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-wr0-f199.google.com ([209.85.128.199]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1ewWKJ-00053f-Uo for gclcip-std-proposals@m.gmane.org; Thu, 15 Mar 2018 18:06:04 +0100 Original-Received: by mail-wr0-f199.google.com with SMTP id d18sf4170521wre.6 for ; Thu, 15 Mar 2018 10:08:11 -0700 (PDT) ARC-Seal: i=2; a=rsa-sha256; t=1521133691; cv=pass; d=google.com; s=arc-20160816; b=OOQ6oAQLile2n40pyoAyeJDdoDbEwRlDOf4amjQak+gzgjqBpV+RnZj1N+TSGIBKhB ZsbMHMi9DhQ+2zBwFM8x+1iSNj5ZfaukKkC5EQH1Q6YcwafUEFA1Qmp+k25oOeZeyRwQ Z3hruB4nii5XKDWwyEF1+q8LVI1bIKbNRUUECPdL+LRYhZ7D9xuK+l6guUdTxdAS6mu/ 3TondPyiaUVe63ACd25TK+3Dnb3N5IXBZGvoAvwdlumI9betQdVjKYvIPBmZWFOG9WMo ZAYuPOeLkKG4YjLFceN8G2EEvGXdjM6TeCifA7iGKuX7ybkX75IkYuMfvvYDbgGmwR21 MJvQ== ARC-Message-Signature: i=2; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=list-unsubscribe:list-subscribe:list-archive:list-help:list-post :list-id:mailing-list:precedence:reply-to:to:subject:message-id:date :from:references:in-reply-to:mime-version:arc-authentication-results :arc-message-signature:dkim-signature:arc-authentication-results; bh=SXwz+h1rEhxOZfNLyI1h/7KznPLP+7TpTqmHa2ySuf0=; b=yWzoKewYBmhfUSNYZdyCM9gPbzQDRi1gbCNCDRZWWMKl3w+kf3ktn/K9VNAzq5VS9a 5AcYpgjOw2S6Zh2BnLKtcbs7ZPddTRVEUjdOKIy3pvxt6nFfOdKLrcOWkCw2ulpILlMb WT8MK0BihhhT2UnCqQ4l8AJ4QGdOqESysByjzL/fuLzp+Mq/XCm5sapg/USW9hTDU7sH zkRA/jHvV4CRDwLKcSjgCpAQ+2NIxKusgpC7LiZizQYGUNV++5tIdDxi9pBPXZADap7q 5yhqAGkI2Pd1u7QQLIo7P0rBdu7uUTuW3GKSdTVWLZfU62jKcy8dEqavZYHOkz42RXog q+9A== ARC-Authentication-Results: i=2; mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=SzWJMkrA; spf=pass (google.com: domain of hyman.rosen@gmail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=hyman.rosen@gmail.com; dmarc=pass (p=NONE sp=QUARANTINE dis=NONE) header.from=gmail.com DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=mime-version:in-reply-to:references:from:date:message-id:subject:to :x-original-sender:x-original-authentication-results:reply-to :precedence:mailing-list:list-id:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=SXwz+h1rEhxOZfNLyI1h/7KznPLP+7TpTqmHa2ySuf0=; b=AkgGiolc8c64TYVyJ9StHtF7Z+vHHYojfw0zhf3IECE4A0CEwxuwrFb6XNU7Lv236E /jPy1hU/WpQLIoMOUHeH9V9N+0hyEakA85RhmPG8DE0rSyszyequaB+WOCW5vERAtqbD tYxLmDRGSQCer8ZnXIc+PmjhMGDZ+taGsuK59qs2wt+DLt72xyxX5Xee/5U+4ynKcyjR 1U1A5cfsZT/+WeEssgmd/Qk5Iuoj2ZFvEW05y4gSN55lDDF/Noy3iTo2KNZAADKMfN3V 7KqRYWbMMF2UDKoCKFMRCqZ3bTyaVpguMWlJhfufv4TD9lnmB0IVcT4+oHDfOMs1nGEW 0tmg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:in-reply-to:references:from:date :message-id:subject:to:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=SXwz+h1rEhxOZfNLyI1h/7KznPLP+7TpTqmHa2ySuf0=; b=UJZvFEqjQLv8zFgz4zRnxiM5LxDeFdmfiCP7Lfqn42SCczgRRxeZJAyUKIv8F5VScn GGw9tS7Ey++SBLdGDltZINLvznz4a2GcejjXtdFCN1dSJAzspNnZ5uneLCdKcWRS+tGg R8Hgjidl52qkRQX26N0+JMW9hTi7IL5RV8aQoGkJcMIQCRO82y2+zk5sCDnXpeD5ez0p JWutPpMxhCo0W8lMZLQjEUlYKphz8EzCKEE8OMYUMLbMVWHF/qQwFhFAAw2nlccw5Cdu tzRlkRpsZAgGlJntUkfEg0L4ejhUW7n9bai33ItT+O/GajMHkmayLBlp/ewtH0wyUXLk Kueg== X-Gm-Message-State: AElRT7GgWozHJemfDXCTWfsMfXDTu8d2/wRw4oy1FoaCOyTG8vxr5fII AECjz8E3JJhEgVdSugmY7QGFDQ== X-Google-Smtp-Source: AG47ELsnjliCTlZECEnSjaiEOhbiSoY9vi5i6Ufxack0zO1iBWBK6RlxpOqwCGzzrzvtVSLNiVo1lA== X-Received: by 10.223.187.6 with SMTP id r6mr1067591wrg.9.1521133690980; Thu, 15 Mar 2018 10:08:10 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.28.4.212 with SMTP id 203ls1118562wme.0.canary-gmail; Thu, 15 Mar 2018 10:08:09 -0700 (PDT) X-Received: by 10.28.184.8 with SMTP id i8mr5836494wmf.52.1521133689140; Thu, 15 Mar 2018 10:08:09 -0700 (PDT) ARC-Seal: i=1; a=rsa-sha256; t=1521133689; cv=none; d=google.com; s=arc-20160816; b=aep8TzGRkH+i+1tFic0Qe8IoogGraUQjwGmFgsl3EsmCp+bgOEWYI90qJCejiyvBuk hTYZ8BrSdc6aRLOtAWl0weg4To9qDkmbZZoogMJLkKXmxT1H8wi9ZVVOE117fxZZ5fgl v7iXBV3gnb3IZbyw6IyfI1/GjMu5W8WkDUZpRovPdabFig+q3BhXFeS2HT33ovkjGeDi nj6hgealOkqJxeJFEhmnkKI/bJsCy3Ujesg1Fc0/320dFouZIrQIDYxRzxdooARy5RIq 31rw45Fke4wuhUHosC0TorefsBlzfpEM4n/guTyjWDx+8CMO1AqAJw9HpP1w34NhA6Nh ynMg== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=to:subject:message-id:date:from:references:in-reply-to:mime-version :dkim-signature:arc-authentication-results; bh=pAZOArYBcvJUiYbDB+waQQKYTvUP8XVJdhKqInABr5k=; b=jGCDcpihUaf7V8wfOpvpsWI0CtCaK+Jd4F1x3MLa6sYtHJ0VhlfAQout9eJfQFBK4R c24O5fLdOA22beWEnrO2hUgCmKDcI7zcJQfAeiDtKgzTZTOqV/jUelgHKuM6chpEGK7z 0JbxE7pVQbrmQ/j/sRT4/68Olyi8yUq15fJ4r33HiJFMO9OIfc+AToceqbb8HM47OQVk UdbSGUNqIgTs89jLmvh7ff6ng+mOAN4brmYz9JOjfs++lPNvJfF4WFvUBoUBMrWFFF5M wJn7leMSHuunPzWxwiHAzIiaEaTFCD74Vo4I2ie6nhC6ktkpK/yMZ2FjoORj+J9rg7/w bN/w== ARC-Authentication-Results: i=1; mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=SzWJMkrA; spf=pass (google.com: domain of hyman.rosen@gmail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=hyman.rosen@gmail.com; dmarc=pass (p=NONE sp=QUARANTINE dis=NONE) header.from=gmail.com Original-Received: from mail-sor-f41.google.com (mail-sor-f41.google.com. [209.85.220.41]) by mx.google.com with SMTPS id v13sor2606836wrg.44.2018.03.15.10.08.09 for (Google Transport Security); Thu, 15 Mar 2018 10:08:09 -0700 (PDT) Received-SPF: pass (google.com: domain of hyman.rosen@gmail.com designates 209.85.220.41 as permitted sender) client-ip=209.85.220.41; X-Received: by 10.223.209.65 with SMTP id b1mr7449217wri.277.1521133688588; Thu, 15 Mar 2018 10:08:08 -0700 (PDT) Original-Received: by 10.28.55.19 with HTTP; Thu, 15 Mar 2018 10:07:48 -0700 (PDT) In-Reply-To: X-Original-Sender: hyman.rosen@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=SzWJMkrA; spf=pass (google.com: domain of hyman.rosen@gmail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=hyman.rosen@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:37337 Archived-At: --f4f5e80aede42a0db10567768908 Content-Type: text/plain; charset="UTF-8" On Thu, Mar 15, 2018 at 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)? > The Standard requires (in *[headers]*) that offsetof be a macro, so a library that bans macros perforce cannot use offsetof (or assert, setjmp, va_arg, va_end, and va_start). If you want to change that, then make offsetof a built-in keyword, just like decltype and sizeof. Also, what does it mean, "a modular standard library that doesn't use macros"? A module can (or must, I suppose, for now) have an exported interface that does not provide macros to its users, but that does not prevent the code in the module from including header files and using macros as part of its implementation. -- 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/CAHSYqdbx5W6zu%2B0SuOFU7OhiYD7jAcRY-mrw%2BNY%3Dm%2BAAAh5_qw%40mail.gmail.com. --f4f5e80aede42a0db10567768908 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
On T= hu, Mar 15, 2018 at 12:12 PM, Nicol Bolas <jmckesson@gmail.com> wrote:
The C standard library defines a large number of things that are m= acros. In the vast majority of cases, these are constants or functions, whi= ch can in C++ code be converted into `constexpr` variables or actual functi= ons. For all intents and purposes, such constructs can be drop-in replaceme= nts 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 spec= ial case because it's not a function. Or at least, it's not a funct= ion that C++ at present allows you to write. Without reflection, there is n= o way to pass a typename or member variable as a function parameter. And wh= ile 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++ rep= lacement for `offsetof`. As modules moves forward, are there plans to resol= ve this issue and similar ones (beside the "let's get modules to e= xport macros" plan)?

The Standard requires = (in [headers]) that offsetof be a macro, so a library that bans macros perforce cannot use offsetof=C2=A0(or assert, setjmp, va_arg, va_end, and va_s= tart).=C2=A0 If you want to change that, then make offsetof a built-in keyword, just like decltype and sizeof.

Also, what does it mean, "a modular stand= ard library that doesn't use macros"?=C2=A0 A module can (or must,= I suppose, for now) have an exported interface that does not provide macro= s to its users, but that does not prevent the code in the module from inclu= ding header files and using macros as part of its implementation.

--
You received this message because you are subscribed to the Google Groups &= quot;ISO C++ Standard - Future Proposals" group.
To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposa= ls+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/CAHSYqdbx= 5W6zu%2B0SuOFU7OhiYD7jAcRY-mrw%2BNY%3Dm%2BAAAh5_qw%40mail.gmail.com. --f4f5e80aede42a0db10567768908-- .