220 27728 <6322c6eb-76f7-47cc-b012-db8c5d8892fc@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: rhalbersma@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: if constexpr in class declaration Date: Tue, 9 Aug 2016 15:01:33 -0700 (PDT) Lines: 78 Approved: news@gmane.org Message-ID: <6322c6eb-76f7-47cc-b012-db8c5d8892fc@isocpp.org> References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_761_1141610826.1470780093348" X-Trace: blaine.gmane.org 1470780100 20345 195.159.176.226 (9 Aug 2016 22:01:40 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 9 Aug 2016 22:01:40 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCMYZKMJV4IRBPVFVG6QKGQERJZHD2I@isocpp.org Wed Aug 10 00:01:36 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ua0-f197.google.com ([209.85.217.197]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1bXF5b-0005Ad-P9 for gclcip-std-proposals@m.gmane.org; Wed, 10 Aug 2016 00:01:35 +0200 Original-Received: by mail-ua0-f197.google.com with SMTP id u13sf50946088uau.2 for ; Tue, 09 Aug 2016 15:01:36 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=9Hv0iQN0W7TBOejxQa2WQaCmlepLsKb5QHh02Q4Ij7w=; b=EcYxfchH38YZp+bSoafCj/qFpOfPcBLE0wG48O3HmZFNDbWqVl7sRX+hJAQ4jOgFKe Zkrk/ainICA6zAPk8S4qDfQ5ZY5DOX24y7hvaRtVZpS6FW7vlMV+ZOzYNDq99GvijIqY CZgwWkcHHE4pzxPmRGdDqwNYRNryjt/LoPSnMmFUPWTsg/9jJ/k/hfiZrE8XxJMOcPWH jivsYKfo0sJPmBYbKQvg0zVD7b3LCWVV4ONSdGcMhF0F2iQIXFBVS1h9pawOgTU1MINF PH0PBDGSHV41oxnE5csl9nH829BurFxGiAWpP7+jlO/YLmWCgnTy60ImxhJXKkZ/FkQI 7eUg== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=9Hv0iQN0W7TBOejxQa2WQaCmlepLsKb5QHh02Q4Ij7w=; b=D6BdHNHRpO/G4M1ToHsnNtKdbNm9fcpStvkZne41lKCrgTo6P9mmqOWx/El89g+SMC pWmwqVeaLq+bd8JI4Z3tXkD4uO4aHEAkR3EfHy4Temk+cIDpehHTRpGcXWvD8SUk2x9d n7p9WWbTNchTQrRS1+WheNvOxM5qZ4CGtiJyGncHfmzC2MUpIy1F9P6f+OyC8Hw8jTTu xUeGXxzhZZT63ILTpdgIk3mnq9Aq0SLvBXtg0HsWkBqjIiOZYxCzZRKCCOtJmBBCsN2B otwM8GSb/IiPE78sgfrmgDtucOUouE/fy7fnbo8gRWRJQlPaFj6o7S3aINe/xSpcg7Ci iLPQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:date:from:to:message-id:in-reply-to:references :subject:mime-version:x-original-sender:reply-to:precedence :mailing-list:list-id:x-spam-checked-in-group:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=9Hv0iQN0W7TBOejxQa2WQaCmlepLsKb5QHh02Q4Ij7w=; b=KBdQlKxaylgEk3qsl7n/CftHyo/gZnsJx6nim7eZNMNjgXck0R5edTVtb+ldEIrvl2 +h6FpbM8lE5imARl21WHO8NGALTCFlGjVyrCwATVFmlFM5lyB4ZMDGrco1ILIZTZPwsL nV1WIrQna/Ypt+os9u9EpipAT032c37En71EZlLGydOZGyQ1AQ2VvaAEeswaoxc2vZV7 8OVQg6JqaIBsMD6ZSOvQtbdENuDNq6YdIwMWvejuDzLnzEUaUsDawLjhevCqZQk8kqjJ 0oP3kEFqrAIQUC5IS35fjHs530as4v73SiYbt3Ynuiym6+KIzOsQfLk4rwxae7nGg0HK hCdQ== X-Gm-Message-State: AEkooutBV5aDsFWh6xW0KZKtOnmUUL/ECVcVT3LCPYMM9UgZ0/UOnl1UR10klkXuz2CF0A== X-Received: by 10.31.166.196 with SMTP id p187mr498190vke.4.1470780095838; Tue, 09 Aug 2016 15:01:35 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.55.214 with SMTP id e205ls5185877ioa.11.gmail; Tue, 09 Aug 2016 15:01:34 -0700 (PDT) X-Received: by 10.36.50.145 with SMTP id j139mr107303ita.9.1470780094634; Tue, 09 Aug 2016 15:01:34 -0700 (PDT) In-Reply-To: X-Original-Sender: rhalbersma@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:27728 Archived-At: ------=_Part_761_1141610826.1470780093348 Content-Type: multipart/alternative; boundary="----=_Part_762_1490687678.1470780093348" ------=_Part_762_1490687678.1470780093348 Content-Type: text/plain; charset=UTF-8 On Tuesday, August 9, 2016 at 7:56:22 PM UTC+2, Richard Smith wrote: > > > Yes, it's been thought about, but this is doing something fundamentally > different from what 'if constexpr' does, and it has major practical > problems. You can look through the history of papers on constexpr if and > static if for a taste of the problems. > I understand that applying if-constexpr to conditional data member layout is different because a regular if cannot appear at class scope. But reading the static if papers and their critiques, the supplied alternative to the OP's example using partial specializations, tuple and empty base class trickery do not really provide a superior alternative. Optimal class layout is currently cumbersome, using if-constexpr would make this a lot easier for users. What exactly would be making it hard (compared to if-constexpr at function scope) for the compiler to do the conditional data layout using if-constexpr? -- 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/6322c6eb-76f7-47cc-b012-db8c5d8892fc%40isocpp.org. ------=_Part_762_1490687678.1470780093348 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable


On Tuesday, August 9, 2016 at 7:56:22 PM UTC+2, Ri= chard Smith wrote:

Yes, it's been thou= ght about, but this is doing something fundamentally different from what &#= 39;if constexpr' does, and it has major practical problems. You can loo= k through the history of papers on constexpr if and static if for a taste o= f the problems.

I underst= and that applying if-constexpr to conditional data member layout is differe= nt because a regular if cannot appear at class scope. But reading the stati= c if papers and their critiques, the supplied alternative to the OP's e= xample using partial specializations, tuple and empty base class trickery d= o not really provide a superior alternative. Optimal class layout is curren= tly cumbersome, using if-constexpr would make this a lot easier for users.= =C2=A0

What exactly would be making it hard (compa= red to if-constexpr at function scope) for the compiler to do the condition= al data layout using if-constexpr?

--
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/6322c6eb-76f7-47cc-b012-db8c5d8892fc= %40isocpp.org.
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