220 27796 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: Wed, 17 Aug 2016 02:44:05 -0700 (PDT) Lines: 134 Approved: news@gmane.org Message-ID: References: <6322c6eb-76f7-47cc-b012-db8c5d8892fc@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_369_2061463580.1471427045606" X-Trace: blaine.gmane.org 1471427054 22990 195.159.176.226 (17 Aug 2016 09:44:14 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Wed, 17 Aug 2016 09:44:14 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCMYZKMJV4IRBZXD2C6QKGQENUUYIKA@isocpp.org Wed Aug 17 11:44:10 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qk0-f197.google.com ([209.85.220.197]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1bZxOL-0005gp-Rk for gclcip-std-proposals@m.gmane.org; Wed, 17 Aug 2016 11:44:10 +0200 Original-Received: by mail-qk0-f197.google.com with SMTP id i140sf225694196qke.0 for ; Wed, 17 Aug 2016 02:44:10 -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=BvsQ90vZIZNrp7w0nP/4nlob6vHHvSiQfwK2jQe5wu0=; b=0xd3OV2HurFB2jDiwJUzNnPrYPpndaDqLMEBFc/EKfS8cCRTGKwWfNC1hObEi3Cy1Q xWFEElRSqp3WOHKWUx6RAg/4qOEs+UsweyMFZhleTvN65xgrH5Sp7uI2lhd/F7VArzXn eLZFzUcCLN9D99zcWiT038o8fIWCHVTKXphy/QJG+q0QlV9rfMhaRZYgZ3iJyav36HbL /ex2DU/5WywqXp6yEpESLqzywz/hdmWnEiwUavkySTC4jDqYgPmVB6kUl8fO/FtBVPds +XqslV9b7zd8Xre4/g4XhoBXZWUkbInMllW4ij9FwGMkKCZODGC3Kbn15u8YMY+44KXP 8NKg== 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=BvsQ90vZIZNrp7w0nP/4nlob6vHHvSiQfwK2jQe5wu0=; b=Rwf2Nlk/BSmn7vLq8s1p7XAsm4pSJmOI/T7tIbOsHioyocF9rTF6oNrusSTK0F9I8L 9qhQGN+OBIDvYjJjzoGQYiz8J4+uVGS9hDB9X3Wi4faK3zdEDNK8WTlPg50RL713KZkO JTIppfZW3W+GouDnvFyOcAw8S4V81vrbh1vSxp8DKcxYDNqjIxgX7pfwSifLw7p4BoPh o8Sm7RCuwqEVxQUy5DjSWQqVL4sn7orgcQ6dKHW2/9alUPt6YLGNamdbmHQLmZxRhzn8 lAriOYEj4MUjH/LEw4Cj1rDynvfCRbvLiiIPC8n9X3maaKluixOnycy3o+zxc+N44sHV neGQ== 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=BvsQ90vZIZNrp7w0nP/4nlob6vHHvSiQfwK2jQe5wu0=; b=QWzAa6MZsenIL6fpEeS0uOA+4Teg76pED3Amf6nBV6LkGMQe/9Ucv+7nCptluLu7A6 pnUzBRXBtTR5ar5RCfwn4uFWA+nSn8ScD2BvqGKUntS7FNJPxygJ0/zGSDAAnOZKmdOU yEcbKTPiuxmRBt90HPqmOOiaP3mbPBaeaCnhiwObGniXAW3hlthGrHjbaRbsdz6PHjbm hUMZvOImzxn296L29RP5YVpf6lYF28wK9hHASXApxQsU8DdKIAMirCEtR344Arekfnu4 tsX0USv5vAHKYTQlK5cFdEWZg0mlCQ1NcB6S35VF5b92SvAi3535iTa3y1y8ym+sW9ID W1yw== X-Gm-Message-State: AEkoousneTdLAPXY+SZ0nVMhBBim+zlA2GFaPIBkS31uw+i5Suf3pW/z6wMjjJJbRDhK0g== X-Received: by 10.200.56.115 with SMTP id r48mr28275196qtb.28.1471427050136; Wed, 17 Aug 2016 02:44:10 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.55.214 with SMTP id e205ls558633ioa.11.gmail; Wed, 17 Aug 2016 02:44:06 -0700 (PDT) X-Received: by 10.36.103.4 with SMTP id u4mr374975itc.9.1471427046612; Wed, 17 Aug 2016 02:44:06 -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:27796 Archived-At: ------=_Part_369_2061463580.1471427045606 Content-Type: multipart/alternative; boundary="----=_Part_370_2018173270.1471427045607" ------=_Part_370_2018173270.1471427045607 Content-Type: text/plain; charset=UTF-8 On Wednesday, August 10, 2016 at 1:12:16 AM UTC+2, Richard Smith wrote: > > On Tue, Aug 9, 2016 at 3:01 PM, > wrote: > >> >> >> 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? >> > > Just a taste: > > int value; > template struct X { > some_kind_of_static_if (is_fooable_v) { > T value; > } > int f() { return value; } > }; > How is that different from int value; template int f() { some_kind_of_static_if(is_fooable_v) { return value; } else { int value = 42; return value; } } Won't there be a warning about shadowing a global variable in both cases? -- 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/a4dd84d3-45b6-4c0e-a1cc-6c88c46cbe35%40isocpp.org. ------=_Part_370_2018173270.1471427045607 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable


On Wednesday, August 10, 2016 at 1:12:16 AM UTC+2,= Richard Smith wrote:
On Tue, Aug 9, 2016 at 3:01 PM, <= span dir=3D"ltr"><rhalb...@gmail.com> wrote:


On Tuesday, August 9, 2016 a= t 7:56:22 PM UTC+2, Richard Smith wrote:

Yes, i= t's been thought about, but this is doing something fundamentally diffe= rent from what 'if constexpr' does, and it has major practical prob= lems. 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 scop= e. But reading the static if papers and their critiques, the supplied alter= native to the OP's example using partial specializations, tuple and emp= ty base class trickery do not really provide a superior alternative. Optima= l class layout is currently cumbersome, using if-constexpr would make this = a lot easier for users.=C2=A0

What exactly would b= e making it hard (compared to if-constexpr at function scope) for the compi= ler to do the conditional data layout using if-constexpr?

Just a taste:

int value= ;
template<typename T> struct X {
=C2=A0 some_kin= d_of_static_if (is_fooable_v<T>) {
=C2=A0 =C2=A0 T value;
=C2=A0 }
=C2=A0 int f() { return value; }
};

How is that differen= t from

int value;

templat= e<typename T> int f()
{
=C2=A0 =C2=A0 some_kind_o= f_static_if(is_fooable_v<T>) {
=C2=A0 =C2=A0 =C2=A0 =C2=A0 = return value;
=C2=A0 =C2=A0 } else {
=C2=A0 =C2=A0 =C2= =A0 =C2=A0 int value =3D 42;
=C2=A0 =C2=A0 =C2=A0 =C2=A0 return v= alue;
=C2=A0 =C2=A0}
}=C2=A0

W= on't there be a warning about shadowing a global variable in both cases= ?

--
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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/a4dd84d3-45b6-4c0e-a1cc-6c88c46cbe35= %40isocpp.org.
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