220 29390 article Path: news.gmane.org!.POSTED!not-for-mail From: Nicol Bolas Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: why not formally support meta programming. Date: Wed, 9 Nov 2016 16:45:16 -0800 (PST) Lines: 235 Approved: news@gmane.org Message-ID: References: <668e0a04-c988-4354-ae32-30f626f75403@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_135_419760284.1478738717035" X-Trace: blaine.gmane.org 1478738740 8742 195.159.176.226 (10 Nov 2016 00:45:40 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Thu, 10 Nov 2016 00:45:40 +0000 (UTC) Cc: billconan@gmail.com To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCEKFTV6ZUMBBHMGR7AQKGQEZL4EX6Y@isocpp.org Thu Nov 10 01:45:36 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ua0-f199.google.com ([209.85.217.199]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1c4dUR-0007Hc-Rl for gclcip-std-proposals@m.gmane.org; Thu, 10 Nov 2016 01:45:16 +0100 Original-Received: by mail-ua0-f199.google.com with SMTP id d33sf183656231uad.2 for ; Wed, 09 Nov 2016 16:45:19 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:cc: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=hEoPqXd26qzLxqkVTwYlMKQrrnjLDP97S8iAWQHVQ5o=; b=GkO0umNUIuC++PDkVm9lAKZs6XSb/JYGE+pt4fS55XphyDDzSDOBBafdwGhbyci3vM Mo0WNgU4VCI1CGm5TXDAelnbCgjcSScwU2cfykfe7d0eLPbjZtFcQVOqMdBb4yZYd8yQ BZA3mqesjLinxGaq9WEtvz3klyQl006WU+HumoPV1IXg4ogd+5nJcIu4bT75I6Hu5o95 AVofpyN7+ygdX1zF1B9JFnzNozD2DyZseEQMvF7gXZH+xCSQHGywe4jCNR5+jp3lOb36 5vmAoHd7cObla1SOXUNef1LKCW4T1rS7IoWNrr7XrQppmgKudJYMXSfa8rl1dsudE58b qazA== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=date:from:to:cc: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=hEoPqXd26qzLxqkVTwYlMKQrrnjLDP97S8iAWQHVQ5o=; b=ks73RRP5vKjjcnOu7vjXkBKv2Qcr/+rwoL/I1Sfi7kgSG5lO8CVQzHEqw2oEgTKqeA NpRR2QDmms6iOr1T0un7SylbjHu0u3Du55IAfjtA3fynGB3Jtpq68CsCDNbKWH8vKex6 mXK3qd6l3/oKQmhK4xbcaLYJxMwjW7/YpwCZlmbKsfTJTxWZbE8tFiaUY12v2GTgWIfL IwGn3tRbz51AwTs4Ix7dL8Una3ROYS1+0vSnzBA0qRkn+/7rejlLcveC+gGQC+0Lldrz Tx5wmMZMVYUT+nF+oEid9YH2lHsATGZlhdTB5z4mFnoO+gStxRHWNPQCRjQNGDLLffHH OMAQ== 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:cc: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=hEoPqXd26qzLxqkVTwYlMKQrrnjLDP97S8iAWQHVQ5o=; b=JGjbOPqAD2HSeNrQvEJ0W0j9Svc0oaT1DTkckUTBBfQhEe/xMDylU5s2vcbhp459WF g3nw6GAP1lUvY9MfbsPiFdLbNhnelTW8ST5AxZYQuTHQqyC5bxAIcPtNgfd6sgPSslRy B7mrhCFTnBo3hP/ouK2mIRo1oYwUOdBzTeD68iXH6+iorPWJa+gnpSZ+7y3hBqLYX2sY qNlMmosN0c7AVtkTk9vmny98lRs0PIkRfsTYP0aRxLq1c42pVad5QTTY+U3M+JlHVMM9 CVVaSgEtnrJpTmW6HQ9wICfZBi2/TFUvkfx7bv5vCO4geBeWbvJXziM4w1HXOzlCK1EZ hZIQ== X-Gm-Message-State: ABUngvekTbpEJl6VeBzo2SCbP7TzvD5mGMi1Dn+jlVPPoZsjcj8A6aLUYcwMWGEQvCsWvQ== X-Received: by 10.176.84.72 with SMTP id o8mr19915uaa.8.1478738718671; Wed, 09 Nov 2016 16:45:18 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.14.179 with SMTP id 48ls7206592otj.31.gmail; Wed, 09 Nov 2016 16:45:17 -0800 (PST) X-Received: by 10.157.3.235 with SMTP id f98mr408894otf.14.1478738717709; Wed, 09 Nov 2016 16:45:17 -0800 (PST) In-Reply-To: X-Original-Sender: jmckesson@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:29390 Archived-At: ------=_Part_135_419760284.1478738717035 Content-Type: multipart/alternative; boundary="----=_Part_136_1451348335.1478738717035" ------=_Part_136_1451348335.1478738717035 Content-Type: text/plain; charset=UTF-8 A combination of `if constexpr` and template specialization can handle what you're doing: struct CPU_Device {}; struct GPU_Device {} template class BlahData; template class BlahData { DataType member_only_used_by_cpu; void function_on_cpu(); }; template class BlahData { DataType member_only_used_by_gpu; void function_on_gpu(); }; template class Blah : public BlahData { DataType shared_member; void shared_function() { ... if constexpr(std::is_same) { } } }; If you want to have `BlahData` functions be able to access the shared members of `Blah`, then you'll need to have `BlahData` use the CRTP trick to do so. On Wednesday, November 9, 2016 at 3:11:58 PM UTC-5, Shi Yan wrote: > > So what's the reason for rejection? > That it was really, really difficult to specify. -- 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/f2877a3a-dd59-4a52-ab25-6458b9de3128%40isocpp.org. ------=_Part_136_1451348335.1478738717035 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
A combination of `if constexpr` and template specializatio= n can handle what you're doing:

struct CPU_Device {};
struct GPU_Device {}

template<= typename <= /span>Device, typename DataType>
class BlahData;
template<= /span><= typename DataType>
class BlahData<CPU_Device, DataT= ype>
{
=C2=A0 DataType member_only_used_by_cpu;
=C2=A0 void function_on_cpu();
};

template<typename DataType>
class BlahData<GPU_Device, DataType>
{
=C2=A0 DataType member_only_used_by_gpu;
=C2=A0 void function_on_gpu();
};<= br>
tem= plate<<= /span>typename Device, typename DataType>
class Blah : public BlahData<Device,= DataType<= /span>>=
{
=C2=A0 DataType shared_member;

=C2=A0 void shared_function()
=C2=A0 {
=C2=A0 =C2=A0 ...
=C2=A0 =C2=A0 if constexpr(std::is_sa= me<Device<= span style=3D"color: #660;" class=3D"styled-by-prettify">, GPU_Device>)
=C2=A0 =C2=A0 {
=C2=A0 =C2=A0 }
=C2=A0 }
};

If you want to = have `BlahData` functions be able to access the shared members of `Blah`, t= hen you'll need to have `BlahData` use the CRTP trick to do so.

= On Wednesday, November 9, 2016 at 3:11:58 PM UTC-5, Shi Yan wrote:
So what's the reas= on for rejection?=C2=A0

That it was really, real= ly difficult to specify.

--
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/f2877a3a-dd59-4a52-ab25-6458b9de3128= %40isocpp.org.
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