220 8063 <52A4065B.2010806@gmail.com> article Path: news.gmane.org!not-for-mail From: David Krauss Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Subsets of constexpr functions should be considered constant expressions (in certain cases) Date: Sun, 08 Dec 2013 13:40:43 +0800 Lines: 52 Approved: news@gmane.org Message-ID: <52A4065B.2010806@gmail.com> References: <8c654488-9a6d-4d14-ae94-c854bf9fcf8b@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed X-Trace: ger.gmane.org 1386481246 32239 80.91.229.3 (8 Dec 2013 05:40:46 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sun, 8 Dec 2013 05:40:46 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCW25A7E3QCRBYEMSCKQKGQEFISZJKI@isocpp.org Sun Dec 08 06:40:52 2013 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pd0-f197.google.com ([209.85.192.197]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1VpX6p-0002Vz-CI for gclcip-std-proposals@m.gmane.org; Sun, 08 Dec 2013 06:40:51 +0100 Original-Received: by mail-pd0-f197.google.com with SMTP id v10sf11244694pde.4 for ; Sat, 07 Dec 2013 21:40:49 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:message-id:date:from:user-agent:mime-version:to :subject:references:in-reply-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:content-type; bh=KbZSgoLE5C8sd6uly1t8WBWGk2m4bM9k4L/zxHG8Jc4=; b=dNNd2I7XuR7ZoxDdW0pqcvflhLNUZQV0dxBdg11WRIFuYpMpY8S5HQ2Z397xPBOSqv t7gcD3bWcgvXRAFKnEqIr7t1uajZHI7y9Xd8Z1Axo9fyFowKrKgu4GKizdM8gKJPvmFb +JJWwygp/G2jxaRWXOcT2vXG43mZR12ptL8p22XAsCszKEdwYD+D2q/aYHKhFC82BYK6 xgEdzQhFdJ2b98+pcFkYVk6hCW6cljEKEpasG5nT7+7HB0w3tfQ4kSjYB0UZc7/s9CYA xtDEzwOeFXPSHBi6y/3cTvfPuNVqRhDsP9rmcTTb1s7Uqv1QK6o+sJrW48QNJ1/uSWBU 7U+A== X-Gm-Message-State: ALoCoQk78TZfKBWzy9ESTseoLeb6IJFzpjFsHuiUry8DhFMPPl3t0eaCe0ByLgDcIKy617sYLIVP X-Received: by 10.67.3.34 with SMTP id bt2mr94909pad.41.1386481249218; Sat, 07 Dec 2013 21:40:49 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.49.58.229 with SMTP id u5ls1820701qeq.69.gmail; Sat, 07 Dec 2013 21:40:48 -0800 (PST) X-Received: by 10.236.220.195 with SMTP id o63mr580877yhp.149.1386481248393; Sat, 07 Dec 2013 21:40:48 -0800 (PST) Original-Received: from mail-pb0-x236.google.com (mail-pb0-x236.google.com [2607:f8b0:400e:c01::236]) by mx.google.com with ESMTPS id k1si4502302yhm.118.2013.12.07.21.40.48 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Sat, 07 Dec 2013 21:40:48 -0800 (PST) Received-SPF: pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:400e:c01::236 as permitted sender) client-ip=2607:f8b0:400e:c01::236; Original-Received: by mail-pb0-f54.google.com with SMTP id un15so3421504pbc.13 for ; Sat, 07 Dec 2013 21:40:47 -0800 (PST) X-Received: by 10.66.218.226 with SMTP id pj2mr13478073pac.62.1386481247548; Sat, 07 Dec 2013 21:40:47 -0800 (PST) Original-Received: from Davids-MacBook-Pro.local ([121.54.54.152]) by mx.google.com with ESMTPSA id nn6sm8529800pbc.29.2013.12.07.21.40.44 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Sat, 07 Dec 2013 21:40:46 -0800 (PST) User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.8; rv:24.0) Gecko/20100101 Thunderbird/24.1.1 In-Reply-To: <8c654488-9a6d-4d14-ae94-c854bf9fcf8b@isocpp.org> X-Original-Sender: potswa@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:400e:c01::236 as permitted sender) smtp.mail=potswa@gmail.com; dkim=pass header.i=@gmail.com; dmarc=pass (p=NONE dis=NONE) header.from=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:8063 Archived-At: On 12/7/13 12:31 PM, Mark wrote: > I've been toying around with the relaxed constexpr rules of C++14 in an SVN > build of clang. The thought occurred to me that I might be able to use the > 'natural' syntax of a for loop to iterate over the contents in a std::tuple > at compile-time in a constexpr function. Currently, each name is declared once with a definite type, and looked up exactly once per use, or once per template instantiation in the case of names following a template argument dependent nested-name-specifier. Resolving a new type for a local name once per loop iteration would be quite a radical change. The most similar proposal is called static_if, to allow a section of source code to be interpreted either zero or one times. Static loops are a logical next step. The idea is very controversial, and it's not clear that such a construct is conducive to type-safe generic programming. (The main problem is that it attempts to ignore scoping, which your idea doesn't require.) The for_each idiom is doable in C++14 using the limited local template facility in polymorphic lambdas. template< typename tuple, typename ftor, std::size_t ... index > void tuple_for_each_impl( tuple & t, ftor f, std::integer_sequence< std::size_t, index ... > ) { int q[] = { ( f( std::get< index >( t ) ), 0 ) ... }; } template< typename tuple, typename ftor > void tuple_for_each( tuple && t, ftor f ) { tuple_for_each_impl( t, std::move( f ), std::make_integer_sequence< std::size_t, std::tuple_size< typename std::decay< tuple >::type >::value >() ); } tuple_for_each( std::make_tuple( "Hello ", true, std::string( " world!" ), 1 ), []( auto x ) { std::cout << x; } ); (I was not able to test this because my GCC doesn't have make_integer_sequence and my Clang doesn't have polymorphic lambdas.) -- --- 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. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposals/. .