220 8135 <52AE9F22.1070403@gmail.com> article Path: news.gmane.org!not-for-mail From: David Krauss Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: A compile-time string literal library type Date: Mon, 16 Dec 2013 14:35:14 +0800 Lines: 52 Approved: news@gmane.org Message-ID: <52AE9F22.1070403@gmail.com> References: 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 1387175719 11990 80.91.229.3 (16 Dec 2013 06:35:19 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Mon, 16 Dec 2013 06:35:19 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCW25A7E3QCRBKV6XKKQKGQECBCAWLY@isocpp.org Mon Dec 16 07:35:25 2013 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pd0-f198.google.com ([209.85.192.198]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1VsRm0-0000rH-RO for gclcip-std-proposals@m.gmane.org; Mon, 16 Dec 2013 07:35:25 +0100 Original-Received: by mail-pd0-f198.google.com with SMTP id g10sf14192245pdj.1 for ; Sun, 15 Dec 2013 22:35:23 -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=wg+YyftgFfkuMNgfrNa287QrOLS7UysmB41L2j0G7h0=; b=ijlKqFsXMvezmJ0lZcA8O+izFzqmPNl90qTdboDb12Dy3xvGF8gu97OaQZwsdmQ15K t+SRze3Xy+74dT0md3esdHoU09Fd7FYIjIhIa8AmL+ZLH1Fzi5rq2x20mTKDczT+XqIZ qLjaT62TUBc5k1QvYrvr6h6wNiM/0HIGzV/LyZaXtCjVJQcxPzdxVGpHS7NjeJrDoM/o vdw1wJhGY4zvcPNjLuwB4+9yMWZHTiZLy3hRCt4P4zm/GApH5KorKTA7sAxv+2Qn/1QR h4mGtUPXX5UeJ9Tm8t9CIzc1/z2JoPLzbBaASyR14CB72DwdahnCvlqKlbRpX1YaB6r/ kGaQ== X-Gm-Message-State: ALoCoQkdIE0WMIkStGuKpG6FrpwqXuYxfNk8XRUrPxoHpnP0DNrjTPLhwt8+X+1zi9ke2UqR+mzM X-Received: by 10.66.218.99 with SMTP id pf3mr8773075pac.8.1387175723657; Sun, 15 Dec 2013 22:35:23 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.49.127.175 with SMTP id nh15ls1999034qeb.9.gmail; Sun, 15 Dec 2013 22:35:22 -0800 (PST) X-Received: by 10.236.190.199 with SMTP id e47mr11317455yhn.26.1387175722676; Sun, 15 Dec 2013 22:35:22 -0800 (PST) Original-Received: from mail-pa0-x22f.google.com (mail-pa0-x22f.google.com [2607:f8b0:400e:c03::22f]) by mx.google.com with ESMTPS id x29si11614819yha.161.2013.12.15.22.35.22 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Sun, 15 Dec 2013 22:35:22 -0800 (PST) Received-SPF: pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:400e:c03::22f as permitted sender) client-ip=2607:f8b0:400e:c03::22f; Original-Received: by mail-pa0-f47.google.com with SMTP id kq14so2519952pab.20 for ; Sun, 15 Dec 2013 22:35:21 -0800 (PST) X-Received: by 10.68.244.2 with SMTP id xc2mr18445573pbc.58.1387175721743; Sun, 15 Dec 2013 22:35:21 -0800 (PST) Original-Received: from Davids-MacBook-Pro.local ([121.54.54.58]) by mx.google.com with ESMTPSA id wp8sm23607789pbc.26.2013.12.15.22.35.18 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Sun, 15 Dec 2013 22:35:20 -0800 (PST) User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.8; rv:24.0) Gecko/20100101 Thunderbird/24.2.0 In-Reply-To: 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:c03::22f 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:8135 Archived-At: On 12/16/13 12:52 PM, Zhihao Yuan wrote: > So what is the problem you want to solve? Compile-time parsing? > If so, Bristol EWG says: > > 1. don't think template instantiations is the only way to do it; > 2. doubt user to overuse it (a compile time SQL parser is fun, > but what you gain?); > 3. need use cases. I need a template to map compile-time strings to types. The use case is essentially an exception class template. The program, a parser, has countless exception types, representing the various syntax errors and such. It's nice to be able to declare a new, unique, named exception type, without making a boilerplate declaration at namespace scope. There are also other generic use cases in the same program. Right now I'm making due with an imperfect constexpr hash: typedef std::uintmax_t hash_t; constexpr hash_t parameter_name_hash( char const *str, std::size_t offset = 0 ) { return str[ offset ]? /* recursive hash iteration */ : 0; } constexpr hash_t operator "" _param ( char const * str, std::size_t ) { return parameter_name_hash( str ); } The ud-string-literal is used directly as a non-type template argument, so it must have scalar type, which cannot contain more information than a std::intmax_t, or else be a reference to global, which seemingly cannot be generated suitably. Another way of looking at this is not the need for template string arguments, but the need for a non-type template argument that can uniquely encode arbitrarily much data, and can be returned from a ud-string-literal function. It would be a lot easier if C style arrays were value-semantic. But because these uniquefying tags used very heavily in my metaprocessing (not as strings, just as hashed ID numbers), I wouldn't really want to sacrifice much efficiency just to avoid hash collisions. I suspect that one-parameter-per-character string packs could cause compile time to suffer. (If such an extension is already part of the g++1y dialect, I should try and see.) -- --- 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/. .