220 8478 <52CDFB2D.4050703@gmail.com> article Path: news.gmane.org!not-for-mail From: David Krauss Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: A new keyword for either a type or a variable (typical use case for variadic template template)? Date: Thu, 09 Jan 2014 09:28:13 +0800 Lines: 58 Approved: news@gmane.org Message-ID: <52CDFB2D.4050703@gmail.com> References: <-5123564187529511709@gmail297201516> <52CB7506.1000906@gmail.com> <62bda680-3dd7-4be8-8ac5-0d2e08bf6d99@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 1389230894 19467 80.91.229.3 (9 Jan 2014 01:28:14 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Thu, 9 Jan 2014 01:28:14 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCW25A7E3QCRBMPWW6LAKGQECFRKCMI@isocpp.org Thu Jan 09 02:28:21 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ob0-f200.google.com ([209.85.214.200]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1W14Pz-0002Vo-Uw for gclcip-std-proposals@m.gmane.org; Thu, 09 Jan 2014 02:28:20 +0100 Original-Received: by mail-ob0-f200.google.com with SMTP id wm4sf8796829obc.7 for ; Wed, 08 Jan 2014 17:28:18 -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=CqoBN038h4kb264iAQjNb/yXvGqGNeuJVZ6dEn+LPM0=; b=VnVpFkGoIJVuNf19ONKP7hcRPhw5mJFFCNTFTgPc0CvHZ98w+uHBsrlba65+DK/pnx PQOjkdYdU/Akl6sLN0O5jb72/jcZHbcXVI1/OFM5y5QlJrTPwIL8obYfRpct2C1DRBxL B6Pjt2NFf+vXNvSEyLnFoEIrfpw9Dj08w2ZZEG11HHXIVg+NMhV51X59AVtsml3SiprM DUaKnnqi9J/HwvZYqSh+ZRSXrRrm9x06uAqBrRHiIoFwE2eaq7SdvweHlvJ3UDKsN+cp 8/qDY7TmB9NwBfWLmAsZ4BfYmyX+NOrdnFJ2J/0ykjlxQYgTaQShSChAs5WgMd0BjbB4 OuSg== X-Gm-Message-State: ALoCoQn2bTf5+6LTDcWXQ6F3yBJ2bdm4mcLJEScCdObZ7sjrDsGmCQ4hsCzuAO4NjPGiIrqpxtkW X-Received: by 10.182.241.129 with SMTP id wi1mr176659obc.10.1389230898691; Wed, 08 Jan 2014 17:28:18 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.49.48.105 with SMTP id k9ls741526qen.96.gmail; Wed, 08 Jan 2014 17:28:17 -0800 (PST) X-Received: by 10.236.152.233 with SMTP id d69mr409238yhk.11.1389230897809; Wed, 08 Jan 2014 17:28:17 -0800 (PST) Original-Received: from mail-oa0-x22d.google.com (mail-oa0-x22d.google.com [2607:f8b0:4003:c02::22d]) by mx.google.com with ESMTPS id z48si2869341yha.6.2014.01.08.17.28.17 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Wed, 08 Jan 2014 17:28:17 -0800 (PST) Received-SPF: pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:4003:c02::22d as permitted sender) client-ip=2607:f8b0:4003:c02::22d; Original-Received: by mail-oa0-f45.google.com with SMTP id j17so27484oag.18 for ; Wed, 08 Jan 2014 17:28:17 -0800 (PST) X-Received: by 10.60.52.14 with SMTP id p14mr279709oeo.28.1389230897434; Wed, 08 Jan 2014 17:28:17 -0800 (PST) Original-Received: from Davids-MacBook-Pro.local ([121.54.54.58]) by mx.google.com with ESMTPSA id g4sm3292290obe.5.2014.01.08.17.28.15 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Wed, 08 Jan 2014 17:28:16 -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:f8b0:4003:c02::22d 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:8478 Archived-At: On 1/9/14 8:44 AM, Richard Smith wrote: > Here's an implementation: > > template struct is_type : std::false_type {}; > template struct is_type : std::true_type {}; This appears to be using ? like auto, not to mean "type or object." Also, as you know, class templates cannot currently be overloaded like this. Functions can be, so: template< typename > std::true_type is_type(); template< int > // Insufficient generality, to be fixed by "auto"-like type deduction. std::false_type is_type(); template< template< typename ... > class > // Insufficient generality, to be fixed by "?"-like kind deduction. std::false_type is_type(); bool three_is_type = decltype( is_type< 3 >() )::value, double_is_type = decltype( is_type< double >() )::value, vector_is_type = decltype( is_type< std::vector >() )::value; > Yes, I think the ? extension alone is sufficient for this. > > With the addition of some motivating examples, this seems ready to be > written up as a paper to me. I'm somewhat optimistic that the most > contentious part of this will be the syntax ('?' or 'auto' or a new keyword > or something else). I still don't see the value of an identifier, as opposed to a pack expansion element, that can be either a type or a value. And such a name would throw a wrench into template parsing. They could *only* be used as explicit template arguments to functions as in my example above, or to template template parameters with "?"-like parameter lists. If you end up needing to use is_type, you're already in deep shit. On the other hand, parameter lists are already unpacked using overloading, or partial specialization which works the same way. This gives you a branching point to different code paths for type, value, or template, which can already be written and parsed without redesigning dependent name lookup. This is why I think "..." is sufficient with no motivation for "?" or any provision for a single argument. -- --- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. 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