220 8502 <628bba42-9a56-42d7-8182-0de7c10c437b@isocpp.org> article Path: news.gmane.org!not-for-mail From: Alex B 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: Fri, 10 Jan 2014 07:35:15 -0800 (PST) Lines: 125 Approved: news@gmane.org Message-ID: <628bba42-9a56-42d7-8182-0de7c10c437b@isocpp.org> References: <-5123564187529511709@gmail297201516> <52CB7506.1000906@gmail.com> <62bda680-3dd7-4be8-8ac5-0d2e08bf6d99@isocpp.org> <52CDFB2D.4050703@gmail.com> <-9126987679220332623@gmail297201516> <52CE0F54.5020800@gmail.com> <52CF3AB0.7090102@gmail.com> <52CF3D80.9090702@gmail.com> <52CF4DC0.20100@gmail.com> <031149b3-1c5b-4c6d-a827-82d29bc73d6f@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_Part_349_5201527.1389368115307" X-Trace: ger.gmane.org 1389368112 23119 80.91.229.3 (10 Jan 2014 15:35:12 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Fri, 10 Jan 2014 15:35:12 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBC7JZJ43WYOBBNFGYCLAKGQECQV5STA@isocpp.org Fri Jan 10 16:35:20 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-oa0-f72.google.com ([209.85.219.72]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1W1e7B-0003E6-Mg for gclcip-std-proposals@m.gmane.org; Fri, 10 Jan 2014 16:35:18 +0100 Original-Received: by mail-oa0-f72.google.com with SMTP id o6sf17189280oag.11 for ; Fri, 10 Jan 2014 07:35:16 -0800 (PST) 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 :list-post:list-help:list-archive:list-subscribe:list-unsubscribe :content-type; bh=UzLzgysTJZefAPp9e/W0UhLX4yQc9JrboZJbg4PsE/Y=; b=oaQ+EveWpzxx0QgcCEG/EGiPptRvZ6bMGkzkK1A+zFiIvcPDdiCO+1OC6gpLza1Qc8 44b//COkcpVzloI/7LUFWS0zoSlQ12LiGGCcOkKQTFc+exTe7QebN/1N7pyOfsVjQyA1 aRvVTDG/CgMhTU+zHymgSP5nI1WI0OxBkHFT0pQ/8J3vwcWkzn9Bba+5p1j24WVrcqhp K0Ht0wZWotJFWho0sS3UQI5xu5KsFCGON//x6JkDs99fGmJmSflx12lKGzQk+aqYxgca 7sC0rrjyRCY+dsJAtjFWJFpUTD3wBCarZezHiboRF2nwzaMe6GeN5c6S8SW4LtrLl9pa guCQ== 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:list-post:list-help:list-archive :list-subscribe:list-unsubscribe:content-type; bh=UzLzgysTJZefAPp9e/W0UhLX4yQc9JrboZJbg4PsE/Y=; b=NFBXxkO0bRhH4ocXJg9fkM330F+s7rY7NluX9OWguwnOCetpSy/iFor2VS8VMVcJTw YPYmhS4RE+7AuDVOwGUeGIltQzhFusxgWTSg/DpsK2fXRNaH1HOUkijZdDeOH2o8iSt/ Z99oRI5auGai79mvT6Qa9OgGd6GBDw9V8DXXAndOBRDPrYXGk1yZLO3xQnEKRuGshl0I n6sxzYlwrUBtz4PV8E4XaamZz4USgxTzPbNBmxTiDRZj393rgWVH4Grd7BFtbAOT3xk+ DlGVrY6z3GAWjjQvbvuUMUzky5TAXICLPKL9RXaBPPoCN2xgSjlCmzn8kbC5Oh+ZVmxP DL1A== X-Gm-Message-State: ALoCoQmg8hO+8zIFo+ENcg7Ub/d/EvknoJCBXiXgd5a2TRBCWsnAqjszKf/pkw2FJtGUW4rS5dTx X-Received: by 10.42.46.3 with SMTP id i3mr797742icf.31.1389368116730; Fri, 10 Jan 2014 07:35:16 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.182.121.193 with SMTP id lm1ls529328obb.35.gmail; Fri, 10 Jan 2014 07:35:15 -0800 (PST) X-Received: by 10.182.183.74 with SMTP id ek10mr73975obc.21.1389368115870; Fri, 10 Jan 2014 07:35:15 -0800 (PST) In-Reply-To: <031149b3-1c5b-4c6d-a827-82d29bc73d6f@isocpp.org> X-Original-Sender: devalexb@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:8502 Archived-At: ------=_Part_349_5201527.1389368115307 Content-Type: text/plain; charset=ISO-8859-1 On Friday, January 10, 2014 5:01:31 AM UTC-5, Bengt Gustafsson wrote: > > template<...> struct HeadInfo; > template struct HeadInfo { > static const bool is_type = true; > typedef T type; > }; > template struct HeadInfo { > static const bool is_type = false; > typedef decltype(V) type; > static const auto value = V; > }; > > That syntax without ? would still require being able to name the generic pack. Your example should be: template struct HeadInfo { static const bool is_type = true; typedef T type; }; Giving a name would be required to disambiguate cases where there is more than one generic pack: template <...> struct A {}; template struct B; template <...ElemsT, ...ElemsU> struct B, A> { }; As for actual use cases (with or without ?), there is more than metaprogramming refactoring sugar. I can see at least one case that it would allow to fix which is boost recursive variants: *http://www.boost.org/doc/libs/1_55_0/doc/html/boost/make_recursive_variant.html* Unless I'm wrong, recursive variants currently cannot contain a template type with non-type parameters, which is a bit annoying. This feature would make it possible. -- --- 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. 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On Friday, January 10, 2014 5:01:31 AM UTC-5, Bengt G= ustafsson wrote:
&n= bsp;
   template<...> struct HeadInfo;
=    template<typename T, ...> struct HeadInfo<T, ...> = {
       static const bool is_type =3D true;<= /div>
       typedef T type;
   = ;};
   template<auto V, ...> struct HeadInfo<V= , ...> {
       static const bool is_type = =3D false;
       typedef decltype(V) type;
       static const auto value =3D V;
   };
 
 
That syntax without ? would still require being able to name the gene= ric pack. Your example should be:
 
template<typename T, ...Tail> struct HeadInfo<T, Tail...> {
= static const bool is_type =3D true;
typedef T type;
};
 
 
Giving a name would be required to disambiguate cases wh= ere there is more than one generic pack:
 
template <...> struct A {};
 
template <class T, class U>
struct B;
 
templ= ate <...ElemsT, ...ElemsU>
struct B<A<ElemsT...>, A<ElemsU...>>
{
};
 
As for actual use cases (with or without ?), there is more than = ;metaprogramming refactoring sugar. I can see at least one case t= hat it would allow to fix which is boost recursive variants:
Unles= s I'm wrong, recursive variants currently cannot contain a template type wi= th non-type parameters, which is a bit annoying. This feature would ma= ke it possible.

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