220 8481 article Path: news.gmane.org!not-for-mail From: Richard Smith 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: Wed, 8 Jan 2014 19:18:08 -0800 Lines: 174 Approved: news@gmane.org Message-ID: References: <-5123564187529511709@gmail297201516> <52CB7506.1000906@gmail.com> <62bda680-3dd7-4be8-8ac5-0d2e08bf6d99@isocpp.org> <52CDFB2D.4050703@gmail.com> <-9126987679220332623@gmail297201516> <52CE0F54.5020800@gmail.com> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary=047d7bd6b5b4015a7604ef810f40 X-Trace: ger.gmane.org 1389237485 19672 80.91.229.3 (9 Jan 2014 03:18:05 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Thu, 9 Jan 2014 03:18:05 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDVNBJG4YAIBB4FJXCLAKGQEMXQXF3I@isocpp.org Thu Jan 09 04:18:12 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-oa0-f69.google.com ([209.85.219.69]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1W168I-0004yw-Gy for gclcip-std-proposals@m.gmane.org; Thu, 09 Jan 2014 04:18:10 +0100 Original-Received: by mail-oa0-f69.google.com with SMTP id m1sf9559344oag.8 for ; Wed, 08 Jan 2014 19:18:09 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:mime-version:sender:in-reply-to:references:date :message-id:subject:from: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=4XKNJBY2g5/SsUhpB0+eQ0u8ZIGxEM14b7sRr26UAs0=; b=KvtHGZSvkZzS/RfG4DcJcWaRdfFSkbBryb718XvJrxRMWhAK3/QMUARgy+fokbudBO qfjC0mDUn4m+lHBDHGbHjLL0kvjNgWxO6PRVsQTSwBR0fMFsnj1djpxyr9P41Isb+kyu pfeMLJ8cLBKELB7UwrdylXfF3IqCcICizgjlEa4qcI6s+qlMVnpqiKWHDLzjFRoWYs/X 9QxkgifAVZCF0TQ03x3GD8zf0mMgKTm0dNDiG9KkbwozjwHdAuE3HJfyR6FrKTgYlG/i 8Rqtys49h5NrDy/VhOF3WWj/y4K1XOLDkasSO9UUrjhiDmemHdnuFq+QIG/JWr/u/UZQ Xykg== X-Gm-Message-State: ALoCoQnS8KoyGaaEjnD4XCVmE8ViAHBpb/ZzMEJx7Ff/sS14fQcqjuR0uR/QriRi7etH3mEHPm7h X-Received: by 10.50.47.18 with SMTP id z18mr17210475igm.1.1389237489556; Wed, 08 Jan 2014 19:18:09 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.49.131.4 with SMTP id oi4ls901870qeb.76.gmail; Wed, 08 Jan 2014 19:18:08 -0800 (PST) X-Received: by 10.236.160.9 with SMTP id t9mr1033846yhk.10.1389237488766; Wed, 08 Jan 2014 19:18:08 -0800 (PST) Original-Received: from mail-ve0-x22b.google.com (mail-ve0-x22b.google.com [2607:f8b0:400c:c01::22b]) by mx.google.com with ESMTPS id x4si3663708qad.60.2014.01.08.19.18.08 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Wed, 08 Jan 2014 19:18:08 -0800 (PST) Received-SPF: pass (google.com: domain of metafoo@gmail.com designates 2607:f8b0:400c:c01::22b as permitted sender) client-ip=2607:f8b0:400c:c01::22b; Original-Received: by mail-ve0-f171.google.com with SMTP id pa12so1948825veb.30 for ; Wed, 08 Jan 2014 19:18:08 -0800 (PST) X-Received: by 10.58.227.38 with SMTP id rx6mr485748vec.10.1389237488391; Wed, 08 Jan 2014 19:18:08 -0800 (PST) Original-Sender: metafoo@gmail.com Original-Received: by 10.58.136.227 with HTTP; Wed, 8 Jan 2014 19:18:08 -0800 (PST) In-Reply-To: <52CE0F54.5020800@gmail.com> X-Original-Sender: richard@metafoo.co.uk X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of metafoo@gmail.com designates 2607:f8b0:400c:c01::22b as permitted sender) smtp.mail=metafoo@gmail.com; dkim=pass header.i=@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:8481 Archived-At: --047d7bd6b5b4015a7604ef810f40 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable On Wed, Jan 8, 2014 at 6:54 PM, David Krauss wrote: > On 1/9/14 10:41 AM, Richard Smith wrote: > >> On Wed Jan 08 2014 at 5:28:21 PM, David Krauss wrote: >> >> It's not an overload, it's a partial specialization. >> > > OK, that wasn't clear from the example. So do you intend that an > identifier declared with "?" never be used? What could you possibly do wi= th > X? You would only be able to use it as a template-argument. > Quoting the previous message: > > 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 nam= e >>> would throw a wrench into template parsing. They could *only* be used a= s >>> explicit template arguments to functions as in my example above, or to >>> template template parameters with "?"-like parameter lists. >>> >> > Edit: You couldn't even use X in these cases, because parse time ambiguit= y > of a template argument is always resolved in favor of a typename > ([temp.arg] =A714.3/2). The useful thing about introducing a new feature is that you get to specify how it works =3D) Naturally, we would specify that passing a generic templa= te argument to another template preserves the kind of template argument. > 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. > 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/. > --=20 ---=20 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 e= mail 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-proposa= ls/. --047d7bd6b5b4015a7604ef810f40 Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable
On W= ed, Jan 8, 2014 at 6:54 PM, David Krauss <potswa@gmail.com> w= rote:
On 1/9/14 10:41 AM, Richar= d Smith wrote:
On Wed Jan 08 2014 at 5:28:21 PM, David Krauss <potswa@gmail.com> wrote:

It's not an overload, it's a partial specialization.

OK, that wasn't clear from the example. So do you intend that an identi= fier declared with "?" never be used? What could you possibly do = with X?

You would only be able to use it as= a template-argument.
=A0
Quoting the previous message:

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.

Edit: You couldn't even use X in these cases, because parse time ambigu= ity of a template argument is always resolved in favor of a typename ([temp= ..arg] =A714.3/2).

The useful thing about in= troducing a new feature is that you get to specify how it works =3D) Natura= lly, we would specify that passing a generic template argument to another t= emplate preserves the kind of template argument.
=A0
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 &q= uot;?" or
any provision for a single argument.


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