220 39134 article Path: news.gmane.org!.POSTED!not-for-mail From: Zhihao Yuan Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: RE: Re: Concept-defined placeholder type Date: Fri, 13 Jul 2018 00:35:22 -0400 Lines: 91 Approved: news@gmane.org Message-ID: References: <0fe92570-1bc7-4a8b-aaf4-b272950f20af@isocpp.org> <8ba6d594-0aa8-4c6c-87b1-b785cc519570@isocpp.org> <9df59e26-2363-4119-9794-090f958c4499@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Trace: blaine.gmane.org 1531456405 17716 195.159.176.226 (13 Jul 2018 04:33:25 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Fri, 13 Jul 2018 04:33:25 +0000 (UTC) To: 'Nicol Bolas' , 'ISO C++ Standard - Future Proposals' Original-X-From: std-proposals+bncBCSKRWMD4EHBBEWYUDNAKGQELSLGUNQ@isocpp.org Fri Jul 13 06:33:20 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-wr1-f69.google.com ([209.85.221.69]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1fdplg-0004XA-Ml for gclcip-std-proposals@m.gmane.org; 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[185.70.40.18]) by mx.google.com with ESMTPS id z140-v6si4818589wmd.190.2018.07.12.21.35.30 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 12 Jul 2018 21:35:30 -0700 (PDT) Received-SPF: pass (google.com: domain of zy@miator.net designates 185.70.40.18 as permitted sender) client-ip=185.70.40.18; In-Reply-To: <9df59e26-2363-4119-9794-090f958c4499@isocpp.org> Feedback-ID: xu1i2ADzKwGyrUl8KbXpsa1PQSI0Bc21qlX1V7gpMylRDQPR42YBpWcLHW4brubvCmGM18xDo5tgsD-vWFu_cQ==:Ext:ProtonMail X-Original-Sender: zy@miator.net X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@miator.net header.s=protonmail header.b="v18ex/4Q"; spf=pass (google.com: domain of zy@miator.net designates 185.70.40.18 as permitted sender) smtp.mailfrom=zy@miator.net Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Spam-Checked-In-Group: std-proposals@isocpp.org 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:39134 Archived-At: From: Nicol Bolas =20 Sent: Thursday, July 12, 2018 7:50 PM >=20 > In order for your tool to be superior, you need to explain why: >=20 > Concept T; > T var =3D ...; >=20 > is better than >=20 > Concept auto var =3D ...; > using T =3D decltype(var); >=20 > There is no question that they both accomplish the same end. They both ar= e two lines long. >=20 The first one has 5 tokens, while the second one has 11 tokens? > No evidence shows that the time we use `auto` is=20 > getting close to the time we constrained-type-specifiers,=20 > there are plenty of cases where constraining is=20 > unnecessary. >=20 > Um, if what you're saying is true, you do realize that you're arguing aga= inst your own feature, right? Because if people don't want to constrain the= ir deduced variables... what do we need your feature for? I showed that there are cases you want to constrain, it doesn't mean you *always* want to. > My point is that your statement that there's no way for terse syntax to b= e used for multiple deductions is incorrect. Maybe that particular syntax w= ouldn't work (though since it's new syntax, we could require that constrain= ed declarations make the number of variables match the number of template p= arameters), but I mentioned other syntaxes that certainly could work. It's not syntax's problem; logically it's not going to work. When you are defining two variables you may have two declarations, how do you connect and evaluate information from both without naming them in some way? If relying on variable names then that's two decltypes, if relying on type- names then that's my idea. > Everything we're talking about is syntactic sugar. The effectiveness of s= yntactic sugar is ultimately based on how commonly it will be used, and how= much of a pain the alternative is. >=20 First, constrained-type-name is not a syntax sugar; it's new entity. Second, "shorthand" isn't the way I look at the features. Syntax must match intention, that's the first criteria. When you are deducing and constraining two variables and want them to be of the same type, the intention is "declaring two variables of the same constrained deduced type." And you should be able to read your code in that way. Now look at the following code: Copyable auto a =3D ...; decltype(a) b =3D ...; How do you read the code, "declare a Copyable a, then declare b to be of a's type?" What about this: Copyable T; T a =3D ...; T b =3D ...; "declare a and b to be of a Copyable T." You have a balanced intention, so you write code of this balanced structure, that kind of syntax is what I'm looking for. -- Zhihao --=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. To view this discussion on the web visit https://groups.google.com/a/isocpp= ..org/d/msgid/std-proposals/zIsxha1LpjzhmMmtmntyw_j6dgG5s8r00BGQarW44h4_5u9E= k0MIqB3gQ5UTA5UmP_SPuHQGszrvXWCJulOp4vtuhN3QNNeUU3h8LJzAmLk%3D%40miator.net= .. .