220 31821 article Path: news.gmane.org!.POSTED!not-for-mail From: Zhihao Yuan Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: This variable should not be named: an identifier (not) to remember Date: Tue, 28 Mar 2017 11:53:51 -0500 Lines: 106 Approved: news@gmane.org Message-ID: References: <985b9b2a-c734-45eb-95f4-db4dc0d309a1@isocpp.org> <14211602.1P4LjlnQMh@tjmaciei-mobl1> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary=94eb2c1aaab8f7e8d1054bcd4d1c X-Trace: blaine.gmane.org 1490720042 8918 195.159.176.226 (28 Mar 2017 16:54:02 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 28 Mar 2017 16:54:02 +0000 (UTC) To: "std-proposals@isocpp.org" Original-X-From: std-proposals+bncBCSKRWMD4EHBBIVK5LDAKGQEU2LHXLA@isocpp.org Tue Mar 28 18:53:57 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pg0-f69.google.com ([74.125.83.69]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1csuNR-000161-F2 for gclcip-std-proposals@m.gmane.org; Tue, 28 Mar 2017 18:53:49 +0200 Original-Received: by mail-pg0-f69.google.com with SMTP id u3sf116861562pgn.12 for ; Tue, 28 Mar 2017 09:53:55 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:sender:mime-version:in-reply-to:references:from :date:message-id:subject:to:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=jOZaG9BXbzCFEzqn6g2QoNauAgHulZ/cftcWuC0TL0c=; b=U12271bnyjSh5WfR7DUwOxAc0Fe+hnwUjnDurlpnC9qGuLeUiSZoasRFFnqWjFnrQG dTfFkeBZPtDJ+YjrnLWP2W+sP5NgkRDoQENqdJdCTmJCkPMQlM8YNzrwiw0BjaSROIb6 +UgGF4awcVZ2O17t+1msPjUzftUOfzwVvj+bPUubWbyxezoWNoyoGN6j1vxgg8Lm3xZu MQE37oG+Fb0DTAajYDjKlH77oVi6miysDiUmacAo8VD27rLmvhior2Ie2JV5vqqFALrt 6AO1e+84d0VbU6a+MdKzYYHBBDl/oCtc2Xy/FVkjOczuaV6DsloFf+09dugMKUcqH2Lu oCtg== X-Gm-Message-State: AFeK/H0oIOf2KQCMTG5UES9YRQpIxCUhZtwrSd4uXEYqqGrbaZOEBPmYRpD2D6pBvi3aPQ== X-Received: by 10.99.115.16 with SMTP id o16mr9076978pgc.63.1490720035224; Tue, 28 Mar 2017 09:53:55 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.47.171 with SMTP id r40ls5864770otb.0.gmail; Tue, 28 Mar 2017 09:53:54 -0700 (PDT) X-Received: by 10.237.43.162 with SMTP id e31mr30015068qtd.189.1490720034313; Tue, 28 Mar 2017 09:53:54 -0700 (PDT) Original-Received: from mail-s68.mailgun.info (mail-s68.mailgun.info. [184.173.153.196]) by mx.google.com with ESMTPS id y5si3891325qkc.328.2017.03.28.09.53.53 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 28 Mar 2017 09:53:53 -0700 (PDT) Received-SPF: pass (google.com: domain of bounce+3f9131.69110-std-proposals=isocpp.org@miator.net designates 184.173.153.196 as permitted sender) client-ip=184.173.153.196; Original-Sender: zy@miator.net X-Mailgun-Sending-Ip: 184.173.153.196 X-Mailgun-Sid: WyI3MTBkYiIsICJzdGQtcHJvcG9zYWxzQGlzb2NwcC5vcmciLCAiNjkxMTAiXQ== Original-Received: from mail-pg0-f50.google.com (mail-pg0-f50.google.com [74.125.83.50]) by mxa.mailgun.org with ESMTP id 58da9520.7fcc001adab0-smtp-out-n03; Tue, 28 Mar 2017 16:53:52 -0000 (UTC) Original-Received: by mail-pg0-f50.google.com with SMTP id 21so78043303pgg.1 for ; Tue, 28 Mar 2017 09:53:52 -0700 (PDT) X-Received: by 10.84.210.167 with SMTP id a36mr37313877pli.40.1490720032021; Tue, 28 Mar 2017 09:53:52 -0700 (PDT) Original-Received: by 10.100.159.10 with HTTP; Tue, 28 Mar 2017 09:53:51 -0700 (PDT) In-Reply-To: <14211602.1P4LjlnQMh@tjmaciei-mobl1> X-Gmail-Original-Message-ID: X-Original-Sender: zy@miator.net X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@miator.net; spf=pass (google.com: domain of bounce+3f9131.69110-std-proposals=isocpp.org@miator.net designates 184.173.153.196 as permitted sender) smtp.mailfrom=bounce+3f9131.69110-std-proposals=isocpp.org@miator.net 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:31821 Archived-At: --94eb2c1aaab8f7e8d1054bcd4d1c Content-Type: text/plain; charset=UTF-8 On Tue, Mar 28, 2017 at 11:43 AM, Thiago Macieira wrote: > just like std::ignore, I don't expect __ here > > to issue a call to get(obj), nor an access to a subobject. > > I don't think the *preserving* semantics can fulfill the > > *ignoring* demand. > > It has no difference, because the structured binding is actually a > *binding*, > so there's difference to object creation or retention. The result from > get_tuple is assigned to an anonymous object and all its members are > retained. > Then it's decomposed into two elements: an anonymous one (can't be > accessed) > and a. > Whether a call to get<0>(anon) being made certainly has difference, because it can have observable effect. And, just like I showed in a previous discussion, the purpose of the anonymous object is for decomposition. If nothing is decomposition it, it needs not to be created, therefore auto [__, __] = get_tuple(...); should be physically equivalent to get_tuple(...); thus, the result object is dropped on the floor. -- Zhihao Yuan, ID lichray The best way to predict the future is to invent it. ___________________________________________________ -- 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. To view this discussion on the web visit https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/CAGsORuBepjjTQxa0w%2B9Z_v%2Birrc6QwxqmUv98dTRarUCVHHY%2BA%40mail.gmail.com. --94eb2c1aaab8f7e8d1054bcd4d1c Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable

= On Tue, Mar 28, 2017 at 11:43 AM, Thiago Macieira <thiago@macieira.org> wrote:
jus= t like std::ignore, I don't expect __ here
> to issue a call to get(obj), nor an access to a subobject.
> I don't think the *preserving* semantics can fulfill the
> *ignoring* demand.

It has no difference, because the structured binding is actually a *= binding*,
so there's difference to object creation or retention. The result from<= br> get_tuple is assigned to an anonymous object and all its members are retain= ed.
Then it's decomposed into two elements: an anonymous one (can't be = accessed)
and a.

Whether a call to get<0>(anon) being made
certainly has diffe= rence, because it can have
observable effect.=C2=A0 And, just like I sho= wed
in a previous discussion, the purpo= se of the
anonymous object is for decomposition.=C2=A0 If
nothing is = decomposition it, it needs not to be
created, therefore

=C2=A0 auto [__, __] =3D get_tuple(...);

should be physically equivalent to

=C2=A0 get_tuple(...);

thus, the result object is dropped on the floor.

--
Zhihao Yuan, ID lichray
The best way to predict the future= is to invent it.
___________________________________________________

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