220 24592 article Path: news.gmane.org!not-for-mail From: Matthew Woehlke Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: throwaway variable Date: Mon, 22 Feb 2016 12:56:16 -0500 Lines: 84 Approved: news@gmane.org Message-ID: References: <9658e852-6254-4ecf-a6d4-879160d4de7d@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-Trace: ger.gmane.org 1456163814 23157 80.91.229.3 (22 Feb 2016 17:56:54 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Mon, 22 Feb 2016 17:56:54 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBC37LBFWUIFBBU4XVW3AKGQEGBWFTSY@isocpp.org Mon Feb 22 18:56:42 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-lf0-f69.google.com ([209.85.215.69]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1aXuir-0007gF-CE for gclcip-std-proposals@m.gmane.org; Mon, 22 Feb 2016 18:56:37 +0100 Original-Received: by mail-lf0-f69.google.com with SMTP id l83sf39234275lfd.0 for ; Mon, 22 Feb 2016 09:56:37 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=to:from:subject:date:lines:message-id:references:mime-version :content-type:content-transfer-encoding:user-agent:in-reply-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=jjMaPMRu8X/KloZdUXNA0STVWDujJ1vn23rw1cOqMGo=; b=yJbqITvDbmjIopqRQ8tTyFcLba99+xHG7sPod4KdkrDK2ujnt19qKeZtKSlCEEYpWP VeUUv0PX52aVic32eOL68u8B401CFoFqAdznYvEdl9WgfDtMiVPPEKpl2noCg2umoU0l fSdqE7hYv9hgjYKF+NmKxqmbgXuqATJuAMqmUWcNuSVSt4Y/HsWfHjZJtVDEQjB0A7Er ibLajVGO0jmAv9LJ6LzhP/NLpPCz7a68grhG6iP3FFmhxy+QAmm0WSU3n7QjGx2uk6G+ EzDRVPJyXvVnOMEwi5Py74m7+n1Pmk18TxQHZcpymnvog7lv0amgItR X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:to:from:subject:date:lines:message-id:references :mime-version:content-type:content-transfer-encoding:user-agent :in-reply-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=jjMaPMRu8X/KloZdUXNA0STVWDujJ1vn23rw1cOqMGo=; b=Qpw+GjZ4uqKz+16GA3TUs6pLcfoSTF4np9jpUyyecaaMZQRnp4N2kHI9mWmMd0ltCP AmmmgCDa6xnW705qbeyyFdaZzhijNXpxkUkzSMfemV99aLDy8xIaT8FYYYEMVf5xNd56 GqsDwrWA/iNDQyPF8vReicIBtPBqAGlyQbwZpytNuklHvoJZiGOtfjH4ixGjfmWxEBVO i5+D2sdlATdZUNtsRgbILXdWJ9HKUHgpZZN4WZ4ApqFdfwXbho38EkPslAQCcZMVhObU X0h+n4qsCh7lptXEDhFlpbwZ//lp7nzPmjer1L94nnUnGHuCPTZ X-Gm-Message-State: AG10YOTyvrYNcaam/DSSV/pPGCD05jx6QVdsmpgMPjuEK4tfUX0JSk1/PX6MsHhRVz+8dA== X-Received: by 10.28.47.211 with SMTP id v202mr1361846wmv.3.1456163796751; Mon, 22 Feb 2016 09:56:36 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.25.28.76 with SMTP id c73ls379230lfc.71.gmail; Mon, 22 Feb 2016 09:56:34 -0800 (PST) X-Received: by 10.112.10.131 with SMTP id i3mr10719138lbb.100.1456163794914; Mon, 22 Feb 2016 09:56:34 -0800 (PST) Original-Received: from plane.gmane.org (plane.gmane.org. [80.91.229.3]) by mx.google.com with ESMTPS id o9si13895108lfa.135.2016.02.22.09.56.34 for (version=TLS1 cipher=AES128-SHA bits=128/128); Mon, 22 Feb 2016 09:56:34 -0800 (PST) Received-SPF: pass (google.com: domain of gclcip-std-proposals@m.gmane.org designates 80.91.229.3 as permitted sender) client-ip=80.91.229.3; Original-Received: from list by plane.gmane.org with local (Exim 4.69) (envelope-from ) id 1aXuim-0007Xw-Kx for std-proposals@isocpp.org; Mon, 22 Feb 2016 18:56:32 +0100 Original-Received: from tripoint.kitware.com ([66.194.253.20]) by main.gmane.org with esmtp (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Mon, 22 Feb 2016 18:56:27 +0100 Original-Received: from mwoehlke.floss by tripoint.kitware.com with local (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Mon, 22 Feb 2016 18:56:27 +0100 X-Injected-Via-Gmane: http://gmane.org/ Original-Lines: 71 Original-X-Complaints-To: usenet@ger.gmane.org X-Gmane-NNTP-Posting-Host: tripoint.kitware.com User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 In-Reply-To: X-Original-Sender: mwoehlke.floss@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of gclcip-std-proposals@m.gmane.org designates 80.91.229.3 as permitted sender) smtp.mailfrom=gclcip-std-proposals@m.gmane.org; dmarc=fail (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-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:24592 Archived-At: On 2016-02-22 12:00, Viacheslav Usov wrote: > On Mon, Feb 22, 2016 at 4:35 PM, Matthew Woehlke wrote: >> On 2016-02-21 08:55, Viacheslav Usov wrote: >>> On Fri, Feb 19, 2016 at 9:06 PM, Patrice Roy wrote: >>>> I often use _ as =C2=ABuninteresting but needed=C2=BB variables such a= s >>>> lock_guards. Please don't break my code :) >>> >>> I would even say that this case, "uninteresting but needed" is=20 >>> more interesting. It would be nice to have a language construct=20 >>> that creates an "anonymous" variable in the current scope.=20 >>> Alternatively, this could be conceptualized as a temporary whose >>> lifetime is extended till the end of the current scope. >>=20 >> Is that not what we're proposing? >=20 > I am not sure. The discussion definitely started with a very different > idea, and I do not think I saw a clear statement similar to what I wrote > above. I'm not sure I'd call it "different" or not. The OP (who curiously seems to have dropped out of the conversation) originally only mentioned being implicitly `[[maybe_unused]]`. Ville immediately brought up http://cplusplus.github.io/EWG/ewg-active.html#35, which dealt with "nameless" variables, which seems consistent with the OP's intent as well as much more useful (though Dmitry brings up the interesting point that these should be `[[maybe_unused]]`, now that that seems likely to be in C++17). I've generally read the rest of the thread as referring to such 'anonymous variables'. > For the record, I dislike the special names that have been proposed. I > believe the lifetime-extended temporary approach is more appropriate, so > this should probably be phrased in a language involving temporaries; say = if > produces a temporary, then & makes it an anonymo= us > variable (and an l-value) in the current scope. The use of & is just an > example and not essential. I see at least one issue with that, namely that it doesn't solve the OP's problem (anonymous variables as loop iterators). Otherwise, I fail to see a semantic difference between: &lock_guard{...}; // life extended until end of scope auto&& . =3D lock_guard{...}; // anonymous variable In both cases, a temporary that would otherwise be immediately destroyed is lifetime-extended to the end of the scope in which the statement appears. Said temporary is anonymous and cannot be named (i.e. used again). The only difference I see=C2=B9 is syntactic. Do I miss something? (=C2=B9 Again, ignoring that reuse of declaration syntax means I can do things with 'auto .' that I can't do with pure lifetime extension.) Note also I'm not a big fan of changing the meaning of an existing identifier, even `__`, to be something other than an identifier. That's why I suggested using `.` instead; the proposed syntax is not (AFAIK) currently legal, and `.` is definitely not an identifier. Just for fun... here's how a potential 'do something X times' loop might look in the future: for (.: [0..X)) do_something(); (And no, this isn't completely theoretical code; it might well appear in timing tests.) --=20 Matthew --=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/nafi40%24lve%241%40ger.gmane.org. .