220 15346 article Path: news.gmane.org!not-for-mail From: David Krauss Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: get count of std::weak_ptr objects tracking a shared resource Date: Sun, 28 Dec 2014 14:29:48 +0800 Lines: 43 Approved: news@gmane.org Message-ID: References: <4797942.dAdOve4yz0@tjmaciei-mobl4> <549C50C1.8060008@gmail.com> <71d4ea44-aec9-4bfa-a1a8-ba81ca745b84@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 (Mac OS X Mail 8.1 \(1993\)) Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-Trace: ger.gmane.org 1419748210 23422 80.91.229.3 (28 Dec 2014 06:30:10 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sun, 28 Dec 2014 06:30:10 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCW25A7E3QCRB2OG72SAKGQE7Y5K2KY@isocpp.org Sun Dec 28 07:30:03 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ie0-f199.google.com ([209.85.223.199]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1Y57MY-0004ad-St for gclcip-std-proposals@m.gmane.org; Sun, 28 Dec 2014 07:30:03 +0100 Original-Received: by mail-ie0-f199.google.com with SMTP id rp18sf70867509iec.10 for ; Sat, 27 Dec 2014 22:30:01 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:content-type:mime-version:subject:from :in-reply-to:date:content-transfer-encoding:message-id:references: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; bh=WBo1PBC2ouqKjzM24TIHDeB32LYtQ8TPF8vCVMLriz0=; b=N+2nRm8owHfYd1E+4I3knsluVRy7XYmhc5LxFfwmMFJWFM41rU7nKCnTq2qjdqCCS0 tl4a7uBb7RLWG7+sKGByx22NlsriZ47ukXm4eu9QXEJApu5DShkm26r95tbkgs2PL9yu OnXzFsBIDK+X5JIodNN8amXd2SbWr5ZbbbQeBNB1WfGfIb51haqGjOpKGDTvB7NLLYmK d5WnZkDyCr1398J2RkkJffFxR1H283yAamG4RUd0mgpRoiBEeSOcaGEY8B4NSWeXuoQq qtOinxByJx5+JjditLJAg81CryrARxndPMHtF5jaAgISopEeVRuX/A35U/aMew9HRvUd ynEg== X-Gm-Message-State: ALoCoQmDO2dbCgczUxEIO4drG0uqZ2lDxEfWxMoCgsy6JKyFz2XpfGhwAxFmAHh9eNoAc5nzSuFl X-Received: by 10.182.252.164 with SMTP id zt4mr38205010obc.1.1419748201862; Sat, 27 Dec 2014 22:30:01 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.157.194 with SMTP id g185ls4139580ioe.25.gmail; Sat, 27 Dec 2014 22:30:01 -0800 (PST) X-Received: by 10.66.171.206 with SMTP id aw14mr79945502pac.40.1419748201007; Sat, 27 Dec 2014 22:30:01 -0800 (PST) Original-Received: from mail-pa0-x22d.google.com (mail-pa0-x22d.google.com. [2607:f8b0:400e:c03::22d]) by mx.google.com with ESMTPS id ns5si10018562pbc.178.2014.12.27.22.30.00 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Sat, 27 Dec 2014 22:30:01 -0800 (PST) Received-SPF: pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:400e:c03::22d as permitted sender) client-ip=2607:f8b0:400e:c03::22d; Original-Received: by mail-pa0-f45.google.com with SMTP id lf10so15502765pab.32 for ; Sat, 27 Dec 2014 22:30:00 -0800 (PST) X-Received: by 10.70.56.37 with SMTP id x5mr47772357pdp.86.1419748200800; Sat, 27 Dec 2014 22:30:00 -0800 (PST) Original-Received: from [172.20.10.2] ([121.54.44.91]) by mx.google.com with ESMTPSA id qq6sm32259251pbb.20.2014.12.27.22.29.57 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Sat, 27 Dec 2014 22:30:00 -0800 (PST) In-Reply-To: <71d4ea44-aec9-4bfa-a1a8-ba81ca745b84@isocpp.org> X-Mailer: Apple Mail (2.1993) X-Original-Sender: potswa@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:400e:c03::22d as permitted sender) smtp.mail=potswa@gmail.com; dkim=pass header.i=@gmail.com; dmarc=pass (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-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:15346 Archived-At: > On 2014=E2=80=9312=E2=80=9328, at 1:07 PM, Ramkumar Revanur wrote: >=20 > Just as new shared_ptr can be created to the same resource from existing = shared_ptr. The reliability of the "weak_count" is same as the reliability = of use_count on a shared_ptr. A combination of use_count and weak_count is safer, though, because a poten= tially-last owner can check that there are no other potential owners on oth= er threads. The weak count would need to be checked (evaluated) first thoug= h, which is a bit tricky with unordered expression evaluation semantics. For that matter, a total reference_count which is the sum of strong and wea= k references would accomplish this more elegantly. It would be a drop-in re= placement for use_count. Applications that want to safely invalidate unsynchronized weak references = need another new primitive, perhaps call it expire(), which atomically rese= ts if unique. Then the caller can check whether the reset actually occurred= .. This is needed to make the design in the OP safe against a race between a= cache eviction (or clearing) and a cache access. Whether cache control is a valid use-case of weak_ptr, I=E2=80=99m not real= ly sure. Why not? It=E2=80=99s unusual but clever. Control blocks introduce= some inefficiency, but it sounds like a big shortcut and efficiency gain v= s. shared_ptr alone. Anyway, the proposed extensions won=E2=80=99t add any overhead, and there a= re surely other multithreaded applications out there with unsafe weak refer= ences. --=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/. .