220 23702 <9160999F-C7E2-4D3F-B343-96CF587BEF61@gmail.com> article Path: news.gmane.org!not-for-mail From: Howard Hinnant Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Default swap/hash_value Date: Wed, 13 Jan 2016 11:09:41 -0500 Lines: 47 Approved: news@gmane.org Message-ID: <9160999F-C7E2-4D3F-B343-96CF587BEF61@gmail.com> References: <569582E5.1040108@wanadoo.fr> <4072A7C6-B09D-4847-971B-2914643B91D7@gmail.com> <569594AD.6000505@wanadoo.fr> <2170C30C-830F-47C2-8397-29DD4ED35E97@gmail.com> <56960730.9000403@wanadoo.fr> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 (Mac OS X Mail 9.2 \(3112\)) Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-Trace: ger.gmane.org 1452701398 30408 80.91.229.3 (13 Jan 2016 16:09:58 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 13 Jan 2016 16:09:58 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCK2HM4L6YERBR7N3G2AKGQEFF7BB7Q@isocpp.org Wed Jan 13 17:09:49 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pf0-f200.google.com ([209.85.192.200]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1aJNzV-00089O-KZ for gclcip-std-proposals@m.gmane.org; Wed, 13 Jan 2016 17:09:45 +0100 Original-Received: by mail-pf0-f200.google.com with SMTP id 78sf361176087pfw.0 for ; Wed, 13 Jan 2016 08:09:45 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=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:x-spam-checked-in-group:list-post :list-help:list-archive:list-subscribe:list-unsubscribe; bh=vj/Lf4nVpEapQk/FL/tStM6DScjSyDYPLOBUxxzInvY=; b=OkpLHFpSRtS+Xj4hRrwUpCfEZUfhMbTu7jXcx0yNssTQXwqMqyloJr0zTuV2ZLS6AO cX/sSXywL4PUHbklRv5ABtfLlqntmxpB8TrME3iPEOaqyh8cbSG5Wi1pcXuZuAWXrQwb yYTt79DEIhuGYC0hEG5a42PBNo17QiH39QZMFF4EvoiGiZuUZgjPwdHeste+pAkaGZxi +qOKPqJXr40GowRrJZsoFeondRxUdIigOfrznypDu4rrbK6+cD1p1GBTtcTLsfzr5fC/ UtJT0xkUb0z5sm+GPEpNjSR5Q6kJpDTReesA/StogtP7rUIa0QEZFSaG2BpsSBtBnw+A 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:x-spam-checked-in-group:list-post :list-help:list-archive:list-subscribe:list-unsubscribe; bh=vj/Lf4nVpEapQk/FL/tStM6DScjSyDYPLOBUxxzInvY=; b=l9nszpZLKI4KJANrnJtKSxf8cnzI2/49fjwweLoYIT5WuozOv+XGEQNVpOnnd/d5iq ZaA4gDkD4SAFKpQlGQx3imE6cPJvF3J4ua6P3fvA2PybhsjXh8lx/c92vysXS57Ef/1n ZOjIWU1OT7F9Ayqiz5vRNPlTLDcy2HrWkbZpM6i8NdG7K3jtxSUJ4BA0vcPwrpuu8UPx czRLtVDnjL8XU5/oGYbpE8zFPG5WMzSvz0CmDK5bnAAWeDNwQNejtqJmLAbJMRexYgIG IvYsMOT1eJCmIpglNfyjhj4dMGMA0FQ4JFyWW6YrE1QMwp2GCHRLVzf0VkLnLWJRCCLK X-Gm-Message-State: ALoCoQmUV6e4d3tpRWrXtk24cqOzZfyb1qhurj5tAbTNNMCfrFxgLWXNM/FfISJ/950ADJbc2Dtg2eicAuyVstaey3bn8gLqUQ== X-Received: by 10.66.186.201 with SMTP id fm9mr22022016pac.41.1452701384459; Wed, 13 Jan 2016 08:09:44 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.140.93.245 with SMTP id d108ls4694667qge.89.gmail; Wed, 13 Jan 2016 08:09:42 -0800 (PST) X-Received: by 10.140.25.169 with SMTP id 38mr173112697qgt.73.1452701382932; Wed, 13 Jan 2016 08:09:42 -0800 (PST) Original-Received: from mail-qg0-x234.google.com (mail-qg0-x234.google.com. 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[104.229.168.124]) by smtp.gmail.com with ESMTPSA id p19sm740333qgp.9.2016.01.13.08.09.41 for (version=TLS1 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Wed, 13 Jan 2016 08:09:41 -0800 (PST) In-Reply-To: <56960730.9000403@wanadoo.fr> X-Mailer: Apple Mail (2.3112) X-Original-Sender: howard.hinnant@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of howard.hinnant@gmail.com designates 2607:f8b0:400d:c04::234 as permitted sender) smtp.mailfrom=howard.hinnant@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-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:23702 Archived-At: On Jan 13, 2016, at 3:13 AM, Vicente J. Botet Escriba wrote: >=20 > I've some additional concerns about the possible generation of specializa= tion of is_uniquely_represented. I would like it to be generated the spe= cialization when possible but while multiple definition of hash_value could= be check at link time, I believe that multiple specializations of is_uniqu= ely_represented would result in ODR, which would be a showstopper. So at= the end, it would be the responsibility of the developer of class C to add= a specialization of is_uniquely_represented. What do you think about th= is point? I believe you are correct that it will be necessary for the author of C to = specialize is_uniquely_represented. There are semantics to this propert= y that do not show up in the C++ type system. For example the hashing of a= std::vector should never include the vector=E2=80=99s capacity(). And the= hashing of an IEEE floating point should hash -0. and 0. to the same value= (despite the different bit layouts). C may have similar rules on how it m= ust be hashed and only the author of C can know this. In the case that is_uniquely_represented answers true, I believe it is a= simple library function to automate hash_append/hash_value. Here is an up= dated N3980 implementation of such a function: https://github.com/HowardHinnant/hash_append/blob/master/hash_append.h#L203= -L212 If something is uniquely represented, you can ship its bytes off to the has= h algorithm without concern about how its bases and members are laid out. = Not only is this easier than dealing with the bases and members individuall= y, it is also more efficient for most modern hash algorithms. Hash algorit= hms like to see as many bytes as possible at once for best performance. Th= is leads to less buffering and more bit crunching within the algorithm. Howard --=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 https://groups.google.com/a/isocpp.org/group/std-propos= als/. .