220 23723 <2BCBD0F7-9AD8-4CF1-AEBC-184DD56DBA5A@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: Thu, 14 Jan 2016 14:03:27 -0500 Lines: 68 Approved: news@gmane.org Message-ID: <2BCBD0F7-9AD8-4CF1-AEBC-184DD56DBA5A@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> <9160999F-C7E2-4D3F-B343-96CF587BEF61@gmail.com> <5696CC0A.5040709@wanadoo.fr> <753EF830-97D0-4653-B056-BDB2CD4B0760@gmail.com> <601027C6-EDBF-42C5-9AAB-766B31945702@gmail.com> <5697EA27.7090603@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 1452798219 11618 80.91.229.3 (14 Jan 2016 19:03:39 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Thu, 14 Jan 2016 19:03:39 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCK2HM4L6YERBAXC362AKGQE5TGY7QQ@isocpp.org Thu Jan 14 20:03:33 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pa0-f71.google.com ([209.85.220.71]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1aJnBE-0004x3-GB for gclcip-std-proposals@m.gmane.org; Thu, 14 Jan 2016 20:03:32 +0100 Original-Received: by mail-pa0-f71.google.com with SMTP id a20sf313311384pag.3 for ; Thu, 14 Jan 2016 11:03:32 -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=sHlqDNPRDtTa3C845ephMGOZSdwzeZrvVwEbibQmOwA=; b=ufrQVU9waqNmvYzHUcDNkm2Kb7pi+X4pN1XzzFrTUgXok5zHoDT79zRyIgsFma75CI AwQoztGEEPK5KhtwCENvX6URetvTNssc5Xp+IHL3p+WvzYvdTlPpmigA3rqfI7a7dTCp VtPBZ9t2g0PQiFc5oJ4ohJqOPl+Q8JmzL3pMleFMVo+543Y/ayzXfjCCN+0liF7JGZH+ H3d+FsbtPUTR58oUS7rr6GYOzNwbl4TQ7PDewr3h9x6z+DLzKUyt/1SVL/1zbr/2FPlx +YQfS2oPKS31+v+N8I4mkk1xo7Y301XvTZsswPtdLoovrPNUtRRpG3dt7w5pjoh/ptZz 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=sHlqDNPRDtTa3C845ephMGOZSdwzeZrvVwEbibQmOwA=; b=Z4MztFg/E5a1QC1D9wKHXFGJF6k3IDJ22yyRU5HAdynpMbf/e4RLbD1ytasEKZXWg2 nv4n8HNM3RxCZbFFBB4GH+sNzuSEeoMa4/2Yamyv5v/jQWJJw7MmfGLVfPVGHDXllkNq brE4PkozYvBwaiVkVtf92oGC0YbguSU1aiC3SUlVRx5s77A9FBrU7Y21ny7g6OusJb7T K1S4jlLchFwtD0WrbIJZU1Z7GhCfT04VvJamktF0SCvJ5XCnO4YuLpuXGOoBpzcxjIKL mqLS463d3d7Ac+7Zt31tZCs6XY/TMZxOAcG4h/hQRI6FfI9Biy2lEwjnem4TnFG+OyVx X-Gm-Message-State: ALoCoQnj/uKGNzLDeQIVleKkOUcgRTZwNpw64OpHLkXe3R99qZTjhdpEePFtAxAUhDFLjzgL2zcbOPt8QmyYcxNSIHrnQQ/yqQ== X-Received: by 10.98.79.143 with SMTP id f15mr5052736pfj.6.1452798211382; Thu, 14 Jan 2016 11:03:31 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.140.96.72 with SMTP id j66ls610591qge.88.gmail; Thu, 14 Jan 2016 11:03:29 -0800 (PST) X-Received: by 10.140.18.167 with SMTP id 36mr8139391qgf.38.1452798209712; Thu, 14 Jan 2016 11:03:29 -0800 (PST) Original-Received: from mail-qg0-x235.google.com (mail-qg0-x235.google.com. 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[104.229.168.124]) by smtp.gmail.com with ESMTPSA id 140sm2983274qhm.10.2016.01.14.11.03.28 for (version=TLS1 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Thu, 14 Jan 2016 11:03:28 -0800 (PST) In-Reply-To: <5697EA27.7090603@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::235 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:23723 Archived-At: On Jan 14, 2016, at 1:34 PM, Vicente J. Botet Escriba wrote: >=20 > Le 14/01/2016 18:47, Howard Hinnant a =C3=A9crit : >> On Jan 14, 2016, at 12:43 PM, Jonathan Coe wrot= e: >>> How will is_uniquely_represented work with architecture dependant align= ment requirements and padding? >> It will have to be up to the author of the type to correctly specialize = is_uniquely_represented for all architectures of interest to them. Padding= bytes can be detected by assuring that the sum of the size of your bases a= nd members is equal to the size of your type. For example: >>=20 >> https://github.com/HowardHinnant/hash_append/blob/master/hash_append.h#L= 143-L151 >>=20 >>=20 > Howard. I don't see why the author should be the single one able to do it= correctly. Do you see a case for classes like std::pair for which the spec= ialization your showed is not a good one? No, I think the std::pair specialization should be fine for all cases, with= the possible exception of if someone specialized pair itself. But I=E2=80= =99ve never seen a motivating use case to specialize pair itself, and I wou= ld likely reject any such code in a code review. I may not be stating my position clearly. My position is that given some u= ser-defined class X: class X { std::size_t x_; std::size_t y_; public: friend bool operator=3D(const X&, const X&); }; I see no way to automatically do anything other than claim X is *not* uniqu= ely represented, but of course allow the author of X to say otherwise. Tha= t being said, if we has this instead: class X { std::size_t x_; std::size_t y_; public: friend bool operator=3D(const X&, const X&) =3D default; }; Then I might get talked into believing that it would be safe for the compil= er/std::lib to declare X uniquely represented. For this second case I have= n=E2=80=99t given it enough thought to be positive one way or the other. Howard PS: I agree that it will be less problematic to split this proposal. --=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/. .