220 34935 article Path: news.gmane.org!.POSTED!not-for-mail From: Mingxin Wang Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Idea about "std::pmr::memory_resource" Date: Sat, 14 Oct 2017 20:40:29 -0700 (PDT) Lines: 95 Approved: news@gmane.org Message-ID: References: <0a8293b4-3246-47a5-9881-bc1ca4b91772@isocpp.org> <62cd9e1e-03b8-4125-a032-72624a9f5254@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_14848_403107053.1508038829323" X-Trace: blaine.gmane.org 1508038828 32189 195.159.176.226 (15 Oct 2017 03:40:28 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sun, 15 Oct 2017 03:40:28 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBDNMBNHJWIGBBLNRRPHQKGQEQNYA3YY@isocpp.org Sun Oct 15 05:40:24 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ua0-f198.google.com ([209.85.217.198]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1e3Zmp-0007rz-Og for gclcip-std-proposals@m.gmane.org; Sun, 15 Oct 2017 05:40:23 +0200 Original-Received: by mail-ua0-f198.google.com with SMTP id c28sf5780181uaa.17 for ; Sat, 14 Oct 2017 20:40:31 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :list-post:list-help:list-archive:list-subscribe:list-unsubscribe; bh=te2bVdhfOah1S+6CmIGH3u3ZfuxvQ7CjXyp65XDPVBA=; b=lJ9RxDMTh3k6y3I6Rdw1GPSvuo9F56iD7CL9JxicIYZEX2jCUWPkWGNBQHbPhFNCaT d5ZuefgTMo8ODMw7eFVrsOAk70oJAP2oRGePiUa3pYQeT65uDLbyWu3YQ6YXVhmzakYK IIZQXc+92ee7wOctEwSvwSC5OmkM2gEb/VUNToSqUuWM2Mzgzo45/ZkQucmcl/ujpCx8 466+Feiw/+HmG5ELhamh1P5GDnrKA1heWnRZhQIA1sIEex2cuCedv80wYKqmrGGRdrUl RwOaS28v+aiguCxINby/z/58T7iMb5XBSJa7SWZcHiKF58DkvNbmexp9WsGIRaEqVH7C cy3w== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:message-id:in-reply-to:references :subject:mime-version:x-original-sender:reply-to:precedence :mailing-list:list-id:x-spam-checked-in-group:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=te2bVdhfOah1S+6CmIGH3u3ZfuxvQ7CjXyp65XDPVBA=; b=m7VkumNLsZYmmwyCCMdZ6ZwU2vTA8u8Bo2vkTfG0/RjgpqGVTpyAX3q5/V4Fh6tTOP 0TGrTfv29J/EWqqdbh5PpqRr9D9wQfv0KQUJArAXdUgtRc7Db7GcFci3Un8Fogvr7fFg BihLiDjDlHyifIrQyWmGupBbE/yooamadQuByxwHa/xhxFDsHQBRKJBGzBVe/uBDCAc1 tlDr/s8PWpUxMBn903577j/xXeR1leX/nPDxaFEzJQMXnOGBUM8jdGMAFt/kL4SWwC3A e7TVUd/gYBNC/CHDg/psSgjGvuRLJRyDa5DuBrRRgfB/Z+uhI97QmEqo4JV1tC55wqME JXvQ== X-Gm-Message-State: AMCzsaWug9xf0QvH4VC43TOKIjP49NEqSvcDoPl3WNFa4BttpNaRnkXp RjSQJ0OXkZNxRqHxUD2C3EA2NQ== X-Google-Smtp-Source: AOwi7QBGk62cgM1DxTlW3PscC6DouVliORjtakDdhn8THR+8FxodVfTiPi+fYkfUUlAwgcBrPBjZkw== X-Received: by 10.31.98.66 with SMTP id w63mr3177459vkb.0.1508038831181; Sat, 14 Oct 2017 20:40:31 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.159.45.140 with SMTP id v12ls1305656uaj.20.gmail; Sat, 14 Oct 2017 20:40:29 -0700 (PDT) X-Received: by 10.31.96.146 with SMTP id u140mr395072vkb.2.1508038829786; Sat, 14 Oct 2017 20:40:29 -0700 (PDT) In-Reply-To: <62cd9e1e-03b8-4125-a032-72624a9f5254@isocpp.org> X-Original-Sender: wmx16835vv@163.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:34935 Archived-At: ------=_Part_14848_403107053.1508038829323 Content-Type: multipart/alternative; boundary="----=_Part_14849_1016021062.1508038829323" ------=_Part_14849_1016021062.1508038829323 Content-Type: text/plain; charset="UTF-8" Thanks for your comments! On Sunday, October 15, 2017 at 3:54:41 AM UTC+8, T. C. wrote: > > For simple memory resources, including the three in the standard, is_equal > implementations can simply do an address comparison. There's no need for > dynamic_cast or RTTI. > However, this conclusion only holds in "simple designs", and it is apparent that `std::pmr::memory_resource` is desined to have good extendability for more complex implementations in the future, as what is illustrated in the latest draft: "The most-derived type of other might not match the type of this. For a derived class D, a typical implementation of this function will immediately return false if dynamic_cast(&other) == nullptr". Meanwhile, the `operator==` defined in `std::pmr::memory_resource` seems not so well-designed: bool operator==(const memory_resource& a, const memory_resource& b) > noexcept; > Returns: &a == &b || a.is_equal(b). > For "simple designs", as you said, `&a == &b` and `a.is_equal(b)` seems to be equivalent, so that redundant operations are introduced. > Mingxin Wang -- 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/ac2e1965-b697-4be3-bd33-c9431dd2619d%40isocpp.org. ------=_Part_14849_1016021062.1508038829323 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
Thanks for your comments!

On Sunday, October 15, 20= 17 at 3:54:41 AM UTC+8, T. C. wrote:
For simple memory resources, including the three in = the standard, is_equal implementations can simply do an address comparison.= There's no need for dynamic_cast or RTTI.=C2=A0

However, this conclusion only holds in "simple designs= ", and it is apparent that `std::pmr::memory_resource` is desined to h= ave good extendability for more complex implementations in the future, as w= hat is illustrated in the latest draft: "The most-derived type of othe= r might not match the type of this. For a derived class D, a typical implem= entation of this function will immediately return false if dynamic_cast<= const D*>(&other) =3D=3D nullptr".

Mea= nwhile, the `operator=3D=3D` defined in `std::pmr::memory_resource` seems n= ot so well-designed:

bool operator=3D=3D(const memory_resource& a, c= onst memory_resource& b) noexcept;
Returns: &a =3D=3D &b || a.is= _equal(b).

For "simple designs&quo= t;, as you said, `&a =3D=3D &b` and `a.is_equal(b)` seems to be=C2= =A0equivalent, so that redundant operations are introduced.

Mingxin Wang=C2=A0

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