220 26517 <633216ff-c3df-4d6f-b33f-3457c87c7996@isocpp.org> article Path: news.gmane.org!not-for-mail From: pohilets@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Pointer casts with move semantics Date: Fri, 1 Jul 2016 03:50:49 -0700 (PDT) Lines: 300 Approved: news@gmane.org Message-ID: <633216ff-c3df-4d6f-b33f-3457c87c7996@isocpp.org> References: <7c02ff86-424d-48aa-bd5e-a2f768bd5484@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_267_1398178785.1467370249386" X-Trace: ger.gmane.org 1467370269 20452 80.91.229.3 (1 Jul 2016 10:51:09 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Fri, 1 Jul 2016 10:51:09 +0000 (UTC) Cc: pohilets@gmail.com To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBDT6TQHDA4IIVFWZXMCRUBEEZWD6U@isocpp.org Fri Jul 01 12:50:54 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-vk0-f71.google.com ([209.85.213.71]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1bIw28-0005uG-V7 for gclcip-std-proposals@m.gmane.org; Fri, 01 Jul 2016 12:50:53 +0200 Original-Received: by mail-vk0-f71.google.com with SMTP id v6sf272246770vkb.2 for ; Fri, 01 Jul 2016 03:50:52 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:cc: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=4MGxTc69QI/ZDxazprRU8wUTh0sspNy0OQ4cHS5MAGI=; b=u5ybKTrzZw6l4TIlAy0cUT4/ypEhMGMPKQT6wTUN/9WT6YOwzbpkBFK9zlE3CVKeZy 30ORdZwggzmcwRXwjdWoQWEpkhgvgygCd2Pe7Diud9VRSUrY8O8EptIv/kPAy1sRTMuQ qEe4+zImTJLa9fzr1TXD/CumImiy35mmyRTqnSfUtaZRRv7dv1Tqa9mgkMeu1IsFk556 iouEpWv+1CT8yCPr/hy3d7kLoLkLHZvZpaUHcp0pSXwsiq6KY2N45yANLt/A5zfRVJIh jioWJloO6YWuYCOahRQqqFBesRv60HNfHKxH1VmFdGIK6/ovSjRk2SYoEReRA0ojy+2B PkNw== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=date:from:to:cc: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=4MGxTc69QI/ZDxazprRU8wUTh0sspNy0OQ4cHS5MAGI=; b=M3M1CVqlYowOaP8pll12pAv8/xFkSV0kVUCkt14nNl4SdV0L/VyyYxsfwEggjKHCzU t0VJ7A81F9taJTMP7U8EZBpnvpz9gBVH/OGWemz3oaBX5v/suOxzpzgssmUBBkKqJ/OF E1aqFnMjDShyz9GQ3Rm60+49dUxEJYV0wGfOZTudtzojQEa/TfuC8nP73W/2CJ71GAy+ 5hv7AWi+xvOjK3dq86oqweEjXB7jNVvrP69bfoXTafjmJZQqlUfh49WOKH1UAGNlDkVP qzmJ4UcxRpK4DLXew+yx9X59jTfgz6wODwfzQNZPZrn5yZEch/B5Wu1omGD4Lmrlw5bB PfKQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:date:from:to:cc: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=4MGxTc69QI/ZDxazprRU8wUTh0sspNy0OQ4cHS5MAGI=; b=Zpb5qNms677Qer00VWQ+YuXZ+UmdoHcoT7NRh4tBksqoZ/XKIxqXVOjAamH6s4eqhV WIsSwknlV48vopWK86yiedlXuYiH7j148qXrYon8vRZudI50siknxlr8rf49NWbndxGj hPElrv3ppclxURWaLIQsx6R9xAQ1b4irrtQoigxPnVeeT3OGhzWx/G70i/o3TqKNM56A /5+5mUwB7XZpX2rpyfomlPdP7+hex1PkSDwI7vCJRmqVaQI2ZJST9T+oOB5V+429Nawa JFXkZESkkuQGsTRWP9RTjC1Bf7xS6p/odjBDthk03pKew5YXXGVEWErwtZEssXc+CXwN XjWQ== X-Gm-Message-State: ALyK8tLGdPw5r2RRbeab/WGM8dWhRZiLZU7WZy9M9NCM99ncP2lVdz9NXdMmRejC6oEmPQ== X-Received: by 10.237.57.70 with SMTP id l64mr16403412qte.5.1467370251935; Fri, 01 Jul 2016 03:50:51 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.137.82 with SMTP id l79ls245663iod.74.gmail; Fri, 01 Jul 2016 03:50:50 -0700 (PDT) X-Received: by 10.36.87.213 with SMTP id u204mr524710ita.10.1467370250526; Fri, 01 Jul 2016 03:50:50 -0700 (PDT) In-Reply-To: <7c02ff86-424d-48aa-bd5e-a2f768bd5484@isocpp.org> X-Original-Sender: pohilets@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:26517 Archived-At: ------=_Part_267_1398178785.1467370249386 Content-Type: multipart/alternative; boundary="----=_Part_268_874370203.1467370249387" ------=_Part_268_874370203.1467370249387 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Submitted as P0390R0. Should be published in the next mailing =D1=81=D1=83=D0=B1=D0=BE=D1=82=D0=B0, 21 =D1=82=D1=80=D0=B0=D0=B2=D0=BD=D1= =8F 2016 =D1=80. 19:19:59 UTC+2 =D0=BA=D0=BE=D1=80=D0=B8=D1=81=D1=82=D1=83= =D0=B2=D0=B0=D1=87 pohi...@gmail.com=20 =D0=BD=D0=B0=D0=BF=D0=B8=D1=81=D0=B0=D0=B2: > > Recently I was optimizing reference counting in a tight loop, and was=20 > surprised to discover that all pointer cast functions take their argument= =20 > by const reference and there are no versions that would take argument by= =20 > value or by rvalue reference. As a result, the following code makes a=20 > completely unnecessary atomic increment and decrement: > > struct Foo {}; > > struct Bar : public Foo {}; > > std::shared_ptr foo(); > > auto bar =3D std::static_pointer_cast(foo()); > > > An obvious solution would be add versions that take parameter by rvalue= =20 > reference: > > > template std::shared_ptr static_pointer_cast(const= =20 > std::shared_ptr& r); // since C++11 > > template std::shared_ptr dynamic_pointer_cast(const= =20 > std::shared_ptr& r); // since C++11 > > template std::shared_ptr const_pointer_cast(const st= d > ::shared_ptr& r); // since C++11 > > template std::shared_ptr reinterpret_pointer_cast( > const std::shared_ptr& r); // since C++17 > > > template std::shared_ptr static_pointer_cast(std:: > shared_ptr&& r); // proposed addition > > template std::shared_ptr dynamic_pointer_cast(std:: > shared_ptr&& r); // proposed addition > > template std::shared_ptr const_pointer_cast(std:: > shared_ptr&& r); // proposed addition > > template std::shared_ptr reinterpret_pointer_cast(st= d > ::shared_ptr&& r); // proposed addition > > > Or just have a single version of each function, that takes argument by=20 > value: > > > template std::shared_ptr static_pointer_cast(std:: > shared_ptr r); // proposed replacement > > template std::shared_ptr dynamic_pointer_cast(std:: > shared_ptr r); // proposed replacement > > template std::shared_ptr const_pointer_cast(std:: > shared_ptr r); // proposed replacement > > template std::shared_ptr reinterpret_pointer_cast(st= d > ::shared_ptr r); // proposed replacement > > > The later makes a redundant increment/decrement in one case: when doing= =20 > dynamic_pointer_cast on a l-value and cast fails > > > > --=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/633216ff-c3df-4d6f-b33f-3457c87c7996%40isocpp.or= g. ------=_Part_268_874370203.1467370249387 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
Submitted as=C2=A0P0390R0. Should be published in the next= mailing

=D1=81=D1=83=D0=B1=D0=BE=D1=82=D0=B0, 21 =D1=82= =D1=80=D0=B0=D0=B2=D0=BD=D1=8F 2016 =D1=80. 19:19:59 UTC+2 =D0=BA=D0=BE=D1= =80=D0=B8=D1=81=D1=82=D1=83=D0=B2=D0=B0=D1=87 pohi...@gmail.com =D0=BD=D0= =B0=D0=BF=D0=B8=D1=81=D0=B0=D0=B2:
Recently I was optimizing reference counting in a ti= ght loop, and was surprised to discover that all pointer cast functions tak= e their argument by const reference and there are no versions that would ta= ke argument by value or by rvalue reference. As a result, the following cod= e makes a completely unnecessary atomic increment and decrement:

=C2=A0 =C2=A0struct Foo {};

=C2=A0 =C2=A0struct Bar : public Foo {};

=C2=A0 =C2=A0std::shared_ptr<Foo> foo();

=C2=A0 =C2=A0auto bar =3D std::static_pointer_cast<Bar>(= foo());


An obvious solution would be add versions that ta= ke parameter by rvalue reference:


template<= /span><class T, = class U> std::shared_ptr<T> static_pointer_cast(const = std::shared_ptr<U>& r);=C2=A0 =C2=A0 =C2=A0 // since C++11

template<<= /span>class T, class U> std::shared_ptr<T> dynamic_pointer_cast(const std= ::shared_ptr<U>&= ; r); =C2=A0 =C2=A0 // since C++11

template<class T, cl= ass U> std::shared_ptr<T> co= nst_pointer_cast(const std::shared_ptr<U>& r); =C2=A0 =C2=A0 =C2= =A0 // since C++11

template<c= lass T, class U> std::shared_ptr<T> reinterpret_point= er_cast(const std::shared_ptr<U>& r); // since C++17


template<class<= /span> T, class U&g= t; std::shared_ptr<T> static_pointer_cast(std::shared_ptr<U>&& r); =C2=A0 = =C2=A0 =C2=A0// proposed addition

template<class T, cla= ss U> std= ::shared_ptr<T> dyn= amic_pointer_cast(std::shared_ptr<U>&am= p;& r);=C2=A0 =C2=A0 =C2=A0// prop= osed addition

template<class T, class U> std::shared_ptr<= span><T> const_pointer_cast(std<= /span>::shared_ptr<U>&& r);=C2=A0 =C2=A0 =C2=A0 =C2=A0// proposed addition

template<class T, class= U> std::= shared_ptr<T> reint= erpret_pointer_cast(std::= shared_ptr<U>&= amp;& r);=C2=A0// proposed additio= n


Or just have a single version of each function, that takes argument = by value:


template<class=C2=A0T,=C2=A0= class=C2=A0U>=C2=A0std::shared_ptr<T> static= _pointer_cast(std= ::shared_ptr= <U> r);=C2=A0=C2=A0 =C2=A0 =C2=A0// proposed replacement

templat= e<class= =C2=A0T,=C2=A0class=C2=A0U>=C2=A0std::shared_ptr<T> dynamic_pointer_cast(std::= shared_ptr<U> r);=C2=A0 =C2=A0 =C2=A0// proposed=C2=A0replacement

template<class= =C2=A0T,=C2=A0class=C2=A0U>=C2=A0std<= span>::shared_ptr<= span><T> const_pointer_cast(std::shared_ptr<U> r);=C2=A0 =C2=A0 =C2=A0 =C2=A0// proposed=C2=A0replacement

template<class=C2=A0T,=C2=A0class=C2=A0U>=C2=A0std::shared_ptr<T> reinterpret_po= inter_cast(std::<= /span>shared_ptr<= ;U> r);=C2=A0// proposed=C2=A0= replacement


The later make= s a=C2=A0redundant=C2=A0increment/decrement in one case: when doing=C2= =A0dynamic_pointer_cast=C2= =A0on a l-value and cast fails



=

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To view this discussion on the web visit https://groups.google.= com/a/isocpp.org/d/msgid/std-proposals/633216ff-c3df-4d6f-b33f-3457c87c7996= %40isocpp.org.
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