220 25983 <7c02ff86-424d-48aa-bd5e-a2f768bd5484@isocpp.org> article Path: news.gmane.org!not-for-mail From: pohilets@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Pointer casts with move semantics Date: Sat, 21 May 2016 10:19:59 -0700 (PDT) Lines: 419 Approved: news@gmane.org Message-ID: <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_3165_1371086206.1463851199841" X-Trace: ger.gmane.org 1463851203 7762 80.91.229.3 (21 May 2016 17:20:03 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sat, 21 May 2016 17:20:03 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBDT6TQHDA4IMBMMCXICRUBEAXT4BM@isocpp.org Sat May 21 19:20:02 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ig0-f199.google.com ([209.85.213.199]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1b4AZG-0006yx-Bf for gclcip-std-proposals@m.gmane.org; Sat, 21 May 2016 19:20:02 +0200 Original-Received: by mail-ig0-f199.google.com with SMTP id u5sf38266766igk.2 for ; Sat, 21 May 2016 10:20:01 -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: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=Yv+iDio3d38OaP4jTkHfQzzvukJNOtsUy6P2WKqaaqY=; b=z3NQQ7G8Q45ruhGlSpbcFDBd3MspiEW5ELyp4pzrTPlQofApuaPK78im4kNgLgF1Ih ABkRsl51rfZv5h79cyCFMdEOypO92Zps2Ud863gwWZ4HRMpQTer/Q36bvlQQziWICIDA XUeE4v4VgmPcouPpfdNbUA0KqMhG/S3FY0ngsQu6Aw4mOPfMExviAp9YmPRYoP7gNSz6 J3gOvbPiuH10CwGsLkoKGstlrB2K2eVIDBRgc6PUJcBwAtaCiryLWtCjy2QGgAmy/Cta nXfri4aJkEIniLXTKW9rO+NnYkfoRvuoWrUAwOpHmSuEgUL3d5m57k12IrtX51Bk4j9P Esew== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=date:from:to:message-id: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=Yv+iDio3d38OaP4jTkHfQzzvukJNOtsUy6P2WKqaaqY=; b=S1kXZMGHV3BJr2EkJIxjuN4JZdA9jVlhVwP7eT5VXMzA/nX0kp+8KryN1SgboaJIUE tjeIxFhyXwT7Cb0pcrHkXpvTAgKsQ9jKHLIuRGTt+Cs6GWQts7zxP62xzUD/VPWXmelO 4iemUiBzUmvFFJWmD2Sny7AOwNmmsw2Fm80IomMNLoBZjlA2SUsmnnsfZrN/pX0fteJx hiZgeXSntx+b5p54FCBW7R6bR6Xh8Xi5kUdyvfF0Cc/Dm3kwvy4jsxj3rYiODmpw/YO3 P5zaJD7ouRAiJ0U3kNNPnHBZIEMq9eEYrR0SePN9S1JW+mK07NYwxxkko+OapMNb2D0W oiyg== 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:message-id: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=Yv+iDio3d38OaP4jTkHfQzzvukJNOtsUy6P2WKqaaqY=; b=amZGgp0a8Sg9Vq5U7ady+H1lwYUgNXl4v2rctqmDZApeNAl6cGhGvg2H/j8NeFS1no +4jhNoj9qnFaGKENDgxC2/3wDp7fI4e/g0tS/WdsO/LpkofdWwSZEQwzFaICamr0jDYA gsK/TtCKnmHySDXk7ElBKsqy1+o72wdDrusGdpOk0jlt/PBHTRKEg2sQvgtQ+buu3Zbm tAVMw4WA0QjKKDDlWtUSzYCrQvmRJcFCLkj60Dk2ZzkituNanlWZX0/uX619glFhsUYt MB7nSEf2H4amKiUbHS2EI1IajjMUCcYZ6PUfOd/5dT1kunChQdchrOYNHTH1wnBEt8Jn Mkrg== X-Gm-Message-State: AOPr4FV/jvK1FzmxIf61++SVkJ3AOGVn7HgZNJ6qKHydicovZ14xHzIrKA8QC17TuI7Ujg== X-Received: by 10.157.27.226 with SMTP id v31mr6251373otv.44.1463851201246; Sat, 21 May 2016 10:20:01 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.140.109.195 with SMTP id l61ls263397qgf.1.gmail; Sat, 21 May 2016 10:20:00 -0700 (PDT) X-Received: by 10.140.44.9 with SMTP id f9mr284419qga.20.1463851200317; Sat, 21 May 2016 10:20:00 -0700 (PDT) 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:25983 Archived-At: ------=_Part_3165_1371086206.1463851199841 Content-Type: multipart/alternative; boundary="----=_Part_3166_828223911.1463851199842" ------=_Part_3166_828223911.1463851199842 Content-Type: text/plain; charset=UTF-8 Recently I was optimizing reference counting in a tight loop, and was surprised to discover that all pointer cast functions take their argument by const reference and there are no versions that would take argument by value or by rvalue reference. As a result, the following code makes a completely unnecessary atomic increment and decrement: struct Foo {}; struct Bar : public Foo {}; std::shared_ptr foo(); auto bar = std::static_pointer_cast(foo()); An obvious solution would be add versions that take parameter by rvalue reference: template std::shared_ptr static_pointer_cast(const std ::shared_ptr& r); // since C++11 template std::shared_ptr dynamic_pointer_cast(const std ::shared_ptr& r); // since C++11 template std::shared_ptr const_pointer_cast(const std:: 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(std:: shared_ptr&& r); // proposed addition Or just have a single version of each function, that takes argument by 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(std:: shared_ptr r); // proposed replacement The later makes a redundant increment/decrement in one case: when doing dynamic_pointer_cast on a l-value and cast fails -- 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/7c02ff86-424d-48aa-bd5e-a2f768bd5484%40isocpp.org. ------=_Part_3166_828223911.1463851199842 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
Recently I was optimizing reference counting in a tight lo= op, and was surprised to discover that all pointer cast functions take thei= r argument by const reference and there are no versions that would take arg= ument by value or by rvalue reference. As a result, the following code make= s a completely unnecessary atomic increment and decrement:

=C2=A0 =C2=A0st= ruct Foo= {};

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

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

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


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


template<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<class T, class U> std::shared_ptr<T> dynamic_poin= ter_cast(const std::shared_ptr<U>& r); =C2=A0 =C2=A0 // s= ince C++11

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

template<class T, class U> std::shared_pt= r<T&g= t; reinterpret_pointer_cast(const std::shared_ptr<U>& r); = // since C++17


template<class T, class U> std::<= span style=3D"font-variant-ligatures: no-common-ligatures; color: #703daa">= shared_ptrstd::shared_ptr<U>&& r); = =C2=A0 =C2=A0 =C2=A0// proposed addition

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

<= p style=3D"font-size: 11px; line-height: normal; font-family: Menlo;">temp= late<= class T, class U> std::shared_ptr<T> const_pointer_cast(st= d::shared_ptr<U>&& r);=C2=A0 =C2=A0 =C2=A0 =C2=A0// propose= d addition

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


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


template<class<= /span>=C2=A0T,= =C2=A0class=C2=A0U>=C2=A0std::shared_ptr<T> static_pointer_cast(std::shar= ed_ptr&= lt;U> r);=C2=A0=C2=A0 =C2=A0 =C2=A0// proposed replaceme= nt

template<class=C2=A0T,=C2=A0class= =C2=A0U>=C2=A0std::shared_ptr<T> dynamic_p= ointer_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::shared_ptr<T> const_pointer_cast(std::shar= ed_ptr&= lt;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&g= t; reinterpret_pointer_cast(std::shared_ptr<= /span><U>= ; r);=C2=A0// proposed=C2=A0replacement

The later makes a=C2=A0redund= ant=C2=A0increment/decrement in one case: when doing=C2=A0dynamic_pointer_cast=C2=A0on a l= -value and cast fails


=


--
You received this message because you are subscribed to the Google Groups &= quot;ISO C++ Standard - Future Proposals" group.
To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposa= ls+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/7c02ff86-424d-48aa-bd5e-a2f768bd5484= %40isocpp.org.
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