220 20296 <55EB2F54.3060008@gmail.com> article Path: news.gmane.org!not-for-mail From: Andrey Semashev Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Draft proposal - std::recover_cast: undoing type erasure Date: Sat, 5 Sep 2015 21:07:16 +0300 Lines: 66 Approved: news@gmane.org Message-ID: <55EB2F54.3060008@gmail.com> References: <7935FD03-ACA3-4D49-90B2-BDD107C2EF0B@gmail.com> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: quoted-printable X-Trace: ger.gmane.org 1441476444 6051 80.91.229.3 (5 Sep 2015 18:07:24 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sat, 5 Sep 2015 18:07:24 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCXY5TXHWYJRBVW6VSXQKGQE2CTGSJA@isocpp.org Sat Sep 05 20:07:18 2015 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-wi0-f197.google.com ([209.85.212.197]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1ZYHry-0000kx-Al for gclcip-std-proposals@m.gmane.org; Sat, 05 Sep 2015 20:07:18 +0200 Original-Received: by wisv5 with SMTP id v5sf15204840wis.0 for ; Sat, 05 Sep 2015 11:07:19 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:subject:to:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-type :content-transfer-encoding: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=et8Jym4ww9Lh+8r65mBO25jF39wCdbLhvX7J+zICLwY=; b=GTr0qHaBxYptPMZOPf+d3d2Rr+eBkM3aNDdReBPz3aAmiw8KomjoKZZMU82rNU81L2 xcmO3ZxCod+YxzNAU95fZUGX651qUMpk8cjx5YC93fzPHeoqj6vHP+4Ajr5mR7U7xvJY olXchGgBNzFkU2ehfD7o/p4VkAP32Uzj7yoDKBIy4SHl0d/lKORuIlONOOhU657OX8Ki aLH1XdQjTsfT94wm4vYFXfNwrasBzfpVwgYLc//oQSdCBfGEJ+pz9dL96mWM0ZyN8CtE 6ofUj1XeSyOiwKw6QLTs64A1Q52FLNvOFMR73zgA/eI8dk8 X-Gm-Message-State: ALoCoQkpj2yb80kYcJatQbbLP0V+2RWgtRQyBF54smtLQoVEVeFdAhLNypCq5csYPSwFaZ/dUofp X-Received: by 10.112.149.39 with SMTP id tx7mr2703495lbb.11.1441476439188; Sat, 05 Sep 2015 11:07:19 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.152.6.196 with SMTP id d4ls508912laa.91.gmail; Sat, 05 Sep 2015 11:07:17 -0700 (PDT) X-Received: by 10.112.54.132 with SMTP id j4mr9404501lbp.84.1441476437652; Sat, 05 Sep 2015 11:07:17 -0700 (PDT) Original-Received: from mail-la0-x232.google.com (mail-la0-x232.google.com. [2a00:1450:4010:c03::232]) by mx.google.com with ESMTPS id d9si4929334laf.139.2015.09.05.11.07.17 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sat, 05 Sep 2015 11:07:17 -0700 (PDT) Received-SPF: pass (google.com: domain of andrey.semashev@gmail.com designates 2a00:1450:4010:c03::232 as permitted sender) client-ip=2a00:1450:4010:c03::232; Original-Received: by laeb10 with SMTP id b10so30812448lae.1 for ; Sat, 05 Sep 2015 11:07:17 -0700 (PDT) X-Received: by 10.112.171.227 with SMTP id ax3mr9200823lbc.118.1441476437561; Sat, 05 Sep 2015 11:07:17 -0700 (PDT) Original-Received: from [192.168.1.2] (broadband-178-140-129-233.nationalcablenetworks.ru. [178.140.129.233]) by smtp.googlemail.com with ESMTPSA id n6sm1568806laf.18.2015.09.05.11.07.16 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sat, 05 Sep 2015 11:07:17 -0700 (PDT) User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.2.0 In-Reply-To: <7935FD03-ACA3-4D49-90B2-BDD107C2EF0B@gmail.com> X-Original-Sender: andrey.semashev@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of andrey.semashev@gmail.com designates 2a00:1450:4010:c03::232 as permitted sender) smtp.mailfrom=andrey.semashev@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:20296 Archived-At: On 05.09.2015 12:12, David Krauss wrote: > Abstract: > > C++ has a couple built-in type erasure mechanisms, with respective > facilities to recover the original object: polymorphic classes with > dynamic_cast, and void pointers with static_cast or reinterpret_cast. A > library facility recover_cast is proposed to analogously handle erasure > classes like std::function, any, and variant. I can also add std::locale, which can be considered as a collection of=20 type erased facets. > Besides being a uniform, > generic interface, it simplifies implementation and and even improves > performance. > > > Highlight: > > template< typename T, typename ErasureClass > > constexpr auto * > > recover_cast( ErasureClass /cq /* e ) noexcept;// /cq//is /const/or empty= ../ > > /Requires:/ ErasureClass is capable of erasing T. (Thus, T shall not be > void. A class may not assign semantics to =E2=80=9Cerased values of type = void,=E2=80=9D > and the implementation should diagnose an attempt to use recover_cast< > void >( & e ) as a substitute for static_cast< void * >( e ).) Why the special treatment of type void? > /Returns:/ ( remove_reference_t< T /cq/ > * ) e.operator void const * > ()if econtains type Taccording to e.verify_type< T >(). Otherwise, nullpt= r. > > template< typename T, typename ErasureClass > > auto &&// /cq//is /const/or empty. //ref//is /&/or /&&/./ > > recover_cast( ErasureClass /cq ref/ e ) noexcept; > > /Requires:/ ErasureClass is capable of erasing T. > > /Effects:/ Let /p/ be recover_cast< T >( & e ). If /p/ is a null > pointer, throw an object of class bad_type_recovery. Otherwise, return > static_cast< T /cq ref/ >( * /p/ ). I didn't like that the protocol you described in pdf is intrusive (i.e.=20 requires support built into the type erasing type). I would very much=20 preferred it to be decoupled from the type erasing wrapper. This would=20 allow to support the tool in non-standard type erasing types. Just=20 remove the ErasureClass concept and use functional API, in the=20 std::begin/end style. --=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 http://groups.google.com/a/isocpp.org/group/std-proposa= ls/. .