220 20307 <229EBC6B-8CA5-45DF-B4AE-BAD809C27A3B@gmail.com> article Path: news.gmane.org!not-for-mail From: David Krauss Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Draft proposal - std::recover_cast: undoing type erasure Date: Sun, 6 Sep 2015 10:17:30 +0800 Lines: 111 Approved: news@gmane.org Message-ID: <229EBC6B-8CA5-45DF-B4AE-BAD809C27A3B@gmail.com> References: <7935FD03-ACA3-4D49-90B2-BDD107C2EF0B@gmail.com> <55EB974B.4000004@wanadoo.fr> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 (Mac OS X Mail 8.2 \(2098\)) Content-Type: multipart/alternative; boundary="Apple-Mail=_DD73C85C-EE70-440D-87F9-D43F4777FFC6" X-Trace: ger.gmane.org 1441505872 24322 80.91.229.3 (6 Sep 2015 02:17:52 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sun, 6 Sep 2015 02:17:52 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCW25A7E3QCRBQOEV2XQKGQEOUA4TCQ@isocpp.org Sun Sep 06 04:17:42 2015 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ob0-f199.google.com ([209.85.214.199]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1ZYPWV-0004vS-4S for gclcip-std-proposals@m.gmane.org; Sun, 06 Sep 2015 04:17:39 +0200 Original-Received: by obbhk5 with SMTP id hk5sf101639758obb.2 for ; Sat, 05 Sep 2015 19:17:38 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:from:content-type:message-id:mime-version :subject:date:references:to:in-reply-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=0InVf1wWiJkvVJSsiLbBytmiYT16uvf9/gZa2fvTX0Y=; b=GuZzwzzaPkpXOsNjIWXyA9CP1WPwG8kPtgH2zA6y3PuTc2de9L0UKrIZVAYvjV0M5c SCwEqEiAKxK9sRgG8+srW2bEq0ufpE3FLKbKbkoCLPNq88NL4coJsusuVkgb85w9a4Ay 0Cizxsjui0K8fzEdSWoOYygJt92eGvlj36rxQy7Ds6ioN+R6GADkxGWx3yEAK61P9dgV 4vimbXTH6CtXwo25GTrNU/alI1Q6MYl0wl2puAdJ2YA1NBFKuTQ0CssDQy/8Mro+3bMO f9iVG702VWhT8JIjT/XPbq1QDo9O1K3p+4Q4dUnA/R05/MF8pPeon8K60izIqBTaOSQ2 zQ7w== X-Gm-Message-State: ALoCoQn9nyIa8bu3etXh4kUsIDTUXEeuDGPtpcf7nrbCIwJZMTlxlSa/7GoahH1xcAkXPx6Aawa3 X-Received: by 10.107.163.81 with SMTP id m78mr11502276ioe.18.1441505858399; Sat, 05 Sep 2015 19:17:38 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.50.142.42 with SMTP id rt10ls499701igb.44.gmail; Sat, 05 Sep 2015 19:17:37 -0700 (PDT) X-Received: by 10.68.131.33 with SMTP id oj1mr27569722pbb.96.1441505857645; Sat, 05 Sep 2015 19:17:37 -0700 (PDT) Original-Received: from mail-pa0-x22b.google.com (mail-pa0-x22b.google.com. [2607:f8b0:400e:c03::22b]) by mx.google.com with ESMTPS id da4si12766461pad.91.2015.09.05.19.17.37 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sat, 05 Sep 2015 19:17:37 -0700 (PDT) Received-SPF: pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:400e:c03::22b as permitted sender) client-ip=2607:f8b0:400e:c03::22b; Original-Received: by padhy16 with SMTP id hy16so58454923pad.1 for ; Sat, 05 Sep 2015 19:17:37 -0700 (PDT) X-Received: by 10.66.144.165 with SMTP id sn5mr28354800pab.122.1441505857332; Sat, 05 Sep 2015 19:17:37 -0700 (PDT) Original-Received: from [172.20.10.2] ([121.54.44.89]) by smtp.gmail.com with ESMTPSA id dh1sm7201795pbd.49.2015.09.05.19.17.35 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Sat, 05 Sep 2015 19:17:36 -0700 (PDT) In-Reply-To: <55EB974B.4000004@wanadoo.fr> X-Mailer: Apple Mail (2.2098) X-Original-Sender: potswa@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:400e:c03::22b as permitted sender) smtp.mailfrom=potswa@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:20307 Archived-At: --Apple-Mail=_DD73C85C-EE70-440D-87F9-D43F4777FFC6 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 > On 2015=E2=80=9309=E2=80=9306, at 9:30 AM, Vicente J. Botet Escriba wrote: >=20 > As Andrey, I don't believe that we need to go too far. I don't see the ne= ed to require anything on some TypeErase concept.=20 > We just need to allow to overload this function for several types. This d= oesn't mean that we can not have a common implementation for some types. The problem is that it=E2=80=99s difficult to implement the common protocol= .. Besides any_cast, optional::operator*, and get(variant) having different = specs, there have been implementation bugs and DRs against the handling of = const and value category. Consider const&& references, for example. The implementation experience so far shows that it=E2=80=99s a bad idea to = have each class provide its own overload set. Separately, most users call standard library functions, including cast func= tions, as std::dynamic_pointer_cast, not considering ADL overloading =E2=80= =94 and ADL can be fragile when template arguments bring in overloads from = various libraries. The common implementation is more usable because std:: i= s guaranteed to work. I touch on this in the paper, but perhaps should elab= orate a little. --=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/. --Apple-Mail=_DD73C85C-EE70-440D-87F9-D43F4777FFC6 Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=UTF-8
On 2015=E2=80=9309= =E2=80=9306, at 9:30 AM, Vicente J. Botet Escriba <vicente.botet@wanadoo.fr> wrote:
As Andrey, I don't believe that we need to go too= far. I don't see the need to require anything on some TypeErase concept. 
We just need to allow to overload th= is function for several types. This doesn't mean that we can not have a com= mon implementation for some types.

The problem is that it=E2=80=99s dif= ficult to implement the common protocol. Besides any_cast, optional::operat= or*, and get(variant) havin= g different specs, there have been implementation bugs and DRs against the = handling of const and value catego= ry. Consider const&& refer= ences, for example.

The implementation experience so far shows that it=E2=80=99s a bad idea = to have each class provide its own overload set.

Separately, most users call standard librar= y functions, including cast functions, as std::dynamic_pointer_cast, not considering ADL overloading =E2=80= =94 and ADL can be fragile when template arguments bring in overloads from = various libraries. The common implementation is more usable because std:: is guaranteed to work. I touch on t= his in the paper, but perhaps should elaborate a little.

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