220 27332 article Path: news.gmane.org!not-for-mail From: demiobenour@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Relocation as a solution for the valueless variant problem? Date: Wed, 27 Jul 2016 18:44:16 -0700 (PDT) Lines: 259 Approved: news@gmane.org Message-ID: References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_6220_1247268716.1469670256782" X-Trace: ger.gmane.org 1469670272 12565 80.91.229.3 (28 Jul 2016 01:44:32 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Thu, 28 Jul 2016 01:44:32 +0000 (UTC) Cc: isocppgroup@denisbider.com To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCD5RBOM2UBBB4WG4W6AKGQEEE5YMJQ@isocpp.org Thu Jul 28 03:44:25 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-it0-f72.google.com ([209.85.214.72]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1bSaN6-00016B-IG for gclcip-std-proposals@m.gmane.org; Thu, 28 Jul 2016 03:44:24 +0200 Original-Received: by mail-it0-f72.google.com with SMTP id j124sf37757586ith.1 for ; Wed, 27 Jul 2016 18:44:24 -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=bkeKFUehM+TdXWtL7inWHi2Z7gPRQmnvBFttFaH8Y5o=; b=sbr4iB2JhIN+8CDJxF8ROaleYHj7ZpEbQjcyPRB/l1Tyo8RuJq9E0SfaoyXRYrDcAz FLt1B6+og/pLfVSEPraTYOKeujjwhiHInxu5UHPUybt8m67+/7Qd4GVkVHKzyiHUm8lj EigSXDs3giZSUKWNhLNXPp/uInTLmiFuvJSqgM0nY0e8xaUlBsqv6+jsQ35MHIR27ZPF d/yUNTG/UAE4hE/vtSONNNy6T30RSiA9kN9JpNh19vOn60ds0ETKdB0DZpH/R2sy0627 Lcmlgkh1dlCKCMnpqSt0VKAA5q186VASFABRO17/HDq36+NVkxmGj1ZBqLPD70NJvvjx zwtQ== 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=bkeKFUehM+TdXWtL7inWHi2Z7gPRQmnvBFttFaH8Y5o=; b=vFd/JLZulRebRp1otV3THe6KkVx8DXdjED5rmS3NK0LsQ00l5Xb5v6t6q1QsVRdkN/ gbtUu/35pFRspU8LcVghsUNjIRhLJusZl5EJVSol0VPo4LM5Iziap3UxEdmvyS27r112 cJ7VAhPA6VnDBnofLbl6LFaIdhH2Wg3io5GmQdCa9g4Rwa0RwhGW0WDXiyKSrvtF0mcu FUkXG9Ses3a8AykzAgdvwHMxPMIs2pCuz0WqW5Waqws0CC+mjyXnZb5oiBzXfUQeewHN bzoBeM0SKw2uFao0P4fpBN9dUakvHk029W0P8060PGC+eHzttRG9vy9js+SDJFfAv/ZM iVDw== 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=bkeKFUehM+TdXWtL7inWHi2Z7gPRQmnvBFttFaH8Y5o=; b=ORN8eXI1nH2dq7+Xw1qeqpqmkLN6YXLD891bwwcv4SfwZRZv/NjXSkB/9DthU22HSg 1R1exUNLkTdKtXbWkUZTdbQRLzLas9Kx3bOnyJLwTAqAMrwFqJRAzlbTQNY72ua1Ck7E 1VN8TDU0a7S1Ea4vAHlgobfZl7kFirRMfuqq/StwR2OUjKfbV9RI3PMyvo0LEsgga/1j kYYAx32yq1ZGBZ14MaIbSos6jEZHFIKOu6SVQniye3SFMm5HImY31zXx5TXkX6JaneH2 H9CecBcyItN6rSX3GE3OuZCUMYAStcnT3qfZEKewuxYnTO54vJZimV4ZARp4+OQHgOAk HiuQ== X-Gm-Message-State: AEkoout6+ydJw+9rqFgtD/ExW0FT+YvCBaXUoMRpYWgMdohHRYHU8OZbiCZhNBKPWCP/rQ== X-Received: by 10.157.5.37 with SMTP id 34mr28368528otw.43.1469670263616; Wed, 27 Jul 2016 18:44:23 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.160.75 with SMTP id j72ls1118979ioe.14.gmail; Wed, 27 Jul 2016 18:44:18 -0700 (PDT) X-Received: by 10.36.107.7 with SMTP id v7mr1334172itc.10.1469670258148; Wed, 27 Jul 2016 18:44:18 -0700 (PDT) In-Reply-To: X-Original-Sender: demiobenour@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:27332 Archived-At: ------=_Part_6220_1247268716.1469670256782 Content-Type: multipart/alternative; boundary="----=_Part_6221_970988934.1469670256783" ------=_Part_6221_970988934.1469670256783 Content-Type: text/plain; charset=UTF-8 Here is another option: require move constructors to be noexcept for use in variants. On Tuesday, April 5, 2016 at 7:58:18 PM UTC-4, isocp...@denisbider.com wrote: > > A proposal has recently been floated: > > http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0308r0.html > > ... which proposes pilfering as a solution to the valueless variant > problem. > > Please correct me if I'm wrong - my understanding of the valueless variant > problem is that it arises if a move constructor throws during assignment > and emplacement, such as this: > > std::variant a = X(); > std::variant b = Y(); > try { b = std::move(a); } // bad_alloc here > catch (std::bad_alloc) {} > > > In this case, variant's assignment operator has to do something like this: > > b.y.~Y(); > new (&b.x) X(std::move(a.x)); > > > To avoid a valueless result in case of exception, one's first thought > might be to move first into a temporary: > > X temp { std::move(a.x) }; > b.y.~Y(); > new (&b.x) X(std::move(temp.x); > > > Unfortunately, this does not gain anything. Now the second move > constructor may throw, resulting in the same situation. > > However! > > If C++ implemented relocation, as described in this proposal P0023: > > http://denisbider.com/Relocator.pdf > > Then the solution involving a temporary would work: > > std::aligned_storage_t tempData; > X* temp = new (tempData) { std::move(a.x) }; > b.y.~Y(); > new (&b.x) >>X(*temp); > > > This would now be exception-safe: > > - without requiring pilfering; > - and without a potentially, but almost never, valueless variant. > > Food for thought? > > I believe the relocation proposal has so far not progressed for, at least, > these two reasons: > > (1) Proposals require champions to travel to meetings to represent them. > (2) I am not willing to travel for this proposal, because I think the C++ > committee should just do the right thing, independent of who travels to a > meeting. > > The language should consider ideas based on their merit - not conditional > on the appearance of valiant knights who will cross swamps and deserts, to > rescue ideas from a castle. :-) > > > -- 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/e9fdc297-6cb0-4b6e-84d3-bc0b04c003a6%40isocpp.org. ------=_Part_6221_970988934.1469670256783 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
Here is another option: require move constructors to be no= except for use in variants.

On Tuesday, April 5, 2016 at 7:58:18 PM = UTC-4, isocp...@denisbider.com wrote:
A proposal has recently been floated:


... which proposes pilfering as a= solution to the valueless variant problem.

Please= correct me if I'm wrong - my understanding of the valueless variant pr= oblem is that it arises if a move constructor throws during assignment and = emplacement, such as this:

  std::variant<X, Y> a =3D X();
  std::variant<=
X, Y> b =3D Y=
();
  try { b =3D std::move(a); }    // bad_alloc here
  catch (std::bad_alloc) {}

In this case, variant's assignment oper= ator has to do something like this:

  b.y.~Y();
  new (=
&b.x) X(std::move(a.x));

To avoid a valueless result in case of exce= ption, one's first thought might be to move first into a temporary:

  X temp { std::=
move(a.x) };
  b.y.~Y();
  new (=
&b.x) X(std::move(temp.x);

Unfortunately, this does not gain anything.= Now the second move constructor may throw, resulting in the same situation= ..

However!

If C++ impleme= nted relocation, as described in this proposal P0023:

<= div>http://denisbider.com/Relocator.pdf
=

Then the solution involving a temporary would work:

  std::aligned_storage_t<size=
of(X), alignof(X)> tempData;
  X* temp =3D new (tempData) { std::move<=
/span>(a.x) };
  b.y.~Y();
  new (=
&b.x) ><=
/span>>X(*te=
mp);

This would now be exception-safe:

- without requiring pilfering;
- and without a=C2= =A0potentially, but almost never, valueless variant.

Food for thought?

<= div>I believe the relocation proposal has so far not progressed for, at lea= st, these two reasons:

(1)=C2=A0Proposals require = champions to travel to meetings=C2=A0to represent them.
(2)=C2=A0= I am not willing to travel for this proposal, because I think the C++ commi= ttee should just do the right thing, independent of who travels to a meetin= g.

The language should=C2=A0consider ideas based= =C2=A0on their merit -=C2=A0not conditional on the appearance of valiant kn= ights who=C2=A0will cross swamps and deserts, to rescue=C2=A0ideas=C2=A0fro= m a castle. :-)


--
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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/e9fdc297-6cb0-4b6e-84d3-bc0b04c003a6= %40isocpp.org.
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