220 13942 article Path: news.gmane.org!not-for-mail From: Miro Knejp Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Reconstruct function idea Date: Thu, 16 Oct 2014 00:31:34 +0200 Lines: 189 Approved: news@gmane.org Message-ID: References: <8a74f08b-7453-4d1a-9f20-bc1d3866617c@isocpp.org> <27e2ae9c-8b1f-4688-9bb3-9caef6bf4f54@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 (Mac OS X Mail 7.3 \(1878.6\)) Content-Type: multipart/alternative; boundary="Apple-Mail=_E13665DB-02DB-4780-A11D-E6BA09991BF3" X-Trace: ger.gmane.org 1413412306 865 80.91.229.3 (15 Oct 2014 22:31:46 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 15 Oct 2014 22:31:46 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBC6ONSXJ54LBBSXL7OQQKGQETNZWPQI@isocpp.org Thu Oct 16 00:31:39 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ee0-f71.google.com ([74.125.83.71]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1XeX6Z-0002LE-4M for gclcip-std-proposals@m.gmane.org; Thu, 16 Oct 2014 00:31:39 +0200 Original-Received: by mail-ee0-f71.google.com with SMTP id d17sf1351407eek.6 for ; Wed, 15 Oct 2014 15:31:38 -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: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:list-post:list-help:list-archive:list-subscribe :list-unsubscribe:content-type; bh=mHQO+hg62/DMjfhaDje/cq+N0gEOMn6ykhowjY+BQ+I=; b=kR3tQBSdNAjtxgZ2hko8iQ4/KfZuTAy+1KhzKDZGk0DS2KB4g6+67aiUndYJihMSL8 vj0W+klvll0hq4aApffoQJhdbto5/G1gY6y4p2pORH6T6+ADps189fj8PPcHp2Pc0Gsg 2R54fyidCoZT/nRKl8DcKzcEpngnlNGZldVfKy5CYJx+hHOT7BVtIX5GKXKc7vO8rkDS k7ChK3uCIiNoJVKwWIIG2KsNrZQPAgroxi94kZLrlpA+vIMT/rYjzFuoKkg//8szZ4wR DGuwhFKNWcXer8qMGXtj392YLc6v1Of08F9xeFi1Dp6mRiAWAr8ArrHfjAaQrD3lTLIC nFpQ== X-Gm-Message-State: ALoCoQlCAi15HYLAmnW1Yb8PBtpEG6T0GrHT0QY0J+m6mM1AJrUJPGeP7KYWqRBRpyuSL1Flz3Wq X-Received: by 10.112.95.133 with SMTP id dk5mr1161582lbb.3.1413412298799; Wed, 15 Oct 2014 15:31:38 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.180.97.231 with SMTP id ed7ls192768wib.3.gmail; Wed, 15 Oct 2014 15:31:38 -0700 (PDT) X-Received: by 10.180.21.230 with SMTP id y6mr651973wie.78.1413412298148; Wed, 15 Oct 2014 15:31:38 -0700 (PDT) Original-Received: from mail-wg0-x22e.google.com (mail-wg0-x22e.google.com [2a00:1450:400c:c00::22e]) by mx.google.com with ESMTPS id yx2si27256093wjc.11.2014.10.15.15.31.38 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Wed, 15 Oct 2014 15:31:38 -0700 (PDT) Received-SPF: pass (google.com: domain of miro.knejp@gmail.com designates 2a00:1450:400c:c00::22e as permitted sender) client-ip=2a00:1450:400c:c00::22e; Original-Received: by mail-wg0-f46.google.com with SMTP id l18so2421679wgh.5 for ; Wed, 15 Oct 2014 15:31:38 -0700 (PDT) X-Received: by 10.180.219.106 with SMTP id pn10mr593744wic.63.1413412297932; Wed, 15 Oct 2014 15:31:37 -0700 (PDT) Original-Received: from [192.168.42.4] (ppp-93-104-177-254.dynamic.mnet-online.de. [93.104.177.254]) by mx.google.com with ESMTPSA id fx2sm25293099wjb.37.2014.10.15.15.31.36 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Wed, 15 Oct 2014 15:31:37 -0700 (PDT) In-Reply-To: <27e2ae9c-8b1f-4688-9bb3-9caef6bf4f54@isocpp.org> X-Mailer: Apple Mail (2.1878.6) X-Original-Sender: miro.knejp@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of miro.knejp@gmail.com designates 2a00:1450:400c:c00::22e as permitted sender) smtp.mail=miro.knejp@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-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:13942 Archived-At: --Apple-Mail=_E13665DB-02DB-4780-A11D-E6BA09991BF3 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=ISO-8859-1 On 16 Oct 2014, at 00:22 , Myriachan wrote: > On Wednesday, October 15, 2014 3:02:28 PM UTC-7, Ville Voutilainen wrote: > On 16 October 2014 00:17, Myriachan wrote:=20 > >> It's called std::experimental::optional.=20 > > While that is a nice feature, it's not the same as needing to do this t= o=20 > > types that weren't created as std::optional.=20 >=20 >=20 > Some questions:=20 >=20 > 1) if you can refactor code to take into use this reconstruction=20 > function, do you=20 > think you can't change the same code to start using optional? If so, why?= =20 >=20 >=20 > The main reason to use something like this is for copy constructors. Som= etimes, the most convenient way to implement a copy or move constructor is = like this: >=20 > T &T::operator =3D(const T ©) > { > if (© !=3D this) > { > this->~T(); > new(this) T(copy); > } > return *this; > } >=20 > ...and equivalently with std::forward for move constructors. This would never pass my code review. If that constructor throws you're doo= med. Use copy-and-swap instead, it provides strong exception guarantee and = won't leave you with a destroyed object. Move constructors are a different = game though and the compiler-defaulted one should do it's job just fine in = most cases. T& T::operator =3D(const T& copy) { if (© !=3D this) { T x{copy}; swap(*this, x); } return *this; } --=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=_E13665DB-02DB-4780-A11D-E6BA09991BF3 Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=ISO-8859-1
On 16 Oct 20= 14, at 00:22 , Myriachan <myriach= an@gmail.com> wrote:

On Wednesday, October 15, 2014 3:0= 2:28 PM UTC-7, Ville Voutilainen wrote:
On 16 October 2014 00:17, Myriachan <myri...@gmail.com> wrote:
>> It's called std::experimental::optional.
> While that is a nice feature, it's not the same as needing to do t= his to
> types that weren't created as std::optional<T>.


Some questions:

1) if you can refactor code to take into use this reconstruction
function, do you
think you can't change the same code to start using optional? If so, wh= y?


The main reason to use something like this is for= copy constructors.  Sometimes, the most convenient way to implement a= copy or move constructor is like this:

T &T= ::operator= =3D(const T = &copy<= span style=3D"color: #660;" class=3D"styled-by-prettify">)
{
  &nbs= p; if (&copy !=3D this)
    {
  &nb= sp;     this->= ~T();
        new(this) T(copy= );
&nbs= p;   }
    return *= this;
}

...and equivalently with std::forward for move constructors.

This would never pass my code review. = If that constructor throws you're doomed. Use copy-and-swap instead, it pro= vides strong exception guarantee and won't leave you with a destroyed objec= t. Move constructors are a different game though and the compiler-defaulted= one should do it's job just fine in most cases.

<= div dir=3D"ltr">
T& T::= = operator =3D(const T& copy)
{
&nbs= p;   if (&copy !=3D this)
  &nbs= p; {
        T x{copy};
  &nbs= p;     swap(*this, x);
    }
&= nbsp;   return *this;
}

--

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