220 8277 <4d4d821b-029b-42c9-81b2-e41e4a36aaae@isocpp.org> article Path: news.gmane.org!not-for-mail From: Mikhail Semenov Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: Generalized lambda capture: returning values not captured by reference, should not they be moved? Date: Sat, 28 Dec 2013 09:37:46 -0800 (PST) Lines: 132 Approved: news@gmane.org Message-ID: <4d4d821b-029b-42c9-81b2-e41e4a36aaae@isocpp.org> References: <9a553579-08d1-4d60-a288-b1cb95056127@isocpp.org> <02f29ec5-63e7-4ac9-9cef-1e4c374b3614@isocpp.org> <52BE834B.1020608@gmail.com> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_Part_201_12863888.1388252267005" X-Trace: ger.gmane.org 1388252262 2655 80.91.229.3 (28 Dec 2013 17:37:42 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sat, 28 Dec 2013 17:37:42 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDC55PNFRYGRB24Y7SKQKGQEYGIHYAQ@isocpp.org Sat Dec 28 18:37:49 2013 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-oa0-f71.google.com ([209.85.219.71]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1Vwxpd-00047m-4u for gclcip-std-proposals@m.gmane.org; Sat, 28 Dec 2013 18:37:49 +0100 Original-Received: by mail-oa0-f71.google.com with SMTP id i4sf49288882oah.2 for ; Sat, 28 Dec 2013 09:37:48 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :list-post:list-help:list-archive:list-subscribe:list-unsubscribe :content-type; bh=rIzToldOlPDZKYnsBNTmI3Va4vwI8NQU06NOToN12yY=; b=dVqo5XNtc0JTmRgloTdpOnhKiStVQuFay3N4nAH+ZteE/VJuQr2rvK2khnOVt91Oy7 YZP+16E8W9z/1rTizDr0mjr6wXFB3V9UHJsaXp7B914F7Wh0hn7W5sMMXo6y2s6P2z51 je9w2xSQ3iwYqao+tDrBiBizfGRKre8nlcImUnkBB4o7f1x4i+8h6vY57aypz3td1XtS FK3iEOunADN7DhEVwtIgNr14CI5aipBkaYLbDgcZcbYkfWO1b2nRDliRi+ZUmoT3cLZS OTQ84LTO+iPtCdviL2iE4/wIGNAGI2tiuFreVbxr3HiQ5eRbM333DDkQiSAzPKexiD35 ZfZw== 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:in-reply-to:references :subject:mime-version:x-original-sender:reply-to:precedence :mailing-list:list-id:list-post:list-help:list-archive :list-subscribe:list-unsubscribe:content-type; bh=rIzToldOlPDZKYnsBNTmI3Va4vwI8NQU06NOToN12yY=; b=GQwEma7psMlaE9wYQg6yfRi7tPb3q1OdgEtS7+DVJbArY6yiv0h3T+ydnE11WsS8JU AZ+TEEb76PCr5tyM1IBdmPYMCqS2wgv8N15c5DJvcan+4U7YuzCTOxLxL2OPfT7FYsDj OBvjZLiLdwtpJEDlFPKBbjPun1JSSO47dz8o+0tR15O5/A1ReqRJE8g32/qh7no3fx3c BzH+ii/na16T1cSYQ/TRjrNQEGD+QjWbKYo+5xMJzLQJxTRuZqJegGYVtNtaeDlOZgmP AcgrmmWn2gR0uTOJA+1rVki2VB6OCczvQr+3CDeNpqinHGWKjV09oLqZzcFXX+unNOaX C1sw== X-Gm-Message-State: ALoCoQmKes1UoiMMzHEbv2oTdRIEjr2OA0SzxjssXwUKtHx1VPQ6k0V8FnY6asPiCkMI+Hkok5wn X-Received: by 10.182.108.136 with SMTP id hk8mr22200160obb.11.1388252268166; Sat, 28 Dec 2013 09:37:48 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.49.37.195 with SMTP id a3ls3029605qek.88.gmail; Sat, 28 Dec 2013 09:37:47 -0800 (PST) X-Received: by 10.49.24.109 with SMTP id t13mr169805qef.7.1388252267537; Sat, 28 Dec 2013 09:37:47 -0800 (PST) In-Reply-To: <52BE834B.1020608@gmail.com> X-Original-Sender: mikhailsemenov1957@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:8277 Archived-At: ------=_Part_201_12863888.1388252267005 Content-Type: text/plain; charset=ISO-8859-1 I guess something positive might come out of this discussion. I think that for lambdas using && is not necessary, it's better to use *return std::move(x);* But for && member functions this approach can be useful: class A { double *v; std::size_t n; public: .... A operator+(A x) const & { for (std::size_t i = 0; i < n; i++) { x.v[i] += v[i]; } return x; // copy elision according to the present standard } A operator+(const A& x) *&&* { for (std::size_t i = 0; i < n; i++) { v[i] += x.v[i]; } *return *this; // copy elision should be here as well!!! You have to write return std::move(*this); to get the desired affect* } }; On Saturday, December 28, 2013 7:52:43 AM UTC, David Krauss wrote: > On 12/28/13 4:36 AM, Mikhail Semenov wrote: > > I agree with your comments. My argument was fallacious. > > For the sake of argument, we could have ref-qualified lambdas > > auto fn = [x]() mutable && { return x; } // Automatic move would be > reasonable here > std::move( fn ) (); // because functor can only be called as an rvalue. > fn(); // Error: no matching operator() overload for lvalue fn. > > One-shot functions do come up often enough that ref-qualfied lambdas > could be a useful language feature, but I don't know if the destructive > return rule in particular is worth implementing. Arguably if specified > it should apply to all rvalue ref-qualified functions returning a > member, not only lambdas, and that would be a serious breaking change. > > -- --- 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. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposals/. ------=_Part_201_12863888.1388252267005 Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable
I guess something positive might come out of this dis= cussion. I think that for lambdas using && is not necessary, it's b= etter to use return std::move(x);
 
But for && member functions this approach can be useful:
 
class A
{
    double *v;
 =    std::size_t n;
public:
...
    A opera= tor+(A x) const &
    {
    &n= bsp;   for (std::size_t i =3D 0; i < n; i++)
  &n= bsp;     {
       = ;     x.v[i] +=3D v[i];
    &nbs= p;   }
        return x; //= copy elision according to the present standard
    }
 
    A operator+(const A& x) &&
    {    &nbs= p; 
        for (std::size_t i = =3D 0; i < n; i++)
        {
&n= bsp;            v[i]= +=3D x.v[i];
        }
 &nbs= p;      return *this; // copy elision shou= ld be here as well!!! You have to write return std::move(*this); to ge= t the desired affect
    }
};
 = ;

 

On Saturday, December 28, 2013 7:52:43= AM UTC, David Krauss wrote:
On 12/28/1= 3 4:36 AM, Mikhail Semenov wrote:
> I agree with your comments. My argument was fallacious.

For the sake of argument, we could have ref-qualified lambdas

auto fn =3D [x]() mutable && { return x; } // Automatic move wo= uld be=20
reasonable here
std::move( fn ) (); // because functor can only be called as an rvalue.
fn(); // Error: no matching operator() overload for lvalue fn.

One-shot functions do come up often enough that ref-qualfied lambdas=20
could be a useful language feature, but I don't know if the destructive= =20
return rule in particular is worth implementing. Arguably if specified= =20
it should apply to all rvalue ref-qualified functions returning a=20
member, not only lambdas, and that would be a serious breaking change.

--
 
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