220 26368 article Path: news.gmane.org!not-for-mail From: FrankHB1989 Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Swapping with volatile lvalues Date: Tue, 21 Jun 2016 16:33:54 -0700 (PDT) Lines: 234 Approved: news@gmane.org Message-ID: References: <81998cb5-722e-49a1-be98-7ca1e219b72a@isocpp.org> <9608d5c4-e60f-449e-9078-f27fed1f79fd@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_104_2136466998.1466552034679" X-Trace: ger.gmane.org 1466552052 14910 80.91.229.3 (21 Jun 2016 23:34:12 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Tue, 21 Jun 2016 23:34:12 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCTJVBPG3QIBBY45U65QKGQET5P5CRY@isocpp.org Wed Jun 22 01:34:04 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qk0-f197.google.com ([209.85.220.197]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1bFVBC-0005GP-Qg for gclcip-std-proposals@m.gmane.org; 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Tue, 21 Jun 2016 16:33:56 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.174.7 with SMTP id x7ls2596445ioe.53.gmail; Tue, 21 Jun 2016 16:33:55 -0700 (PDT) X-Received: by 10.36.196.5 with SMTP id v5mr244123itf.4.1466552035706; Tue, 21 Jun 2016 16:33:55 -0700 (PDT) In-Reply-To: X-Original-Sender: frankhb1989@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:26368 Archived-At: ------=_Part_104_2136466998.1466552034679 Content-Type: multipart/alternative; boundary="----=_Part_105_1551560846.1466552034680" ------=_Part_105_1551560846.1466552034680 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable =E5=9C=A8 2016=E5=B9=B46=E6=9C=8822=E6=97=A5=E6=98=9F=E6=9C=9F=E4=B8=89 UTC= +8=E4=B8=8A=E5=8D=882:37:36=EF=BC=8CFarid Mehrabi=E5=86=99=E9=81=93=EF=BC= =9A > > Whenever I face volatile, the atomic bells start to ring in my ears.=20 > Should swapping a volatile object mean atomic exchange, for non-trivial= =20 > types we might get in trouble, but for in-built integrals I would rather= =20 > use std::atomic_exchange. > Well, atomic operations are another monsters... Actually I don't want=20 "volatile" as possible. =20 > Otherwise (if atomicity is not a concern), a cv-cast followed by=20 > conventional swap can do. > > I'm afraid you can't. "Reading a volatile object through a non-volatile= =20 glvalue has undefined behavior." I find no effect way to avoid a volatile= =20 glvalue referring a volatile object. =20 > regards, > FM. > > 2016-06-21 14:48 GMT+04:30 Daniel Kr=C3=BCgler >: > >> 2016-06-21 12:12 GMT+02:00 FrankHB1989 >> >: >> > Call of std::exchange can work, but it seems not so ideal. >> >> This is a misunderstanding of my possibly ambiguous response. I didn't >> mean to use std::exchange, but instead I was trying to suggest that if >> you were considering to reuse an existing name it should better be >> exchange instead of swap. >> >> > The return type >> > of std::exchange is template parameter type T, so when T is a volatile= =20 >> type, >> > lvalue-to-rvalue conversion would be performed, and there are likely= =20 >> more >> > side effects than needed. >> > I also don't think using the volatile lvalue return value (which is no= t=20 >> the >> > case for std::exchange) a good idea. Discarded-value expressions can= =20 >> work in >> > such cases, but is seems to be strange and too error-prone. So perhaps= =20 >> the >> > return type of the call should be void. >> > Regarding with Swappable requriements and symmetry... how about=20 >> providing 2 >> > overloading? >> >> I still feel uneasy about it, especially when trying to make a >> consistent sense with your suggested overloads (even assuming that >> both overloads would be provided) and the Swappable post-conditions. >> But again, I haven't thought carefully through it. >> >> - Daniel >> >> -- >> You received this message because you are subscribed to the Google Group= s=20 >> "ISO C++ Standard - Future Proposals" group. >> To unsubscribe from this group and stop receiving emails from it, send a= n=20 >> email to std-proposal...@isocpp.org . >> To post to this group, send email to std-pr...@isocpp.org . >> To view this discussion on the web visit=20 >> https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/CAGNvRgCYLy= wvtFSO65-NKpeycpT4cPnw0SjvQPhE_DRL98UO7g%40mail.gmail.com >> . >> > > > > --=20 > how am I supposed to end the twisted road of your hair in such a dark=20 > night?? > unless the candle of your face does shed some light upon my way!!! > --=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. To view this discussion on the web visit https://groups.google.com/a/isocpp= ..org/d/msgid/std-proposals/b45ad971-0874-4372-b204-699767d25c21%40isocpp.or= g. ------=_Part_105_1551560846.1466552034680 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable


=E5=9C=A8 2016=E5=B9=B46=E6=9C=8822=E6=97=A5=E6=98= =9F=E6=9C=9F=E4=B8=89 UTC+8=E4=B8=8A=E5=8D=882:37:36=EF=BC=8CFarid Mehrabi= =E5=86=99=E9=81=93=EF=BC=9A
Whenever I face volatile, the atomic bells st= art to ring in my ears. Should swapping a volatile object mean atomic excha= nge, for non-trivial types we might get in trouble, but for in-built integr= als I would rather use std::atomic_exchange.
W= ell, atomic operations are another monsters... Actually I don't want &q= uot;volatile" as possible.
=C2=A0
Otherwise (if atomici= ty is not a concern), a cv-cast followed by conventional swap can do.
=

I'm afraid you can= 9;t. "Reading a volatile object through a non-volatile glvalue has und= efined behavior." I find no effect way to avoid a volatile glvalue ref= erring a volatile object.

=C2=A0
r= egards,
FM.

2016-06-21 14:48 GMT+04:30 Daniel Kr=C3=BCgler <daniel....@gmail.com>:
2016-06-21 12:12 GMT+02:00 FrankHB1989 <frank...@gmail.com= >:
> Call of std::exchange can work, but it seems not so ideal.

This is a misunderstanding of my possibly ambiguous response. I didn= 't
mean to use std::exchange, but instead I was trying to suggest that if
you were considering to reuse an existing name it should better be
exchange instead of swap.

> The return type
> of std::exchange is template parameter type T, so when T is a volatile= type,
> lvalue-to-rvalue conversion would be performed, and there are likely m= ore
> side effects than needed.
> I also don't think using the volatile lvalue return value (which i= s not the
> case for std::exchange) a good idea. Discarded-value expressions can w= ork in
> such cases, but is seems to be strange and too error-prone. So perhaps= the
> return type of the call should be void.
> Regarding with Swappable requriements and symmetry... how about provid= ing 2
> overloading?

I still feel uneasy about it, especially when trying to make a
consistent sense with your suggested overloads (even assuming that
both overloads would be provided) and the Swappable post-conditions.
But again, I haven't thought carefully through it.

- Daniel

--
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To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposal...@isocpp.org.
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To view this discussion on the web visit https://groups.go= ogle.com/a/isocpp.org/d/msgid/std-proposals/CAGNvRgCYLywvtFSO65-<= wbr>NKpeycpT4cPnw0SjvQPhE_DRL98UO7g%40mail.gmail.com.



--
how am I supposed to end the twisted road o= f=C2=A0 your hair in such a dark night??
unless the candle of your face = does shed some light upon my way!!!

--
You received this message because you are subscribed to the Google Groups &= quot;ISO C++ Standard - Future Proposals" group.
To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposa= ls+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/b45ad971-0874-4372-b204-699767d25c21= %40isocpp.org.
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