Re: An argument against (the liberal use of) references

From	Juha Nieminen <nospam@thanks.invalid>
Newsgroups	comp.lang.c++
Subject	Re: An argument against (the liberal use of) references
Date	2022-11-23 13:46 +0000
Organization	Aioe.org NNTP Server
Message-ID	<tll88a$13vs$1@gioia.aioe.org> (permalink)
References	<tli64v$1k4l$1@gioia.aioe.org> <tlibnk$3298$1@dont-email.me> <tlkfv5$rss$1@gioia.aioe.org> <tll502$ce5l$1@dont-email.me>

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Alf P. Steinbach <alf.p.steinbach@gmail.com> wrote:
> On 23 Nov 2022 07:52, Juha Nieminen wrote:
>> Paavo Helde <eesnimi@osa.pri.ee> wrote:
>>> If the code passing a reference is not thread-safe, then just changing
>>> it to pass by value will not magically make it thread-safe. Without
>>> proper synchronization, the object may be changed by another thread at
>>> any moment, for example in the middle of the copy operation, and thus
>>> the copy might become internally inconsistent.
>> 
>> I think you have a point there. I was thinking that since the function is
>> getting a local copy of the object then mutual exclusion problems go away
>> because that local copy is completely independent of the original.
>> 
>> I didn't think that *copying* the object for the function in itself isn't
>> automatically thread-safe. Yet still, for a few seconds, I had the strong
>> instinct that there has to be something "more thread-safe" about making
>> a copy of the value for the function than have the function take a
>> reference to it... but the more I try to figure out how, I fail. Copying
>> merely moves the mutual exclusion problem to a slightly different place,
>> but it doesn't solve it. The calling code *still* needs to solve the
>> mutual exclusion problem regardless of which way the function takes the
>> parameter.
> 
> Depends on when the copying is done.
> 
> Copying to the tread instantiation is safe.

If you are calling a function and giving as parameter a variable that may
be modified by another thread, you need to take care of the mutual
exclusion problem regardless of whether that function takes the parameter
by value or by reference.

I originally didn't think about the fact that the function taking it by
value doesn't solve the problem because now copying the value needs the
mutual exclusion. The only thing that happens is that the point of
potential conflict has been moved to a slightly different place.

You could make a local copy of the value before passing it to the
function (which could be quite efficient if the variable is atomic),
but even then it doesn't really matter if the function takes it by
value or by reference. (OTOH this may be much more efficient if
the function takes a significant amount of time because you don't
need to keep the mutex locked for the duration of the function.)

Thread

An argument *against* (the liberal use of) references Juha Nieminen <nospam@thanks.invalid> - 2022-11-22 09:52 +0000
  Re: An argument *against* (the liberal use of) references "Fred. Zwarts" <F.Zwarts@KVI.nl> - 2022-11-22 11:23 +0100
  Re: An argument *against* (the liberal use of) references Paavo Helde <eesnimi@osa.pri.ee> - 2022-11-22 13:27 +0200
    Re: An argument *against* (the liberal use of) references Juha Nieminen <nospam@thanks.invalid> - 2022-11-23 06:52 +0000
      Re: An argument *against* (the liberal use of) references "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-11-23 13:51 +0100
        Re: An argument *against* (the liberal use of) references Juha Nieminen <nospam@thanks.invalid> - 2022-11-23 13:46 +0000
          Re: An argument *against* (the liberal use of) references Paavo Helde <eesnimi@osa.pri.ee> - 2022-11-30 13:59 +0200
            Re: An argument *against* (the liberal use of) references Juha Nieminen <nospam@thanks.invalid> - 2022-12-01 06:45 +0000
              Re: An argument *against* (the liberal use of) references Paavo Helde <eesnimi@osa.pri.ee> - 2022-12-01 09:19 +0200
                Re: An argument *against* (the liberal use of) references Stuart Redmann <DerTopper@web.de> - 2022-12-01 14:06 +0100
                Re: An argument *against* (the liberal use of) references Paavo Helde <eesnimi@osa.pri.ee> - 2022-12-01 16:45 +0200
                Re: An argument *against* (the liberal use of) references scott@slp53.sl.home (Scott Lurndal) - 2022-12-01 16:02 +0000
                Re: An argument *against* (the liberal use of) references Paavo Helde <eesnimi@osa.pri.ee> - 2022-12-01 20:21 +0200
                Re: An argument *against* (the liberal use of) references scott@slp53.sl.home (Scott Lurndal) - 2022-12-01 19:09 +0000
                Re: An argument *against* (the liberal use of) references Michael S <already5chosen@yahoo.com> - 2022-12-02 05:53 -0800
                Re: An argument *against* (the liberal use of) references scott@slp53.sl.home (Scott Lurndal) - 2022-12-02 14:36 +0000
                Re: An argument *against* (the liberal use of) references "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-12-02 12:29 -0800
                Re: An argument *against* (the liberal use of) references scott@slp53.sl.home (Scott Lurndal) - 2022-12-02 20:55 +0000
                Re: An argument *against* (the liberal use of) references "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-12-02 12:58 -0800
                Re: An argument *against* (the liberal use of) references scott@slp53.sl.home (Scott Lurndal) - 2022-12-02 21:18 +0000
                Re: An argument *against* (the liberal use of) references "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-12-02 14:16 -0800
                Re: An argument *against* (the liberal use of) references Branimir Maksimovic <branimir.maksimovic@icloud.com> - 2022-12-02 21:09 +0000
                Re: An argument *against* (the liberal use of) references "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-12-02 13:22 -0800
                Re: An argument *against* (the liberal use of) references Branimir Maksimovic <branimir.maksimovic@icloud.com> - 2022-12-02 21:36 +0000
                Re: An argument *against* (the liberal use of) references "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-12-02 14:13 -0800
                Re: An argument *against* (the liberal use of) references Paavo Helde <eesnimi@osa.pri.ee> - 2022-12-02 18:33 +0200
                Re: An argument *against* (the liberal use of) references Michael S <already5chosen@yahoo.com> - 2022-12-01 11:59 -0800
                Re: An argument *against* (the liberal use of) references Paavo Helde <eesnimi@osa.pri.ee> - 2022-12-01 23:53 +0200
                Re: An argument *against* (the liberal use of) references Michael S <already5chosen@yahoo.com> - 2022-12-02 04:11 -0800
                Re: An argument *against* (the liberal use of) references "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-12-01 12:21 -0800
                Re: An argument *against* (the liberal use of) references Juha Nieminen <nospam@thanks.invalid> - 2022-12-06 11:36 +0000
                Re: An argument *against* (the liberal use of) references "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-12-06 12:26 -0800
                Re: An argument *against* (the liberal use of) references scott@slp53.sl.home (Scott Lurndal) - 2022-12-06 20:39 +0000
                Re: An argument *against* (the liberal use of) references Paavo Helde <eesnimi@osa.pri.ee> - 2022-12-06 23:53 +0200
                Re: An argument *against* (the liberal use of) references "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-12-06 13:59 -0800
                Re: An argument *against* (the liberal use of) references scott@slp53.sl.home (Scott Lurndal) - 2022-12-06 22:04 +0000
                Re: An argument *against* (the liberal use of) references Öö Tiib <ootiib@hot.ee> - 2022-12-06 21:38 -0800
                Re: An argument *against* (the liberal use of) references scott@slp53.sl.home (Scott Lurndal) - 2022-12-07 15:22 +0000
                Re: An argument *against* (the liberal use of) references Paavo Helde <eesnimi@osa.pri.ee> - 2022-12-07 18:37 +0200
                Re: An argument *against* (the liberal use of) references "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-12-06 14:01 -0800
                Re: An argument *against* (the liberal use of) references Juha Nieminen <nospam@thanks.invalid> - 2022-12-07 09:05 +0000
                Re: An argument *against* (the liberal use of) references "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-12-07 12:48 -0800
                Re: An argument *against* (the liberal use of) references "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-12-07 12:50 -0800
                Re: An argument *against* (the liberal use of) references Juha Nieminen <nospam@thanks.invalid> - 2022-12-08 07:52 +0000
                Re: An argument *against* (the liberal use of) references Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-12-08 05:50 -0800
                Re: An argument *against* (the liberal use of) references Michael S <already5chosen@yahoo.com> - 2022-12-08 13:30 -0800
                Re: An argument *against* (the liberal use of) references Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-12-09 12:50 -0800
                Re: An argument *against* (the liberal use of) references David Brown <david.brown@hesbynett.no> - 2022-12-11 12:18 +0100
                Re: An argument *against* (the liberal use of) references Paavo Helde <eesnimi@osa.pri.ee> - 2022-12-08 13:38 +0200
                Re: An argument *against* (the liberal use of) references "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-12-08 16:45 -0800
                Re: An argument *against* (the liberal use of) references Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2022-12-09 04:35 -0800
  Re: An argument *against* (the liberal use of) references Bonita Montero <Bonita.Montero@gmail.com> - 2022-11-22 15:36 +0100
  Re: An argument *against* (the liberal use of) references Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2022-11-22 15:21 +0000
  Re: An argument *against* (the liberal use of) references Richard Damon <Richard@Damon-Family.org> - 2022-11-22 10:48 -0500
    Re: An argument *against* (the liberal use of) references "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-11-22 13:18 -0800
  Re: An argument *against* (the liberal use of) references El Jo <giorgio.zoppi@gmail.com> - 2022-11-22 13:41 -0800
    Re: An argument *against* (the liberal use of) references "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-11-22 13:51 -0800
  Re: An argument *against* (the liberal use of) references Bonita Montero <Bonita.Montero@gmail.com> - 2022-11-23 20:08 +0100

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