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| DATE | 2016-11-19 |
| FROM | ruben safir
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| SUBJECT | Subject: [Learn] Fwd: ref use
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From learn-bounces-at-nylxs.com Sat Nov 19 11:49:44 2016
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From: Popping mad
Newsgroups: comp.lang.c++
Subject: ref use
Date: Wed, 16 Nov 2016 10:38:09 +0000 (UTC)
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Xref: panix comp.lang.c++:1125257
I'm reading up on concurrency with the thread libraryis C++ Concurrency
in action by Anthony Williams and I'm puzzled by one example having to do
with sending references of objects through the threads. If you own the
text, it is one page 24 where he is callable functions is
void update_data_for_widget(widget_id w, widget_data& data);
the call for the thread is
std::thread t (update_data_for_widget, w, data); where data is an object,
of data I suppose ;)
to do this correctly he says we need to use ref and a reference wrapper
std::thread t(update_data_for_widget, w, std::ref(data) );
Why can't you just send a reference?
std::thread t(update_data_for_widget, w, &data );
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From: ruben safir
Newsgroups: comp.lang.c++
Subject: Re: ref use
Date: Fri, 18 Nov 2016 21:13:03 -0500
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Xref: panix comp.lang.c++:1125335
On 11/16/2016 05:38 AM, Popping mad wrote:
> I'm reading up on concurrency with the thread libraryis C++ Concurrency
> in action by Anthony Williams and I'm puzzled by one example having to do
> with sending references of objects through the threads. If you own the
> text, it is one page 24 where he is callable functions is
>
> void update_data_for_widget(widget_id w, widget_data& data);
>
> the call for the thread is
>
> std::thread t (update_data_for_widget, w, data); where data is an object,
> of data I suppose ;)
>
> to do this correctly he says we need to use ref and a reference wrapper
>
> std::thread t(update_data_for_widget, w, std::ref(data) );
>
> Why can't you just send a reference?
>
> std::thread t(update_data_for_widget, w, &data );
>
nadah on this?
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From: "Alf P. Steinbach"
Newsgroups: comp.lang.c++
Subject: Re: ref use
Date: Sat, 19 Nov 2016 12:58:40 +0100
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Xref: panix comp.lang.c++:1125339
On 16.11.2016 11:38, Popping mad wrote:
> I'm reading up on concurrency with the thread libraryis C++ Concurrency
> in action by Anthony Williams and I'm puzzled by one example having to do
> with sending references of objects through the threads. If you own the
> text, it is one page 24 where he is callable functions is
>
> void update_data_for_widget(widget_id w, widget_data& data);
>
> the call for the thread is
>
> std::thread t (update_data_for_widget, w, data); where data is an object,
> of data I suppose ;)
>
> to do this correctly he says we need to use ref and a reference wrapper
>
> std::thread t(update_data_for_widget, w, std::ref(data) );
>
> Why can't you just send a reference?
Because these arguments are not passed on directly to the thread
function: they're stored by value, and passed on later.
> std::thread t(update_data_for_widget, w, &data );
This is not a reference, it's a pointer.
A pointer would work fine, it can be stored by value, but then the
thread function signature needs to be adjusted accordingly.
Cheers & hth.,
- Alf
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From: Manfred
Newsgroups: comp.lang.c++
Subject: Re: ref use
Date: Sat, 19 Nov 2016 16:59:46 +0100
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Xref: panix comp.lang.c++:1125340
On 11/19/2016 12:58 PM, Alf P. Steinbach wrote:
>> std::thread t(update_data_for_widget, w, std::ref(data) );
>>
>> Why can't you just send a reference?
>
> Because these arguments are not passed on directly to the thread
> function: they're stored by value, and passed on later.
Reminds me of a similar question I had:
http://thread.gmane.org/gmane.comp.gcc.help/49709
see also:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68784
I was also surprised; in principle the language semantics could allow
using a reference type, but this is intentionally disabled by the
standard for std::thread - you have to explicitly use std::ref().
Apparently this was motivated by 'helping' the programmer not pass data
by reference accidentally between threads.
I am not sure I like this approach, though. I think that it is a basic
knowledge of any skilled C++ programmer to know how to use references.
What I like of C++ is its solid foundation of self-consistent language
logic. Introducing this type of exception to the basic semantics is not
really attractive to me.
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