Re: [Boost-users] Passing reference params to boost::thread, parameter copies.

Subject: Re: [Boost-users] Passing reference params to boost::thread, parameter copies.
From: Jason Cipriani (jason.cipriani_at_[hidden])
Date: 2009-09-25 14:03:22

On Fri, Sep 25, 2009 at 11:54 AM, OvermindDL1 <overminddl1_at_[hidden]> wrote:
> On Fri, Sep 25, 2009 at 7:43 AM, Roman Perepelitsa
> <roman.perepelitsa_at_[hidden]> wrote:
>> 2009/9/25 Jason Cipriani <jason.cipriani_at_[hidden]>
>>>
>>> At the end of this message is a test program that uses boost::thread
>>> to create two threads. The first is one that takes a parameter by
>>> value, the second takes a parameter by reference. I have a couple of
>>> questions about some behavior here:
>>>
>>> 1) When passing by value, why are so many copies of the data made? The
>>> program output indicates that there are 9 (!) objects created (at most
>>> 5 exist simultaneously), when ideally there would be only 2 (the
>>> original and the copy passed to the thread).
>>
>> That's common for generic boost libraries. If you care about the number of
>> copies made, create your own function object that uses reference counting or
>> something similar to hold an argument.
>> struct runner {
>>   runner(const test& t) : arg_(new test(t)) {}
>>   void operator()() const {
>>      expected_nocopy(*arg_);
>>   }
>>   boost::shared_ptr<test> arg_;
>> };
>> test t;
>> runner r(t);
>> boost::thread thr(r);
>> By the way, there is no point in accepting argument by value, unless you
>> want to modify it. Since your function expected_copy does not modify its
>> argument, it's better to change pass-by-value to pass-by-const-ref.
>>>
>>> 2) When passing by reference, why is the data being copied? The
>>> copying behavior when passed by reference is identical to when the
>>> param is passed by value (9 objects created, 5 simultaneously). Also,
>>> and this isn't shown in the program below, but I would therefore
>>> expect the program to crash if the boost::thread outlived the
>>> referenced object (of course, it does not, since the object is
>>> copied).
>>
>> Use boost::cref to pass by reference.
>> boost::thread thr(expected_nocopy, boost::cref(t));
>> Roman Perepelitsa.
>
> You can also use Boost.Phoenix (a replacement for Boost.Bind and
> Boost.Lambda that still does even more).  Wrapping the things with
> ref() will send the pointer along so no copies are ever made.

Thanks for the explanation and tips; that answers my questions. :-)

Jason