Re: [boost] Re: Re: Re: Re: thread::current() ?

From: William E. Kempf (wekempf_at_[hidden])
Date: 2003-07-01 09:00:46

Philippe A. Bouchard said:
> William E. Kempf wrote:
>
>>
>> Philippe A. Bouchard said:
>>> William E. Kempf wrote:
>>>
>>> [...]
>>>
>>>> As already pointed out, to associate data with a thread you use
>>>> thread_specific_ptr.  BTW, you still have to remember that the
>>>> functor is copied, and data passed to/in the functor is not
>>>> considered part of the thread in any event.
>>>
>>> Ok, how do you find out the data of the current thread?  The key in
>>> boost::detail::tss is not the same as the one in boost::thread.
>>
>> What data?  The actual thread data (there's not much, beyond the
>> thread id which isn't direclty accessible) is found by calling the
>> default c-tor on the thread.  The data passed in the functor is your
>> responsibility. Either you explicitly pass it everywhere that's
>> needed, or the thread stores it in a tss key.
>>
>
> Suppose you have:
>
> struct functor1
> {
>         list<void *> m_list;
>
>         void operator () ()
>         {
>                 ...
>         }
> };
>
> struct functor2
> {
>         list<void *> m_list;
>
>         void operator () ()
>         {
>                 ...
>         }
> };
>
> int main()
> {
>         thread f1(functor1());
>         thread f2(functor2());
>
>         ...
>
>         // I would like to access m_list of the current thread's
> functor:
>
>         lock()...
>         if (thread() == f1 || thread() == f2)
>         {
>                 thread()..(whatever casts)...m_list;
>         }
>         unlock()...
>
>         // I think the only way to do this is by mapping the thread's id
>  //  with the object's address (map<key, functor1 *>) but there
> is //  no standard key publicly accessible and map<> creates
> overhead.
> }

The functor may be copied any number of times, so this is problematic.  If
you passed it by reference, possibly using boost::ref, then the answer
would be to just retain the functor as a named variable instead of passing
a temporary.  I'd say that's typically the answer for most use cases.  For
some, you may want to place the functor in a thread_specific_ptr<>, as
suggested, but this only allows you to recover the data for the current
thread.

The only way I can see to give the interface your suggesting is to either
type the thread, which is problematic for so many reasons, or to employ
something similar to boost::any.

-- 
William E. Kempf