Re: [Boost-users] thread strange

From: Howard Hinnant (hinnant_at_[hidden])
Date: 2008-01-07 13:05:10

On Jan 7, 2008, at 7:16 AM, Chameleon wrote:

> I have a strange problem with the code below.
>
>> From outside the function operator()(), I can have the pointer to
> thread object 'thread'.
>> From inside operator()() 'thread' is always 0.
>
> Why does this happen?
>
> ------------------------------------------------------------------
> #include <cstdio>
> #include <boost/thread/thread.hpp>
>
> class A
> {
>    boost::thread *thread;
>
> public:
>    A() : thread(0) {}
>    ~A() { join(); }
>
>    void createThread() { thread = new boost::thread(*this); }
>
>    void operator()() {
>        printf("Starting thread with pointer: %d\n",  
> thread);           //<<<<<<<<<<<<<<HERE!
>        delete thread; thread = 0;
>    }
>
>    void join() { if (thread) thread->join(); }
> };
>
> int main(int, char**) {
>    A a;
>    a.createThread();
>    a.join();
>    return 0;
> }

When you create "a", thread is 0.  Inside of createThread you *first*  
copy *this, passing the copy to create a boost::thread, and *then*  
assign the local thread* a value.

-Howard