Subj : Re: regarding threads in VC++, for a beginnner
To   : comp.os.ms-windows.programmer.win32,comp.programming.threads
From : Scott McPhillips [MVP]
Date : Sat Jan 22 2005 09:14 am

Ian T wrote:
> Scott McPhillips [MVP] wrote:
>> The recommended way to start such a "worker" thread is to call 
>> _beginthreadex.  One of its parameters is a function pointer to your 
>> thread function.  
> 
> Another parameter (void*) permits you to pass data to
> 
>> the thread function.  Then your thread function runs concurrently.
>>
> I have been taught (by someone else) to pass an object reference in 
> lparam (as a void *), then convert it back to the object and call a 
> member function.
> 
> So my questions are these.
> 1. Is this bad practice? Should I push my objects onto a queue and 
> process them in the thread funtion (dealing with contention for the 
> queue with a mutex or semaphore)?
> 
> 2. Does the member function that I call in the thread function execute 
> in the thread's "space"? Or does it execute in in the main thread's 
> "space"?
> 
> Thanks,
> Ian

The void* thread parameter can be used to pass a pointer to any desired 
data structure, including an object reference.  Converting it back to 
the object so you can call a C++ member function is not bad practice, it 
is a good C++ technique.

Passing the void* is only possible at thread start.  If you want to pass 
data to the thread more than once then you need an additional mechanism, 
such as a queue or passing messages in the Windows queue.  Queues are 
not a replacement for the thread parameter: Each serves a different purpose.

If "space" means memory space: all threads in a process use the same 
memory space.  If this were not so the void* thread parameter would be 
useless.

-- 
Scott McPhillips [VC++ MVP]

.