Re: Accessing a thread's object

James Kanze <>
15 Aug 2006 14:12:41 -0400
Ryan wrote:

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Given that Boost is multi-platform, and very much a part of C++,
I would have thought that some discussions concerning its use
would be appropriate here. In the case of threading,
particularly: I'm pretty sure that the next version of the
standard will address threading in some way or another, and
there is a definite possibility that Boost threads will be part
of that.

I'm using boost::threads and trying to access the members of
the functor that I start the thread on and can't find
information on how to do it anywhere.

It took me a while to even figure out what was going on but
eventually I determined that creating a new thread actually
just COPIES your functor object. Therefore changing the
object's member values from within the thread has NO effect
whatsoever on the object you used to initiate the thread.

This is actually documented, although it surprised me at first
as well, as it is different from every other threading library
I've seen.

Having played around with it some, I"ve come to conclude that
this is a feature, and not a defect. To begin with, it
certainly fits into the basic C++ model, in which everything is
a value (and gets copied), unless you specifically say
otherwise. In addition, boost::thread can start a detached
thread, which can continue running beyond the lifetime of the
thread object---and the functional object you passed it. (I'm
not sure that this can be considered a feature. Detached and
joinable threads are two different beasts, and having a thread
that suddenly becomes detached because you forgot to catch a
signal is not what I consider a good idea. On the other hand,
what do you do if the destructor is called before the thread is

I'm still experimenting with the best way to handle joinable
threads, with return values, and possibly exceptions that you
want to propagate accross the thread boundary. The key,
however, seems to turn on std::auto_ptr in the functional
object: this means that everything must be dynamically
allocated, but it also means that the lifetime is managed, and
an object used by the thread is neither accessible nor destroyed
as long as the thread is running. For return values, it also
means that the invoker cannot access the value before the join
has occurred. (Note that I've not had the time yet to do any
concrete coding; I've been too involved with other things for
the moment. So this is very much a thought experiment, which
might not work out in practice---auto_ptr can be a tricky beast,
even if its semantics correspond to exactly what you want here.)

If you're allocating the thread object on the stack, I'd also
suggest wrapping it in some sort of user defined class which
will abort if the destructor is called before the join. In
specific systems you might be able to do better, and cancel the
thread in some way (which can also be tricky). Two things you
definitly don't want to do however: 1) detach the thread, and
let it continue running, and 2) wait for its normal termination
in the destructor.

Basically what I'm doing is I create object A. Object A has
the () operator that boost threads automatically start

I tell boost I want to start A on it's own thread.

A does some stuff (in this case 'A' is a thread dedicated to
serving a client and my program creates a new A for every
client that connects so that it can cater to everyone at

The problem is, if I then call A.getWhatever() it returns null
because everything that has been changed, to my absolute
delight, has been changed on the boost::thread's copy of A and
completely ignored the one I passed into the thread()

Somewhere I read that the solution to every programming problem
is an additonal level of indirection. It certainly is the
answer here. For a joinable thread, with a result, the
functional object which you pass to boost::thread should be a
simple forwarder to the actual object. Note, however, that
lifetime management and access control for this object can be
decidedly non-trivial. Consider what happens, for example, in
the following cases:

  -- the thread is still running, and the invoking code gets an
     exception, and

  -- the thread has actually terminated, and the invoking code
     gets an exception before having done the join (and regained
     possesion of the object).

In the first case, you cannot destruct the object, since the
thread is still using it; on the other hand, the thread has to
destruct it in some way, since the invoking thread is no longer
there to do it. In the second case, the invoking thread has to
destroy the object, although it doesn't yet know about it.

Basically all I want to know is if there is any way to just
access the thread's local copy of the object. If not it looks
like my only other option is to pass in a pointer to another
object that will hold all of the state data which will mean
rewriting a HUGE amount of code :(

I don't see how it could be such a HUGE amoung of code; all you
need to do is provide wrappers, and use them when invoking the
thread or joining with it. And you shouldn't be invoking or
joining in that many places.

And I suspect that the original code had a few errors anyway, in
cases where the boost::thread object was destroyed by stack
clean-up after an exception.

The problem is not simple. Boost only offers some very, very
low level wrappers for the basic thread primitives, without
pretending to solve it. Probably because there is no
universally recognized solution; each problem requires
considerable customization.

Let me know if I've left anything out. I can barely
comprehend most of the stuff I'm doing in this program myself

In which case, the code won't work. Threading is NOT trivial,
and very much requires that you know what you are doing. Unless
you carefully control who accesses what when, your code will
contain subtle errors, which generally won't show up during

James Kanze
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