But wait. From http://www.boost.org/doc/libs/1_47_0/libs/smart_ptr/weak_ptr.htm :

"When the last shared_ptr to the object goes away and the object is deleted, the attempt to obtain a shared_ptr from the weak_ptr instances that refer to the deleted object will fail: the constructor will throw an exception of type boost::bad_weak_ptr, and weak_ptr::lock will return an empty shared_ptr."

Wouldn't panel.lock()->addUsers(...) fail in that case? Shouldn't we test with an idiom like this?

boost::shared_ptr<impl> spanel(panel.lock());
if (spanel) { spanel->addUsers(...); }

Or hmm! The weak_ptr page cited above suggests this cool syntax:

if(boost::shared_ptr<impl> spanel = panel.lock())
{
    spanel->addUsers(...);
}

note that this block is protected by a if(!panel.expired()) above.  Expired means the object pointed to is no longer in use, so if it isn't expired the weak pointer will return a shared pointer to the real object.