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copied!<blockquote> <p>If there are locks in both the parent and the children (very likely in a multi-threaded non-trivial application), locking order becomes an issue: the parent invokes a method on the child, which in turn experiences a state transition and tries to notify the parent: dead-lock.</p> </blockquote> <p>It's not clear to me why notifying the parent would cause a deadlock unless</p> <ol> <li>the parent lock is held in thread A</li> <li>thread A is waiting for the child to return a signal via some means</li> <li>the child is signalling the parent in thread B</li> <li>the parent when it receives the signal from the child in (3) attempts to obtain its lock</li> </ol> <p>That's a lot of ifs. And it's a naturally problematic design: one thread (A) is holding a lock, and waiting for another thread (B) to do something.</p> <p>There's no magic solution to avoid this problem - you just have to avoid it. The best answer is probably to not signal back to the parent from a separate thread; or, to distinguish between signals which will or will not be called with the parent lock already held.</p> <blockquote> <p>During destruction of the parent, it must be on the lookout for ongoing callbacks from its children. Especially if those callbacks are fired off in a separate thread. If it is not, it might be gone by the time the callback is invoked.</p> </blockquote> <p>The trick here is probably that the children should have a method (perhaps the destructor) which gaurentees that, after it returns, the child will make no further callbacks. When the parent is being destroyed it calls that method for each of its children. </p> <p>I know you asked for "as few restrictions as possible" but realistically, when working in a multi-threaded environment, you have to have rules to prevent deadlocks and races.</p>

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