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PO
 

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  1. PO
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    <p>Would something like this be satisfactory? It still isn't safe to unsubscribe an observer while being notified though, for that you would need an interface like you mentioned (as far as I can tell).</p> <pre><code>Subscribe(Observer *x) { mutex.lock(); // add x to the list mutex.unlock(); } Unsubscribe(Observer *x) { mutex.lock(); while (!ok_to_delete) cond.wait(mutex); // remove x from list mutex.unlock(); } NotifyLoop() { while (true) { // wait for something to trigger a notify mutex.lock(); ok_to_delete = false; // build a list of observers to notify mutex.unlock(); // notify all observers from the list saved earlier mutex.lock(); ok_to_delete = true; cond.notify_all(); mutex.unlock(); } } </code></pre> <p>If you want to be able to Unsubscribe() inside Notify() - <em>(a bad design decision on the client IMO...)</em> you can add the thread id of the notifier thread into your data structure. In the Unsubscribe function you can check that thread id against the current thread's id (most threading libraries provide this - eg. pthread_self). If they are the same, you can proceed without waiting on the condition variable.</p> <p>NOTE: If the client is responsible for deleting the observer, this means you run into the situation where inside the Notify callback, you will have unsubscribed and deleted the observer, but are still executing something with that junked this pointer. It is something the client will have to be aware of and to only delete it at the end of the Notify().</p>
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    1. POHow do I tear down observer relationship in multithreaded C++?
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    1. USGreg Rogers
    1. USGreg Rogers
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    1. COBut would'nt that deadlock if an Observer Unsubscribe()s itself from its Notify callback?
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      1. USÉric Malenfant
    2. CO+1 as it addresses the crux of the problem. I'd like clients to be able to subscribe from callbacks, though, so I'll need a bit more thinking. It's scary that there's not a well-known idiom for this problem. I see this pattern every day, always broken.
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      1. USfizzer
    3. COfizzer - subscribing from callbacks is fine. You don't need to ever wait on the condition variable when subscribing, only the mutex (which is explicitly release before calling Notify(). It is only unsubscribing that is the problem.
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      1. USGreg Rogers
 

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