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    <p>Here is an approach I've successfully used in the past:</p> <p>Implement your "completion" task as a reference counted object. Each worker thread holds a reference to this object while it is doing its work, then releases it when finished. The completion task does its work when the ref count reaches 0.</p> <h2>Example</h2> <p><em>Note: my C++ is rusty after years of working primarily in C#, so treat the example below as pseudo-code</em></p> <h2>Completion Task</h2> <pre><code>class MyCompletionTask { private: long _refCount; public: MyCompletionTask() { _refCount = 0; } public: // Reference counting implementation // Note ref-counting mechanism must be thread-safe, // so we use the Interlocked functions. void AddRef() { InterlockedIncrement(&amp;_refCount); } void Release() { long newCount = InterlockedDecrement(&amp;_refCount); if (newCount == 0) { DoCompletionTask(); delete this; } } private: void DoCompletionTask() { // TODO: Do your thing here } } </code></pre> <h2>Calling Code</h2> <pre><code>MyCompletionTask *task = new MyCompletionTask(); task-&gt;AddRef(); // add a reference for the main thread for( &lt;loop thru some items &gt;) { task-&gt;AddRef(); // Add a reference on behalf of the worker // thread. The worker thread is responsible // for releasing when it is done. QueueUserWorkItem(ThreadProc, (PVOID)task, &lt;etc&gt; ); } task-&gt;Release(); // release main thread reference // Note: this thread can exit. The completion task will run // on the thread that does the last Release. </code></pre> <h2>Thread Proc</h2> <pre><code>void ThreadProc(void *context) { MyCompletionTask *task = (MyCompletionTask)context; // TODO: Do your thing here task-&gt;Release(); } </code></pre> <p>One thing to keep in mind with this approach is that the thread on which the completion task completes is non-deterministic. It will depend on which worker thread finishes first (or the main thread, if all the worker threads finish before the main thread calls Release)</p>
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    1. POwait for works item to complete pooled using QueueUserWorkItem (not .NET)
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    1. CObut won't this give me the same problem? i'm stuck with waiting in the calling thread for the completion task to finish. sorry if i don't get it.
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    2. COWith this approach, you don't do a wait in the main thread. In the completion task, you put the code that you want to run after all the worker threads have finished, and it will be invoked when the last Release is called.
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