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POHow do I use an arbitrary string as a lock in C++?
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copied!<p>Let's say I have a multithreaded C++ program that handles requests in the form of a function call to <code>handleRequest(string key)</code>. Each call to <code>handleRequest</code> occurs in a separate thread, and there are an arbitrarily large number of possible values for <code>key</code>.</p> <p>I want the following behavior:</p> <ul> <li>Simultaneous calls to <code>handleRequest(key)</code> are serialized when they have the same value for <code>key</code>.</li> <li>Global serialization is minimized.</li> </ul> <p>The body of <code>handleRequest</code> might look like this:</p> <pre><code>void handleRequest(string key) { KeyLock lock(key); // Handle the request. } </code></pre> <p><b>Question:</b> How would I implement <code>KeyLock</code> to get the required behavior?</p> <p>A naive implementation might start off like this:</p> <pre><code>KeyLock::KeyLock(string key) { global_lock->Lock(); internal_lock_ = global_key_map[key]; if (internal_lock_ == NULL) { internal_lock_ = new Lock(); global_key_map[key] = internal_lock_; } global_lock->Unlock(); internal_lock_->Lock(); } KeyLock::~KeyLock() { internal_lock_->Unlock(); // Remove internal_lock_ from global_key_map iff no other threads are waiting for it. } </code></pre> <p>...but that requires a global lock at the beginning and end of each request, and the creation of a separate <code>Lock</code> object for each request. If contention is high between calls to <code>handleRequest</code>, that might not be a problem, but it could impose a lot of overhead if contention is low.</p>

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