How do I use an arbitrary string as a lock in C++?

Question

Let\'s say I have a multithreaded C++ program that handles requests in the form of a function call to handleRequest(string key). Each call to handleRequest

Answer 1

After thinking about it, another approach might go something like this:

• In handleRequest, create a Callback that does the actual work.
• Create a multimap global_key_map, protected by a mutex.
• If a thread sees that key is already being processed, it adds its Callback* to the global_key_map and returns.
• Otherwise, it calls its callback immediately, and then calls the callbacks that have shown up in the meantime for the same key.

Implemented something like this:

LockAndCall(string key, Callback* callback) {
    global_lock.Lock();
    if (global_key_map.contains(key)) {
        iterator iter = global_key_map.insert(key, callback);
        while (true) {
            global_lock.Unlock();
            iter->second->Call();
            global_lock.Lock();
            global_key_map.erase(iter);
            iter = global_key_map.find(key);
            if (iter == global_key_map.end()) {
                global_lock.Unlock();
                return;
            }
        }
    } else {
        global_key_map.insert(key, callback);
        global_lock.Unlock();
    }
}

This has the advantage of freeing up threads that would otherwise be waiting for a key lock, but apart from that it's pretty much the same as the naive solution I posted in the question.

It could be combined with the answers given by Mike B and Constantin, though.