Implementing a simple, generic thread pool in C++11

I want to create a thread pool for experimental purposes (and for the fun factor). It should be able to process a wide variety of tasks (so I can possibly use it in later projects).

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In my thread pool class I'm going to need some sort of task queue. Since the Standard Library provides std::packaged_task since the C++11 standard, my queue will look like std::deque<std::packaged_task<?()> > task_queue, so the client can push std::packaged_tasks into the queue via some sort of public interface function (and then one of the threads in the pool will be notified with a condition variable to execute it, etc.).

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My question is related to the template argument of the std::packaged_task<?()>s in the deque.

The function signature ?() should be able to deal with any type/number of parameters, because the client can do something like:

std::packaged_task<int()> t(std::bind(factorial, 342)); thread_pool.add_task(t);

So I don't have to deal with the type/number of parameters.

But what should the return value be? (hence the question mark)

• If I make my whole thread pool class a template class, one instance of it will only be able to deal with tasks with a specific signature (like std::packaged_task<int()>).

  I want one thread pool object to be able to deal with any kind of task.

• If I go with std::packaged_task<void()> and the function invoked returns an integer, or anything at all, then thats undefined behaviour.