Question

This question comes from:
C++11 thread doesn't work with virtual member function

As suggested in a comment, my question in previous post may not the right one to ask, so here is the original question:

I want to make a capturing system, which will query a few sources in a constant/dynamic frequency (varies by sources, say 10 times / sec), and pull data to each’s queues. while the sources are not fixed, they may add/remove during run time.

and there is a monitor which pulls from queues at a constant freq and display the data.

So what is the best design pattern or structure for this problem.

I’m trying to make a list for all the sources pullers, and each puller holds a thread, and a specified pulling function (somehow the pulling function may interact with the puller, say if the source is drain, it will ask to stop the pulling process on that thread.)

Answer 1

Unless the operation where you query a source is blocking (or you have lots of them), you don’t need to use threads for this. We could start with a Producer which will work with either synchronous or asynchronous (threaded) dispatch:

template <typename OutputType>
class Producer
{
    std::list<OutputType> output;

protected:
    int poll_interval; // seconds? milliseconds?
    virtual OutputType query() = 0;

public:
    virtual ~Producer();

    int next_poll_interval() const { return poll_interval; }
    void poll() { output.push_back(this->query()); }

    std::size_t size() { return output.size(); }
    // whatever accessors you need for the queue here:
    // pop_front, swap entire list, etc.
};

Now we can derive from this Producer and just implement the query method in each subtype. You can set poll_interval in the constructor and leave it alone, or change it on every call to query. There’s your general producer component, with no dependency on the dispatch mechanism.

template <typename OutputType>
class ThreadDispatcher
{
    Producer<OutputType> *producer;
    bool shutdown;
    std::thread thread;

    static void loop(ThreadDispatcher *self)
    {
        Producer<OutputType> *producer = self->producer;

        while (!self->shutdown)
        {
            producer->poll();
            // some mechanism to pass the produced values back to the owner
            auto delay = // assume millis for sake of argument
                std::chrono::milliseconds(producer->next_poll_interval());
            std::this_thread::sleep_for(delay);
        }
    }

public:
    explicit ThreadDispatcher(Producer<OutputType> *p)
      : producer(p), shutdown(false), thread(loop, this)
    {
    }

    ~ThreadDispatcher()
    {
        shutdown = true;
        thread.join();
    }

    // again, the accessors you need for reading produced values go here
    // Producer::output isn't synchronised, so you can't expose it directly
    // to the calling thread
};

This is a quick sketch of a simple dispatcher that would run your producer in a thread, polling it however often you ask it to. Note that passing produced values back to the owner isn’t shown, because I don’t know how you want to access them.

Also note I haven’t synchronized access to the shutdown flag – it should probably be atomic, but it might be implicitly synchronized by whatever you choose to do with the produced values.

With this organization, it’d also be easy to write a synchronous dispatcher to query multiple producers in a single thread, for example from a select/poll loop, or using something like Boost.Asio and a deadline timer per producer.