Reusing thread in loop c++

Question

I need to parallelize some tasks in a C++ program and am completely new to parallel programming. I\'ve made some progress through internet searches so far, but am a bit stu

Answer 1

The easiest way is to use a waitable queue of std::function objects. Like this:

#include 
#include 
#include 
#include 
#include 
#include 
#include 


class ThreadPool
{
    public:

    ThreadPool (int threads) : shutdown_ (false)
    {
        // Create the specified number of threads
        threads_.reserve (threads);
        for (int i = 0; i < threads; ++i)
            threads_.emplace_back (std::bind (&ThreadPool::threadEntry, this, i));
    }

    ~ThreadPool ()
    {
        {
            // Unblock any threads and tell them to stop
            std::unique_lock  l (lock_);

            shutdown_ = true;
            condVar_.notify_all();
        }

        // Wait for all threads to stop
        std::cerr << "Joining threads" << std::endl;
        for (auto& thread : threads_)
            thread.join();
    }

    void doJob (std::function  func)
    {
        // Place a job on the queu and unblock a thread
        std::unique_lock  l (lock_);

        jobs_.emplace (std::move (func));
        condVar_.notify_one();
    }

    protected:

    void threadEntry (int i)
    {
        std::function  job;

        while (1)
        {
            {
                std::unique_lock  l (lock_);

                while (! shutdown_ && jobs_.empty())
                    condVar_.wait (l);

                if (jobs_.empty ())
                {
                    // No jobs to do and we are shutting down
                    std::cerr << "Thread " << i << " terminates" << std::endl;
                    return;
                 }

                std::cerr << "Thread " << i << " does a job" << std::endl;
                job = std::move (jobs_.front ());
                jobs_.pop();
            }

            // Do the job without holding any locks
            job ();
        }

    }

    std::mutex lock_;
    std::condition_variable condVar_;
    bool shutdown_;
    std::queue > jobs_;
    std::vector  threads_;
};

void silly (int n)
{
    // A silly job for demonstration purposes
    std::cerr << "Sleeping for " << n << " seconds" << std::endl;
    std::this_thread::sleep_for (std::chrono::seconds (n));
}

int main()
{
    // Create two threads
    ThreadPool p (2);

    // Assign them 4 jobs
    p.doJob (std::bind (silly, 1));
    p.doJob (std::bind (silly, 2));
    p.doJob (std::bind (silly, 3));
    p.doJob (std::bind (silly, 4));
}