# dlib C++ Library - thread_pool_ex.cpp

    // The contents of this file are in the public domain. See LICENSE_FOR_EXAMPLE_PROGRAMS.txt
    /*

        This is an example illustrating the use of the thread_pool
        object from the dlib C++ Library.

        In this example we will crate a thread pool with 3 threads and then show a
        few different ways to send tasks to the pool.
    */

    #include <dlib/threads.h>
    #include <dlib/misc_api.h>  // for dlib::sleep
    #include <dlib/logger.h>
    #include <vector>

    using namespace dlib;

    // We will be using the dlib logger object to print messages in this example
    // because its output is timestamped and labeled with the thread that the log
    // message came from.  This will make it easier to see what is going on in this
    // example.  Here we make an instance of the logger.  See the logger
    // documentation and examples for detailed information regarding its use.
    logger dlog("main");

    // Here we make an instance of the thread pool object.  You could also use the
    // global dlib::default_thread_pool(), which automatically selects the number of
    // threads based on your hardware.  But here let's make our own.
    thread_pool tp(3);

    // ----------------------------------------------------------------------------------------

    class test
    {
        /*
            The thread_pool accepts "tasks" from the user and schedules them for
            execution in one of its threads when one becomes available.  Each task
            is just a request to call a function.  So here we create a class called
            test with a few member functions, which we will have the thread pool call
            as tasks.
        */
    public:

        void mytask()
        {
            dlog << LINFO << "mytask start";

            dlib::future<int> var;

            var = 1;

            // Here we ask the thread pool to call this->subtask() and this->subtask2().
            // Note that calls to add_task() will return immediately if there is an
            // available thread.  However, if there isn't a thread ready then
            // add_task() blocks until there is such a thread.  Also, note that if
            // mytask() is executed within the thread pool then calls to add_task()
            // will execute the requested task within the calling thread in cases
            // where the thread pool is full.  This means it is always safe to spawn
            // subtasks from within another task, which is what we are doing here.
            tp.add_task(*this,&test::subtask,var); // schedule call to this->subtask(var)
            tp.add_task(*this,&test::subtask2);    // schedule call to this->subtask2()

            // Since var is a future, this line will wait for the test::subtask task to
            // finish before allowing us to access the contents of var.  Then var will
            // return the integer it contains.  In this case result will be assigned
            // the value 2 since var was incremented by subtask().
            int result = var;
            dlog << LINFO << "var = " << result;

            // Wait for all the tasks we have started to finish.  Note that
            // wait_for_all_tasks() only waits for tasks which were started by the
            // calling thread.  So you don't have to worry about other unrelated
            // parts of your application interfering.  In this case it just waits
            // for subtask2() to finish.
            tp.wait_for_all_tasks();

            dlog << LINFO << "mytask end" ;
        }

        void subtask(int& a)
        {
            dlib::sleep(200);
            a = a + 1;
            dlog << LINFO << "subtask end ";
        }

        void subtask2()
        {
            dlib::sleep(300);
            dlog << LINFO << "subtask2 end ";
        }

    };

    // ----------------------------------------------------------------------------------------

    int main() try
    {
        // tell the logger to print out everything
        dlog.set_level(LALL);

        dlog << LINFO << "schedule a few tasks";

        test taskobj;
        // Schedule the thread pool to call taskobj.mytask().  Note that all forms of
        // add_task() pass in the task object by reference.  This means you must make sure,
        // in this case, that taskobj isn't destructed until after the task has finished
        // executing.
        tp.add_task(taskobj, &test::mytask);

        // This behavior of add_task() enables it to guarantee that no memory allocations
        // occur after the thread_pool has been constructed, so long as the user doesn't
        // call any of the add_task_by_value() routines.  The future object also doesn't
        // perform any memory allocations or contain any system resources such as mutex
        // objects.  If you don't care about memory allocations then you will likely find
        // the add_task_by_value() interface more convenient to use, which is shown below.

        // If we call add_task_by_value() we pass task objects to a thread pool by value.
        // So in this case we don't have to worry about keeping our own instance of the
        // task.  Here we create a lambda function and pass it right in and everything
        // works like it should.
        dlib::future<int> num = 3;
        tp.add_task_by_value([](int& val){val += 7;}, num);  // adds 7 to num
        int result = num.get();
        dlog << LINFO << "result = " << result;   // prints result = 10

        // dlib also contains dlib::async(), which is essentially identical to std::async()
        // except that it launches tasks to a dlib::thread_pool (using add_task_by_value)
        // rather than starting an unbounded number of threads.  As an example, here we
        // make 10 different tasks, each assigns a different value into the elements of the
        // vector vect.
        std::vector<std::future<unsigned long>> vect(10);
        for (unsigned long i = 0; i < vect.size(); ++i)
            vect[i] = dlib::async(tp, [i]() { return i*i; });
        // Print the results
        for (unsigned long i = 0; i < vect.size(); ++i)
            dlog << LINFO << "vect["<<i<<"]: " << vect[i].get();

        // Finally, it's usually a good idea to wait for all your tasks to complete.
        // Moreover, if any of your tasks threw an exception then waiting for the tasks
        // will rethrow the exception in the calling context, allowing you to handle it in
        // your local thread.  Also, if you don't wait for the tasks and there is an
        // exception and you allow the thread pool to be destructed your program will be
        // terminated.  So don't ignore exceptions :)
        tp.wait_for_all_tasks();

        /* A possible run of this program might produce the following output (the first
           column is the time the log message occurred and the value in [] is the thread
           id for the thread that generated the log message):

        0 INFO  [0] main: schedule a few tasks
        0 INFO  [1] main: task start
        0 INFO  [0] main: result = 10
      200 INFO  [2] main: subtask end
      200 INFO  [1] main: var = 2
      200 INFO  [0] main: vect[0]: 0
      200 INFO  [0] main: vect[1]: 1
      200 INFO  [0] main: vect[2]: 4
      200 INFO  [0] main: vect[3]: 9
      200 INFO  [0] main: vect[4]: 16
      200 INFO  [0] main: vect[5]: 25
      200 INFO  [0] main: vect[6]: 36
      200 INFO  [0] main: vect[7]: 49
      200 INFO  [0] main: vect[8]: 64
      200 INFO  [0] main: vect[9]: 81
      300 INFO  [3] main: subtask2 end
      300 INFO  [1] main: task end
        */
    }
    catch(std::exception& e)
    {
        std::cout << e.what() << std::endl;
    }