    // Copyright (C) 2008  Davis E. King (davis@dlib.net)
    // License: Boost Software License   See LICENSE.txt for the full license.
    #ifndef DLIB_THREAD_POOl_Hh_
    #define DLIB_THREAD_POOl_Hh_

    #include <exception>
    #include <memory>
    #include <thread>

    #include "thread_pool_extension_abstract.h"
    #include "multithreaded_object_extension.h"
    #include "../member_function_pointer.h"
    #include "../bound_function_pointer.h"
    #include "threads_kernel.h"
    #include "auto_mutex_extension.h"
    #include "../uintn.h"
    #include "../array.h"

    namespace dlib
    {

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

        class thread_pool_implementation;

        template <
            typename T
            >
        class future
        {
            /*!
                INITIAL VALUE
                    - task_id == 0
                    - tp.get() == 0

                CONVENTION
                    - is_ready() == (tp.get() == 0)
                    - get() == var

                    - if (tp.get() != 0)
                        - tp == a pointer to the thread_pool_implementation that is using this future object
                        - task_id == the task id of the task in the thread pool tp that is using
                          this future object.
            !*/
        public:

            future (
            ) : task_id(0) {}

            future (
                const T& item
            ) : task_id(0), var(item) {}

            future (
                const future& item
            ) :task_id(0), var(item.get()) {}

            ~future (
            ) { wait(); }

            future& operator=(
                const T& item
            ) { get() = item; return *this; }

            future& operator=(
                const future& item
            ) { get() = item.get(); return *this; }

            operator T& (
            ) { return get(); }

            operator const T& (
            ) const { return get(); }

            T& get (
            ) { wait(); return var; }

            const T& get (
            ) const { wait(); return var; }

            bool is_ready (
            ) const { return tp.get() == 0; }

        private:

            friend class thread_pool;

            inline void wait () const;

            mutable uint64 task_id;
            mutable std::shared_ptr<thread_pool_implementation> tp;

            T var;
        };

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

        template <typename T>
        inline void swap (
            future<T>& a,
            future<T>& b
        ) { dlib::exchange(a.get(), b.get()); }
        // Note that dlib::exchange() just calls std::swap.  I'm only using it because
        // this works around some bugs in certain compilers.

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

        template <typename T> bool operator== (const future<T>& a, const future<T>& b) { return a.get() == b.get(); }
        template <typename T> bool operator!= (const future<T>& a, const future<T>& b) { return a.get() != b.get(); }
        template <typename T> bool operator<= (const future<T>& a, const future<T>& b) { return a.get() <= b.get(); }
        template <typename T> bool operator>= (const future<T>& a, const future<T>& b) { return a.get() >= b.get(); }
        template <typename T> bool operator<  (const future<T>& a, const future<T>& b) { return a.get() <  b.get(); }
        template <typename T> bool operator>  (const future<T>& a, const future<T>& b) { return a.get() >  b.get(); }

        template <typename T> bool operator== (const future<T>& a, const T& b)         { return a.get() == b; }
        template <typename T> bool operator== (const T& a,         const future<T>& b) { return a       == b.get(); }
        template <typename T> bool operator!= (const future<T>& a, const T& b)         { return a.get() != b; }
        template <typename T> bool operator!= (const T& a,         const future<T>& b) { return a       != b.get(); }
        template <typename T> bool operator<= (const future<T>& a, const T& b)         { return a.get() <= b; }
        template <typename T> bool operator<= (const T& a,         const future<T>& b) { return a       <= b.get(); }
        template <typename T> bool operator>= (const future<T>& a, const T& b)         { return a.get() >= b; }
        template <typename T> bool operator>= (const T& a,         const future<T>& b) { return a       >= b.get(); }
        template <typename T> bool operator<  (const future<T>& a, const T& b)         { return a.get() <  b; }
        template <typename T> bool operator<  (const T& a,         const future<T>& b) { return a       <  b.get(); }
        template <typename T> bool operator>  (const future<T>& a, const T& b)         { return a.get() >  b; }
        template <typename T> bool operator>  (const T& a,         const future<T>& b) { return a       >  b.get(); }

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

        class thread_pool_implementation
        {
            /*!
                CONVENTION
                    - num_threads_in_pool() == tasks.size()
                    - if (the destructor has been called) then
                        - we_are_destructing == true
                    - else
                        - we_are_destructing == false

                    - is_task_thread() == is_worker_thread(get_thread_id())

                    - m == the mutex used to protect everything in this object
                    - worker_thread_ids == an array that contains the thread ids for
                      all the threads in the thread pool
            !*/
            typedef bound_function_pointer::kernel_1a_c bfp_type;

            friend class thread_pool;
            explicit thread_pool_implementation (
                unsigned long num_threads
            );

        public:
            ~thread_pool_implementation(
            );

            void wait_for_task (
                uint64 task_id
            ) const;

            unsigned long num_threads_in_pool (
            ) const;

            void wait_for_all_tasks (
            ) const;

            bool is_task_thread (
            ) const;

            template <typename T>
            uint64 add_task (
                T& obj,
                void (T::*funct)()
            )
            {
                auto_mutex M(m);
                const thread_id_type my_thread_id = get_thread_id();

                // find a thread that isn't doing anything
                long idx = find_empty_task_slot();
                if (idx == -1 && is_worker_thread(my_thread_id))
                {
                    // this function is being called from within a worker thread and there
                    // aren't any other worker threads free so just perform the task right
                    // here

                    M.unlock();
                    (obj.*funct)();

                    // return a task id that is both non-zero and also one
                    // that is never normally returned.  This way calls
                    // to wait_for_task() will never block given this id.
                    return 1;
                }

                // wait until there is a thread that isn't doing anything
                while (idx == -1)
                {
                    task_done_signaler.wait();
                    idx = find_empty_task_slot();
                }

                tasks[idx].thread_id = my_thread_id;
                tasks[idx].task_id = make_next_task_id(idx);
                tasks[idx].mfp0.set(obj,funct);

                task_ready_signaler.signal();

                return tasks[idx].task_id;
            }

            template <typename T>
            uint64 add_task (
                T& obj,
                void (T::*funct)(long),
                long arg1
            )
            {
                auto_mutex M(m);
                const thread_id_type my_thread_id = get_thread_id();

                // find a thread that isn't doing anything
                long idx = find_empty_task_slot();
                if (idx == -1 && is_worker_thread(my_thread_id))
                {
                    // this function is being called from within a worker thread and there
                    // aren't any other worker threads free so just perform the task right
                    // here

                    M.unlock();
                    (obj.*funct)(arg1);

                    // return a task id that is both non-zero and also one
                    // that is never normally returned.  This way calls
                    // to wait_for_task() will never block given this id.
                    return 1;
                }

                // wait until there is a thread that isn't doing anything
                while (idx == -1)
                {
                    task_done_signaler.wait();
                    idx = find_empty_task_slot();
                }

                tasks[idx].thread_id = my_thread_id;
                tasks[idx].task_id = make_next_task_id(idx);
                tasks[idx].mfp1.set(obj,funct);
                tasks[idx].arg1 = arg1;

                task_ready_signaler.signal();

                return tasks[idx].task_id;
            }

            template <typename T>
            uint64 add_task (
                T& obj,
                void (T::*funct)(long,long),
                long arg1,
                long arg2
            )
            {
                auto_mutex M(m);
                const thread_id_type my_thread_id = get_thread_id();

                // find a thread that isn't doing anything
                long idx = find_empty_task_slot();
                if (idx == -1 && is_worker_thread(my_thread_id))
                {
                    // this function is being called from within a worker thread and there
                    // aren't any other worker threads free so just perform the task right
                    // here

                    M.unlock();
                    (obj.*funct)(arg1, arg2);

                    // return a task id that is both non-zero and also one
                    // that is never normally returned.  This way calls
                    // to wait_for_task() will never block given this id.
                    return 1;
                }

                // wait until there is a thread that isn't doing anything
                while (idx == -1)
                {
                    task_done_signaler.wait();
                    idx = find_empty_task_slot();
                }

                tasks[idx].thread_id = my_thread_id;
                tasks[idx].task_id = make_next_task_id(idx);
                tasks[idx].mfp2.set(obj,funct);
                tasks[idx].arg1 = arg1;
                tasks[idx].arg2 = arg2;

                task_ready_signaler.signal();

                return tasks[idx].task_id;
            }

            struct function_object_copy
            {
                virtual ~function_object_copy(){}
            };

            template <typename T>
            struct function_object_copy_instance : function_object_copy
            {
                function_object_copy_instance(const T& item_) : item(item_) {}
                T item;
                virtual ~function_object_copy_instance(){}
            };

            uint64 add_task_internal (
                const bfp_type& bfp,
                std::shared_ptr<function_object_copy>& item
            );
            /*!
                ensures
                    - adds a task to call the given bfp object.
                    - swaps item into the internal task object which will have a lifetime
                      at least as long as the running task.
                    - returns the task id for this new task
            !*/

            uint64 add_task_internal (
                const bfp_type& bfp
            ) { std::shared_ptr<function_object_copy> temp; return add_task_internal(bfp, temp); }
            /*!
                ensures
                    - adds a task to call the given bfp object.
                    - returns the task id for this new task
            !*/

            void shutdown_pool (
            );
            /*!
                ensures
                    - causes all threads to terminate and blocks the
                      caller until this happens.
            !*/

        private:

            bool is_worker_thread (
                const thread_id_type id
            ) const;
            /*!
                requires
                    - m is locked
                ensures
                    - if (thread with given id is one of the thread pool's worker threads or num_threads_in_pool() == 0) then
                        - returns true
                    - else
                        - returns false
            !*/

            void thread (
            );
            /*!
                this is the function that executes the threads in the thread pool
            !*/

            long find_empty_task_slot (
            ) const;
            /*!
                requires
                    - m is locked
                ensures
                    - if (there is currently a empty task slot) then
                        - returns the index of that task slot in tasks
                        - there is a task slot
                    - else
                        - returns -1
            !*/

            long find_ready_task (
            ) const;
            /*!
                requires
                    - m is locked
                ensures
                    - if (there is currently a task to do) then
                        - returns the index of that task in tasks
                    - else
                        - returns -1
            !*/

            uint64 make_next_task_id (
                long idx
            );
            /*!
                requires
                    - m is locked
                    - 0 <= idx < tasks.size()
                ensures
                    - returns the next index to be used for tasks that are placed in
                      tasks[idx]
            !*/

            unsigned long task_id_to_index (
                uint64 id
            ) const;
            /*!
                requires
                    - m is locked
                    - num_threads_in_pool() != 0
                ensures
                    - returns the index in tasks corresponding to the given id
            !*/

            struct task_state_type
            {
                task_state_type() : is_being_processed(false), task_id(0), next_task_id(2), arg1(0), arg2(0), eptr(nullptr) {}

                bool is_ready () const
                /*!
                    ensures
                        - if (is_empty() == false && no thread is currently processing this task) then
                            - returns true
                        - else
                            - returns false
                !*/
                {
                    return !is_being_processed && !is_empty();
                }

                bool is_empty () const
                /*!
                    ensures
                        - if (this task state is empty.  i.e. it doesn't contain a task to be processed) then
                            - returns true
                        - else
                            - returns false
                !*/
                {
                    return task_id == 0;
                }

                bool is_being_processed;  // true when a thread is working on this task
                uint64 task_id; // the id of this task.  0 means this task is empty
                thread_id_type thread_id; // the id of the thread that requested this task

                uint64 next_task_id;

                long arg1;
                long arg2;

                member_function_pointer<> mfp0;
                member_function_pointer<long> mfp1;
                member_function_pointer<long,long> mfp2;
                bfp_type bfp;

                std::shared_ptr<function_object_copy> function_copy;
                mutable std::exception_ptr eptr; // non-null if the task threw an exception

                void propagate_exception() const
                {
                    if (eptr)
                    {
                        auto tmp = eptr;
                        eptr = nullptr;
                        std::rethrow_exception(tmp);
                    }
                }

            };

            array<task_state_type> tasks;
            array<thread_id_type> worker_thread_ids;

            mutex m;
            signaler task_done_signaler;
            signaler task_ready_signaler;
            bool we_are_destructing;

            std::vector<std::thread> threads;

            // restricted functions
            thread_pool_implementation(thread_pool_implementation&);        // copy constructor
            thread_pool_implementation& operator=(thread_pool_implementation&);    // assignment operator

        };

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

        class thread_pool
        {
            /*!
                This object is just a shell that holds a std::shared_ptr
                to the real thread_pool_implementation object.  The reason for doing
                it this way is so that we can allow any mixture of destruction orders
                between thread_pool objects and futures.  Whoever gets destroyed
                last cleans up the thread_pool_implementation resources.
            !*/
            typedef bound_function_pointer::kernel_1a_c bfp_type;

        public:
            explicit thread_pool (
                unsigned long num_threads
            )
            {
                impl.reset(new thread_pool_implementation(num_threads));
            }

            ~thread_pool (
            )
            {
                try
                {
                    impl->shutdown_pool();
                }
                catch (std::exception& e)
                {
                    std::cerr << "An unhandled exception was inside a dlib::thread_pool when it was destructed." << std::endl;
                    std::cerr << "It's what string is: \n" << e.what() << std::endl;
                    assert(false);
                    abort();
                }
                catch (...)
                {
                    std::cerr << "An unhandled exception was inside a dlib::thread_pool when it was destructed." << std::endl;
                    assert(false);
                    abort();
                }
            }

            void wait_for_task (
                uint64 task_id
            ) const { impl->wait_for_task(task_id); }

            unsigned long num_threads_in_pool (
            ) const { return impl->num_threads_in_pool(); }

            void wait_for_all_tasks (
            ) const { impl->wait_for_all_tasks(); }

            bool is_task_thread (
            ) const { return impl->is_task_thread(); }

            template <typename T>
            uint64 add_task (
                T& obj,
                void (T::*funct)()
            )
            {
                return impl->add_task(obj, funct);
            }

            template <typename T>
            uint64 add_task (
                T& obj,
                void (T::*funct)(long),
                long arg1
            )
            {
                return impl->add_task(obj, funct, arg1);
            }

            template <typename T>
            uint64 add_task (
                T& obj,
                void (T::*funct)(long,long),
                long arg1,
                long arg2
            )
            {
                return impl->add_task(obj, funct, arg1, arg2);
            }

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

            template <typename F>
            uint64 add_task (
                F& function_object
            )
            {
                COMPILE_TIME_ASSERT(std::is_function<F>::value == false);
                COMPILE_TIME_ASSERT(std::is_pointer<F>::value == false);

                bfp_type temp;
                temp.set(function_object);
                uint64 id = impl->add_task_internal(temp);

                return id;
            }

            template <typename F>
            uint64 add_task_by_value (
                const F& function_object
            )
            {
                thread_pool_implementation::function_object_copy_instance<F>* ptr = 0;
                ptr = new thread_pool_implementation::function_object_copy_instance<F>(function_object);
                std::shared_ptr<thread_pool_implementation::function_object_copy> function_copy(ptr);

                bfp_type temp;
                temp.set(ptr->item);
                uint64 id = impl->add_task_internal(temp, function_copy);

                return id;
            }

            template <typename T>
            uint64 add_task (
                const T& obj,
                void (T::*funct)() const
            )
            {
                bfp_type temp;
                temp.set(obj,funct);
                uint64 id = impl->add_task_internal(temp);

                return id;
            }

            template <typename T>
            uint64 add_task_by_value (
                const T& obj,
                void (T::*funct)() const
            )
            {
                thread_pool_implementation::function_object_copy_instance<const T>* ptr = 0;
                ptr = new thread_pool_implementation::function_object_copy_instance<const T>(obj);
                std::shared_ptr<thread_pool_implementation::function_object_copy> function_copy(ptr);

                bfp_type temp;
                temp.set(ptr->item,funct);
                uint64 id = impl->add_task_internal(temp, function_copy);

                return id;
            }

            template <typename T>
            uint64 add_task_by_value (
                const T& obj,
                void (T::*funct)()
            )
            {
                thread_pool_implementation::function_object_copy_instance<T>* ptr = 0;
                ptr = new thread_pool_implementation::function_object_copy_instance<T>(obj);
                std::shared_ptr<thread_pool_implementation::function_object_copy> function_copy(ptr);

                bfp_type temp;
                temp.set(ptr->item,funct);
                uint64 id = impl->add_task_internal(temp, function_copy);

                return id;
            }

            uint64 add_task (
                void (*funct)()
            )
            {
                bfp_type temp;
                temp.set(funct);
                uint64 id = impl->add_task_internal(temp);

                return id;
            }

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

            template <typename F, typename A1>
            uint64 add_task (
                F& function_object,
                future<A1>& arg1
            )
            {
                COMPILE_TIME_ASSERT(std::is_function<F>::value == false);
                COMPILE_TIME_ASSERT(std::is_pointer<F>::value == false);

                bfp_type temp;
                temp.set(function_object,arg1.get());
                uint64 id = impl->add_task_internal(temp);

                // tie the future to this task
                arg1.task_id = id;
                arg1.tp = impl;
                return id;
            }

            template <typename F, typename A1>
            uint64 add_task_by_value (
                const F& function_object,
                future<A1>& arg1
            )
            {
                thread_pool_implementation::function_object_copy_instance<F>* ptr = 0;
                ptr = new thread_pool_implementation::function_object_copy_instance<F>(function_object);
                std::shared_ptr<thread_pool_implementation::function_object_copy> function_copy(ptr);

                bfp_type temp;
                temp.set(ptr->item, arg1.get());
                uint64 id = impl->add_task_internal(temp, function_copy);

                // tie the future to this task
                arg1.task_id = id;
                arg1.tp = impl;
                return id;
            }

            template <typename T, typename T1, typename A1>
            uint64 add_task (
                T& obj,
                void (T::*funct)(T1),
                future<A1>& arg1
            )
            {
                bfp_type temp;
                temp.set(obj,funct,arg1.get());
                uint64 id = impl->add_task_internal(temp);

                // tie the future to this task
                arg1.task_id = id;
                arg1.tp = impl;
                return id;
            }

            template <typename T, typename T1, typename A1>
            uint64 add_task_by_value (
                const T& obj,
                void (T::*funct)(T1),
                future<A1>& arg1
            )
            {
                thread_pool_implementation::function_object_copy_instance<T>* ptr = 0;
                ptr = new thread_pool_implementation::function_object_copy_instance<T>(obj);
                std::shared_ptr<thread_pool_implementation::function_object_copy> function_copy(ptr);

                bfp_type temp;
                temp.set(ptr->item,funct,arg1.get());
                uint64 id = impl->add_task_internal(temp, function_copy);

                // tie the future to this task
                arg1.task_id = id;
                arg1.tp = impl;
                return id;
            }

            template <typename T, typename T1, typename A1>
            uint64 add_task (
                const T& obj,
                void (T::*funct)(T1) const,
                future<A1>& arg1
            )
            {
                bfp_type temp;
                temp.set(obj,funct,arg1.get());
                uint64 id = impl->add_task_internal(temp);

                // tie the future to this task
                arg1.task_id = id;
                arg1.tp = impl;
                return id;
            }

            template <typename T, typename T1, typename A1>
            uint64 add_task_by_value (
                const T& obj,
                void (T::*funct)(T1) const,
                future<A1>& arg1
            )
            {
                thread_pool_implementation::function_object_copy_instance<const T>* ptr = 0;
                ptr = new thread_pool_implementation::function_object_copy_instance<const T>(obj);
                std::shared_ptr<thread_pool_implementation::function_object_copy> function_copy(ptr);

                bfp_type temp;
                temp.set(ptr->item,funct,arg1.get());
                uint64 id = impl->add_task_internal(temp, function_copy);

                // tie the future to this task
                arg1.task_id = id;
                arg1.tp = impl;
                return id;
            }

            template <typename T1, typename A1>
            uint64 add_task (
                void (*funct)(T1),
                future<A1>& arg1
            )
            {
                bfp_type temp;
                temp.set(funct,arg1.get());
                uint64 id = impl->add_task_internal(temp);

                // tie the future to this task
                arg1.task_id = id;
                arg1.tp = impl;
                return id;
            }

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

            template <typename F, typename A1, typename A2>
            uint64 add_task (
                F& function_object,
                future<A1>& arg1,
                future<A2>& arg2
            )
            {
                COMPILE_TIME_ASSERT(std::is_function<F>::value == false);
                COMPILE_TIME_ASSERT(std::is_pointer<F>::value == false);

                bfp_type temp;
                temp.set(function_object, arg1.get(), arg2.get());
                uint64 id = impl->add_task_internal(temp);

                // tie the future to this task
                arg1.task_id = id;
                arg1.tp = impl;
                arg2.task_id = id;
                arg2.tp = impl;
                return id;
            }

            template <typename F, typename A1, typename A2>
            uint64 add_task_by_value (
                const F& function_object,
                future<A1>& arg1,
                future<A2>& arg2
            )
            {
                thread_pool_implementation::function_object_copy_instance<F>* ptr = 0;
                ptr = new thread_pool_implementation::function_object_copy_instance<F>(function_object);
                std::shared_ptr<thread_pool_implementation::function_object_copy> function_copy(ptr);

                bfp_type temp;
                temp.set(ptr->item, arg1.get(), arg2.get());
                uint64 id = impl->add_task_internal(temp, function_copy);

                // tie the future to this task
                arg1.task_id = id;
                arg1.tp = impl;
                arg2.task_id = id;
                arg2.tp = impl;
                return id;
            }

            template <typename T, typename T1, typename A1,
                                  typename T2, typename A2>
            uint64 add_task (
                T& obj,
                void (T::*funct)(T1,T2),
                future<A1>& arg1,
                future<A2>& arg2
            )
            {
                bfp_type temp;
                temp.set(obj, funct, arg1.get(), arg2.get());
                uint64 id = impl->add_task_internal(temp);

                // tie the futures to this task
                arg1.task_id = id;
                arg1.tp = impl;
                arg2.task_id = id;
                arg2.tp = impl;
                return id;
            }

            template <typename T, typename T1, typename A1,
                                  typename T2, typename A2>
            uint64 add_task_by_value (
                const T& obj,
                void (T::*funct)(T1,T2),
                future<A1>& arg1,
                future<A2>& arg2
            )
            {
                thread_pool_implementation::function_object_copy_instance<T>* ptr = 0;
                ptr = new thread_pool_implementation::function_object_copy_instance<T>(obj);
                std::shared_ptr<thread_pool_implementation::function_object_copy> function_copy(ptr);

                bfp_type temp;
                temp.set(ptr->item, funct, arg1.get(), arg2.get());
                uint64 id = impl->add_task_internal(temp, function_copy);

                // tie the futures to this task
                arg1.task_id = id;
                arg1.tp = impl;
                arg2.task_id = id;
                arg2.tp = impl;
                return id;
            }

            template <typename T, typename T1, typename A1,
                                  typename T2, typename A2>
            uint64 add_task (
                const T& obj,
                void (T::*funct)(T1,T2) const,
                future<A1>& arg1,
                future<A2>& arg2
            )
            {
                bfp_type temp;
                temp.set(obj, funct, arg1.get(), arg2.get());
                uint64 id = impl->add_task_internal(temp);

                // tie the futures to this task
                arg1.task_id = id;
                arg1.tp = impl;
                arg2.task_id = id;
                arg2.tp = impl;
                return id;
            }

            template <typename T, typename T1, typename A1,
                                  typename T2, typename A2>
            uint64 add_task_by_value (
                const T& obj,
                void (T::*funct)(T1,T2) const,
                future<A1>& arg1,
                future<A2>& arg2
            )
            {
                thread_pool_implementation::function_object_copy_instance<const T>* ptr = 0;
                ptr = new thread_pool_implementation::function_object_copy_instance<const T>(obj);
                std::shared_ptr<thread_pool_implementation::function_object_copy> function_copy(ptr);

                bfp_type temp;
                temp.set(ptr->item, funct, arg1.get(), arg2.get());
                uint64 id = impl->add_task_internal(temp, function_copy);

                // tie the futures to this task
                arg1.task_id = id;
                arg1.tp = impl;
                arg2.task_id = id;
                arg2.tp = impl;
                return id;
            }

            template <typename T1, typename A1,
                      typename T2, typename A2>
            uint64 add_task (
                void (*funct)(T1,T2),
                future<A1>& arg1,
                future<A2>& arg2
            )
            {
                bfp_type temp;
                temp.set(funct, arg1.get(), arg2.get());
                uint64 id = impl->add_task_internal(temp);

                // tie the futures to this task
                arg1.task_id = id;
                arg1.tp = impl;
                arg2.task_id = id;
                arg2.tp = impl;
                return id;
            }

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

            template <typename F, typename A1, typename A2, typename A3>
            uint64 add_task (
                F& function_object,
                future<A1>& arg1,
                future<A2>& arg2,
                future<A3>& arg3
            )
            {
                COMPILE_TIME_ASSERT(std::is_function<F>::value == false);
                COMPILE_TIME_ASSERT(std::is_pointer<F>::value == false);

                bfp_type temp;
                temp.set(function_object, arg1.get(), arg2.get(), arg3.get());
                uint64 id = impl->add_task_internal(temp);

                // tie the future to this task
                arg1.task_id = id;
                arg1.tp = impl;
                arg2.task_id = id;
                arg2.tp = impl;
                arg3.task_id = id;
                arg3.tp = impl;
                return id;
            }

            template <typename F, typename A1, typename A2, typename A3>
            uint64 add_task_by_value (
                const F& function_object,
                future<A1>& arg1,
                future<A2>& arg2,
                future<A3>& arg3
            )
            {
                thread_pool_implementation::function_object_copy_instance<F>* ptr = 0;
                ptr = new thread_pool_implementation::function_object_copy_instance<F>(function_object);
                std::shared_ptr<thread_pool_implementation::function_object_copy> function_copy(ptr);

                bfp_type temp;
                temp.set(ptr->item, arg1.get(), arg2.get(), arg3.get());
                uint64 id = impl->add_task_internal(temp, function_copy);

                // tie the future to this task
                arg1.task_id = id;
                arg1.tp = impl;
                arg2.task_id = id;
                arg2.tp = impl;
                arg3.task_id = id;
                arg3.tp = impl;
                return id;
            }

            template <typename T, typename T1, typename A1,
                                  typename T2, typename A2,
                                  typename T3, typename A3>
            uint64 add_task (
                T& obj,
                void (T::*funct)(T1,T2,T3),
                future<A1>& arg1,
                future<A2>& arg2,
                future<A3>& arg3
            )
            {
                bfp_type temp;
                temp.set(obj, funct, arg1.get(), arg2.get(), arg3.get());
                uint64 id = impl->add_task_internal(temp);

                // tie the futures to this task
                arg1.task_id = id;
                arg1.tp = impl;
                arg2.task_id = id;
                arg2.tp = impl;
                arg3.task_id = id;
                arg3.tp = impl;
                return id;
            }

            template <typename T, typename T1, typename A1,
                                  typename T2, typename A2,
                                  typename T3, typename A3>
            uint64 add_task_by_value (
                const T& obj,
                void (T::*funct)(T1,T2,T3),
                future<A1>& arg1,
                future<A2>& arg2,
                future<A3>& arg3
            )
            {
                thread_pool_implementation::function_object_copy_instance<T>* ptr = 0;
                ptr = new thread_pool_implementation::function_object_copy_instance<T>(obj);
                std::shared_ptr<thread_pool_implementation::function_object_copy> function_copy(ptr);

                bfp_type temp;
                temp.set(ptr->item, funct, arg1.get(), arg2.get(), arg3.get());
                uint64 id = impl->add_task_internal(temp, function_copy);

                // tie the futures to this task
                arg1.task_id = id;
                arg1.tp = impl;
                arg2.task_id = id;
                arg2.tp = impl;
                arg3.task_id = id;
                arg3.tp = impl;
                return id;
            }

            template <typename T, typename T1, typename A1,
                                  typename T2, typename A2,
                                  typename T3, typename A3>
            uint64 add_task (
                const T& obj,
                void (T::*funct)(T1,T2,T3) const,
                future<A1>& arg1,
                future<A2>& arg2,
                future<A3>& arg3
            )
            {
                bfp_type temp;
                temp.set(obj, funct, arg1.get(), arg2.get(), arg3.get());
                uint64 id = impl->add_task_internal(temp);

                // tie the futures to this task
                arg1.task_id = id;
                arg1.tp = impl;
                arg2.task_id = id;
                arg2.tp = impl;
                arg3.task_id = id;
                arg3.tp = impl;
                return id;
            }

            template <typename T, typename T1, typename A1,
                                  typename T2, typename A2,
                                  typename T3, typename A3>
            uint64 add_task_by_value (
                const T& obj,
                void (T::*funct)(T1,T2,T3) const,
                future<A1>& arg1,
                future<A2>& arg2,
                future<A3>& arg3
            )
            {
                thread_pool_implementation::function_object_copy_instance<const T>* ptr = 0;
                ptr = new thread_pool_implementation::function_object_copy_instance<const T>(obj);
                std::shared_ptr<thread_pool_implementation::function_object_copy> function_copy(ptr);

                bfp_type temp;
                temp.set(ptr->item, funct, arg1.get(), arg2.get(), arg3.get());
                uint64 id = impl->add_task_internal(temp, function_copy);

                // tie the futures to this task
                arg1.task_id = id;
                arg1.tp = impl;
                arg2.task_id = id;
                arg2.tp = impl;
                arg3.task_id = id;
                arg3.tp = impl;
                return id;
            }

            template <typename T1, typename A1,
                      typename T2, typename A2,
                      typename T3, typename A3>
            uint64 add_task (
                void (*funct)(T1,T2,T3),
                future<A1>& arg1,
                future<A2>& arg2,
                future<A3>& arg3
            )
            {
                bfp_type temp;
                temp.set(funct, arg1.get(), arg2.get(), arg3.get());
                uint64 id = impl->add_task_internal(temp);

                // tie the futures to this task
                arg1.task_id = id;
                arg1.tp = impl;
                arg2.task_id = id;
                arg2.tp = impl;
                arg3.task_id = id;
                arg3.tp = impl;
                return id;
            }

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

            template <typename F, typename A1, typename A2, typename A3, typename A4>
            uint64 add_task (
                F& function_object,
                future<A1>& arg1,
                future<A2>& arg2,
                future<A3>& arg3,
                future<A4>& arg4
            )
            {
                COMPILE_TIME_ASSERT(std::is_function<F>::value == false);
                COMPILE_TIME_ASSERT(std::is_pointer<F>::value == false);

                bfp_type temp;
                temp.set(function_object, arg1.get(), arg2.get(), arg3.get(), arg4.get());
                uint64 id = impl->add_task_internal(temp);

                // tie the future to this task
                arg1.task_id = id;
                arg1.tp = impl;
                arg2.task_id = id;
                arg2.tp = impl;
                arg3.task_id = id;
                arg3.tp = impl;
                arg4.task_id = id;
                arg4.tp = impl;
                return id;
            }

            template <typename F, typename A1, typename A2, typename A3, typename A4>
            uint64 add_task_by_value (
                const F& function_object,
                future<A1>& arg1,
                future<A2>& arg2,
                future<A3>& arg3,
                future<A4>& arg4
            )
            {
                thread_pool_implementation::function_object_copy_instance<F>* ptr = 0;
                ptr = new thread_pool_implementation::function_object_copy_instance<F>(function_object);
                std::shared_ptr<thread_pool_implementation::function_object_copy> function_copy(ptr);

                bfp_type temp;
                temp.set(ptr->item, arg1.get(), arg2.get(), arg3.get(), arg4.get());
                uint64 id = impl->add_task_internal(temp, function_copy);

                // tie the future to this task
                arg1.task_id = id;
                arg1.tp = impl;
                arg2.task_id = id;
                arg2.tp = impl;
                arg3.task_id = id;
                arg3.tp = impl;
                arg4.task_id = id;
                arg4.tp = impl;
                return id;
            }

            template <typename T, typename T1, typename A1,
                                  typename T2, typename A2,
                                  typename T3, typename A3,
                                  typename T4, typename A4>
            uint64 add_task (
                T& obj,
                void (T::*funct)(T1,T2,T3,T4),
                future<A1>& arg1,
                future<A2>& arg2,
                future<A3>& arg3,
                future<A4>& arg4
            )
            {
                bfp_type temp;
                temp.set(obj, funct, arg1.get(), arg2.get(), arg3.get(), arg4.get());
                uint64 id = impl->add_task_internal(temp);

                // tie the futures to this task
                arg1.task_id = id;
                arg1.tp = impl;
                arg2.task_id = id;
                arg2.tp = impl;
                arg3.task_id = id;
                arg3.tp = impl;
                arg4.task_id = id;
                arg4.tp = impl;
                return id;
            }

            template <typename T, typename T1, typename A1,
                                  typename T2, typename A2,
                                  typename T3, typename A3,
                                  typename T4, typename A4>
            uint64 add_task_by_value (
                const T& obj,
                void (T::*funct)(T1,T2,T3,T4),
                future<A1>& arg1,
                future<A2>& arg2,
                future<A3>& arg3,
                future<A4>& arg4
            )
            {
                thread_pool_implementation::function_object_copy_instance<T>* ptr = 0;
                ptr = new thread_pool_implementation::function_object_copy_instance<T>(obj);
                std::shared_ptr<thread_pool_implementation::function_object_copy> function_copy(ptr);

                bfp_type temp;
                temp.set(ptr->item, funct, arg1.get(), arg2.get(), arg3.get(), arg4.get());
                uint64 id = impl->add_task_internal(temp, function_copy);

                // tie the futures to this task
                arg1.task_id = id;
                arg1.tp = impl;
                arg2.task_id = id;
                arg2.tp = impl;
                arg3.task_id = id;
                arg3.tp = impl;
                arg4.task_id = id;
                arg4.tp = impl;
                return id;
            }

            template <typename T, typename T1, typename A1,
                                  typename T2, typename A2,
                                  typename T3, typename A3,
                                  typename T4, typename A4>
            uint64 add_task (
                const T& obj,
                void (T::*funct)(T1,T2,T3,T4) const,
                future<A1>& arg1,
                future<A2>& arg2,
                future<A3>& arg3,
                future<A4>& arg4
            )
            {
                bfp_type temp;
                temp.set(obj, funct, arg1.get(), arg2.get(), arg3.get(), arg4.get());
                uint64 id = impl->add_task_internal(temp);

                // tie the futures to this task
                arg1.task_id = id;
                arg1.tp = impl;
                arg2.task_id = id;
                arg2.tp = impl;
                arg3.task_id = id;
                arg3.tp = impl;
                arg4.task_id = id;
                arg4.tp = impl;
                return id;
            }

            template <typename T, typename T1, typename A1,
                                  typename T2, typename A2,
                                  typename T3, typename A3,
                                  typename T4, typename A4>
            uint64 add_task_by_value (
                const T& obj,
                void (T::*funct)(T1,T2,T3,T4) const,
                future<A1>& arg1,
                future<A2>& arg2,
                future<A3>& arg3,
                future<A4>& arg4
            )
            {
                thread_pool_implementation::function_object_copy_instance<const T>* ptr = 0;
                ptr = new thread_pool_implementation::function_object_copy_instance<const T>(obj);
                std::shared_ptr<thread_pool_implementation::function_object_copy> function_copy(ptr);

                bfp_type temp;
                temp.set(ptr->item, funct, arg1.get(), arg2.get(), arg3.get(), arg4.get());
                uint64 id = impl->add_task_internal(temp, function_copy);

                // tie the futures to this task
                arg1.task_id = id;
                arg1.tp = impl;
                arg2.task_id = id;
                arg2.tp = impl;
                arg3.task_id = id;
                arg3.tp = impl;
                arg4.task_id = id;
                arg4.tp = impl;
                return id;
            }

            template <typename T1, typename A1,
                      typename T2, typename A2,
                      typename T3, typename A3,
                      typename T4, typename A4>
            uint64 add_task (
                void (*funct)(T1,T2,T3,T4),
                future<A1>& arg1,
                future<A2>& arg2,
                future<A3>& arg3,
                future<A4>& arg4
            )
            {
                bfp_type temp;
                temp.set(funct, arg1.get(), arg2.get(), arg3.get(), arg4.get());
                uint64 id = impl->add_task_internal(temp);

                // tie the futures to this task
                arg1.task_id = id;
                arg1.tp = impl;
                arg2.task_id = id;
                arg2.tp = impl;
                arg3.task_id = id;
                arg3.tp = impl;
                arg4.task_id = id;
                arg4.tp = impl;
                return id;
            }

        private:

            std::shared_ptr<thread_pool_implementation> impl;

            // restricted functions
            thread_pool(thread_pool&);        // copy constructor
            thread_pool& operator=(thread_pool&);    // assignment operator

        };

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

        template <typename T>
        void future<T>::
        wait (
        ) const
        {
            if (tp)
            {
                tp->wait_for_task(task_id);
                tp.reset();
                task_id = 0;
            }
        }

    }

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

    #ifdef NO_MAKEFILE
    #include "thread_pool_extension.cpp"
    #endif

    #endif // DLIB_THREAD_POOl_Hh_