threadpool

#ifndef THREAD_POOL_HPP
#define THREAD_POOL_HPP

#include <vector>
#include <queue>
#include <memory>
#include <thread>
#include <mutex>
#include <condition_variable>
#include <future>
#include <functional>
#include <stdexcept>

class thread_pool {
    public:
        thread_pool(size_t);
        template<class F, class... Args>
        auto enqueue(F&& f, Args&& ... args)-> std::future<typename std::result_of<F(Args...)>::type>;
        ~thread_pool();
    private:
        // need to keep track of threads so we can join them
        std::vector<std::thread> workers;
        // the task queue
        std::queue<std::function<void()>> tasks;
        // synchronization
        std::mutex queue_mutex;
        std::condition_variable condition;
        bool stop;
};

// the constructor just launches some amount of workers
inline thread_pool::thread_pool(size_t threads)
    : stop(false) {
    for(size_t i = 0; i<threads; ++i)
        workers.emplace_back(
        [this] {
        for(;;) {
            std::function<void()> task;
            {
                std::unique_lock<std::mutex> lock(this->queue_mutex);
                this->condition.wait(lock,
                [this] { return this->stop || !this->tasks.empty(); });
                if(this->stop && this->tasks.empty()) {
                    return;
                }
                task = std::move(this->tasks.front());
                this->tasks.pop();
            }
            task();
        }
    }
    );
}

// add new work item to the pool
template<class F, class... Args>
auto thread_pool::enqueue(F&& f, Args&& ... args) -> std::future<typename std::result_of<F(Args...)>::type> {
    using return_type = typename std::result_of<F(Args...)>::type;

    auto task = std::make_shared<std::packaged_task<return_type()>>(
        std::bind(std::forward<F>(f), std::forward<Args>(args)...)
    );
    std::future<return_type> res = task->get_future();
    {
        std::unique_lock<std::mutex> lock(queue_mutex);
        // don't allow enqueueing after stopping the pool
        if(stop) {
            throw std::runtime_error("enqueue on stopped thread pool");
        }
        tasks.emplace([task]() {
            (*task)();
        });
    }
    condition.notify_one();
    return res;
}

// the destructor joins all threads
inline thread_pool::~thread_pool() {
    {
        std::unique_lock<std::mutex> lock(queue_mutex);
        stop = true;
    }
    condition.notify_all();
    for(std::thread& worker: workers) {
        worker.join();
    }
}

#endif