ThreadPool.hpp

#ifndef THREAD_POOL_HPP
#define THREAD_POOL_HPP

#include <functional>
#include <future>
#include <queue>

class ThreadPool {
public:
    explicit ThreadPool(size_t);
    template<class F, class... Args>
    decltype(auto) enqueue(F&& f, Args&&... args);
    ~ThreadPool();
private:
    // need to keep track of threads so we can join them
    std::vector< std::thread > workers;
    // the task queue
    std::queue< std::packaged_task<void()> > tasks;

    // synchronization
    std::mutex queue_mutex;
    std::condition_variable condition;
    bool stop;
};

// add new work item to the pool
template<class F, class... Args>
decltype(auto) ThreadPool::enqueue(F&& f, Args&&... args)
{
    using return_type = std::invoke_result_t<F, Args...>;

    std::packaged_task<return_type()> task(
            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 ThreadPool");

        tasks.emplace(std::move(task));
    }
    condition.notify_one();
    return res;
}

#endif