ThreadPool.h

//
// Created by erick on 12/24/18.
// code from https://github.com/progschj/ThreadPool/blob/master/ThreadPool.h
//

#ifndef SERVER_THREADPOOL_H
#define SERVER_THREADPOOL_H

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

class ThreadPool {
public:
    explicit ThreadPool(size_t);

    template<typename F, typename ...Args>
    auto enqueue(F &&f, Args &&... args)
    -> std::future<typename std::result_of<F(Args...)>::type>;

    ~ThreadPool();

private:
    std::vector<std::thread> workers;
    std::queue<std::function<void()>> tasks;

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

inline ThreadPool::ThreadPool(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();
                    }
                }
        );
    }
}

template<typename F, typename ...Args>
auto ThreadPool::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);
        if (stop) {
            throw std::runtime_error("enqueue on stopped ThreadPool");
        }
        tasks.emplace([task]() { (*task)(); });
    }
    condition.notify_one();
    return res;
}

inline ThreadPool::~ThreadPool() {
    {
        std::unique_lock<std::mutex> lock(queue_mutex);
        stop = true;
    }
    condition.notify_all();
    std::for_each(workers.begin(), workers.end(),
                  std::mem_fn(&std::thread::join));
}

#endif //SERVER_THREADPOOL_H