thread_pool.cpp

#include <functional>
#include <future>
#include <thread>
#include <vector>
#include <queue>

class ThreadPool {
  public:
    explicit ThreadPool(
        size_t thread_count = std::thread::hardware_concurrency())
        : stop_(false) {
        for (size_t i = 0; i < thread_count; ++i) {
            workers_.emplace_back([this] {
                while (true) {
                    std::function<void()> task;
                    {
                        std::unique_lock<std::mutex> lock(queue_mutex_);
                        condition_.wait(
                            lock, [this] { return stop_ || !tasks_.empty(); });
                        if (stop_ && tasks_.empty())
                            return;
                        task = std::move(tasks_.front());
                        tasks_.pop();
                    }
                    task();
                }
            });
        }
    }

    template <typename F, typename... Args>
    auto enqueue(F &&f, Args &&...args) -> std::future<decltype(f(args...))> {
        using return_type = decltype(f(args...));

        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;
    }

    ~ThreadPool() {
        {
            std::unique_lock<std::mutex> lock(queue_mutex_);
            stop_ = true;
        }
        condition_.notify_all();
        for (std::thread &worker : workers_) {
            if (worker.joinable())
                worker.join();
        }
    }

  private:
    std::vector<std::thread> workers_;
    std::queue<std::function<void()>> tasks_;
    std::mutex queue_mutex_;
    std::condition_variable condition_;
    bool stop_;
};