ThreadPool

A thread pool maintains multiple threads waiting for tasks to be allocated for concurrent execution by the supervising program.By maintaining a pool of threads, the model increases performance and avoids latency in execution due to frequent creation and destruction of threads for short-lived tasks.

通过提前创建好若干个线程，然后将这些线程统一管理，放在一个容器中。当有异步任务需要执行的时候，从该容器中取得空闲（当前不处于执行异步任务的状态）的线程，将这个异步任务交给这个线程（唤醒）进行处理。当这个任务执行完成之后，该线程不进行销毁，重新回到空闲状态（阻塞），等待新的任务到来。

避免了线程频繁创建和销毁带来的性能开销。

¶progschj/ThreadPool

class ThreadPool {
public:
    ThreadPool(size_t);
    template<class F, class... Args>
    auto enqueue(F&& f, Args&&... args) 
        -> std::future<typename std::result_of<F(Args...)>::type>;
    ~ThreadPool();
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; // 统一为无参数无返回值的函数
    
    // synchronization
    std::mutex queue_mutex;
    std::condition_variable condition;
    bool stop;
};

// the constructor just launches some amount of workers
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(); });
                        // 等价于，std::wait会检查并避免假醒情况
                        // while (!stop_waiting()) {
                        //     wait(lock);
                        // }
                        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 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()> >( // 这里使用shared_ptr应该是为了保活，否则emplace到tasks中的lambda还没执行，捕获的task地址就被析构调用了
                                                                       // 而如果用拷贝的话，又不是对应前面提前取到的futrue了
                                                                       // 此外，shared_ptr的包装使得packaged_task可以被拷贝和移动
                                                                       // packaged_task本身是不可拷贝，只能移动的，但是std::function又只能接收可拷贝的lambda
            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");

        // packaged_task内部有一个std::promise，在执行完包装的task后会调用promise的set_value，从而future可以通过get拿到结果
        tasks.emplace([task](){ (*task)(); }); // 额外用一个lambda包装成无返回值的
    }
    condition.notify_one();
    return res;
}

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

另一个类似的C++版本：https://github.com/mtrebi/thread-pool