ThreadPool.h

#ifndef THREAD_POOL_H
#define THREAD_POOL_H

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

class ThreadPool {
public:
    ThreadPool(size_t);
    template <class F, class... Args>
    // 这里返回值要指明是一个future类型的对象
    // 这里result_of可以用于推导返回值类型
    // 在比较新的C++版本里可以用invoke_result代替！
    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;

    // 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)
        // 每个线程都去任务队列中取任务：只要不为空就取任务执行
        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;
                        // 到此就能保证任务队列里有任务，并且线程池未被停止
                        task = std::move(this->tasks.front());
                        // 删除队列第一个数据
                        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;

    // 这里用packaged_task，类型为函数类型，返回类型是return_type，输入参数是空，因为bind存在
    // 首先通过bind将函数和参数绑定在一起，成为一个function对象
    // 用该对象初始化一个packaged_task对象，并用智能指针管理生命周期
    auto task = std::make_shared<std::packaged_task<return_type()>>(std::bind(std::forward<F>(f), std::forward<Args>(args)...));

    // 这里就是获取返回值的关键！
    // 通过future联系上packaged_task
    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");

        // 将函数调用封装成匿名函数传入
        tasks.emplace([task]() { (*task)(); });
    }
    condition.notify_one();
    // 返回的是一个future类型！
    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();
}

#endif