c++ 写一个线程池类并附带例程

好的，下面是线程池类的代码和附带的例程：

代码：

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

class ThreadPool {
public:
    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<class F, class... Args>
    auto 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;
    }

    ~ThreadPool() {
        {
            std::unique_lock<std::mutex> lock(queue_mutex);
            stop = true;
        }
        condition.notify_all();
        for (std::thread &worker : workers)
            worker.join();
    }

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

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

例程：

#include <iostream>
#include <chrono>
#include <thread>
#include "ThreadPool.h"

int main() {
    ThreadPool pool(4);

    std::vector<std::future<int>> results;

    for (int i = 0; i < 8; ++i) {
        results.emplace_back(
                pool.enqueue([i] {
                    std::cout << "task " << i << " started\n";
                    std::this_thread::sleep_for(std::chrono::seconds(1));
                    std::cout << "task " << i << " finished\n";
                    return i * i;
                })
        );
    }

    for (auto && result : results)
        std::cout << result.get() << ' ';
    std::cout << std::endl;

    return 0;
}

这个例程中，我们定义了一个线程池，使用大小为4的线程池，并在其中运行8个任务，每个任务都是一个延时1秒的函数，每个任务完成后返回其自身的平方值。运行完毕后，打印输出这8个任务的结果。