写一个C++ 线程池

好的，下面是一个简单的 C++ 线程池的实现：

#include <iostream>
#include <queue>
#include <thread>
#include <mutex>
#include <condition_variable>
#include <functional>

class ThreadPool {
public:
    ThreadPool(size_t num_threads) {
        for (size_t i = 0; i < num_threads; ++i) {
            threads_.emplace_back([this] {
                while (true) {
                    std::function<void()> task;
                    {
                        std::unique_lock<std::mutex> lock(mutex_);
                        condition_.wait(lock, [this] {
                            return !tasks_.empty() || stop_;
                        });
                        if (stop_ && tasks_.empty()) {
                            return;
                        }
                        task = std::move(tasks_.front());
                        tasks_.pop();
                    }
                    task();
                }
            });
        }
    }
    
    ~ThreadPool() {
        {
            std::unique_lock<std::mutex> lock(mutex_);
            stop_ = true;
        }
        condition_.notify_all();
        for (std::thread& thread : threads_) {
            thread.join();
        }
    }
    
    template<typename F, typename... Args>
    void enqueue(F&& f, Args&&... args) {
        {
            std::unique_lock<std::mutex> lock(mutex_);
            tasks_.emplace(std::bind(std::forward<F>(f), std::forward<Args>(args)...));
        }
        condition_.notify_one();
    }

private:
    std::vector<std::thread> threads_;
    std::queue<std::function<void()>> tasks_;
    std::mutex mutex_;
    std::condition_variable condition_;
    bool stop_ = false;
};

int main() {
    ThreadPool pool(4);
    for (int i = 0; i < 8; ++i) {
        pool.enqueue([](int id) {
            std::cout << "Task " << id << " is running." << std::endl;
        }, i);
    }
    return 0;
}

这个线程池使用一个队列来存储任务，每个线程都会不停地从队列中取出任务并执行。当队列为空时，线程会等待直到有新的任务加入。当线程池被销毁时，所有线程都会退出。我们可以通过调用 `enqueue` 方法向线程池中添加任务。