c++11之100行实现简单线程池

代码从github上拷的,写了一些理解,如有错误请指正

Threadpool.h

#ifndef THREAD_POOL_H
#define THREAD_POOL_H

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

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;
    
    // 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(                //这里一下启动threads个,即使lambda阻塞(在已启动子线程内的阻塞),主线程还是会循环
            [this]
            {
                for(;;)
                {
                    std::function<void()> task;

                    {
                        std::unique_lock<std::mutex> lock(this->queue_mutex);
                        
                        //当前线程被阻塞, 直到condition.notify_one()调用,如果lambda返回false,wait会解锁互斥元lock并置阻塞或等待状态,如果条件满足互斥元仍被锁定
                        //而这里锁用的是std::unique_lock而不是std::lock_guard,是因为std::lock_guard不能在wait等待中解锁,并在之后重新锁定
                        //如果互斥元在线程休眠期间始终被锁定,enqueue就无法锁定互斥元往下执行,则造成死锁
                        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();
                    
                }
            }
        );
}

// add new work item to the pool            //函数后跟throw()代表不抛出任何异常,跟thorw(...)代表可以抛出任何异常
template<class F, class... Args>            //... Args这个代表不限类型,不限数量
auto ThreadPool::enqueue(F&& f, Args&&... args)                    //&&代表右值引用(可以用常量做参数)
    -> std::future<typename std::result_of<F(Args...)>::type>            //<F(Args...)>代表这个是个返回类型为F,参数不确定多的函数;放入类型(如int)会报错
{
    using return_type = std::result_of<F(Args...)>::type;                //std::result_of::type 获得函数返回类型,直接用decltype会报错

    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();
    {                                                                //这加一个作用域的作用是出了这个作用域就解锁
        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");

        tasks.emplace([task](){ (*task)(); });
    }
    condition.notify_one();        ////选择一个wait状态的线程进行唤醒，并使他获得对象上的锁来完成任务(即其他线程无法访问对象)
    return res;
}

// the destructor joins all threads
inline ThreadPool::~ThreadPool()
{
    {
        std::unique_lock<std::mutex> lock(queue_mutex);
        stop = true;
    }
    condition.notify_all();                        //通知所有wait状态的线程竞争对象的控制权，唤醒所有线程执行
    for(std::thread &worker: workers)
        worker.join();
}

#endif

运行代码

#include "stdafx.h"
#include <iostream>
#include <vector>
#include <chrono>

#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 << "hello " << i << std::endl;
                std::this_thread::sleep_for(std::chrono::seconds(1));
                std::cout << "world " << i << std::endl;
                return i*i;
            })
        );
    }

    for(auto && result: results)
        std::cout << result.get() << ' ';
    std::cout << std::endl;
    system("pause");
    return 0;
}