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 = ;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();
     std::vector< std::future<int> > results;

     for(int i = ; i < ; ++i) {
         results.emplace_back(
             pool.enqueue([i] {
                 std::cout << "hello " << i << std::endl;
                 std::this_thread::sleep_for(std::chrono::seconds());
                 std::cout << "world " << i << std::endl;
                 return i*i;
             })
         );
     }

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