作为一个c++菜鸟，研究半天这个代码的实现原理，发现好多语法不太熟悉，因此加了一大堆注释，仅供参考。该段代码主要通过继承workthread类来实现自己的线程代码，通过thread_pool类来管理线程池，线程池不能够实现动态改变线程数目，存在一定局限性。目前可能还有缺陷，毕竟c++来封装这个东西，资源释放什么的必须想清楚，比如vector存储了基类指针实现多态，那么如何释放对象仍需要考虑，后续我可能会更进一步修改完善该代码，下面贡献一下自己的劳动成果。

#include <pthread.h>
#include <semaphore.h>
#include <iostream>
#include <vector>

using namespace std;
/*
WorkerThread class
This class needs to be sobclassed by the user.
*/
class WorkerThread{
public:
    int id;
    unsigned virtual executeThis()
	{
		return 0;
	}

    WorkerThread(int id) : id(id) {}
    virtual ~WorkerThread(){}
};

/*
ThreadPool class manages all the ThreadPool related activities. This includes keeping track of idle threads and synchronizations between all threads.
*/
class ThreadPool{
public:
    ThreadPool();
    ThreadPool(int maxThreadsTemp);
    virtual ~ThreadPool();
	
	void destroyPool(int maxPollSecs);

    bool assignWork(WorkerThread *worker);
    bool fetchWork(WorkerThread **worker);

	void initializeThreads();
	
    static void *threadExecute(void *param); // pthread_create()调用的函数必须为静态的 
    static pthread_mutex_t mutexSync;
    static pthread_mutex_t mutexWorkCompletion;//工作完成个数互斥量
	
    
private:
    int maxThreads;
    
    pthread_cond_t  condCrit;
    sem_t availableWork;
    sem_t availableThreads;

    vector<WorkerThread *> workerQueue;

    int topIndex;
    int bottomIndex;
	int incompleteWork;
    int queueSize;
};

#include <stdlib.h>
#include "threadpool.h"
using namespace std;


//初始化类的静态成员必须加上类型和作用域，static数据成员必须在类定义体的外部定义，不像不同数据成员可以用构造函数初始化
//应该在定义时进行初始化，注意是定义，这个定义应该放在包含类的非内联成员函数定义的文件中。
//注：静态成员函数只能使用静态变量，非静态没有限制，静态变量必须在外部定义和初始化，没初始化就为默认数值
pthread_mutex_t ThreadPool::mutexSync = PTHREAD_MUTEX_INITIALIZER;
pthread_mutex_t ThreadPool::mutexWorkCompletion = PTHREAD_MUTEX_INITIALIZER;


ThreadPool::ThreadPool()
{
	ThreadPool(2);
}

ThreadPool::ThreadPool(int maxThreads)
{
   if (maxThreads < 1)  
       maxThreads=1;
   
   pthread_mutex_lock(&mutexSync);
   this->maxThreads = maxThreads;
   this->queueSize = maxThreads;
   workerQueue.resize(maxThreads, NULL);//调整容器大小，然后用默认构造函数初始化新的空间
   topIndex = 0;
   bottomIndex = 0;
   incompleteWork = 0;
   sem_init(&availableWork, 0, 0); //工作队列信号量，表示已经加入队列的工作，初始时没有工作
   sem_init(&availableThreads, 0, queueSize);  //空闲线程信号量，一开始就有quisize个线程可以使用
   pthread_mutex_unlock(&mutexSync);
}

//调用pthread_create()让线程跑起来，threadExecute是类的静态函数，因为pthread_create()第三个参数必须为静态函数
void ThreadPool::initializeThreads()
{
   for(int i = 0; i<maxThreads; ++i)
	{
		pthread_t tempThread;
		pthread_create(&tempThread, NULL, ThreadPool::threadExecute, (void*)this );
   }
}

ThreadPool::~ThreadPool()
{
	//因为对于vector，clear并不真正释放内存(这是为优化效率所做的事)，clear实际所做的是为vector中所保存的所有对象调用析构函数(如果有的话),
	//然后初始化size这些东西，让你觉得把所有的对象清除了。。。
    //真正释放内存是在vector的析构函数里进行的，所以一旦超出vector的作用域（如函数返回)，首先它所保存的所有对象会被析构，
	//然后会调用allocator中的deallocate函数回收对象本身的内存。。。
    workerQueue.clear();
}


void ThreadPool::destroyPool(int maxPollSecs = 2)
{
	while(incompleteWork>0 )
	{
	    //cout << "Work is still incomplete=" << incompleteWork << endl;
		sleep(maxPollSecs);
	}
	cout << "All Done!! Wow! That was a lot of work!" << endl;
	sem_destroy(&availableWork);
	sem_destroy(&availableThreads);
    pthread_mutex_destroy(&mutexSync);
    pthread_mutex_destroy(&mutexWorkCompletion);

}

//分配人物到top，然后通知有任务需要执行。
bool ThreadPool::assignWork(WorkerThread *workerThread)
{
    pthread_mutex_lock(&mutexWorkCompletion);
    incompleteWork++;
		//cout << "assignWork...incomapleteWork=" << incompleteWork << endl;
	pthread_mutex_unlock(&mutexWorkCompletion);
	sem_wait(&availableThreads);
	pthread_mutex_lock(&mutexSync);
    //workerVec[topIndex] = workerThread;
    workerQueue[topIndex] = workerThread;
    //cout << "Assigning Worker[" << workerThread->id << "] Address:[" << workerThread << "] to Queue index [" << topIndex << "]" << endl;
	if(queueSize !=1 )
		topIndex = (topIndex+1) % (queueSize-1);
    sem_post(&availableWork);
	pthread_mutex_unlock(&mutexSync);
	return true;
}

//当已经有人物放到队列里面后，就会受到通知，然后从底部拿走工作，在workerArg中返回
bool ThreadPool::fetchWork(WorkerThread **workerArg)
{
	sem_wait(&availableWork);

	pthread_mutex_lock(&mutexSync);
	WorkerThread * workerThread = workerQueue[bottomIndex];
    workerQueue[bottomIndex] = NULL;
	*workerArg = workerThread;
	if(queueSize !=1 )
		bottomIndex = (bottomIndex+1) % (queueSize-1);
	sem_post(&availableThreads);
	pthread_mutex_unlock(&mutexSync);
    return true;
}

//每个线程运行的静态函数实体，executeThis 方法将会被继承累从写，之后实现具体线程的工作。
void *ThreadPool::threadExecute(void *param)
{
	WorkerThread *worker = NULL;
	while(((ThreadPool *)param)->fetchWork(&worker))
	{
		if(worker)
        {
			worker->executeThis();
            //cout << "worker[" << worker->id << "]\tdelete address: [" << worker << "]" << endl;
            delete worker;
            worker = NULL;
        }

		pthread_mutex_lock( &(((ThreadPool *)param)->mutexWorkCompletion) );
        //cout << "Thread " << pthread_self() << " has completed a Job !" << endl;
	 	((ThreadPool *)param)->incompleteWork--;
		pthread_mutex_unlock( &(((ThreadPool *)param)->mutexWorkCompletion) );
	}
	return 0;
}

#include <iostream>
#include "threadpool.h"

using namespace std;


#define ITERATIONS 20

class SampleWorkerThread : public WorkerThread
{
public:
    int id;
	unsigned virtual executeThis()
	{
	// Instead of sleep() we could do anytime consuming work here.
	// Using ThreadPools is advantageous only when the work to be done is really time consuming. (atleast 1 or 2 seconds)
		cout<<"This is SampleWorkerThread sleep 2s"<<endl;
		sleep(2);
		return(0);
	}

    SampleWorkerThread(int id) : WorkerThread(id), id(id)
    {
//       cout << "Creating SampleWorkerThread " << id << "\t address=" << this << endl;
    }

    ~SampleWorkerThread()
    {
//       cout << "Deleting SampleWorkerThread " << id << "\t address=" << this << endl;
    }
};


int main(int argc, char **argv)
{
	
	cout<<"Thread pool"<<endl;
	ThreadPool* myPool = new ThreadPool(25);
	//pthread_create()执行，开始等待任务分配
	myPool->initializeThreads();

	//用来计算时间间隔。
    time_t t1=time(NULL);

	//分配具体工作到线程池
	for(unsigned int i=0;i<ITERATIONS;i++){
		SampleWorkerThread* myThreathreadExecuted = new SampleWorkerThread(i);
		myPool->assignWork(myThreathreadExecuted);
	}
	
	//销毁钱等待所有线程结束，等待间隔为2秒。
    myPool->destroyPool(2);

    time_t t2=time(NULL);
    cout << t2-t1 << " seconds elapsed\n" << endl;
	delete myPool;
	
    return 0;
}

linux 下c++线程池的简单实现（在老外代码上添加注释）