簡介
執行緒池（thread pool）：一種執行緒的使用模式，執行緒過多會帶來排程開銷，進而影響快取區域性性和整體效能。而執行緒池維護著多個執行緒，等待著監督管理者分配可併發執行的任務。這避免了在處理短時間任務時建立與銷燬執行緒的代價。執行緒池不僅能夠保證核心的充分利用，還能防止過分排程。可用執行緒數量應該取決於可用的併發處理器、處理器核心、記憶體、網路sockets等的數量。
執行緒池的組成
1、執行緒池管理器
　　建立一定數量的執行緒，啟動執行緒，調配任務，管理著執行緒池。
　　本篇執行緒池目前只需要啟動(start())，停止方法(stop())，及任務新增方法(addTask).
　　start()建立一定數量的執行緒池,進行執行緒迴圈.

程式碼實現：

ThreadPool.hpp：

#ifndef _THREAD_POOL_H_
#define _THREAD_POOL_H_

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

class ThreadPool
{
public:
    using Task = std::function<void()>;

    explicit ThreadPool(int num) : _thread_num(num), _is_running(false
 
)
    {}

    ~ThreadPool()
    {
        if (_is_running)
            stop();
    }

    void start()
    {
        _is_running = true;

        // start threads
        for (int i = 0; i < _thread_num; i++)
            _threads.emplace_back(std::thread(&ThreadPool::work, this));
    }

    void stop()
    {
        {
            // stop thread pool, should notify all threads to wake
            std::unique_lock<std::mutex> lk(_mtx);
            _is_running = false;
            _cond.notify_all(); // must do this to avoid thread block
        }

        // terminate every thread job
        for (std::thread& t : _threads)
        {
            if (t.joinable())
                t.join();
        }
    }

    void appendTask(const Task& task)
    {
        if (_is_running)
        {
            std::unique_lock<std::mutex> lk(_mtx);
            _tasks.push(task);
            _cond.notify_one(); // wake a thread to to the task
        }
    }

private:
    void work()
    {
        printf("begin work thread: %d\n", std::this_thread::get_id());

        // every thread will compete to pick up task from the queue to do the task
        while (_is_running)
        {
            Task task;
            {
                std::unique_lock<std::mutex> lk(_mtx);
                if (!_tasks.empty())
                {
                    // if tasks not empty, 
                    // must finish the task whether thread pool is running or not
                    task = _tasks.front();
                    _tasks.pop(); // remove the task
                }
                else if (_is_running && _tasks.empty())
                    _cond.wait(lk);
            }

            if (task)
                task(); // do the task
        }

        printf("end work thread: %d\n", std::this_thread::get_id());
    }

public:
    // disable copy and assign construct
    ThreadPool(const ThreadPool&) = delete;
    ThreadPool& operator=(const ThreadPool& other) = delete;

private:
    bool _is_running; // thread pool manager status
    std::mutex _mtx;
    std::condition_variable _cond;
    int _thread_num;
    std::vector<std::thread> _threads;
    std::queue<Task> _tasks;
};


#endif // !_THREAD_POOL_H_

main.cpp

#include "stdafx.h"
#include <iostream>
#include <chrono>
#include "ThreadPool.hpp"

void fun1()
{
    std::cout << "working in thread " << std::this_thread::get_id() << std::endl;
}

void fun2(int x)
{
    std::cout << "task " << x << " working in thread " << std::this_thread::get_id() << std::endl;
}

int main(int argc, char* argv[])
{
    ThreadPool thread_pool(3);
    thread_pool.start();
    std::this_thread::sleep_for(std::chrono::milliseconds(500));

    for (int i = 0; i < 6; i++)
    {
        //thread_pool.appendTask(fun1);
        thread_pool.appendTask(std::bind(fun2, i));
        //std::this_thread::sleep_for(std::chrono::milliseconds(500));
    }

    thread_pool.stop();

    getchar();
    return 0;
}