简单的C++11线程池实现

线程池的C++11简单实现，源代码来自Github上作者progschj，地址为：A simple C++11 Thread Pool implementation，具体博客可以参见Jakob’s Devlog，地址为：A Thread Pool with C++11

1、线程池的实现代码如下：

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(
         [this]
         {
             for(;;)
             {
                 std::function<void()> task;

                 {
                     std::unique_lock<std::mutex> lock(this->queue_mutex);
                     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
template<class F, class... Args>
auto ThreadPool::enqueue(F&& f, Args&&... args) 
 -> std::future<typename std::result_of<F(Args...)>::type>
{
 using return_type = typename std::result_of<F(Args...)>::type;

 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();
 {
     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();
 return res;
}

// the destructor joins all threads
inline ThreadPool::~ThreadPool()
{
 {
     std::unique_lock<std::mutex> lock(queue_mutex);
     stop = true;
 }
 condition.notify_all();
 for(std::thread &worker: workers)
     worker.join();
}

#endif

2、基本的用法：

// create thread pool with 4 worker threads
ThreadPool pool(4);

// enqueue and store future
auto result = pool.enqueue([](int answer) { return answer; }, 42);

// get result from future
std::cout << result.get() << std::endl;

3、测试C++11线程池的例子程序如下：

example.cpp

#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;
    
    return 0;
}

编译运行example.cpp时需要添加C++11的支持，并且要加入pthread库的链接。
在Linux下的编译的命令为: g++ example.cpp -o example -std=c++11 -lpthread；如果在VS2017中使用，由于默认支持C++11，可以直接创建一个控制台程序，添加ThreadPool.h和example.cpp到项目，编译运行即可。

在Ubuntu18.10下测试如下：

havealex@havealex:~/GithubProjects/ThreadPool$ ls
COPYING  example.cpp  README.md  ThreadPool.h
havealex@havealex:~/GithubProjects/ThreadPool$ g++ example.cpp -o example -std=c++11
/usr/bin/ld: /tmp/ccduRGse.o: in function `std::thread::thread<ThreadPool::ThreadPool(unsigned long)::{lambda()#1}, , void>(ThreadPool::ThreadPool(unsigned long)::{lambda()#1}&&)':
example.cpp:(.text._ZNSt6threadC2IZN10ThreadPoolC4EmEUlvE_JEvEEOT_DpOT0_[_ZNSt6threadC5IZN10ThreadPoolC4EmEUlvE_JEvEEOT_DpOT0_]+0x2f): undefined reference to `pthread_create'
collect2: error: ld returned 1 exit status
havealex@havealex:~/GithubProjects/ThreadPool$ g++ example.cpp -o example -std=c++11 -lpthread
havealex@havealex:~/GithubProjects/ThreadPool$ ls
> ^C
havealex@havealex:~/GithubProjects/ThreadPool$ ls
COPYING  example  example.cpp  README.md  ThreadPool.h
havealex@havealex:~/GithubProjects/ThreadPool$ ./example 
hello 0
hello 1
hello 2
hello 3
world 1
hello 4
world 0
hello 5
0 1 world 2
hello 6
4 world 3
hello 7
9 world 5
world 4
16 25 world 6
36 world 7
49 

可以看出，结果的输出是无序并行的。