在C++11之前，我们要在程序中新建一个线程是很麻烦的，不过好在现在借助C++11和boost库，我们可以非常方便的以各种姿势往程序里新加几个线程了。本文为你总结了以下3种方式来新建线程：

  • std::thread：c++11提供的线程类，使用它可以很容易的新建和运行一个线程
  • std::async：c++11提供的创建任务的方法，是比std::thread更加高一层的工具，用它既可以创建线程，也能让标准库“智能”的帮你判断是否是新建一个线程还是延迟处
  • boost::asio::thread_pool：通常在需要创建大量线程的时候，使用线程池可以避免线程的重复创建。可惜c++11没有现成的方法创建线程池，我们可以借助boost库来实现。

    std::thread

    ```cpp

    include

    include

    include

    include

std::string GetThreadIdString(const std::thread::id& id) { std::stringstream ss; ss << id; return ss.str(); }

int Mytask(int iNum) { // 这里不使用”cout << A << B << C”而使用”cout << A + B + C”，因为前者是3个cout的步骤，多线程下容易输出混乱，而后者只有一个cout步骤 std::cout << “tid(“+ GetThreadIdString(std::this_thread::get_id()) +”) “ + “do:” + std::to_string(iNum) + “\n”; return iNum; }

//演示std::promise void MyTask2(int iNum, std::promise& pr) { int iRet = Mytask(iNum); pr.set_value_at_thread_exit(iRet); //在线程中对std::promise中的future赋值 }

int main(int argc, char *argv[]) { std::promise pr1; std::promise pr2; std::promise pr3; std::promise xxpr4; std::promise pr5; std::packaged_task task6(Mytask);

  1. std::thread t1(MyTask2, 1, std::ref(pr1));
  2. std::thread t2(MyTask2, 2, std::ref(pr2));
  3. std::thread t3(MyTask2, 3, std::ref(pr3));
  4. std::thread t4(MyTask2, 4, std::ref(pr4));
  5. std::thread t5(MyTask2, 5, std::ref(pr5));
  6. std::thread t6(std::ref(task6), 6);
  8. //t.join(); //阻塞线程池，避免main函数结束导致线程中的任务没运行
  9. std::this_thread::sleep_for(std::chrono::milliseconds(1000)); //阻塞main函数线程1s，给时间让线程池中的任务运行
  11. std::cout << "rst1:" << pr1.get_future().get() << std::endl;
  12. std::cout << "rst2:" << pr2.get_future().get() << std::endl;
  13. std::cout << "rst3:" << pr3.get_future().get() << std::endl;
  14. std::cout << "rst4:" << pr4.get_future().get() << std::endl;
  15. std::cout << "rst5:" << pr5.get_future().get() << std::endl;
  16. std::cout << "rst6:" << task6.get_future().get() << std::endl;
  18. t1.join();
  19. t2.join();
  20. t3.join();
  21. t4.join();
  22. t5.join();
  23. t6.join();

}

  1. 运行结果：<br />![image.png](https://cdn.nlark.com/yuque/0/2022/png/675577/1649152764417-c4c96dda-316d-4639-8ded-33ffceecb099.png#clientId=u679589ee-d091-4&crop=0&crop=0&crop=1&crop=1&from=paste&height=252&id=u2c0ac452&margin=%5Bobject%20Object%5D&name=image.png&originHeight=504&originWidth=1436&originalType=binary&ratio=1&rotation=0&showTitle=false&size=71388&status=done&style=none&taskId=udf6cfaa5-c87b-4113-aefc-0f42ffc4619&title=&width=718)
  2. <a name="oKJXy"></a>
  3. ## std::async
  4. ```cpp
  5. #include <iostream>
  6. #include <thread>
  7. #include <future>
  8. #include <sstream>
  10. std::string GetThreadIdString(const std::thread::id& id)
  11. {
  12.     std::stringstream ss;
  13.     ss << id;
  14.     return ss.str();
  15. }
  17. int Mytask(int iNum)
  18. {
  19.     // 这里不使用"cout << A << B << C"而使用"cout << A + B + C"，因为前者是3个cout的步骤，多线程下容易输出混乱，而后者只有一个cout步骤
  20.     std::cout << "tid("+ GetThreadIdString(std::this_thread::get_id()) +") " + "do:" + std::to_string(iNum) + "\n";
  21.     return iNum;
  22. }
  24. int main(int argc, char *argv[]) {
  26.     std::future<int> fut1 = std::async(std::launch::async, Mytask, 1);
  27.     std::future<int> fut2 = std::async(std::launch::async, Mytask, 2);
  28.     std::future<int> fut3 = std::async(std::launch::async, Mytask, 3);
  29.     std::future<int> fut4 = std::async(std::launch::async, Mytask, 4);
  30.     std::future<int> fut5 = std::async(std::launch::async, Mytask, 5);
  31.     std::future<int> fut6 = std::async(std::launch::async, Mytask, 6);
  33.     std::this_thread::sleep_for(std::chrono::milliseconds(1000)); //阻塞main函数线程1s，给时间让线程池中的任务运行
  35.     std::cout << "rst1:" << fut1.get() << std::endl;
  36.     std::cout << "rst2:" << fut2.get() << std::endl;
  37.     std::cout << "rst3:" << fut3.get() << std::endl;
  38.     std::cout << "rst4:" << fut4.get() << std::endl;
  39.     std::cout << "rst5:" << fut5.get() << std::endl;
  40.     std::cout << "rst6:" << fut6.get() << std::endl;
  41. }

运行结果：
image.png

boost::asio::thread_pool

  1. #include <iostream>
  2. #include <boost/asio.hpp>
  3. #include <boost/asio/thread_pool.hpp>
  4. #include <thread>
  5. #include <sstream>
  7. std::string GetThreadIdString(const std::thread::id& id)
  8. {
  9.     std::stringstream ss;
  10.     ss << id;
  11.     return ss.str();
  12. }
  14. int MyTask(int iNum)
  15. {
  16.     // 这里不使用"cout << A << B << C"而使用"cout << A + B + C"，因为前者是3个cout的步骤，多线程下容易输出混乱，而后者只有一个cout步骤
  17.     std::cout << "tid("+ GetThreadIdString(std::this_thread::get_id()) +") " + "do:" + std::to_string(iNum) + "\n";
  18.     return iNum;
  19. }
  21. int main(int argc, char *argv[]) {
  22.     boost::asio::thread_pool pool(4); //4线程的线程池
  24.     //陆续将任务塞入线程池
  25.     boost::asio::post(pool, std::bind(MyTask, 1));
  26.     boost::asio::post(pool, std::bind(MyTask, 2));
  27.     boost::asio::post(pool, std::bind(MyTask, 3));
  28.     boost::asio::post(pool, std::bind(MyTask, 4));
  29.     boost::asio::post(pool, std::bind(MyTask, 5));
  30.     boost::asio::post(pool, std::bind(MyTask, 6));
  31.     boost::asio::post(pool, std::bind(MyTask, 7));
  32.     boost::asio::post(pool, std::bind(MyTask, 8));
  33.     boost::asio::post(pool, std::bind(MyTask, 9));
  34.     boost::asio::post(pool, std::bind(MyTask, 10));
  36.     //pool.join(); //阻塞线程池，避免main函数结束导致线程中的任务没运行
  37.     std::this_thread::sleep_for(std::chrono::milliseconds(1000)); //阻塞main函数线程1s，给时间让线程池中的任务运行
  39.     pool.join();
  40. }

运行结果：
image.png