C++多线程编程第十一讲--std::atomic续谈、std::async深入谈

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//(1)原子操作std::atomic续谈 // 原子操作针对++，--，+=，-=，&=，|=，~=是支持的，其他的可能不支持。   #include<iostream> #include<mutex> #include<thread> #include<future>   using namespace std;     //int g_mycount = 0; std::atomic<int> g_mycount = 0;   //具备原子操作的整型值   void my_thread() {     int i = 0;     for (; i < 1000000; ++i)     {         g_mycount++;         //g_mycount += 1;  正确         //g_mycount = g_mycount + 1;   //结果错误     } }     int main() {     cout << "main() start... " << "thread id = " << std::this_thread::get_id() << endl;       thread mythread(my_thread);       thread mythread2(my_thread);       mythread.join();     mythread2.join();       cout << g_mycount << endl;     cout << "main() end... " << "thread id = " << std::this_thread::get_id() << endl;     return 0; }   //(2)std::async深入谈 // std::async和thread最大的不同就是std::async可能不创建新的线程，具体的执行动作可以由参数决定。     //(2.1) std::async参数详解     // std::launch::deferred  延迟调用,不会创建一个新的线程     // std::launch::async    立即创建一个线程     // 如果什么也不写，默认值是std::launch::async | std::launch::deferred     // 系统会根据实际情况选择其中之一来执行。系统资源紧缺时，会使用std::launch::deferred，因为不需要创建新的线程；     // 如果系统资源宽松时，就会选择std::launch::async，创建一个新的线程来执行代码。     //(2.2) std::async和std::thread的区别     // 在资源紧张时，使用thread来创建线程，如果发生错误，程序会直接崩溃。     // 这个时候如果使用std::async就可能不创建新的线程。     // std::async也很容易拿到线程的返回值。   #include<iostream>     #include<mutex>     #include<thread>     #include<future>           using namespace std;                 int my_thread()     {         cout << "my_thread start... " << "thread id = " << std::this_thread::get_id() << endl;         std::chrono::milliseconds dura(3000);         std::this_thread::sleep_for(dura);         cout << "my_thread end... " << "thread id = " << std::this_thread::get_id() << endl;               return 5;     }                 int main()     {         cout << "main() start... " << "thread id = " << std::this_thread::get_id() << endl;               std::future<int> result = std::async(my_thread);               cout << result.get() << endl;               cout << "main() end... " << "thread id = " << std::this_thread::get_id() << endl;         return 0;     }   //(2.3) std::async不确定性问题的解决     //可以用std::future的wait_for方法来判断线程到底有没有创建新的线程   #include<iostream> #include<mutex> #include<thread> #include<future>   using namespace std;     int my_thread() {     cout << "my_thread start... " << "thread id = " << std::this_thread::get_id() << endl;     std::chrono::milliseconds dura(3000);     std::this_thread::sleep_for(dura);     cout << "my_thread end... " << "thread id = " << std::this_thread::get_id() << endl;       return 5; }     int main() {     cout << "main() start... " << "thread id = " << std::this_thread::get_id() << endl;       std::future<int> result = std::async(my_thread);       // std::future_status status = result.wait_for(0s);     std::future_status status = result.wait_for(std::chrono::seconds(0));   //等待0秒      if(status == std::future_status::deferred)     {         cout << "no thread created..." << endl;     }      else      {          cout << "new thread start..." << endl;      }       cout << result.get() << endl;       cout << "main() end... " << "thread id = " << std::this_thread::get_id() << endl;     return 0; }