C++多线程编程第九讲--async、future、packaged_task、promise

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//(1)std::async、 std::future创建后台任务并返回值 // async是一个函数模板，用来启动一个异步任务，返回一个future类型的对象。用future // 的get方法来获得线程的返回值。 // 异步任务：自动创建一个线程，并开始执行对应线程的入口函数。 #include<iostream> #include<mutex> #include<thread> #include<future>   using namespace std;   int my_thread() {     cout << "my_thread start...  " << "thread id = " << this_thread::get_id() << endl;     std::chrono::milliseconds duro(5000);     std::this_thread::sleep_for(duro);     cout << "my_thread end...  " << "thread id = " << 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;              //主线程会在get这里一致等到新的线程返回结果     //cout << result.get() << endl;            //不能调用多次，调用多次会报异常     //result.wait();  //等待新的线程执行结束，但并不会储存返回值。     cout << "main() end... " << "thread id = " << std::this_thread::get_id() << endl;     return 0; }   #include<iostream> #include<mutex> #include<thread> #include<future>   using namespace std; class A { public:     int my_thread(int mypar)     {         cout << "my_thread start...  " << "thread id = " << this_thread::get_id() << endl;         std::chrono::milliseconds duro(5000);         std::this_thread::sleep_for(duro);         cout << "my_thread end...  " << "thread id = " << this_thread::get_id() << endl;         return mypar;     } };       int main() {     A myobja;     cout << "main() start... " << "thread id = " << std::this_thread::get_id() << endl;     std::future<int> result = std::async(&A::my_thread, std::ref(myobja), 5);     //主线程会在get这里一致等到新的线程返回结果,如果没有这一句的话     //在主线程return的时候，会等待新的线程执行结束。     cout << result.get() << endl;     //cout << result.get() << endl;            //不能调用多次，调用多次会报异常     //result.wait();  //等待新的线程执行结束，但并不会储存返回值。     cout << "main() end... " << "thread id = " << std::this_thread::get_id() << endl;     return 0; }   // 可以额外向std::async()传递一个参数，参数类型是std::lunch类型（枚举）。 // std::launch::deferred  新线程执行时间延迟到调用wait或者get时才执行（创建）   #include<iostream> #include<mutex> #include<thread> #include<future>   using namespace std; class A { public:     int my_thread(int mypar)     {         cout << "my_thread start...  " << "thread id = " << this_thread::get_id() << endl;         std::chrono::milliseconds duro(5000);         std::this_thread::sleep_for(duro);         cout << "my_thread end...  " << "thread id = " << this_thread::get_id() << endl;         return mypar;     } };       int main() {     A myobja;     cout << "main() start... " << "thread id = " << std::this_thread::get_id() << endl;     //新线程延迟到调用wait或者get的时候再启动，但使用std::launch::deferred这个参数     //直接没有创建新的线程，在主线程中调用的入口函数。     std::future<int> result = std::async(std::launch::deferred, &A::my_thread, std::ref(myobja), 5);     //主线程会在get这里一致等到新的线程返回结果,如果没有这一句的话     //在主线程return的时候，会等待新的线程执行结束。     cout << result.get() << endl;                   //在主线程中执行。     //cout << result.get() << endl;            //不能调用多次，调用多次会报异常     //result.wait();  //等待新的线程执行结束，但并不会储存返回值。     cout << "main() end... " << "thread id = " << std::this_thread::get_id() << endl;     return 0; }   // std::launch::async 在调用async这个函数的时候即开始创建新的线程，执行入口函数。默认async方法的第一个参数 //就是std::launch::async // //(2)std::packaged_task   是个模板类，将各种可调用对象包装起来。   #include<iostream> #include<mutex> #include<thread> #include<future>   using namespace std;   int my_thread(int mypar) {     cout << "my_thread start...  " << "thread id = " << this_thread::get_id() << endl;     std::chrono::milliseconds duro(5000);     std::this_thread::sleep_for(duro);     cout << "my_thread end...  " << "thread id = " << this_thread::get_id() << endl;     return mypar; }   int main() {     cout << "main() start... " << "thread id = " << std::this_thread::get_id() << endl;       std::packaged_task<int(int)> mypt(my_thread);           //把函数my_thread包装起来     std::thread t1(std::ref(mypt), 5);                      //用一个package_task对象作为线程的参数       t1.join();       std::future<int> result = mypt.get_future();       cout << result.get() << endl;              //打印返回的结果       cout << "main() end... " << "thread id = " << std::this_thread::get_id() << endl;     return 0; }    #include<iostream> #include<mutex> #include<thread> #include<future>   using namespace std;   int main() {     cout << "main() start... " << "thread id = " << std::this_thread::get_id() << endl;       //包装lamda表达式     std::packaged_task<int(int)> mypt([](int mypar)         {             cout << "my_thread start...  " << "thread id = " << this_thread::get_id() << endl;             std::chrono::milliseconds duro(5000);             std::this_thread::sleep_for(duro);             cout << "my_thread end...  " << "thread id = " << this_thread::get_id() << endl;             return mypar;         }     );       std::thread t1(std::ref(mypt), 5);                      //用一个package_task对象作为线程的参数       t1.join();       std::future<int> result = mypt.get_future();       cout << result.get() << endl;              //打印返回的结果       cout << "main() end... " << "thread id = " << std::this_thread::get_id() << endl;     return 0; }   #include<iostream> #include<mutex> #include<thread> #include<future>   using namespace std;   int main() {     cout << "main() start... " << "thread id = " << std::this_thread::get_id() << endl;       //包装lamda表达式     std::packaged_task<int(int)> mypt([](int mypar)         {             cout << "my_thread start...  " << "thread id = " << this_thread::get_id() << endl;             std::chrono::milliseconds duro(5000);             std::this_thread::sleep_for(duro);             cout << "my_thread end...  " << "thread id = " << this_thread::get_id() << endl;             return mypar;         }     );       mypt(5);            //不创建新的线程，直接调用也可以。       std::future<int> result = mypt.get_future();       cout << result.get() << endl;              //打印返回的结果       cout << "main() end... " << "thread id = " << std::this_thread::get_id() << endl;     return 0; }   #include<iostream> #include<mutex> #include<thread> #include<future> #include<vector>   using namespace std;   vector<std::packaged_task<int(int)>> mytasks;   int main() {     cout << "main() start... " << "thread id = " << std::this_thread::get_id() << endl;       //包装lamda表达式     std::packaged_task<int(int)> mypt([](int mypar)         {             cout << "my_thread start...  " << "thread id = " << this_thread::get_id() << endl;             std::chrono::milliseconds duro(5000);             std::this_thread::sleep_for(duro);             cout << "my_thread end...  " << "thread id = " << this_thread::get_id() << endl;             return mypar;         }     );       mytasks.push_back(std::move(mypt));     //使用移动语义       std::packaged_task<int(int)> mypt2;     auto iter = mytasks.begin();     mypt2 = std::move(*iter);       mytasks.erase(iter);              //迭代器失效，后面不可以使用iter       mypt2(123);     std::future<int> result = mypt2.get_future();     cout << result.get() << endl;         cout << "main() end... " << "thread id = " << std::this_thread::get_id() << endl;     return 0; }   //(3)std::promise // 我们可以在某个线程中给他赋值，在其他线程中把这个值取出来。 #include<iostream> #include<mutex> #include<thread> #include<future> #include<vector> #include<memory>   using namespace std;   void my_thread(std::promise<int>& tmp, int calc) {     cout << "my_thread start..." << endl;     cout << "my_thread id = " << std::this_thread::get_id() << endl;     cout << calc << endl;     std::chrono::milliseconds duro(5000);   //休眠5秒，模拟计算过程     std::this_thread::sleep_for(duro);         int result = calc;       tmp.set_value(result);    //将结果保存到promise的对象中       cout << "my_thread end..." << endl; }   void my_thread2(std::future<int>& tmpf) {     cout << "my_thread2 start..." << endl;     cout << "thread id = " << std::this_thread::get_id() << endl;     cout << tmpf.get() << endl;   //获得另外一个线程传过来的值       cout << "my_thread2 end..." << endl; }   int main() {     cout << "main() start... " << "thread id = " << std::this_thread::get_id() << endl;           std::promise<int> result;     //std::thread t1(my_thread, result, 123);                 //错误     std::thread t1(my_thread, std::ref(result), 123);         //使用ref来传递，不能直接赋值对象。     auto fut = result.get_future();     std::thread t2(my_thread2, std::ref(fut));       t1.join();                                    t2.join();       cout << "main() end... " << "thread id = " << std::this_thread::get_id() << endl;     return 0; }