C++多线程基础学习笔记（七）

一、std::async和std::future的用法

std::async是一个函数模板，std::future是一个类模板

 #include <iostream>
 #include <thread>
 #include <future>
 #include <Windows.h>

 using namespace std;

 int mythread()
 {
     cout << "my thread:" << std::this_thread::get_id() << " start" << endl;
     int temp = ;
     temp++;
     Sleep();
     cout << "my thread:" << std::this_thread::get_id() << " end" << endl;
     return temp;
 }
 int main()
 {
     cout << "main thread:" << std::this_thread::get_id() << " start" << endl;
     std::future<int> result = std::async(std::launch::async, mythread);
     //std::async(mythread);相当于std::async(std::launch::async|std::lauch::deferred, mythread);
     //自动创建一个线程开始执行线程入口函数，并返回一个std::future对象给result

     cout << result.get() << endl;        //等待入口函数执行，直到拿到返回值，即temp
     //result.wait();                     //只等待入口函数执行完，但不返回结果
     cout << "main thread:" << std::this_thread::get_id() << " end" << endl;
     system("pause");
     return ;
 }

std::async(std::launch::async, mythread);如果std::async（）的第一个参数改成std::lauch::deferred,那么线程不会被马上执行，而是延迟到std::future的wait()或者get()函数调用时才执行线程的入口函数。实际上，并没有创建
子线程，而只是在主函数中调用了入口函数。
二、std::packaged_task的用法

std::packaged_task是一个类模板，模板参数各种可调用对象，通过它把各种可调用对象包装起来，方便作为线程入口函数来调用

 #include <iostream>
 #include <thread>
 #include <future>
 #include <Windows.h>

 using namespace std;

 int fun(int val)
 {
     cout << "my thread:" << std::this_thread::get_id() << " start" << endl;
     val++;
     cout << "my thread:" << std::this_thread::get_id() << " end" << endl;
     return val;
 }
 int main()
 {
     cout << "main thread:" << std::this_thread::get_id() << " start" << endl;
     std::packaged_task<int(int)> pack(fun);
     thread t1(std::ref(pack), );
     t1.join();
     std::future<int> result = pack.get_future();
     cout << result.get() << endl;
     cout << "main thread:" << std::this_thread::get_id() << " end" << endl;
     system("pause");
     return ;
 }

三、std::promise的用法

std::promise是一个类模板，能够在某个线程中给它赋值，然后可以在其他线程中，在将来的某个时刻，可以把这个值取出来。

 #include <iostream>
 #include <thread>
 #include <future>
 #include <Windows.h>

 using namespace std;

 void mythread1(std::promise<int> &pro, int val)
 {
     cout << "my thread1:" << std::this_thread::get_id() << " start" << endl;
     val++;
     val--;
     Sleep();   //假设运算了500毫秒
     int result = val;
     pro.set_value(val);//保存结果
     cout << "my thread1:" << std::this_thread::get_id() << " end" << endl;
 }

 void mythread2(std::future<int> &getful)
 {
     cout << "my thread2:" << std::this_thread::get_id() << " start" << endl;
     auto getval = getful.get();
     cout << getval << endl;
     cout << "my thread2:" << std::this_thread::get_id() << " end" << endl;
 }
 int main()
 {
     cout << "main thread:" << std::this_thread::get_id() << " start" << endl;
     std::promise<int> prom;  //声明一个promise对象，保存类型为int
     thread t1(mythread1, std::ref(prom), );
     t1.join();

     std::future<int> ful = prom.get_future();  //promise和future绑定，用于返回保存的结果

     //在主线程获取结果
     //auto result = ful.get();   //get()只能使用一次，如果主线程中使用了，t2就不能使用了
     //cout << result << endl;

     //在线程t2中获取结果
     thread t2(mythread2, std::ref(ful));
     t2.join();

     cout << "main thread:" << std::this_thread::get_id() << " end" << endl;
     system("pause");
     return ;
 }

std::ref 用于包装按引用传递的值。