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

 1 #include <iostream>
 2 #include <thread>
 3 #include <future>
 4 #include <Windows.h>
 5 
 6 using namespace std;
 7 
 8 int mythread()
 9 {
10     cout << "my thread:" << std::this_thread::get_id() << " start" << endl;
11     int temp = 0;
12     temp++;
13     Sleep(5000);   
14     cout << "my thread:" << std::this_thread::get_id() << " end" << endl;
15     return temp;
16 }
17 int main()
18 {
19     cout << "main thread:" << std::this_thread::get_id() << " start" << endl;
20     std::future<int> result = std::async(std::launch::async, mythread);
21     //std::async(mythread);相当于std::async(std::launch::async, mythread);
22     //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
23 
24     cout << result.get() << endl;        //等待入口函数执行，直到拿到返回值，即temp
25     //result.wait();                     //只等待入口函数执行完，但不返回结果
26     cout << "main thread:" << std::this_thread::get_id() << " end" << endl;
27     system("pause");
28     return 0;
29 }

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

 1 #include <iostream>
 2 #include <thread>
 3 #include <future>
 4 #include <Windows.h>
 5 
 6 using namespace std;
 7 
 8 int fun(int val)
 9 {
10     cout << "my thread:" << std::this_thread::get_id() << " start" << endl;
11     val++;
12     cout << "my thread:" << std::this_thread::get_id() << " end" << endl;
13     return val;
14 }
15 int main()
16 {
17     cout << "main thread:" << std::this_thread::get_id() << " start" << endl;
18     std::packaged_task<int(int)> pack(fun);
19     thread t1(std::ref(pack), 0);
20     t1.join();
21     std::future<int> result = pack.get_future();
22     cout << result.get() << endl;
23     cout << "main thread:" << std::this_thread::get_id() << " end" << endl;
24     system("pause");
25     return 0;
26 }

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。

 1 #include <iostream>
 2 #include <thread>
 3 #include <future>
 4 #include <Windows.h>
 5 
 6 using namespace std;
 7 
 8 void mythread1(std::promise<int> &pro, int val)
 9 {
10     cout << "my thread1:" << std::this_thread::get_id() << " start" << endl;
11     val++;
12     val--;
13     Sleep(500);   //假设运算了500毫秒
14     int result = val;
15     pro.set_value(val);//保存结果
16     cout << "my thread1:" << std::this_thread::get_id() << " end" << endl;
17 }
18 
19 
20 void mythread2(std::future<int> &getful)
21 {
22     cout << "my thread2:" << std::this_thread::get_id() << " start" << endl;
23     auto getval = getful.get();
24     cout << getval << endl;
25     cout << "my thread2:" << std::this_thread::get_id() << " end" << endl;
26 }
27 int main()
28 {
29     cout << "main thread:" << std::this_thread::get_id() << " start" << endl;
30     std::promise<int> prom;  //声明一个promise对象，保存类型为int
31     thread t1(mythread1, std::ref(prom), 10);
32     t1.join();
33 
34     std::future<int> ful = prom.get_future();  //promise和future绑定，用于返回保存的结果
35 
36     //在主线程获取结果
37     //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
38     //cout << result << endl;
39 
40     //在线程t2中获取结果
41     thread t2(mythread2, std::ref(ful));
42     t2.join();
43 
44     cout << "main thread:" << std::this_thread::get_id() << " end" << endl;
45     system("pause");
46     return 0;
47 }

std::ref 用于包装按引用传递的值。