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 = 0;
    temp++;
    Sleep(5000);   
    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 0;
}

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), 0);
    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 0;
}

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(500);   //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), 10);
    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 0;
}

std::ref 用于包装按引用传递的值。