C++并发与多线程(四)async、future、packaged_task、promise、shared

本文系列大部分来自c++11并发与多线程视频课程的学习笔记，系列文章有(不定期更新维护)：

C++并发与多线程(一)线程传参
C++并发与多线程(二) 创建多个线程、数据共享问题分析、案例代码
C++并发与多线程(三)单例设计模式与共享数据分析、call_once、condition_variable使用
C++并发与多线程(四)async、future、packaged_task、promise、shared_future
C++并发与多线程(五)互斥量，atomic、与线程池

std::async、std::future创建后台任务并返回值

之前，我们用std::thread创建一个线程，用join()等待这个线程结束，如果希望线程返回一个结果呢？

std::async是一个函数模板，用来启动一个异步任务，启动起来一个异步任务之后，它返回一个std::future对象，这个对象也是个类模板。什么叫“启动一个异步任务”？就是自动创建一个线程，并开始执行对应的线程入口函数，它返回一个std::future对象，这个std::future对象中就含有线程入口函数所返回的结果，我们可以通过调用future对象的成员函数get()来获取结果。

“future”将来的意思，也有人称呼std::future提供了一种访问异步操作结果的机制，就是说这个结果你可能没办法马上拿到，但是在不久的将来，这个线程执行完毕的时候，你就能够拿到结果了，所以，大家这么理解：future中保存着一个值，这个值是在将来的某个时刻能够拿到。

#include <iostream>
#include <future>
using namespace std;
int mythread(){
    cout << "mythread() started and the thread id is " << std::this_thread::get_id() << endl;
    std::chrono::milliseconds dura(5000); // 休息五秒
    std::this_thread::sleep_for(dura);
    cout << "mythread() ended and the thread id is " << std::this_thread::get_id() << endl;
    return 5;
}
int main(){
    cout << "main started and the thread id is " << std::this_thread::get_id() << endl;
    std::future<int> res = std::async(mythread); // 创建一个线程并开始执行
    cout << "continue ....." << endl;
    cout << "res.get() is : " << res.get() << endl; // 执行到get的时候，会卡在此行，等待mythread执行完毕。
    cout << "main ended and the thread id is " << std::this_thread::get_id() << endl;
}

程序输出结果为：

main started and the thread id is 0x1000e3d40
continue .....
res.get() is : mythread() started and the thread id is 0x16fe87000
mythread() ended and the thread id is 0x16fe87000
5
main ended and the thread id is 0x1000e3d40

std::future对象的get()成员函数会等待线程执行结束并返回结果，拿不到结果它就会一直等待，感觉有点像join()但是，它是可以获取结果的。而std::future对象的wait()成员函数，用于等待线程返回，本身并不返回结果，这个效果和std::thread的join()更像。

#include <iostream>
#include <future>
using namespace std;
int mythread(){
    cout << "mythread() started and the thread id is " << std::this_thread::get_id() << endl;
    std::chrono::milliseconds dura(5000); // 休息五秒
    std::this_thread::sleep_for(dura);
    cout << "mythread() ended and the thread id is " << std::this_thread::get_id() << endl;
    return 5;
}
int main(){
    cout << "main started and the thread id is " << std::this_thread::get_id() << endl;
    std::future<int> res = std::async(mythread); // 创建一个线程并开始执行
    cout << "continue ....." << endl;
    //cout << "res.get() is : " << res.get() << endl; // 执行到get的时候，会卡在此行，等待mythread执行完毕。
    res.wait(); // 等待线程返回，但是拿不到返回值，类似join。
    cout << "main ended and the thread id is " << std::this_thread::get_id() << endl;
}

同样的，我们可以使用类成员函数作为线程的入口函数：

#include <iostream>
#include <future>
using namespace std;
class A{
public:
    int mythread(int num){
        cout << "mythread() started and the thread id is " << std::this_thread::get_id() << endl;
        cout << "num is: " << num << endl;
        std::chrono::milliseconds dura(5000); // 休息五秒
        std::this_thread::sleep_for(dura);
        cout << "mythread() ended and the thread id is " << std::this_thread::get_id() << endl;
        return 5;
    }
};
int main(){
    A a;
    cout << "main started and the thread id is " << std::this_thread::get_id() << endl;
    std::future<int> res = std::async(&A::mythread, &a, 10); // 第二个参数是对象引用，如果不用引用的话，就会创建一个新的类A的对象。
    cout << "continue ....." << endl;
    cout << "res.get() is : " << res.get() << endl; // 执行到get的时候，会卡在此行，等待mythread执行完毕。
    cout << "main ended and the thread id is " << std::this_thread::get_id() << endl;
}

我们可以通过向std::async()额外传递一个参数，该参数是std::launch类型（枚举类型），来达到一些特殊的目的：

std::lunch::deferred：（defer推迟，延期）表示线程入口函数的调用会被延迟，一直到std::future的wait()或者get()函数被调用时（由主线程调用）才会执行；如果wait()或者get()没有被调用，则不会执行。

#include <iostream>
#include <future>
using namespace std;
class A{
public:
    int mythread(int num){
        cout << "mythread() started and the thread id is " << std::this_thread::get_id() << endl;
        cout << "num is: " << num << endl;
        std::chrono::milliseconds dura(5000); // 休息五秒
        std::this_thread::sleep_for(dura);
        cout << "mythread() ended and the thread id is " << std::this_thread::get_id() << endl;
        return 5;
    }
};
int main(){
    A a;
    cout << "main started and the thread id is " << std::this_thread::get_id() << endl;
    std::future<int> res = std::async(std::launch::deferred,&A::mythread, &a, 10); // 第二个参数是对象引用，如果不用引用的话，就会创建一个新的类A的对象。
    cout << "continue ....." << endl;
    cout << "res.get() is : " << res.get() << endl; // 执行到get的时候，会卡在此行，等待mythread执行完毕。
    cout << "main ended and the thread id is " << std::this_thread::get_id() << endl;
}

程序输出结果为：

main started and the thread id is 0x1000e7d40
continue .....
res.get() is : mythread() started and the thread id is 0x1000e7d40
num is: 10
mythread() ended and the thread id is 0x1000e7d40
5
main ended and the thread id is 0x1000e7d40
Program ended with exit code: 0

可以看到主线程id为0x1000e7d40，子线程id同样为0x1000e7d40，也就是说，实际上根本就没有创建新线程。std::lunch::deferred意思时延迟调用，并没有创建新线程，是在主线程中调用的线程入口函数。上述代码永远都会先打印出continue…，然后才会打印出mythread() start和mythread() end等信息。

std::launch::async，在调用async函数的时候就开始创建新线程。

#include <iostream>
#include <future>
using namespace std;
class A{
public:
    int mythread(int num){
        cout << "mythread() started and the thread id is " << std::this_thread::get_id() << endl;
        cout << "num is: " << num << endl;
        std::chrono::milliseconds dura(5000); // 休息五秒
        std::this_thread::sleep_for(dura);
        cout << "mythread() ended and the thread id is " << std::this_thread::get_id() << endl;
        return 5;
    }
};
int main(){
    A a;
    cout << "main started and the thread id is " << std::this_thread::get_id() << endl;
    std::future<int> res = std::async(std::launch::async,&A::mythread, &a, 10); // 第二个参数是对象引用，如果不用引用的话，就会创建一个新的类A的对象。
    cout << "continue ....." << endl;
    cout << "res.get() is : " << res.get() << endl; // 执行到get的时候，会卡在此行，等待mythread执行完毕。
    cout << "main ended and the thread id is " << std::this_thread::get_id() << endl;
}

程序输出结果为：

main started and the thread id is 0x1000e7d40
continue .....
res.get() is : mythread() started and the thread id is 0x16fe87000
num is: 10
mythread() ended and the thread id is 0x16fe87000
5
main ended and the thread id is 0x1000e7d40
Program ended with exit code: 0

std::packaged_task

std::packaged_task打包任务，把任务包装起来。也是一个类模板，它的模板参数是各种可调用对象，通过packaged_task把各种可调用对象包装起来，方便将来作为线程入口函数来调用。

#include <iostream>
#include <future>
using namespace std;
int mythread(int num){
    cout << "mythread() started and the thread id is " << std::this_thread::get_id() << endl;
    cout << "num is: " << num << endl;
    std::chrono::milliseconds dura(5000); // 休息五秒
    std::this_thread::sleep_for(dura);
    cout << "mythread() ended and the thread id is " << std::this_thread::get_id() << endl;
    return 5;
}
int main(){
    cout << "main started and the thread id is " << std::this_thread::get_id() << endl;
    //我们把函数mythread通过packaged_task包装起来。参数是一个int，返回值类型是int
    std::packaged_task<int(int)> mypt(mythread);
    std::thread mythread(std::ref(mypt), 10); // 线程直接开始执行，第二个参数为线程入口参数
    mythread.join();
    //std::future对象里包含有线程入口函数的返回结果，这里result保存mythread返回的结果。
    std::future<int> res = mypt.get_future();
    cout << "res.get() is : " << res.get() << endl;
    cout << "main ended and the thread id is " << std::this_thread::get_id() << endl;
}

输出结果为：

main started and the thread id is 0x1000e3d40
mythread() started and the thread id is 0x16fe87000
num is: 10
mythread() ended and the thread id is 0x16fe87000
res.get() is : 5
main ended and the thread id is 0x1000e3d40
Program ended with exit code: 0

可调用对象可由函数换成lambda表达式：

#include <iostream>
#include <future>
using namespace std;
int main(){
    cout << "main started and the thread id is " << std::this_thread::get_id() << endl;
    //我们把函数mythread通过packaged_task包装起来。参数是一个int，返回值类型是int
    std::packaged_task<int(int)> mypt([](int num){
        cout << "mythread() started and the thread id is " << std::this_thread::get_id() << endl;
        cout << "num is: " << num << endl;
        std::chrono::milliseconds dura(5000); // 休息五秒
        std::this_thread::sleep_for(dura);
        cout << "mythread() ended and the thread id is " << std::this_thread::get_id() << endl;
        return 5;
    });
    std::thread mythread(std::ref(mypt), 10); // 线程直接开始执行，第二个参数为线程入口参数
    mythread.join();
    //std::future对象里包含有线程入口函数的返回结果，这里result保存mythread返回的结果。
    std::future<int> res = mypt.get_future();
    cout << "res.get() is : " << res.get() << endl;
    cout << "main ended and the thread id is " << std::this_thread::get_id() << endl;
}

输出结果为：

main started and the thread id is 0x1000e3d40
mythread() started and the thread id is 0x16fe87000
num is: 10
mythread() ended and the thread id is 0x16fe87000
res.get() is : 5
main ended and the thread id is 0x1000e3d40
Program ended with exit code: 0

C++并发与多线程(四)async、future、packaged_task、promise、shared_future（上）

std::async、std::future创建后台任务并返回值

std::packaged_task

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C++并发与多线程(四)async、future、packaged_task、promise、shared_future（上）

std::async、std::future创建后台任务并返回值

std::packaged_task

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