55 C++ 多线程返回值问题。引出的 async,future,packaged

一前提，thread返回值的写法

在之前的代码中，我们并没有讨论子线程的返回值问题。

这一章就考虑这个问题怎么处理。

下面我们先按照之前的写法，我们需要返回值时的可能的fix方案。

//如果线程有返回值，并且主线程要得到这个返回值，

//方案一：使用全局变量去接这个子线程的返回值。然后在main函数中取。


//如果线程有返回值，并且主线程要得到这个返回值，

//方案一：使用全局变量去接这个子线程的返回值。然后在main函数中取。
int teacher176readfuncreturnnumber = -1;
int teacher176writefuncreturnnumber = -1;

class Teacher176 {
public:
	mutex mymutex;
	list<int> mylist;

public:
	int readfunc(string &tempstr) {
		for (size_t i = 0; i < 10; i++)
		{
			mymutex.lock();
			cout << "read func tempstr = " << tempstr << "  i = " << i << " threadid = " << this_thread::get_id()<<  "  &tempstr = " << &tempstr <<  "  tempstr = " << tempstr << endl;
			mymutex.unlock();
		}
		int tempreturnvalue = 10;
		teacher176readfuncreturnnumber = tempreturnvalue;

		return tempreturnvalue;
	}

	int writefunc(double &tempdouble) {
		for (size_t i = 0; i < 10; i++)
		{
			mymutex.lock();
			cout << "write func tempdouble = " << tempdouble << "  i = " << i <<" threadid = " << this_thread::get_id() <<  "  &tempdouble = " << &tempdouble <<"   tempdouble = " << tempdouble << endl;
			mymutex.unlock();
		}
		tempdouble = tempdouble * 2;
		teacher176writefuncreturnnumber = (int)tempdouble;
		return (int)tempdouble;
	}
};

void main() {
	cout << "main thread start " << endl;
	Teacher176 tea;
	string name = "nihao";
	double dou = 60.8;
	cout << "main threadid = " << this_thread::get_id() << " &name = " << &name << "   name = " << name << "  dou = " << dou << "   &dou = " << &dou << endl;
	thread readthread(&Teacher176::readfunc,&tea, ref(name));//注意要第二个传递的 &tea,不会调用copy构造函数。注意第三个参数，如果不用ref包裹，那么传递的也是copy，即使readfunc的参数是引用，thread内部也会搞一个copy，且readfunc的参数必须是const的。
	thread writethread(&Teacher176::writefunc, &tea, ref(dou));
	readthread.join();
	writethread.join();

	cout << "teacher176readfuncreturnnumber = " << teacher176readfuncreturnnumber <<  "  teacher176writefuncreturnnumber = " << teacher176writefuncreturnnumber << endl;
	cout << "main thread end" << endl;
}

从上述代码中，虽然可以获得子线程的返回值，但是这样并不方便。

C++给考虑到这种case，提供了 async函数，返回值是 future的方案。