扩展代码，

#include <future>
#include <iostream>
#include <thread>

using namespace std;

struct X {
    int value;

    X(int val) : value(val) {}

    friend ostream& operator<<(ostream& os, const X& x) {
        os << x.value;
        return os;
    }
};

void f(promise<X>& px) {
    try {
        X res(42);  // 为了示例，计算一个固定的结果
        cout << "Calculating result..." << endl;
        this_thread::sleep_for(chrono::seconds(2));  // 模拟计算过程
        px.set_value(res);
        cout << "Result calculated and set." << endl;
    } catch (...) {
        px.set_exception(current_exception());
    }
}

void g(future<X>& fx) {
    try {
        cout << "Waiting for result..." << endl;
        X v = fx.get();  // if necessary, wait for the value to get computed
        cout << "Result received: " << v << endl;
        // ... use v ...
    } catch (...) {
        // ... handle error ...
    }
}

int main() {
    promise<X> px;
    future<X> fx = px.get_future();

    thread t1(f, ref(px));  // start a thread for f(px)
    thread t2(g, ref(fx));  // start a thread for g(fx)

    t1.join();  // wait for the thread to complete
    t2.join();  // wait for the thread to complete

    return 0;
}