promise_future.cpp

#include <future>
#include <iostream>
#include <thread>
using namespace std;


int factorial(std::future<int>& f){
    int N = f.get();
    int res = 1;
    for(int i=2;i<=N;i++){
        res *= i;
    }

    return res;
}

int main(){

    int x;
    std::promise<int> p;
    std::future<int> f = p.get_future();

    std::future<int> fu = std::async(factorial, std::ref(f));

    // do something else
    std::this_thread::sleep_for(chrono::milliseconds(1000));

    p.set_value(6);

    x = fu.get();
    cout <<"the result is : " << x << endl;

}