Future test

#include <iostream>
#include <future>

using namespace std;

long factorial (int num) {
    long prod = 1;
    for (int i = 1; i <= num; ++i) {
        prod *= i;
    }
    return prod;
}

int main () {
    future <long> fut = async (factorial, 100000);
    fut.wait ();

    long ans = fut.get ();

    cout << ans << endl;

    cout << "Done\n";
}

Error:
/data/data/com.n0n3m4.droidc/files/temp.c: In function 'int main()':
/data/data/com.n0n3m4.droidc/files/temp.c:19:16: error: variable 'std::future<long int> fut' has initializer but incomplete type
  future <long> fut = async (factorial, 100000);
                ^
compilation terminated due to -Wfatal-errors.