下面程序的输出结果是（）
#include<iostream>
using namespace std;
class ClassA {
friend long fun (ClassA a) {
if (a.i < 2) return 1;
return a.i * fun(ClassA(a.i-1));
}
public:
ClassA(long a) { i = a; }
private:
long i;
};
int main() {
int sum = 0;
for (int i = 0; i < 4; i++) {
sum += fun(ClassA(i));
}
cout << sum;
} 
