class s1 {
public:
    int CALLBACK f1(int i);
};

class Delegate
{
    s1* obj;
    int (CALLBACK s1::*fn);  
public:
    Delegate(s1* obj_, int(CALLBACK s1::*fn_)) : obj(obj_), fn(fn_) {}
    int operator(int i) { return (obj->::*fn)(i); }
};

...
s1 s;
Delegate d(&s, s1::f1);
int k=d(10);
...