Help with templates in C++

Why doesn't this code work???

Code:

---

template<class t>
t sum( t a, t b )
{
        return a + b;
}

template<class t>
class c_x {
public:
        t a, b, c;

        void sum()
        {
                c = sum( a, b );
        }
};

void main()
{
        c_x<int> x;
        x.a = 10;
        x.b = 20;
        x.sum();
}

---

Error
C2660: 'c_x<t>::sum' : function does not take 2 arguments
with
[
t=int
]

Code:

---

template<class t>
t sum( t a, t b )
{
        return a + b;
}

---

Code:

---

        void sum()
        {
                c = sum( a, b );
        }

---

in case you don't get it, what does:

sum(a,b) resolve to in class c_x...