Pointer conversion

I'm simulating the .Net delegates.

Currently it only supports non-void return types and two arguments (because that's what I currently need). It consists of two template classes, one for normal functions and one for member functions:

Code:

---

// Pointer to any global or static 2-arg function
template <typename RET, typename ARG1, typename ARG2>
class two_arg_fxn_ptr
{
protected:
        // void * to allow for other pointer types in derived classes
        void *ptr;

        two_arg_fxn_ptr()
                : ptr(0)
        {
        }

public:
        typedef RET (*ptr_type)(ARG1, ARG2);

        explicit two_arg_fxn_ptr(ptr_type p)
        {
                ptr = reinterpret_cast<void *>(p);
        }

        virtual RET call(ARG1 a1, ARG2 a2)
        {
                ptr_type p = reinterpret_cast<ptr_type>(ptr);
                return (*p)(a1, a2);
        }

        RET operator ()(ARG1 a1, ARG2 a2)
        {
                return call(a1, a2);
        }
};

// Pointer to any 2-arg member function.
template <typename OBJ, typename RET, typename ARG1, typename ARG2>
class two_arg_member_fxn_ptr
        : public two_arg_fxn_ptr<RET, ARG1, ARG2>
{
        OBJ *obj;

public:
        typedef RET (OBJ::*ptr_type)(ARG1, ARG2);
               
        two_arg_member_fxn_ptr(OBJ *o, ptr_type p)
                : obj(o)
        {
                ptr = reinterpret_cast<void *>(p);
        }

        virtual RET call(ARG1 a1, ARG2 a2)
        {
                ptr_type p = reinterpret_cast<ptr_type>(ptr);
                return (obj->*p)(a1, a2);
        }
};

---

You see that the member function class derives from the other to allow for storage of both in a single container.

The normal version compiles fine, but the member function version fails at the two red lines with an impossible-conversion error. VC++, cryptic as always, says something about "there is no context in which this conversion would be possible".

Which is really confusing me...