typecast for member function pointer

<code>
struct nullptr_t
{
    nullptr_t() : ptr_(0) {}

    // pointer
    template < typename T >
    operator T*() const
    { return (T*)0; }

    // function pointer
    template < typename R >
    struct typedef_fun_ptr
    {
        typedef R (*type)();
    };
    template < typename R >
    operator typename typedef_fun_ptr<R>::type () const
    {
        return (typedef_fun_ptr<R>::type)0;
    }
    // arg1 ... arg10

    // for member function pointer (doesn't works)
    template < typename C, typename R >
    struct typedef_memfun_ptr
    {
        typedef R (C::*type)();
    };
    template < typename C, typename R >
    operator typename typedef_memfun_ptr<C,R>::type () const
    {
        return (typedef_memfun_ptr<C,R>::type)0;
    }
    // arg1 ... arg10

private:
    void* ptr_;
};
const nullptr_t nullptr;

</code>

this is my code for nullptr in C++98.
it works for normal pointers, function pointers but not for member
pointers.
anybody knows why it doesn't works?

sample code is follows
<code>
    int* p = nullptr;
    std::string* s = nullptr;
    void* vp = nullptr;
    assert( p == nullptr );
    assert( s == nullptr );
    assert( sizeof(nullptr) == sizeof(void*) );

// int i = nullptr; // compile-time error: ok

    // function pointer
    int (*fptr1)() = nullptr;
    void* (*fptr2)() = nullptr;

    int (nullptr_test::*memfunp2)() =
nullptr.get_nullptr<nullptr_test,int>();
    int (nullptr_test::*memfunp3)() = nullptr; // don't works
</code>

the error message is
<error_msg>
1>d:\_work\qoom-sr\qoomserver\trunk\worldserver\asioserver
\win_main.cpp(162) : error C2440: 'initializing' : couln't convert
from 'const nullptr_t' to 'int (__thiscall nullptr_test::* )(void)'
</error_msg>