Pointer conversion

[CornedBee]

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:

// 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...