How to enforce a virtual fn is overloaded by derived class?

Hi All,
I need to enforce requirement that the programmer overloads a particular
virtual function in all derived classes.
i.e.
class base {
public:
virtual void foo() {} // base has some handler
class deriv1 : public base {
void foo() {} // good overridden
class deriv2 : public base {
I am using VS2008 (VC9) (not /clr) so it adheres to _some_ of the standard