TypeID for derived classes of a common base class

I'm attempting to implement some mechanism in C++ whereby all classes derived from a common base class are assigned a unique "class ID". For example:

class BaseClass  
{  
    //...
    public: unsigned int GetID( void );
    //...
};
class DerivedClass : public BaseClass
{
}

Class DerivedClass, and all other children of BaseClass, should be able to return unique identifiers without any additional code added to DerivedClass...C++ is making this rather difficult for me, however. Any ideas would be appreciated.

Thanks in advance! ---Dan