class A1 
{ 
public: 
       virtual void k(){std::cout << "A1";} 
}; 
 
 
class B1: public A1 
{ 
public: 
        void k(){std::cout << "B1";} 
}; 
 
 
void function(A1 a) 
{ 
       a.k(); 
} 
 
int main() 
{ 
   B1 b; 
   function(b); 
}