:)

gistfile1.txt

//in file ClassA.cpp
#include <stdio.h>
class ClassA {
    public:
        virtual void f( void );
};
class ClassB : public ClassA{
    public:
         void f( int x = 0);
};
void ClassA::f( void )
{
   printf("Hello class A");
   //implementation is irrelevant
}

void ClassB::f( int x = 0 )
{
   printf("Hello Class B");
   //implementation is irrelevant
}
int main(int argc, char **argv){
    ClassA *instanceOfClassB = new ClassB();
    instanceOfClassB->f();
    return 0;
}