what

#include <stdio.h>
class B
{
public:
    int x;
    B(int v = 0) : x(v) {}
    virtual void f(int a = 1, int b = 2) {printf("\n%d", a);}
};

class D : public B
{
    public:
    D(int v) : B(v) {}
    void f(int a = 3, int b = 4) {printf("\n%d", a);}
};

int main()
{
    B b, *pb;
    D d(15);
    pb = &d;
    pb->f(10,20);
    b.f();
    pb->f();
    return 0;
}