Вопрос по динамическому полиморфизму

#include <iostream>
 
class base
{
    ...
    public:
        base(...) { ... };
        virtual ~base(){};
        virtual void DoSmth();
};
 
void base::DoSmth()
{
       cout<<"Base"<<endl; 
}
 
class derived: public base
{
    ...
    public:
        derived(...): base(...) { ... };
        ~derived(){}
        void DoSmth();
};
 
void derived::DoSmth()
{
       cout<<"Derived"<<endl;
}
 
int main()
{
 
    base* p;
    derived d(...);
    p = &d;
    p->DoSmth();
 
    return 0;
}

class Base { public: virtual void f(){} };
 
class A : public Base { public: virtual void f(){std::cout << "A";} };
class B : public Base { public: virtual void f(){std::cout << "B";} };
class C : public Base { public: virtual void f(){std::cout << "C";} };
 
int main()
{
   Base* base = 0;
   switch( rand() % 3 )
   {
       case 0: base = new A; break;
       case 1: base = new B; break;
       case 2: base = new C; break;
   }
   base->f();
}