query on dynamic cast

Hello All,
I was trying the below example for which I have a doubt. My doubt is
that why is virtual needed for function "dummy" in class CBase which
makes program compile?
Secondly, As per my current understanding of dynamic cast, pba is a
object of CDerived which RTTI knows and hence cast should be
successful. Isn't it? Which concept I didn't understand here ?

//Program to understand the dynamic cast.
#include <iostream>
#include <exception>
using namespace std;

class CBase
{
     void dummy() {}
};
class CDerived: public CBase
{
};

int main () {
   try {
     CBase * pba = new CDerived;
     CDerived * pd;

     pd = dynamic_cast<CDerived*>(pba);
     if (pd==0) cout << "Null pointer on first type-cast" << endl;
   } catch (exception& e) {cout << "Exception: " << e.what();}
   return 0;
}

Thanks.

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