C++ dynamic_cast dynamic cast to base class

//RTTI must be enabled in compiler
#include <iostream>
#include <typeinfo>            //for dynamic_cast
using namespace std;
class Base/*from   www .j av a2 s .com*/
{
   protected:
   int ba;
   public:
   Base() : ba(0)
   {  }
   Base(int b) : ba(b)
   {  }
   virtual void vertFunc()  //needed for dynamic_cast
   {  }
   void show()
   { cout << "Base: ba=" << ba << endl; }
};
class Derv : public Base
{
   private:
   int da;
   public:
   Derv(int b, int d) : da(d)
   { ba = b; }
   void show()
   { cout << "Derv: ba=" << ba << ", da=" << da << endl; }
};
int main()
{
   Base* pBase = new Base(10);          //pointer to Base
   Derv* pDerv = new Derv(21, 22);      //pointer to Derv
   //derived-to-base: upcast -- points to Base subobject of Derv
   pBase = dynamic_cast<Base*>(pDerv);
   pBase->show();                       //"Base: ba=21"
   pBase = new Derv(31, 32);            //normal
   //base-to-derived: downcast -- (pBase must point to a Derv)
   pDerv = dynamic_cast<Derv*>(pBase);
   pDerv->show();                       //"Derv: ba=31, da=32"
   return 0;
}