Regarding Base & Derived Class

For runtime the instances "d" and "db" are exactly similar. The difference is, that "bd" is declared to hold all derivated class types and the base class itself. This does matter if you have several derivated.

i.g.

Base
|- Derived
|- Derived2
|- Dervied3


Script:

Base bd;

if(userinput == 1)
        bd = new Derived();
else if(userinput == 2);
        bd = new Derived2();
else if(userinput == 3)
        bd = new Derived3();
else
        bd = new Base();

You see "bd" can hold all derived classes.