http://www.csse.monash.edu.au/~jonmc/CSE2305/Topics/05.10.VirtualFns/html/text.html
C++ provides a simple mechanism for class¤-based polymorphism
Member functions can be marked as polymorphic using the virtual keyword
Calls to such functions then get dispatched according to the actual type of the object which owns them, rather than the apparent type of the pointer (or reference) through which they are invoked