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─────C/C++ Language─────────────────────────────────────────────────────────
 
     A virtual function is a member function that you expect to be
     redefined in derived classes. When you refer to a derived class
     object using a pointer or a reference to the base class, you can
     call a virtual function for that object and execute the derived
     class's version of the function.
 
     For example:
 
          class WageEmployee
          {
          public:
              virtual float computePay();
          };
 
          class SalesPerson : public WageEmployee
          {
          public:
              float computePay();
          };
 
     You can execute different versions of computePay() depending on the
     type of object you're calling it for:
 
          WageEmployee aWorker;
          SalesPerson aSeller;
          WageEmployee *wagePtr;
 
          wagePtr = &aWorker;
          wagePtr->computePay();   // call WageEmployee::computePay
          wagePtr = &aSeller;
          wagePtr->computePay();   // call SalesPerson::computePay
 
     The virtual keyword is needed only in the base class's declaration
     of the function; any subsequent declarations in derived classes
     are virtual by default.
 
     A derived class's version of a virtual function must have the same
     parameter list and return type as those of the base class. If
     these are different, the function is not considered a redefinition
     of the virtual function. A redefined virtual function cannot
     differ from the original only by return type.
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