Virtual Function

Visual Studio 6.0

C++ Specific —>

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.

END C++ Specific

Example 1

class WageEmployee
   virtual float computePay();

class SalesPerson : public WageEmployee
   float computePay();

You can execute different versions of computePay( ) depending on the type of object you're calling it for.

Example 2

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.