ch08_Notes02_VirtFuncs

(Latest Revision: Wed Apr 2 23:34:56 PST 2003 ) ch08_Notes02_VirtFuncs

ch08 Notes on Virtual Functions

Notes on the text of chapter 8 -- VIRTUAL FUNCTIONS

As we noted in the discussion of inheritance, it can be difficult or impossible for a compiler to determine at compile time what the data type of an object is. In particular, if this segment of code is executed:

 
sphereClass sphere ;
sphereClass * SPtr = &sphere ;
ballClass ball ;
SPtr->DisplayStatistics();
SPtr = &ball ;
SPtr->DisplayStatistics();

Then the sphereClass version of DisplayStatistics may be invoked both times.

The situation is even worse, if this segment of code is executed:


sphereClass * X[26] ; // 26 pointers
init (X) ; // Some are made to point to balls, others to plain spheres.
int n ;
cin >> n ;
   // how does the compiler know whether *X[n] is a ball? 
X[n]->DisplayStatistics() ;

The virtual keyword is basically a directive that the programmer uses to mark a method in a base class. It warns the compiler that a derived class may revise the method. When the compiler gets this warning, it generates code that uses dynamic binding at runtime to aid in choosing the right version of the method to execute.

DisplayStatistics is an example of a polymorphic method: The meaning of the method depends on the data type of the object to which the message is sent. (A ball responds to a message to display itself one way, a plain sphere another way, a pyramid still another.)

If you define a method in a base class whose definition will be changed by some derived class or classes, begin the declaration of the method in the base class with the keyword virtual. For example:

 
class sphereClass
{
  public:
         ...  // everything as before, except DisplayStatistics
       virtual void DisplayStatistics() const ;
        {
            // put definition of function here.
        }
         ... // all the rest of the declarations
} ; // end class

(Unfortunately this forces the designer of the base class to worry about the possible derived classes that may be invented.)