Programming and Graphics

6.15 Pointers and virtual functions 191

If instead we include in the public section of the flower the non-virtual
functionprintrtypeimplementation:

void printrtype() const{};

then the statement

pointer2 -> printrtype();

will invoke theprintrtypefunction of the flower class, which is idle.

When a virtual function is declared, a v-table is constructed for the class
consisting of addresses to the virtual functions for classes and pointers to the
functions from each of the objects of the derived class. Whenever a function
call is made to the virtual function, the v-table is used to resolve to the function
address by way ofdynamic binding.

Virtual functions exemplify intriguing concepts underlying the notion of
object-oriented programming. A class that declares or inherits virtual functions
is called apolymorphic.

Student tuition

The following code exemplifies the use of virtual functions with a base
class of students and two derived classes of resident and non-resident students.
For convenience, the class definition and implementation are consolidated. The
base class is defined and implemented as:

#include <iostream>

using namespace std;

//--- STUDENT CLASS

class student

{

public:

virtual void payment()=0;

};

Note that the base class is endowed with only one idle virtual function, and lacks
a constructor. A class with such minimum functionality is called anabstract
base class.