PURE VIRTUAL FUNCTIONS
Pure Virtual Functions
As the examples in the preceding section illustrate, when a virtual function is not redefined by
a derived class, the version defined in the base class will be used. However, in many situations
there can be no meaningful definition of a virtual function within a base class. For example, a
base class may not be able to define an object sufficiently to allow a base-class virtual function
to be created. Further, in some situations you will want to ensure that all derived classes
override a virtual function. To handle these two cases, C++ supports the pure virtual function.
Apure virtual function is a virtual function that has no definition within the base class.
To declare a pure virtual function, use this general form:
virtual type
func-name(parameter-list)
= 0;
When a virtual function is made pure, any derived class must provide its own definition. If the
derived class fails to override the pure virtual function, a compile-time error will result.
The following program contains a simple example of a pure virtual function. The base class,
number, contains an integer called val, the function setval( ), and the pure virtual function
show( ). The derived classes hextype,dectype, and octtype inherit number and redefine
show( ) so that it outputs the value of val in each respective number base (that is, hexadecimal,
decimal, or octal).
#include <iostream>
using namespace std;
class number {
protected:
int val;
public:
void setval(int i) { val = i; }
// show() is a pure virtual function
virtual void show() = 0;
};
class hextype : public number {
public:
