Sams Teach Yourself C++ in 21 Days

478 Day 14

5: Shape(){}

6: ~Shape(){}

7: virtual long GetArea() = 0;

8: virtual long GetPerim()= 0;

9: virtual void Draw() = 0;

10: private:

11: };

(1)Circle (2)Rectangle (3)Square (0)Quit: 2

x x x x x x

x x x x x x

x x x x x x

x x x x x x

(1)Circle (2)Rectangle (3)Square (0)Quit: 3

x x x x x

x x x x x

x x x x x

x x x x x

x x x x x

(1)Circle (2)Rectangle (3)Square (0)Quit: 0

As you can see, the workings of the program are totally unaffected. The only dif-

ference is that it would now be impossible to make an object of class Shape.

OUTPUT

LISTING14.8 continued

ANALYSIS

Abstract Data Types

Declare a class to be an abstract class (also called an abstract data type) by including one

or more pure virtual functions in the class declaration. Declare a pure virtual function by

writing = 0after the function declaration.

Example

class Shape

{

virtual void Draw() = 0; // pure virtual

};

Implementing Pure Virtual Functions ............................................................

Typically, the pure virtual functions in an abstract base class are never implemented.

Because no objects of that type are ever created, no reason exists to provide implementa-

tions, and the abstract class works purely as the definition of an interface to objects,

which derive from it.