program inheritanceExample;

​

{$MODE OBJFPC} //directive to be used for creating classes

{$M+} //directive that allows class constructors and destructors

​

type

   Books = Class 

   protected 

      title : String; 

      price: real;

   public

      constructor Create(t : String; p: real); //default constructor

      procedure setTitle(t : String); //sets title for a book

      function getTitle() : String; //retrieves title

      procedure setPrice(p : real); //sets price for a book

      function getPrice() : real; //retrieves price

      procedure Display(); virtual; // display details of a book

end;

(* Creating a derived class *)

​

type

   Novels = Class(Books)

   private

      author: String;

   public

      constructor Create(t: String); overload;

      constructor Create(a: String; t: String; p: real); overload;

      procedure setAuthor(a: String); // sets author for a book

      function getAuthor(): String; // retrieves author name

      procedure Display(); override;

end;

var

   n1, n2: Novels;

​

//default constructor 

constructor Books.Create(t : String; p: real);

begin

   title := t;

   price := p;

end;

​

procedure Books.setTitle(t : String); //sets title for a book

begin

   title := t;

end;

​

function Books.getTitle() : String; //retrieves title

begin

   getTitle := title;

end;

​

procedure Books.setPrice(p : real); //sets price for a book

begin

   price := p;

end;

​

function Books.getPrice() : real; //retrieves price

begin

   getPrice:= price;

end;

​

procedure Books.Display();

begin

   writeln('Title: ', title);

   writeln('Price: ', price);

end;

​

(* Now the derived class methods  *)

constructor Novels.Create(t: String);

begin

   inherited Create(t, 0.0);

   author:= ' ';

end;

​

constructor Novels.Create(a: String; t: String; p: real);

begin

   inherited Create(t, p);

   author:= a;

end;

​

procedure Novels.setAuthor(a : String); //sets author for a book

begin

   author := a;

end;

​

function Novels.getAuthor() : String; //retrieves author

begin

   getAuthor := author;

end;

​

procedure Novels.Display();

begin

   writeln('Title: ', title);

   writeln('Price: ', price:5:2);

   writeln('Author: ', author);

end;

​

begin 

   n1 := Novels.Create('Gone with the Wind');

   n2 := Novels.Create('Ayn Rand','Atlas Shrugged', 467.75);

   n1.setAuthor('Margaret Mitchell');

   n1.setPrice(375.99);

   n1.Display;

   n2.Display;

end.

​