Constructor

unit Unit1;

interface

uses
Forms, Dialogs, Classes, Controls, StdCtrls;

type
// Define a parent class, base on TObject by default
TFruit = class
public
name : string;
Constructor Create; overload; // This constructor uses defaults
Constructor Create(name : string); overload;
end;

// Define a descendant types
TApple = class(TFruit)
public
diameter : Integer;
published
Constructor Create(name : string; diameter : Integer);
end;

// The class for the form used by this unit
TForm1 = class(TForm)
procedure FormCreate(Sender: TObject);
end;

var
Form1: TForm1;

implementation
{$R *.dfm} // Include form definitions

// Create a fruit object — parameterless version
constructor TFruit.Create;
begin
// Execute the parent (TObject) constructor first
inherited; // Call the parent Create method

// Now set a default fruit name
self.name := ‘Fruit’;
end;

// Create a fruit object — parameterised version
constructor TFruit.Create(name: string);
begin
// Cannot execute the parent constructor — parms differ

// And save the fruit name
self.name := name;
end;

// Create an apple object
constructor TApple.Create(name: string; diameter : Integer);
begin
// Execute the parent (TFruit) constructor first
inherited Create(name); // Call the parent method

// Now save the passed apple diameter
self.diameter := diameter;
end;

// Main line code
procedure TForm1.FormCreate(Sender: TObject);
var
fruit : TFruit;
banana : TFruit;
apple : TApple;

begin
// Create 3 different fruit objects
fruit := TFruit.Create;
banana := TFruit.Create(‘Banana’);
apple := TApple.Create(‘Pink Lady’, 12);

// See which of our objects are fruits
if fruit Is TFruit then ShowMessage(fruit.name +’ is a fruit’);
if banana Is TFruit then ShowMessage(banana.name +’ is a fruit’);
if apple Is TFruit then ShowMessage(apple.name +’ is a fruit’);

// See which objects are apples
if fruit Is TApple then ShowMessage(fruit.name +’ is an apple’);
if banana Is TApple then ShowMessage(banana.name +’ is an apple’);
if apple Is TApple then ShowMessage(apple.name +’ is an apple’);
end;
end.