Working Delphi 2009 Override Example

The following code assumes a Windows application with a single form�with a button. It�demonstrates using virtual and override to override a parent method in a descendant class.

unit Unit1;

interface  

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
     Button1: TButton;
     procedure Button1Click(Sender: TObject);
  end;

  TRobot = class(TObject)
  public
    procedure Speak; virtual;
  end;

  TCyborg = class(TRobot)
    procedure Speak; override;
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TRobot.Speak;

begin

  ShowMessage('Robot says hi');

end;
  

procedure TCyborg.Speak;

begin

  ShowMessage('Cyborg says hi');

end;

procedure TForm1.Button1Click(Sender: TObject);
var
  Robot: TRobot;
  Cyborg: TCyborg;
begin
  Robot := TRobot.Create;
  Cyborg := TCyborg.Create;

  Robot.Speak;
  Cyborg.Speak;

  FreeAndNil(Robot);
  FreeAndNil(Cyborg);
end;  

end.

Hiding a Method with 'reintroduce'

Use the reintroduce keyword to introduce a new implementation of a parent method (this hides the parent method). You can hide a method without using�reintroduce but you will get a compiler warning. Using�reintroduce will suppress the warning.

The�reintroduce and override modifiers have different meanings. The�reintroduce modifier creates a new member with the same name, signature, and visibility and hides the original member. The override modifier extends the implementation for an inherited member and allows you to implement inheritance-based polymorphism.

Avoid Introducing New Members: Sometimes there are clear reasons to introduce a new method with the same name, signature, and visibility of a parent method. In those clear cases, introducing a new member is a powerful feature. However, if you do not have a clear reason, then avoid introducing a new version of a method by naming the new method something unique and appropriate.

//interface section:

TRobot = class(TObject)
public
  procedure Speak;
end;

TCyborg = class(TRobot)
  procedure Speak; reintroduce;
end;

//implementation section:

procedure TRobot.Speak;
begin
  ShowMessage('Robot says hi');
end;

procedure TCyborg.Speak;
begin

  ShowMessage('Cyborg says hi');
end;

Invoking the Parent Method with 'inherited'

A common task In OO is to extend a method by first executing the parent method code and then adding code. Use the Inherited keyword to invoke the parent class method.

method Robot.Speak;
begin
  MessageBox.Show("Robot says hi");
end;

method Cyborg.Speak;
begin
  inherited;
  MessageBox.Show("Cyborg says hi");
end;