General Info: Method Overriding

Where you define or implement a virtual method in a parent class and then replace it in a descendant class.

When you decide to declare a method as virtual, you are giving permission to derived classes to extend and override the method with their own implementation. You can have the extended method call the parent method's code too.

In most OO languages you can also choose to hide a parent method. When you introduce a new implementation of the same named method with the same signature without overriding, you are hiding the parent method.

Delphi Overriding

In Delphi, you specify a virtual method with the virtual keyword in a parent class and extend (or replace) it in a descendant class using the override keyword. Call Inherited in the descendant method to execute the code in the parent method.

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

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

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;