Delphi OOP Details

Abstraction

Delphi:   abstract, override

Delphi for Win32 supports abstract class members using the abstract keyword. You can even instantiate instances of a class that contains abstract members. Then you override each abstract member in a descendant class with Override.

Delphi does not support setting an entire class as abstract. You can create an abstract class (a class with one or more abstract methods), but there is no way to tell the compiler to not allow the instantiation of the abstract class.

Delphi does not support abstract member properties directly. To implement an abstract properity, make use of abstract methods. That is, you can read a GetPropertyX abstract function and write to a SetPropertyX abstract procedure. In effect, creating  an abstract property.

TCyborg = class(TObject)
public
  procedure Speak(pMessage: String); virtual; abstract;
  procedure Walk; virtual; abstract;
end;

TSeries600 = class(TCyborg)
public
  procedure Speak(pMessage: String); override;
  procedure Walk; override;
end;

 Delphi Abstraction (abstract, override)

Now let's dig deeper...

Virtual Abstract versus Dynamic Abstract

For abstract methods, you must specify either regular virtual with the virtual keyword or dynamic virtual with the dynamic keyword. In Delphi for Win32, virtual methods are optimized for speed and dynamic methods are optimized for size. The Delphi help indicates to use virtual for most situations.

It is true that the compiler could make virtual the default and therefore optional but requiring one or the other is consistent with Object Pascal's strong typing. However, with Delphi Prism, the use of the virtual keyword with abstract methods is optional.

An Abstract Example with an Abstract Property

The following demonstrates the abstract method above and also contains an abstract property (or rather, a regular property that makes use of an abstract method). The following form unit assumes a form with a button.

unit AbstractionUnit;

interface

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

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

  TCyborg = class(TObject)
  private
    FCyborgName: String;
  protected
    procedure SetCyborgName(const Value: String); virtual; abstract;
  public
    property CyborgName: String read FCyborgName write SetCyborgName;
    procedure Speak(pMessage: String); virtual; abstract;
    procedure Walk; virtual; abstract;
  end;

  TSeries600 = class(TCyborg)
  protected
    procedure SetCyborgName(const Value: String); override;
  public
    procedure Speak(pMessage: String); override;
    procedure Walk; override;
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TSeries600.Speak(pMessage: String);
begin
  ShowMessage(pMessage);
end;

procedure TSeries600.SetCyborgName(const Value: String);
begin
  FCyborgName := Value;
end;

procedure TSeries600.Walk;
begin

  //Implement here.

end;

procedure TForm1.Button1Click(Sender: TObject);
var
  MyKiller: TSeries600;
begin
  MyKiller := TSeries600.Create;

  MyKiller.CyborgName := 'John';

  MyKiller.Speak('I am ' + MyKiller.CyborgName + '.');
  MyKiller.Speak('I''ll be back.');
end;

end.

More Info

 To/From Code Guide: Abstraction (Cross Ref > OOP Details) vs. Other Languages

Definition:	Abstract Class / Abstract Member
Code:	Delphi Abstraction (abstract, override)