Forward in Klassen funktioniert nicht

[ATS3788]

Hallo ich mache da wohl einen Denkfehler und komme nicht drauf

Delphi-Quellcode:

type
  // Define the classes in this Unit at the very start for clarity
  TForm1 = Class;          // This is a forward class definition


  TFruit = Class(TObject)  // This is an actual class definition :
    // Internal class field definitions - only accessible in this unit
     private
       t        : TForm1;  // Warum ist die variable ungültig !
       isRound  : Boolean;
       length   : single;
       width    : single;
       diameter : single;
    // Fields and methods only accessible by this class and descendants
    procedure SetForm(const Value : TForm1);
     protected
    // Externally accessible fields and methods
     public

      // 2 constructors - one for round fruit, the other long fruit
       constructor Create(diameter : single);               overload;
       constructor Create(length : single; width : single); overload;
    // Externally accessible and inspectable fields and methods
     published
      // Note that properties must use different names to local defs
       property round : Boolean
         read   isRound;
       property len   : single
         read   length;
       property wide  : single
         read   width;
       property diam  : single
         read   diameter;
  end;                    // End of the TFruit class definition

  // The actual TForm1 class is now defined
   TForm1 = Class(TForm)
    Button1: TButton;
    Button2: TButton;
     procedure FormCreate(Sender: TObject);
     procedure ShowFruit(fruit : TFruit);
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
   private
    // No local data
    f : TFruit;
   public
    // Uses just the TForm ancestor class public definitions
   end;

 var
   Form1: TForm1;

 implementation

 {$R *.dfm}

// Create a round fruit object
 constructor TFruit.Create(diameter: single);
 begin
  // Indicate that we have a round fruit, and set its size
   isRound       := true;
   self.diameter := diameter;
 end;

// Create a long fruit object
 constructor TFruit.Create(length, width: single);
 begin
  // Indicate that we have a long fruit, and set its size
   isRound     := false;
   self.length := length;
   self.width  := width;
 end;


 procedure TFruit.SetForm(const Value : TForm1);
 var
 f1 : TForm1;
 begin
   t := nil;  // Nicht mal das geht
   f1 := Value; // Das würde gehen
   t := Value; // und das ist der Grund für den Eintrag
 end;


// Form object - action taken when the form is created
 procedure TForm1.Button1Click(Sender: TObject);
begin
FormCreate(Sender);
end;

procedure TForm1.Button2Click(Sender: TObject);
 var
 Fruit : TFruit;
begin
 Fruit := TFruit.Create(23);

  ShowFruit(Fruit);
end;

procedure TForm1.FormCreate(Sender: TObject);
 var
   apple, banana : TFruit;
 begin
 f.SetForm(self);

  // Let us create our fruit objects
   apple  := TFruit.Create(3.5);
   banana := TFruit.Create(7.0, 1.75);

  // Show details about our fruits
   ShowFruit(apple);
   ShowFruit(banana);
 end;

// Show what the characteristics of our fruit are
 procedure TForm1.ShowFruit(fruit: TFruit);
 begin
   if fruit.round
   then ShowMessage('We have a round fruit, with diam = '+
                    FloatToStr(fruit.diam))
   else
   begin
     ShowMessage('We have a long fruit');
     ShowMessage('    it has length = '+FloatToStr(fruit.len));
     ShowMessage('    it has width  = '+FloatToStr(fruit.wide));
   end;
 end;

 end.

http://www.delphibasics.co.uk/RTL.asp?Name=Class
Ich habe das Beispiel ein wenig abgeändert
Meine Frage wie kann ich dem Object TFruit
mitteilen das TForm1 Object mitteilen.