多态性赋值能力

unit Unit1;

interface

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

type
  TParent = class(TObject)
    procedure Draw;virtual;
  end;
  TChild = class(TParent)
    procedure Draw;override;
    procedure  ShowHierarchy ;virtual;
  end;

type
  TForm1 = class(TForm)
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}


{ TParent }

procedure TParent.Draw;
begin
     ShowMessage('ParentDraw');
end;

{ TChild }

procedure TChild.Draw;
begin
   ShowMessage('ChildDraw');

end;

procedure TChild.ShowHierarchy;
begin
   ShowMessage('Child show hierarchy');
end;

procedure ChildDraw(O:TParent);
begin
     o.Draw;
end;
procedure FreeObject(O:TObject);
begin
    O.Free;
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  Parent:TParent;
  Child :TChild;

begin
  Parent := TParent.Create;
  Child :=TChild.Create;
  ChildDraw(Parent);
  ChildDraw(Child);
  FreeObject(Child);
  FreeObject(parent);

end;

end.