Abstract methods and virtual methods and override

Type Tplane = class public procedure fly; Virtual           Abstract
Declares the parent class abstract method, op polymorphic end; Type Tcopter = Class (Tplane) public procedure fly;                    Override
Subclass rewrite end; Type Tjet = Class (Tplane) public procedure fly;                    Override

Subclass rewrite end;
var Form1:tform1;                           Implementation {$R *.DFM} procedure tcopter.fly;
The instantiation of the subroutine begin ShowMessage (' I ' M tcopter ');
                                                 End
Subclass instantiation of Procedure Tjet.fly;
Begin ShowMessage (' I ' M tjet ');

End
Procedure Plane_fly (const pplane:tplane);
Begin Pplane.fly;

End
Procedure Tform1.button1click (Sender:tobject);
var Copter:tplane;           Begin Copter: = Tcopter.create;
  Depending on the invocation of the actual type, the non-defined variable type plane_fly (copter); Copter.
Destroy;

End
Procedure Tform1.button2click (Sender:tobject);
var Jet:tplane;                 Begin Jet: = Tjet.create;
  Depending on the invocation of the actual type, the non-defined variable type plane_fly (Jet);
Jet.free; End