T_ClassA=class
public
  procedure ShowMessage;virtual;
end;T_ClassB=class(T_ClassA)
public
  procedure ShowMessage;override;
end;
procedure T_ClassA.ShowMessage;
begin
  ShowMessage('This is A_class');
end;procedure T_ClassB.ShowMessage;
begin
  inherited;
  ShowMessage('This is B_class');
end;
procedure Test;
var
  T1,T2:T_ClassA;
begin
  T1:=T_ClassA.Create;
  T2:=T_ClassB.Create;
  T1.ShowMessage;
  T2.ShowMessage;  
end;