virtual override

//不带virtual override的type
  TForm9 = class(TForm)
    btn1: TButton;
    mmo1: TMemo;
    procedure btn1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;
  a = class
    procedure funct1; // virtual;
  end;
  b = class(a)
    procedure funct1;// override;
  end;
var
  Form9: TForm9;
implementation
{$R *.dfm}
procedure TForm9.btn1Click(Sender: TObject);
var
  b1: b;
  a1: a;
begin
  b.Create.funct1; //a  b
  a(b.Create).funct1;  //a
  a.Create.funct1; //a
end;
{ a }
procedure a.funct1;
begin
  Form9.mmo1.Lines.Add('a');
end;
{ b }
procedure b.funct1;
begin
  inherited;
  Form9.mmo1.Lines.Add('b');
end;
end.

//带
type
  TForm9 = class(TForm)
    btn1: TButton;
    mmo1: TMemo;
    procedure btn1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;
  a = class
    procedure funct1;virtual;
  end;
  b = class(a)
    procedure funct1; override;
  end;
var
  Form9: TForm9;
implementation
{$R *.dfm}
procedure TForm9.btn1Click(Sender: TObject);
var
  b1: b;
  a1: a;
begin
  a(b.Create).funct1; //也是输出a  b 
  b.Create.funct1; // a  b
end;
{ a }
procedure a.funct1;
begin
  Form9.mmo1.Lines.Add('a');
end;
{ b }
procedure b.funct1;
begin
  inherited;
  Form9.mmo1.Lines.Add('b');
end;
end.

实际中：

function GetSubForm(SubFormName:string):TForm;virtual;abstract;

//子类中

function  GetSubForm(SubFormName:string):TForm;override;