unit Unit1;    interface    uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;    type
  TForm1 = class(TForm)
    Edit1: TEdit;
    procedure FormCreate(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;    var
  Form1: TForm1;    implementation    {$R *.dfm}    Type
  TMyHintWindow = Class (THintWindow)
   Constructor Create (AOwner: TComponent); override;
end;    Constructor TMyHintWindow.Create (AOwner: TComponent);
Begin
  Inherited Create (Aowner);
  Canvas.Font.Name := 'Times New Roman';
  Canvas.Font.Size := 14;
end;    procedure TForm1.FormCreate(Sender: TObject);
begin
  Application.ShowHint := False;
  HintWindowClass := TMyHintWindow;
  Application.ShowHint := True;
end;    end.    

Application.ShowHint := False;
  HintWindowClass := TMyHintWindow;
这里我苦思不得其解，
HintWindowClass是全局变量，感觉就是ThintWindow的对象：
  HintWindowClass: THintWindowClass = THintWindow;
TMyHintWindow是从THintWindow派生的类，两者之间为什么可以赋值操作？      Application.ShowHint := True;