unit Unit1;interfaceuses  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,  Dialogs, StdCtrls;type  TForm1 = class(TForm)    Button1: TButton;    procedure Button1Click(Sender: TObject);  end;var  Form1: TForm1;implementation{$R *.dfm}function fun(str: string): string;begin  if str = '' then Exit('空'); {Delphi 2009 支持}  Result := str + str;end;procedure TForm1.Button1Click(Sender: TObject);var  s: string;begin  s := fun('ABC');  ShowMessage(s);  {ABCABC}  s := fun('');  ShowMessage(s);  {空}end;end.