本文整理自网络，侵删。
unit Unit1;
interface
uses
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.
--------------------------------------------------------------------------------------------------------------------------------------------------------------
其中的 Exit('空') 相当于
result := '空'
exit;
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