assigned(对象名)判断对象有没有被实例化？

assigned 是用来判断某一指针(pointer)或过程引用是否为nil(空)，如果为空则返回假（false）。
用法示例(防止窗体被实例化多次)：

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

uses Unit2;

{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
begin
  if (Not assigned(form2)) then
  begin
    form2:=Tform2.Create(Self);
  end;
  form2.show;
end;

end.

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unit Unit2;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs;

type
  TForm2 = class(TForm)
    procedure FormClose(Sender: TObject; var Action: TCloseAction);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form2: TForm2;

implementation

{$R *.dfm}

procedure TForm2.FormClose(Sender: TObject; var Action: TCloseAction);
begin
  Action:=caFree;
  form2:=nil;  //这句比较重要，窗体被释放时，窗体变量并不会自动变空
end;

end.