delphi assigned函数的用法

if not Assigned(Modeless) then Assigned()什么意思！

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.

很详细呢，不过不是我的原创，我转的，希望对你有用啦。。。。。

另一篇网络文章

关于Delphi中的Assigned

2008-04-05 10:53

关于Delphi中的Assigned

 

function Assigned(var P): Boolean;

Description

Use Assigned to determine whether the pointer or procedure referenced by P is nil. P must be a variable reference of a pointer or procedural type. Assigned(P) corresponds to the test P<> nil for a pointer variable, and @P <> nil for a procedural variable.

Assigned returns False if P is nil, True otherwise.

检查指针指向的参考变量或过程是否为nil

每次我通常的处理方法都是：

if assigned(frm) then frm.close;    但是当下次调用时就会出错。为什么呢，直到咋天我才知道原因

frm.close;frm.free;   只是指定这块内存可以重写，并未释放为NIL 因此当下次调用时即使frm.free已经

执行过assigned(frm)仍为TRUE；

正确的处理方法：

if assigned(frm) then
begin
    frm.close;
    frm:=nil;
end;

或:

if assigned(frm) then
begin
   frm.close;
   freeandnil(frm);
end;

freeandnil的说明：

procedure FreeAndNil(var Obj);

Description

Use FreeAndNil to ensure that a variable is nil after you free the object it references. Pass any variable that represents an object as the Obj parameter.