delphiassigned函数的用法 delphi assigned

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;

procedureButton1Click(Sender: TObject);

private

{ Privatedeclarations }

public

{ Publicdeclarations }

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)

procedureFormClose(Sender: TObject; var Action: TCloseAction);

private

{ Privatedeclarations }

public

{ Publicdeclarations }

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 function Assigned(var P): Boolean; Description Use Assigned to determine whether the pointer or procedurereferenced by P is nil. P must be a variable reference of a pointeror procedural type. Assigned(P) corresponds to the testP<> nil for a pointer variable, and@P <> nil for a proceduralvariable. 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) thenfrm.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 theobject it references. Pass any variable that represents an objectas the Obj parameter.

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