delphi assigned函数的用法

勒石發表於 2019-5-6 13:11:00

<pre>if not Assigned(Modeless) then Assigned()什么意思！</pre>
 
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
<div>关于Delphi中的Assigned</div>
<div>2008-04-05 10:53</div>

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<div>关于Delphi中的Assigned</div>
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<td>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. </td>

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<div id="MySignature" role="contentinfo">
好的代码像粥一样，都是用时间熬出来的 
来源：https://www.cnblogs.com/jijm123/p/10818999.html

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delphi assigned函数的用法