學習官方樣本 – System.Assigned

unit Unit1;interfaceuses  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,  Dialogs, StdCtrls;type  TForm1 = class(TForm)    Button1: TButton;    Button2: TButton;    procedure Button1Click(Sender: TObject);    procedure Button2Click(Sender: TObject);  end;var  Form1: TForm1;implementation{$R *.dfm}var  p1: Pointer;procedure TForm1.Button1Click(Sender: TObject);var  b1,b2,b3,b4: Boolean;begin  //p1 := nil;  b1 := Assigned(p1);  GetMem(p1, 1024);  b2 := Assigned(p1);  FreeMem(p1, 1024);  b3 := Assigned(p1);  p1 := nil;  b4 := Assigned(p1);  ShowMessageFmt('%s, %s, %s, %s', [BoolToStr(b1,True),                                    BoolToStr(b2,True),                                    BoolToStr(b3,True),                                    BoolToStr(b4,True)]); {False, True, True, False}end;procedure TForm1.Button2Click(Sender: TObject);var  p2: Pointer;  b1,b2,b3,b4: Boolean;begin  p2 := nil;  b1 := Assigned(p2);  GetMem(p2, 1024);  b2 := Assigned(p2);  FreeMem(p2, 1024);  b3 := Assigned(p2);  p2 := nil;  b4 := Assigned(p2);  ShowMessageFmt('%s, %s, %s, %s', [BoolToStr(b1,True),                                    BoolToStr(b2,True),                                    BoolToStr(b3,True),                                    BoolToStr(b4,True)]); {False, True, True, False}end;end.