学习官方示例 - System.Assigned

unit Unit1;

interface

uses
  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.