CODEWITHSUNDEEP
delphiruntime-errordelphi-2009
Marus Gradinaru
Marus Gradinaru

Reputation: 3120

Why I don't receive Exception if I use the object after I destroy it?

The following code works just fine, but it shouldn't ! When I click the Button1, the object is destroyed first, and then its Value is used and I don't receive any Access Violation or something... Even more, the multiply operation gives the correct result, that proves that Obj1 is not destroyed ! But then again, this is not true either, because when I close the program it does'n report any memory leakage. I'm very confused.

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  MyObj = class(TObject)
   Value: Cardinal;
  end;

  TForm1 = class(TForm)
    Button1: TButton;
    procedure FormCreate(Sender: TObject);
    procedure Button1Click(Sender: TObject);
  public
   Obj1:MyObj;
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
begin
 Obj1.Free;
 Obj1.Value:=Obj1.Value * 5;
 Caption:=IntToStr(Obj1.Value);
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
 ReportMemoryLeaksOnShutdown:=true;
 Obj1:=MyObj.Create;
 Obj1.Value:=10;
end;

end.

Upvotes: 3

Views: 197

Answers (1)

David Heffernan
David Heffernan

Reputation: 613441

The object is destroyed. The memory is returned to the memory manager. What happens next is out of your control. The memory could be returned to the system. In which case you'd see a runtime error. Or, the memory could be kept alive by the memory manager ready to reuse the next time the program asks for a block of that size. This is what happens here.

Your program exhibits undefined behaviour. Anything could happen, including the program appearing to work. Obviously the program is wrong and you must not access objects after they have been destroyed.

If you use the full debug version of FastMM then you should see an error because in that scenario steps are taken to detect access after free. That's a useful debugging tool.

Upvotes: 5

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