Deleting TList within a TList

I am trying to free Tlist within a Tlist in a onDestroy event and FastMM4 is raising an access violation error. Here is the code snippet.

procedure TSignalFrm.FormDestroy(Sender: TObject);
var
  x,y: integer;
begin
  for x := 0 to signalList.Count - 1 do
  begin
    for y:=0 to TSignal(SignalList.Items[x]).alarmList.Count-1 do
    begin
      TAlarm(TSignal(SignalList.Items[x]).alarmList.Items[y]).Free;
    end;
    TSignal(SignalList.Items[x]).AlarmList.Free;
    TSignal(SignalList.Items[x]).Free;
  end;
  SignalList.Free;
end;

I get access violation error at TSignal(SignalList.items[x]).Free; line. Freeing AlarmList items before freeing SignalList items raises the access violation error, but WHY?

Update: I am using Delphi 7.0 on Windows XP. The actual FastMM4 messages is as follows.

FastMM has detected an attempt to call a virtual method on a freed object. An access viloation will now be raised in order to abort the current operation.

Freed Object class: TList

Virtual method: Destroy

Virtual method address:427CF0

The allocation number was: 80055

Followed by a lots of memory dump.

According to this FastMM4 error, if you free an object within 开发者_StackOverflowan another object, you automatically free the owner as well. I know that can't be true, but correct me if I am wrong.

Does TSignal not free its AlarmList member in its destructor? (That’s how I would do this).

Update: does it work if you remove the TSignal(SignalList.Items[x]).AlarmList.Free; line?

Second update: Each TList's items need to be freed, if it contains pointers to objects.

Your problem was that TSignal is not a TList. Since it takes care of freeing its members (such as the Alarmlist), that Alarmlist should not be freed explicitly.

Since TAlam and TSignal are both objects (not records) I believe you should use TObjectList instead of TList. TObjectList has a special property calld OwnsObjects that allows it to free it's contents properly as it is being freed. Check this out http://docwiki.embarcadero.com/VCL/XE/en/Contnrs.TObjectList.OwnsObjects

As an advice do not use TList unless you need to store pointers not objects.