Tuesday, July 5, 2011

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 an another object, you automatically free the owner as well. I know that can't be true, but correct me if I am wrong.

Answer:
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.
 

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