replace an object in a TObjectList with freeing it (Delphi7)

I have a TObjectList with few elements. I have to replace one of them with one new at the same item (without change a count). I do the following:

procedure TForm1.Button1Click(Sender: TObject);
var
  TObj: TObjectList;
  LPoint: TPPoint;
  i: integer;
begin
  TObj:= TObjectList.Create;
  try

    for i:= 0 to 3 do
    begin
        LPoint:= TPPoint.Create(i, i+ 1);
      TObj.Add(LPoint);
    end;

    LPoint:= TPPoint.Create(21, 22);
    TObj.Items[1]:= nil;
    TObj.Items[1]:= LPoint;

    for i:= 0 to 3 do
    begin
        ShowMessage(IntToStr(TPPoint(TObj.Items[i]).X));
    end;

  finally
    TObj.Free;
  end;
end;

My question is: how can I free the replaced element in the memory? The help says "an Objekt will be freed if his Index will be reassigned". Is the command TObj.Items[1]:= nil; enough?

Thanks in advance for any information.

TObj.Items[1]:= nil;
TObj.Items[1]:= LPoint;

Here you perform two assignments, so the class attempts to free two items.

TObj.Items[1]:= nil;

At this point the previous item is a non-nil reference added in your earlier loop. That object is thus destroyed.

TObj.Items[1]:= LPoint;

When this line executes, TObj.Items[1] = nil and so the Free method is called on nil. Nothing happens.

The bottom line is that your code is overcomplicated. You can replace

TObj.Items[1]:= nil;
TObj.Items[1]:= LPoint;

with

TObj.Items[1]:= LPoint;

The class will destroy the object currently stored in TObj.Items[1] and then replace it with LPoint. Just as you want.