Delphi - How to free Memory from a TDataSet?

Question

D2010, Win7 64bit. Hello,

I have a buttonClick event with needs to process a TDataSet opened in another routine... GetDBGenericData.

The function GetDBGenericData returns a TDataSet. This routine basically takes a tQuery component, sets it's SQL property, and opens it. It then returns the TDataSet to my buttonclick.

procedure TForm1.Button2Click(Sender: TObject);
var
DS : TDataSet;
begin

DS := TDataSet.Create(nil);
DS := GetDBGenericData(dbSOURCE, 'LIST_ALL_SCHEMAS', [] );

while Not DS.EOF do
   begin
   ShowMessage(DS.FieldByName('USERNAME').AsString);
   DS.Next;
   end;

DS.Close;
DS.Free;

My problem is -- Understanding DS. I am creating it here in this routine. I am "assigning" it to a TDataSet that points to a component. If I DON'T Free it, I have a memory leak (as reported by EurekaLog). If I do free it, I get an AV the next time I run this routine. (specifically inside the GetDBGenericData routine).

What I think is happening is that DS is getting assigned to (as opposed to copying) to the TDataSet that is being returned, so in effect, I am FREEING BOTH DS in this routine, and the tQuery in GetDBGenericData, when I do a free.

How do I "break" the linkage, and then delete the memory associated to ONLY the one I am dynamically creating.

Thanks, GS

Answer 1

If your DS variable is being assigned another TDataSet by GetDBGenericData, you should neither Create or Free it. You're just using it to refer to an existing dataset.

procedure TForm1.Button2Click(Sender: TObject);
var
  DS : TDataSet;
  UserNameField: TField;  // Minor change for efficiency
begin
  DS := GetDBGenericData(dbSOURCE, 'LIST_ALL_SCHEMAS', [] );

  // Call FieldByName only once; no need to create or
  // free this either.
  UserNameField := DS.FieldByName('USERNAME');

  while not DS.Eof do
  begin
    ShowMessage(UserNameField.AsString);
    DS.Next;
  end;

  // I'd probably remove the `Close` unless the function call
  // above specifically opened it before returning it.
  DS.Close;
end;