Default parameters in COM

Hi,

I would like to add a default parameter to a COM function. I defined
it in the IDL file like this:
HRESULT GetName( [in]          BSTR              id1
                              , [in]          BSTR              id2
                              , [in,
defaultvalue("False")]              BOOL
disableSignin
                           , [out, retval] BSTR** pName );

But when I try to implement this function in COM, I can't set the
default value because it is not the last parameter... and I have to
keep the return value as the last parameter (to stop from breaking
client's codes etc.).

Am I wrong in thinking it should be possible to set a default
parameter in a COM function? How do you get around having the return
value at the end of the list of parameters or is it only possible on a
void function?

Thanks.