First, I find no T_OBJECT in my typemap file. Perhaps you meant T_PTROBJ?

If so, then no, RETVAL= PL_sv__undef isn't the thing to do. When in doubt, use the source! From typemap:

INPUT
T_PTROBJ
    if (sv_derived_from($arg, \"${ntype}\")) {
        IV tmp = SvIV((SV*)SvRV($arg));
        $var = INT2PTR($type,tmp);
    }
    else
        croak(\"$var is not of type ${ntype}\")

OUTPUT
T_PTROBJ
    sv_setref_pv($arg, \"${ntype}\", (void*)$var);
[download]

so your code ends up doing: sv_setref_pv(ST(0),"cstruct",(void*)&PL_sv__undef); so you'd get a new object that is a blessed reference to undef (well, unless that attempt fails, which it probably does).

You have a couple of ways you can go. You could extend the typemap to handle a failure case:

OUTPUT
OUTPUT
T_PTROBJ
    if(  NULL != $var  )
        sv_setref_pv($arg, \"${ntype}\", (void*)$var);
    else
        sv_setsv( $arg, &PL_sv__undef );
[download]

Or something close to that.

Update: The "other ways" I decided not to describe since they seemed inferior after I saw how easy that was.

        - tye (but my friends call me "Tye")