Thanks for the help, to be honest I didn't consider the preprocessor since I tend to avoid it as much as possible in the interests of readability.. but in this case it may have a use.

Just so you know, it will be more a case of:

int
MyClass::countItemsIn (...)
  CODE:
    if (SV_IS_MYLIST(ST(0))) {
      THIS->countItemsIn (
        CAST_SV_TO_MYLIST(ST(0))
      );
    } elsif (SV_IS_MYHASH(ST(0)) {
      THIS->countItemsIn (
        CAST_SV_TO_MYHASH(ST(0))
      );
    };
  OUTPUT:
   RETVAL
[download]

..where the SV_IS_MYLIST/CAST_SV_TO_MYLIST parts are all macros.