np_vtable_fwd.h

#ifndef NP_VTABLE_FWD
typedef struct np_vtable np_vtable;
#define NP_VTABLE_FWD
#endif

np_vtable.h

#include "np_vtable_fwd.h"
struct np_vtable {
    // blah blah
};
// blah blah

classobject.h

#include "np_vtable_fwd.h"
// blah blah