#define TRIGGER_OBJECT 1
#define OBJECT         0
typedef int type_object;

void trigger_update(trigger_object *);
void object_update(object *);

typedef struct {
      static const type_object type = TRIGGER_OBJECT;
      static const void (*update)(struct trigger_object *) = trigger_update;
} object;

typedef struct {
      static const type_object type = OBJECT;
      static const void (*update)(struct object *) = object_update;
} trigger_object;

...

int main()
{
      trigger_object t_obj;
      object obj;
      obj.update(obj);
      t_obj.update(t_obj);

      return 0;
}