impl.cpp

// interface

struct object {
  object * create();
  void release();
  int getState();
};

// implementation

struct object_data : object {
  int state = 0;
};

static inline object_data * cast_this(object *p) { return (object_data *)p; }

int
object::getState() {
  auto self = cast_this(this);
  return self->state;
}

object *
object::create() {
  return new object_data;
}

void
object::release() {
  auto self = cast_this(this);
  delete(self);
}