maybe.cpp

template <class T> struct Maybe { virtual ~Maybe() {}; };
template <class T> struct Nothing : public Maybe<T> {};
template <class T> struct Just : public Maybe<T> { Just(T v) : v(v) {} T v; };

template <class T>
T fromMaybe(const Maybe<T>& maybeVal, const T& defaultVal)
{
  if (const Just<T>* j = dynamic_cast<const Just<T>*>(&maybeVal)) {
      return j->v;
  }

  return defaultVal;
}