Class fetcher

src/python/histogram.cpp:172–202

template <typename T>
struct fetcher {
  long n = -1;
  union {
    T value = 0;
    const T* carray;
  };
  bp::object keep_alive;

  void assign(bp::object o) {
    // skipping check for currently held type, since it is always value
    bp::extract<T> get_value(o);
    if (get_value.check()) {
      value = get_value();
      n = 0;
      return;
    }
#ifdef HAVE_NUMPY
    np::ndarray a = np::from_object(o, np::dtype::get_builtin<T>(), 1);
    carray = reinterpret_cast<const T*>(a.get_data());
    n = a.shape(0);
    keep_alive = a; // this may be a temporary object
    return;
#endif
    throw std::invalid_argument("argument must be a number");
  }

  const T& operator[](long i) const noexcept {
    if (n > 0) return carray[i];
    return value;
  }
};

template <typename T>
struct span {