namespace std {
  template <class T> class slice_array {
  public:
    using value_type = T;
    void operator=  (const valarray<T>&) const;
    void operator*= (const valarray<T>&) const;
    void operator/= (const valarray<T>&) const;
    void operator%= (const valarray<T>&) const;
    void operator+= (const valarray<T>&) const;
    void operator-= (const valarray<T>&) const;
    void operator^= (const valarray<T>&) const;
    void operator&= (const valarray<T>&) const;
    void operator|= (const valarray<T>&) const;
    void operator<<=(const valarray<T>&) const;
    void operator>>=(const valarray<T>&) const;
    slice_array(const slice_array&);
    ~slice_array();
    const slice_array& operator=(const slice_array&) const;
    void operator=(const T&) const;
    slice_array() = delete;       // as implied by declaring copy constructor above
  };
}The slice_array template is a helper template used by the slice subscript operator
slice_array<T> valarray<T>::operator[](slice);
It has reference semantics to a subset of an array specified by a slice object.