indirect_array

template<class T>
    class indirect_array {
public:
    typedef T value_type;
    void operator=(const valarray<T> x) const;
    void operator=(const T& x);
    void operator*=(const valarray<T> x) const;
    void operator/=(const valarray<T> x) const;
    void operator%=(const valarray<T> x) const;
    void operator+=(const valarray<T> x) const;
    void operator-=(const valarray<T> x) const;
    void operator^=(const valarray<T> x) const;
    void operator&=(const valarray<T> x) const;
    void operator|=(const valarray<T> x) const;
    void operator<<=(const valarray<T> x) const;
    void operator>>=(const valarray<T> x) const;
    void fill();
    };

The class describes an object that stores a reference to an object x of class valarray<T>, along with an object xa of class valarray<size_t>, which describes the sequence of elements to select from the valarray<T> object.

You construct an indirect_array<T> object only by writing an expression of the form x[xa]. The member functions of class indirect_array then behave like the corresponding function signatures defined for valarray<T>, except that only the sequence of selected elements is affected.

The sequence consists of xa.size() elements, where element i becomes the index xa[i] within x. For example:

const size_t vi[] = {7, 5, 2, 3, 8};
// x[valarray<size_t>(vi, 5)] selects elements with indices
//   7, 5, 2, 3, 8