Class holder

cpp/nd/iterator.hpp:80–90

{
public:
    struct holder
    {
        virtual ~holder() = default;
        virtual array value() const = 0;
        virtual void operator++() = 0;
        virtual void operator+=(int) = 0;
        virtual void operator--() = 0;
        virtual void operator-=(int) = 0;
        virtual std::ptrdiff_t operator-(const holder& other) const = 0;
        virtual bool operator==(const holder& other) const = 0;
    };

    template <typename T>
    class concrete_holder : public holder