Class iterator

library/include/BitArray.h:288–395

        class const_iterator;

        class iterator
        {
            L *root;
            L *cur;
            friend struct DfLinkedList<L, I>;
            friend class const_iterator;
            iterator(L *root, L *cur) : root(root), cur(cur) {}
        public:
            using difference_type = std::ptrdiff_t;
            using value_type = I *;
            using pointer = I **;
            using reference = I *&;
            using iterator_category = std::bidirectional_iterator_tag;

            iterator() : root(nullptr), cur(nullptr) {}
            iterator(const iterator & other) : root(other.root), cur(other.cur) {}

            iterator & operator++()
            {
                CHECK_NULL_POINTER(root);
                CHECK_NULL_POINTER(cur);

                cur = cur->next;
                return *this;
            }
            iterator & operator--()
            {
                CHECK_NULL_POINTER(root);

                if (!cur)
                {
                    // find end() - 1
                    for (cur = root->next; cur && cur->next; cur = cur->next)
                    {
                    }
                    return *this;
                }

                CHECK_NULL_POINTER(cur);
                CHECK_NULL_POINTER(cur->prev);

                cur = cur->prev;
                return *this;
            }
            iterator operator++(int)
            {
                iterator copy(*this);
                ++*this;
                return copy;
            }
            iterator operator--(int)
            {
                iterator copy(*this);
                --*this;
                return copy;
            }
            iterator & operator=(const iterator & other)
            {