Class iterator

include/proxy/hdrs/MIME.h:256–304

   * This iterator should be stable over field deletes, but not insertions.
   */
  class iterator
  {
    using self_type = iterator; ///< Self reference types.

  public:
    iterator() = default;

    // STL iterator compliance types.
    using difference_type   = void;
    using value_type        = MIMEField;
    using pointer           = value_type *;
    using reference         = value_type &;
    using iterator_category = std::forward_iterator_tag;

    pointer   operator->();
    reference operator*();

    self_type &operator++();
    self_type  operator++(int);

    bool operator==(self_type const &that) const;
    bool operator!=(self_type const &that) const;

  protected:
    MIMEFieldBlockImpl *_block = nullptr; ///< Current block.
    unsigned            _slot  = 0;       ///< Slot in @a _block

    /** Internal constructor.
     *
     * @param block Block containing current field.
     * @param slot Index of current field.
     */
    iterator(MIMEFieldBlockImpl *block, unsigned slot) : _block(block), _slot(slot) { this->step(); }

    /** Move to a valid (live) slot.
     *
     * This enforces the invariant that the iterator is exactly one of
     * 1. referencing a valid slot
     * 2. equal to the @c end iterator
     *
     * Therefore if called when the iterator is in state (1) the iterator is unchanged.
     *
     * @return @a this
     */
    self_type &step();

    friend class MIMEHdr;
    friend struct MIMEHdrImpl;
  };

  // HdrHeapObjImpl is 4 bytes, so this will result in 4 bytes padding
  uint64_t m_presence_bits;