/**
	 * const bidirectional iterator
	 */
	class const_iterator : public std::iterator<std::bidirectional_iterator_tag, T>
	{
	private:
		typename std::vector<std::vector<T> >::const_iterator _rowIt, _rowsEnd;
		typename std::vector<T>::const_iterator _colIt;
	public:
		/**
		 * @param rowIt points to a row
		 * @param rowsEnd points to the end of the rows
		 * @param colIt points to a cell within that row
		 */
		const_iterator(typename std::vector<std::vector<T> >::const_iterator rowIt,
						typename std::vector<std::vector<T> >::const_iterator rowsEnd,
						typename std::vector<T>::const_iterator colIt)
						: _rowIt(rowIt), _rowsEnd(rowsEnd), _colIt(colIt) {}

		/**
		 * default c-tor
		 */
		const_iterator() {}

		/**
		 * prefix
		 * @return ref to the incremented iterator
		 */
		const_iterator& operator++();
		/**
		 * postfix
		 * @return the non-incremented iterator
		 */
		const_iterator operator++(int);
		/**
		 * prefix
		 * @return ref to the decremented iterator
		 */
		const_iterator& operator--();
		/**
		 * postfix
		 * @return the non-decremented iterator
		 */
		const_iterator operator--(int);

		/**
		 * @return whether other is equal
		 */
		bool operator==(const const_iterator& other) const
		{
			return _rowIt == other._rowIt && _colIt == other._colIt;
		}

		/**
		 * @return whether other is not equal
		 */
		bool operator!=(const const_iterator& other) const
		{
			return !(*this == other);
		}

		const T& operator*() const
		{
			return *_colIt;
		}

		T* operator->() const
		{
			return &(*_colIt);
		}
	};