// element access
/**
 *  @brief  Subscript access to the data contained in the %vector.
 *  @param __n The index of the element for which data should be
 *  accessed.
 *  @return  Read/write reference to data.
 *
 *  This operator allows for easy, array-style, data access.
 *  Note that data access with this operator is unchecked and
 *  out_of_range lookups are not defined. (For checked lookups
 *  see at().)
 */
 reference
 operator[](size_type __n) _GLIBCXX_NOEXCEPT
 {
     __glibcxx_requires_subscript(__n);
     return *(this->_M_impl._M_start + __n);  // overflow???
 }

/// Safety check used only from at().
 void
 _M_range_check(size_type __n) const
 {
     if (__n >= this->size())
         __throw_out_of_range_fmt(__N("vector::_M_range_check: __n "
                                  "(which is %zu) >= this->size() "
                                  "(which is %zu)"),
                                  __n, this->size());
}

/**
 *  @brief  Provides access to the data contained in the %vector.
 *  @param __n The index of the element for which data should be
 *  accessed.
 *  @return  Read/write reference to data.
 *  @throw  std::out_of_range  If @a __n is an invalid index.
 *
 *  This function provides for safer data access.  The parameter
 *  is first checked that it is in the range of the vector.  The
 *  function throws out_of_range if the check fails.
 */
 reference
 at(size_type __n)
 {
      _M_range_check(__n);
      return (*this)[__n];
 }