value_type_wrap operator[](difference_type _Off) const
Iterator index operator.
If _Off not in a valid range, the returned value will be invalid. Note that you should use a value_type_wrap type to hold the returned value.
The valid index relative to this iterator.
Return the element which is at position *this + _Off. The returned value can only be used to read its referenced element.
db_vector_base_iterator