reference at(index_type n)
Index function.
The reference to the element at specified position, can act as both a left value and a right value.
const_reference at(index_type n) const
Read only index function.
Only used as a right value, no need for assignment capability. The return value can't be used to update the element.
The operator[] and at() only come from std::vector and std::deque, If you are using db_vector as std::list, you don't have to set DB_RENUMBER flag to the backing database handle, and you get better performance, but at the same time you can't use these functions.
Otherwise if you have set the DB_RENUMBER flag to the backing database handle, you can use this function though it is an std::list equivalent.