Four different types of iterators are provided, namely, const_iterator, const_valid_iterator, const_data_iterator, and const_valid_data_iterator.
const_iterator and const_valid_iterator point to an Entry structure, the difference being that the latter only iterates over valid entries - i.e., entries whose data are not null. The same difference exists between const_data_iterator and const_valid_data_iterator which point directly to historical values without reference to the date they are associated to.