This is the interface for iterator objects. They implement an interface to a collection or stream of data items and a cursor that can be used to iterate over and examine individual items in the data set.
Iterators are typically created to access a data set in some specific object, array, mapping, multiset or string. An object can have several iterators that access different data sets in it, or the same in different ways. E.g. strings have both an iterator for access char-by-char (String.Iterator), and another for access over splitted substrings (String.SplitIterator). lfun::_get_iterator may be defined in an object to get an instance of the canonical iterator type for it. It's used by e.g. foreach to iterate over objects conveniently.
It's not an error to advance an iterator past the beginning or end of the data set; `!() will only return true then, and index and value will return UNDEFINED. An iterator in that state need not keep track of positions, so it's undefined what happens if it's "moved back" into the set of items.
Backward movement for iterators is optional. It's supported if and only if `-() is defined, but even then it's undefined how far back the iterator can move. Therefore iterators may support only a limited amount of backward movement, e.g. when they access a stream through a limited buffer. If such an iterator is moved back past the limit then it'll behave as if it's pointing entirely outside the data set (see above).
An iterator that doesn't support backward movement at all should throw an error if it's attempted.
- See also