An Iterator is a simple example of a situation where the
interaction with an object is not memoryless. Normally, when an
object provides access to a private variable localdata through
an accessor method get_localdata(), each call to
get_localdata() is independent and the value returned does not
depend on how many times, if any, get_localdata() has been
invoked earlier.
In contrast, the accessor method get_next() provided by the
Iterator interface returns different values depending on the
history of previous calls to the same method. Thus, some state
information has to be maintained by the object to ensure that the
value returned by get_next() is accurate.
In general, the OO paradigm does not provide any direct way of specifying such ``interactions with state'' with an object. The word port has been used recently in certain specification methodologies (e.g., UML) to refer to a point of interaction with an object that is accompanied by a state. As we have seen with Iterators, ports can be simulated indirectly by exporting objects from within a class that implement a specified interface.