Iterator

Iterator patterns are common in STL. They provide a uniform and simple way of traversing a complicated object, such as a graph. The interaction of an iteration and the object can be divided into two groups: If the iterator itself controls the traversing procedure, it is called an external iterator while if the object controls it, it is called an internal iterator.

Usually, different ConcreteIterators can be applied to traverse the ConcreteAggregate in different styles. A robust iterator should ensure modification of ConcreteAggregate during traversal won't hinder itself. The polymorphism of Iterator is not really necessary sometimes, as STL has told us.

The Iterator must know a lot about the object it traverses hence they are tightly coupled. We should avoid giving too much privilege to the Iterator by making them friends if we are writing a library in which new iterators can be added by users.