Iterator pattern

In object-oriented programming, the iterator pattern is a design pattern in which an iterator is used to traverse a container and access the container's elements. The iterator pattern decouples algorithms from containers; in some cases, algorithms are necessarily container-specific and thus cannot be decoupled.

For example, the hypothetical algorithm SearchForElement can be implemented generally using a specified type of iterator rather than implementing it as a container-specific algorithm. This allows SearchForElement to be used on any container that supports the required type of iterator.

Definition

The essence of the Iterator Factory method Pattern is to "Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.".

Language-specific implementation

Main article: Iterator

Some languages standardize syntax. C++ and Python are notable examples.

C++

C++ implements iterators with the semantics of pointers in that language. In C++, a class can overload all of the pointer operations, so an iterator can be implemented that acts more or less like a pointer, complete with dereference, increment, and decrement. This has the advantage that C++ algorithms such as std::sort can immediately be applied to plain old memory buffers, and that there is no new syntax to learn. However, it requires an "end" iterator to test for equality, rather than allowing an iterator to know that it has reached the end. In C++ language, we say that an iterator models the iterator concept.

Java

Java has an Iterator interface that the Collections should implement in order to traverse the elements of the collection. Classic implementations were using the next() method, which is the same for the Java interface. However, there are no currentItem(), first(), and isDone() methods defined. Instead, the Java interface adds the hasNext() and remove() methods.

The Java language is flexible enough that one can implement the iterator design pattern without implementing the Iterator interface that is a part of the Java API.

Python

Python prescribes a syntax for iterators as part of the language itself, so that language keywords such as for work with what Python calls sequences. A sequence has an __iter__() method that returns an iterator object. The "iterator protocol" requires next() return the next element or raise a StopIteration exception upon reaching the end of the sequence. Iterators also provide an __iter__() method returning themselves so that they can also be iterated over e.g., using a for loop. (In Python 3, next() was replaced with __next__().)

See also

• Iterator
• Composite pattern
• Container (data structure)
• Design pattern (computer science)
• Observer pattern

External links

• Object iteration in PHP
• Iterator Pattern in C#
• Iterator pattern in UML and in LePUS3 (a formal modelling language)
• SourceMaking tutorial
• Iterator Pattern