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.
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 
Some languages standardize syntax. C++ and Python are notable examples.
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 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
isDone() methods defined. Instead, the Java interface adds the
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 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
See also 
- Composite pattern
- Container (data structure)
- Design pattern (computer science)
- Observer pattern
- Gang Of Four
- "Python v2.7.1 documentation: The Python Standard Library: 5. Built-in Types". Retrieved 2 May 2011.
|The Wikibook Computer Science Design Patterns has a page on the topic of: Iterator implementations in various languages|
- Object iteration in PHP
- Iterator Pattern in C#
- Iterator pattern in UML and in LePUS3 (a formal modelling language)
- SourceMaking tutorial
- Iterator Pattern