Iterator pattern
Encyclopedia
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 algorithm
s 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 which supports the required type of iterator.
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
.
prescribes a syntax for iterators as part of the language itself, so that language keywords such as
Object-oriented programming
Object-oriented programming is a programming paradigm using "objects" – data structures consisting of data fields and methods together with their interactions – to design applications and computer programs. Programming techniques may include features such as data abstraction,...
, the iterator pattern is a design pattern
Design pattern (computer science)
In software engineering, a design pattern is a general reusable solution to a commonly occurring problem within a given context in software design. A design pattern is not a finished design that can be transformed directly into code. It is a description or template for how to solve a problem that...
in which an iterator
Iterator
In computer programming, an iterator is an object that enables a programmer to traverse a container. Various types of iterators are often provided via a container's interface...
is used to traverse a container
Container (data structure)
In computer science, a container is a class, a data structure, or an abstract data type whose instances are collections of other objects. In other words; they are used for storing objects in an organized way following specific access rules...
and access the container's elements. The iterator pattern decouples algorithm
Algorithm
In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...
s 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 which 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
Some languages standardize syntax. C++ and Python are notable examples.C++
C++C++
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
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 conceptConcept (generic programming)
In generic programming, a concept is a description of supported operations on a type, including syntax and semantics. In this way, concepts are related to abstract base classes but concepts do not require a subtype relationship.-Languages using:...
.
Python
PythonPython (programming language)
Python is a general-purpose, high-level programming language whose design philosophy emphasizes code readability. Python claims to "[combine] remarkable power with very clear syntax", and its standard library is large and comprehensive...
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" which 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__.)
Java
Java has an Iterator interface that the Collections should implement in order to transverse the elements of the collection
See also
- IteratorIteratorIn computer programming, an iterator is an object that enables a programmer to traverse a container. Various types of iterators are often provided via a container's interface...
- Container (data structure)Container (data structure)In computer science, a container is a class, a data structure, or an abstract data type whose instances are collections of other objects. In other words; they are used for storing objects in an organized way following specific access rules...
- Observer patternObserver patternThe observer pattern is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods...
- Composite patternComposite patternIn software engineering, the composite pattern is a partitioning design pattern. The composite pattern describes that a group of objects are to be treated in the same way as a single instance of an object. The intent of a composite is to "compose" objects into tree structures to represent...
- Design pattern (computer science)Design pattern (computer science)In software engineering, a design pattern is a general reusable solution to a commonly occurring problem within a given context in software design. A design pattern is not a finished design that can be transformed directly into code. It is a description or template for how to solve a problem that...
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