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Iterator pattern

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In <a href="Object-oriented_programming" title="Object-oriented programming">object-oriented programming</a>, the iterator pattern is a <a href="Design_pattern_(computer_science)" class="mw-redirect" title="Design pattern (computer science)">design pattern</a> in which an <a href="Iterator" title="Iterator">iterator</a> is used to traverse a <a href="Collection_(abstract_data_type)" title="Collection (abstract data type)">container</a> and access the container's elements. The iterator pattern decouples <a href="Algorithm" title="Algorithm">algorithms</a> 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.

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<div class="mw-heading mw-heading2"><h2 id="Overview">Overview</h2></div>

The Iterator

design pattern is one of the 23 well-known

<a href="Design_Patterns" title="Design Patterns">"Gang of Four" design patterns</a>

that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change, test, and reuse.

<div class="mw-heading mw-heading3"><h3 id="What_problems_can_the_Iterator_design_pattern_solve?">What problems can the Iterator design pattern solve?</h3></div>

<ul><li>The elements of an aggregate object should be accessed and traversed without exposing its representation (data structures).</li>

<li>New traversal operations should be defined for an aggregate object without changing its interface.</li></ul>

Defining access and traversal operations in the aggregate interface is inflexible because it commits the aggregate to particular access and traversal operations and makes it impossible to add new operations

later without having to change the aggregate interface.

<div class="mw-heading mw-heading3"><h3 id="What_solution_does_the_Iterator_design_pattern_describe?">What solution does the Iterator design pattern describe?</h3></div>

<ul><li>Define a separate (iterator) object that encapsulates accessing and traversing an aggregate object.</li>

<li>Clients use an iterator to access and traverse an aggregate without knowing its representation (data structures).</li></ul>

Different iterators can be used to access and traverse an aggregate in different ways.

New access and traversal operations can be defined independently by defining new iterators.

See also the UML class and sequence diagram below.

<div class="mw-heading mw-heading2"><h2 id="Definition">Definition</h2></div>

The essence of the Iterator Pattern is to "Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.".<a href="#cite_note-3">[3]</a>

<div class="mw-heading mw-heading2"><h2 id="Structure">Structure</h2></div>

<div class="mw-heading mw-heading3"><h3 id="UML_class_and_sequence_diagram">UML class and sequence diagram</h3></div>

In the above <a href="UML" class="mw-redirect" title="UML">UML</a> <a href="Class_diagram" title="Class diagram">class diagram</a>, the <code>Client</code> class refers (1) to the <code>Aggregate</code> interface for creating an <code>Iterator</code> object (<code>createIterator()</code>) and (2) to the <code>Iterator</code> interface for traversing an <code>Aggregate</code> object (<code>next(),hasNext()</code>).

The <code>Iterator1</code> class implements the <code>Iterator</code> interface by accessing the <code>Aggregate1</code> class.

The <a href="UML" class="mw-redirect" title="UML">UML</a> <a href="Sequence_diagram" title="Sequence diagram">sequence diagram</a>

shows the run-time interactions: The <code>Client</code> object calls <code>createIterator()</code> on an <code>Aggregate1</code> object, which creates an <code>Iterator1</code> object and returns it

to the <code>Client</code>.

The <code>Client</code> uses then <code>Iterator1</code> to traverse the elements of the <code>Aggregate1</code> object.

<div class="mw-heading mw-heading3"><h3 id="UML_class_diagram">UML class diagram</h3></div>

<div class="mw-heading mw-heading2"><h2 id="Example">Example</h2></div>

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</style><div role="note" class="hatnote navigation-not-searchable">Main article: <a href="Iterator" title="Iterator">Iterator</a></div>

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

<a href="C%2B%2B" title="C++">C++</a> implements iterators with the semantics of <a href="Pointer_(computer_programming)" title="Pointer (computer programming)">pointers</a> 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 <code>std::sort</code> 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 <a href="Concept_(generic_programming)" title="Concept (generic programming)">concept</a>.

This C++11 implementation is based on chapter "Generalizing vector yet again".<a href="#cite_note-5">[5]</a>

<div class="mw-highlight mw-highlight-lang-c++ mw-content-ltr" dir="ltr"><pre>#include <iostream>

#include <stdexcept>

#include <initializer_list>

class Vector {

public:

using iterator = double*;

iterator begin() { return elem; }

iterator end() { return elem + sz; }

Vector(std::initializer_list<double> lst) :elem(nullptr), sz(0) {

sz = lst.size();

elem = new double[sz];

double* p = elem;

for (auto i = lst.begin(); i != lst.end(); ++i, ++p) {

*p = *i;

}

}

~Vector() { delete[] elem; }

int size() const { return sz; }

double& operator[](int n) {

if (n < 0 || n >= sz) throw std::out_of_range("Vector::operator[]");

return elem[n];

}

Vector(const Vector&) = delete; // rule of three

Vector& operator=(const Vector&) = delete;

private:

double* elem;

int sz;

};

int main() {

Vector v = {1.1*1.1, 2.2*2.2};

for (const auto& x : v) {

std::cout << x << '\n';

}

for (auto i = v.begin(); i != v.end(); ++i) {

std::cout << *i << '\n';

}

for (auto i = 0; i <= v.size(); ++i) {

std::cout << v[i] << '\n';

}

}

</pre></div>

The program output is

<div class="mw-highlight mw-highlight-lang-c++ mw-content-ltr" dir="ltr"><pre>1.21

4.84

1.21

4.84

1.21

4.84

terminate called after throwing an instance of 'std::out_of_range'

what(): Vector::operator[]

</pre></div>

<ul><li><a href="Composite_pattern" title="Composite pattern">Composite pattern</a></li>

<li><a href="Container_(data_structure)" class="mw-redirect" title="Container (data structure)">Container (data structure)</a></li>

<li><a href="Design_pattern_(computer_science)" class="mw-redirect" title="Design pattern (computer science)">Design pattern (computer science)</a></li>

<li><a href="Iterator" title="Iterator">Iterator</a></li>

<li><a href="Observer_pattern" title="Observer pattern">Observer pattern</a></li></ul>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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</style><cite id="CITEREFErich_GammaRichard_HelmRalph_JohnsonJohn_Vlissides1994" class="citation book cs1">Erich Gamma; Richard Helm; Ralph Johnson; John Vlissides (1994). <a rel="nofollow" class="external text" href="https://archive.org/details/designpatternsel00gamm/page/257">Design Patterns: Elements of Reusable Object-Oriented Software</a>. Addison Wesley. pp. <a rel="nofollow" class="external text" href="https://archive.org/details/designpatternsel00gamm/page/257">257ff</a>. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>0-201-63361-2</bdi>.</cite>

</li>

<li id="cite_note-2"><a href="#cite_ref-2">^</a> <cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://w3sdesign.com/?gr=b04&ugr=proble">"The Iterator design pattern - Problem, Solution, and Applicability"</a>. w3sDesign.com. Retrieved 2017-08-12.</cite>

</li>

</li>

<li id="cite_note-4"><a href="#cite_ref-4">^</a> <cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://w3sdesign.com/?gr=b04&ugr=struct">"The Iterator design pattern - Structure and Collaboration"</a>. w3sDesign.com. Retrieved 2017-08-12.</cite>

</li>

<li id="cite_note-5"><a href="#cite_ref-5">^</a> <cite id="CITEREFBjarne_Stroustrup2014" class="citation book cs1">Bjarne Stroustrup (2014). Programming: Principles and Practice using C++ (2 ed.). Addison Wesley. pp. 729 ff. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>978-0-321-99278-9</bdi>.</cite>

</li>

</ol></div></div>

<div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2></div>

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<div class="side-box-text plainlist">The Wikibook <a href="https://en.wikibooks.org/wiki/Computer_Science_Design_Patterns" class="extiw external" title="wikibooks:Computer Science Design Patterns">Computer Science Design Patterns</a> has a page on the topic of: <a href="https://en.wikibooks.org/wiki/Computer_Science_Design_Patterns/Iterator" class="extiw external" title="wikibooks:Computer Science Design Patterns/Iterator">Iterator implementations in various languages</a></div></div>

</div>

<ul><li><a rel="nofollow" class="external text" href="http://us3.php.net/manual/en/language.oop5.iterations.php">Object iteration</a> in PHP</li>

<li><a rel="nofollow" class="external text" href="http://www.dofactory.com/Patterns/PatternIterator.aspx">Iterator Pattern</a> in C#</li>

<li><a rel="nofollow" class="external text" href="https://web.archive.org/web/20160303172550/http://www.lepus.org.uk/ref/companion/Iterator.xml">Iterator pattern in UML and in LePUS3 (a formal modelling language)</a></li>

<li><a rel="nofollow" class="external text" href="http://sourcemaking.com/design_patterns/iterator">SourceMaking tutorial</a></li>

<li><a rel="nofollow" class="external text" href="https://web.archive.org/web/20150520003129/http://www.patterns.pl/iterator.html">Design Patterns implementation examples tutorial</a></li>

<li><a rel="nofollow" class="external text" href="http://c2.com/cgi/wiki?IteratorPattern">Iterator Pattern</a></li></ul>

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</style><div id="Software_design_patterns225" style="font-size:114%;margin:0 4em"><a href="Software_design_pattern" title="Software design pattern">Software design patterns</a></div></th></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Design_Patterns" title="Design Patterns">Gang of Four patterns</a></th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"></div><table class="nowraplinks navbox-subgroup" style="border-spacing:0"><tbody><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Creational_pattern" title="Creational pattern">Creational</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Abstract_factory_pattern" title="Abstract factory pattern">Abstract factory</a></li>

<li><a href="Builder_pattern" title="Builder pattern">Builder</a></li>

<li><a href="Factory_method_pattern" title="Factory method pattern">Factory method</a></li>

<li><a href="Prototype_pattern" title="Prototype pattern">Prototype</a></li>

<li><a href="Singleton_pattern" title="Singleton pattern">Singleton</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Structural_pattern" title="Structural pattern">Structural</a></th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Adapter_pattern" title="Adapter pattern">Adapter</a></li>

<li><a href="Bridge_pattern" title="Bridge pattern">Bridge</a></li>

<li><a href="Composite_pattern" title="Composite pattern">Composite</a></li>

<li><a href="Decorator_pattern" title="Decorator pattern">Decorator</a></li>

<li><a href="Facade_pattern" title="Facade pattern">Facade</a></li>

<li><a href="Flyweight_pattern" title="Flyweight pattern">Flyweight</a></li>

<li><a href="Proxy_pattern" title="Proxy pattern">Proxy</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Behavioral_pattern" title="Behavioral pattern">Behavioral</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Chain-of-responsibility_pattern" title="Chain-of-responsibility pattern">Chain of responsibility</a></li>

<li><a href="Command_pattern" title="Command pattern">Command</a></li>

<li><a href="Interpreter_pattern" title="Interpreter pattern">Interpreter</a></li>

<li><a href="Mediator_pattern" title="Mediator pattern">Mediator</a></li>

<li><a href="Memento_pattern" title="Memento pattern">Memento</a></li>

<li><a href="Observer_pattern" title="Observer pattern">Observer</a></li>

<li><a href="State_pattern" title="State pattern">State</a></li>

<li><a href="Strategy_pattern" title="Strategy pattern">Strategy</a></li>

<li><a href="Template_method_pattern" title="Template method pattern">Template method</a></li>

<li><a href="Visitor_pattern" title="Visitor pattern">Visitor</a></li></ul>

</div></td></tr></tbody></table><div></div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Concurrency_pattern" title="Concurrency pattern">Concurrency patterns</a></th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Active_object" title="Active object">Active object</a></li>

<li><a href="Balking_pattern" title="Balking pattern">Balking</a></li>

<li><a href="Binding_properties_pattern" title="Binding properties pattern">Binding properties</a></li>

<li><a href="Double-checked_locking" title="Double-checked locking">Double-checked locking</a></li>

<li><a href="Asynchronous_method_invocation" title="Asynchronous method invocation">Event-based asynchronous</a></li>

<li><a href="Guarded_suspension" title="Guarded suspension">Guarded suspension</a></li>

<li><a href="Monitor_(synchronization)" title="Monitor (synchronization)">Monitor</a></li>

<li><a href="Proactor_pattern" title="Proactor pattern">Proactor</a></li>

<li><a href="Reactor_pattern" title="Reactor pattern">Reactor</a></li>

<li><a href="Readers%E2%80%93writer_lock" title="Readers–writer lock">Read–write lock</a></li>

<li><a href="Scheduler_pattern" class="mw-redirect" title="Scheduler pattern">Scheduler</a></li>

<li><a href="Scheduled-task_pattern" title="Scheduled-task pattern">Scheduled-task pattern</a></li>

<li><a href="Semaphore_(programming)" title="Semaphore (programming)">Semaphore</a></li>

<li><a href="Thread_pool" title="Thread pool">Thread pool</a></li>

<li><a href="Thread-local_storage" title="Thread-local storage">Thread-local storage</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Architectural_pattern" title="Architectural pattern">Architectural patterns</a></th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Front_controller" title="Front controller">Front controller</a></li>

<li><a href="Interceptor_pattern" title="Interceptor pattern">Interceptor</a></li>

<li><a href="Specification_pattern" title="Specification pattern">Specification</a></li>

<li><a href="Publish%E2%80%93subscribe_pattern" title="Publish–subscribe pattern">Publish–subscribe</a></li>

<li><a href="Naked_objects" title="Naked objects">Naked objects</a></li>

<li><a href="Service_locator_pattern" title="Service locator pattern">Service locator</a></li>

<li><a href="Active_record_pattern" title="Active record pattern">Active record</a></li>

<li><a href="Identity_map_pattern" title="Identity map pattern">Identity map</a></li>

<li><a href="Data_access_object" title="Data access object">Data access object</a></li>

<li><a href="Data_transfer_object" title="Data transfer object">Data transfer object</a></li>

<li><a href="Inversion_of_control" title="Inversion of control">Inversion of control</a></li>

<li><a href="JSP_model_2_architecture" title="JSP model 2 architecture">Model 2</a></li>

<li><a href="Broker_pattern" title="Broker pattern">Broker</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Other patterns</th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Blackboard_design_pattern" class="mw-redirect" title="Blackboard design pattern">Blackboard</a></li>

<li><a href="Business_delegate_pattern" title="Business delegate pattern">Business delegate</a></li>

<li><a href="Composite_entity_pattern" title="Composite entity pattern">Composite entity</a></li>

<li><a href="Dependency_injection" title="Dependency injection">Dependency injection</a></li>

<li><a href="Guard_(computer_science)" title="Guard (computer science)">Guard clause</a></li>

<li><a href="Intercepting_filter_pattern" title="Intercepting filter pattern">Intercepting filter</a></li>

<li><a href="Lazy_loading" title="Lazy loading">Lazy loading</a></li>

<li><a href="Mock_object" title="Mock object">Mock object</a></li>

<li><a href="Null_object_pattern" title="Null object pattern">Null object</a></li>

<li><a href="Object_pool_pattern" title="Object pool pattern">Object pool</a></li>

<li><a href="Servant_(design_pattern)" title="Servant (design pattern)">Servant</a></li>

<li><a href="Type_Tunnel_pattern" class="mw-redirect" title="Type Tunnel pattern">Type tunnel</a></li>

<li><a href="Method_chaining" title="Method chaining">Method chaining</a></li>

<li><a href="Delegation_pattern" title="Delegation pattern">Delegation</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Books</th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Design_Patterns" title="Design Patterns">Design Patterns</a></li>

<li><a href="Enterprise_Integration_Patterns" title="Enterprise Integration Patterns">Enterprise Integration Patterns</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">People</th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Christopher_Alexander" title="Christopher Alexander">Christopher Alexander</a></li>

<li><a href="Erich_Gamma" title="Erich Gamma">Erich Gamma</a></li>

<li><a href="Ralph_Johnson_(computer_scientist)" title="Ralph Johnson (computer scientist)">Ralph Johnson</a></li>

<li><a href="John_Vlissides" title="John Vlissides">John Vlissides</a></li>

<li><a href="Grady_Booch" title="Grady Booch">Grady Booch</a></li>

<li><a href="Ward_Cunningham" title="Ward Cunningham">Ward Cunningham</a></li>

<li><a href="Martin_Fowler_(software_engineer)" title="Martin Fowler (software engineer)">Martin Fowler</a></li>

<li><a href="Robert_C._Martin" title="Robert C. Martin">Robert Martin</a></li>

<li><a href="Jim_Coplien" title="Jim Coplien">Jim Coplien</a></li>

<li><a href="Douglas_C._Schmidt" title="Douglas C. Schmidt">Douglas Schmidt</a></li>

<li><a href="Linda_Rising" title="Linda Rising">Linda Rising</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Communities</th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="The_Hillside_Group" title="The Hillside Group">The Hillside Group</a></li>

<li><a href="Portland_Pattern_Repository" title="Portland Pattern Repository">Portland Pattern Repository</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">See also</th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Anti-pattern" title="Anti-pattern">Anti-pattern</a></li>

<li><a href="Architectural_pattern" title="Architectural pattern">Architectural pattern</a></li></ul>

</div></td></tr></tbody></table></div></div><div><div class="zim-footer">

This article is issued from <a class="external text" title="Last edited on 2024-09-16" href="https://en.wikipedia.org/wiki/?title=Iterator_pattern&oldid=1246079515">Wikipedia</a>. The text is available under <a class="external text" href="https://creativecommons.org/licenses/by-sa/4.0/deed.en">Creative Commons Attribution-Share Alike 4.0</a> unless otherwise noted. Additional terms may apply for the media files.

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