Editing Visitor pattern (section)

== Structure ==
=== UML class and sequence diagram ===
[[File:w3sDesign Visitor Design Pattern UML.jpg|frame|none|A sample [[Unified Modeling Language|UML]] class diagram and [[sequence diagram]] for the Visitor design pattern.<ref>{{Cite web |title=The Visitor design pattern - Structure and Collaboration |url=http://w3sdesign.com/?gr=b11&ugr=struct |url-status=dead |access-date=2017-08-12 |website=w3sDesign.com}}</ref>]]

In the [[Unified Modeling Language|UML]] [[class diagram]] above, the <code>ElementA</code> class doesn't implement a new operation directly.
Instead, <code>ElementA</code> implements a ''dispatching operation''  <code>accept(visitor)</code> that "dispatches" (delegates) a request to the "accepted visitor object" (<code>visitor.visitElementA(this)</code>). The <code>Visitor1</code> class implements the operation (<code>visitElementA(e:ElementA)</code>).
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<code>ElementB</code> then implements <code>accept(visitor)</code> by dispatching to <code>visitor.visitElementB(this)</code>. The <code>Visitor1</code> class implements the operation (<code>visitElementB(e:ElementB)</code>).

The [[Unified Modeling Language|UML]] [[sequence diagram]]
shows the run-time interactions: The <code>Client</code> object traverses the elements of an object structure (<code>ElementA,ElementB</code>) and calls <code>accept(visitor)</code> on each element.
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First, the <code>Client</code> calls <code>accept(visitor)</code> on
<code>ElementA</code>, which calls <code>visitElementA(this)</code> on the accepted <code>visitor</code> object.
The element itself (<code>this</code>) is passed to the <code>visitor</code> so that 
it can "visit" <code>ElementA</code> (call <code>operationA()</code>).
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Thereafter, the <code>Client</code> calls <code>accept(visitor)</code> on
<code>ElementB</code>, which calls <code>visitElementB(this)</code> on the <code>visitor</code> that "visits" <code>ElementB</code> (calls <code>operationB()</code>).

=== Class diagram ===
[[File:Visitor design pattern.svg|frame|none|Visitor in [[Unified Modeling Language]] (UML).<ref>{{Cite book |last=Reddy |first=Martin |url=https://www.worldcat.org/oclc/704559821 |title=API design for C++ |date=2011 |publisher=Morgan Kaufmann |isbn=978-0-12-385004-1 |location=Boston |oclc=704559821}}</ref>{{Rp|page=381}}]]
[[File:Visitor pattern class diagram in LePUS3.gif|frame|none|Visitor in [[Lepus3|LePUS3]] ([http://lepus.org.uk/ref/legend/legend.xml legend])]]