Editing Visitor pattern (section)

==Overview==
The Visitor<ref name="GoF">{{Cite book |last=Gamma |first=Erich |author-link=Erich Gamma |url=https://archive.org/details/designpatternsel00gamm/page/331 |title=Design Patterns: Elements of Reusable Object-Oriented Software |last2=Helm |first2=Richard |author-link2=Richard Helm |last3=Johnson |first3=Ralph |author-link3=Ralph Johnson (computer scientist) |last4=Vlissides |first4=John |author-link4=John Vlissides |publisher=Addison Wesley |year=1994 |isbn=0-201-63361-2 |pages=[https://archive.org/details/designpatternsel00gamm/page/331 331ff] |url-access=registration}}</ref>
design pattern is one of the twenty-three well-known ''[[Design Patterns|Gang of Four design patterns]]'' 
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.

=== Problems, the Visitor design pattern can solve ===

* It should be possible to define a new operation for (some) classes of an object structure without changing the classes.

When new operations are needed frequently and the object structure consists of many unrelated classes,
it's inflexible to add new subclasses each time a new operation is required
because "[..] distributing all these operations across the various node classes leads to a system that's hard to understand, maintain, and change."<ref name="GoF" />

=== Solution, the Visitor design pattern describes ===
* Define a separate (visitor) object that implements an operation to be performed on elements of an object structure.
* Clients traverse the object structure and call a ''dispatching operation accept (visitor)'' on an element — that "dispatches" (delegates) the request to the "accepted visitor object". The visitor object then performs the operation on the element ("visits the element").
            
This makes it possible to create new operations independently from the classes of an object structure
by adding new visitor objects.

See also the UML class and sequence diagram below.