43 lines
1.7 KiB
Markdown
43 lines
1.7 KiB
Markdown
---
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layout: pattern
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title: Acyclic Visitor
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folder: acyclic-visitor
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permalink: /patterns/acyclic-visitor/
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categories: Behavioral
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tags:
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- Extensibility
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---
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## Intent
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Allow new functions to be added to existing class hierarchies without affecting those hierarchies, and without creating the troublesome dependency cycles that are inherent to the GOF VISITOR Pattern.
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## Class diagram
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## Applicability
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This pattern can be used:
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* When you need to add a new function to an existing hierarchy without the need to alter or affect that hierarchy.
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* When there are functions that operate upon a hierarchy, but which do not belong in the hierarchy itself. e.g. the ConfigureForDOS / ConfigureForUnix / ConfigureForX issue.
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* When you need to perform very different operations on an object depending upon its type.
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* When the visited class hierarchy will be frequently extended with new derivatives of the Element class.
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* When the recompilation, relinking, retesting or redistribution of the derivatives of Element is very expensive.
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## Consequences
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The good:
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* No dependency cycles between class hierarchies.
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* No need to recompile all the visitors if a new one is added.
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* Does not cause compilation failure in existing visitors if class hierarchy has a new member.
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The bad:
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* Violates the principle of least surprise or Liskov's Substitution principle by showing that it can accept all visitors but actually only being interested in particular visitors.
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* Parallel hierarchy of visitors has to be created for all members in visitable class hierarchy.
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## Related patterns
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* [Visitor Pattern](../visitor/)
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## Credits
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* [Acyclic Visitor](http://condor.depaul.edu/dmumaugh/OOT/Design-Principles/acv.pdf)
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