# Visitor Pattern

> A design pattern that places each operation over a type hierarchy in its own visitor, so an operation is added without editing the element types.

Record: `architecture:visitor-pattern`
Kind: pattern
Layer: [Design Patterns Core](https://banes-lab.com/records/layer/design-patterns-core.md)
Severity: contextual
Scope: behavior, operation, type hierarchy
Canonical: https://banes-lab.com/ontology#architecture-visitor-pattern

Listed in [Architecture principles](https://banes-lab.com/api/records/architecture.md), after [Iterator Pattern](https://banes-lab.com/records/architecture/iterator-pattern.md) and before [Memento Pattern](https://banes-lab.com/records/architecture/memento-pattern.md).

## Repair

- Refactored by: Introduce Visitor
- Detected by: type-tag switches repeated per operation
- Violated by: operations added by editing every element type
- Measured by: type-switch duplication across operations
- Enforced by: [design review](https://banes-lab.com/records/architecture/design-review.md)

## Requires

- [Stable Element Hierarchy](https://banes-lab.com/records/lexicon/stable-element-hierarchy.md)

## Reinforces

- [Open/Closed Principle (OCP)](https://banes-lab.com/records/architecture/open-closed.md)
- [Separation of Concerns](https://banes-lab.com/records/architecture/separation-of-concerns.md)

## Enables

- [Operation Extension Without Element Change](https://banes-lab.com/records/lexicon/operation-extension-without-element-change.md)

## Conflicts with

- [Type-Switch Dispatch](https://banes-lab.com/records/lexicon/type-switch-dispatch.md)

## In tension with

- [Element Stability](https://banes-lab.com/records/lexicon/element-stability.md)

## Tensions

- [Visitor Pattern / Element Stability](https://banes-lab.com/records/tension/element-stability-visitor-pattern.md)

## Severity

- [contextual](https://banes-lab.com/records/vocabulary/severity-contextual.md)

## Category

- [Behavioral Patterns](https://banes-lab.com/records/architecture-category/behavioral-patterns.md)

## Linked from

- [Core Modular Design](https://banes-lab.com/ontology/principles/architecture-category-core-modular-design.md)
- [SOLID / Object-Oriented Design](https://banes-lab.com/ontology/principles/architecture-category-solid-object-oriented-design.md)
- [Behavioral Patterns](https://banes-lab.com/ontology/lexicon/lexicon-category-behavioral-patterns.md)
- [Severity levels](https://banes-lab.com/ontology/schema/the-vocabulary-severity.md)
- [The resolutions](https://banes-lab.com/ontology/schema/the-resolutions.md)