Extensibility with Visitors

Visitor improves one kind of extensibility and weakens another.

Easy To Extend

Adding a new operation is often easy:

• add a new visitor type
• keep element classes unchanged

Harder To Extend

Adding a new element type is often expensive:

• update the visitor interface
• update every concrete visitor

Review Rule

Choose Visitor only when operation growth is the dominant force and element-type churn is relatively low.