Visitor Pattern

Lets you add new operations to a class hierarchy without modifying any of the existing classes — by moving the operation logic into a separate "visitor" object.

#behavioral#visitor#double dispatch#open closed#gof

Field Note - Real-world Analogy

Think of it like this

“Like a tax assessor visiting different property types. The assessor (visitor) visits a house, an apartment, and a commercial building (elements). The assessment logic is completely different for each type. The properties don't know how they're assessed — they just let the assessor in (accept). To add a "property insurance calculator", you create a new assessor type without touching the house or apartment classes at all.”

Field Observations - Key Topics Covered

The core problem Visitor solves: you have a stable class hierarchy (like Shape → Circle, Square, Triangle) and you want to perform many different operations on it (calculate area, export to SVG, export to PDF, generate a bounding box, etc.). Without Visitor, every time you add a new operation, you have to open and modify every class in the hierarchy — violating the Open-Closed Principle.

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