Visitor Pattern

Adds new operations and behaviours to existing objects without modifying them.

Able to separate an algorithm from the object on which it operates.

"Double Dispatch" technique
- An object takes in a visitor, and calls the right visitor method with the needed data

ie Car

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visitor = Visitor()
Car.accept(visitor)
-> for each element in car:
   -> element.accept(visitor)
      -> visitor.visit(this)
visitor.visit(this)

Define a Visitor class which knows of the classes it will visit
- Has a visit(T) method for each class type
- In the implementation, do whatever you need
Define an interface Visitable which exposes an accept(Visitor) method
- In the implementation, <Visitor>.visit(this)

A new class would require a new visit method to be added to each visitor...

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