Explain the Visitor pattern and its applications.

Visitor pattern separates algorithms from object structures, allowing new operations without modifying classes. Elements accept visitor; visitor implements operation for each element type. Double dispatch: element type determines method, visitor provides implementation. Use cases: compilers (AST operations), document processing, reporting across hierarchies. Benefits: add operations easily, gather related operations. Drawbacks: hard to add element types, breaks encapsulation. Consider pattern matching in modern languages as alternative. Often used with Composite pattern.