State
Aside from potentially improving separation of concerns, the visitor pattern has an additional advantage over simply calling a polymorphic method: a visitor object can have state. This is extremely useful in many cases where the action performed on the object depends on previous such actions.
An example of this is a pretty-printer in a programming language implementation (such as a compiler or interpreter). Such a pretty-printer object (implemented as a visitor, in this example), will visit nodes in a data structure that represents a parsed and processed program. The pretty-printer will then generate a textual representation of the program tree. To make the representation human-readable, the pretty-printer should properly indent program statements and expressions. The current indentation level can then be tracked by the visitor as its state, correctly applying encapsulation, whereas in a simple polymorphic method invocation, the indentation level would have to be exposed as a parameter and the caller would rely on the method implementation to use and propagate this parameter correctly.
Read more about this topic: Visitor Pattern
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—Administration in the State of Sout, U.S. public relief program (1935-1943)
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—Anna Quindlen (20th century)
“[T]he Congregational minister in a neighboring town definitely stated that ‘the same spirit which drove the herd of swine into the sea drove the Baptists into the water, and that they were hurried along by the devil until the rite was performed.’”
—For the State of Vermont, U.S. public relief program (1935-1943)