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
Famous quotes containing the word state:
“There is nothing worse than an idle hour, with no occupation offering. People who have many such hours are simply animals waiting docilely for death. We all come to that state soon or late. It is the curse of senility.”
—H.L. (Henry Lewis)
“Alexander Woollcott broadcasts the story of the wife who returned a dog to the Seeing Eye with this note attached: “I am sending the dog back. My husband used to depend on me. Now he is independent, and I never know where he is.””
—For the State of New Jersey, U.S. public relief program (1935-1943)
“In the most high and palmy state of Rome,
A little ere the mightiest Julius fell,
The graves stood tenantless and the sheeted dead
Did squeak and gibber in the Roman streets.”
—William Shakespeare (1564–1616)