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:
“But is it not the fact that religion emanates from the nature, from the moral state of the individual? Is it not therefore true that unless the nature be completely exercised, the moral state harmonised, the religion cannot be healthy?”
—Harriet Martineau (1802–1876)
“For I have learned, in whatsoever state I am, therewith to be content.”
—Bible: New Testament St. Paul, in Philippians, 4:11.
“Inferiors revolt in order that they may be equal, and equals that they may be superior. Such is the state of mind which creates revolutions.”
—Aristotle (384–322 B.C.)