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:
“In the years of the Roman Republic, before the Christian era, Roman education was meant to produce those character traits that would make the ideal family man. Children were taught primarily to be good to their families. To revere gods, one’s parents, and the laws of the state were the primary lessons for Roman boys. Cicero described the goal of their child rearing as “self- control, combined with dutiful affection to parents, and kindliness to kindred.””
—C. John Sommerville (20th century)
“Scholars who become politicians are usually assigned the comic role of having to be the good conscience of state policy.”
—Friedrich Nietzsche (1844–1900)
“Once, when lying in bed with no paper at hand, he began to sketch the idea for a new machine on the back of his wife’s nightgown. He asked her if she knew the figure he was drawing. “Yes,” she answered, “the figure of a fool.””
—For the State of New Jersey, U.S. public relief program (1935-1943)