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:
“The State has but one face for me: that of the police. To my eyes, all of the State’s ministries have this single face, and I cannot imagine the ministry of culture other than as the police of culture, with its prefect and commissioners.”
—Jean Dubuffet (1901–1985)
“Mine was, as it were, the connecting link between wild and cultivated fields; as some states are civilized, and others half-civilized, and others savage or barbarous, so my field was, though not in a bad sense, a half-cultivated field. They were beans cheerfully returning to their wild and primitive state that I cultivated, and my hoe played the Ranz des Vaches for them.”
—Henry David Thoreau (1817–1862)
“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)