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 idea of a classless society is ... a disastrous mirage which cannot be maintained without tyranny of the few over the many. It is even more pernicious culturally than politically, not because the monolithic state forces the party line upon its intellectuals and artists, but because it has no social patterns to reflect.”
—Agnes E. Meyer (1887–1970)
“Across Parker Avenue from the fort is the Site of the Old Gallows, where 83 men “stood on nothin’, a-lookin’ up a rope.” The platform had a trap wide enought to “accommodate” 12 men, but half that number was the highest ever reached. On two occasions six miscreants were executed. There were several groups of five, some quartets and trios.”
—Administration in the State of Arka, U.S. public relief program. Arkansas: A Guide to the State (The WPA Guide to Arkansas)
“A State, in idea, is the opposite of a Church. A State regards classes, and not individuals; and it estimates classes, not by internal merit, but external accidents, as property, birth, etc. But a church does the reverse of this, and disregards all external accidents, and looks at men as individual persons, allowing no gradations of ranks, but such as greater or less wisdom, learning, and holiness ought to confer. A Church is, therefore, in idea, the only pure democracy.”
—Samuel Taylor Coleridge (1772–1834)