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 best way of learning to be an independent sovereign state is to be an independent sovereign state.”
—Kwame Nkrumah (1900–1972)
“Wags try to invent new stories to tell about the legislature, and end by telling the old one about the senator who explained his unaccustomed possession of a large roll of bills by saying that someone pushed it over the transom while he slept. The expression “It came over the transom,” to explain any unusual good fortune, is part of local folklore.”
—For the State of Montana, U.S. public relief program (1935-1943)
“Feminism, like Boston, is a state of mind. It is the state of mind of women who realize that their whole position in the social order is antiquated, as a woman cooking over an open fire with heavy iron pots would know that her entire housekeeping was out of date.”
—Rheta Childe Dorr (1866–1948)