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:
“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)
“In the most high and palmy state of Rome,
A little ere the mightiest Julius fell,
The graves stood tenantless and the sheeted dead
Did squeak and gibber in the Roman streets.”
—William Shakespeare (1564–1616)
“Neighboring farmers and visitors at White Sulphur drove out occasionally to watch ‘those funny Scotchmen’ with amused superiority; when one member imported clubs from Scotland, they were held for three weeks by customs officials who could not believe that any game could be played with ‘such elongated blackjacks or implements of murder.’”
—For the State of West Virginia, U.S. public relief program (1935-1943)