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:
“Wooing, wedding, and repenting, is as a Scotch jig, a measure, and a cinquepace; the first suit is hot and hasty, like a Scotch jig, and full as fantastical; the wedding, mannerly-modest, as a measure, full of state and ancientry; and then comes repentance and, with his bad legs, falls into the cinquepace faster and faster, till he sink into his grave.”
—William Shakespeare (1564–1616)
“The recent attempt to secure a charter from the State of North Dakota for a lottery company, the pending effort to obtain from the State of Louisiana a renewal of the charter of the Louisiana State Lottery, and the establishment of one or more lottery companies at Mexican towns near our border, have served the good purpose of calling public attention to an evil of vast proportions.”
—Benjamin Harrison (1833–1901)
“Such is my jealousy and such my hate
though I would state it otherwise.”
—Hilda Doolittle (1886–1961)