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:
“Scepticism is an ability, or mental attitude, which opposes appearances to judgments in any way whatsoever, with the result that, owing to the equipollence of the objects and reasons thus opposed we are brought firstly to a state of mental suspense and next to a state of “unperturbedness” or quietude.”
—Sextus Empiricus (2nd or 3rd cen., A.d.)
“Thou knowest in the state of innocency Adam fell, and what
should poor Jack Falstaff do in the days in villainy?”
—William Shakespeare (1564–1616)
“The present century has not dealt kindly with the farmer. His legends are all but obsolete, and his beliefs have been pared away by the professors at colleges of agriculture. Even the farm- bred bards who twang guitars before radio microphones prefer “I’m Headin’ for the Last Roundup” to “Turkey in the Straw” or “Father Put the Cows Away.””
—For the State of Kansas, U.S. public relief program (1935-1943)