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:
“A ship is a bit of terra firma cut off from the main; it is a state in itself; and the captain is its king.”
—Herman Melville (1819–1891)
“To reduce the imagination to a state of slavery—even though it would mean the elimination of what is commonly called happiness—is to betray all sense of absolute justice within oneself. Imagination alone offers me some intimation of what can be.”
—André Breton (1896–1966)
“No state can build
A literature that shall at once be sound
And sad on a foundation of well-being.”
—Robert Frost (1874–1963)