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:
“Our ancestors were savages. The story of Romulus and Remus being suckled by a wolf is not a meaningless fable. The founders of every state which has risen to eminence have drawn their nourishment and vigor from a similar wild source. It was because the children of the Empire were not suckled by the wolf that they were conquered and displaced by the children of the northern forests who were.”
—Henry David Thoreau (1817–1862)
“Feign then what’s by a decent tact believed
And act that state is only so conceived,
And build an edifice of form
For house where phantoms may keep warm.”
—William Empson (1906–1984)
“An alliance is like a chain. It is not made stronger by adding weak links to it. A great power like the United States gains no advantage and it loses prestige by offering, indeed peddling, its alliances to all and sundry. An alliance should be hard diplomatic currency, valuable and hard to get, and not inflationary paper from the mimeograph machine in the State Department.”
—Walter Lippmann (1889–1974)