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:
“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)
“[17th-century] Puritans were the first modern parents. Like many of us, they looked on their treatment of children as a test of their own self-control. Their goal was not to simply to ensure the child’s duty to the family, but to help him or her make personal, individual commitments. They were the first authors to state that children must obey God rather than parents, in case of a clear conflict.”
—C. John Sommerville (20th century)
“Melancholy has ceased to be an individual phenomenon, an exception. It has become the class privilege of the wage earner, a mass state of mind that finds its cause wherever life is governed by production quotas.”
—Günther Grass (b. 1927)