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:
“Worn down by the hoofs of millions of half-wild Texas cattle driven along it to the railheads in Kansas, the trail was a bare, brown, dusty strip hundreds of miles long, lined with the bleaching bones of longhorns and cow ponies. Here and there a broken-down chuck wagon or a small mound marking the grave of some cowhand buried by his partners “on the lone prairie” gave evidence to the hardships of the journey.”
—For the State of Kansas, U.S. public relief program (1935-1943)
“The most absurd apology for authority and law is that they serve to diminish crime. Aside from the fact that the State is itself the greatest criminal, breaking every written and natural law, stealing in the form of taxes, killing in the form of war and capital punishment, it has come to an absolute standstill in coping with crime. It has failed utterly to destroy or even minimize the horrible scourge of its own creation.”
—Emma Goldman (1869–1940)
“When the Revolutionaries ran short of gun wadding the Rev. James Caldwell ... broke open the church doors and seized an armful of Watts’ hymnbooks. The preacher threw them to the soldiers and shouted, “Give ‘em Watts, boys—give ‘em Watts!””
—For the State of New Jersey, U.S. public relief program (1935-1943)