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:
“Society in every state is a blessing, but Government, even in its best state, is but a necessary evil; in its worst state, an intolerable one.”
—Thomas Paine (1737–1809)
“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)
“The nonconformist and the rebel say all manner of unanswerable things against the existing republic, but discover to our sense no plan of house or state of their own.”
—Ralph Waldo Emerson (1803–1882)