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:
“Art is identical with a state of capacity to make, involving a true course of reasoning. All art is concerned with coming into being ... for art is concerned neither with things that are, or come into being, by necessity, nor with things that do so in accordance with nature.”
—Aristotle (384–323 B.C.)
“We are living in a demented world. And we know it. It would not come as a surprise to anyone if tomorrow the madness gave way to a frenzy which would leave our poor Europe in a state of distracted stupor, with engines still turning and flags streaming in the breeze, but with the spirit gone.”
—Johan Huizinga (1872–1945)
“Realizing that his time was nearly spent, he gave full oral instructions about his burial and the manner in which he wished to be remembered.... A few minutes later, feeling very tired, he left the room, remarking, ‘I have no disposition to leave this precious circle. I love to be here surrounded by my family and friends.’ Then he gave them his blessing and said, ‘I am ready to go and I wish you goodnight.’”
—For the State of New Hampshire, U.S. public relief program (1935-1943)