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:
“In common with other rural regions much of the Iowa farm lore concerns the coming of company. When the rooster crows in the doorway, or the cat licks his fur, company is on the way.”
—For the State of Iowa, U.S. public relief program (1935-1943)
“He believes without reservation that Kentucky is the garden spot of the world, and is ready to dispute with anyone who questions his claim. In his enthusiasm for his State he compares with the Methodist preacher whom Timothy Flint heard tell a congregation that “Heaven is a Kentucky of a place.””
—For the State of Kentucky, U.S. public relief program (1935-1943)
“From this elevation, just on the skirts of the clouds, we could overlook the country, west and south, for a hundred miles. There it was, the State of Maine, which we had seen on the map, but not much like that,—immeasurable forest for the sun to shine on, the eastern stuff we hear of in Massachusetts. No clearing, no house. It did not look as if a solitary traveler had cut so much as a walking-stick there.”
—Henry David Thoreau (1817–1862)