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:
“An alliance is like a chain. It is not made stronger by adding weak links to it. A great power like the United States gains no advantage and it loses prestige by offering, indeed peddling, its alliances to all and sundry. An alliance should be hard diplomatic currency, valuable and hard to get, and not inflationary paper from the mimeograph machine in the State Department.”
—Walter Lippmann (1889–1974)
“If Los Angeles has been called “the capital of crackpots” and “the metropolis of isms,” the native Angeleno can not fairly attribute all of the city’s idiosyncrasies to the newcomer—at least not so long as he consults the crystal ball for guidance in his business dealings and his wife goes shopping downtown in beach pajamas.”
—For the State of California, U.S. public relief program (1935-1943)
“Cole’s Hill was the scene of the secret night burials of those who died during the first year of the settlement. Corn was planted over their graves so that the Indians should not know how many of their number had perished.”
—For the State of Massachusetts, U.S. public relief program (1935-1943)