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:
“I was put into jail as I was going to the shoemaker’s to get a shoe which was mended. When I was let out the next morning, I proceeded to finish my errand, and, having put on my mended shoe, joined a huckleberry party, who were impatient to put themselves under my conduct; and in half an hour ... was in the midst of a huckleberry field, on one of our highest hills, two miles off, and then the State was nowhere to be seen.”
—Henry David Thoreau (1817–1862)
“In time the scouring of wind and rain will wear down the ranges and plane off the region until it has the drab monotony of the older deserts. In the meantime—a two-million-year meantime—travelers may enjoy the cruel beauties of a desert in its youth,....”
—For the State of California, U.S. public relief program (1935-1943)
“Thou knowest in the state of innocency Adam fell, and what
should poor Jack Falstaff do in the days in villainy?”
—William Shakespeare (1564–1616)