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:
“The essence of the modern state is that the universal be bound up with the complete freedom of its particular members and with private well-being, that thus the interests of family and civil society must concentrate themselves on the state.... It is only when both these moments subsist in their strength that the state can be regarded as articulated and genuinely organized.”
—Georg Wilhelm Friedrich Hegel (1770–1831)
“A perfect personality ... is only possible in a state of society where man is free to choose the mode of work, the conditions of work, and the freedom to work. One to whom the making of a table, the building of a house, or the tilling of the soil, is what the painting is to the artist and the discovery to the scientist,—the result of inspiration, of intense longing, and deep interest in work as a creative force.”
—Emma Goldman (1869–1940)
“Our object in the construction of the state is the greatest happiness of the whole, and not that of any one class.”
—Plato (c. 427–347 B.C.)