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Ruby Programming
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Object-Specific Classes

Ruby allows you to create a class tied to a particular object. In the following example, we create two String objects. We then associate an anonymous class with one of them, overriding one of the methods in the object's base class and adding a new method.

a = "hello"
b = a.dup
class <<a
  def to_s
    "The value is '#{self}'"
  end
  def twoTimes
    self + self
  end
end
a.to_s "The value is 'hello'"
a.twoTimes "hellohello"
b.to_s "hello"

This example uses the ``class << obj'' notation, which basically says ``build me a new class just for object obj.'' We could also have written it as:

a = "hello"
b = a.dup
def a.to_s
  "The value is '#{self}'"
end
def a.twoTimes
  self + self
end
a.to_s "The value is 'hello'"
a.twoTimes "hellohello"
b.to_s "hello"

The effect is the same in both cases: a class is added to the object ``a''. This gives us a strong hint about the Ruby implementation: a singleton class is created and inserted as a's direct class. a's original class, String, is made this singleton's superclass. The before and after pictures are shown in Figure 19.3 on page 242.

Ruby performs a slight optimization with these singleton classes. If an object's klass reference already points to a singleton class, a new one will not be created. This means that the first of the two method definitions in the previous example will create a singleton class, but the second will simply add a method to it.

Figure not available...

Ruby Programming
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  Published under the terms of the Open Publication License Design by Interspire