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Ruby Programming
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Class Variables

A class variable is shared among all objects of a class, and it is also accessible to the class methods that we'll describe later. There is only one copy of a particular class variable for a given class. Class variable names start with two ``at'' signs, such as ``@@count''. Unlike global and instance variables, class variables must be initialized before they are used. Often this initialization is just a simple assignment in the body of the class definition.

For example, our jukebox may want to record how many times each particular song has been played. This count would probably be an instance variable of the Song object. When a song is played, the value in the instance is incremented. But say we also want to know how many songs have been played in total. We could do this by searching for all the Song objects and adding up their counts, or we could risk excommunication from the Church of Good Design and use a global variable. Instead, we'll use a class variable.

class Song
  @@plays = 0
  def initialize(name, artist, duration)
    @name     = name
    @artist   = artist
    @duration = duration
    @plays    = 0
  end
  def play
    @plays += 1
    @@plays += 1
    "This  song: #@plays plays. Total #@@plays plays."
  end
end

For debugging purposes, we've arranged for Song#play to return a string containing the number of times this song has been played, along with the total number of plays for all songs. We can test this easily.

s1 = Song.new("Song1", "Artist1", 234)  # test songs..
s2 = Song.new("Song2", "Artist2", 345)
s1.play "This  song: 1 plays. Total 1 plays."
s2.play "This  song: 1 plays. Total 2 plays."
s1.play "This  song: 2 plays. Total 3 plays."
s1.play "This  song: 3 plays. Total 4 plays."

Class variables are private to a class and its instances. If you want to make them accessible to the outside world, you'll need to write an accessor method. This method could be either an instance method or, leading us neatly to the next section, a class method.
Ruby Programming
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  Published under the terms of the Open Publication License Design by Interspire