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    <p>there is a clear description in the book "The ruby programming language", read it will be very helpful. I paste it here(from chapter 7.1.16):</p> <blockquote> <p>An instance variable used inside a class definition but outside an instance method definition is a <strong>class instance variable</strong>.</p> </blockquote> <pre><code>class Point # Initialize our class instance variables in the class definition itself @n = 0 # How many points have been created @totalX = 0 # The sum of all X coordinates @totalY = 0 # The sum of all Y coordinates def initialize(x,y) # Initialize method @x,@y = x, y # Sets initial values for instance variables end def self.new(x,y) # Class method to create new Point objects # Use the class instance variables in this class method to collect data @n += 1 # Keep track of how many Points have been created @totalX += x # Add these coordinates to the totals @totalY += y super # Invoke the real definition of new to create a Point # More about super later in the chapter end # A class method to report the data we collected def self.report # Here we use the class instance variables in a class method puts "Number of points created: #@n" puts "Average X coordinate: #{@totalX.to_f/@n}" puts "Average Y coordinate: #{@totalY.to_f/@n}" end end </code></pre> <p>......</p> <blockquote> <p>Because class instance variables are just instance variables of class objects, we can use attr, attr_reader, and attr_accessor to create accessor methods for them.</p> </blockquote> <pre><code>class &lt;&lt; self attr_accessor :n, :totalX, :totalY end </code></pre> <p>With these accessors defined, we can refer to our raw data as Point.n, Point.totalX, and Point.totalY.</p>
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