Ruby Language Tutorial => Variable Scope and Visibility

Syntax

Class variables are shared in the class hierarchy. This can result in surprising behavior.

class A
  @@variable = :x

  def self.variable
    @@variable
  end
end

class B < A
  @@variable = :y
end

A.variable  # :y

Classes are objects, so instance variables can be used to provide state that is specific to each class.

class A
  @variable = :x

  def self.variable
    @variable
  end
end

class B < A
  @variable = :y
end

A.variable  # :x

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