class_variable_test.rb

class RubyClassInstanceVariableTest
  def self.attr1
    @attr1 = "*att1*"
  end

  def self.test1
    attr1
    puts "Try directly print @attr1: #{@attr1}"
  end

  def self.test2
    puts "Try print @attr1 via attr1 method: #{attr1}"
  end
end

class ChildRubyClassInstanceVariableTest < RubyClassInstanceVariableTest
  def self.test3
    puts "If I can see attr1 from parent? #{@attr1}"
  end
end

puts "Testcase 1:"
RubyClassInstanceVariableTest.test1
RubyClassInstanceVariableTest.test2
puts "instance_variable_get: #{RubyClassInstanceVariableTest.instance_variable_get(:@attr1)}"

puts "\nIn SubClass:"
ChildRubyClassInstanceVariableTest.test1
ChildRubyClassInstanceVariableTest.test2
ChildRubyClassInstanceVariableTest.test3
puts "child instance_variable_get: #{ChildRubyClassInstanceVariableTest.instance_variable_get(:@attr1)}"

## @@variable

puts "\nTestcase 2:"

class RubyClassInstanceVariableTest2

  def self.attr1
    @@attr1 = "[attr1]"
  end

  def self.test1
    attr1
    puts "Try directly print @@attr1: #{@@attr1}"
  end

  def self.test2
    puts "Try print @attr1 via attr1 method: #{attr1}"
  end
end

class ChildRubyClassInstanceVariableTest2 < RubyClassInstanceVariableTest2
  def self.test3
    puts "If I can see attr1 from parent? #{@@attr1}"
  end
end

RubyClassInstanceVariableTest2.test1
RubyClassInstanceVariableTest2.test2
puts "class_variable_get: #{RubyClassInstanceVariableTest2.class_variable_get(:@@attr1)}"

puts "\nIn SubClass:"
ChildRubyClassInstanceVariableTest2.test1
ChildRubyClassInstanceVariableTest2.test2
ChildRubyClassInstanceVariableTest2.test3
puts "child class_variable_get: #{ChildRubyClassInstanceVariableTest2.class_variable_get(:@@attr1)}"

Result:

Testcase 1:
Try directly print @attr1: *att1*
Try print @attr1 via attr1 method: *att1*
instance_variable_get: *att1*

In SubClass:
Try directly print @attr1: *att1*
Try print @attr1 via attr1 method: *att1*
If I can see attr1 from parent? *att1*
child instance_variable_get: *att1*

Testcase 2:
Try directly print @@attr1: [attr1]
Try print @attr1 via attr1 method: [attr1]
class_variable_get: [attr1]

In SubClass:
Try directly print @@attr1: [attr1]
Try print @attr1 via attr1 method: [attr1]
If I can see attr1 from parent? [attr1]
child class_variable_get: [attr1]