gistfile1.rb

def set_my_ivar
  @my_ivar = 'set'
end

class A
  attr_reader :my_ivar

  def test_the_setting
    set_my_ivar
  end
end

a = A.new

a.test_the_setting
puts "a: #{a.my_ivar.inspect}"
puts "main: #{@my_ivar.inspect}"

# a: "set"
# main: nil