lazy_class_variable.rb

def init_var
  puts 'init var...'
  'var value'
end

puts 'define class A'

class A
  @@var = init_var

  class << self
    def use_var
      puts @@var
    end
  end
end

puts 'class A defined'

puts 'using var'
A.use_var

puts

puts 'define class B'

class B
  class << self
    def use_var
      puts var
    end

    def var
      @var ||= init_var
    end
  end
end

puts 'class B defined'

puts 'using var'
B.use_var
    
__END__
Output:

  define class A
  init var...
  class A defined
  using var
  var value

  define class B
  class B defined
  using var
  init var...
  var value