How deny creating instance variables in Ruby

Question

I would like to deny creating instance variables in Ruby,to prevent unattended variables being created 'by mistake'.

My class:

class Test
  def initialize
    @a = 'Var A'
  end

  def make_new
    @b = 'Var B' <-- I would like to deny creation of any variables that were not defined during the init
  end
end

Answer 1

You can freeze object instances at the end of initialize method:

class Test
  def initialize
    @a = 'Var A'
    freeze
  end

  def make_new
    @b = 'Var B' # I would like to deny creation of any variables that were not defined during the init
  end
end

t=Test.new
p t.instance_variable_get :@a
# "Var A"
t.make_new
#a.rb:24:in `make_new': can't modify frozen Test (RuntimeError)
#        from a.rb:30:in `<main>'
t.instance_variable_set :@c, 'Var C'
# a.rb:31:in `instance_variable_set': can't modify frozen Test (RuntimeError)
#        from a.rb:31:in `<main>'
class << t
  @d = 'Var D'
end
#a.rb:33:in `singletonclass': can't modify frozen Class (RuntimeError)
#        from a.rb:32:in `<main>'
p t.instance_variable_get :@d