How deny creating instance variables in Ruby - 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

I don't claim this is a good idea, but just b/c it's kind of interesting, here is a solution that will throw an exception when a new ivar is created, but will also let you modify defined instance variables (unlike freezing the class). Just threw this together, there are undoubtably some issues w/ it, including the fact that it duplicates every method :)
module IvarBlocker
def method_added(method)
alias_name = "__#{method}_orig"
return if method == :initialize || method_defined?(alias_name) || method.match(/__.*_orig/)
alias_method alias_name, method
define_method(method) do |*args|
ivars_before = instance_variables.dup
send(alias_name, *args).tap { raise "New Ivar assigned" if !(instance_variables - ivars_before).empty? }
end
end
end
Usage
class Test
extend IvarBlocker
def initialize
#a = 1
end
def set_b
#b = 2
end
def set_a
#a = 6
end
end
t = Test.new #=> #<Test:0x007f87f13c41e8 #a=1>
t.set_b #=> RuntimeError: New Ivar assigned
t.set_a #=> 6

Answer 2

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

Answer 3

There is a way - a hacky (but fun) way which is not meant for production (and is relatively slow). My sample implementation works for a single object only, but can be extended to support many objects.
Let's assume the following setup:
class Foo
def initialize
#a = :foo
end
def set_b; #b = 3; end
def set_c; #c = 7; end
end
def freeze_variables_of(obj)
frozen_variables = obj.instance_variables
set_trace_func lambda {|event, file, line, id, binding, classname|
if classname == obj.class
this = binding.eval 'self'
if this == obj
(this.instance_variables - frozen_variables).each {|var| this.remove_instance_variable var}
end
end
}
end
With the use of set_trace_func we can set a Proc which is called very often (usually more than once per statement). In that Proc we can check instance variables and remove unwanted variables.
Let's look at an example:
a = Foo.new
# => #<Foo:0x007f6f9db75cc8 #a=:foo>
a.set_b; a
# => #<Foo:0x007f6f9db75cc8 #a=:foo, #b=3>
freeze_variables_of a
a.set_c; a
# => #<Foo:0x007f6f9db75cc8 #a=:foo, #b=3>
We see that after doing the "freeze", set_c cannot set the instance variable (in fact the variable is removed at the very moment the set_c method returns).
In contrast to freezing the object (with a.freeze) (which I'd recommend for any real world application), this way allows you to modify all allowed instance variables while forbidding new ones.
This even works, if you directly assign instance variables (while Alex' method is probably faster, but relies on accessor methods).

Answer 4

There is no way to prevent creation of accidental instance variables defined that way. Why do you want to do this? What would you want such code to achieve?