Where is my instance variable in Object?

Question

Why can't I see @obj.instance_variables in Object when method_missing is invoked?

module Arena
  class Place
    def initialize obj
      @obj = obj
      method_missing_in_obj
      @obj.instance_variable_set(:@unit, '10')
      puts @obj.instance_variables
      yield @obj
    end
    def method_missing_in_obj
      def @obj.method_missing method, *args, &blk
        puts @obj.instance_variables
        super
      end
      self
    end
  end
end

Arena::Place.new(Object.new) do |obj|
  puts obj.instance_variable_get(:@unit)
  puts obj.foo
end

$> ruby test_me.rb:

=> @unit
=> 10
=> in `method_missing': undefined method `foo' for #<Object:0x007fd89b1c96e0 @unit="10"> (NoMethodError)

Max · Accepted Answer · 2014-10-04 20:45:42Z

2

This is a subtle bug! The problem is that you're calling @obj.instance_variables when you define @obj.method_missing. Remember, that defines a method in @obj's singleton class, so when you use @obj inside the method definition, you're asking for @obj's instance variable @obj which is nil (and nil has no instance variables).

All you need to do is remove the explicit receiver, since @obj is implicitly the receiver for methods defined in its singleton class.

def method_missing_in_obj
  def @obj.method_missing method, *args, &blk
    puts instance_variables
    super
  end
  self
end

answered Oct 4, 2014 at 20:45

Max

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