Overriding instance variable array’s operators in Ruby and scoping

Question

I have a test class and a box class, in the test class i have a var called boxHolder, which is an array, i want to override the << method for this array. Inside the singleton how can i access moski_call ?

class Test
  attr_accessor :boxHolder

  def initialize()
   super
   self.boxHolder = Array.new

   class << @boxHolder
     def <<(box)
       box.setPositionWithinColumn(moski_call)
       super(box)
     end
   end
  end   

  def moski_call
    "YAAAAAAAAAAAAAAAAAAAA"
  end
end

class Box
  def initialize
  end

  def setPositionWithinColumn(str)
    puts "got a string #{str}"
  end
end

# test
box = Box.new
test = Test.new
test.boxHolder

By convention, rubyists don't use camelCase; it's box_holder, not boxHolder, etc... — Marc-André Lafortune
– Marc-André Lafortune, Commented Jul 22, 2010 at 16:02

horseyguy · Accepted Answer · 2010-07-22 12:54:38Z

0

like this:

# need this or else `moski_call` method is looked up in context of @boxholder
moski_call_output = moski_call

class << @boxholder; self; end.send(:define_method, :<<) { |box| 
     box.setPositionWithinColumn(moski_call_output)
     super(box)
}

edited Jul 22, 2010 at 12:54

answered Jul 22, 2010 at 12:34

horseyguy

30k20 gold badges110 silver badges151 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Moski Doski Over a year ago

i am getting the following error: "<<': undefined local variable or method moski_call' for []:Array (NameError)"

Moski Doski Over a year ago

Ah, this works, many thanks ... can you please explain why this works, defining moski_call_output solved the problem, but whats the difference between moski_call and moski_call_output ?

horseyguy Over a year ago

@moski, moski_call is a method and moski_call_output is a local variable. In a define_method block methods are looked up in the object the method is defined on - in this case @boxholder. However @boxholder does not have such a method and so it will error. local variables on the other hand are captured by the define_method block (closure) and will be accessible.

Marc-André Lafortune Over a year ago

moski_call_output won't get updated if moski_call changes... I posted a solution I prefer.

Edu · Accepted Answer · 2010-07-22 12:06:17Z

0

What about:

def self.boxHolder.<< (box)
   box.setPositionWithinColumn(moski_call)
   super(box)
end

This would declare a method for your instance boxHolder. But boxHolder does not have access to the method moski_call

answered Jul 22, 2010 at 12:06

Edu

1,9691 gold badge16 silver badges19 bronze badges

1 Comment

Moski Doski Over a year ago

it seems the syntax is not correct, i am getting "syntax error, unexpected '.', expecting '\n' or ';'"

Marc-André Lafortune · Accepted Answer · 2013-03-11 01:25:41Z

0

You need to maintain access to the "parent" Test object. This can be done using the fact that blocks are closures:

parent = self # to be accessible in the closure

@boxHolder.define_singleton_method(:<<) do |box|
  box.setPositionWithinColumn(parent.moski_call)
  super(box)
end

Note: define_singleton_method is new in Ruby 1.9, so either upgrade, require 'backports/1.9.1/kernel/define_singleton_method' or do class << @boxHolder; define_method(:<<){ "..." } end if using an older Ruby.

edited Mar 11, 2013 at 1:25

answered Jul 22, 2010 at 15:57

Marc-André Lafortune

79.8k17 gold badges172 silver badges167 bronze badges

Collectives™ on Stack Overflow

Overriding instance variable array’s operators in Ruby and scoping

3 Answers 3

4 Comments

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

4 Comments

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related