Only setting getter's apparently doesn't prevent manipulation of instance variables. The shovel is a great and dangerous too.

example.rb

# create a class with an attr_reader for
# a string and an array
class Thing
  attr_reader :arr
  attr_reader :str

  def initialize
    @arr = []
    @str = ""
  end
end

# initialize the class
thing = Thing.new


# putting the array and string work
# as we would expect

p thing.arr
# => []

p thing.str
# => ""


# but if we shovel into them, we can
# alter their state -- even though we
# only exposed a getter method

thing.arr << "hello"
p thing.arr
# => ["hello"]

thing.str << "hello"
p thing.str
# => "hello"


# Lets try protecting our vars using dup

class Protected
  attr_accessor :exposed

  def initialize
    @exposed = "safe"
    @safe = "safe"
  end

  def safe
    @safe.dup
  end
end

# again, a sanity check to see that
# our values both equal "safe"

example = Protected.new
p example.safe == example.exposed
# => true

# we'll shovel to try and modify them
example.exposed << " or not"
example.safe << " or not"

# and test equality again
p example.safe == example.exposed
# => false

# we can see @safe keeps its original value
p example.safe
# => "safe"