From: Adam Bender <abender@...>
Date: 2008-04-15T02:25:07+09:00
Subject: Mutable member variables -- surprising behavior

I noticed some surprising behavior in one of my Ruby programs, and am
wondering what the rationale is.  Basically,
given read-only access to a member variable (using attr_reader) of
class Array, I can modify that Array by modifying (what I think should
be) a local copy of it.  Shouldn't the accessor return a copy of the
variable instead of a reference to it?  Or is it standard practice to
hand-write an accessor for arrays that returns a clone of the array?

#!/usr/bin/ruby

class Example
  attr_reader :elems

  def initialize
    @elems = []
    3.times { @elems << Object.new }
  end

  def to_s
    @elems.inspect
  end
end

e = Example.new
puts e
# output is:
# -604282308
# [#<Object:0xb7f6c450>, #<Object:0xb7f6c43c>, #<Object:0xb7f6c428>]

list = e.elems
puts list.object_id
# output is: -604282308
list.delete_at(0)

puts e
# output is
# -604282308
# [#<Object:0xb7f6c43c>, #<Object:0xb7f6c428>]