instance_variables.rb

class Shopper
  def initialize(list)
    @list = list
  end

  attr_accessor :list
end

string = "foo"
a = Shopper.new(string)
b = Shopper.new(string)

p a.object_id # => 6504360
p b.object_id # => 6504340

p a.list.object_id # => 6504380
p b.list.object_id # => 6504380

b.list.concat("baz") # => "foobaz"
p b.list # => "foobaz"
p a.list # => "foobaz"

a.list.concat("hello") # => "foobazhello"
p a.list # => "foobazhello"
p b.list # => "foobazhello"

# >> 6504360
# >> 6504340
# >> 6504380
# >> 6504380
# >> "foobaz"
# >> "foobaz"
# >> "foobazhello"
# >> "foobazhello"

Now try with :

s = "foo"
a = Shopper.new(s)
b = Shopper.new(s)

... and compare s.object_id, with a.list.object_id, and b.list.object_id

I see the difference. I updated the gist to reflect these differences.

Thanks!