Question

class Foo
  def bar
    @instance_variable = [['first']]

    # make a duplicate object with the :dup method
    local_variable=@instance_variable.dup

    # They have different object_id
    p @instance_variable.object_id
    p local_variable.object_id


    local_variable.each{|n|n.push('second')}
    @instance_variable
  end
end

f=Foo.new
p f.bar

=> 2000
=> 2002
=> [["first", "second"]]

It seems that the local_variable still references to the @instance_variable, although it is a different object. This behaviour is both with the push and unshift in the each block. With a normal assignment like local_variable='second', the result is as expected => [['first']]

I don’t understand why local_variable.each{|n|n.push('second')} has an effect on the @instance_variable

Using Ruby-1.9.2p318

Answer 1

Both local_variable and @instance_variable have references to the same object, the inner array ['first']. And because it’s a mutable Array, you can effect changes to one array through the other.

Object#dup in Ruby provides a shallow copy. In order to make a deep copy of an Array, you’d need to write some code (or find a library) that recursively walks the data structure, deep-cloning its pieces of mutable state.