How do I use default on a hash of empty arrays?

Question

I want to use default to reset my ary when I need to. But I can't figure out how to not have default's values changed when ary's values change.

> default = {"a"=>[], "b"=>[], "c"=>[]}
=> {"a"=>[], "b"=>[], "c"=>[]} 

> ary = default.clone
=> {"a"=>[], "b"=>[], "c"=>[]} 

> ary["a"] << "foo"
=> ["foo"] 

> default
=> {"a"=>["foo"], "b"=>[], "c"=>[]} 

Answer 1

What you've discovered here is that Hash#clone only does a shallow clone, that is it only replicates itself but not the objects that are referenced within it.

There are a number of "deep clone" gems that address this specific problem, or you can write your own to work around it:

class Hash
  def deep_clone
    Hash[collect { |k,v| [ k, v.respond_to?(:deep_clone) ? v.deep_clone : v ] }]
  end
end

class Array
  def deep_clone
    collect { |v| v.respond_to?(:deep_clone) ? v.deep_clone : v }
  end
end

This will let you clone arbitrary Hash and Array objects as required.

Answer 2

Both clone and dup create a shallow copy of your object, which results in this behaviour. I'm not sure what the proper way to achieve a deep copy is, but instead of:

ary = default.clone

Try:

ary = Marshal.load(Marshal.dump(default))

This is taken from a live 2.3.8 environment on ruby 1.8.7