How to create a deep copy of an object in Ruby?

Question

I did some searching found some different methods and posts about creating a deep copy operator.

Is there a quick and easy (built-in) way to deep copy objects in Ruby? The fields are not arrays or hashes.

Working in Ruby 1.9.2.

Solution 1

Deep copy isn't built into vanilla Ruby, but you can hack it by marshalling and unmarshalling the object:

Marshal.load(Marshal.dump(@object))

This isn't perfect though, and won't work for all objects. A more robust method:

class Object
  def deep_clone
    return @deep_cloning_obj if @deep_cloning
    @deep_cloning_obj = clone
    @deep_cloning_obj.instance_variables.each do |var|
      val = @deep_cloning_obj.instance_variable_get(var)
      begin
        @deep_cloning = true
        val = val.deep_clone
      rescue TypeError
        next
      ensure
        @deep_cloning = false
      end
      @deep_cloning_obj.instance_variable_set(var, val)
    end
    deep_cloning_obj = @deep_cloning_obj
    @deep_cloning_obj = nil
    deep_cloning_obj
  end
end

Source:

http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-list/43424

Solution 2

I've created a native implementation to perform deep clones of ruby objects.

It's approximately 6 to 7 times faster than the Marshal approach.

https://github.com/balmma/ruby-deepclone

Note that this project is not maintained anymore (last commit in 2017, there are reported issues)

Solution 3

Rails has a recursive method named deep_dup that will return a deep copy of an object and, on the contrary of dup and clone, works even on composite objects (array/hash of arrays/hashes). It's as easy as:

def deep_dup
  map { |it| it.deep_dup }
end

Solution 4

There is a native implementation to perform deep clones of ruby objects: ruby_deep_clone

Install it with gem:

gem install ruby_deep_clone

Example usage:

require "deep_clone"
object = SomeComplexClass.new()
cloned_object = DeepClone.clone(object)

It's approximately 6 to 7 times faster than the Marshal approach and event works with frozen objects.

Note that this project is not maintained anymore (last commit in 2017, there are reported issues)

Solution 5

Automatic deep clone is not always what you want. Often you need to define a selected few attributes to deep clone. A flexible way to do this is to implement the initialize_copy, initialize_dup and initialize_clone methods.

If you have a class:

class Foo
  attr_accessor :a, :b
end

and you only want to only deep clone :b, you override the initialize_* method:

class Foo
  attr_accessor :a, :b

  def initialize_dup(source)
    @b = @b.dup
    super
  end
end

Of course if you want @b to also deep clone some of its own attributes, you do the same in b's class.

Rails does this (see https://github.com/rails/rails/blob/0951306ca5edbaec10edf3440d5ba11062a4f2e5/activemodel/lib/active_model/errors.rb#L78)

For more complete explanation, I learned it here from this post https://aaronlasseigne.com/2014/07/20/know-ruby-clone-and-dup/

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