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.

related: http://stackoverflow.com/questions/14104542/ruby-object-deep-copying — , Aug 14 '14 at 19:43

score 72 · Accepted Answer · answered Nov 21 '11 at 02:06

72

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

answered Nov 21 '11 at 02:06

Alex Peattie

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OK, thanks. I found a post about marshalling but didn't know if it was a reasonable solution. `Marshal.load(Marshal.dump(@object))` worked great. – B Seven Nov 21 '11 at 02:13
2

Yeah, it's only slow relatively speaking - you'd only notice if you were cloning lots of objects at a time – Alex Peattie Nov 21 '11 at 02:17
4

Obviously, it also assumes that the object is marshallable in the first place, which not all objects are. – Jörg W Mittag Nov 21 '11 at 12:48
1

How comes and there isn't any ruby built in method for that? – vasilakisfil Aug 05 '14 at 19:54

score 20 · Answer 2 · edited Apr 19 '19 at 16:57

20

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)

edited Apr 19 '19 at 16:57

reducing activity

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answered Apr 08 '13 at 08:58

user2256822

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This project is not maintained any more and not production ready, according to the author. – bbozo May 09 '17 at 17:41

score 7 · Answer 3 · edited Apr 19 '19 at 17:51

7

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)

edited Apr 19 '19 at 17:51

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answered Apr 08 '13 at 10:40

Mat

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Unfortunately, this gem does not handle custom Collection classes that inherit from Array and contain their own complex nested classes. Ruby_deep_clone turns those objects into Arrays and they lose their custom attributes. – Abe Heward Mar 14 '14 at 16:40
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This project is not maintained any more and not production ready, according to the author. – bbozo May 09 '17 at 17:42
@bbozo It claims to support up to Ruby 2.4. Is there a source saying it is no longer maintained? – lulalala Mar 19 '18 at 12:51
This answer is a duplicate of https://stackoverflow.com/questions/8206523/how-to-create-a-deep-copy-of-an-object-in-ruby/15875069#15875069 that was posted earlier – reducing activity Apr 19 '19 at 12:29

score 7 · Answer 4 · answered Sep 03 '15 at 14:55

7

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

answered Sep 03 '15 at 14:55

Claudio Floreani

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Rails' Object#deep_dup is actually a shallow `dup` call. Besdie Hash and Array, you need to implement your own `deep_dup` for it to work. – lulalala Mar 19 '18 at 12:10

score 3 · Answer 5 · answered Mar 31 '16 at 12:17

You can use duplicate gem for this.

It's a small pure ruby gem that able to recursively duplicate object It will duplicate it's object references too to the new duplication.

require 'duplicate'
duplicate('target object')

https://rubygems.org/gems/duplicate

https://github.com/adamluzsi/duplicate.rb

score 3 · Answer 6 · answered Mar 19 '18 at 12:47

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/

score 2 · Answer 7 · answered Aug 27 '15 at 07:16

2

I would suggest you use the ActiveSupport gem which adds a lot of sugar to your native Ruby core, not just a deep clone method.

You can look into the documentation for more information regarding the methods that have been added.

answered Aug 27 '15 at 07:16

Noman Ur Rehman

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score 2 · Answer 8 · answered Feb 27 '19 at 14:17

You really don't need a Gem for this. This couldn't be much simpler than this, which is not worth the overhead of a Gem!

def deep_clone(obj)
  obj.clone.tap do |new_obj|
    new_obj.each do |key, val|
      new_obj[key] = deep_clone(val) if val.is_a?(Hash)
    end
  end
end

score 1 · Answer 9 · answered Jan 19 '14 at 19:35

1

Also check out deep_dive. This allows you to do controlled deep copies of your object graphs.

https://rubygems.org/gems/deep_dive

answered Jan 19 '14 at 19:35

Lord Alveric

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

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