Feature #4514: #deep_clone and #deep_dup for Objects

• Status: Assigned
• Priority: Normal
• Assignee: matz (Yukihiro Matsumoto)
• Target version: next minor

[wardrop (Tom Wardrop)]

=begin
There's often a need to do a deep clone of an object, especially of Hash/Array trees. The typical work around to the lack of this functionality is to Marshall and then Unmarshall (e.g. Marshal::load(Marshal::dump(self)) ), which incurs more overhead than it probably should, and is not very semantic. My suggestion is to either provide #deep_clone and #deep_dup methods on the Object class, or to at least provide equivalent functionality for Hashes and Arrays, such as possibly a #deep_merge method for Hash. The exact implantation is not a large concern of mine; I'll let the experts determine the best method of achieving the desired outcome.
=end

[wardrop (Tom Wardrop)]

No one wants to add anything?

[shyouhei (Shyouhei Urabe)]

Some thoughts

• #deep_merge is a Rails method. If you only need Arrays + Hashs to be deep_dup'able, chances are they also should go into Rails.
• It is not always obvious what a "deep" copy is. For instance it is very hard to define one for a Proc instance.
• Recursive duplication may not be that simple to implement than you imagine. For instance an Array can contain itself: r = [].tap {|r| r << r } How do you copy it deeply?

[marcandre (Marc-Andre Lafortune)]

Hi,

Shyouhei Urabe wrote:

• It is not always obvious what a "deep" copy is. For instance it is very hard to define one for a Proc instance.

Agreed, but since Procs do no "contain" anything, I'd say Proc#deep_dup == Proc#dup (except that it would call deep_dup on any instance variables, I imagine).

• Recursive duplication may not be that simple to implement than you imagine. For instance an Array can contain itself: r = [].tap {|r| r << r } How do you copy it deeply?

That shouldn't be too hard, we simply maintain a hash with the ids of objects being cloned as keys and with the corresponding new copies as values. I would have fun implementing it. Note that Marshal::load(Marshal::dump(r)) works for recursive arrays and so does YAML serialization.

I can attest that the question comes up from time to time on StackOverflow, so it is something quite a bit of people wonder about.

[kstephens (Kurt Stephens)]

Marc-Andre Lafortune wrote:

Shyouhei Urabe wrote:

• It is not always obvious what a "deep" copy is. For instance it is very hard to define one for a Proc instance.
• Recursive duplication may not be that simple to implement than you imagine. For instance an Array can contain itself: r = [].tap {|r| r << r } How do you copy it deeply?

That shouldn't be too hard, we simply maintain a hash with the ids of objects being cloned as keys and with the corresponding new copies as values. I would have fun implementing it. Note that Marshal::load(Marshal::dump(r)) works for recursive arrays and so does YAML serialization.

See https://github.com/kstephens/red_steak/blob/master/lib/red_steak/copier.rb

The problem is how to control how "deep" a copier should go, which objects need to have identity maintained and which objects are simply containers. The programmer needs control via some protocol with the copier.

[mame (Yusuke Endoh)]

• Status changed from Open to Assigned
• Assignee set to matz (Yukihiro Matsumoto)

[mame (Yusuke Endoh)]

• Target version set to next minor