Ruby / Rails: clone vs. dup vs. deep_dup

Ruby and Rails have several methods for creating a new object that looks like another: clone, dup, deep_dup. When using them you should be aware of their differences so that you can select the method you really need.

clone

• Shallow copy: references to other objects/values are copied (instead of cloning those objects/values)
• Clones the object and all its "special object attributes" like frozen, tainted and modules that the object has been extended with
• Ruby 2.6 documentation for clone Show archive.org snapshot

dup

• Shallow copy: references to other objects/values are copied (instead of cloning those objects/values)
• Clones the object, but ignores "special object attributes" like frozen, tainted and modules that the object has been extended with
• Ruby 2.6 documentation for dup Show archive.org snapshot

deep_dup

• Provided by ActiveSupport
• "Special object attributes" like frozen, tainted and singleton methods are ignored (like dup)
• Behavior depends on implementation
  • Hash Show archive.org snapshot : returns a deep copy, i.e. referenced objects/values will be deep_duped as well
  • Array Show archive.org snapshot : returns a new array where each object is deep_duped
  • any other Object Show archive.org snapshot : calls dup, or self when frozen. If your object has instance variables that are hashes or arrays, those will not be deep_duped.

Note that deep_dup is not a silver bullet and needs to be implemented properly by the object you call it on.

Generally speaking, you should prefer deep_dup over dup, but need to know how the object itself implements it.

Further Reading

• https://en.wikipedia.org/wiki/Object_copy#Shallow_copy Show archive.org snapshot
• https://makandracards.com/makandra/477919-using-deep_dup-for-copying-whole-hashes-and-array