# Shadow copy and deep copy

Hello,

x=[1,2,3]
y=x.dup

this is a shadow copy, I know the theory behind it.

z=x

is this a deep copy? (z is the deep copy of x?)

Thanks

Eva wrote in post #960269:

Hello,

x=[1,2,3]
y=x.dup

this is a shadow copy, I know the theory behind it.

z=x

is this a deep copy? (z is the deep copy of x?)

Thanks

I don't think it's deep copy.
irb(main):010:0> print z.__id__
137429210=> nil
irb(main):011:0> print x.__id__
137429210=> nil
irb(main):012:0> print y.__id__
137515980=> nil
irb(main):013:0> z.push 4
=> [1, 2, 3, 4]
irb(main):014:0> p x
[1, 2, 3, 4]
=> [1, 2, 3, 4]

···

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x=[1,2,3]
y=x.dup

this is a shadow copy, I know the theory behind it.

Actually it's called "shallow copy". And what #dup really does
depends on the class so it could even implement a deep copy (although
that is normally not the case). In the case of Array you are correct,
it's a shallow copy.

z=x

is this a deep copy? (z is the deep copy of x?)

No, there is no copy of state at all. This is called "aliasing": now
both z and x point to the same object.

Example for deep copy: