From: Kevin Smith <sent@...>
Date: 2000-12-30T16:37:47+09:00
Subject: [ruby-talk:8347] How to copy an object?

I have a case where I would like to make a deep 
copy of an object (it happens to be an array, but 
I don't think that's really important). Both dup 
and clone make shallow copies, according to the 
pickaxe book.

My goal is to create a copy of my data that I can 
hand to another class, confident that the other 
class cannot possibly mess up my precious data. 
Here's my test code to show what I want:

a = 'this is a test'
b = 'nope'
c = [a, b]
d = c
p "#{c.inspect}, #{d.inspect}"
d[0].chop!
p "#{c.inspect}, #{d.inspect}"
c[1].chop!
p "#{c.inspect}, #{d.inspect}"
d[0] = b
p "#{c.inspect}, #{d.inspect}"

In this case, access through d damages the data, 
as can be seen via c. When I replace d[0], it 
also replaces c[0], and when I alter the contents 
of d[0], it also affects the contents of c[0]. I 
tried d=c.dup and d=c.clone, but as I expected 
they didn't change the results.

In my real case, the data will not all be 
strings, so no shortcuts there.

Do I have to write my own deep_copy method in 
every class? Or is there some sneaky shortcut I 
am missing?

Kevin