From: "Benjamin J. Tilly" <ben_tilly@...>
Date: 2001-03-23T13:22:59+09:00
Subject: [ruby-talk:13113] Re: alias method

>===== Original Message From "Nathaniel Talbott" <ntalbott@rolemodelsoft.com> 
=====
>Kenichi Komiya [mailto:kom@mail1.accsnet.ne.jp] wrote:
[...]
>Ah, I see. If we think of the class as just holding a reference to a method
>instance, alias_method is just adding a new method reference to the class
>and pointing it at the same method instance as the original reference.
>Changing what the original reference points at obviously doesn't change what
>the new reference points at. Now that I understand that, the current
>behavior makes sense, but it certainly doesn't seem obvious at first blush.
>Dave & Andy, do you think it would be valuable to clarify this in the book?

Ruby generally works this way.  When you copy a variable, copies
by default are shallow.  But modification creates a new instance
so this usually works out as you would expect.

>It still seems that, whatever it's doing underneath, from a usage standpoint
>the current alias_method() is copying the method, since I don't think
>there's a way to change the original method such that it affects the
>'alias'. Is this true?
[...]

I don't know how to do it off of the top of my head with methods,
but the following example with arrays may be enlightening:

  a = [1, 2, 3, 4, 5]
  p "a", a
  b = a
  p "b", b
  a.push(6)
  p b
  a = a + [7]
  p b
  p a

You see that when you copied a to b, you get an alias.  If you
modify a in place (eg with push()) you will modify b as well.
But attempt a common operation (eg +) on a, and a new object is
created which protects b from seeing the change.

Therefore while copying usually copies by reference, in practice
you see few surprises.  (Though they certainly exist.)

Cheers,
Ben