From: Florian Gilcher <flo@...>
Date: 2009-11-29T12:05:29+09:00
Subject: Re: Class variables, instance variables, singleton; Ruby v.

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1


On Nov 28, 2009, at 5:12 PM, Ralph Shnelvar wrote:
>
> So, basically, the reason that instance_variable_get is private to
> Module is that one does not wish make breaking encapsulation too easy?
>
> I mean, it seems easy enough to break encapsulation by adding an
> accessor, right?

The reason that instance_variable_get exists is solely to allow dynamic
lookup of instance variables to work in cases like define_method. It  
is not
intended to be used from the outside, because it would unintentionally  
publish
all internal state of a class. This can be easily circumvented in such a
dynamic language like Ruby, but it has a big, fat warning sign  
attached to it.


But even from the outside, it doesn't break one of the main properties  
of the
ruby language: the uniform access principle. There is only one way to
retrieve or change a value from/in an Object: by calling a method.
C++ and Java have two: directly accessing a member or calling a method.
In Ruby, this is especially true, because:

   a.foo = "bar"

Is the same as sending the message :foo= to a.

   a.send(:foo=, "bar")

where foo= is implemented as:

   class A
     def foo=(val)
       #internal assignment
     end
   end

In every language it is easy to publish a value by adding a method. This
is not "breaking encapsulation", it is intentional publishing. If you  
don't
know what you are doing when implementing this method, no language can  
help
you. The way Ruby handles instance variables only makes it harder to  
access
them in ways that the class implementer intended.

But even with instance_variable_get public, your only way of getting a  
value
would still be to call that method. Which would give the class  
implementer
the possibility to delete, change or hook that method.

Regards,
Florian

[1]: http://en.wikipedia.org/wiki/Uniform_access_principle
- --
Florian Gilcher

smtp:   flo@andersground.net
jabber: Skade@jabber.ccc.de
gpg:    533148E2

-----BEGIN PGP SIGNATURE-----
Version: GnuPG/MacGPG2 v2.0.12 (Darwin)

iEYEARECAAYFAksR5PMACgkQyLKU2FMxSOLjWACdH+UwQoPd9NvQdXcxygsjDVBI
PLgAniLm2EMTmTnlZGne9Ep4I02D72Gb
=FlTZ
-----END PGP SIGNATURE-----