From: Rick DeNatale <rick.denatale@...>
Date: 2009-05-27T22:32:16+09:00
Subject: Re: How do Class definitions in Ruby differ from other OO languages?

On Wed, May 27, 2009 at 3:01 AM, Charles Oliver Nutter
<charles.nutter@sun.com> wrote:
> kunjaan wrote:
>>
>> What do you mean when you say"Class definitions are executable code".
>> How do classes differ from other OO languages?
>
> I like to say it like this:
>
> Ruby doesn't have class declarations. "Declaring" a class is actually just
> constructing a new Class object and running code against it. All classes
> start out blank and must have methods and instance variables added to them
> at runtime.

As I tried to point out earlier in this thread, that last phrase is a
bit ambiguous.

If we're talking about "class instance variables" then yes, Ruby
classes have instance variables added to them at runtime, but classes
don't "know" about the set of instance variables that their INSTANCEs
have, in the same way that the do in Java or Smalltalk.

Each instance acquires instance variables only when a method is run
with that instance as self which defines the iv by initializing it.
At least this is what it must look like to the Ruby program,  defined?
@iv must return nil for each instance until @iv has been defined in
THAT instance:

class Foo
  attr_accessor :iv
  def iv_defined?
    defined? @iv
  end
end

f1 = Foo.new
f2 = Foo.new

f1.iv_defined? # => nil
f2.iv_defined? # => nil
f1.iv = 1
f1.iv_defined? # => "instance-variable"
f2.iv_defined? # => nil

Implementations might play some tricks due to the underlying VM
implementation like pre-allocating slots to hold some ivs, but that
has to be hidden from the Ruby programmer. If used it needs to be one
of those VM implementation 'cheats' which the VM implementer needs to
avoid being caught using.

MagLev does, or maybe did, do something like this.  It assigns the
first few ivs it sees to fixed slots, since  the Smallalk object model
prefers fixed iv name to slot offset mappings.  They had some issues
with not handling defined? properly something which the RubySpec
didn't cover until I added that http://rubyspec.org/issues/show/82
I'm not sure exactly what changes that caused in their implementation.

I don't know what JRuby does.


Ruby 1.8, uses a hash to implement ivs, Ruby 1.9 uses a dynamic
allocation technique which DOEs move the hash to the instance's
immediate class, and maps iv names to slot indexes, in an array of
slots either directly inside the object's 'box' for an instance with a
very small number of IVs or to a separate array, with slots
initialized to a value which represents not nil, but undefined.  One
of the things that's different in this implemenation is that in a case
like

class A
   attr_accessor :a, :b
end

class B < A
end

the offset of @a might be different for instances of A than for
instances of B, i.e. the offsets are NOT inherited.

-- 
Rick DeNatale

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