From: Robert Klemme <shortcutter@...>
Date: 2011-02-23T17:25:41+09:00
Subject: Re: Ruby Hash Keys and Related Questions

On Wed, Feb 23, 2011 at 7:42 AM, Terry Michaels <cmhoward@frigidcode.com> wrote:
> I'm still a bit new to Ruby, so humor me a bit. But I discovered today
> (through trial and error) that not only can Strings, numbers, and
> symbols be keys for hashes, but also any object, or even a class name!
> Ruby is the first language I've used in which I would have even thought
> to try that, let alone it actually working:

> Anyway, this raised a few related questions in my mind:
>
> 1. If the "key" taken by hash[key]= can be any object, and the key still
> works even after it is aliased to another variable, does that mean that
> the "key" is just a reference?

Yes, in Ruby you always only ever see references.  This means, that if
you modify an instance which is used in a Hash as key the Hash likely
needs updating since the hash code of the key usually also changes
(see Hash#rehash).

> 2. If I pass in a number, say an Integer, as a key, does Ruby actually
> use the Integer? Or does it use a reference to an Integer object?
> (Numbers are objects too, right?)

Yes, everything is an object in Ruby.  There are some optimizations
internally but as a user of the language you do not see them.
Anything, and I mean _anything_, in Ruby is an object and can be
referenced by any variable.

> 3. If I am allowed to pass in a class as a key, does that mean that
> classes are objects too? If not, what exactly is being stored as the
> key?

Yes, classes are objects, too.  Classes are instances of class Class:

irb(main):002:0> String.class
=> Class
irb(main):003:0> String.class.ancestors
=> [Class, Module, Object, Kernel]
irb(main):004:0> String.kind_of? Object
=> true

There is a tad of recursion involved but you should just accept it and
not think about it too much.  Otherwise serious brain damage could be
the consequence. :-)

> 4. When I use irb, and a line returns an object, irb shows me the
> object's hexadecimal reference address (or at least, that's what it
> looks like). Is there a method one can call on an object to get that
> reference when one is not in irb? Just curious.

No, IRB shows the result of obj.inspect.  The default implementation
returns something which is related to #object_id which in turn is
derived from the address (I forgot the details):

irb(main):010:0> o=Object.new
=> #<Object:0x7ff72c5c>
irb(main):011:0> o.object_id.to_s 16
=> "3ffb962e"
irb(main):012:0> 0x7ff72c5c / 0x3ffb962e
=> 2

Anyway, since you cannot access memory directly from Ruby it's
worthless to know the memory address.  And in other Ruby
implementations (e.g. JRuby) that memory address may even change.
Forget C (until you write your first Ruby extension).

Ah, and one note: Ruby's Hash will apply special treatment to String
keys if they are not frozen.  In this case the key is copied so you
can safely modify the key you passed:

irb(main):015:0> h={}
=> {}
irb(main):016:0> k="foo"
=> "foo"
irb(main):017:0> h[k]="x"
=> "x"
irb(main):018:0> h
=> {"foo"=>"x"}
irb(main):019:0> k << "_modified"
=> "foo_modified"
irb(main):020:0> k
=> "foo_modified"
irb(main):021:0> h
=> {"foo"=>"x"}
irb(main):022:0> k.object_id
=> 1073420300
irb(main):023:0> h.each {|k,v| puts k, k.object_id}
foo
1073420320
=> {"foo"=>"x"}

If you know you do not need to modify a String key afterwards you can
gain a few CPU cycles by freezing the String.

Kind regards

robert

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/