From: Terry Michaels <cmhoward@...>
Date: 2011-02-23T15:42:16+09:00
Subject: Ruby Hash Keys and Related Questions

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:


irb(main):001:0> hsh = {}
=> {}
irb(main):002:0> obj = Object.new
=> #<Object:0x7fa4b83ab1f0>
irb(main):003:0> obj2 = Object.new
=> #<Object:0x7fa4b83a7320>
irb(main):004:0> hsh[obj] = "blah"
=> "blah"
irb(main):005:0> hsh[obj2] = "ble"
=> "ble"
irb(main):006:0> puts hsh[obj2]
ble
=> nil
irb(main):007:0> puts hsh[obj1]
NameError: undefined local variable or method `obj1' for main:Object
  from (irb):7
irb(main):008:0> puts hsh[obj]
blah
=> nil
irb(main):009:0> clone = obj
=> #<Object:0x7fa4b83ab1f0>
irb(main):010:0> puts hsh[clone]
blah
=> nil
irb(main):011:0> class Cl
irb(main):012:1> end
=> nil
irb(main):013:0> hsh[Cl] = "blo"
=> "blo"
irb(main):014:0> puts hsh[obj]
blah
=> nil
irb(main):015:0> puts hsh[Cl]
blo
=> nil
irb(main):016:0> class Cl2
irb(main):017:1> end
=> nil
irb(main):018:0> hsh[Cl2] = "blu"
=> "blu"
irb(main):019:0> puts hsh[Cl]
blo
=> nil
irb(main):020:0> puts hsh[Cl2]
blu

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?

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?)

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?

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.

-- 
Posted via http://www.ruby-forum.com/.