From: m4dc4p <jgbailey@...>
Date: 2005-11-24T04:22:25+09:00
Subject: How to modify Hash to track key insertions?

I am trying to figure out how I can modify the Hash object so that I am
notified each time a key is inserted in a Hash when it is constructed
using the syntax below:

  a = { "x" => "1", "y" => "2" }

I've tried overriding []=, but that will only get called when the Hash
is constructed explicitly.

Here's a little bit of code showing what I've tried and the output I'd
like to see. Thanks for any help!

class Hash
 alias :old_assignment :[]=
 def []=(key, value)
  puts "#{key} => #{value}"
  old_assignment(key, value)
 end
end

puts "Creating a"
a = { "x" => "1", "y" => "2" }
puts "Creating b"
b = Hash.new
b["1"] = "x"
b["2"] = "y"

# Expected output
#  Creating a
#  x => 1
#  y => 2
#  Creating b
#  1 => x
#  2 => y

# Actual output
#  Creating a
#  Creating b
#  1 => x
#  2 => y

p.s. I'm ultimately doing this to get to an OrderedHash. I looked at
Ara's code but the problem is I need to *replace* the functionality of
Hash - it's not an option for me to explicitly create an OrderedHash
object.
p.p.s. Is there a way to avoid the alias statement for []=? For some
reason, calling super in there would give a NoMethodError.