From: Phrogz <gavin@...>
Date: 2006-10-17T13:30:11+09:00
Subject: Re: dynamically assigning instance variables (redux)

Phrogz wrote:
> class Foo
>   def initialize( hash={} )
>     hash.each{ |key,value|
>       var_name = "@#{key}"
>       instance_variable_set( var_name, value )
>       singleton_class = class << self; self; end
>       singleton_class.class_eval{
>         define_method( key ){
>           instance_variable_get( var_name )
>         }
>         define_method( "#{key}=" ){ |new_value|
>           instance_variable_set( var_name, new_value )
>         }
>       }
>     }
>   end
> end

Note that the above defines distinct readers/writers for each instance:

  f1 = Foo.new :name=>'whee', :age=>12
  f2 = Foo.new :chicken=>'gorkbo'
  p f1, f2
  #=> #<Foo:0x32b900 @age=12, @name="whee">
  #=> #<Foo:0x32b360 @chicken="gorkbo">

  p f1.name
  #=> "whee"

  p f2.name
  #=> NoMethodError: undefined method `name' for #<Foo:0x32b34c
@chicken="gorkbo">
  #=> at top level in tmp.rb at line 26

You can also open up the main class (instead of the singleton class) to
define the methods shared by all instances, or store the hash as an
instance variable and use method_missing to dynamically catch and
report values as appropriate.

Here's an example of that for Hash objects in general:

class Hash
  # Causes *all* Hash objects to behave similar to Object literals in
JS,
  # with their keys accessible via dot notation.
  #    foo = { 'name'=>'Gavin', 'age'=>33 }
  #    foo.weight = 180
  #    p foo.name, foo.weight
  #    #=> "Gavin"
  #    #=> 180
  #
  #    p foo.zzzz
  #    #=> NoMethodError: undefined method `zzzz' for
  #    #=> {"name"=>"Gavin", "weight"=>180, "age"=>33}:Hash
  alias_method :__mMm__, :method_missing
  def method_missing(meth,*args)
    if /=$/ =~ (meth=meth.id2name)
      # Name ends with an equals sign; shove value to the hash
      self[ meth[0...-1] ] = (args.length<2 ? args[0] : args)
    else
      # Retrieve the value
      if self.key?( meth )
        self[ meth ]
      else
        # Let the real method_missing handle/pass on up
        __mMm__( meth.intern, *args )
      end
    end
  end
end

TIMTOWTDI.