From: Tim Pease <tim.pease@...>
Date: 2011-07-22T07:56:11+09:00
Subject: Re: attr_accessor for hash keys


On Jul 21, 2011, at 4:41 PM, Sean O'Halpin wrote:

> On Thu, Jul 21, 2011 at 10:17 PM, Chad Perrin <code@apotheon.net> wrote:
>> Is there a way to use attr_accessor, or something like it (without
>> writing it from scratch), for hash keys?  For instance:
>> 
>>    class Foo
>>      attr_accessor :bar[:one], :bar[:two]
>>      def initialize(bar)
>>        @bar = bar
>>      end
>>    end
>> 
>> Obviously, :bar[:one] and :bar[:two] won't work, because symbols don't
>> have [] methods, but that should illustrate the general gist of what I'm
>> asking (I hope).  I'd just like to avoid the clutter of writing a bunch
>> of basic accessors myself, the long way, in a class definition where I'd
>> like to keep all the data in a hash but have direct accessor method ways
>> to work with specific key=>value pairs.
>> 
>> --
>> Chad Perrin [ original content licensed OWL: http://owl.apotheon.org ]
>> 
> 
> This is reasonably close to what you described:
> 
> module HashAccessor
>  def hash_accessor(hash_name, *key_names)
>    key_names.each do |key_name|
>      define_method key_name do
>        instance_variable_get("@#{hash_name}")[key_name]
>      end
>      define_method "#{key_name}=" do |value|
>        instance_variable_get("@#{hash_name}")[key_name] = value
>      end
>    end
>  end
> end
> 
> class Foo
>  extend HashAccessor
>  hash_accessor :bar, :one, :two
>  def initialize(bar)
>    @bar = bar
>  end
> end
> 
> foo = Foo.new(:one => 1, :two => 2) # => #<Foo:0x9c28310
> @bar={:one=>1, :two=>2}>
> foo.one                             # => 1
> foo.one = 42
> foo                                 # => #<Foo:0x9c28310
> @bar={:one=>42, :two=>2}>
> 

This sounds like a job for method_missing ...

  class Foo
    def initialize( bar )
      @bar = bar
    end

    def method_missing( key, *args, &block )
      super unless @bar.has_key? key
      @bar[key]
    end
  end


Also, take a look at Ara Howard's "Map" gem and see if that will get you what you need.

https://github.com/ahoward/map

  m = Map(:one => 1, :two => 2)
  s = m.struct
  s.one  #=> 1
  s.two  #=> 2

Blessings,
TwP