From: Pat Maddox <pergesu@...>
Date: 2007-11-15T11:26:18+09:00
Subject: Re: adding an attribute to an object

On Nov 14, 2007 6:10 PM, Mark Volkmann <r.mark.volkmann@mac.com> wrote:
> I know how to add new methods to an existing object, but is there a way to add an attribute?
> I know I can do it if the object was created with ObjectStruct, but what if it is a "normal" Ruby object?
>
> R. Mark Volkmann
> Object Computing, Inc.
>
>

Not quite sure what you mean...but anyway, maybe this irb session will
help you along:

irb(main):001:0> class Dog; end
=> nil
irb(main):002:0> d = Dog.new
=> #<Dog:0x2063cc>
irb(main):003:0> Dog.class_eval { attr_accessor :name }
=> nil
irb(main):004:0> d.name = "Fido"
=> "Fido"
irb(main):005:0> (class << d; self; end).class_eval { attr_accessor :age }
=> nil
irb(main):006:0> d.age = 12
=> 12
irb(main):007:0> d2 = Dog.new
=> #<Dog:0x265afc>
irb(main):008:0> d2.name = "Spike"
=> "Spike"
irb(main):009:0> d2.age = 7
NoMethodError: undefined method `age=' for #<Dog:0x265afc @name="Spike">
	from (irb):9
	from :0


First we call attr_accessor for Dog, which defines #name and #name=
for all dogs.  Then we define #age and #age= for only the one dog, by
calling attr_accessor on that object's singleton class.

However, there's not really a concept of "attributes" in Ruby (afaik).
 attr_* are just macros that define methods that look like

class Dog
  def age=(val)
    @age = val
  end

  def age
    @age
  end
end

You can also dig in to an object with instance_eval and do your work:

irb(main):001:0> class Dog
irb(main):002:1> attr_reader :name
irb(main):003:1> end
=> nil
irb(main):004:0> d = Dog.new
=> #<Dog:0x287eb8>
irb(main):005:0> d.name
=> nil
irb(main):006:0> d.instance_eval { @name = "Spot" }
=> "Spot"
irb(main):007:0> d.name
=> "Spot"

hth

Pat