From: Robert Klemme <bob.news@...>
Date: 2005-10-09T18:21:50+09:00
Subject: Re: Quick module include question.

Chris Eskow <chriseskow@gmail.com> wrote:
> Hi, I'm semi-new to Ruby and I have a question.
>
> I have a class, World, that basically looks like this:
>
> class World
>
> # Convenience method that takes one or more symbols and defines an
> # accessor+mutator method for each one.
> def self.att *atts
> atts.each do |att|
> class_eval %{
> def #{att} val=nil
> val == nil ? @#{att} : @#{att} = val
> end
> }
> end
> end
>
> # Now I can define some attributes.
> att :title, :author
>
> # Other methods here...
>
> end
>
> ..which allows me to do things like this:
>
> world.title "My Title"
> world.title #=> "My Title"
>
> The att method creates an instance method that sets an attribute if
> an argument is given, or returns it if no arguments are given. This
> is nice and all, but now I want to use this att method with another
> class. Since it's very short I could just copy and paste it, but I'd
> rather use a module, Attributes, that I could include into any class:
>
> class World
> include Attributes
> att :title, :author
> end
>
> class Thing
> include Attributes
> att :name, :desc
> end
>
> I'm having trouble getting it to work, however. I have a feeling I
> have to use metaclasses or something, but I'm not quite sure how I'm
> suppose to do that. Everything I tried resulted in "undefined method
> `att'" errors. Any thoughts?

You're basically reimplementing attr_accessor, attr_reader, attr_writer and 
attr.  These come predefined and you can even use them on an instance level:

>> o=Object.new
=> #<Object:0x10192dc8>
>> class<<o
>> attr_accessor :name
>> end
=> nil
>> o.name="foo"
=> "foo"
>> o.name
=> "foo"

Of course you can wrap that in an instance method:

>> class Object
>> def att(*a)
>> cl=class<<self;self;end
>> a.each {|at| cl.send(:attr_accessor,at)}
>> end
>> end
=> nil
>> o=Object.new
=> #<Object:0x10164808>
>> o.att :name
=> [:name]
>> o.name="foo"
=> "foo"
>> o.name
=> "foo"

Also note that if you use OpenStruct you don't even need to define 
attributes - you can just use them.

>> require 'ostruct'
=> true
>> o=OpenStruct.new
=> <OpenStruct>
>> o.name="bar"
=> "bar"
>> o.name
=> "bar"


Kind regards

    robert