From: Robert Klemme <bob.news@...>
Date: 2005-08-29T00:21:22+09:00
Subject: Re: Dynamic methods / accessors

Robert Klemme <bob.news@gmx.net> wrote:
> Frodo Larik <lists@elasto.nl> wrote:
>> Hi all,
>>
>> I'm trying to setup some kind of dynamic method / accessor assignment
>> to objects. But I don't want the other objects from the same class to
>> know about "personal" methods. I'm just learning Ruby, so it could be
>> I'm missing some crucial knowledge.
>>
>> For example:
>>
>> class MyThing
>>    def create_method(method)
>>       # I found two things on internet which can acchieve this
>>       #    self.class.class_eval { attr_accessor method }
>>       # AND
>>       #    self.class.send(:define_method, method, &block)
>>       # but I'm not sure which is the best one to use, but I think
>>       # I prefer the attr_accessor, because that's the functionality
>>       # I need
>>    end
>> end
>>
>>
>> # Create first object
>> a = MyThing.new
>> a.create_method('box')       # Adds a method box to a
>> a.box = 'something inside'
>>
>> # The nicest thing would be if I could do something like this:
>> a.create_method('box','some_value'), so a.box is populated with 'some
>> value'
>>
>> # Create second object
>> b = MyThing.new
>>
>> # I don't want the following to happen
>> b.box = 'something other'
>>
>> I don't want the method 'box' be available to b, only to a
>>
>> How can I achieve this?
> You need so called singleton methods. These are present in just a
> single instance.  You define them via the singleton class.
> (Disclaimer: there has been some debate about the naming of these
> because "singleton" can be misleading here, but AFAIK this is still
> the officiel term.)  You access the singleton class (a class that is
> associated with a single instance only) like this
>
> class <<some_instance
> end
>
> So you can do:
>
> class MyThing
> end
> a = MyThing.new
> class <<a
>  attr_accessor :box
> end
> a.box = 'something inside'
>
> If you want to have it a bit more convenient you can pack this into a
> method like this:
>
> class MyThing
>  def add_attribute(name, val=nil)
>    class <<self;self;end.send( :attr_accessor, name )
>    self.send("#{name}=", val)
>  end
> end
> a = MyThing.new
> a.add_attribute :foo, "hello"
> a.foo # returns "hello"
>
> Kind regards
>
>    robert

PS: There is also OpenStruct which adds methods on the fly.  So you can 
directly do

o = OpenStruct.new
o.bar = 10

Kind regards

    robert