From: Robert Klemme <shortcutter@...>
Date: 2007-02-24T22:10:07+09:00
Subject: Re: Extending object instances with <<

On 24.02.2007 13:52, Tim Becker wrote:
> Hi,
> 
> I'm trying to write a class with a method to extend instances of
> itself to contain additional accessors. I thought using `class <<
> self` would be the most elegant way to go about it, but I'm running
> into some problems. To illustrate:
> 
> class Test
>  # takes an array of symbols to add to the instance.
>  def add syms
>    syms.each { |sym|
>      @@__tmp = sym
>      $__tmp = sym
>      class << self
>        #attr_accessor sym  # this would be my preferance, but sym
> isn't in scope here
>        #attr_accessor $__tmp  # this works, but uses globals
>        attr_accessor @@__tmp   # this is nearly as bad as using globals
>      end # <<
>    } # each
>  end # add
> end # Test
> 
> t = Test.new
> t.add [:thingie, :thingie2]
> 
> t.thingie="whatever"
> t.thingie2="bla"
> puts t.thingie
> puts t.thingie2
> 
> I don't like the idea of using globals to transport the symbol
> information and the class members approach is nearly as bad
> (synchronization issues mainly, apart from elegance). But I can't
> think of another way to transport dynamic data into the `class<<self`
> block.

There is:

irb(main):017:0> class Bar
irb(main):018:1>   def add(*syms)
irb(main):019:2>     cl = class<<self;self;end
irb(main):020:2>     cl.instance_eval { attr_accessor *syms }
irb(main):021:2>   end
irb(main):022:1> end
=> nil
irb(main):023:0> f=Bar.new
=> #<Bar:0x3c1a40>
irb(main):024:0> f.add :bar
=> nil
irb(main):025:0> f.bar=10
=> 10
irb(main):026:0> f.bar
=> 10

> Alternatives would be to handle this using `method_missing` though
> that wouldn't just affect a single instance or using `eval` which
> would involve executing strings I'm banging together.
> 
> Another thing I tried was:
> 
> ..
> self.class.attr_accessor sym
> ..
> 
> but that doesn't work because `attr_accessor` is private (contrary to
> what it says in the documentation...)
> 
> Any ideas? Am I missing something?

See above.  Apart from that you could simply use OpenStruct or inherit 
OpenStruct which does all this for you already automagically:

irb(main):013:0> require 'ostruct'
=> true
irb(main):014:0> f=OpenStruct.new
=> #<OpenStruct>
irb(main):015:0> f.bar=10
=> 10
irb(main):016:0> f.bar
=> 10

Major difference is that you do not explicitly control accessor creation 
but automatically get *all* - even spelling errors.

Kind regards

	robert