From: Robert Klemme <shortcutter@...>
Date: 2006-04-10T20:57:04+09:00
Subject: Re: Adding in class attribute with a Module

2006/4/10, zdennis <zdennis@mktec.com>:
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> I think the OP wanted class level attribute accessor, not instance level accessors. You showed how-to do instance level accessors:
>
>   zdennis@lima:~$ irb
>   irb(main):001:0> class A ; attr_accessor :foo ; end
>   => nil
>   irb(main):002:0> a = A.new
>   => #<A:0xb7cdc8a8>
>   irb(main):003:0> a.foo = 5
>   => 5
>   irb(main):004:0> a.foo
>   => 5
>
> but using mixins:
>
>   irb(main):007:0> module M ; attr_accessor :foo ; end
>   => nil
>   irb(main):008:0> class B ; include M; end
>   => B
>   irb(main):009:0> b = B.new
>   => #<B:0xb7cb20a4>
>   irb(main):010:0> b.foo = 5
>   => 5
>   irb(main):011:0> b.foo
>   => 5
>
> I think OP wants the equivalent of:
>
>   irb(main):012:0> class C ; class << self ; attr_accessor :foo ; end ; end
>   => nil
>   irb(main):013:0> C.foo = 5
>   => 5
>   irb(main):014:0> C.foo
>   => 5
>
> But with using mixins. The OP should just open up the instance of the class and include the module there. For example:
>
>   irb(main):015:0> class D
>   irb(main):016:1>   class << self
>   irb(main):017:2>     include M
>   irb(main):018:2>   end
>   irb(main):019:1> end
>   => #<Class:D>
>   irb(main):020:0> D.foo = 99
>   => 99
>   irb(main):021:0> D.foo
>   => 99

There's an easier way:

class D
 extend M
end

or

class D;end
D.extend M

There are other options (e.g. use #included) but it depends what the
OP really wants.

Kind regards

robert