From: Mark Hubbart <discordantus@...>
Date: 2005-11-10T03:29:16+09:00
Subject: Re: Writing my own attr_* methods

On 11/9/05, David Brady <ruby_talk@shinybit.com> wrote:
> Hi All,
>
> I frequently find myself creating member vars that contain a status flag
> that want me to write a ? method.  I know ruby can be tricked into
> thinking it has an additional attr_ method, but off the top of my head I
> can't think where I'd inject that method and how.
>
> I figure some guru will know what to do to Kernel, Class or Object to
> make this work:
>
> ----------8<----------8<----------8<----------
> class Foo
>   attr_flag  :bar, :baz
>   attr_flag_ro  :qaz, :qux, :quux
>
>   def initialize(qaz, qux, quux)
>     @qaz=qaz; @qux=qux; @quux=quux
>     @bar = @baz = false
>   end
> end
>
> f = Foo.new(true, true, true)
>
> f.bar = true
> f.baz = true
>
> puts "Yay!" if f.bar?
> puts "Yay!" if f.baz?
> puts "Yay!" if f.qaz?
> puts "Yay!" if f.qux?
> puts "Yay!" if f.quux?
> ----------8<----------8<----------8<----------
>
> I don't know if attr_flag, attr_flag_ro is the right pairing--perhaps
> attr_flag would be read only and if you want it to be settable you want
> to call attr_accessor, etc.
>
> Anyway, I'm not sure this is a perfect design, mostly I'm interested in
> seeing *how* to write an attr_ method.
>
> Thoughts?

attr_ methods are instance methods of Class. They define the desired
accessor methods for that class when called:

class Class
  def my_attr_reader(name)
    define_method(name){instance_variable_get "@#{name}" }
  end
  def my_attr_writer(name)
    define_method("#{name}="){|value| instance_variable_set "@#{name}", value}
  end
end

class Foo
  my_attr_reader :bar
  my_attr_writer :bar
end

f = Foo.new
f.bar = 23
f.bar     #==> 23

cheers,
Mark

> Thanks!
>
> -dB
>
> --
> David Brady
> ruby_talk@shinybit.com
> C++ Guru. Ruby nuby.  Apply salt as needed.
>
>
>
>