[ruby-talk:11957] Re: singleton classes?

"Christoph Rippel" <[e-mail]> wrote:
>
> > From: Ben Tilly [mailto:[e-mail]]
>[...]
>
> > >Sometimes it's simply a notational convenience. Othertimes, you
> > >genuinely need to be able to add features to objects.[...]
> > >
> > >I have to admit I use this rarely, but it's always a nice solution to
> > >have in your back pocket.
> >
> > I actually tried putting this in my back pocket a while
> > ago, and failed.
> >
> > Can someone give me an example of how you would write a
> > method that extends an object and gives it a new method?
> > (I kept on seeing "nested method definition" warnings
> > when I tried it the naive way.)

I just found a way.  I don't really like it, but I
will live.

>Since methods are not objects you will have hard time
>doing this (I know you can create a method object but
>this is not the same thing).

You can do it with eval.

>Probably the closed is the #extend method from Object -
>for example.
>
>module Bla
>def do_nothing
>end
>def also_useless
>end
>
>x = Object.new
>x.extend Bla
>
>x.do_nothing
>x.also_useless
>
That was basically what was already shown by Dave.
So you can have a limited list of modules around, and
you can add modules to classes wherever you want.

That isn't what I want.

What I got to work was this:

    class SomeClass
      def add_hello (my_msg="Hello, world")
        eval <<-"end_eval"
          class << self
            def hello (msg="#{my_msg}")
              puts msg
            end
          end
        end_eval
      end
    end

    a = SomeClass.new
    b = SomeClass.new
    c = SomeClass.new

    a.add_hello
    b.add_hello "This works also"

    a.hello         # Hello, world
    b.hello         # This works also
    c.hello         # Oops. :-)

Ruby just really doesn't like seeing "def" as a key
word within a method definition.

Cheers,
Ben
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