From: Stefan Lang <langstefan@...>
Date: 2005-11-05T20:57:13+09:00
Subject: Re: what does "class << self" do?

On Saturday 05 November 2005 08:32, petermichaux@yahoo.com wrote:
> Hi,
>
> I'm looking at the rails activerecord base.rb file it's main
> structure is shown below. Why have a class inside a class like
> this? What advantages are there?
>
> Thanks,
> Peter
>
> class Base
>
>     # methods ...
>
>     class << self # Class methods
>       # methods ...
>     end
>
>     # methods ...
>
> end

You can define methods like attr_accessor, has_many etc.
Example:

    class Base
        class << self
            # At this level we are defining methods
            # for the Class instance Base. These methods
            # are also called class methods (of the Base class).
            def flag(attr_name)
                class_eval <<-EOD
                    def #{attr_name}?
                        @#{attr_name} == true
                    end
                    def #{attr_name}
                        @#{attr_name} = true
                    end
                    def disable_#{attr_name}
                        @#{attr_name} = false
                    end
                EOD
            end
        end
    end

    class Foo < Base
        flag :quiet
        def initialize
            @quiet = false
        end
    end

    f = Foo.new
    p f.quiet?   # => false
    f.quiet
    p f.quiet?   # => true
    f.disable_quiet
    p f.quiet?   # => false

-- 
Stefan