2008/8/21 Niklas Baumstark <niklas.baumstark / googlemail.com>:
> Hello,
>
> i just wondered where the difference between the following code parts is?
>
>  ---- code ----
>
> class << Array
>  p self.inspect
>  define_method :foo do
>    p "inside foo"
>  end
> end
>
> Array.class.module_eval do
>  p self.inspect
>  define_method :bar do
>    p "inside bar"
>  end
> end
>
> Array.foo
> Array.bar
>
>  ---- output ----
>
> "#<Class:Array>"
> "Class"
> "inside foo"
> "inside bar"
>
>  ---- end ----
>
>
> it obviously makes a difference because self.inspect returns two different values. but how is this practically relevant?

"class << Array" opens the eigenclass of the Array object.
Thus the foo method is only defined for the Array object itself.

"Array.class" returns the class of the Array object, which is Class.
Defining a method with module_eval in there means that it
is defined for _all_ class objects.

To demonstrate this, try to call the methods on another class:

    irb(main):017:0> String.foo
    NoMethodError: undefined method `foo' for String:Class
            from (irb):17
    irb(main):018:0> String.bar
    "inside bar"

Stefan