Re: Use of extend <module> from within a Class

On Fri, Dec 4, 2009 at 6:37 PM, Mike Papper <[e-mail]> wrote:
> I would like to include a module into a 'static' class - such that all
> the modules methods are class methods. The problem is that code in the
> module that uses "self.class.XYZ' breaks and when this code is included,
> ruby cannot find the XYZ method.
>
> Here is an example:
>
> module Basic
> =A0def methoda
> =A0 =A0puts "AAAAAAAAAAA"
> =A0end
>
> =A0def mb
> =A0 =A0self.class.cee
> =A0end
>
> =A0module ClassMethods
> =A0 =A0attr_writer :cee
>
> =A0 =A0def cee
> =A0 =A0 =A0puts "CCCCCCCCCCCCC"
> =A0 =A0end
> =A0end
> end

> class Use
> =A0extend Basic

Okay, at this point in the class definition self =3D=3D Use

and extend Basic

mixes the Basic module into the singleton class of Use, so all of the
methods of Basic (i.e. mb and methoda) are now CLASS methods of Use

There is no actual usage of Basic::ClassMethods

> =A0def self.z
  This is a class method of Use (because of def self.z)  inside this
method self is also the class object Use
> =A0 =A0mb
> =A0end
> end
>
> If I do the following:
> =A0Use.z
>
> I get this error:
> NoMethodError: undefined method `cee' for Class:Class
> =A0 =A0 =A0 =A0from (irb):100:in `mb'
> =A0 =A0 =A0 =A0from (irb):116:in `z'
> =A0 =A0 =A0 =A0from (irb):119

so what happened.

in Basic#mb we have

def mb
   self.class.cee
end

and self is still the class object Use, but Use.class is class, which
is one issue. Even if the mb method had self.cee, Use doesn't have cee
as a class method because although Basic::ClassMethods was defined, it
was never referenced.

As Jesus pointed out, you appeared to be confused with the way Rails,
and lots of other ruby code causes modules to add both instance and
class methods to classes which include them.

The normal pattern is

module SomeModule
   # define any instance methods here

   module ClassMethods
       # define any class methods here
   end

  # and here is the crucial step  this gets called when some class
  # includes the module
  def self.included(some_class)
     some_class.extend(ClassMethods)
  end
end

So if you want to make a module which adds only class methods either

1) define the module 'normally' and in the class which wants the
methods as class methods

     class Foo
         extend MyGreatModuleOfClassMethods
     end

or

 2) Use the pattern above, leave the instance methods out, and put all
of the methods in the inner ClassMethods module (actually the name of
this module is irrelevant except for clarity. then in the using class

     class Foo
         include MyGreatModuleWhichOnlyHasClassMethodsRightNow
     end

HTH




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Rick DeNatale

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