Mixing-in the inherited method

ruby-talk
1

Dear Rubyists,

I fail to understand why the following code doesn't output anything.

module Foobar
  def inherited(klass)
    p klass
  end
end

class Class
  include Foobar
end

class A
end

My intuition says it should work since defining Class#inherited
directly works. I believe I understand the whole "instance methods of
Class become class methods of Class instances" business, but I must be
missing something obvious.

Any ideas?

···

--
Christoffer Sawicki

2

Christoffer Sawicki wrote:

Dear Rubyists,

I fail to understand why the following code doesn't output anything.

module Foobar
def inherited(klass)
   p klass
end
end

class Class
include Foobar
end

class A
end

My intuition says it should work since defining Class#inherited
directly works. I believe I understand the whole "instance methods of
Class become class methods of Class instances" business, but I must be
missing something obvious.

Any ideas?

A is not a subclass of Class, it's an instance of Class.

Also, you need to use extend rather than include, so that the instance
methods of Foobar (particularly, #inherited) become methods of A.
Otherwise, the instance methods of Foobar become instance methods of
_instances_ of A.

These two examples both print "A":

module Foobar
def inherited(klass)
   p klass
end
end

class Object
extend Foobar
end

class A
end

···

--------------------

module Foobar
def inherited(klass)
   p klass
end
end

class Classy
extend Foobar
end

class A < Classy
end

--
      vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407

3

Hello Joel,

A is not a subclass of Class, it's an instance of Class.

Indeed.

Also, you need to use extend rather than include, so that the instance
methods of Foobar (particularly, #inherited) become methods of A.
Otherwise, the instance methods of Foobar become instance methods of
_instances_ of A.

These two examples both print "A":

/.../

I understand your examples and your argument, but you seem to be
missing my point (if I have one). Instance methods of class Class
become class methods of Class instances, so why doesn't the mixed-in
inherited() work? Comparing these two examples, why is the output of
the second different?

= Example 1

class Class
  def inherited(klass)
    p klass
  end

  def test!
    p "test!"
  end
end

class A; end
A.test!

== Output

A
"test!"

= Example 2

module Foo
  def inherited(klass)
    p klass
  end

  def test!
    p "test!"
  end
end

class Class
  include Foo
end

class A; end
A.test!

== Output

"test!"

Thanks,

···

--
Christoffer Sawicki

4

Try this

class Class

     remove_method :inherited

  include Foo
end

Guy Decoux

5

Guy Decoux wrote:

Try this

> class Class

     remove_method :inherited

> include Foo
> end

Aha! That makes sense.

Thanks,

···

--
Christoffer Sawicki