Inherit and override in the same time?

ruby-talk
1

Hi Ruby gurus,

Let's say we have a Parent class, and a Child1 class that inherits from
the Parent class.

I'd like to create a Child2 class that inherits most of the Child1
class, except for one method, which I'd like to override completely. The
problem is that I need to call the method in the Parent class, and
apparently "super.super" is not supported:

···

----------------------------
class Parent
def go
   puts 'parent'
end
end

class Child1 < Parent
def go
   super
   puts 'child1'
end
def a_method_id_like_to_inherit
end
end

class Child2 < Child1
def go
   super.super
   puts 'child2'
end
end

Child2.new.go
----------------------------
in `go': undefined method `super' for nil:NilClass (NoMethodError)
----------------------------

I ended up doing this, which apprently works.

----------------------------
class Parent
def go
   puts 'parent'
end
end

class Child1 < Parent
def go
   super
   puts 'child1'
end
def a_method_id_like_to_inherit
end
end

class Child2 < Child1
def go
   self.class.superclass.superclass.instance_method( :go ).bind( self
).call
   puts 'child2'
end
end

Child2.new.go
----------------------------
parent
child2
----------------------------

Isn't there anything simpler than that? Or is that kind of pattern
considered as bad design maybe?

Philippe

2

I ended up doing this, which apprently works.

----------------------------
class Parent
def go
  puts 'parent'
end
end

class Child1 < Parent
def go
  super
  puts 'child1'
end
def a_method_id_like_to_inherit
end
end

class Child2 < Child1
def go
  self.class.superclass.superclass.instance_method( :go ).bind( self
).call
  puts 'child2'
end
end

Child2.new.go
----------------------------
parent
child2
----------------------------

Isn't there anything simpler than that? Or is that kind of pattern
considered as bad design maybe?

Smells a little of bad design. Consider putting the common methods for
Child1, Child2 into a module:

class Parent
end

class Child1 < Parent
  include ChildStuff
end

class Child2 < Parent
  include ChildStuff
end

Just a suggestion -- it's hard to really comment on a design without knowing
the problem you're trying to solve.

3

It's not clear WHY you want to do this but..

your self.class.superclass.superclass.instance_method(:go).bind(self).call

really doesn't do the same thing, in general as your desired super.super
(which I've not by the way seen in other dynamic OO languages either).

Consider what would happen if you added

class Child3 < Child2
end

Child3.new.go

I think that the right way to do this is to refactor the method to extract
just the behavior you want to control, so in this particular case something
like

class Parent
def go
  puts 'parent'
end
end

class Child1 < Parent

def put_me
   puts 'child1'
end

def go
  super
  put_me
end

end

class Child2 < Child1
def put_me
  puts 'child2'
end
end

class Child3 < Child2
end

Parent.new.go
Child1.new.go
Child2.new.go
Child3.new.go

HTH

···

On Mon, Feb 16, 2009 at 10:29 AM, Philippe Lang <philippe.lang@attiksystem.ch> wrote:

Hi Ruby gurus,

Let's say we have a Parent class, and a Child1 class that inherits from
the Parent class.

I'd like to create a Child2 class that inherits most of the Child1
class, except for one method, which I'd like to override completely. The
problem is that I need to call the method in the Parent class, and
apparently "super.super" is not supported:

----------------------------
class Parent
def go
  puts 'parent'
end
end

class Child1 < Parent
def go
  super
  puts 'child1'
end
def a_method_id_like_to_inherit
end
end

class Child2 < Child1
def go
  super.super
  puts 'child2'
end
end

Child2.new.go
----------------------------
in `go': undefined method `super' for nil:NilClass (NoMethodError)
----------------------------

I ended up doing this, which apprently works.

----------------------------
class Parent
def go
  puts 'parent'
end
end

class Child1 < Parent
def go
  super
  puts 'child1'
end
def a_method_id_like_to_inherit
end
end

class Child2 < Child1
def go
  self.class.superclass.superclass.instance_method( :go ).bind( self
).call
  puts 'child2'
end
end

Child2.new.go
----------------------------
parent
child2
----------------------------

Isn't there anything simpler than that? Or is that kind of pattern
considered as bad design maybe?

--
Rick DeNatale

Blog: http://talklikeaduck.denhaven2.com/
Twitter: http://twitter.com/RickDeNatale

4

IMHO the "bug" is in your inheritance design. You probably should rather do

class Parent
   def go
     puts 'parent'
   end
end

class BaseChild < Parent
  def a_method_id_like_to_inherit
    puts 'Boo!'
  end
end

class Child1 < BaseChild
  def go
    super
    puts 'child1'
  end
end

class Child2 < BaseChild
  def go
    super
    puts 'child2'
  end
end

Now, if there are no other classes that should derive from Parent you might as well merge BaseChild and Parent.

Kind regards

  robert

···

On 16.02.2009 16:29, Philippe Lang wrote:

Hi Ruby gurus,

Let's say we have a Parent class, and a Child1 class that inherits from
the Parent class.

I'd like to create a Child2 class that inherits most of the Child1
class, except for one method, which I'd like to override completely. The
problem is that I need to call the method in the Parent class, and
apparently "super.super" is not supported:

----------------------------
class Parent
def go
   puts 'parent'
end
end

class Child1 < Parent
def go
   super
   puts 'child1'
end
def a_method_id_like_to_inherit
end
end

class Child2 < Child1
def go
   super.super
   puts 'child2'
end
end

Child2.new.go ----------------------------
in `go': undefined method `super' for nil:NilClass (NoMethodError)
----------------------------

I ended up doing this, which apprently works.

----------------------------
class Parent
def go
   puts 'parent'
end
end

class Child1 < Parent
def go
   super
   puts 'child1'
end
def a_method_id_like_to_inherit
end
end

class Child2 < Child1
def go
   self.class.superclass.superclass.instance_method( :go ).bind( self
).call puts 'child2'
end
end

Child2.new.go ----------------------------
parent
child2
----------------------------

Isn't there anything simpler than that? Or is that kind of pattern
considered as bad design maybe?

5

Rick DeNatale wrote:

Hi Ruby gurus,

Let's say we have a Parent class, and a Child1 class that inherits
from the Parent class.

I'd like to create a Child2 class that inherits most of the Child1
class, except for one method, which I'd like to override completely.
The problem is that I need to call the method in the Parent class,
and apparently "super.super" is not supported:

It's not clear WHY you want to do this but..

Hi Rick and James,

Thanks for your answer.

I tried to rewrite my example so it shows a little bit better WHY I'm
trying to do that.

···

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

class Form
def initialize
   puts "form preparation"
end
def search_data
   puts "form search preparation"
   yield
   puts "form search cleanup"
end
end

class ProductForm < Form
def initialize
   super
   puts "product form widgets preparation"
end
def search_data
   super do
    puts "product search"
   end
  end
end

class ArchivedProductForm < ProductForm
def search_data
   self.class.superclass.superclass.instance_method( :search_data
).bind( self ).call do
     puts "archived product search"
   end
  end
end

ProductForm.new.search_data
puts "------------------"
ArchivedProductForm.new.search_data

------------------------------------
form preparation
product form widgets preparation
form search preparation
product search
form search cleanup
------------------
form preparation
product form widgets preparation
form search preparation
archived product search
form search cleanup
------------------------------------

Basically, a ProductForm or ArchivedProductForm are identical, except
the way they are being searched for. All the rest, like the widgets on
the form, are exactly the same.

I'm pretty sure now I have to refactor somehow my code, to keep things
clean. I guess the following code, with an extra class, is the way to
go:

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

class Form
def initialize
   puts "form preparation"
end
def search_data
   puts "form search preparation"
   yield
   puts "form search cleanup"
end
end

class ProductBaseForm < Form
def initialize
   super
   puts "product form widgets preparation"
end
end

class ProductForm < ProductBaseForm
def search_data
   super do
     puts "product search"
   end
  end
end

class ArchivedProductForm < ProductBaseForm
def search_data
   super do
     puts "archived product search"
   end
  end
end

ProductForm.new.search_data
puts "------------------"
ArchivedProductForm.new.search_data

------------------------------------
form preparation
product form widgets preparation
form search preparation
product search
form search cleanup
------------------
form preparation
product form widgets preparation
form search preparation
archived product search
form search cleanup
------------------------------------

Best regards,

Philippe

6

Philippe, you could introduce a new method, for example #product_search:

class Form
  def initialize
    puts "form preparation"
  end
  def search_data
    puts "form search preparation"
    yield
    puts "form search cleanup"
  end
end

class ProductForm < Form
  def initialize
    super
    puts "product form widgets preparation"
  end
  def search_data
    super do
      product_search
    end
  end
  def product_search
    puts "product search"
  end
end

class ArchivedProductForm < ProductForm
  def product_search
    puts "archived product search"
  end
end

(not tested)

Regards,
Pit

···

2009/2/16 Philippe Lang <philippe.lang@attiksystem.ch>:

I tried to rewrite my example so it shows a little bit better WHY I'm
trying to do that.

7

Exactly!

This is what I recommended, recast into the "new" problem.

···

On Mon, Feb 16, 2009 at 1:28 PM, Pit Capitain <pit.capitain@gmail.com>wrote:

2009/2/16 Philippe Lang <philippe.lang@attiksystem.ch>:
> I tried to rewrite my example so it shows a little bit better WHY I'm
> trying to do that.

Philippe, you could introduce a new method, for example #product_search:

class Form
def initialize
   puts "form preparation"
end
def search_data
   puts "form search preparation"
   yield
   puts "form search cleanup"
end
end

class ProductForm < Form
def initialize
   super
   puts "product form widgets preparation"
end
def search_data
   super do
      product_search
   end
end
def product_search
    puts "product search"
end
end

class ArchivedProductForm < ProductForm
def product_search
    puts "archived product search"
end
end

--
Rick DeNatale

Blog: http://talklikeaduck.denhaven2.com/
Twitter: http://twitter.com/RickDeNatale