From: Florian Frank <flori@...>
Date: 2006-09-26T03:57:01+09:00
Subject: Re: How to copy a method from one class to another

MonkeeSage wrote:
> Sam Kong wrote:
>   
>> Let's say that there's a class (C1) in a library and I'm building my
>> own class hierarchy including C2 (class C2 < SomeOtherClass).
>> In such a case, Inheritance or mix-in is not an option.
>>     
>
> Unless I misunderstand, a mixin is fine:
>   
This wouldn't work in the situation Sam describes:

class Rubyforge::C1 # for example in some rubyforge library
  def interesting_method
    if @foo > 3
      @bar += 2
    else
      @baz /= 2
    end
  end
end

Sam wonders how to call interesting_method from his class C2, that 
cannot inherit
from C1, because it already inherits from SomeOtherClass:

class C2 < SomeOtherClass
  def my_method
    interesting_method # ???
  end
end

This isn't possible in Ruby, because of single inheritance and the fact 
that Sam doesn't control the rubyforge library and thus cannot create a 
module to include in C2. The only way to get interesting_method's 
functionality into C2 is to copy & paste it.

If there was a way in Ruby to do that like:

class C2 < SomeOtherClass
  include Rubyforge::C1.to_module(:interesting_method)

  def my_method
    interesting_method
  end
end

This would introduce a tight coupling between C2 and C1 on the same 
level like inheritance would do. In some way it would be equivalent to 
real multiple inheritance. Now the author of C1 can break your code by 
refactoring e. g.:

class Rubyforge::C1 # for example in some rubyforge library
  def interesting_method
    if @foo > 3
       add_two
    else
      @baz /= 2
    end
  end

  private

  def add_two
      @bar += 2
  end
end

I was in the same situation and ended up copying & pasting. I wished at 
the time, that it would have been possible to just import those methods, 
even if it meant that I might ending up shooting myself into the foot.

There is an additional problem, that occurs when someone attempts to 
import methods from a Ruby class implemented in C, that uses a macro 
like RSTRING(foo)->len. In these cases it would be possible to trigger a 
Ruby segmentation fault by calling those methods. Of course it would 
perhaps be possible to mark C implemented methods and let Ruby refuse to 
export them. This would at least cause an ordinary exception during 
class loading time.

-- 
Florian Frank