From: Martin Hart <martin@...>
Date: 2002-12-12T10:46:29+09:00
Subject: Re: How to detect Module Inclusion for Class Instances (like Module#extend_object)?



On 12/12/02, 1:08:36 AM, Joel VanderWerf <vjoel@PATH.Berkeley.EDU> wrote 
regarding Re: How to detect Module Inclusion for Class Instances (like 
Module#extend_object)?:


> > Is there a way to tell that a module is being included by a class?
> > Specifically I want to detect this for instances of the class in question
> > - sort of like using #extend_object.

> Take a look at Module#append_features.

AFAICT that works for the class - but not for instances of it...

irb(main):001:0> module P
irb(main):002:1> def P.extend_object(o)
irb(main):003:2> puts "EXTENDED"
irb(main):004:2> end
irb(main):005:1>
irb(main):006:1* def P.append_features(o)
irb(main):007:2> puts "FEATURES APPENDED"
irb(main):008:2> end
irb(main):009:1> end
nil
irb(main):010:0> class A
irb(main):011:1> end
nil
irb(main):012:0>
irb(main):013:0* a = A.new.extend(P)
EXTENDED
#<A:0x401e8628>
irb(main):014:0> class B
irb(main):015:1> include P
irb(main):016:1> end
FEATURES APPENDED
B
irb(main):017:0> a = B.new
#<B:0x401e3088>
irb(main):018:0>

What I want is for the module "P" to do something to instances of B when 
they (the instances) are created - not when the class is created.  In 
other words - all my problems would be solved if extend_object is called 
when an instance of B is created.  However, it isn't :-)

I can see a way to achieve this by using #append_features to create a 
B.new function which could then do the work - but this seems a bit 'arse 
about face' if you'll excuse the slang :-)

I have a temporary work around - which is to define a Module function 
that creates an instance of the object, and then calls extend on it...  

class Factory
 def Factory.createB
   B.new.extend(P)
 end
end

... the problem with this approach is remembering to call it.


I would appreciate any further comments on this - perhaps I am doing 
something that I really shouldn't be trying to do?  Trouble is, Ruby is 
so cool that it opens avenues I would not even have considered when using 
C++ :-)


TIA

Martin