Forum: Ruby extending object with another object

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Ittay D. (Guest)
on 2008-10-22 11:54
(Received via mailing list)
Hi,


How is it possible to extend an object using another object?


I know I can iterate the public methods of one and define them (or maybe
clone, and bind) in the other, but I'm looking for something that will
work more closely to how 'extend' works.


Thanks,

Ittay
Brian C. (Guest)
on 2008-10-22 12:21
Ittay D. wrote:
> How is it possible to extend an object using another object?

It depends exactly on what you're trying to achieve.

You have two separate objects, both with their own sets of methods *and*
their own sets of instance variables. In what way do you want to
"extend" the first with the second? What happens to instance variables
belonging to the second object?

Now, if you simply want a call to method X on object A to be forwarded
to object B (in which context it would use object B's instance
variables), then look at delegate.rb, or you can synthesise this pattern
yourself using method_missing

class A
  def hello
    puts "hello"
  end
end

class B
  def bye
    puts "bye"
  end
end

a = A.new
b = B.new

# This is a rough implementation of SimpleDelegator
def a.other=(x)
  @other=x
end
def a.method_missing(*args)
  @other.send(*args)
end

a.other = b
a.hello
a.bye

The important thing here is that when methods are delegated to the other
object, they will be in the context of that object and hence have access
to that object's instance variables. Put simply, that object will
"work".

In cases where the methods by themselves make sense, then generally
you'd stick those methods into a Module so that both a and b could make
use of them.

class A
  include MyStuff
end
...
b = B.new
b.extend MyStuff

But clearly you know about this already.
Ittay D. (Guest)
on 2008-10-22 13:41
(Received via mailing list)
Brian C. wrote:

> belonging to the second object?
>
I want just to get the methods defined in the second object, something
like
object.extend(as_module(other_object))

where 'as_module' is some method that returns a module representing the
class hierarchy (with singletons) of the other object.

class A
  def initialize(name)
     @name = name
  end
  def hi
     puts @name
  end
end

class B
  def initialize(name)
     @name = name
  end
end

a = A.new('a')
b = B.new('b')
b.object_extend(a)
b.hi  #=> 'b'

Ittay


>
> def a.other=(x)
> The important thing here is that when methods are delegated to the other
> end
> ...
> b = B.new
> b.extend MyStuff
>
> But clearly you know about this already.
>

--
Ittay D. <removed_email_address@domain.invalid>
Tikal <http://www.tikalk.com>
Tikal Project <http://tikal.sourceforge.net>
Brian C. (Guest)
on 2008-10-22 22:34
Ittay D. wrote:
> Brian C. wrote:
>
>> belonging to the second object?
>>
> I want just to get the methods defined in the second object, something
> like
> object.extend(as_module(other_object))
>
> where 'as_module' is some method that returns a module representing the
> class hierarchy (with singletons) of the other object.

But I still don't know what the *purpose* of this is, i.e. what actual
problem you're trying to solve, and without that I can't propose a
better solution.

What you're asking for is not especially easy to do. I guess you could
iterate over the methods of B, obtain UnboundMethod objects, and bind
them to A. But I've never come across any situation where I've needed
(or wanted) to do this.
Thomas S. (Guest)
on 2008-10-22 23:19
(Received via mailing list)
On Oct 22, 3:52 am, Ittay D. <removed_email_address@domain.invalid> wrote:
> Hi,
>
> How is it possible to extend an object using another object?
>
> I know I can iterate the public methods of one and define them (or maybe
> clone, and bind) in the other, but I'm looking for something that will
> work more closely to how 'extend' works.

You have to use delegation. Look up delegate.rb.

T.
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