From: Liam <ruby-talk@...>
Date: 2007-11-18T07:40:38+09:00
Subject: Re: Composition: Build objects from other objects

On Sun, Nov 18, 2007 at 05:02:12AM +0900, Thufir wrote:
> 
> Hehe, fair enough.  Ok, enough Java, it was just an entry point (although 
> I would be interested to hear how this is addressed in other OOP oriented 
> languages).  What stands out about the above is:
> 
> 1.)  I don't follow all of the syntax, some of it's over my head (which 
> is certainly ok).

It makes use of fowardable[1] from the standard library.

> 
> 2.) The engine is being instantiated *outside* of the class definition.  
> I was assuming that the engine would be created in the vehicle's 
> initialize method.  The above technique is to specify the engine when the 
> car is "manufactured" by passing a new instance of Engine into the new 
> Vehicle instance.

This is more of a design decision.  Providing outside access allows for
inversion of control.  Perhaps you would prefer to use a factory for
your car?

class Factory
	def self.build_car
		Car.new(Engine.new)
	end
end
class Mechanic
	def soup_up(car)
		car.engine=MustangEngine.new
	end
end

falcon=Factory.build_car
car.vroom
joe=Mechanic.new
joe.soup_up falcon
car.vroom

The interface is of course rather arbitrary.  Take a look at logr4[2]
for some interesting composition.


[1] http://ruby-doc.org/stdlib/libdoc/forwardable/rdoc/files/forwardable_rb.html
[2] http://log4r.sourceforge.net/