From: "David A. Black" <dblack@...>
Date: 2005-08-26T22:10:15+09:00
Subject: Re: Circular dependecy on classes

Hi --

On Fri, 26 Aug 2005, Anatol Pomozov wrote:

> I have several classes that have circular dependencies on each other.
> See following example.
>
> class Solder

s/der/dier/g :-)

>  @can_upgrade_to = [Knight]
>  #or use constant from Knight class

There's a big difference, though.  What you're doing here is assigning
to an instance variable of the class instance Soldier.  And what
you're doing here:

>  def upgrade!(to)
>    raise "Unable to upgrade" unless @can_upgrade_to.contain(to)
>  end
> end

is looking up an instance variable of an *instance* of class Soldier.

Simplified example:

class C
   @x = 1
   def meth
     p @x
   end
   p @x         # 1
end

C.new.meth     # nil

There are two @x's here.  They are both instance variables, but they
belong to different objects.  One belongs to the object C (which is an
instance of Class).  The other belongs to whatever objects are
instances of C.

Constants are a different thing entirely.  If you define a constant in
a class, it's visible throughout.  It doesn't belong to the class in
the same private way that a class's instance variables do.


David

-- 
David A. Black
dblack@wobblini.net