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comp.lang.ruby

Instance Variable in Mixins

Dale Martenson

5/19/2008 10:55:00 PM

I am trying to get access to an instance variable. I have simplified
my usage down to the following example.

=== start-of-code

module A
def A.a
puts "A:#{@u}"
end
end

module B
def b
puts "B:#{@u}"
end
end

module C
include A
include B
end

class Blah
include C

def initialize(u)
@u = u
end

def go
A.a
b
end
end

blah = Blah.new("bob")

blah.go

=== end-of-code

My main question is that how can I get at the instance variable @u
from within module A. I can use it fine from module B so I assume it
is a namespace problem. It turns out that due to the way my mixins are
used, I need to guard them against duplication. I guess I could use a
prefix "def moduleA_a ... end" (El Yucko).

I am using ruby 1.8.6.

Thanks in advance,
Dale
1 Answer

Rick DeNatale

5/19/2008 11:37:00 PM

0

On Mon, May 19, 2008 at 7:00 PM, Dale Martenson
<dale.martenson@gmail.com> wrote:
> I am trying to get access to an instance variable. I have simplified
> my usage down to the following example.
>

Hint. What do you think the difference is between:

> module A
> def A.a

and

> module B
> def b

Why do you need to write:

A.a
but just
b

in the go method in Blah?

Let's instrument your code a little bit:

module A
def A.a
puts "self is #{self} A:#{@u}"
end
end

module B
def b
puts "self is #{self} B:#{@u}"
end
end

module C
include A
include B
end

class Blah
include C

def initialize(u)
puts "Initializing #{self} u:#{u}"
@u = u
end

def go
A.a
b
end
end

blah = Blah.new("bob")

blah.go

RubyMate r8136 running Ruby r1.8.6
(/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby)
>>> untitled

Initializing #<Blah:0x8e2b0> u:bob
self is A A:
self is #<Blah:0x8e2b0> B:bob
Program exited.


The first method is defined ON the module A, not in the module, so
it's not a method of the instance of Blah, and within the A.a method
@u refers to an instance variable of the object named A, i.e. the
module itself, where as the b method, being in an instance method
provided by the B module, is referring to the instance variable in the
instance of Blah.

--
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denh...