Object singleton problem

Rails
1

Hi All

I’m trying to add an instance variable and a method to a object
The CODE:

#! /usr/bin/ruby

class Xyz
def initialize
p “initializing”
end
end

my_inst = Xyz.new

class <<my_inst
@new_var = “Hello world”

def get_new_var
p “instance method”
@new_var
end
end

p my_inst.get_new_var

=========

prints:
instance method
nil

Any suggestions why the variable is ‘nil’ ??

Thnx a lot
LuCa

2

@new_var is a Class attribute only accessible in class methods.

check this out:

On Jul 17, 11:04 am, Luca S. [email protected]

3

I thought I opened the object so I could add instance variables to it,
but I guess this should be done like

my_inst.instance_variable_set(…)

or can this be done like I’m trying ?

4

thnx, but how would I access the @x variable which belongs to the class
object C ?

5

Hi –

On Sat, 19 Jul 2008, Luca S. wrote:

thnx, but how would I access the @x variable which belongs to the class
object C ?

Like any object, C can expose its instance variable through accessor
methods:

class << C
attr_accessor :x
end

C.x = 1 # etc.

You can also use instance_eval or instance_variable_get, but those are
“impolite” ways of making an object show you its data. (Useful, but
impolite :slight_smile:

David

–
Rails training from David A. Black and Ruby Power and Light:
Intro to Ruby on Rails July 21-24 Edison, NJ
Advancing With Rails August 18-21 Edison, NJ
See http://www.rubypal.com for details and updates!

6

Hi –

On Sat, 19 Jul 2008, Luca S. wrote:

I thought I opened the object so I could add instance variables to it,
but I guess this should be done like

my_inst.instance_variable_set(…)

or can this be done like I’m trying ?

There’s a very simple rule: every time you see this:

@var

you are seeing an instance variable that belongs to ‘self’ – that is,
whatever ‘self’ happens to be at the time.

Your example looks like this:

class C
end

c = C.new

class << c
@x = 1
end

If you check for self at that point in execution, you’ll find that
self is the class you’ve just opened up – namely, the singleton class
of the object c:

class << c
p self # #<Class:#<C:0x7aeb8>>
end

So the instance variable defined at that point belongs to the class.

It’s not specific to singleton classes; it’s about self.

class C
@x = 1 # belongs to the class object C

 def instance_x
   @x           # belongs to whatever instance of C is 'self'
 end

end

David

–
Rails training from David A. Black and Ruby Power and Light:
Intro to Ruby on Rails July 21-24 Edison, NJ
Advancing With Rails August 18-21 Edison, NJ
See http://www.rubypal.com for details and updates!

7

Hi –

On Sun, 20 Jul 2008, Luca S. wrote:

my examples uses an object not the class, so if I would do

obj = Xyz.new

class << obj
attr_accessor :x
end

how would I assign a value to x and how would I retrieve it ?

obj.x = 1
puts obj.x

David

–
Rails training from David A. Black and Ruby Power and Light:
Intro to Ruby on Rails July 21-24 Edison, NJ
Advancing With Rails August 18-21 Edison, NJ
See http://www.rubypal.com for details and updates!

8

my examples uses an object not the class, so if I would do

obj = Xyz.new

class << obj
attr_accessor :x
end

how would I assign a value to x and how would I retrieve it ?

thnx a lot