object singleton problem

rubyonrails-talk
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:

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

Hi --

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

5

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

6

Hi --

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

7

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

8

Hi --