From: Stefano Crocco <stefano.crocco@...>
Date: 2007-11-24T19:24:50+09:00
Subject: Re: Access modifier : private

Alle sabato 24 novembre 2007, Dharmarth Shah ha scritto:
> Hi!
>
> I'm missing some core ruby concept here that I just can't figure out;
> any assistance would be excellent and welcome:
>
> I came to know from various posts that Class level varibales are
> shared among its subclasses in 1.8.
>
> But my confusion is, even though i mark it as private, it is
> accessible in subclass. See example below for more clarity on doubt.
>
> class Test1
>  private
>  @@v=10
> end
>
> class Test2 < Test1
>  def pr
>    @@v=12
>    puts @@v
>  end
> end
>
> Test2.new.pr
>
> ~Dharmarth
> --
> "When I let go of what I am, I become what I might be."

First of all, private only works for instance methods, not for variables, 
either instance variables or class variables, which are always private. 
Second, the meaning of "private" in ruby is different from what you have in 
other languages such as C++. In ruby, "private" only means you can't call it 
with a receiver (i.e, using the form receiver.method). This means that a 
method declared private in a class can be called in a subclass. The same is 
true for instance variables (writing rec.@var gives a syntax error, which is 
the reason instance variables are always private). Here are some examples:

class A
  def initialize
    @x=1
  end

  def m1
    puts "m1 calling m"
    m
  end

  def m2
    puts "m2 calling self.m"
    self.m
  end

  def m3 other
    puts "m3 calling m on other instance of class A"
    other.m
  end

  private
  def m
    puts "m"
  end

end

class B < A

  def mb1
    puts "@x is: #{@x}"
  end

  def mb2
    puts "mb2 calling m"
    m
  end

end

After loading the previous code in IRB (from the file test_private.rb),  I run 
the methods defined for A and B, with the following results:
irb(main):001:0> require 'test_private'
=> true
irb(main):002:0> a = A.new
=> #<A:0xb78da7b8 @x=1>
irb(main):003:0> a.m1
m1 calling m
m
=> nil
irb(main):004:0> a.m2
m2 calling self.m
NoMethodError: private method `m' called for #<A:0xb78da7b8 @x=1>
        from ./prova.rb:14:in `m2'
        from (irb):4
irb(main):005:0> a.m3 A.new
m3 calling m on other instance of class A
NoMethodError: private method `m' called for #<A:0xb78d44f8 @x=1>
        from ./prova.rb:19:in `m3'
        from (irb):6
irb(main):006:0> b = B.new
=> #<B:0xb78d257c @x=1>
irb(main):007:0> b.mb1
@x is: 1
=> nil
irb(main):008:0> b.mb2
mb2 calling m
m
=> nil

A#m1, obviously, works. A#m2 doesn't work because it calls m using a receiver, 
which is forbidden (even when the receiver is the implicit receiver, self). 
A#m3 doesn't work because it calls m with a receiver (even if of the same 
class as self). B#bm1 works, because the instance variable is called without 
a receiver. B#mb2 works, because it's calling the method without the 
receiver. The fact that the method is private doesn't matter.

I hope this helps

Stefano