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
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
