Freezing variables in Ruby doesn't work [duplicate]

Question

I am learning Ruby, and I just found interesting behaviour when using the Object#freeze method with variables.

After I freeze a variable (either Fixnum or Array), I am still able to modify it! It is strange, since as far as I am concerned this should not happen and TypeError should be raised.

Here's my code:

test = 666
var = 90
#ok
var += 5

puts "var.frozen? #{var.frozen?}"    
var.freeze    
puts "var.frozen? #{var.frozen?}"

var = test
puts "var = #{var}"

The same is for array:

test = [666]
var = [90]
#ok
var += [5]

puts "var.frozen? #{var.frozen?}"    
var.freeze    
puts "var.frozen? #{var.frozen?}"

var = test
puts "var = #{var}"

But when I try to push something into the array after freezing, it raises an arror, as expected:

test = [666]
var = [90]
#ok
var += [5]

puts "var.frozen? #{var.frozen?}"    
var.freeze    
puts "var.frozen? #{var.frozen?}"

var << test
puts "var = #{var}"

Can somebody explain to me this issue? It seems strange.

Edit I am using Windows XP + Ruby 1.9.3-p429

Answer 1

You freeze objects, not variables, i.e. you can't update a frozen object but you can assign a new object to the same variable. Consider this:

a = [1,2,3]
a.freeze
a << 4
# RuntimeError: can't modify frozen Array

# `b` and `a` references the same frozen object
b = a
b << 4    
# RuntimeError: can't modify frozen Array

# You can replace the object referenced by `a` with an unfrozen one
a = [4, 5, 6]
a << 7
# => [4, 5, 6, 7]

As an aside: it is quite useless to freeze Fixnums, since they are immutable objects.

Answer 2

In Ruby, variables are references to objects. You freeze the object, not the variable.

Please note also that

a = [1, 2]
a.freeze
a += [3]

is not an error because + for arrays creates a new object.