Digging Deeper

There may be times when you want to make sure that your Ruby objects cannot be modified in the ways described in this chapter. Here you will learn how to do this.

Freezing Objects

With all these ways of modifying objects at your disposal, you may be concerned that objects are at risk of being modified unintentionally. In fact, you can specifically fix the state of an object by “freezing” it using the freeze method, which you first encountered in Chapter 12. Once frozen, the data contained by an object cannot be modified, and if an attempt is made to do so, a TypeError exception will be raised. Take care when freezing an object, however, because, once frozen, it cannot be “unfrozen.”

freeze.rb

s = "Hello"
s << " world"
s.freeze
s << " !!!"   # Error: "can't modify frozen string"

You can specifically check whether an object is frozen using the frozen? method:

a = [1,2,3]
a.freeze
if !(a.frozen?) then
    a << [4,5,6]
end

Be aware that although the data of a frozen object cannot be modified, the class from which it is defined can be modified. Let’s suppose you have a class X that contains the method addMethod, which can create new methods with the name given by the symbol m:

cant_freeze.rb

def addMethod( m, &block )
    self.class.send( :define_method, m , &block )
end

Now, if you have an object, ob, created from the M class, then it is perfectly legitimate to call addMethod to add a new method to class M:

ob.freeze
ob.addMethod( :abc ) { puts("This is the abc method") }

If you want to prevent a frozen object from modifying its class, you could, of course, test its state using the frozen? method:

if not( ob.frozen? ) then
  ob.addMethod(:def){puts("'def' is not a good name for a method")}
end

You can also freeze the class itself (remember, a class is also an object):

freeze_class.rb

X.freeze
if not( X.frozen? ) then
  ob.addMethod(:def){puts("'def' is not a good name for a method")}
end