Ruby: freezing your object

Before diving in, lets look into what are mutable and immutable objects.

Mutable object is object that can be modified anytime during its life time. Immutable object is object that cannot be modified after it is created.

In ruby object can be made immutable anytime using freeze method. Once the object is frozen, it cannot be modified further in its life time. Any attempt to modify frozen object results in TypeError. Let's take a look at simple example of freezing a string object.


Once str is frozen there is no way it can be unfrozen.

Ruby also provides frozen? method to check whether object is frozen or not.


One important point to take a note about freeze method here is that freeze makes object immutable and not object reference variable. Take a look at below code.


obj1 and obj3 points to one instance of FreezeTest and obj2 points to another instance of FreezeTest. Once obj1.freeze is invoked, instance of FreezeTest to which obj1 and obj3 are pointing is frozen and not obj1 and obj3 variables. Both these variables can be modified to point to any other instance as seen in above code.


So where can freeze method be useful? Two places I can think of

1. Creating a true constant object: By true constant, I mean constant variable that refer to immutable object. Lets take example from rails. request.rb defines LOCALHOST constant as
   
   LOCALHOST = [/^127\.0\.0\.\d{1,3}$/, "::1", /^0:0:0:0:0:0:0:1(%.*)?$/].freeze
   
   By freezing array to which LOCALHOST points, rails makes sure that no one should be able to modify definition of localhost by removing and adding elements to array.
   
   
2. Using object as hash key: Once object is used as hash key, we want to make sure that it's hash is not changed otherwise it will be difficult to find value associated with it. One of the way to achieve this is by freezing object and then using it as hash key.