Freeze Objects in Ruby

Sometimes a situation arises where we would want to freeze the object instance so that it cannot be instantiated or modified and in Ruby, we can do that with the help of the freeze keyword.

The approach is to invoke the Object#freeze statement.

When we freeze an object, we are basically turning it into a constant and it should be noted that once an object is frozen, cannot be unfrozen.

Syntax

The syntax to freeze an object is shown below.

Object.freeze

Now that we know a little about freeze, let's take a couple of example to understand how it works.

Example 1

Consider the code shown below

veggies = ["spinach", "cucumber", "potato"]
veggies.freeze
veggies << "cauliflower"

In the above code, there's an array named veggies which contains three strings, and then on the next line, I invoked the freeze method on it and then lastly, I tried adding another string to the array.

Output

main.rb:3:in `<main>': can't modify frozen Array (RuntimeError)

Example 2

Let's consider a slightly more complex example where we would make use of getters and setters and then set one of the methods as frozen. Consider the code shown below.

# Freezing object example

# define a class
class Subtract

   # constructor method
   def initialize(x, y)
      @first, @second = x, y
   end

   # accessor methods
   def getA
      @first
   end

   def getB
      @second
   end

   # setter methods
   def setA=(value)
      @first = value
   end

   def setB=(value)
      @second = value
   end
end

# create an object
add = Subtract.new(10, 20)

# let us freez this object
add.freeze
if( add.frozen? )
   puts "Frozen object"
else
   puts "Normal object"
end

# now try using setter methods
sub.setA = 30
sub.setB = 50

# use accessor methods
sub.getA()
sub.getB()

puts "First is : #{sub.getA()}"
puts "Second is : #{sub.getB()}"

Output

Frozen object
main.rb:37:in `<main>': undefined local variable or method `sub' for main:Object (NameError)