This post will cover the differences between immutability and mutability in Ruby.

Variable References

An object has state which is often referred to as a value. When a variable is assigned to an object it has a reference to that object. Ex:

>> a = "hello world"
=> "hello world"

>> a.object_id
=> 70113614077220

This means the variable a has a reference to the String object with the value “hello world”. #object_id is a method that Ruby provides and it tells us what the id of the object it is refering to. In this example a is pointing to the string object with the id 70113614077220.


There are two types of objects - those that are immutable and those that are mutable

Immutable Objects

These are objects whose state cannot be changed. In Ruby, the Integers class is immutable. Here’s an example

>> number = 5
=> 5

>> number.object_id
=> 11

>> number = 12
=> 12

>> number.object_id
=> 25

When we assign number to the value 5 the object id is 11. But when we change number to the value of 12 the object id is 25. So number is referring to a different object after being reassigned.

Internally it looks like this


Even something like this will cause number to have a different reference point

>> number = 5
=> 5

>> number.object_id
=> 11

>> number += 2
=> 7

>> number.object_id
=> 15

Even when we are incrementing number it actually references a new integer (7) which is a different instance of the Integer class.

Mutable Objects

These are objects whose state can be changed. In Ruby, Strings, Hashes, and Arrays are a few of the classes that are mutable. Here’s an example

# create an empty hash
>> a = {}
=> {}
>> a.object_id
=> 70113613908440
# add a value to the hash
>> a[1] = "foo"
=> "foo"
>> a
=> {1=>"foo"}
# the variable 'a' still has the same object id
>> a.object_id
=> 70113613908440
# the key of '1' has a different object id than a 
>> a[1].object_id
=> 70113613877980

Here’s what’s happening internally mutable_diagram

a has a reference to an instance of Hash and each key of the hash is pointing to an instance of String with the corresponding hash value.

Because a Hash is mutable if we add another key, value pair to the hash, a will have the same reference point (the same object id).

# add a key value pair to the hash
>> a[2] = "bar"
=> "bar"
>> a
=> {1=>"foo", 2=>"bar"}
# a has the same object id 
>> a.object_id
=> 70113613908440
# the new key of '2' has a new object id
>> a[2].object_id
=> 70113613831220

Again here’s what’s happening internally