What is self ?

The topic of these posts are in reference to the Ruby language. While the concept of self is simple, the confusion that can occur depends on the context of self in code. Taking the time to recognize where self is used in code can let you know if self is referencing a class or an object.

Self in a Class Method

By definition a class method is defined by prepending self to a method definition, as a result self references the class.

class Person
   def self.species
     'Homo sapiens'
   end
 end
 ​
 bob = Person.new
 bob.species
 => NoMethodError #(undefined method `species' for #<Person:0x0000000001068d48>)
 ​
 Person.species
 => "Homo sapiens"
 ​

Self in an Instance Method

Implicitly inside an Instance Method

self is not written in the code below, but it is implied. Although not written, self references an object defined by a class.

Per Ruby documentation, self is the “current object” and the default receiver of method calls for which no explicitly receiver is specified. In other words receiver = object, if no object is specified in the method call then self is the default object. In the example below, line 9 name is equivalent to self.name and bob.name.

class Person
   attr_reader :name
   
   def initialize(name)
     @name = name
   end
   
   def get_name
     name
   end
 end
 ​
 bob = Person.new('Bob')
 bob.name
 => 'Bob'
 bob.get_name
 => 'Bob'

Explicitly inside an Instance Method

self is written in the code below. Just like before self references an object defined by a class. Calling the method with self tells Ruby we are calling a setter method and not initiating a new local variable. ie object.method = value.

class Person
   attr_accessor :name
   
   def initialize(name)
     @name = name
   end
   
   def change_name(new_name)
     self.name = new_name
   end
 end
 ​
 bob = Person.new('Bob')
 bob.name
 => 'Bob'
 bob.change_name('Rick')
 => 'Rick'
 bob.name
 => 'Rick'

What happens if we do not use self on line 9?

class Person
   attr_accessor :name
   
   def initialize(name)
     @name = name
   end
   
   def change_name(new_name)
     name = new_name
   end
 end
 ​
 bob = Person.new('Bob')
 bob.name
 => 'Bob'
 bob.change_name('Rick')
 => 'Rick'
 bob.name
 => 'Bob'

The object bob's name would not change because Ruby thinks you have initialized the local variable name and not invoked the method bob.name.

If you know of other uses of self or find this information conflicting please leave a comment.

Ruby document on self