What does "self" mean in Ruby?

Experiments with self

Assumptions

  • Perhaps you have heard of ‘object / receiver / sender’ terminology. If not, please consider this tutorial post

  • It is assumed you know what a class means, and perhaps what an “instance method” is.

What is self?

It is not uncommon to see self bandied around in Ruby. What does it mean?

Asking this is analogous to asking: “who is winning” when watching a sporting contest: the answer depends on the context. Similarly, self can be different things, based on the context.

Student Exercise

I would encourage you to copy and paste the following in a ruby file, and to complete the exercise for yourself:

puts "what is self right now? It is #{self} - we are guessing: ....insert_your_answer_here"

class Car
	puts "what is self right now? It is #{self} - we are guessing: ....insert_your_answer_here"

	class << self	
		puts "what is self right now? It is #{self} - we are guessing: ....insert_your_answer_here"
		puts "Are we in the eigenclass? #{self == Car.singleton_class}"
		
		def speed
			puts "what is self right now? - we are guessing: ....insert_your_answer_here"
		end
	end

	def start
		puts "what is self right now? It is #{self} - we are guessing: ....insert_your_answer_here"
	end
end

ferrari = Car.new
ferrari.start
Car.speed

The Answers

Consider the following:

puts "what is self right now? It is #{self} - we are guessing main "

class Car
	puts "what is self right now? It is #{self} - we are guessing 'Car'"

	class << self	
		puts "what is self right now? It is #{self} - we are guessing 'eigenclass'"
		puts "Are we in the eigenclass? #{self == Car.singleton_class}"
		
		def speed
			puts "In speed: what is self right now? It is #{self} - we are guessing Car"			
		end
	end

	def start
		puts "what is self right now? It is #{self} - we are guessing: an instance of car"
	end
end

ferrari = Car.new
ferrari.start
Car.speed


## The output
#=> what is self right now? It is main - we are guessing main 
#=> what is self right now? It is Car - we are guessing 'Car'
#=> what is self right now? It is #<Class:Car> - we are guessing 'eigenclass'
#=> Are we in the 'eigenclass? true
#=> what is self right now? It is #<Car:0x0000561c55362de8> - we are guessing: an instance of car
#=> in speed - what is self right now? - we are guessing Car

Implicit Receiver

You may have heard the expression: everything is an object, in ruby. If that is the case, when we call a method - what if there is no object? In this case, the object is “implicit”: in other words, the implicit receiver is self.


def test
	puts "what is the receiver here?"
end

test
# => what is the receiver here?
# the receiver is self, which in this case is the main object
Written on November 20, 2023