You can’t see me.. like the eigenclass..

Understanding the Eigenclass in less than 5 minutes

In this article, we’re going to explore the following topics:

• what’s the eigenclass ?
• the eigenclass behind the scene

Introduction

In Ruby, a class is an object. Indeed, a class is literally an instance of the class Class.

Feel free to have a look to the Short guide to learning how Classes work in Ruby article if you’re not familiar with the class Class.

So, how a class makes the difference between class-level and object-level methods, variables & constants?

By using the eigenclass (a.k.a meta-class, shadow-class, singleton class, etc..).

What’s the eigenclass?

In Ruby, methods can be called in three ways:

• the method is called by a class: MyClass.my_method
• the method is called by an object: my_object.my_method
• with an implicit caller: puts('puts is an implicit call to self.puts')

By default, when a method is defined in a class this method becomes an instance method. This instance method is accessible by all the instances (a.k.a objects) of this class

Here, we can see that the Greeting#hello method is considered as an instance method.

This method is shared among all the instances of Greeting.

Now let’s try to add a class method to our Greeting class

Here Greeting.hello is defined at a class level. That’s why we can invoke this method by using theGreeting class as the caller. Also, we can see that the method is defined as a singleton method.

To keep it simple — and related to this article — let’s assume that singleton methods are class methods.

But what if I tell you that a singleton method is also an instance method?

Well, this makes sense as (almost) everything in Ruby is an object. What you commonly call a class method is, behind the scene, an instance method of the eigenclass.

By the way, the eigenclass is also referred as singleton class.

Ok... But what exactly is the eigenclass?

Let’s access the eigenclass and inspect what it contains

First, we define a Greeting.eigenclass method. This method returns self in the context of the eigenclass — by using the class << self ... end syntax. In this case, self contains an unnamed instance of the class Class (a.k.a an anonymous class). This anonymous class keeps track of the class to which it is attached — the Greeting class in our case.

Then, we can see that the singleton methods of the Greeting class are the instance methods of the Greeting eigenclass.

Note that defining our singleton method using self.hello or Greeting.hello produces the same result.

That’s cool. We now understand what’s the purpose of the eigenclass. We also know that it’s an anonymous class. So, now let’s have a deeper look at the properties of this anonymous class.

The eigenclass behind the scene

Let’s detail how the eigenclass is built behind the scene

1- As seen in the previous section, the eigenclass is an unnamed instance of the class Class.

This anonymous class is defined within Greeting. To be certain, we can check the return value of the Module.nesting method. This method returns the list of modules & classes nested at the point of call.

2- We can see that the eigenclass inherits from the Object eigenclass —the #<Class:Object>.

Indeed, as a class inherits from Object by default, then it seems natural that the eigenclass inherits from the Object eigenclass by default.

Feel free to have a look at the Ruby Object Model article if you’re not familiar with this ancestor chain and the Object class in Ruby.

Conclusion

The eigenclass tends to be pretty difficult to apprehend. But, in fact, it’s very well designed within the context of Ruby. Indeed, when you know a bit about Ruby — especially about the Ruby Object Model — the understanding of this notion becomes natural. In another article, I’ll cover more advanced topics about the eigenclass.

Finally, let’s try to give a proper definition of the eigenclass in Ruby:

The eigenclass is an unnamed instance of the class Class attached to an object and which instance methods are used as singleton methods of the defined object.

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