Today I'd like to talk about self . If you've been programming Ruby for a while, you've likely internalized the idea of self . Whenever you read or write a program, self is there in the back of your mind.

But for less-experienced Rubyists, self can be baffling. It's always changing, but it's never explicitly shown in the code. You're just expected to know.

A lot of the problems beginners face are caused by not understanding self . If you've ever "lost" an instance variable or puzzled over what data is visible to a mixin, then it's because you didn't understand self in that context.

In this post, we're going to look at self in a variety of every-day situations.

What is self ?

You may have heard people say that everything in Ruby is an object. If that's true it means that every piece of code you write "belongs" to some object.

self is a special variable that points to the object that "owns" the currently executing code. Ruby uses self everwhere:

For instance variables: @myvar

For method and constant lookup

When defining methods, classes and modules.

In theory, self is pretty obvious. But in practice, it's easy for tricky situations to pop up. That's why I wrote this post.

Examples of self

We're going to step through several examples now. If the first ones seem too basic for you, just keep reading. They get more advanced.

Inside of an instance method

In the code below, reflect is an instance method. It belongs to the object we created via Ghost.new . So self points to that object.

class Ghost def reflect self end end g = Ghost . new g . reflect == g # => true

Inside of a class method

For this example, reflect is a class method of Ghost . With class methods, the class itself "owns" the method. self points to the class.

class Ghost def self . reflect self end end Ghost . reflect == Ghost # => true

It works the same with "class" methods inside of modules. For example:

module Ghost def self . reflect self end end Ghost . reflect == Ghost # => true

Remember, classes and modules are treated as objects in Ruby. So this behavior isn't that different from the instance method behavior we saw in the first example.

Inside of a class or module definition

One feature of Ruby that makes it such a good fit for frameworks like Rails is that you can execute arbitrary code inside class and module definitions. When you put code inside of a class/module definition, it runs just like any other Ruby code. The only real difference is the value of self .

As you can see below, self points to the class or module that's in the process of being defined.

class Ghost self == Ghost # => true end module Mummy self == Mummy # => true end

Inside mixin methods

Mixed-in methods behave just like "normal" instance or class methods when it comes to self . This makes sense. Otherwise the mixin wouldn't be able to interact with the class you mixed it into.

Instance methods

Even though the reflect method was defined in the module, its self is the instance of the class it was mixed into.

module Reflection def reflect self end end class Ghost include Reflection end g = Ghost . new g . reflect == g # => true

Class methods

When we extend a class to mix in class methods, self behaves exactly like it does in normal class methods.

module Reflection def reflect self end end class Ghost extend Reflection end Ghost . reflect == Ghost # => true

Inside the metaclass

Chances are you've seen this popular shortcut for defining lots of class methods at once.

class Ghost class << self def method1 end def method2 end end end

The class << foo syntax is actually pretty interesting. It lets you access an object's metaclass - which is also called the "singleton class" or "eigenclass." I plan on covering metaclasses more deeply in a future post. But for now, you just need to know that the metaclass is where Ruby stores methods that are unique to a specific object.

If you access self from inside the class << foo block, you get the metaclass.

class << "test" puts self . inspect end # => #<Class:#<String:0x007f8de283bd88>

Outside of any class

If you're running code outside of any class, Ruby still provides self . It points to "main", which is an instance of Object :