Self in Ruby

The keyword self in Ruby gives you access to the current object – the object that is receiving the current message. To explain: a method call in Ruby is actually the sending of a message to a receiver. When you write obj.meth, you're sending the meth message to the object obj. obj will respond to meth if there is a method body defined for it. And inside that method body, self refers to obj. When I started with Ruby, I learned this pretty quickly, but it wasn't totally apparent when you might actually need to use self. I will outline the two most common use cases I've found for it.

Class methods

The first usage I ran into was to define class methods. Inside a class, the def keyword will create a new instance method, when used without an explicit receiver.

class Post
  attr_writer :title

  def print_title
    puts "The title of this post is #{@title}"
  end
end

pst = Post.new
pst.title = "Green Beans"
pst.print_title
# "The title of this post is Green Beans"

In the context of a class, self refers to the current class, which is simply an instance of the class Class. Defining a method on self creates a class method.

class Post
  def self.print_author
    puts "The author of all posts is Jimmy"
  end
end

Post.print_author
# "The author of all posts is Jimmy"

Another more advanced way to do this is to define a method inside the Class instance itself. This is referred to as the eigenclass or the singleton class and it uses the self keyword to open a new context where the Class instance is held in self.

class Post
  class << self
    def print_author
      puts "The author of all posts is Jimmy"
    end
  end
end

Post.print_author
# "The author of all posts is Jimmy"

Disambiguation

When you call a method without an explicit receiving object, the method is implicitly called on self. So if self is assumed for us, why do we ever need to use self.meth outside of a class method definition? As it turns out, it may not always be clear which method you're trying to call. Consider this example:

class Post
  attr_writer :title

  def self.author
    "Jimmy"
  end

  def full_title
    "#{@title} by #{class.author}"
  end
end

pst = Post.new
pst.title = "Delicious Ham"
puts pst.full_title

When we call full_title, we get a syntax error because class.author in the method body attempts to use the class keyword instead of the class method on the pst object, which is what we want – the object's class. If we use self.class.author instead, Ruby knows that we want the class method of pst, and we get the result we expect.

Another time when self is needed for disambiguation is when assigning a value to one of the object's attributes. Here is a contrived example:

class Post
  attr_accessor :title

  def replace_title(new_title)
    title = new_title
  end

  def print_title
    puts title
  end
end

pst = Post.new
pst.title = "Cream of Broccoli"
pst.replace_title("Cream of Spinach")
pst.print_title
# "Cream of Broccoli"

Even though we replaced the title of the post with "Cream of Spinach," it remained set to "Cream of Broccoli" and that's what we see when calling print_title. This is because the assignment inside replace_title is simply assigning to a local variable called title which is not used for anything. If we change that line to self.title = new_title, then the call to print_title at the end will give us "Cream of Spinach" as we were expecting. Note that it is not necessary to use self.title explicitly when using the accessor method inside the definition of print_title, because Ruby will see that there is no local variable with that name and then send self the message title. In the case of assignment, Ruby must assume you want to assign to a local variable, because if it sends the title= message to self, you are left with no way to set a local variable.

If you have more examples of when you might use self, feel free to leave a comment.

Comments

Aziz Light Aziz Light commented
February 03, 2011

Great article! I have one little question though:

In the last code sample it makes sense to use self.title instead of title in the replace_title method. However, I tried using @title instead of title (or self.title) and it also worked. So in that case what is the different between self.title and @title ?

Jimmy Cuadra Jimmy Cuadra commented
February 03, 2011

In such a contrived example, there is no difference between self.title and @title, since the "title" method is just an attribute reader. If you defined your own accessor method for title which did more than simply return the value of @title, the difference would be more clear.

AzizLight AzizLight commented
February 05, 2011

Jimmy Cuadra: Sorry for the late response, and thanks for your answer. Now it all makes sense :)

seydar seydar commented
February 05, 2011

AzizLight: self.title= calls a method, and @title refers specifically to the instance variable. Often, people will add some kind of check to restrict the actual value of @title. Assigning directly to @title would bypass these checks, while self.title= would abide by any checks. You can define your own method as such:

def title=(val)
val = "SUPER DUPER COOL" if val == "cool"
@title = val
end

Assigning directly to @title would allow you to set @title to be "cool", which would normally not be allowed (since it would always be replaced with "SUPER DUPER COOL").