How to Use attr_accessor, attr_writer & attr_reader

This article is about attribute accessors (attr_accessor) in Ruby.

If you’re in a hurry, scroll down.

Because I would like to start by explaining:

Why we use attribute accessors!

Let’s say that you have a class with instance variables & you want to expose them to the outside world.

How?

You have to define a method.

Only methods can access instance variables.

Why?

Because you’ll get an error if you don’t do this.

Here’s an example:

class Food
  def initialize(protein)
    @protein = protein
  end
end

bacon = Food.new(21)
bacon.protein

# NoMethodError: undefined method `protein'

NoMethodError is the error you get when you ask for the value of protein without the proper setup.

What’s the solution?

You can define your own method like this:

class Food
  def protein
    @protein
  end
end

bacon.protein
# 21

In other OOP languages this is known as a “getter” method. You define a method that gets you the value of the instance variable.

You may also want to change the value.

For that, you’ll need another method, like this one:

class Food
  def protein=(value)
    @protein = value
  end
end

bacon.protein = 25

Imagine you’re opening a portal into the object so that you can change the value.

That’s what this is doing.

Now:

Is there a better way to define this kind of method?

Like some kind of shortcut?

Yes!

There is 🙂

That’s where attr_accessor comes in.

Ruby attr_accessor Examples

You can tell Ruby to create these methods for you with attr_accessor.

Here’s how:

class Food
 attr_accessor :protein

 def initialize(protein)
   @protein = protein
 end
end

Look at this line:

attr_accessor :protein

This is a Ruby method that creates other methods for you.

What methods?

For this example, it creates:

protein
protein=

These are the same methods we created before…

But now you don’t have to type them out.

It’s a shortcut!

attr_accessor vs attr_reader

Besides attr_accessor, you also have other kinds of accessors.

Three of them to be exact:

attr_accessor
attr_reader
attr_writer

What are the differences between them?

Well, attr_accessor creates both the READER & WRITER methods.

attr_reader creates only the reader.
attr_writer creates only the writer.

In other words:

With attr_reader you can only read the value, but not change it. With attr_writer you can only change a value but not read it.

Multiple Instance Variables

Want to define multiple attribute methods?

You can do it.

But you have to use the right syntax.

Like this:

attr_reader :name, :value, :ready?

You can create as many as you want.

Watch Video Tutorial

Summary

You have learned about attribute accessors in Ruby. A set of 3 methods (attr_accessor, attr_writer & attr_reader) to access & set instance variables from outside the class.

Now it’s time to practice!