What is attr_accessor in Ruby?

Question

I am having a hard time understanding attr_accessor in Ruby. Can someone explain this to me?

Answer 1

I faced this problem as well and wrote a somewhat lengthy answer to this question. There are some great answers on this already, but anyone looking for more clarification, I hope my answer can help

Initialize Method

Initialize allows you to set data to an instance of an object upon creation of the instance rather than having to set them on a separate line in your code each time you create a new instance of the class.

class Person
  attr_accessor :name

  def initialize(name)
    @name = name
  end


  def greeting
    "Hello #{@name}"
  end
end

person = Person.new("Denis")
puts person.greeting

In the code above we are setting the name “Denis” using the initialize method by passing Dennis through the parameter in Initialize. If we wanted to set the name without the initialize method we could do so like this:

class Person
  attr_accessor :name

  # def initialize(name)
  #     @name = name
  # end

  def greeting
    "Hello #{name}"
  end
end

person = Person.new
person.name = "Dennis"
puts person.greeting

In the code above, we set the name by calling on the attr_accessor setter method using person.name, rather than setting the values upon initialization of the object.

Both “methods” of doing this work, but initialize saves us time and lines of code.

This is the only job of initialize. You cannot call on initialize as a method. To actually get the values of an instance object you need to use getters and setters (attr_reader (get), attr_writer(set), and attr_accessor(both)). See below for more detail on those.

Getters, Setters (attr_reader, attr_writer, attr_accessor)

Getters, attr_reader: The entire purpose of a getter is to return the value of a particular instance variable. Visit the sample code below for a breakdown on this.

class Item

  def initialize(item_name, quantity)
    @item_name = item_name
    @quantity = quantity
  end

  def item_name
    @item_name
  end

  def quantity
     @quantity
  end
end

example = Item.new("TV",2)
puts example.item_name
puts example.quantity

In the code above you are calling the methods “item_name” and “quantity” on the instance of Item “example”. The “puts example.item_name” and “example.quantity” will return (or “get”) the value for the parameters that were passed into the “example” and display them to the screen.

Luckily in Ruby there is an inherent method that allows us to write this code more succinctly; the attr_reader method. See the code below;

class Item

attr_reader :item_name, :quantity

  def initialize(item_name, quantity)
    @item_name = item_name
    @quantity = quantity
  end

end

item = Item.new("TV",2)
puts item.item_name
puts item.quantity

This syntax works exactly the same way, only it saves us six lines of code. Imagine if you had 5 more state attributable to the Item class? The code would get long quickly.

Setters, attr_writer: What crossed me up at first with setter methods is that in my eyes it seemed to perform an identical function to the initialize method. Below I explain the difference based on my understanding;

As stated before, the initialize method allows you to set the values for an instance of an object upon object creation.

But what if you wanted to set the values later, after the instance was created, or change them after they have been initialized? This would be a scenario where you would use a setter method. THAT IS THE DIFFERENCE. You don’t have to “set” a particular state when you are using the attr_writer method initially.

The code below is an example of using a setter method to declare the value item_name for this instance of the Item class. Notice that we continue to use the getter method attr_reader so that we can get the values and print them to the screen, just in case you want to test the code on your own.

class Item

attr_reader :item_name

  def item_name=(str)
    @item_name = (str)
  end

end

The code below is an example of using attr_writer to once again shorten our code and save us time.

class Item

attr_reader :item_name
attr_writer :item_name

end

item = Item.new
puts item.item_name = "TV"

The code below is a reiteration of the initialize example above of where we are using initialize to set the objects value of item_name upon creation.

class Item

attr_reader :item_name

  def initialize(item_name)
    @item_name = item_name
  end

end

item = Item.new("TV")
puts item.item_name

attr_accessor: Performs the functions of both attr_reader and attr_writer, saving you one more line of code.

Answer 2

Attributes and accessor methods

Attributes are class components that can be accessed from outside the object. They are known as properties in many other programming languages. Their values are accessible by using the "dot notation", as in object_name.attribute_name. Unlike Python and a few other languages, Ruby does not allow instance variables to be accessed directly from outside the object.

class Car
  def initialize
    @wheels = 4  # This is an instance variable
  end
end

c = Car.new
c.wheels     # Output: NoMethodError: undefined method `wheels' for #<Car:0x00000000d43500>

In the above example, c is an instance (object) of the Car class. We tried unsuccessfully to read the value of the wheels instance variable from outside the object. What happened is that Ruby attempted to call a method named wheels within the c object, but no such method was defined. In short, object_name.attribute_name tries to call a method named attribute_name within the object. To access the value of the wheels variable from the outside, we need to implement an instance method by that name, which will return the value of that variable when called. That's called an accessor method. In the general programming context, the usual way to access an instance variable from outside the object is to implement accessor methods, also known as getter and setter methods. A getter allows the value of a variable defined within a class to be read from the outside and a setter allows it to be written from the outside.

In the following example, we have added getter and setter methods to the Car class to access the wheels variable from outside the object. This is not the "Ruby way" of defining getters and setters; it serves only to illustrate what getter and setter methods do.

class Car
  def wheels  # getter method
    @wheels
  end

  def wheels=(val)  # setter method
    @wheels = val
  end
end

f = Car.new
f.wheels = 4  # The setter method was invoked
f.wheels  # The getter method was invoked
# Output: => 4

The above example works and similar code is commonly used to create getter and setter methods in other languages. However, Ruby provides a simpler way to do this: three built-in methods called attr_reader, attr_writer and attr_acessor. The attr_reader method makes an instance variable readable from the outside, attr_writer makes it writeable, and attr_acessor makes it readable and writeable.

The above example can be rewritten like this.

class Car
  attr_accessor :wheels
end

f = Car.new
f.wheels = 4
f.wheels  # Output: => 4

In the above example, the wheels attribute will be readable and writable from outside the object. If instead of attr_accessor, we used attr_reader, it would be read-only. If we used attr_writer, it would be write-only. Those three methods are not getters and setters in themselves but, when called, they create getter and setter methods for us. They are methods that dynamically (programmatically) generate other methods; that's called metaprogramming.

The first (longer) example, which does not employ Ruby's built-in methods, should only be used when additional code is required in the getter and setter methods. For instance, a setter method may need to validate data or do some calculation before assigning a value to an instance variable.

It is possible to access (read and write) instance variables from outside the object, by using the instance_variable_get and instance_variable_set built-in methods. However, this is rarely justifiable and usually a bad idea, as bypassing encapsulation tends to wreak all sorts of havoc.

Answer 3

Let's say you have a class Person.

class Person
end

person = Person.new
person.name # => no method error

Obviously we never defined method name. Let's do that.

class Person
  def name
    @name # simply returning an instance variable @name
  end
end

person = Person.new
person.name # => nil
person.name = "Dennis" # => no method error

Aha, we can read the name, but that doesn't mean we can assign the name. Those are two different methods. The former is called reader and latter is called writer. We didn't create the writer yet so let's do that.

class Person
  def name
    @name
  end

  def name=(str)
    @name = str
  end
end

person = Person.new
person.name = 'Dennis'
person.name # => "Dennis"

Awesome. Now we can write and read instance variable @name using reader and writer methods. Except, this is done so frequently, why waste time writing these methods every time? We can do it easier.

class Person
  attr_reader :name
  attr_writer :name
end

Even this can get repetitive. When you want both reader and writer just use accessor!

class Person
  attr_accessor :name
end

person = Person.new
person.name = "Dennis"
person.name # => "Dennis"

Works the same way! And guess what: the instance variable @name in our person object will be set just like when we did it manually, so you can use it in other methods.

class Person
  attr_accessor :name

  def greeting
    "Hello #{@name}"
  end
end

person = Person.new
person.name = "Dennis"
person.greeting # => "Hello Dennis"

That's it. In order to understand how attr_reader, attr_writer, and attr_accessor methods actually generate methods for you, read other answers, books, ruby docs.

Answer 4

attr_accessor is just a method. (The link should provide more insight with how it works - look at the pairs of methods generated, and a tutorial should show you how to use it.)

The trick is that class is not a definition in Ruby (it is "just a definition" in languages like C++ and Java), but it is an expression that evaluates. It is during this evaluation when the attr_accessor method is invoked which in turn modifies the current class - remember the implicit receiver: self.attr_accessor, where self is the "open" class object at this point.

The need for attr_accessor and friends, is, well:

1. Ruby, like Smalltalk, does not allow instance variables to be accessed outside of methods¹ for that object. That is, instance variables cannot be accessed in the x.y form as is common in say, Java or even Python. In Ruby y is always taken as a message to send (or "method to call"). Thus the attr_* methods create wrappers which proxy the instance @variable access through dynamically created methods.

2. Boilerplate sucks

Hope this clarifies some of the little details. Happy coding.

---

¹ This isn't strictly true and there are some "techniques" around this, but there is no syntax support for "public instance variable" access.

Answer 5

Hmmm. Lots of good answers. Here is my few cents on it.

• attr_accessor is a simple method that helps us in cleaning(DRY-ing) up the repeating getter and setter methods.

• So that we can focus more on writing business logic and not worry about the setters and getters.

Answer 6

If you are familiar with OOP concept, You must familiar with getter and setter method. attr_accessor does the same in Ruby.

Getter and Setter in General Way

class Person
  def name
    @name
  end

  def name=(str)
    @name = str
  end
end

person = Person.new
person.name = 'Eshaan'
person.name # => "Eshaan"

Setter Method

def name=(val)
  @name = val
end

Getter method

def name
  @name
end

Getter and Setter method in Ruby

class Person
  attr_accessor :name
end

person = Person.new
person.name = "Eshaan"
person.name # => "Eshaan"