Understanding Accessors in Ruby

One thing that puzzled me as a newbie (disclaimer: I still am) are accessors in Ruby, more commonly known as setters and getters or explicitly described as attr_reader, attr_writer and attr_accessor. Now let’s dive into the code first and describe the concepts of accessors after we’re done with coding.

Initializing a Class

Let’s say we want to create a class to resemble a Person with a name, and finally let’s try to access that name outside the class:

# Class definition
class Person
def initialize(name)
@name = name
end

end

# Initialize class and inspect it
p1 = Person.new('Anne')

# Can't access 'name' variable outside class
puts p1.name

But it seems that we can’t access the variable @name even though we initialized right:

# Output:
test.rb:13:in `<main>': undefined method `name' for #<Person:0x00000002278100 @name="name"> (NoMethodError)

This is because we have to define a method to access the variable inside the class.

Getters and Setters

Let’s grab our code and add two methods:

  • A method to update the person’s name
  • And a method to read the person’s name
# Class definition
class Person
def initialize(name)
@name = name
end

# Update 'name'
# use =() to make a method behave
# like an attribute assignment
def name=(name)
@name = name
end

# Get value of 'name'
def name
@name
end
end

# Initialize class and the value 'name'
p1 = Person.new('Anne')
puts p1.name

# Change 'name'
p1.name = 'Johnson'
puts p1.name
# Output
Anne
Johnson

Now this code works perfectly, but a person doesn’t have just one attribute like name, they have age, height, eye color, skin color, hair color. Could you imagine writing endless methods about each attribute? Gladly we have accessors

Write and Read Accessor

Before we disclose the technical concept of accessors let’s first try them, shall we? We’re changing our code into a more sophisticated and short one with attr_accessor:

# Class definition
class Person
attr_accessor :name

def initialize(name)
@name = name
end
end

# Initialize class and the value 'name'
p1 = Person.new('Anne')
puts p1.name

# Change name
p1.name = 'Johnson'
puts p1.name

# Output
Anne
Johnson

What happened here? We wrote an accessor that allow us to read and update the attribute name in the class Person, this way we can add more attributes like height and weight in a single line:

# Class definition
class Person
attr_accessor :name, :height, :weight

def initialize(name)
@name = name
end
end

# Initialize class with a name and
# add values outside the class
p1 = Person.new('Anne')
p1.height = '1.80m'
p1.weight = '180lbs'

puts "Name: #{p1.name}"
puts "Height: #{p1.height}"
puts "Weight: #{p1.weight}"

# Output
Name: Anne
Height: 1.80m
Weight: 180lbs

Write-Only Accessor

Now that’s a lot easier than to write and read each method, but what about if want to set an attribute but not read them? For example, a person can have many thoughts but no one else can read them, let’s give this person a thoughts attribute and let’s try to access this person thoughts outside the class:

# Class definition
class Person
attr_accessor :name, :height, :weight
attr_writer :thoughts

def initialize(name)
@name = name
end
end

# Initialize class with a name and
# add values outside the class
p1 = Person.new('Anne')
p1.height = '1.80m'
p1.weight = '180lbs'

# Set a thought and inspect the class
p1.thoughts = 'pizza <3'
puts p1.inspect

puts "Name: #{p1.name}"
puts "Height: #{p1.height}"
puts "Weight: #{p1.weight}"

# Try to access that thought
puts "#{p1.name} is thinking about: #{p1.thoughts}"

# Output
#<Person:0x00000000e5f538 @name="Anne", @height="1.80 meters", @weight="180 lbs", @thoughts="pizza <3">
Name: Anne
Height: 1.80 meters
Weight: 180 lbs
test.rb:26:in `<main>': undefined method `thoughts' for #<Person:0x00000000e5f538> (NoMethodError)

See? We are able to set a value on the thought attribute, but outside the class we are unable to read it (line 26)

Read-Only Accessor

Now, a person have a lot of things that they can’t change… not by conventional means at least. Let’s add a read-only attribute to this person with attr_reader, like eye_color in line 5 and initialize it:

# Class definition
class Person
attr_accessor :name, :height, :weight
attr_writer :thoughts

# read-only accessor
attr_reader :eye_color

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

# Initialize class with a name and
# add values outside the class
p1 = Person.new('Anne', 'Green')
p1.height = '1.80m'
p1.weight = '180lbs'
p1.thoughts = 'pizza <3'

puts "Name: #{p1.name}"
puts "Height: #{p1.height}"
puts "Weight: #{p1.weight}"

# Read eye color
puts "Eye Color: #{p1.eye_color}"
# Output
Name: Anne
Height: 1.80m
Weight: 180lbs
Eye Color: Green

As you can see there’s no problem in initializing and accessing the eye_color attribute, but as soon as we try to change it with p1.eye_color = 'red', we can expect the following response:

test.rb:30:in `<main>': undefined method `eye_color=' for #<Person:0x000000019defa8> (NoMethodError)

That, of course, is because we set the accessor as read only.

Conclusion and Technical Definition

I cannot describe better what an accessor is more than the definition found in the official Ruby user’s guide:

An object’s instance variables are its attributes, the things that generally distinguish it from other objects of the same class. It is important to be able to write and read these attributes; doing so requires writing methods called attribute accessors.

Basically, attr_accessor, attr_writer and attr_reader is a short way to define and set class variables without having to define the methods to access and read them outside the class, with:

  1. attr_accessor we are able to read and write values outside the class.
  2. attr_writer allow us to write values without being able to read them outside the class.
  3. and with attr_reader we can initialize and read the class attributes without being able to reassign it.

You can read more about accessors and how useful they are in the following documentation:

If I got something wrong, you can correct me in the comments bellow.