What is a getter method?

A getter method is a method that gets a value of an instance variable.
Without a getter method, you can not retrieve a value of an instance variable outside the class the instance variable is instantiated from.

Here is an example.

class Movie
  def initialize(name)
    @name = name
  end
end

obj1 = Movie.new('Forrest Gump')
p obj1.name #=> undefined method `name' for #<Movie:0x007fecd08cb288 @name="Forrest Gump"> (NoMethodError)

As you can see, the value of obj1 (name) can not be retrieved outside Movie class. if you try to retrive a value of an instance variable outside its class without a getter method, Ruby raises No Mothod Error.

Now, Let's see how to retrieve the value of obj1 outside Movie class with a getter method.
All you have to do here is to define a getter method named name. Though the name of a getter method can be anything, it is common practice to name a getter method the instance variable’s name.

class Movie
  def initialize(name)
    @name = name
  end

  def name
    @name
  end
end

obj1 = Movie.new('Forrest Gump')
p obj1.name #=> "Forrest Gump"

What is a setter method?

A setter is a method that sets a value of an instance variable.
Without a setter method, you can not assign a value to an instance variable outside its class.
if you try to set a value of an instance variable outside its class, Ruby raises No Method Error just like it does when you try to retrieve a value of an instance variable outside its class without a getter method.

class Movie
  def initialize(name)
    @name = name
  end

  def name #getter method
    @name
  end
end

obj1 = Movie.new('Forrest Gump')
p obj1.name #=> "Forrest Gump"
obj1.name = 'Fight Club' #=> undefined method `name=' for #<Movie:0x007f9937053368 @name="Forrest Gump"> (NoMethodError)

Defining a setter method inside a class makes it possible to set a value of an instance variable outside the class.
You can define a setter method like the code below.

class Movie
  def initialize(name)
    @name = name
  end

  def name #getter method
    @name
  end

  def name=(name) #setter method
    @name = name
  end
end

obj1 = Movie.new('Forrest Gump')
p obj1.name #=> "Forrest Gump"
obj1.name = 'Fight Club'
p obj1.name #=> "Fight Club"

By using name=, you can now assign a new value Fight Club to obj1.

What are accessors?

Accessors are a way to create getter and setter methods without explicitly defining them in a class.
There are three types fo accessors in Ruby.

• attr_reader automatically generates a getter method for each given attribute.
• attr_writer automatically generates a setter method for each given attribute.
• attr_accessor automatically generates a getter and setter method for each given attribute.

First, let's take a look at attr_reader!
As you can see in the code below, name and year are retrieved outside Movie class even though there is no getter method for either of them. This is because attr_reader generates a getter method for each given attribute.

class Movie
 attr_reader :name, :year

 def initialize(name, year)
   @name = name
   @year = year
 end
end
obj1 = Movie.new('Forrest Gump', 1994)
p obj1.name #=> Forrest Gump
p obj1.year #=> 1994

Second, let's see how attr_writer works!
As I mentioned above, attr_witer generates a setter method for each given attribute. Therefore you can assign new values to ob1 without explicitly writing setter methods for name and year!

class Movie
  attr_reader :name, :year 
  attr_writer :name, :year

  def initialize(name, year)
    @name = name
    @year = year
  end
end
obj1 = Movie.new('Forrest Gump', 1994)
obj1.name = 'Fight Club'
obj1.year = 1999
p obj1.name #=> "Fight Club"
p obj1.year #=> 1999

Last but certainly not least, attr_accessor does what attr_reader and attr_writer do with just one line of code! It will automatically generate a getter and setter mehod for each given attribute.

class Movie
  attr_accessor :name, :year
  
  def initialize(name, year)
    @name = name
    @year = year
  end
end
obj1 = Movie.new('Forrest Gump', 1994)
obj1.name = 'Fight Club'
obj1.year = 1999
p obj1.name #=> "Fight Club"
p obj1.year #=> 1999

References

• Ruby Getters and Setters
• How getter/setter methods work in Ruby
• [What is attr_accessor in Ruby?]
  (https://stackoverflow.com/questions/4370960/what-is-attr-accessor-in-ruby)
• rubylearning.com