Getter/Setter Methods in Ruby

In Ruby and most programming language, it is not possible to call instance variable directly. To access objects’ attributes, one need to use getter and setter methods.

Getters and setters are instance methods that let you retreive and modify instance variables.

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

  def get_name
    @name
  end

  def set_name=(new_name)
    @name = new_name
  end

  def speak
    puts "けけけけ"
  end
end

ghost = Obake.new('Casper')
puts ghost.get_name    # => Casper
ghost.set_name = 'Arthur'
puts ghost.get_name    # => Arthur

In most programming language, get_* and set_* is used as the method name. In Ruby, by convention we use the variable name itself. Let’s rewrite the code above with Ruby’s convention:

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

  def name
    @name
  end

  def name=(new_name)
    @name = new_name
  end

  def speak
    puts "けけけけ"
  end
end

ghost = Obake.new('Casper')
puts ghost.name        # => Casper
ghost.name = 'Arthur'
puts ghost.get_name    # => Arthur

Attributes Accessor

Writting getter and setter methods takes a lot of room in our program for a somewhat basic feature. With a lot of states, even a basic class can take several hundreds lines.

Fortunately, Ruby has a built-in way to automatically create getter and setter methods, using the attr_accessor method.

class Obake
  attr_accessor :name

  def initialize(name)
    @name = name
  end

  def speak
    puts "けけけけ"
  end
end

ghost = Obake.new('Casper')
puts ghost.name        # => Casper
ghost.name = 'Arthur'
puts ghost.get_name    # => Arthur

There are three attr_ methods:

• attr_accessor read & write
• attr_reader read only
• attr_writer write only

Instance variables are written like symboles with a :. Multiple variables can be written separated by comma: attr_reader :name, :age.

Setter Methods

ghost.name = 'Arthur' is a shorthand for ghost.name=('Arthur') where name= (with =) is the method name and the string 'Arthur' is the argument passed to in to the method. However Ruby recognizes this is a setter method and let us use a more natural assignment syntax.

class BankAccount
  # Rest of the code...
  def balance=(new_balance)
    if new_balance >= 0
      @balance = new_balance
      true
    else
      false
    end
  end
end

The balance= method above will always return its argument (new_balance), even if the instance variable @balance is re-assigned or not.