Instance Variables in

About Instance Variables

Key Points:

• When a class' .new method is called to create an object instance, the .initialize method is passed all arguments to initialize the instance's state.
• instance variable names are prefixed with @.
• instance variables default to nil until they are explicitly set.
• instance variables are private by default, and they should be manipulated with getters and setters

class Backpack
  initialize(owner)
    @owner = owner
  end

  def owner
    @owner
  end

  def owner=(new_owner)
    @owner = new_owner
  end
end

• Methods named with a trailing = are recognized as setters by Ruby, and allow the syntactic "sugar" use of the assignment syntax, e.g. Backpack.new("Sven").owner = "Ayah". Notice the space between owner and = while the actual method name is owner=.
• Getters and setters can be created using the attr_reader, attr_writer, and attr_accessor methods:
  • attr_reader: Create getters for the symbols listed
  • attr_writer: Create setters for the symbols listed
  • attr_accessor: Create getters and setters for the symbols listed

class Backpack
  attr_accessor :owner

  initialize(owner)
    @owner = owner
  end
end

• Why use getters and setters rather than the instance variable directly?
  • If there was a typographical error (we call these "typo") in the previous example (e.g. @ownar), it would fail silently, potentially introducing a bug into the system.
  • Getters and setters make this explicit, and will raise an error when a typo is made

References

Initializing object instances

• Ruby Guides: Initialize Method

Instance variables

• Ruby For Beginners: Instance variables
• Ruby Guides: Instance variables
• Ruby User's Guide: Instance variables
• Geeks for Geeks: Ruby Getters and Setters Methods
• Mix & Go: Ruby's attr_accessor, attr_reader, attr_writer