About

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