Ruby class instance variables and inheritance

Question

I have a Ruby class called LibraryItem. I want to associate with every instance of this class an array of attributes. This array is long and looks something lik

Answer 1

Just as a version:

class LibraryItem < Object
  def initialize
    @attributes = ['one', 'two'];
  end
end

class LibraryBook < LibraryItem
  def initialize
   super
   @attributes.push('three')
 end
end

b = LibraryBook.new

Answer 2

Since you mention that the attributes are "fixed" and "unchanging", I am assuming that you mean that you will never change their value once the object is created. In that case, something like the following should work:

class Foo
    ATTRS = ['title', 'authors', 'location']
    def attributes
        ATTRS
    end
end

class Bar < Foo
    ATTRS = ['ISBN', 'pages']
    def attributes
        super + ATTRS
    end
end

You are manually implementing a reader method (instead of letting attr_accessor create it for you) that disguises the internal name of the array. In your subclass, you simply call the ancestor class' reader function, tack on the additional fields associated with the child class, and return that to the caller. To the user, this appears like a read-only member variable named attributes that has additional values in the sub-class.

Answer 3

Out of curiosity, will something like this work?

class Foo
  ATTRIBUTES = ['title','authors','location']
end

class Bar < Foo
  ATTRIBUTES |= ['ISBN', 'pages']
end

This would seem to produce the desired result - the ATTRIBUTES array is expanded when the class object is created, and the values of ATTRIBUTES varies as expected:

> Foo::ATTRIBUTES
=> ['title','authors','location'] 
> Bar::ATTRIBUTES
=> ['title','authors','location', 'ISBN', 'pages']