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 like
['title', 'authors', 'location', ...]
Note that these attributes are not really supposed to be methods, just a list of attributes that a LibraryItem
has.
Next, I want to make a subclass of LibraryItem
called LibraryBook
that has an array of attributes that includes all the attributes of LibraryItem
but will also include many more.
Eventually I will want several subclasses of LibraryItem
each with their own version of the array @attributes
but each adding on to LibraryItem
's @attributes
(e.g., LibraryBook
, LibraryDVD
, LibraryMap
, etc.).
So, here is my attempt:
class LibraryItem < Object
class << self; attr_accessor :attributes; end
@attributes = ['title', 'authors', 'location',]
end
class LibraryBook < LibraryItem
@attributes.push('ISBN', 'pages')
end
This does not work. I get the error
undefined method `push' for nil:NilClass
If it were to work, I would want something like this
puts LibraryItem.attributes
puts LibraryBook.attributes
to output
['title', 'authors', 'location']
['title', 'authors', 'location', 'ISBN', 'pages']
(Added 02-May-2010)
One solution to this is to make @attributes
a simple instance variable and then add the new attributes for LibraryBoot
in the initialize
method (this was suggested by demas in one of the answers).
While this would certainly work (and is, in fact, what I have been doing all along), I am not happy with this as it is sub-optimal: why should these unchanging arrays be constructed every time an object is created?
What I really want is to have class variables that can inherit from a parent class but when changed in the child class do not change in the the parent class.
Another solution would be to use the inherited hook:
class LibraryItem < Object
class << self
attr_accessor :attributes
def inherit_attributes(attrs)
@attributes ||= []
@attributes.concat attrs
end
def inherited(sublass)
sublass.inherit_attributes(@attributes)
end
end
@attributes = ['title', 'authors', 'location',]
end
class LibraryBook < LibraryItem
@attributes.push('ISBN', 'pages')
end