Class Instance Variables

Ruby has a unique concept, namely Class Instance Variables that is not common in other programming languages. Ruby has instance variables, class variables and class instance variables. Lets take a quick look at what I am talking about.

Example 1

class Resource
  @@developers = []

  def self.developers
    return @@developers
  end

  def add_to_pool
    @@developers << self
  end

  def initialize(resource_name)
    @resource_name = resource_name
  end
end

Resource.new(‘Matz’).add_to_pool
Resource.new(‘David’).add_to_pool
Resource.new(‘James’).add_to_pool

puts Resource.developers.size    # 3

The output is pretty obvious here. Now lets consider a practical problem with this approach.

Example 2

class RubyDeveloper < Resource; end
class JavaDeveloper < Resource; end

RubyDeveloper.new(‘Matz’).add_to_pool
RubyDeveloper.new(‘David’).add_to_pool

JavaDeveloper.new(‘James’).add_to_pool

puts RubyDeveloper.developers.size    # ?
puts JavaDeveloper.developers.size    # ?

The output here is:
>3
>3

Here’s the problem, how do we restrict the class variable to a given class and prevent any modification by its subclass?

Class Instance Variables to the rescue

Example 3

class Resource

  class << self
    def developers
      return @developers
    end

    def developers=(developers)
      @developers | |= [ ]
    end
  end

  def add_to_pool
    self.class. developers | |= [ ]
    self.class. developers << self
  end

  def initialize(resource_name)
    @resource_name = resource_name
  end
end

class RubyDeveloper < Resource; end
class JavaDeveloper < Resource; end

RubyDeveloper.new(‘Matz’).add_to_pool
RubyDeveloper.new(‘David’).add_to_pool

JavaDeveloper.new(‘James’).add_to_pool

puts RubyDeveloper.developers.size    # 2
puts JavaDeveloper.developers.size    # 1

Simplifying the code:

class << self
  def developers
    return @developers
  end

  def developers=(developers)
    @developers | |= [ ]
  end
end

can be achieved by:

class << self
  attr_accessor :developers
end

Rewriting the Resource class

class Resource
  class << self; attr_accessor :developers; end

  def add_to_pool
    self.class. developers | |= [ ]
    self.class. developers << self
  end

  def initialize(resource_name)
    @resource_name = resource_name
  end
end

Just to summarize:

Declaring the Class Instance Variable:

class << self; attr_accessor :developers; end

Accessing the Class Instance Variable:

Inside the class, where self is the class object:

self.developers

Inside an action in the class, where self is the instance:

self.class.developers

Hope that was interesting.
Happy coding!!!

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