class Class

def _stash_object_in_method(object, method, instance_reader = true)

variable and look up the superclass chain manually.
inheritance behavior, without having to store the object in an instance
Take the object being set and store it in a method. This gives us automatic 
def _stash_object_in_method(object, method, instance_reader = true)
  singleton_class.remove_possible_method(method)
  singleton_class.send(:define_method, method) { object }
  remove_possible_method(method)
  define_method(method) { object } if instance_reader
end

def _superclass_delegating_accessor(name, options = {}) 

def _superclass_delegating_accessor(name, options = {})
  singleton_class.send(:define_method, "#{name}=") do |value|
    _stash_object_in_method(value, name, options[:instance_reader] != false)
  end
  send("#{name}=", nil)
end

def cattr_accessor(*syms, &blk)

Person.class_variable_get("@@hair_colors") #=> [:brown, :black, :blonde, :red]

end
end
[:brown, :black, :blonde, :red]
cattr_accessor :hair_colors do
class Person

Also you can pass a block to set up the attribute with a default value.

Person.new.hair_colors # => NoMethodError
Person.new.hair_colors = [:brown] # => NoMethodError

end
cattr_accessor :hair_colors, instance_accessor: false
class Person

Or pass instance_accessor: false, to opt out both instance methods.

Person.new.hair_colors # => NoMethodError
Person.new.hair_colors = [:brown] # => NoMethodError

end
cattr_accessor :hair_colors, instance_writer: false, instance_reader: false
class Person

To opt out of the instance reader method, pass instance_reader: false.
To opt out of the instance writer method, pass instance_writer: false.

Person.hair_colors # => [:brown, :black, :blonde, :red, :blue]
Male.hair_colors << :blue

end
class Male < Person

change the value of subclasses too.
parent class. Similarly if parent class changes the value then that would
If a subclass changes the value then that would also change the value for

Person.new.hair_colors # => [:brown, :black, :blonde, :red]
Person.hair_colors # => [:brown, :black, :blonde, :red]
Person.hair_colors = [:brown, :black, :blonde, :red]

end
cattr_accessor :hair_colors
class Person

Defines both class and instance accessors for class attributes. 
def cattr_accessor(*syms, &blk)
  cattr_reader(*syms)
  cattr_writer(*syms, &blk)
end

def cattr_reader(*syms)

Person.new.hair_colors # => NoMethodError

end
cattr_reader :hair_colors, instance_reader: false
class Person

or instance_accessor: false.
If you want to opt out the instance reader method, you can pass instance_reader: false

# => NameError: invalid attribute name
end
cattr_reader :"1_Badname "
class Person

The attribute name must be a valid method name in Ruby.

Person.new.hair_colors # => [:brown, :black]
Person.hair_colors # => [:brown, :black]
Person.class_variable_set("@@hair_colors", [:brown, :black])

end
cattr_reader :hair_colors
class Person

returns the attribute value.
Defines a class attribute if it's not defined and creates a reader method that 
def cattr_reader(*syms)
  options = syms.extract_options!
  syms.each do |sym|
    raise NameError.new("invalid class attribute name: #{sym}") unless sym =~ /^[_A-Za-z]\w*$/
    class_eval(<<-EOS, __FILE__, __LINE__ + 1)
      unless defined? @@#{sym}
        @@#{sym} = nil
      end
      def self.#{sym}
        @@#{sym}
      end
    EOS
    unless options[:instance_reader] == false || options[:instance_accessor] == false
      class_eval(<<-EOS, __FILE__, __LINE__ + 1)
        def #{sym}
          @@#{sym}
        end
      EOS
    end
  end
end

def cattr_writer(*syms)

Person.class_variable_get("@@hair_colors") # => [:brown, :black, :blonde, :red]

end
end
[:brown, :black, :blonde, :red]
cattr_writer :hair_colors do
class Person

Also, you can pass a block to set up the attribute with a default value.

Person.new.hair_colors = [:blonde, :red] # => NoMethodError

end
cattr_writer :hair_colors, instance_writer: false
class Person

or instance_accessor: false.
If you want to opt out the instance writer method, pass instance_writer: false

# => NameError: invalid attribute name
end
cattr_writer :"1_Badname "
class Person

The attribute name must be a valid method name in Ruby.

Person.class_variable_get("@@hair_colors") # => [:blonde, :red]
Person.new.hair_colors = [:blonde, :red]
Person.class_variable_get("@@hair_colors") # => [:brown, :black]
Person.hair_colors = [:brown, :black]

end
cattr_writer :hair_colors
class Person

assignment to the attribute.
Defines a class attribute if it's not defined and creates a writer method to allow 
def cattr_writer(*syms)
  options = syms.extract_options!
  syms.each do |sym|
    raise NameError.new("invalid class attribute name: #{sym}") unless sym =~ /^[_A-Za-z]\w*$/
    class_eval(<<-EOS, __FILE__, __LINE__ + 1)
      unless defined? @@#{sym}
        @@#{sym} = nil
      end
      def self.#{sym}=(obj)
        @@#{sym} = obj
      end
    EOS
    unless options[:instance_writer] == false || options[:instance_accessor] == false
      class_eval(<<-EOS, __FILE__, __LINE__ + 1)
        def #{sym}=(obj)
          @@#{sym} = obj
        end
      EOS
    end
    send("#{sym}=", yield) if block_given?
  end
end

def class_attribute(*attrs)

To opt out of both instance methods, pass instance_accessor: false.

object.setting = false # => NoMethodError

To opt out of the instance writer method, pass instance_writer: false.

object.setting? # => NoMethodError
object.setting # => NoMethodError

To opt out of the instance reader method, pass instance_reader: false.

Base.setting # => true
object.setting # => false
object.setting = false
object.setting # => true
object = Base.new
Base.setting = true

Instances may overwrite the class value in the same way:

Subclass.setting? # => false

To skip it, pass instance_predicate: false.
For convenience, an instance predicate method is defined as well.

Subclass.setting # => [:foo]
Base.setting # => []
Subclass.setting += [:foo]
Base.setting = []
# Use setters to not propagate changes:

Subclass.setting # => [:foo]
Base.setting # => [:foo]
Subclass.setting << :foo
# Appending in child changes both parent and child because it is the same object:

Subclass.setting # => []
Base.setting # => []
Base.setting = []

In such cases, you don't want to do changes in places but use setters:
when using +class_attribute+ with mutable structures as +Array+ or +Hash+.
on a subclass as overriding the reader method. However, you need to be aware
This matches normal Ruby method inheritance: think of writing an attribute

the value assigned by Subclass would be returned.
would read value assigned to parent class. Once Subclass assigns a value then
by performing Subclass.setting = _something_ , Subclass.setting
In the above case as long as Subclass does not assign a value to setting

Base.setting # => true
Subclass.setting # => false
Subclass.setting = false
Subclass.setting # => true
Base.setting = true

end
class Subclass < Base

end
class_attribute :setting
class Base

Subclasses can change their own value and it will not impact parent class.
Declare a class-level attribute whose value is inheritable by subclasses. 
def class_attribute(*attrs)
  options = attrs.extract_options!
  # double assignment is used to avoid "assigned but unused variable" warning
  instance_reader = instance_reader = options.fetch(:instance_accessor, true) && options.fetch(:instance_reader, true)
  instance_writer = options.fetch(:instance_accessor, true) && options.fetch(:instance_writer, true)
  instance_predicate = options.fetch(:instance_predicate, true)
  attrs.each do |name|
    define_singleton_method(name) { nil }
    define_singleton_method("#{name}?") { !!public_send(name) } if instance_predicate
    ivar = "@#{name}"
    define_singleton_method("#{name}=") do |val|
      singleton_class.class_eval do
        remove_possible_method(name)
        define_method(name) { val }
      end
      if singleton_class?
        class_eval do
          remove_possible_method(name)
          define_method(name) do
            if instance_variable_defined? ivar
              instance_variable_get ivar
            else
              singleton_class.send name
            end
          end
        end
      end
      val
    end
    if instance_reader
      remove_possible_method name
      define_method(name) do
        if instance_variable_defined?(ivar)
          instance_variable_get ivar
        else
          self.class.public_send name
        end
      end
      define_method("#{name}?") { !!public_send(name) } if instance_predicate
    end
    attr_writer name if instance_writer
  end
end

def descendants # :nodoc:

:nodoc: 
def descendants # :nodoc:
  descendants = []
  ObjectSpace.each_object(singleton_class) do |k|
    descendants.unshift k unless k == self
  end
  descendants
end

def descendants # :nodoc:

:nodoc:
JRuby 
def descendants # :nodoc:
  descendants = []
  ObjectSpace.each_object(Class) do |k|
    descendants.unshift k if k < self
  end
  descendants.uniq!
  descendants
end

def singleton_class? 

def singleton_class?
  ancestors.first != self
end

def subclasses

Foo.subclasses # => [Bar]

class Baz < Bar; end
class Bar < Foo; end
class Foo; end

Integer.subclasses # => [Fixnum, Bignum]

Returns an array with the direct children of +self+. 
def subclasses
  subclasses, chain = [], descendants
  chain.each do |k|
    subclasses << k unless chain.any? { |c| c > k }
  end
  subclasses
end

def superclass_delegating_accessor(name, options = {}) 

def superclass_delegating_accessor(name, options = {})
  # Create private _name and _name= methods that can still be used if the public
  # methods are overridden. This allows
  _superclass_delegating_accessor("_#{name}")
  # Generate the public methods name, name=, and name?
  # These methods dispatch to the private _name, and _name= methods, making them
  # overridable
  singleton_class.send(:define_method, name) { send("_#{name}") }
  singleton_class.send(:define_method, "#{name}?") { !!send("_#{name}") }
  singleton_class.send(:define_method, "#{name}=") { |value| send("_#{name}=", value) }
  # If an instance_reader is needed, generate methods for name and name= on the
  # class itself, so instances will be able to see them
  define_method(name) { send("_#{name}") } if options[:instance_reader] != false
  define_method("#{name}?") { !!send("#{name}") } if options[:instance_reader] != false
end