class Class

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!
  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|
    remove_possible_singleton_method(name)
    define_singleton_method(name) { nil }
    remove_possible_singleton_method("#{name}?")
    define_singleton_method("#{name}?") { !!public_send(name) } if instance_predicate
    ivar = "@#{name}"
    remove_possible_singleton_method("#{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
      remove_possible_method "#{name}?"
      define_method("#{name}?") { !!public_send(name) } if instance_predicate
    end
    if instance_writer
      remove_possible_method "#{name}="
      attr_writer name
    end
  end
end