# File activesupport/lib/active_support/core_ext/class/attribute.rb, line 85
  def class_attribute(*attrs, instance_accessor: true,
    instance_reader: instance_accessor, instance_writer: instance_accessor, instance_predicate: true, default: nil)

    class_methods, methods = [], []
    attrs.each do |name|
      unless name.is_a?(Symbol) || name.is_a?(String)
        raise TypeError, "#{name.inspect} is not a symbol nor a string"
      end

      class_methods << <<~RUBY # In case the method exists and is not public
        silence_redefinition_of_method def #{name}
        end
      RUBY

      methods << <<~RUBY if instance_reader
        silence_redefinition_of_method def #{name}
          defined?(@#{name}) ? @#{name} : self.class.#{name}
        end
      RUBY

      class_methods << <<~RUBY
        silence_redefinition_of_method def #{name}=(value)
          redefine_method(:#{name}) { value } if singleton_class?
          redefine_singleton_method(:#{name}) { value }
          value
        end
      RUBY

      methods << <<~RUBY if instance_writer
        silence_redefinition_of_method(:#{name}=)
        attr_writer :#{name}
      RUBY

      if instance_predicate
        class_methods << "silence_redefinition_of_method def #{name}?; !!self.#{name}; end"
        if instance_reader
          methods << "silence_redefinition_of_method def #{name}?; !!self.#{name}; end"
        end
      end
    end

    location = caller_locations(1, 1).first
    class_eval(["class << self", *class_methods, "end", *methods].join(";").tr("\n", ";"), location.path, location.lineno)

    attrs.each { |name| public_send("#{name}=", default) }
  end

Declare a class-level attribute whose value is inheritable by subclasses. Subclasses can change their own value and it will not impact parent class.

Options

• :instance_reader - Sets the instance reader method (defaults to true).

• :instance_writer - Sets the instance writer method (defaults to true).

• :instance_accessor - Sets both instance methods (defaults to true).

• :instance_predicate - Sets a predicate method (defaults to true).

• :default - Sets a default value for the attribute (defaults to nil).

Examples

class Base
  class_attribute :setting
end

class Subclass < Base
end

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

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

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

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

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

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

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

Subclass.setting?       # => false

Instances may overwrite the class value in the same way:

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

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

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

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

object.setting = false  # => NoMethodError

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

To set a default value for the attribute, pass default:, like so:

class_attribute :settings, default: {}

# File activesupport/lib/active_support/core_ext/class/subclasses.rb, line 38
def subclasses
  descendants.select { |descendant| descendant.superclass == self }
end

Returns an array with the direct children of self.

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

Foo.subclasses # => [Bar]