class_attribute(*attrs) public

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

class Base
  class_attribute :setting

class Subclass < Base

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 places but 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 =
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.

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September 15, 2011
2 thanks

beware of trying to dup in subclass inside class context

The example of adding to an array without effecting superclass:

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

That’s right as far as it goes. But beware when you are in context of class definition:

class Subclass < Base
   # possibly wrong, ruby seems to get 
   # confused and think you mean a local 
   # var, not the class ivar
   setting += [:foo]

   # But this will work:
   self.setting += [:foo]

   # Or:
   self.setting = self.setting.dup
   self.setting << :foo

August 20, 2012
0 thanks

To use class attribute with a hash

You can use a setter with merge:

self.settings = settings.merge(key => value)