cattr_accessor

When I was new to ruby and rails, I ran into cattr_accessor and it's lack of documentation.

Simply, cattr_accessor creates a class-level accessor for a class variable. It also adds an instance method for accessing the class variable. There are several options that can be specified to remove the instance_reader and instance_writer. Cattr_accessor is located in the file activesupport/lib/active_support/core_ext/class/attribute_accessors.rb.

These class variables are shared with any subclasses, so be carefull. For example -

Loading development environment (Rails 3.0.0.beta3)

irb(main):001:0> class Pet

irb(main):002:1> cattr_accessor :number

irb(main):003:1> end

=> [:number]

irb(main):004:0> class Dog <>

irb(main):005:1> end

=> nil

irb(main):006:0> class Cat <>

irb(main):007:1> end

=> nil

irb(main):008:0> Dog.class_eval('number = 4')

=> 4

irb(main):009:0> Cat.class_eval('number = 0')

=> 0

irb(main):010:0> puts Dog.class_eval('number')

0

=> nil

irb(main):011:0>

This is because the inherited accessors all reference the Pet's number.