class Module

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

end
include HairColors
class Person

end
end
[:brown, :black, :blonde, :red]
mattr_accessor :hair_colors do
module HairColors

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
include HairColors
class Person

end
mattr_accessor :hair_colors, instance_accessor: false
module HairColors

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

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

end
include HairColors
class Person

end
mattr_accessor :hair_colors, instance_writer: false, instance_reader: false
module HairColors

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.new.hair_colors # => [:brown, :black, :blonde, :red, :blue]
Male.new.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]
HairColors.hair_colors # => [:brown, :black, :blonde, :red]
HairColors.hair_colors = [:brown, :black, :blonde, :red]

end
include HairColors
class Person

end
mattr_accessor :hair_colors
module HairColors

Defines both class and instance accessors for class attributes.

def mattr_accessor(*syms, &blk)
  mattr_reader(*syms, &blk)
  mattr_writer(*syms)
end