eigenclass -

Eigenclasses aka metaclasses or singleton classes in Ruby.

Check out the metaclass implementations in other languages for more examples.

Note: This gem was originally written back in 2008. Since then, Ruby has introduced a couple new methods which provide the same functionality as this gem's eigenclass and edefine_method methods.

• Object#singleton_class
• Object#define_singleton_method

Installation

gem install eigenclass

Usage

Everything in Ruby is an object, including classes.

class SomeObject
end

Every object has an eigenclass.

SomeObject.eigenclass #=> #<Class:#<SomeObject:0x007f9611030300>>

The implementation of the eigenclass method is pretty simple.

class Object
  def eigenclass
    class << self
      self
    end
  end
end

Evaluating code within the context of an object's eigenclass allows us to do some pretty cool things - like defining class level attributes.

SomeObject.eigenclass_eval do
  attr_accessor :example
end

SomeObject.example = :test
SomeObject.example #=> :test

The convenience methods for defining class level methods makes this even easier.

class SomeObject
  eattr_accessor :example_accessor
  eattr_reader :example_reader
  eattr_writer :example_writer

  edefine_method(:example_class_method) do
    1 + 1
  end
end

SomeObject.example_class_method #=> 2

Since all objects have an eigenclass, we can even define methods for a single instance of a class.

object = SomeObject.new
object.eattr_accessor :example
object.example = :test
object.example #=> :test

other_object = SomeObject.new
other_object.example #=> NoMethodError undefined method `example' for #<SomeObject:0x007fee348dde00>

This is pretty incredible! We can hook in and inject behavior into any and all objects - at runtime!

Ruby is like one big plugin framework - with an awesome standard library and amazing community!

API

YARD Documentation

• eattr_accessor
• eattr_reader
• eattr_writer
• edefine_method
• eigenclass
• eigenclass_eval
• eigenclass_exec

Testing

bundle exec rspec

Contributing

• Fork the project.
• Make your feature addition or bug fix.
• Add tests for it. This is important so I don't break it in a future version unintentionally.
• Commit, do not mess with Rakefile, version, or history. (if you want to have your own version, that is fine but bump version in a commit by itself I can ignore when I pull)
• Send me a pull request. Bonus points for topic branches.

License

MIT - Copyright © 2008 Sean Huber