Ruby Class Methods vs. Methods in Eigenclasses

Question

Are class methods and methods in the eigenclass (or metaclass) of that class just two ways to define one thing?

Otherwise, what are the differences?

class X
  # class method
  def self.a
    "a"
  end

  # eigenclass method
  class << self
    def b
      "b"
    end
  end
end

Do X.a and X.b behave differently in any way?

I recognize that I can overwrite or alias class methods by opening the eigenclass:

irb(main):031:0> class X; def self.a; "a"; end; end
=> nil
irb(main):032:0> class X; class << self; alias_method :b, :a; end; end
=> #<Class:X>
irb(main):033:0> X.a
=> "a"
irb(main):034:0> X.b
=> "a"
irb(main):035:0> class X; class << self; def a; "c"; end; end; end
=> nil
irb(main):036:0> X.a
=> "c"

Answer 1

The two methods are equivalent. The 'eigenclass' version is helpful for using the attr_* methods, for example:

class Foo
  @instances = []
  class << self;
    attr_reader :instances
  end
  def initialize
    self.class.instances << self
  end
end

2.times{ Foo.new }
p Foo.instances
#=> [#<Foo:0x2a3f020>, #<Foo:0x2a1a5c0>]

You can also use define_singleton_method to create methods on the class:

Foo.define_singleton_method :bim do "bam!" end

Answer 2

In Ruby there really are no such things as class methods. Since everything is an object in Ruby (including classes), when you say def self.class_method, you are just really defining a singleton method on the instance of the class Class. So to answer your question, saying

class X
  def self.a
    puts "Hi"
  end

  class << self
    def b
      puts "there"
    end
  end
end

X.a # => Hi
X.b # => there

is two ways of saying the same thing. Both these methods are just singeton (eigen, meta, ghost, or whatever you want to call them) methods defined in the instance of your Class object, which in your example was X. This topic is part of metaprogramming, which is a fun topic, that if you have been using Ruby for a while, you should check out. The Pragmatic Programmers have a great book on metaprogramming that you should definitely take a look at if you interested in the topic.

Answer 3

Yet another necromancer here to unearth this old question... One thing you might not be aware of is that marking a class method as private (using the private keyword instead of :private_class_method) is different than marking an eigenclass method as such. :

class Foo
  class << self
    def baz
      puts "Eigenclass public method."
    end

    private
    def qux
      puts "Private method on eigenclass."
    end
  end
  private
  def self.bar
    puts "Private class method."
  end
end

Foo.bar
#=> Private class method.
Foo.baz
#=> Eigenclass public method.
Foo.qux
#=> NoMethodError: private method `qux' called for Foo:Class
#   from (irb)

The following example will work how the previous one intends:

class Foo
  class << self
    def baz
      puts "Eigen class public method."
    end

    private
    def qux
      puts "Private method on eigenclass."
    end
  end
  def bar
    puts "Private class method."
  end
  private_class_method :bar
end
Foo.bar
#=> NoMethodError: private method `bar' called for Foo:Class
#     from (irb)
Foo.baz
#=> Eigen class public method.
Foo.qux
#=> NoMethodError: private method `qux' called for Foo:Class
#     from (irb)

Answer 4

Most instance methods used in Ruby are global methods. That means they are available in all instances of the class on which they were defined. In contrast, a singleton method is implemented on a single object.

There is an apparent contradiction. Ruby stores methods in classes and all methods must be associated with a class. The object on which a singleton method is defined is not a class (it is an instance of a class). If only classes can store methods, how can an object store a singleton method? When a singleton method is created, Ruby automatically creates an anonymous class to store that method. These anonymous classes are called metaclasses, also known as singleton classes or eigenclasses. The singleton method is associated with the metaclass which, in turn, is associated with the object on which the singleton method was defined.

If multiple singleton methods are defined within a single object, they are all stored in the same metaclass.

class Zen
end

z1 = Zen.new
z2 = Zen.new

def z1.say_hello  # Notice that the method name is prefixed with the object name
  puts "Hello!"
end

z1.say_hello    # Output: Hello!
z2.say_hello    # Output: NoMethodError: undefined method `say_hello'…

In the above example, the say_hello method was defined within the z1 instance of the Zen class but not the z2 instance.

The following example shows a different way to define a singleton method, with the same result.

class Zen
end

z1 = Zen.new
z2 = Zen.new

class << z1
  def say_hello
    puts "Hello!"
  end
end

z1.say_hello    # Output: Hello!
z2.say_hello    # Output: NoMethodError: undefined method `say_hello'…

In the above example, class << z1 changes the current self to point to the metaclass of the z1 object; then, it defines the say_hello method within the metaclass.

Both of the above examples serve to illustrate how singleton methods work. There is, however, an easier way to define a singleton method: using a built-in method called define_singleton_method.

class Zen
end

z1 = Zen.new
z2 = Zen.new

z1.define_singleton_method(:say_hello) { puts "Hello!" }

z1.say_hello    # Output: Hello!
z2.say_hello    # Output: NoMethodError: undefined method `say_hello'…

We learned earlier that classes are also objects (instances of the built-in class called Class). We also learned about class methods. Class methods are nothing more than singleton methods associated with a class object.

One more example:

class Zabuton
  class << self
    def stuff
      puts "Stuffing zabuton…"
    end
  end
end

All objects may have metaclasses. That means classes can also have metaclasses. In the above example, class << self modifies self so it points to the metaclass of the Zabuton class. When a method is defined without an explicit receiver (the class/object on which the method will be defined), it is implicitly defined within the current scope, that is, the current value of self. Hence, the stuff method is defined within the metaclass of the Zabuton class. The above example is just another way to define a class method.

Read more at this post about Ruby Classes.