Question

Is the singleton class in Ruby a class in and of itself? Is it the reason why all objects belong to ‘class?’ The concept is fuzzy, but I believe it has something to do with why I can define a class method at all (class foo; def foo.bar ...).

What is the singleton class in Ruby?

Answer 1

First, a little definition: a singleton method is a method that is defined only for a single object. Example:

irb(main):001:0> class Foo; def method1; puts 1; end; end => nil irb(main):002:0> foo = Foo.new => #<Foo:0xb79fa724> irb(main):003:0> def foo.method2; puts 2; end => nil irb(main):004:0> foo.method1 1 => nil irb(main):005:0> foo.method2 2 => nil irb(main):006:0> other_foo = Foo.new => #<Foo:0xb79f0ef4> irb(main):007:0> other_foo.method1 1 => nil irb(main):008:0> other_foo.method2 NoMethodError: undefined method `method2' for #<Foo:0xb79f0ef4>         from (irb):8 

Instance methods are methods of a class (i.e. defined in the class’s definition). Class methods are singleton methods on the Class instance of a class — they are not defined in the class’s definition. Instead, they are defined on the singleton class of the object.

irb(main):009:0> Foo.method_defined? :method1 => true irb(main):010:0> Foo.method_defined? :method2 => false 

You open the singleton class of an object with the syntax class << obj. Here, we see that this singleton class is where the singleton methods are defined:

irb(main):012:0> singleton_class = ( class << foo; self; end ) => #<Class:#<Foo:0xb79fa724>> irb(main):013:0> singleton_class.method_defined? :method1 => true irb(main):014:0> singleton_class.method_defined? :method2 => true irb(main):015:0> other_singleton_class = ( class << other_foo; self; end ) => #<Class:#<Foo:0xb79f0ef4>> irb(main):016:0> other_singleton_class.method_defined? :method1 => true irb(main):017:0> other_singleton_class.method_defined? :method2 => false 

So an alternative means of adding singleton methods to an object would be to define them with the object’s singleton class open:

irb(main):018:0> class << foo; def method3; puts 3; end; end => nil irb(main):019:0> foo.method3 3 => nil irb(main):022:0> Foo.method_defined? :method3 => false 

In summary:

• methods must always belong to a class (or: be instance methods of some class)
• normal methods belong to the class they’re defined in (i.e. are instance methods of the class)
• class methods are just singleton methods of a Class
• singleton methods of an object are not instance methods of the class of the object; rather, they are instance methods of the singleton class of the object.