class and instance methods in ruby

类方法是一个类的单件方法，可以被继承。类方法是单件方法的特例。

def obj.a_singleton_method
end

Way 1


class Foo
  def self.bar
    puts 'class method'
  end
end

Foo.bar # "class method"

Way 2


class Foo
  class << self
    def bar
      puts 'class method'
    end
  end
end

Foo.bar # "class method"

Way 3


class Foo; end
def Foo.bar
  puts 'class method'
end

Foo.bar # "class method"