How can a nested class access a method in the outer class in Ruby?

def class A
  def a
    raise "hi" #can't be reached
  end

  class B
    def b
      a() #doesn't find method a.
    end
  end
end

I want to invoke a from b and raise the exception. How can I?

Ruby doesn't have nested classes.

The only way to inherit behavior is, well, via inheritance.

If you want your code to work, you need to use a language which supports nested classes. While this is an incredibly neat and powerful feature, I unfortunately know of only two languages that have nested classes:

• BETA, the language which introduced nested classes (and its successor gbeta)
• Newspeak

I don't know of any other.

Java has a construct called nested classes, but they have some unfortunate design limitations.

In your example above, it's not the class B that is nested inside A, it is the constant B that is nested inside A. Think about this:

C = A::B

Now, the class is available under two names: A::B and C. It should be immediately obvious that C is global and not nested inside A. (Well, actually, C is nested inside Object, because there aren't really global constants either, but that's beside the point.) But since C and A::B are the same class, it obviously cannot be both nested and not nested. The only logical conclusion is that the class itself isn't nested.

The defining feature of nested classes is that method lookup goes along two dimensions: up the inheritance chain, and outwards through the nesting. Ruby, like 99.9% of all OO languages, only supports the former. (In some sense, nested classes inherit not only the features of their superclass, but also the features of their surrounding class.)