Question

Why there is an error in the following example?

class ClassA
  class ClassB
  end
  class ClassC
    def test
      ClassB.new
    end
  end
end

p ClassA::ClassC.new.test # => #<ClassA::ClassB:0x0000010103f860>

class ClassA
  class ClassD
    def test
      ClassB.new
    end
  end
end

p ClassA::ClassD.new.test # => #<ClassA::ClassB:0x0000010103f010>

class ClassA::ClassE
  def test
    ClassB.new
  end
end

p ClassA::ClassE.new.test # => NameError: uninitialized constant ClassA::ClassE::ClassB

Is there another way to create ClassE, not by typing class ClassA; class ClassE?

Answer 1

Well, yes, if you define your test method to return ClassA::ClassB.new 🙂

You could also play around with const_missing so that it calls ClassA.const_get.

Otherwise ClassB is not in the current scope, which at that point is only ClassA::ClassE and Object. When you first open ClassA, then ClassE, the lookup for ClassB is done first in ClassA::ClassE, then in ClassA (where it is found) and would also look in Object.