Why isn't this simple test class's method inherited in Ruby?

Question

Consider this very simple logging class:

class MockLog
  # ...
end

Let's add logging to all our classes:

class Module
  def has_logging()
    class_eval {
      @log = MockLog.new
      def log
        self.class.instance_variable_get :@log
      end
    }
  end
end

Now, why doesn't this work?

class Foo
  has_logging
end
Foo.new.log.nil?   # => false, as expected

class Bar < Foo
end
Bar.new.log.nil?   # => true?! Why wasn't the `log` method inherited?

Answer 1

The log method was inherited (if it wasn't, you would get a NoMethodError), but your class level instance variable wasn't, so instance_variable_get returned nil. You should declare a regular class variable like:

class Module
  def has_logging()
    class_eval {
      @@log = MockLog.new
      def log
        @@log
      end
    }
  end
end

class Foo
  has_logging
end

class Bar < Foo
end

Now it will be inherited. See Class and Instance Variables In Ruby