Looking at L4Ka::Pistachio, the Haiku kernel, or Symbian's EKA2 might be a good idea, if you're interested in examples of "real" C++-based kernels that have been used as the foundations of complex, feature-laden operating systems.
I've found the latter to be well-structured and consist of very concise and readable code, but others might disagree. However, it's difficult to obtain and build at the moment (since the Symbian Foundation Websites are currently offline), although a copy is mirrored on Google Code.
That aside, I've found L4Ka::Pistachio to be one of the easiest-to-build kernels yet, but customisation and implementation of higher-level components is another manner - although Genode seemed like a promising route to quickly building higher-level components when I tested it, ages ago.
