Wykobi is an excellent example of a geometry library; cross platform and open source under the GPL 2.0 library. This was a conversion of a pascal library and is somewhat procedural by the authors own admission.
The image is a screen-dump from the .NET demo which shows some of what it can do. It's still early days for the library but even now it does a heck of a lot. I've added the link into the C++ code Library.