Form what I have seen all examples provided by Felgo are written purely in Qt Quick + JS fashion.
What is misleading here is the fact that Felgo “only” provides additional plugins/features on top of Qt.
They are plenty of Qt tutorials about c++ and passing data between Qt Quick layer.
All of that is possible while using Felgo.
Separate thing is if all the plugins provided by Felgo can be used on c++ level.
In my opinion Felgo expose only features to Qt Quick, not libraries on c++ level.
But can be wrong here.
While we only offer access to our components via QML, e.g. the Gaming Components for physics, collision detection, etc are implemented in pure C++ and only exposed as QML types for simple and easy use in your QML UI. And as QML runs quite fast as well, many use-cases don’t necessarily require to implement app or game logic in C++.
However, just as Marcin said it is possible to freely mix-in any C++ logic or components to e.g. perform data-intense calculations, threaded operations or use Qt C++ features like e.g. Qt Networking. Please see here to learn more about C++ and QML: http://doc.qt.io/qt-5/qtqml-cppintegration-topic.html