I don't understand why they would parse the XML on the fly. Qt's designer spits out *.ui XML files that are compiled by Qt's uic into the same plain old C++ you

I don't understand why they would parse the XML on the fly. Qt's designer spits out *.ui XML files that are compiled by Qt's uic into the same plain old C++ you would write to create and arrange the widgets. Lots of C++ Qt apps create their UIs from XML. They just do it at compile time.