A mature (and very effective) structural editor is the FLUID user-interface designer, part of the FLTK UI framework (C++). It outputs C++ and works very well.

A mature (and very effective) structural editor is the FLUID user-interface designer, part of the FLTK UI framework (C++). It outputs C++ and works very well. I remember dismissing it as a toy back when starting to work professionally with FLTK. Soon I realised I was mistaken; it is excellent to create UI's with for C++/FLKT.