gtkmm was basically the modern c++ competitor of Qt from 20 years ago. Back in the Qt2-3 era, where gtkmm relied on libsigc++ and templates, Qt relied on meta-object compiler for reflection and macros for connecting signals to slots. It also had threading and database interaction. It provided productivity at the cost of being butt-ugly and you had to be confined to their own editor and build system. Fast-forward 20 years, Qt5 got a big makeover and adapted to C++11. It deprecated hacks like QSignalMapper and provided more 3d and multimedia stuff. And Qt6 adapted to CMake. So Qt is pretty nice looking these days.

The examples all look reasonably modern, to the extent that you could use good C++11 without fighting the library.