ref

https://stackoverflow.com/questions/53926523/gtkmm-label-not-successfully-being-inserted-into-a-listbox

auto app = Gtk::Application::create(argc, argv, "org.gtkmm.examples.base");

  Gtk::Main kit(argc, argv);

  Gtk::Window window;
  Gtk::Label label;
  label.set_text("label text...");
  
  window.add(label);
  window.set_default_size(200, 200);
  window.show_all();

  return app->run(window);

gnome ref:

https://developer-old.gnome.org/gtkmm-tutorial/stable/sec-labels.html.en

source code:

https://gitlab.gnome.org/GNOME/gtkmm-documentation/tree/master/examples/book/label