It completely works

app_win_2_back.hpp

@@ -0,0 +1,31 @@
+#ifndef LEARNGTK4_APP_WIN_2_BACK_HPP
+#define LEARNGTK4_APP_WIN_2_BACK_HPP
+
+#include <gtkmm.h>
+
+namespace learn_gtkmm4 {
+    class HelloWorld;
+}
+
+namespace gui_to_app {
+
+    class AppWin2Back final : public Gtk::Application {
+    public:
+        ~AppWin2Back() noexcept final = default;
+
+        static Glib::RefPtr<AppWin2Back> create();
+
+    protected:
+        AppWin2Back();
+
+        void on_startup() final;
+
+        void on_activate() final;
+
+    private:
+        learn_gtkmm4::HelloWorld *main_window{ nullptr };
+    };
+
+} // gui_to_app
+
+#endif //LEARNGTK4_APP_WIN_2_BACK_HPP