correct menu_bar.ui

main.cpp

@@ -1,7 +1,24 @@
 #include "hello_world.hpp"
 #include <gtkmm/application.h>
+#include <gtkmm/builder.h>
+#include <iostream>
+#include <memory>
 
 int main(int argc, char *argv[]) {
     auto app = Gtk::Application::create("org.gtkmm.example");
-    return app->make_window_and_run<learn_gtkmm4::HelloWorld>(argc, argv);
+
+    const auto menu_model = []() -> Glib::RefPtr<Gio::Menu> {
+        try {
+            const auto builder = Gtk::Builder::create_from_file("menu_bar.ui");
+            return std::dynamic_pointer_cast<Gio::Menu>(builder->get_object("menu"));
+        } catch (const std::exception &e) {
+            std::cerr << e.what() << '\n';
+            return nullptr;
+        }
+    }();
+    if (menu_model == nullptr) {
+        std::cerr << "Menu model object is null. Down-cast failed.\n";
+        return EXIT_FAILURE;
+    }
+    return app->make_window_and_run<learn_gtkmm4::HelloWorld>(argc, argv, menu_model);
 }