line 11, add is part of gtkmm but no namespace or method call on an object?

main_window.cpp

Main_window::Main_window() {

    // /////////////////
    // G U I   S E T U P
    // /////////////////

    set_default_size(400, 200);

    // Put a vertical box container as the Window contents
    Gtk::Box *vbox = Gtk::manage(new Gtk::Box(Gtk::ORIENTATION_VERTICAL, 0));
    add(*vbox);

    // ///////
    // M E N U
    // Add a menu bar as the top item in the vertical box
    Gtk::MenuBar *menubar = Gtk::manage(new Gtk::MenuBar());
    vbox->pack_start(*menubar, Gtk::PACK_SHRINK, 0);