Loading the .glade file

Gtk::Builder must be used via a Glib::RefPtr. Like all such classes, you need to use a create() method to instantiate it. For instance,

auto builder = Gtk::Builder::create_from_file("basic.glade");
This will instantiate the windows defined in the .glade file.

To instantiate just one window, or just one of the child widgets, you can specify the name of a widget as the second parameter. For instance,

auto builder = Gtk::Builder::create_from_file("basic.glade", "treeview_products");