Widgets in widget

[AlannY <m alanny ru>]

Hi there.

I'm writing a custom widget. And I need to place some other widgets in 
my widget. SO:

+-----------------------------+
| My custom widget            |
| +-------------------------+ |
| | GtkVBox                 | |
| | +---------------------+ | |
| | | GtkButton           | | |
| | +---------------------+ | |
| +-------------------------+ |
+-----------------------------+

I'm wring a _new() function like this (my widget is called "grid"):

GtkWidget*
grid_new ()
{
  Grid *grid;

  grid = gtk_type_new (grid_get_type ());

  GtkWidget *widget = GTK_WIDGET (grid);
  GtkWidget *vbox = gtk_vbox_new (FALSE, 0);

  gtk_container_add (GTK_CONTAINER (widget), vbox);

  return widget;
}

But, when running I get a message:

(grid-test:28016): GLib-GObject-WARNING **: invalid cast from `Grid' to 
`GtkContainer'

(grid-test:28016): Gtk-CRITICAL **: gtk_container_add: assertion 
`GTK_IS_CONTAINER (container)' failed

How to put widgets in my custom widget.

Thanks.