|  |  |  | GObject Reference Manual |  | 
|---|
      GObject interfaces can also have
      properties. Declaration of the interface properties is similar to
      declaring the properties of ordinary GObject types as explained in
      the section called “Object properties”, except that
      g_object_interface_install_property
      is used to declare the properties instead of
      g_object_class_install_property.
    
      To include a property named 'name' of type string in the 
      maman_ibaz interface example code above, we only need to
      add one 
      [12]  
      line in the maman_ibaz_default_init as shown below:
static void
maman_ibaz_default_init (gpointer g_iface)
{
  g_object_interface_install_property (g_iface,
                                       g_param_spec_string ("name",
                                                            "Name",
                                                            "Name of the MamanIbaz",
                                                            "maman",
                                                            G_PARAM_READWRITE));
}
One point worth noting is that the declared property wasn't assigned an integer ID. The reason being that integer IDs of properties are used only inside the get and set methods and since interfaces do not implement properties, there is no need to assign integer IDs to interface properties.
      An implementation declares and defines it's properties in the usual
      way as explained in the section called “Object properties”, except for one
      small change: it can declare the properties of the interface it
      implements using g_object_class_override_property
      instead of g_object_class_install_property.
      The following code snippet shows the modifications needed in the
      MamanBaz declaration and implementation above:
struct _MamanBaz
{
  GObject parent_instance;
  gint instance_member;
  gchar *name;
};
enum
{
  PROP_0,
  PROP_NAME
};
static void
maman_baz_set_property (GObject      *object,
                        guint         prop_id,
                        const GValue *value,
                        GParamSpec   *pspec)
{
  MamanBaz *baz = MAMAN_BAZ (object);
  switch (prop_id)
    {
    case PROP_NAME:
      g_free (baz->name);
      baz->name = g_value_dup_string (value);
      break;
    default:
      G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
      break;
    }
}
static void
maman_baz_get_property (GObject    *object,
                        guint       prop_id,
                        GValue     *value,
                        GParamSpec *pspec)
{
  MamanBaz *baz = MAMAN_BAZ (object);
  switch (prop_id)
    {
    case PROP_NAME:
      g_value_set_string (value, baz->name);
      break;
    default:
      G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
      break;
    }
}
static void
maman_baz_class_init (MamanBazClass *klass)
{
  GObjectClass *gobject_class = G_OBJECT_CLASS (klass);
  gobject_class->set_property = maman_baz_set_property;
  gobject_class->get_property = maman_baz_get_property;
  g_object_class_override_property (gobject_class, PROP_NAME, "name");
}