g_type_class_add_private() and G_TYPE_INSTANCE_GET_PRIVATE() were
deprecated in GLib 2.58. Instead, use
GDEFINE[ABSTRACT_]TYPE_WITH_PRIVATE(), and
G_ADD_PRIVATE[_DYNAMIC](), and the implictly-defined
foo_get_instance_private() functions, all of which are available in
the GLib versions we depend on.
g_type_class_add_private() and G_TYPE_INSTANCE_GET_PRIVATE() were deprecated in GLib 2.58. Instead, use GDEFINE[ABSTRACT_]TYPE_WITH_PRIVATE(), and G_ADD_PRIVATE[_DYNAMIC](), and the implictly-defined foo_get_instance_private() functions, all of which are available in the GLib versions we depend on.
https://developer.gnome.org/gobject/stable/gobject-Type-Information.html#G-TYPE-INSTANCE-GET-PRIVATE:CAPS