Developing gMTP - New Instance of GObject Subclass

2026-08-08 by Roman Yepishev.

If you followed GObject 2.0 tutorial, you've seen the following snippet:

/*
 * Type declaration.
 */
#define VIEWER_TYPE_FILE viewer_file_get_type()
G_DECLARE_FINAL_TYPE (ViewerFile, viewer_file, VIEWER, FILE, GObject)

/*
 * Method definitions.
 */
ViewerFile *viewer_file_new (void);

G_END_DECLS
        

Later, ViewerFile *viewer_file_new (void); is never referenced, and it is not immediately clear what should go there.

Do I need to implement the "new" method myself when writing GObject classes in C? - Stack Overflow was asking the same question. After digging more I found an authoritative source at the deprecated Gnome Wiki ( HowDoI/SubclassGObject):

MyAppWindow *
my_app_window_new (void)
{
  return g_object_new (MY_APP_TYPE_WINDOW, NULL);
}
         

...

As a general rule, _new() functions should only call g_object_new() with the passed in arguments as properties. If your class has extra code in _new() then it will be difficult (or impossible) to subclass or to use from language bindings.

So the correct code for viewer_file_new is

ViewerFile *
viewer_file_new (void)
{
    return g_object_new (VIEWER_TYPE_FILE, NULL);
}
         

And nothing else. The NULL in this case signifies the end of properties, none in this case. If you do need properties, you need to specify them:

ViewerFile *
viewer_file_new (void, SomeClass1 *property1_value, SomeClass2 *property2_value)
{
    return g_object_new (VIEWER_TYPE_FILE, "property1", property1_value, "property2", property2_value, NULL);
}