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The GObject base class

The previous chapter discussed the details of GLib's Dynamic Type System. The GObject library also contains an implementation for a base fundamental type named GObject.

GObject is a fundamental classed instantiable type. It implements:

  • Memory management with reference counting

  • Construction/Destruction of instances

  • Generic per-object properties with set/get function pairs

  • Easy use of signals

All the GNOME libraries which use the GLib type system (like GTK+ and GStreamer) inherit from GObject which is why it is important to understand the details of how it works.

Object instantiation

The g_object_new family of functions can be used to instantiate any GType which inherits from the GObject base type. All these functions make sure the class and instance structures have been correctly initialized by GLib's type system and then invoke at one point or another the constructor class method which is used to:

  • Allocate and clear memory through g_type_create_instance,

  • Initialize the object's instance with the construction properties.

Although one can expect all class and instance members (except the fields pointing to the parents) to be set to zero, some consider it good practice to explicitly set them.

Once all construction operations have been completed and constructor properties set, the constructed class method is called.

Objects which inherit from GObject are allowed to override this constructed class method. The example below shows how ViewerFile overrides the parent's construction process:

#define VIEWER_TYPE_FILE viewer_file_get_type ()
G_DECLARE_FINAL_TYPE (ViewerFile, viewer_file, VIEWER, FILE, GObject)

struct _ViewerFile
  GObject parent_instance;

  /* instance members */

/* will create viewer_file_get_type and set viewer_file_parent_class */
G_DEFINE_TYPE (ViewerFile, viewer_file, G_TYPE_OBJECT)

static void
viewer_file_constructed (GObject *obj)
  /* update the object state depending on constructor properties */

  /* Always chain up to the parent constructed function to complete object
   * initialisation. */
  G_OBJECT_CLASS (viewer_file_parent_class)->constructed (obj);

static void
viewer_file_class_init (ViewerFileClass *klass)
  GObjectClass *object_class = G_OBJECT_CLASS (klass);

  object_class->constructed = viewer_file_constructed;

static void
viewer_file_init (ViewerFile *self)
  /* initialize the object */

If the user instantiates an object ViewerFile with:

ViewerFile *file = g_object_new (VIEWER_TYPE_FILE, NULL);

If this is the first instantiation of such an object, the viewer_file_class_init function will be invoked after any viewer_file_base_class_init function. This will make sure the class structure of this new object is correctly initialized. Here, viewer_file_class_init is expected to override the object's class methods and setup the class' own methods. In the example above, the constructor method is the only overridden method: it is set to viewer_file_constructor.

Once g_object_new has obtained a reference to an initialized class structure, it invokes its constructor method to create an instance of the new object, if the constructor has been overridden in viewer_file_class_init. Overridden constructors must chain up to their parent’s constructor. In order to find the parent class and chain up to the parent class constructor, we can use the viewer_file_parent_class pointer that has been set up for us by the G_DEFINE_TYPE macro.

Finally, at one point or another, g_object_constructor is invoked by the last constructor in the chain. This function allocates the object's instance buffer through g_type_create_instance which means that the instance_init function is invoked at this point if one was registered. After instance_init returns, the object is fully initialized and should be ready to have its methods called by the user. When g_type_create_instance returns, g_object_constructor sets the construction properties (i.e. the properties which were given to g_object_new) and returns to the user's constructor.

The process described above might seem a bit complicated, but it can be summarized easily by the table below which lists the functions invoked by g_object_new and their order of invocation:

Table 4. g_object_new

Invocation time Function invoked Function's parameters Remark
First call to g_object_new for target type target type's base_init function On the inheritance tree of classes from fundamental type to target type. base_init is invoked once for each class structure. Never used in practice. Unlikely you will need it.
target type's class_init function On target type's class structure Here, you should make sure to initialize or override class methods (that is, assign to each class' method its function pointer) and create the signals and the properties associated to your object.
interface's base_init function On interface's vtable  
interface's interface_init function On interface's vtable  
Each call to g_object_new for target type target type's class constructor method: GObjectClass->constructor On object's instance If you need to handle construct properties in a custom way, or implement a singleton class, override the constructor method and make sure to chain up to the object's parent class before doing your own initialization. In doubt, do not override the constructor method.
type's instance_init function On the inheritance tree of classes from fundamental type to target type. the instance_init provided for each type is invoked once for each instance structure. Provide an instance_init function to initialize your object before its construction properties are set. This is the preferred way to initialize a GObject instance. This function is equivalent to C++ constructors.
target type's class constructed method: GObjectClass->constructed On object's instance If you need to perform object initialization steps after all construct properties have been set. This is the final step in the object initialization process, and is only called if the constructor method returned a new object instance (rather than, for example, an existing singleton).

Readers should feel concerned about one little twist in the order in which functions are invoked: while, technically, the class' constructor method is called before the GType's instance_init function (since g_type_create_instance which calls instance_init is called by g_object_constructor which is the top-level class constructor method and to which users are expected to chain to), the user's code which runs in a user-provided constructor will always run after GType's instance_init function since the user-provided constructor must (you've been warned) chain up before doing anything useful.

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