Object Construction

People often get confused when trying to construct their GObjects because of the sheer number of different ways to hook into the objects's construction process: it is difficult to figure which is the correct, recommended way.

Table 4, “g_object_new” shows what user-provided functions are invoked during object instantiation and in which order they are invoked. A user looking for the equivalent of the simple C++ constructor function should use the instance_init method. It will be invoked after all the parent's instance_init functions have been invoked. It cannot take arbitrary construction parameters (as in C++) but if your object needs arbitrary parameters to complete initialization, you can use construction properties.

Construction properties will be set only after all instance_init functions have run. No object reference will be returned to the client of g_object_new until all the construction properties have been set.

As such, I would recommend writing the following code first:

static void
maman_bar_init (MamanBar *self)
  self->private = MAMAN_BAR_GET_PRIVATE (self); 

  /* initialize all public and private members to reasonable default values. */

  /* If you need specific construction properties to complete initialization,
   * delay initialization completion until the property is set. 

Now, if you need special construction properties, install the properties in the class_init function, override the set and get methods and implement the get and set methods as described in the section called “Object properties”. Make sure that these properties use a construct only GParamSpec by setting the param spec's flag field to G_PARAM_CONSTRUCT_ONLY: this helps GType ensure that these properties are not set again later by malicious user code.

static void
bar_class_init (MamanBarClass *klass)
  GObjectClass *gobject_class = G_OBJECT_CLASS (klass);
  GParamSpec *maman_param_spec;

  gobject_class->set_property = bar_set_property;
  gobject_class->get_property = bar_get_property;

  maman_param_spec = g_param_spec_string ("maman",
                                          "Maman construct prop",
                                          "Set maman's name",
                                          "no-name-set" /* default value */,
                                          G_PARAM_CONSTRUCT_ONLY | G_PARAM_READWRITE);
  g_object_class_install_property (gobject_class,

If you need this, make sure you can build and run code similar to the code shown above. Make sure your construct properties can set correctly during construction, make sure you cannot set them afterwards and make sure that if your users do not call g_object_new with the required construction properties, these will be initialized with the default values.

I consider good taste to halt program execution if a construction property is set its default value. This allows you to catch client code which does not give a reasonable value to the construction properties. Of course, you are free to disagree but you should have a good reason to do so.

Some people sometimes need to construct their object but only after the construction properties have been set. This is possible through the use of the constructor class method as described in the section called “Object instantiation” or, more simply, using the constructed class method available since GLib 2.12.