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23 <div class="titlepage"><div><div><h2 class="title">
24 <a name="chapter-gobject"></a>The GObject base class</h2></div></div></div>
26 <dt><span class="sect1"><a href="chapter-gobject.html#gobject-instantiation">Object instantiation</a></span></dt>
27 <dt><span class="sect1"><a href="gobject-memory.html">Object memory management</a></span></dt>
29 <dt><span class="sect2"><a href="gobject-memory.html#gobject-memory-refcount">Reference count</a></span></dt>
30 <dt><span class="sect2"><a href="gobject-memory.html#gobject-memory-weakref">Weak References</a></span></dt>
31 <dt><span class="sect2"><a href="gobject-memory.html#gobject-memory-cycles">Reference counts and cycles</a></span></dt>
33 <dt><span class="sect1"><a href="gobject-properties.html">Object properties</a></span></dt>
34 <dd><dl><dt><span class="sect2"><a href="gobject-properties.html#gobject-multi-properties">Accessing multiple properties at once</a></span></dt></dl></dd>
37 The two previous chapters discussed the details of GLib's Dynamic Type System
38 and its signal control system. The GObject library also contains an implementation
39 for a base fundamental type named <a class="link" href="gobject-The-Base-Object-Type.html#GObject"><span class="type">GObject</span></a>.
42 <a class="link" href="gobject-The-Base-Object-Type.html#GObject"><span class="type">GObject</span></a> is a fundamental classed instantiable type. It implements:
44 <div class="itemizedlist"><ul class="itemizedlist" type="disc">
45 <li class="listitem"><p>Memory management with reference counting</p></li>
46 <li class="listitem"><p>Construction/Destruction of instances</p></li>
47 <li class="listitem"><p>Generic per-object properties with set/get function pairs</p></li>
48 <li class="listitem"><p>Easy use of signals</p></li>
51 All the GNOME libraries which use the GLib type system (like GTK+ and GStreamer)
52 inherit from <a class="link" href="gobject-The-Base-Object-Type.html#GObject"><span class="type">GObject</span></a> which is why it is important to understand
53 the details of how it works.
56 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
57 <a name="gobject-instantiation"></a>Object instantiation</h2></div></div></div>
59 The <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-new" title="g_object_new ()">g_object_new</a></code>
60 family of functions can be used to instantiate any GType which inherits
61 from the GObject base type. All these functions make sure the class and
62 instance structures have been correctly initialized by GLib's type system
63 and then invoke at one point or another the constructor class method
66 <div class="itemizedlist"><ul class="itemizedlist" type="disc">
67 <li class="listitem"><p>
68 Allocate and clear memory through <code class="function"><a class="link" href="gobject-Type-Information.html#g-type-create-instance" title="g_type_create_instance ()">g_type_create_instance</a></code>,
70 <li class="listitem"><p>
71 Initialize the object's instance with the construction properties.
75 Although one can expect all class and instance members (except the fields
76 pointing to the parents) to be set to zero, some consider it good practice
77 to explicitly set them.
80 Objects which inherit from GObject are allowed to override this
81 constructor class method: they should however chain to their parent
82 constructor method before doing so:
84 <pre class="programlisting">
85 GObject *(* constructor) (GType gtype,
87 GObjectConstructParam *properties);
92 The example below shows how <span class="type">MamanBar</span> overrides the parent's constructor:
94 <pre class="programlisting">
95 #define MAMAN_TYPE_BAR (maman_bar_get_type ())
96 #define MAMAN_BAR(obj) (G_TYPE_CHECK_INSTANCE_CAST ((obj), MAMAN_TYPE_BAR, MamanBar))
97 #define MAMAN_IS_BAR(obj) (G_TYPE_CHECK_INSTANCE_TYPE ((obj), MAMAN_TYPE_BAR))
98 #define MAMAN_BAR_CLASS(klass) (G_TYPE_CHECK_CLASS_CAST ((klass), MAMAN_TYPE_BAR, MamanBarClass))
99 #define MAMAN_IS_BAR_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE ((klass), MAMAN_TYPE_BAR))
100 #define MAMAN_BAR_GET_CLASS(obj) (G_TYPE_INSTANCE_GET_CLASS ((obj), MAMAN_TYPE_BAR, MamanBarClass))
102 typedef struct _MamanBar MamanBar;
103 typedef struct _MamanBarClass MamanBarClass;
107 GObject parent_instance;
109 /* instance members */
112 struct _MamanBarClass
114 GObjectClass parent_class;
119 /* will create maman_bar_get_type and set maman_bar_parent_class */
120 G_DEFINE_TYPE (MamanBar, maman_bar, G_TYPE_OBJECT);
123 maman_bar_constructor (GType gtype,
125 GObjectConstructParam *properties)
130 /* Always chain up to the parent constructor */
131 obj = G_OBJECT_CLASS (maman_bar_parent_class)->constructor (gtype, n_properties, properties);
134 /* update the object state depending on constructor properties */
140 maman_bar_class_init (MamanBarClass *klass)
142 GObjectClass *gobject_class = G_OBJECT_CLASS (klass);
144 gobject_class->constructor = maman_bar_constructor;
148 maman_bar_init (MamanBar *self)
150 /* initialize the object */
155 If the user instantiates an object <span class="type">MamanBar</span> with:
157 <pre class="programlisting">
158 MamanBar *bar = g_object_new (MAMAN_TYPE_BAR, NULL);
161 If this is the first instantiation of such an object, the
162 <code class="function">maman_bar_class_init</code> function will be invoked
163 after any <code class="function">maman_bar_base_class_init</code> function.
164 This will make sure the class structure of this new object is
165 correctly initialized. Here, <code class="function">maman_bar_class_init</code>
166 is expected to override the object's class methods and setup the
167 class' own methods. In the example above, the constructor method is
168 the only overridden method: it is set to
169 <code class="function">maman_bar_constructor</code>.
172 Once <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-new" title="g_object_new ()">g_object_new</a></code> has obtained a reference to an initialized
173 class structure, it invokes its constructor method to create an instance of the new
174 object. Since it has just been overridden by <code class="function">maman_bar_class_init</code>
175 to <code class="function">maman_bar_constructor</code>, the latter is called and, because it
176 was implemented correctly, it chains up to its parent's constructor. In
177 order to find the parent class and chain up to the parent class
178 constructor, we can use the <code class="literal">maman_bar_parent_class</code>
179 pointer that has been set up for us by the
180 <code class="literal">G_DEFINE_TYPE</code> macro.
183 Finally, at one point or another, <code class="function">g_object_constructor</code> is invoked
184 by the last constructor in the chain. This function allocates the object's instance' buffer
185 through <code class="function"><a class="link" href="gobject-Type-Information.html#g-type-create-instance" title="g_type_create_instance ()">g_type_create_instance</a></code>
186 which means that the instance_init function is invoked at this point if one
187 was registered. After instance_init returns, the object is fully initialized and should be
188 ready to answer any user-request. When <code class="function"><a class="link" href="gobject-Type-Information.html#g-type-create-instance" title="g_type_create_instance ()">g_type_create_instance</a></code>
189 returns, <code class="function">g_object_constructor</code> sets the construction properties
190 (i.e. the properties which were given to <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-new" title="g_object_new ()">g_object_new</a></code>) and returns
191 to the user's constructor which is then allowed to do useful instance initialization...
194 The process described above might seem a bit complicated, but it can be
195 summarized easily by the table below which lists the functions invoked
196 by <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-new" title="g_object_new ()">g_object_new</a></code>
197 and their order of invocation:
202 <a name="gobject-construction-table"></a><p class="title"><b>Table 4. <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-new" title="g_object_new ()">g_object_new</a></code></b></p>
203 <div class="table-contents"><table summary="g_object_new" border="1">
210 <th align="left">Invocation time</th>
211 <th align="left">Function Invoked</th>
212 <th align="left">Function's parameters</th>
217 <td rowspan="4" align="left">First call to <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-new" title="g_object_new ()">g_object_new</a></code> for target type</td>
218 <td align="left">target type's base_init function</td>
219 <td align="left">On the inheritance tree of classes from fundamental type to target type.
220 base_init is invoked once for each class structure.</td>
222 I have no real idea on how this can be used. If you have a good real-life
223 example of how a class' base_init can be used, please, let me know.
227 <td align="left">target type's class_init function</td>
228 <td align="left">On target type's class structure</td>
230 Here, you should make sure to initialize or override class methods (that is,
231 assign to each class' method its function pointer) and create the signals and
232 the properties associated to your object.
236 <td align="left">interface' base_init function</td>
237 <td align="left">On interface' vtable</td>
241 <td align="left">interface' interface_init function</td>
242 <td align="left">On interface' vtable</td>
246 <td rowspan="2" align="left">Each call to <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-new" title="g_object_new ()">g_object_new</a></code> for target type</td>
247 <td align="left">target type's class constructor method: GObjectClass->constructor</td>
248 <td align="left">On object's instance</td>
250 If you need to complete the object initialization after all the construction properties
251 are set, override the constructor method and make sure to chain up to the object's
252 parent class before doing your own initialization.
253 In doubt, do not override the constructor method.
257 <td align="left">type's instance_init function</td>
258 <td align="left">On the inheritance tree of classes from fundamental type to target type.
259 the instance_init provided for each type is invoked once for each instance
262 Provide an instance_init function to initialize your object before its construction
263 properties are set. This is the preferred way to initialize a GObject instance.
264 This function is equivalent to C++ constructors.
270 <p><br class="table-break">
273 Readers should feel concerned about one little twist in the order in
274 which functions are invoked: while, technically, the class' constructor
275 method is called <span class="emphasis"><em>before</em></span> the GType's instance_init
276 function (since <code class="function"><a class="link" href="gobject-Type-Information.html#g-type-create-instance" title="g_type_create_instance ()">g_type_create_instance</a></code> which calls instance_init is called by
277 <code class="function">g_object_constructor</code> which is the top-level class
278 constructor method and to which users are expected to chain to), the
279 user's code which runs in a user-provided constructor will always
280 run <span class="emphasis"><em>after</em></span> GType's instance_init function since the
281 user-provided constructor <span class="emphasis"><em>must</em></span> (you've been warned)
282 chain up <span class="emphasis"><em>before</em></span> doing anything useful.
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