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<div class="chapter">
<div class="titlepage"><div><div><h2 class="title">
<a name="chapter-gobject"></a>The GObject base class</h2></div></div></div>
<div class="toc"><dl>
<dt><span class="sect1"><a href="chapter-gobject.html#gobject-instantiation">Object instantiation</a></span></dt>
<dt><span class="sect1"><a href="gobject-memory.html">Object memory management</a></span></dt>
<dd><dl>
<dt><span class="sect2"><a href="gobject-memory.html#gobject-memory-refcount">Reference count</a></span></dt>
<dt><span class="sect2"><a href="gobject-memory.html#gobject-memory-weakref">Weak References</a></span></dt>
<dt><span class="sect2"><a href="gobject-memory.html#gobject-memory-cycles">Reference counts and cycles</a></span></dt>
</dl></dd>
<dt><span class="sect1"><a href="gobject-properties.html">Object properties</a></span></dt>
<dd><dl><dt><span class="sect2"><a href="gobject-properties.html#gobject-multi-properties">Accessing multiple properties at once</a></span></dt></dl></dd>
</dl></div>
<p>
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 <a class="link" href="gobject-The-Base-Object-Type.html#GObject"><span class="type">GObject</span></a>.
</p>
<p>
<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:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>Memory management with reference counting</p></li>
<li class="listitem"><p>Construction/Destruction of instances</p></li>
<li class="listitem"><p>Generic per-object properties with set/get function pairs</p></li>
<li class="listitem"><p>Easy use of signals</p></li>
</ul></div>
<p>
All the GNOME libraries which use the GLib type system (like GTK+ and GStreamer)
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
the details of how it works.
</p>
<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="gobject-instantiation"></a>Object instantiation</h2></div></div></div>
<p>
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>
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:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>
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>,
</p></li>
<li class="listitem"><p>
Initialize the object's instance with the construction properties.
</p></li>
</ul></div>
<p>
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.
</p>
<p>
Objects which inherit from GObject are allowed to override this
constructor class method: they should however chain to their parent
constructor method before doing so:
</p>
<pre class="programlisting">
GObject *(* constructor) (GType gtype,
guint n_properties,
GObjectConstructParam *properties);
</pre>
<p>
</p>
<p>
The example below shows how <span class="type">MamanBar</span> overrides the parent's constructor:
</p>
<pre class="programlisting">
#define MAMAN_TYPE_BAR (maman_bar_get_type ())
#define MAMAN_BAR(obj) (G_TYPE_CHECK_INSTANCE_CAST ((obj), MAMAN_TYPE_BAR, MamanBar))
#define MAMAN_IS_BAR(obj) (G_TYPE_CHECK_INSTANCE_TYPE ((obj), MAMAN_TYPE_BAR))
#define MAMAN_BAR_CLASS(klass) (G_TYPE_CHECK_CLASS_CAST ((klass), MAMAN_TYPE_BAR, MamanBarClass))
#define MAMAN_IS_BAR_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE ((klass), MAMAN_TYPE_BAR))
#define MAMAN_BAR_GET_CLASS(obj) (G_TYPE_INSTANCE_GET_CLASS ((obj), MAMAN_TYPE_BAR, MamanBarClass))
typedef struct _MamanBar MamanBar;
typedef struct _MamanBarClass MamanBarClass;
struct _MamanBar
{
GObject parent_instance;
/* instance members */
};
struct _MamanBarClass
{
GObjectClass parent_class;
/* class members */
};
/* will create maman_bar_get_type and set maman_bar_parent_class */
G_DEFINE_TYPE (MamanBar, maman_bar, G_TYPE_OBJECT);
static GObject *
maman_bar_constructor (GType gtype,
guint n_properties,
GObjectConstructParam *properties)
{
GObject *obj;
{
/* Always chain up to the parent constructor */
obj = G_OBJECT_CLASS (maman_bar_parent_class)->constructor (gtype, n_properties, properties);
}
/* update the object state depending on constructor properties */
return obj;
}
static void
maman_bar_class_init (MamanBarClass *klass)
{
GObjectClass *gobject_class = G_OBJECT_CLASS (klass);
gobject_class->constructor = maman_bar_constructor;
}
static void
maman_bar_init (MamanBar *self)
{
/* initialize the object */
}
</pre>
<p>
If the user instantiates an object <span class="type">MamanBar</span> with:
</p>
<pre class="programlisting">
MamanBar *bar = g_object_new (MAMAN_TYPE_BAR, NULL);
</pre>
<p>
If this is the first instantiation of such an object, the
<code class="function">maman_bar_class_init</code> function will be invoked
after any <code class="function">maman_bar_base_class_init</code> function.
This will make sure the class structure of this new object is
correctly initialized. Here, <code class="function">maman_bar_class_init</code>
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
<code class="function">maman_bar_constructor</code>.
</p>
<p>
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
class structure, it invokes its constructor method to create an instance of the new
object. Since it has just been overridden by <code class="function">maman_bar_class_init</code>
to <code class="function">maman_bar_constructor</code>, the latter is called and, because it
was implemented correctly, it chains up to its parent's constructor. In
order to find the parent class and chain up to the parent class
constructor, we can use the <code class="literal">maman_bar_parent_class</code>
pointer that has been set up for us by the
<code class="literal">G_DEFINE_TYPE</code> macro.
</p>
<p>
Finally, at one point or another, <code class="function">g_object_constructor</code> is invoked
by the last constructor in the chain. This function allocates the object's instance' buffer
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>
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 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>
returns, <code class="function">g_object_constructor</code> sets the construction properties
(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
to the user's constructor which is then allowed to do useful instance initialization...
</p>
<p>
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 <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 their order of invocation:
</p>
<p>
</p>
<div class="table">
<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>
<div class="table-contents"><table summary="g_object_new" border="1">
<colgroup>
<col align="left">
<col align="left">
<col align="left">
</colgroup>
<thead><tr>
<th align="left">Invocation time</th>
<th align="left">Function Invoked</th>
<th align="left">Function's parameters</th>
<th>Remark</th>
</tr></thead>
<tbody>
<tr>
<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>
<td align="left">target type's base_init function</td>
<td align="left">On the inheritance tree of classes from fundamental type to target type.
base_init is invoked once for each class structure.</td>
<td>
I have no real idea on how this can be used. If you have a good real-life
example of how a class' base_init can be used, please, let me know.
</td>
</tr>
<tr>
<td align="left">target type's class_init function</td>
<td align="left">On target type's class structure</td>
<td>
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.
</td>
</tr>
<tr>
<td align="left">interface' base_init function</td>
<td align="left">On interface' vtable</td>
<td> </td>
</tr>
<tr>
<td align="left">interface' interface_init function</td>
<td align="left">On interface' vtable</td>
<td> </td>
</tr>
<tr>
<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>
<td align="left">target type's class constructor method: GObjectClass->constructor</td>
<td align="left">On object's instance</td>
<td>
If you need to complete the object initialization after all the construction properties
are set, 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.
</td>
</tr>
<tr>
<td align="left">type's instance_init function</td>
<td align="left">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.</td>
<td>
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.
</td>
</tr>
</tbody>
</table></div>
</div>
<p><br class="table-break">
</p>
<p>
Readers should feel concerned about one little twist in the order in
which functions are invoked: while, technically, the class' constructor
method is called <span class="emphasis"><em>before</em></span> the GType's instance_init
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
<code class="function">g_object_constructor</code> 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 <span class="emphasis"><em>after</em></span> GType's instance_init function since the
user-provided constructor <span class="emphasis"><em>must</em></span> (you've been warned)
chain up <span class="emphasis"><em>before</em></span> doing anything useful.
</p>
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