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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<title>
Initializing a New Class
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<div class="SECT1">
<h1 class="SECT1">
<a name="SEC-CLASSINIT">Initializing a New Class</a>
</h1>
<p>
When a type is first used, GTK+ creates an instance of its
class struct (using the information supplied to <tt class=
"FUNCTION">gtk_type_unique()</tt>). To initialize the class
struct for a type, GTK+ first checks that all parent
classes are initialized and initializes them if not. Then
it fills the top portion of the class struct with a
byte-for-byte copy of the parent's class struct. This means
the subclass inherits any function pointers found in the
parent class.
</p>
<p>
Next, the base class initialization functions of each
parent class and that of the class itself are called in
order, starting with <span class="STRUCTNAME">
GtkObject</span>. (The base class init function is the last
argument to <tt class="FUNCTION">gtk_type_unique()</tt>). A
base class initializer is optional; in the <tt class=
"CLASSNAME">GtkButton</tt> case, there is none. If present,
the base class initializer supplements the byte-for-byte
copy of the class struct; for example, some functions
should not be inherited. To prevent class function
inheritance, the base class initializer can zero certain
function pointers. Normally you do not need a base class
initializer.
</p>
<p>
Finally, GTK+ calls the type's own class init function. The
class init function can override functions from the parent
class by replacing them in the class struct. It should also
fill in any functions unique to the subclass, and register
signals and object arguments (discussed later in the
chapter).
</p>
<p>
A concrete example should make the class creation process
clear. The class hierarchy for <tt class="CLASSNAME">
GtkButton</tt> is shown in <a href=
"sec-classinit.html#FIG-GTKBUTTON">Figure 1</a>. When the
<tt class="CLASSNAME">GtkButton</tt> type is registered, an
empty <tt class="CLASSNAME">GtkButtonClass</tt> is created.
This class struct is initialized as follows:
</p>
<ol type="1">
<li>
<p>
The class struct for <tt class="CLASSNAME">GtkBin</tt>,
<tt class="CLASSNAME">GtkButton</tt>'s immediate
parent, is copied into it. This means <tt class=
"CLASSNAME">GtkButton</tt> inherits class functions
from <tt class="CLASSNAME">GtkBin</tt>.
</p>
</li>
<li>
<p>
The base class initialization function for <span class=
"STRUCTNAME">GtkObject</span> is called on it. This
zeroes some <span class="STRUCTNAME">GtkObject</span>
class functions that should not be inherited.
</p>
</li>
<li>
<p>
There is no base class initializer for <tt class=
"CLASSNAME">GtkWidget</tt>, or it would be called.
</p>
</li>
<li>
<p>
The base class initializer for <tt class="CLASSNAME">
GtkContainer</tt> is called. This zeroes some <span
class="STRUCTNAME">GtkContainer</span> class functions
that should not be inherited, and initializes a <span
class="STRUCTNAME">GtkContainerClass</span> data
member.
</p>
</li>
<li>
<p>
There is no base class initializer for <tt class=
"CLASSNAME">GtkBin</tt>, or it would be called.
</p>
</li>
<li>
<p>
There is no base class initializer for <tt class=
"CLASSNAME">GtkButton</tt>, or it would be called.
</p>
</li>
<li>
<p>
The class initializer is called for <tt class=
"CLASSNAME">GtkButton</tt>. This fills in the <span
class="STRUCTNAME">GtkButtonClass</span> structure,
registers signals, and registers object arguments.
</p>
</li>
</ol>
<p>
When writing a new class, you only need to concern yourself
with the final two steps---you should consider whether a
base class initializer is needed, and supply it if so; you
must supply a class initializer in all cases.
</p>
<div class="FIGURE">
<a name="FIG-GTKBUTTON"></a>
<p>
<b>Figure 1. <tt class="CLASSNAME">GtkButton</tt>
Ancestry</b>
</p>
</div>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="PROGRAMLISTING">
GtkObject
|
GtkWidget
|
GtkContainer
|
GtkBin
|
GtkButton
</pre>
</td>
</tr>
</table>
<p>
Here is the <tt class="CLASSNAME">GtkButton</tt> class
initialization function, just to give you an initial sense
of things; read on to learn what this code does.
</p>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="PROGRAMLISTING">
static void
gtk_button_class_init (GtkButtonClass *klass)
{
GtkObjectClass *object_class;
GtkWidgetClass *widget_class;
GtkContainerClass *container_class;
object_class = (GtkObjectClass*) klass;
widget_class = (GtkWidgetClass*) klass;
container_class = (GtkContainerClass*) klass;
parent_class = gtk_type_class (GTK_TYPE_BIN);
gtk_object_add_arg_type ("GtkButton::label",
GTK_TYPE_STRING,
GTK_ARG_READWRITE,
ARG_LABEL);
gtk_object_add_arg_type ("GtkButton::relief",
GTK_TYPE_RELIEF_STYLE,
GTK_ARG_READWRITE,
ARG_RELIEF);
button_signals[PRESSED] =
gtk_signal_new ("pressed",
GTK_RUN_FIRST,
object_class->type,
GTK_SIGNAL_OFFSET (GtkButtonClass, pressed),
gtk_marshal_NONE__NONE,
GTK_TYPE_NONE, 0);
button_signals[RELEASED] =
gtk_signal_new ("released",
GTK_RUN_FIRST,
object_class->type,
GTK_SIGNAL_OFFSET (GtkButtonClass, released),
gtk_marshal_NONE__NONE,
GTK_TYPE_NONE, 0);
button_signals[CLICKED] =
gtk_signal_new ("clicked",
GTK_RUN_FIRST | GTK_RUN_ACTION,
object_class->type,
GTK_SIGNAL_OFFSET (GtkButtonClass, clicked),
gtk_marshal_NONE__NONE,
GTK_TYPE_NONE, 0);
button_signals[ENTER] =
gtk_signal_new ("enter",
GTK_RUN_FIRST,
object_class->type,
GTK_SIGNAL_OFFSET (GtkButtonClass, enter),
gtk_marshal_NONE__NONE,
GTK_TYPE_NONE, 0);
button_signals[LEAVE] =
gtk_signal_new ("leave",
GTK_RUN_FIRST,
object_class->type,
GTK_SIGNAL_OFFSET (GtkButtonClass, leave),
gtk_marshal_NONE__NONE,
GTK_TYPE_NONE, 0);
gtk_object_class_add_signals (object_class, button_signals, LAST_SIGNAL);
object_class->set_arg = gtk_button_set_arg;
object_class->get_arg = gtk_button_get_arg;
widget_class->activate_signal = button_signals[CLICKED];
widget_class->realize = gtk_button_realize;
widget_class->draw = gtk_button_draw;
widget_class->draw_focus = gtk_button_draw_focus;
widget_class->draw_default = gtk_button_draw_default;
widget_class->size_request = gtk_button_size_request;
widget_class->size_allocate = gtk_button_size_allocate;
widget_class->expose_event = gtk_button_expose;
widget_class->button_press_event = gtk_button_button_press;
widget_class->button_release_event = gtk_button_button_release;
widget_class->enter_notify_event = gtk_button_enter_notify;
widget_class->leave_notify_event = gtk_button_leave_notify;
widget_class->focus_in_event = gtk_button_focus_in;
widget_class->focus_out_event = gtk_button_focus_out;
container_class->add = gtk_button_add;
container_class->remove = gtk_button_remove;
container_class->child_type = gtk_button_child_type;
klass->pressed = gtk_real_button_pressed;
klass->released = gtk_real_button_released;
klass->clicked = NULL;
klass->enter = gtk_real_button_enter;
klass->leave = gtk_real_button_leave;
}
</pre>
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