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Type Checking and New Types
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<div class="SECT1">
<h1 class="SECT1">
<a name="Z105">Type Checking and New Types</a>
</h1>
<p>
GTK+ has an extensive type system, which is to some extent
independent of its object system. However, the object
system makes use of the larger type system. Every object
has a type, and every type has a unique integer identifier.
When writing a <span class="STRUCTNAME">GtkObject</span>,
it's customary to provide a function which returns the
type's identifier.
</p>
<p>
In the case of <tt class="CLASSNAME">GtkButton</tt>, the
relevant function is:
</p>
<table border="0" bgcolor="#E0E0E0" width="100%">
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<td>
<pre class="PROGRAMLISTING">
GtkType gtk_button_get_type();
</pre>
</td>
</tr>
</table>
<p>
The first time this function is invoked, it will register a
<span class="STRUCTNAME">GtkButton</span> type with the
object system, and in the process obtain a type identifier.
On subsequent calls, the type identifier is simply
returned. <span class="STRUCTNAME">GtkType</span> is a
typedef (<span class="STRUCTNAME">unsigned int</span> is
the actual type of GTK+'s type identifiers).
</p>
<p>
The type system allows GTK+ to check the validity of casts.
To facilitate this, objects customarily provide macros like
these in their header file:
</p>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="PROGRAMLISTING">
#define GTK_TYPE_BUTTON (gtk_button_get_type ())
#define GTK_BUTTON(obj) (GTK_CHECK_CAST ((obj), \
GTK_TYPE_BUTTON, GtkButton))
#define GTK_BUTTON_CLASS(klass) (GTK_CHECK_CLASS_CAST ((klass), \
GTK_TYPE_BUTTON, GtkButtonClass))
#define GTK_IS_BUTTON(obj) (GTK_CHECK_TYPE ((obj), \
GTK_TYPE_BUTTON))
#define GTK_IS_BUTTON_CLASS(klass) (GTK_CHECK_CLASS_TYPE ((klass), \
GTK_TYPE_BUTTON))
</pre>
</td>
</tr>
</table>
<p>
Instead of simply casting an object, you can use the <tt
class="FUNCTION">GTK_BUTTON()</tt> macro. If <tt class=
"FUNCTION">GTK_NO_CHECK_CASTS</tt> is defined, these macros
are equivalent to simple casts. Otherwise, they retrieve
the type of the object and compare it to the type you're
attempting to cast to.
</p>
<p>
GTK+ also provides convenient runtime type checking, with
the <tt class="FUNCTION">GTK_IS_BUTTON()</tt> macro. This
is often used in preconditions; for example, a function
expecting a button as an argument might have this check at
the beginning:
</p>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="PROGRAMLISTING">
g_return_if_fail(GTK_IS_BUTTON(widget));
</pre>
</td>
</tr>
</table>
<p>
The GTK+ and Gnome library functions have many such checks.
You can also use the macro to make certain code conditional
on an object's type, though this is most likely a poor idea
from a design standpoint.
</p>
<p>
To give you an idea what sort of information GTK+ stores
about each object type, here's the implementation of <tt
class="FUNCTION">gtk_button_get_type()</tt>:
</p>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="PROGRAMLISTING">
GtkType
gtk_button_get_type (void)
{
static GtkType button_type = 0;
if (!button_type)
{
static const GtkTypeInfo button_info =
{
"GtkButton",
sizeof (GtkButton),
sizeof (GtkButtonClass),
(GtkClassInitFunc) gtk_button_class_init,
(GtkObjectInitFunc) gtk_button_init,
/* reserved_1 */ NULL,
/* reserved_2 */ NULL,
(GtkClassInitFunc) NULL,
};
button_type = gtk_type_unique (GTK_TYPE_BIN, &button_info);
gtk_type_set_chunk_alloc (button_type, 16);
}
return button_type;
}
</pre>
</td>
</tr>
</table>
<p>
The code fills in a struct with information about the
class, then hands that struct to GTK+ to get a type
identifier (<span class="STRUCTNAME">GtkType</span>). Only
six components of the <span class="STRUCTNAME">
GtkTypeInfo</span> struct are important. <tt class=
"CLASSNAME">GtkButton</tt> gives GTK+ a human-readable name
for the class, used in error messages and the like; the
size of the instance and class structs; then a function to
initialize the class struct and another to initialize each
new instance. The sixth and seventh members of the struct
(<span class="STRUCTNAME">reserved_1</span> and <span
class="STRUCTNAME">reserved_2</span>) are obsolete and
preserved only for compatibility. The final member is a
pointer to a base class initialization function, used to
initialize the class struct of any subclasses.
</p>
<p>
<tt class="FUNCTION">gtk_type_unique()</tt> registers the
new type and obtains a type identifier. The <span class=
"STRUCTNAME">GTK_TYPE_BIN</span> argument is a macro
containing the type of <tt class="CLASSNAME">
GtkButton</tt>'s parent class, <tt class="CLASSNAME">
GtkBin</tt>. The call to <tt class="FUNCTION">
gtk_type_set_chunk_alloc()</tt> optimizes memory allocation
for this type; it is never required, and should only be
used for frequently-allocated types like <tt class=
"CLASSNAME">GtkButton</tt>.
</p>
<p>
Given a registered <tt class="CLASSNAME">GtkButton</tt>
type, the following code creates a type instance:
</p>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="PROGRAMLISTING">
GtkWidget*
gtk_button_new (void)
{
return GTK_WIDGET (gtk_type_new (gtk_button_get_type ()));
}
</pre>
</td>
</tr>
</table>
<p>
The newborn <tt class="CLASSNAME">GtkButton</tt> will be
initialized by its instance initializer. The instance
initialization function is called each time an instance of
the type is created; it gives the object's data members
reasonable default values:
</p>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="PROGRAMLISTING">
static void
gtk_button_init (GtkButton *button)
{
GTK_WIDGET_SET_FLAGS (button, GTK_CAN_FOCUS);
GTK_WIDGET_UNSET_FLAGS (button, GTK_NO_WINDOW);
button->child = NULL;
button->in_button = FALSE;
button->button_down = FALSE;
button->relief = GTK_RELIEF_NORMAL;
}
</pre>
</td>
</tr>
</table>
<p>
Remember that <tt class="FUNCTION">gtk_button_init()</tt>
was passed to <tt class="FUNCTION">gtk_type_unique()</tt>
when the <tt class="CLASSNAME">GtkButton</tt> type was
created. GTK+ stores the function pointer and uses it to
create <tt class="CLASSNAME">GtkButton</tt> instances.
</p>
<p>
Instance structs are created with all bits set to 0; so
settings members to 0 or <span class="STRUCTNAME">
NULL</span> is not strictly necessary. Still, most GTK+
code does initialize the members, for clarity.
</p>
<p>
The class initialization and base class initialization
functions require some background information to understand
fully; you will know how to write them after you read this
chapter.
</p>
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