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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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Attaching Data to Objects
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<div class="SECT1">
<h1 class="SECT1">
<a name="SEC-OBJECTDATA">Attaching Data to Objects</a>
</h1>
<p>
You can "attach" arbitrary string-pointer pairs to a <span
class="STRUCTNAME">GtkObject</span> instance, in effect
adding a new data member. GTK+ uses this some internally,
but it can also be a convenient way to pass data around in
your application. In particular, it's a nice way to pass
information to callbacks.
</p>
<p>
Here's a simple example:
</p>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="PROGRAMLISTING">
GtkWidget* button = gtk_button_new();
GtkWidget* label = gtk_label_new(_("Foo"));
gtk_object_set_data(GTK_OBJECT(button), "my_label_key", label);
</pre>
</td>
</tr>
</table>
<p>
Later, when you have a pointer to the button but not the
label (perhaps in a callback connected to the button's
<span class="SYMBOL">"clicked"</span> signal), you can do
this:
</p>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="PROGRAMLISTING">
GtkWidget* label = gtk_object_get_data(GTK_OBJECT(button),
"my_label_key");
/* If no data is found for the key, NULL is returned. */
if (label == NULL)
{
g_warning("No data was associated with 'my_label_key'!");
}
</pre>
</td>
</tr>
</table>
<p>
A pair of convenience functions use a predetermined key and
thus save typing (and remembering) the object data key.
These are <tt class="FUNCTION">
gtk_object_set_user_data()</tt> and <tt class="FUNCTION">
gtk_object_get_user_data()</tt>. You can also register a
function to free the data when the data is removed or
replaced, or the <span class="STRUCTNAME">GtkObject</span>
is destroyed; This function should be of type <span class=
"STRUCTNAME">GtkDestroyNotify</span>:
</p>
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<td>
<pre class="PROGRAMLISTING">
typedef void (*GtkDestroyNotify) (gpointer data);
</pre>
</td>
</tr>
</table>
<p>
Conveniently, <tt class="FUNCTION">g_free()</tt> and <tt
class="FUNCTION">gtk_object_unref()</tt> will work here.
You register a "destroy notification" function when you set
the data, using <tt class="FUNCTION">
gtk_object_set_data_full()</tt>. You can remove data before
the object is destroyed with <tt class="FUNCTION">
gtk_object_remove_data()</tt>, or remove it without calling
the destroy function with <tt class="FUNCTION">
gtk_object_remove_no_notify()</tt>. Setting the data to
<span class="STRUCTNAME">NULL</span> is equivalent to
removing it with <tt class="FUNCTION">
gtk_object_remove_data()</tt>, and will also call the
destroy function if you registered one. <a href=
"sec-objectdata.html#FL-OBJECTDATA">Figure 6</a> summarizes
the object data functions.
</p>
<p>
It's worth pointing out that the object data system is a
thin wrapper around the <span class="STRUCTNAME">
GData</span> facility in glib, which can be used
standalone.
</p>
<div class="FIGURE">
<a name="FL-OBJECTDATA"></a>
<div class="FUNCSYNOPSIS">
<a name="FL-OBJECTDATA.SYNOPSIS"></a>
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<pre class="FUNCSYNOPSISINFO">
#include <gtk/gtkobject.h>
</pre>
</td>
</tr>
</table>
<p>
<code><code class="FUNCDEF">void <tt class="FUNCTION">
gtk_object_set_data</tt></code>(GtkObject* <tt class=
"PARAMETER"><i>object</i></tt>, const gchar* <tt class=
"PARAMETER"><i>key</i></tt>, gpointer <tt class=
"PARAMETER"><i>data</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class="FUNCTION">
gtk_object_set_data_full</tt></code>(GtkObject* <tt
class="PARAMETER"><i>object</i></tt>, const gchar* <tt
class="PARAMETER"><i>key</i></tt>, gpointer <tt class=
"PARAMETER"><i>data</i></tt>, GtkDestroyNotify <tt
class="PARAMETER"><i>destroy</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class="FUNCTION">
gtk_object_remove_data</tt></code>(GtkObject* <tt
class="PARAMETER"><i>object</i></tt>, const gchar* <tt
class="PARAMETER"><i>key</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">gpointer <tt class=
"FUNCTION">gtk_object_get_data</tt></code>(GtkObject*
<tt class="PARAMETER"><i>object</i></tt>, const gchar*
<tt class="PARAMETER"><i>key</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class="FUNCTION">
gtk_object_remove_no_notify</tt></code>(GtkObject* <tt
class="PARAMETER"><i>object</i></tt>, const gchar* <tt
class="PARAMETER"><i>key</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class="FUNCTION">
gtk_object_set_user_data</tt></code>(GtkObject* <tt
class="PARAMETER"><i>object</i></tt>, gpointer <tt
class="PARAMETER"><i>data</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">gpointer <tt class=
"FUNCTION">
gtk_object_get_user_data</tt></code>(GtkObject* <tt
class="PARAMETER"><i>object</i></tt>);</code>
</p>
</div>
<p>
<b>Figure 6. Attaching key-value pairs to a <span class=
"STRUCTNAME">GtkObject</span></b>
</p>
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