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<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="howto-interface-implement"></a>Implementing interfaces</h2></div></div></div>

 Once the interface is defined, implementing it is rather trivial.
 

 The first step is to define a normal final GObject class exactly as in
 <a class="xref" href="howto-gobject.html#howto-gobject-header" title="Boilerplate header code">the section called “Boilerplate header code”</a>.
 

 The second step is to implement ViewerFile by defining
 it using
 <code class="function"><a class="link" href="gobject-Type-Information.html#G-DEFINE-TYPE-WITH-CODE:CAPS" title="G_DEFINE_TYPE_WITH_CODE()">G_DEFINE_TYPE_WITH_CODE</a></code>
 and
 <code class="function"><a class="link" href="gobject-Type-Information.html#G-IMPLEMENT-INTERFACE:CAPS" title="G_IMPLEMENT_INTERFACE()">G_IMPLEMENT_INTERFACE</a></code>
 instead of
 <code class="function"><a class="link" href="gobject-Type-Information.html#G-DEFINE-TYPE:CAPS" title="G_DEFINE_TYPE()">G_DEFINE_TYPE</a></code>:

<div class="informalexample">
 <table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
 <tbody>
 <tr>
 <td class="listing_lines" align="right"><pre>1
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 <td class="listing_code"><pre class="programlisting">static void viewer_file_editable_interface_init (ViewerEditableInterface *iface);

G_DEFINE_TYPE_WITH_CODE (ViewerFile, viewer_file, G_TYPE_OBJECT,
 G_IMPLEMENT_INTERFACE (VIEWER_TYPE_EDITABLE,
 viewer_file_editable_interface_init))</pre></td>
 </tr>
 </tbody>
 </table>
</div>

This definition is very much like all the similar functions seen
 previously. The only interface-specific code present here is the use of
 <code class="function"><a class="link" href="gobject-Type-Information.html#G-IMPLEMENT-INTERFACE:CAPS" title="G_IMPLEMENT_INTERFACE()">G_IMPLEMENT_INTERFACE</a></code>.
 
<div class="note">Classes can implement multiple interfaces by using multiple calls to
 <code class="function"><a class="link" href="gobject-Type-Information.html#G-IMPLEMENT-INTERFACE:CAPS" title="G_IMPLEMENT_INTERFACE()">G_IMPLEMENT_INTERFACE</a></code>
 inside the call to
 <code class="function"><a class="link" href="gobject-Type-Information.html#G-DEFINE-TYPE-WITH-CODE:CAPS" title="G_DEFINE_TYPE_WITH_CODE()">G_DEFINE_TYPE_WITH_CODE</a></code>
 </div>

 <code class="function">viewer_file_editable_interface_init</code>, the interface
 initialization function: inside it every virtual method of the interface
 must be assigned to its implementation:

<div class="informalexample">
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 <td class="listing_code"><pre class="programlisting">static void
viewer_file_editable_save (ViewerFile *self,
 GError **error)
{
 g_print ("File implementation of editable interface save method: %s.\n",
 self->filename);
}

static void
viewer_file_editable_undo (ViewerFile *self,
 guint n_steps)
{
 g_print ("File implementation of editable interface undo method: %s.\n",
 self->filename);
}

static void
viewer_file_editable_redo (ViewerFile *self,
 guint n_steps)
{
 g_print ("File implementation of editable interface redo method: %s.\n",
 self->filename);
}

static void
viewer_file_editable_interface_init (ViewerEditableInterface *iface)
{
 iface->save = viewer_file_editable_save;
 iface->undo = viewer_file_editable_undo;
 iface->redo = viewer_file_editable_redo;
}

static void
viewer_file_init (ViewerFile *self)
{
 /* Instance variable initialisation code. */
}</pre></td>
 </tr>
 </tbody>
 </table>
</div>

If the object is not of final type, e.g. was declared using
 <code class="function"><a class="link" href="gobject-Type-Information.html#G-DECLARE-DERIVABLE-TYPE:CAPS" title="G_DECLARE_DERIVABLE_TYPE()">G_DECLARE_DERIVABLE_TYPE</a></code>
 then
 <code class="function"><a class="link" href="gobject-Type-Information.html#G-ADD-PRIVATE:CAPS" title="G_ADD_PRIVATE()">G_ADD_PRIVATE</a></code>
 macro should be added. The private structure should be declared exactly
 as for a normal derivable object, see <a class="xref" href="howto-gobject-code.html" title="Boilerplate code">the section called “Boilerplate code”</a>.

<div class="informalexample">
 <table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
 <tbody>
 <tr>
 <td class="listing_lines" align="right"><pre>1
2
3
4</pre></td>
 <td class="listing_code"><pre class="programlisting">G_DEFINE_TYPE_WITH_CODE (ViewerFile, viewer_file, G_TYPE_OBJECT,
 G_ADD_PRIVATE (ViewerFile)
 G_IMPLEMENT_INTERFACE (VIEWER_TYPE_EDITABLE,
 viewer_file_editable_interface_init))</pre></td>
 </tr>
 </tbody>
 </table>
</div>

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