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<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="gtype-non-instantiable-classed"></a>Non-instantiable classed types: interfaces</h2></div></div></div>

 This section covers the theory behind interfaces. See
 <a class="xref" href="howto-interface.html" title="How to define and implement interfaces">How to define and implement interfaces</a> for the recommended way to define an
 interface.
 

 GType's interfaces are very similar to Java's interfaces. They allow
 to describe a common API that several classes will adhere to.
 Imagine the play, pause and stop buttons on hi-fi equipment — those can
 be seen as a playback interface. Once you know what they do, you can
 control your CD player, MP3 player or anything that uses these symbols.
 To declare an interface you have to register a non-instantiable
 classed type which derives from 
 <a class="link" href="gobject-Type-Information.html#GTypeInterface" title="struct GTypeInterface">GTypeInterface</a>. The following piece of code declares such an interface.

<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">#define VIEWER_TYPE_EDITABLE viewer_editable_get_type ()
G_DECLARE_INTERFACE (ViewerEditable, viewer_editable, VIEWER, EDITABLE, GObject)

struct _ViewerEditableInterface {
 GTypeInterface parent;

void (*save) (ViewerEditable *self,
 GError **error);
};

void viewer_editable_save (ViewerEditable *self,
 GError **error);</pre></td>
 </tr>
 </tbody>
 </table>
</div>

The interface function, <code class="function">viewer_editable_save</code> is implemented
 in a pretty simple way:

<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">void
viewer_editable_save (ViewerEditable *self,
 GError **error)
{
 ViewerEditableinterface *iface;

g_return_if_fail (VIEWER_IS_EDITABLE (self));
 g_return_if_fail (error == NULL || *error == NULL);

iface = VIEWER_EDITABLE_GET_IFACE (self);
 g_return_if_fail (iface->save != NULL);
 iface->save (self);
}</pre></td>
 </tr>
 </tbody>
 </table>
</div>

<code class="function">viewer_editable_get_type</code> registers a type named ViewerEditable
 which inherits from G_TYPE_INTERFACE. All interfaces must
 be children of G_TYPE_INTERFACE in the inheritance tree.
 

 An interface is defined by only one structure which must contain as first member
 a <a class="link" href="gobject-Type-Information.html#GTypeInterface" title="struct GTypeInterface">GTypeInterface</a> structure. The interface structure is expected to
 contain the function pointers of the interface methods. It is good style to 
 define helper functions for each of the interface methods which simply call
 the interface's method directly: <code class="function">viewer_editable_save</code>
 is one of these.
 

 If you have no special requirements you can use the
 <a class="link" href="gobject-Type-Information.html#G-IMPLEMENT-INTERFACE:CAPS" title="G_IMPLEMENT_INTERFACE()">G_IMPLEMENT_INTERFACE</a> macro
 to implement an interface:

<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_save (ViewerEditable *self)
{
 g_print ("File implementation of editable interface save method.\n");
}

static void
viewer_file_editable_interface_init (ViewerEditableInterface *iface)
{
 iface->save = viewer_file_save;
}

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

If your code does have special requirements, you must write a custom
 <code class="function">get_type</code> function to register your GType which
 inherits from some <a class="link" href="gobject-The-Base-Object-Type.html#GObject">GObject</a>
 and which implements the interface ViewerEditable. For
 example, this code registers a new ViewerFile class which
 implements ViewerEditable:

<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_save (ViewerEditable *editable)
{
 g_print ("File implementation of editable interface save method.\n");
}

static void
viewer_file_editable_interface_init (gpointer g_iface,
 gpointer iface_data)
{
 ViewerEditableInterface *iface = g_iface;

iface->save = viewer_file_save;
}

GType 
viewer_file_get_type (void)
{
 static GType type = 0;
 if (type == 0) {
 const GTypeInfo info = {
 sizeof (ViewerFileClass),
 NULL, /* base_init */
 NULL, /* base_finalize */
 NULL, /* class_init */
 NULL, /* class_finalize */
 NULL, /* class_data */
 sizeof (ViewerFile),
 0, /* n_preallocs */
 NULL /* instance_init */
 };
 const GInterfaceInfo editable_info = {
 (GInterfaceInitFunc) viewer_file_editable_interface_init, /* interface_init */
 NULL, /* interface_finalize */
 NULL /* interface_data */
 };
 type = g_type_register_static (VIEWER_TYPE_FILE,
 "ViewerFile",
 &info, 0);
 g_type_add_interface_static (type,
 VIEWER_TYPE_EDITABLE,
 &editable_info);
 }
 return type;
}</pre></td>
 </tr>
 </tbody>
 </table>
</div>

<code class="function"><a class="link" href="gobject-Type-Information.html#g-type-add-interface-static" title="g_type_add_interface_static ()">g_type_add_interface_static</a></code> records in the type system that
 a given type implements also FooInterface 
 (<code class="function">foo_interface_get_type</code> returns the type of 
 FooInterface).
 The <a class="link" href="gobject-Type-Information.html#GInterfaceInfo" title="struct GInterfaceInfo">GInterfaceInfo</a> structure holds
 information about the implementation of the interface:

<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">struct _GInterfaceInfo
{
 GInterfaceInitFunc interface_init;
 GInterfaceFinalizeFunc interface_finalize;
 gpointer interface_data;
};</pre></td>
 </tr>
 </tbody>
 </table>
</div>

<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="gtype-non-instantiable-classed-init"></a>Interface Initialization</h3></div></div></div>

 When an instantiable classed type which implements an interface
 (either directly or by inheriting an implementation from a superclass)
 is created for the first time, its class structure is initialized
 following the process described in <a class="xref" href="gtype-instantiable-classed.html" title="Instantiable classed types: objects">the section called “Instantiable classed types: objects”</a>.
 After that, the interface implementations associated with
 the type are initialized.
 

 First a memory buffer is allocated to hold the interface structure. The parent's
 interface structure is then copied over to the new interface structure (the parent
 interface is already initialized at that point). If there is no parent interface,
 the interface structure is initialized with zeros. The
 <code>g_type</code> and the
 <code>g_instance_type</code> fields are then
 initialized: <code>g_type</code> is set to the type of
 the most-derived interface and
 <code>g_instance_type</code> is set to the type of the
 most derived type which implements this interface.
 

 The interface's <code class="function">base_init</code> function is called,
 and then the interface's <code class="function">default_init</code> is invoked.
 Finally if the type has registered an implementation of the interface,
 the implementation's <code class="function">interface_init</code>
 function is invoked. If there are multiple implementations of an
 interface the <code class="function">base_init</code> and
 <code class="function">interface_init</code> functions will be invoked once
 for each implementation initialized.
 

 It is thus recommended to use a <code class="function">default_init</code> function to
 initialize an interface. This function is called only once for the interface no
 matter how many implementations there are. The
 <code class="function">default_init</code> function is declared by
 <a class="link" href="gobject-Type-Information.html#G-DEFINE-INTERFACE:CAPS" title="G_DEFINE_INTERFACE()">G_DEFINE_INTERFACE</a>
 which can be used to define the interface:

<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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7</pre></td>
 <td class="listing_code"><pre class="programlisting">G_DEFINE_INTERFACE (ViewerEditable, viewer_editable, G_TYPE_OBJECT)

static void
viewer_editable_default_init (ViewerEditableInterface *iface)
{
 /* add properties and signals here, will only be called once */
}</pre></td>
 </tr>
 </tbody>
 </table>
</div>

Or you can do that yourself in a GType function for your interface:

<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">GType
viewer_editable_get_type (void)
{
 static volatile gsize type_id = 0;
 if (g_once_init_enter (&type_id)) {
 const GTypeInfo info = {
 sizeof (ViewerEditableInterface),
 NULL, /* base_init */
 NULL, /* base_finalize */
 viewer_editable_default_init, /* class_init */
 NULL, /* class_finalize */
 NULL, /* class_data */
 0, /* instance_size */
 0, /* n_preallocs */
 NULL /* instance_init */
 };
 GType type = g_type_register_static (G_TYPE_INTERFACE,
 "ViewerEditable",
 &info, 0);
 g_once_init_leave (&type_id, type);
 }
 return type_id;
}

static void
viewer_editable_default_init (ViewerEditableInterface *iface)
{
 /* add properties and signals here, will only called once */
}</pre></td>
 </tr>
 </tbody>
 </table>
</div>

In summary, interface initialization uses the following functions:
 

 
<div class="table">
<a name="ginterface-init-table"></a>Table 2. Interface Initialization
<div class="table-contents"><table class="table" summary="Interface Initialization" 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 align="left">First call to <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>
 for any type implementing interface
 </td>
<td align="left">interface's <code class="function">base_init</code> function</td>
<td align="left">On interface's vtable</td>
<td>Rarely necessary to use this. Called once per instantiated classed type implementing the interface.</td>
</tr>
<tr>
<td align="left">First call to <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>
 for each type implementing interface
 </td>
<td align="left">interface's <code class="function">default_init</code> function</td>
<td align="left">On interface's vtable</td>
<td>Register interface's signals, properties, etc. here. Will be called once.</td>
</tr>
<tr>
<td align="left">First call to <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>
 for any type implementing interface
 </td>
<td align="left">implementation's <code class="function">interface_init</code> function</td>
<td align="left">On interface's vtable</td>
<td>
 Initialize interface implementation. Called for each class that that
 implements the interface. Initialize the interface method pointers
 in the interface structure to the implementing class's implementation.
 </td>
</tr>
</tbody>
</table></div>
</div>
 
 
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="gtype-non-instantiable-classed-dest"></a>Interface Destruction</h3></div></div></div>

 When the last instance of an instantiable type which registered 
 an interface implementation is destroyed, the interface's 
 implementations associated to the type are destroyed.
 

 To destroy an interface implementation, GType first calls the 
 implementation's <code class="function">interface_finalize</code> function 
 and then the interface's most-derived 
 <code class="function">base_finalize</code> function.
 

 Again, it is important to understand, as in 
 <a class="xref" href="gtype-non-instantiable-classed.html#gtype-non-instantiable-classed-init" title="Interface Initialization">the section called “Interface Initialization”</a>,
 that both <code class="function">interface_finalize</code> and <code class="function">base_finalize</code>
 are invoked exactly once for the destruction of each implementation of an interface. Thus,
 if you were to use one of these functions, you would need to use a static integer variable
 which would hold the number of instances of implementations of an interface such that
 the interface's class is destroyed only once (when the integer variable reaches zero).
 

 The above process can be summarized as follows:
 
<div class="table">
<a name="ginterface-fini-table"></a>Table 3. Interface Finalization
<div class="table-contents"><table class="table" summary="Interface Finalization" 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>
</tr></thead>
<tbody>
<tr>
<td rowspan="2" align="left">Last call to <code class="function"><a class="link" href="gobject-Type-Information.html#g-type-free-instance" title="g_type_free_instance ()">g_type_free_instance</a></code> for type
 implementing interface
 </td>
<td align="left">interface's <code class="function">interface_finalize</code> function</td>
<td align="left">On interface's vtable</td>
</tr>
<tr>
<td align="left">interface's <code class="function">base_finalize</code> function</td>
<td align="left">On interface's vtable</td>
</tr>
</tbody>
</table></div>
</div>
 
 
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