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  52 <div class="chapter" title="How to define and implement interfaces">
  53 <div class="titlepage"><div><div><h2 class="title">
  54 <a name="howto-interface"></a>How to define and implement interfaces</h2></div></div></div>
  55 <div class="toc"><dl>
  56 <dt><span class="sect1"><a href="howto-interface.html#howto-interface-define">How to define interfaces</a></span></dt>
  57 <dt><span class="sect1"><a href="howto-interface-implement.html">How To define implement an Interface?</a></span></dt>
  58 <dt><span class="sect1"><a href="ch06s03.html">Interface definition prerequisites</a></span></dt>
  59 <dt><span class="sect1"><a href="howto-interface-properties.html">Interface Properties</a></span></dt>
  60 </dl></div>
  61 <div class="sect1" title="How to define interfaces">
  62 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  63 <a name="howto-interface-define"></a>How to define interfaces</h2></div></div></div>
  64 <p>
  65     The bulk of interface definition has already been shown in <a class="xref" href="gtype-non-instantiable-classed.html" title="Non-instantiable classed types: interfaces">the section called “Non-instantiable classed types: interfaces”</a>
  66     but I feel it is needed to show exactly how to create an interface.
  67   </p>
  68 <p>
  69     As above, the first step is to get the header right:
  70 </p>
  71 <pre class="programlisting">
  72 #ifndef __MAMAN_IBAZ_H__
  73 #define __MAMAN_IBAZ_H__
  74
  75 #include &lt;glib-object.h&gt;
  76
  77 #define MAMAN_TYPE_IBAZ                 (maman_ibaz_get_type ())
  78 #define MAMAN_IBAZ(obj)                 (G_TYPE_CHECK_INSTANCE_CAST ((obj), MAMAN_TYPE_IBAZ, MamanIbaz))
  79 #define MAMAN_IS_IBAZ(obj)              (G_TYPE_CHECK_INSTANCE_TYPE ((obj), MAMAN_TYPE_IBAZ))
  80 #define MAMAN_IBAZ_GET_INTERFACE(inst)  (G_TYPE_INSTANCE_GET_INTERFACE ((inst), MAMAN_TYPE_IBAZ, MamanIbazInterface))
  81
  82
  83 typedef struct _MamanIbaz               MamanIbaz; /* dummy object */
  84 typedef struct _MamanIbazInterface      MamanIbazInterface;
  85
  86 struct _MamanIbazInterface
  87 {
  88   GTypeInterface parent_iface;
  89
  90   void (*do_action) (MamanIbaz *self);
  91 };
  92
  93 GType maman_ibaz_get_type (void);
  94
  95 void maman_ibaz_do_action (MamanIbaz *self);
  96
  97 #endif /* __MAMAN_IBAZ_H__ */
  98 </pre>
  99 <p>
 100     This code is the same as the code for a normal <span class="type"><a class="link" href="gobject-Type-Information.html#GType" title="GType">GType</a></span>
 101     which derives from a <span class="type"><a class="link" href="gobject-The-Base-Object-Type.html#GObject">GObject</a></span> except for a few details:
 102     </p>
 103 <div class="itemizedlist"><ul class="itemizedlist" type="disc">
 104 <li class="listitem"><p>
 105         The <code class="function">_GET_CLASS</code> macro is called <code class="function">_GET_INTERFACE</code>
 106                   and not implemented with <code class="function">G_TYPE_INSTANCE_GET_CLASS</code>
 107                   but with <code class="function">G_TYPE_INSTANCE_GET_INTERFACE</code>.
 108       </p></li>
 109 <li class="listitem"><p>
 110         The instance type, <span class="type">MamanIbaz</span> is not fully defined: it is
 111         used merely as an abstract type which represents an instance of
 112         whatever object which implements the interface.
 113       </p></li>
 114 <li class="listitem"><p>
 115         The parent of the <span class="type">MamanIbazInterface</span> is not
 116         <span class="type">GObjectClass</span> but <span class="type">GTypeInterface</span>.
 117       </p></li>
 118 </ul></div>
 119 <p>
 120   </p>
 121 <p>
 122     The implementation of the <span class="type">MamanIbaz</span> type itself is trivial:
 123     </p>
 124 <div class="itemizedlist"><ul class="itemizedlist" type="disc">
 125 <li class="listitem"><p><code class="function">maman_ibaz_get_type</code> registers the
 126        type in the type system.
 127        </p></li>
 128 <li class="listitem"><p><code class="function">maman_ibaz_base_init</code> is expected
 129       to register the interface's signals if there are any (we will see a bit
 130       (later how to use them). Make sure to use a static local boolean variable
 131       to make sure not to run the initialization code twice (as described in
 132       <a class="xref" href="gtype-non-instantiable-classed.html#gtype-non-instantiable-classed-init" title="Interface Initialization">the section called “Interface Initialization”</a>,
 133       <code class="function">base_init</code> is run once for each interface implementation
 134       instantiation)</p></li>
 135 <li class="listitem"><p><code class="function">maman_ibaz_do_action</code> dereferences
 136       the class structure to access its associated class function and calls it.
 137       </p></li>
 138 </ul></div>
 139 <p>
 140 </p>
 141 <pre class="programlisting">
 142 static void
 143 maman_ibaz_base_init (gpointer g_class)
 144 {
 145   static gboolean is_initialized = FALSE;
 146
 147   if (!is_initialized)
 148     {
 149       /* add properties and signals to the interface here */
 150
 151       is_initialized = TRUE;
 152     }
 153 }
 154
 155 GType
 156 maman_ibaz_get_type (void)
 157 {
 158   static GType iface_type = 0;
 159   if (iface_type == 0)
 160     {
 161       static const GTypeInfo info = {
 162         sizeof (MamanIbazInterface),
 163         maman_ibaz_base_init,   /* base_init */
 164         NULL,   /* base_finalize */
 165       };
 166
 167       iface_type = g_type_register_static (G_TYPE_INTERFACE, "MamanIbaz",
 168                                            &amp;info, 0);
 169     }
 170
 171   return iface_type;
 172 }
 173
 174 void
 175 maman_ibaz_do_action (MamanIbaz *self)
 176 {
 177   g_return_if_fail (MAMAN_IS_IBAZ (self));
 178
 179   MAMAN_IBAZ_GET_INTERFACE (self)-&gt;do_action (self);
 180 }
 181 </pre>
 182 <p>
 183     </p>
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