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  52 <div class="sect1" title="Interface definition prerequisites">
  53 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  54 <a name="id650299"></a>Interface definition prerequisites</h2></div></div></div>
  55 <p>
  56       To specify that an interface requires the presence of other interfaces
  57       when implemented, GObject introduces the concept of
  58       <span class="emphasis"><em>prerequisites</em></span>: it is possible to associate
  59       a list of prerequisite interfaces to an interface. For example, if
  60       object A wishes to implement interface I1, and if interface I1 has a
  61       prerequisite on interface I2, A has to implement both I1 and I2.
  62     </p>
  63 <p>
  64       The mechanism described above is, in practice, very similar to
  65       Java's interface I1 extends interface I2. The example below shows
  66       the GObject equivalent:
  67 </p>
  68 <pre class="programlisting">
  69   /* inside the GType function of the MamanIbar interface */
  70   type = g_type_register_static (G_TYPE_INTERFACE, "MamanIbar", &amp;info, 0);
  71
  72   /* Make the MamanIbar interface require MamanIbaz interface. */
  73   g_type_interface_add_prerequisite (type, MAMAN_TYPE_IBAZ);
  74 </pre>
  75 <p>
  76       The code shown above adds the MamanIbaz interface to the list of
  77       prerequisites of MamanIbar while the code below shows how an
  78       implementation can implement both interfaces and register their
  79       implementations:
  80 </p>
  81 <pre class="programlisting">
  82 static void
  83 maman_ibar_do_another_action (MamanIbar *ibar)
  84 {
  85   MamanBar *self = MAMAN_BAR (ibar);
  86
  87   g_print ("Bar implementation of IBar interface Another Action: 0x%x.\n",
  88            self-&gt;instance_member);
  89 }
  90
  91 static void
  92 maman_ibar_interface_init (MamanIbarInterface *iface)
  93 {
  94   iface-&gt;do_another_action = maman_ibar_do_another_action;
  95 }
  96
  97 static void
  98 maman_ibaz_do_action (MamanIbaz *ibaz)
  99 {
 100   MamanBar *self = MAMAN_BAR (ibaz);
 101
 102   g_print ("Bar implementation of IBaz interface Action: 0x%x.\n",
 103            self-&gt;instance_member);
 104 }
 105
 106 static void
 107 maman_ibaz_interface_init (MamanIbazInterface *iface)
 108 {
 109   iface-&gt;do_action = maman_ibaz_do_action;
 110 }
 111
 112 static void
 113 maman_bar_class_init (MamanBarClass *klass)
 114 {
 115
 116 }
 117
 118 static void
 119 maman_bar_init (MamanBar *self)
 120 {
 121   self-&gt;instance_member = 0x666;
 122 }
 123
 124 G_DEFINE_TYPE_WITH_CODE (MamanBar, maman_bar, G_TYPE_OBJECT,
 125                          G_IMPLEMENT_INTERFACE (MAMAN_TYPE_IBAZ,
 126                                                 maman_ibaz_interface_init)
 127                          G_IMPLEMENT_INTERFACE (MAMAN_TYPE_IBAR,
 128                                                 maman_ibar_interface_init));
 129 </pre>
 130 <p>
 131       It is very important to notice that the order in which interface
 132       implementations are added to the main object is not random:
 133       <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>,
 134       which is called by <code class="function">G_IMPLEMENT_INTERFACE</code>, must be
 135       invoked first on the interfaces which have no prerequisites and then on
 136       the others.
 137     </p>
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