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<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="howto-interface-override"></a>Overriding interface methods</h2></div></div></div>
<p>
      If a base class already implements an interface and a derived
      class needs to implement the same interface but needs to override certain
      methods, you must reimplement the interface and set only the interface
      methods which need overriding.
    </p>
<p>
      In this example, <span class="type">ViewerAudioFile</span> is derived from
      <span class="type">ViewerFile</span>. Both implement the <span class="type">ViewerEditable</span>
      interface. <span class="type">ViewerAudioFile</span> only implements one method of the
      <span class="type">ViewerEditable</span> interface and uses the base class implementation of
      the other.
</p>
<div class="informalexample">
  <table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
    <tbody>
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        <td class="listing_code"><pre class="programlisting"><span class="keyword">static</span><span class="normal"> </span><span class="type">void</span>
<span class="function">viewer_audio_file_editable_save</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerEditable</span><span class="normal">  </span><span class="symbol">*</span><span class="normal">editable</span><span class="symbol">,</span>
<span class="normal">                                 </span><span class="usertype">GError</span><span class="normal">         </span><span class="symbol">**</span><span class="normal">error</span><span class="symbol">)</span>
<span class="cbracket">{</span>
<span class="normal">  </span><span class="usertype">ViewerAudioFile</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">self </span><span class="symbol">=</span><span class="normal"> </span><span class="function">VIEWER_AUDIO_FILE</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">editable</span><span class="symbol">);</span>

<span class="normal">  </span><span class="function"><a href="../glib-Warnings-and-Assertions.html#g-print">g_print</a></span><span class="normal"> </span><span class="symbol">(</span><span class="string">"Audio file implementation of editable interface save method.</span><span class="specialchar">\n</span><span class="string">"</span><span class="symbol">);</span>
<span class="cbracket">}</span>

<span class="keyword">static</span><span class="normal"> </span><span class="type">void</span>
<span class="function">viewer_audio_file_editable_interface_init</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerEditableInterface</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">iface</span><span class="symbol">)</span>
<span class="cbracket">{</span>
<span class="normal">  </span><span class="comment">/* Override the implementation of save(). */</span>
<span class="normal">  iface</span><span class="symbol">-&gt;</span><span class="normal">save </span><span class="symbol">=</span><span class="normal"> viewer_audio_file_editable_save</span><span class="symbol">;</span>

<span class="normal">  </span><span class="comment">/*</span>
<span class="comment">   * Leave iface-&gt;undo and -&gt;redo alone, they are already set to the</span>
<span class="comment">   * base class implementation.</span>
<span class="comment">   */</span>
<span class="cbracket">}</span>

<span class="function"><a href="gobject-Type-Information.html#G-DEFINE-TYPE-WITH-CODE:CAPS">G_DEFINE_TYPE_WITH_CODE</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">ViewerAudioFile</span><span class="symbol">,</span><span class="normal"> viewer_audio_file</span><span class="symbol">,</span><span class="normal"> VIEWER_TYPE_FILE</span><span class="symbol">,</span>
<span class="normal">                         </span><span class="function"><a href="gobject-Type-Information.html#G-IMPLEMENT-INTERFACE:CAPS">G_IMPLEMENT_INTERFACE</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">VIEWER_TYPE_EDITABLE</span><span class="symbol">,</span>
<span class="normal">                                                viewer_audio_file_editable_interface_init</span><span class="symbol">))</span>

<span class="keyword">static</span><span class="normal"> </span><span class="type">void</span>
<span class="function">viewer_audio_file_class_init</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerAudioFileClass</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">klass</span><span class="symbol">)</span>
<span class="cbracket">{</span>
<span class="normal">  </span><span class="comment">/* Nothing here. */</span>
<span class="cbracket">}</span>

<span class="keyword">static</span><span class="normal"> </span><span class="type">void</span>
<span class="function">viewer_audio_file_init</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerAudioFile</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">self</span><span class="symbol">)</span>
<span class="cbracket">{</span>
<span class="normal">  </span><span class="comment">/* Nothing here. */</span>
<span class="cbracket">}</span></pre></td>
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</div>

<p>
    </p>
<p>
      To access the base class interface implementation use
      <code class="function"><a class="link" href="gobject-Type-Information.html#g-type-interface-peek-parent" title="g_type_interface_peek_parent ()">g_type_interface_peek_parent</a></code>
      from within an interface's <code class="function">default_init</code> function.
    </p>
<p>
      To call the base class implementation of an interface
      method from an derived class where than interface method has been
      overridden, stash away the pointer returned from
      <code class="function"><a class="link" href="gobject-Type-Information.html#g-type-interface-peek-parent" title="g_type_interface_peek_parent ()">g_type_interface_peek_parent</a></code>
      in a global variable.
    </p>
<p>
      In this example <span class="type">ViewerAudioFile</span> overrides the
      <code class="function">save</code> interface method. In its overridden method
      it calls the base class implementation of the same interface method.
</p>
<div class="informalexample">
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        <td class="listing_code"><pre class="programlisting"><span class="keyword">static</span><span class="normal"> </span><span class="usertype">ViewerEditableInterface</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">viewer_editable_parent_interface </span><span class="symbol">=</span><span class="normal"> <a href="../glib-Standard-Macros.html#NULL:CAPS">NULL</a></span><span class="symbol">;</span>

<span class="keyword">static</span><span class="normal"> </span><span class="type">void</span>
<span class="function">viewer_audio_file_editable_save</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerEditable</span><span class="normal">  </span><span class="symbol">*</span><span class="normal">editable</span><span class="symbol">,</span>
<span class="normal">                                 </span><span class="usertype">GError</span><span class="normal">         </span><span class="symbol">**</span><span class="normal">error</span><span class="symbol">)</span>
<span class="cbracket">{</span>
<span class="normal">  </span><span class="usertype">ViewerAudioFile</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">self </span><span class="symbol">=</span><span class="normal"> </span><span class="function">VIEWER_AUDIO_FILE</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">editable</span><span class="symbol">);</span>

<span class="normal">  </span><span class="function"><a href="../glib-Warnings-and-Assertions.html#g-print">g_print</a></span><span class="normal"> </span><span class="symbol">(</span><span class="string">"Audio file implementation of editable interface save method.</span><span class="specialchar">\n</span><span class="string">"</span><span class="symbol">);</span>

<span class="normal">  </span><span class="comment">/* Now call the base implementation */</span>
<span class="normal">  viewer_editable_parent_interface</span><span class="symbol">-&gt;</span><span class="function">save</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">editable</span><span class="symbol">,</span><span class="normal"> error</span><span class="symbol">);</span>
<span class="cbracket">}</span>

<span class="keyword">static</span><span class="normal"> </span><span class="type">void</span>
<span class="function">viewer_audio_file_editable_interface_init</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerEditableInterface</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">iface</span><span class="symbol">)</span>
<span class="cbracket">{</span>
<span class="normal">  viewer_editable_parent_interface </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="gobject-Type-Information.html#g-type-interface-peek-parent">g_type_interface_peek_parent</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">iface</span><span class="symbol">);</span>

<span class="normal">  iface</span><span class="symbol">-&gt;</span><span class="normal">save </span><span class="symbol">=</span><span class="normal"> viewer_audio_file_editable_save</span><span class="symbol">;</span>
<span class="cbracket">}</span>

<span class="function"><a href="gobject-Type-Information.html#G-DEFINE-TYPE-WITH-CODE:CAPS">G_DEFINE_TYPE_WITH_CODE</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">ViewerAudioFile</span><span class="symbol">,</span><span class="normal"> viewer_audio_file</span><span class="symbol">,</span><span class="normal"> VIEWER_TYPE_FILE</span><span class="symbol">,</span>
<span class="normal">                         </span><span class="function"><a href="gobject-Type-Information.html#G-IMPLEMENT-INTERFACE:CAPS">G_IMPLEMENT_INTERFACE</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">VIEWER_TYPE_EDITABLE</span><span class="symbol">,</span>
<span class="normal">                                                viewer_audio_file_editable_interface_init</span><span class="symbol">))</span>

<span class="keyword">static</span><span class="normal"> </span><span class="type">void</span>
<span class="function">viewer_audio_file_class_init</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerAudioFileClass</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">klass</span><span class="symbol">)</span>
<span class="cbracket">{</span>
<span class="normal">  </span><span class="comment">/* Nothing here. */</span>
<span class="cbracket">}</span>

<span class="keyword">static</span><span class="normal"> </span><span class="type">void</span>
<span class="function">viewer_audio_file_init</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerAudioFile</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">self</span><span class="symbol">)</span>
<span class="cbracket">{</span>
<span class="normal">  </span><span class="comment">/* Nothing here. */</span>
<span class="cbracket">}</span></pre></td>
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