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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <title>Overriding interface methods: GObject Reference Manual</title> <meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> <link rel="home" href="index.html" title="GObject Reference Manual"> <link rel="up" href="howto-interface.html" title="How to define and implement interfaces"> <link rel="prev" href="howto-interface-properties.html" title="Interface properties"> <link rel="next" href="howto-signals.html" title="How to create and use signals"> <meta name="generator" content="GTK-Doc V1.32 (XML mode)"> <link rel="stylesheet" href="style.css" type="text/css"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table class="navigation" id="top" width="100%" summary="Navigation header" cellpadding="2" cellspacing="5"><tr valign="middle"> <td width="100%" align="left" class="shortcuts"></td> <td><a accesskey="h" href="index.html"><img src="home.png" width="16" height="16" border="0" alt="Home"></a></td> <td><a accesskey="u" href="howto-interface.html"><img src="up.png" width="16" height="16" border="0" alt="Up"></a></td> <td><a accesskey="p" href="howto-interface-properties.html"><img src="left.png" width="16" height="16" border="0" alt="Prev"></a></td> <td><a accesskey="n" href="howto-signals.html"><img src="right.png" width="16" height="16" border="0" alt="Next"></a></td> </tr></table> <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> <tr> <td class="listing_lines" align="right"><pre>1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36</pre></td> <td class="listing_code"><pre class="programlisting"><span class="k">static</span> <span class="kt">void</span> <span class="nf">viewer_audio_file_editable_save</span> <span class="p">(</span><span class="n">ViewerEditable</span> <span class="o">*</span><span class="n">editable</span><span class="p">,</span> <span class="n">GError</span> <span class="o">**</span><span class="n">error</span><span class="p">)</span> <span class="p">{</span> <span class="n">ViewerAudioFile</span> <span class="o">*</span><span class="n">self</span> <span class="o">=</span> <span class="n">VIEWER_AUDIO_FILE</span> <span class="p">(</span><span class="n">editable</span><span class="p">);</span> <span class="n">g_print</span> <span class="p">(</span><span class="s">"Audio file implementation of editable interface save method.</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span> <span class="p">}</span> <span class="k">static</span> <span class="kt">void</span> <span class="nf">viewer_audio_file_editable_interface_init</span> <span class="p">(</span><span class="n">ViewerEditableInterface</span> <span class="o">*</span><span class="n">iface</span><span class="p">)</span> <span class="p">{</span> <span class="cm">/* Override the implementation of save(). */</span> <span class="n">iface</span><span class="o">-></span><span class="n">save</span> <span class="o">=</span> <span class="n">viewer_audio_file_editable_save</span><span class="p">;</span> <span class="cm">/*</span> <span class="cm"> * Leave iface->undo and ->redo alone, they are already set to the</span> <span class="cm"> * base class implementation.</span> <span class="cm"> */</span> <span class="p">}</span> <span class="n">G_DEFINE_TYPE_WITH_CODE</span> <span class="p">(</span><span class="n">ViewerAudioFile</span><span class="p">,</span> <span class="n">viewer_audio_file</span><span class="p">,</span> <span class="n">VIEWER_TYPE_FILE</span><span class="p">,</span> <span class="n">G_IMPLEMENT_INTERFACE</span> <span class="p">(</span><span class="n">VIEWER_TYPE_EDITABLE</span><span class="p">,</span> <span class="n">viewer_audio_file_editable_interface_init</span><span class="p">))</span> <span class="k">static</span> <span class="kt">void</span> <span class="n">viewer_audio_file_class_init</span> <span class="p">(</span><span class="n">ViewerAudioFileClass</span> <span class="o">*</span><span class="n">klass</span><span class="p">)</span> <span class="p">{</span> <span class="cm">/* Nothing here. */</span> <span class="p">}</span> <span class="k">static</span> <span class="kt">void</span> <span class="n">viewer_audio_file_init</span> <span class="p">(</span><span class="n">ViewerAudioFile</span> <span class="o">*</span><span class="n">self</span><span class="p">)</span> <span class="p">{</span> <span class="cm">/* Nothing here. */</span> <span class="p">}</span></pre></td> </tr> </tbody> </table> </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 a 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"> <table class="listing_frame" border="0" cellpadding="0" cellspacing="0"> <tbody> <tr> <td class="listing_lines" align="right"><pre>1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37</pre></td> <td class="listing_code"><pre class="programlisting"><span class="k">static</span> <span class="n">ViewerEditableInterface</span> <span class="o">*</span><span class="n">viewer_editable_parent_interface</span> <span class="o">=</span> <span class="nb">NULL</span><span class="p">;</span> <span class="k">static</span> <span class="kt">void</span> <span class="nf">viewer_audio_file_editable_save</span> <span class="p">(</span><span class="n">ViewerEditable</span> <span class="o">*</span><span class="n">editable</span><span class="p">,</span> <span class="n">GError</span> <span class="o">**</span><span class="n">error</span><span class="p">)</span> <span class="p">{</span> <span class="n">ViewerAudioFile</span> <span class="o">*</span><span class="n">self</span> <span class="o">=</span> <span class="n">VIEWER_AUDIO_FILE</span> <span class="p">(</span><span class="n">editable</span><span class="p">);</span> <span class="n">g_print</span> <span class="p">(</span><span class="s">"Audio file implementation of editable interface save method.</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span> <span class="cm">/* Now call the base implementation */</span> <span class="n">viewer_editable_parent_interface</span><span class="o">-></span><span class="n">save</span> <span class="p">(</span><span class="n">editable</span><span class="p">,</span> <span class="n">error</span><span class="p">);</span> <span class="p">}</span> <span class="k">static</span> <span class="kt">void</span> <span class="nf">viewer_audio_file_editable_interface_init</span> <span class="p">(</span><span class="n">ViewerEditableInterface</span> <span class="o">*</span><span class="n">iface</span><span class="p">)</span> <span class="p">{</span> <span class="n">viewer_editable_parent_interface</span> <span class="o">=</span> <span class="n">g_type_interface_peek_parent</span> <span class="p">(</span><span class="n">iface</span><span class="p">);</span> <span class="n">iface</span><span class="o">-></span><span class="n">save</span> <span class="o">=</span> <span class="n">viewer_audio_file_editable_save</span><span class="p">;</span> <span class="p">}</span> <span class="n">G_DEFINE_TYPE_WITH_CODE</span> <span class="p">(</span><span class="n">ViewerAudioFile</span><span class="p">,</span> <span class="n">viewer_audio_file</span><span class="p">,</span> <span class="n">VIEWER_TYPE_FILE</span><span class="p">,</span> <span class="n">G_IMPLEMENT_INTERFACE</span> <span class="p">(</span><span class="n">VIEWER_TYPE_EDITABLE</span><span class="p">,</span> <span class="n">viewer_audio_file_editable_interface_init</span><span class="p">))</span> <span class="k">static</span> <span class="kt">void</span> <span class="n">viewer_audio_file_class_init</span> <span class="p">(</span><span class="n">ViewerAudioFileClass</span> <span class="o">*</span><span class="n">klass</span><span class="p">)</span> <span class="p">{</span> <span class="cm">/* Nothing here. */</span> <span class="p">}</span> <span class="k">static</span> <span class="kt">void</span> <span class="n">viewer_audio_file_init</span> <span class="p">(</span><span class="n">ViewerAudioFile</span> <span class="o">*</span><span class="n">self</span><span class="p">)</span> <span class="p">{</span> <span class="cm">/* Nothing here. */</span> <span class="p">}</span></pre></td> </tr> </tbody> </table> </div> <p> </p> </div> <div class="footer"> <hr>Generated by GTK-Doc V1.32</div> </body> </html>