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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_lines" align="right"><pre>1 |
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| 36</pre></td> |
| <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">-></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->undo and ->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> |
| </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 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"> |
| <table class="listing_frame" border="0" cellpadding="0" cellspacing="0"> |
| <tbody> |
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| <td class="listing_lines" align="right"><pre>1 |
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| 37</pre></td> |
| <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">-></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">-></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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