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 <div class="sect1">
 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
 <a name="howto-gobject-methods"></a>Object methods</h2></div></div></div>
 <p>
       Just as with C++, there are many different ways to define object
       methods and extend them: the following list and sections draw on
       C++ vocabulary. (Readers are expected to know basic C++ concepts.
       Those who have not had to write C++ code recently can refer to e.g.
       <a class="ulink" href="http://www.cplusplus.com/doc/tutorial/" target="_top">http://www.cplusplus.com/doc/tutorial/</a> to refresh
       their memories.)
       </p>
 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 <li class="listitem"><p>
             non-virtual public methods,
           </p></li>
 <li class="listitem"><p>
             virtual public methods and
           </p></li>
 <li class="listitem"><p>
             virtual private methods
           </p></li>
 </ul></div>
 <p>
     </p>
 <div class="sect2">
 <div class="titlepage"><div><div><h3 class="title">
 <a name="non-virtual-public-methods"></a>Non-virtual public methods</h3></div></div></div>
 <p>
         These are the simplest, providing a simple method which
         acts on the object. Provide a function
         prototype in the header and an implementation of that prototype
         in the source file.
 </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="comment">/* declaration in the header. */</span>
 <span class="type">void</span><span class="normal"> </span><span class="function">viewer_file_open</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerFile</span><span class="normal">  </span><span class="symbol">*</span><span class="normal">self</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="comment">/* implementation in the source file */</span>
 <span class="type">void</span>
 <span class="function">viewer_file_open</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerFile</span><span class="normal">  </span><span class="symbol">*</span><span class="normal">self</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="function"><a href="../glib-Warnings-and-Assertions.html#g-return-if-fail">g_return_if_fail</a></span><span class="normal"> </span><span class="symbol">(</span><span class="function">VIEWER_IS_FILE</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"><a href="../glib-Warnings-and-Assertions.html#g-return-if-fail">g_return_if_fail</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">error </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="normal"> </span><span class="symbol">*</span><span class="normal">error </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="normal">  </span><span class="comment">/* do stuff here. */</span>
 <span class="cbracket">}</span></pre></td>
       </tr>
     </tbody>
   </table>
 </div>

 <p>
       </p>
 </div>
 <div class="sect2">
 <div class="titlepage"><div><div><h3 class="title">
 <a name="virtual-public-methods"></a>Virtual public methods</h3></div></div></div>
 <p>
         This is the preferred way to create GObjects with overridable methods:
         </p>
 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 <li class="listitem"><p>
             Define the common method and its virtual function in the
             class structure in the public header
           </p></li>
 <li class="listitem"><p>
             Define the common method in the header file and implement it in the
             source file
           </p></li>
 <li class="listitem"><p>
             Implement a base version of the virtual function in the source
             file and initialize the virtual function pointer to this
             implementation in the object’s <code class="function">class_init</code>
             function; or leave it as <code class="constant">NULL</code> for a ‘pure
             virtual’ method which must be overridden by derived classes
           </p></li>
 <li class="listitem"><p>
             Re-implement the virtual function in each derived class which needs
             to override it
           </p></li>
 </ul></div>
 <p>
       </p>
 <p>
         Note that virtual functions can only be defined if the class is
         derivable, declared using
         <a class="link" href="gobject-Type-Information.html#G-DECLARE-DERIVABLE-TYPE:CAPS" title="G_DECLARE_DERIVABLE_TYPE()"><code class="function">G_DECLARE_DERIVABLE_TYPE</code></a>
         so the class structure can be defined.
 </p>
 <div class="informalexample">
   <table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
     <tbody>
       <tr>
         <td class="listing_lines" align="right"><pre>1
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         <td class="listing_code"><pre class="programlisting"><span class="comment">/* declaration in viewer-file.h. */</span>
 <span class="preproc">#define</span><span class="normal"> </span><span class="usertype">VIEWER_TYPE_FILE</span><span class="normal"> </span><span class="function">viewer_file_get_type</span><span class="normal"> </span><span class="symbol">()</span>
 <span class="function"><a href="gobject-Type-Information.html#G-DECLARE-DERIVABLE-TYPE:CAPS">G_DECLARE_DERIVABLE_TYPE</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">ViewerFile</span><span class="symbol">,</span><span class="normal"> viewer_file</span><span class="symbol">,</span><span class="normal"> VIEWER</span><span class="symbol">,</span><span class="normal"> FILE</span><span class="symbol">,</span><span class="normal"> <a href="gobject-The-Base-Object-Type.html#GObject-struct">GObject</a></span><span class="symbol">)</span>

 <span class="keyword">struct</span><span class="normal"> </span><span class="classname">_ViewerFileClass</span>
 <span class="cbracket">{</span>
 <span class="normal">  </span><span class="usertype">GObjectClass</span><span class="normal"> parent_class</span><span class="symbol">;</span>

 <span class="normal">  </span><span class="comment">/* stuff */</span>
 <span class="normal">  </span><span class="type">void</span><span class="normal"> </span><span class="symbol">(*</span><span class="normal">open</span><span class="symbol">)</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerFile</span><span class="normal">  </span><span class="symbol">*</span><span class="normal">self</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="normal">  </span><span class="comment">/* Padding to allow adding up to 12 new virtual functions without</span>
 <span class="comment">   * breaking ABI. */</span>
 <span class="normal">  </span><span class="usertype">gpointer</span><span class="normal"> padding</span><span class="symbol">[</span><span class="number">12</span><span class="symbol">];</span>
 <span class="cbracket">}</span><span class="symbol">;</span>

 <span class="type">void</span><span class="normal"> </span><span class="function">viewer_file_open</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerFile</span><span class="normal">  </span><span class="symbol">*</span><span class="normal">self</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="comment">/* implementation in viewer-file.c */</span>
 <span class="type">void</span>
 <span class="function">viewer_file_open</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerFile</span><span class="normal">  </span><span class="symbol">*</span><span class="normal">self</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">ViewerFileClass</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">klass</span><span class="symbol">;</span>

 <span class="normal">  </span><span class="function"><a href="../glib-Warnings-and-Assertions.html#g-return-if-fail">g_return_if_fail</a></span><span class="normal"> </span><span class="symbol">(</span><span class="function">VIEWER_IS_FILE</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"><a href="../glib-Warnings-and-Assertions.html#g-return-if-fail">g_return_if_fail</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">error </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="normal"> </span><span class="symbol">*</span><span class="normal">error </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="normal">  klass </span><span class="symbol">=</span><span class="normal"> </span><span class="function">VIEWER_FILE_GET_CLASS</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"><a href="../glib-Warnings-and-Assertions.html#g-return-if-fail">g_return_if_fail</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">klass</span><span class="symbol">-&gt;</span><span class="normal">open </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="normal">  klass</span><span class="symbol">-&gt;</span><span class="function">open</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">self</span><span class="symbol">,</span><span class="normal"> error</span><span class="symbol">);</span>
 <span class="cbracket">}</span></pre></td>
       </tr>
     </tbody>
   </table>
 </div>

 <p>
         The code above simply redirects the <code class="function">open</code> call
         to the relevant virtual function.
       </p>
 <p>
         It is possible to provide a default
         implementation for this class method in the object's
         <code class="function">class_init</code> function: initialize the
         <code class="function">klass-&gt;open</code> field to a pointer to the
         actual implementation.
         By default, class methods that are not inherited are initialized to
         <code class="function">NULL</code>, and thus are to be considered "pure virtual".
 </p>
 <div class="informalexample">
   <table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
     <tbody>
       <tr>
         <td class="listing_lines" align="right"><pre>1
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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_file_real_close</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerFile</span><span class="normal">  </span><span class="symbol">*</span><span class="normal">self</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="comment">/* Default implementation for the virtual method. */</span>
 <span class="cbracket">}</span>

 <span class="keyword">static</span><span class="normal"> </span><span class="type">void</span>
 <span class="function">viewer_file_class_init</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerFileClass</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">/* this is not necessary, except for demonstration purposes.</span>
 <span class="comment">   *</span>
 <span class="comment">   * pure virtual method: mandates implementation in children.</span>
 <span class="comment">   */</span>
 <span class="normal">  klass</span><span class="symbol">-&gt;</span><span class="normal">open </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="normal">  </span><span class="comment">/* merely virtual method. */</span>
 <span class="normal">  klass</span><span class="symbol">-&gt;</span><span class="normal">close </span><span class="symbol">=</span><span class="normal"> viewer_file_real_close</span><span class="symbol">;</span>
 <span class="cbracket">}</span>

 <span class="type">void</span>
 <span class="function">viewer_file_open</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerFile</span><span class="normal">  </span><span class="symbol">*</span><span class="normal">self</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">ViewerFileClass</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">klass</span><span class="symbol">;</span>

 <span class="normal">  </span><span class="function"><a href="../glib-Warnings-and-Assertions.html#g-return-if-fail">g_return_if_fail</a></span><span class="normal"> </span><span class="symbol">(</span><span class="function">VIEWER_IS_FILE</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"><a href="../glib-Warnings-and-Assertions.html#g-return-if-fail">g_return_if_fail</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">error </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="normal"> </span><span class="symbol">*</span><span class="normal">error </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="normal">  klass </span><span class="symbol">=</span><span class="normal"> </span><span class="function">VIEWER_FILE_GET_CLASS</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">self</span><span class="symbol">);</span>

 <span class="normal">  </span><span class="comment">/* if the method is purely virtual, then it is a good idea to</span>
 <span class="comment">   * check that it has been overridden before calling it, and,</span>
 <span class="comment">   * depending on the intent of the class, either ignore it silently</span>
 <span class="comment">   * or warn the user.</span>
 <span class="comment">   */</span>
 <span class="normal">  </span><span class="function"><a href="../glib-Warnings-and-Assertions.html#g-return-if-fail">g_return_if_fail</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">klass</span><span class="symbol">-&gt;</span><span class="normal">open </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="normal">  klass</span><span class="symbol">-&gt;</span><span class="function">open</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">self</span><span class="symbol">,</span><span class="normal"> error</span><span class="symbol">);</span>
 <span class="cbracket">}</span>

 <span class="type">void</span>
 <span class="function">viewer_file_close</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerFile</span><span class="normal">  </span><span class="symbol">*</span><span class="normal">self</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">ViewerFileClass</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">klass</span><span class="symbol">;</span>

 <span class="normal">  </span><span class="function"><a href="../glib-Warnings-and-Assertions.html#g-return-if-fail">g_return_if_fail</a></span><span class="normal"> </span><span class="symbol">(</span><span class="function">VIEWER_IS_FILE</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"><a href="../glib-Warnings-and-Assertions.html#g-return-if-fail">g_return_if_fail</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">error </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="normal"> </span><span class="symbol">*</span><span class="normal">error </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="normal">  klass </span><span class="symbol">=</span><span class="normal"> </span><span class="function">VIEWER_FILE_GET_CLASS</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">self</span><span class="symbol">);</span>
 <span class="normal">  </span><span class="keyword">if</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">klass</span><span class="symbol">-&gt;</span><span class="normal">close </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="normal">    klass</span><span class="symbol">-&gt;</span><span class="function">close</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">self</span><span class="symbol">,</span><span class="normal"> error</span><span class="symbol">);</span>
 <span class="cbracket">}</span></pre></td>
       </tr>
     </tbody>
   </table>
 </div>

 <p>
       </p>
 </div>
 <div class="sect2">
 <div class="titlepage"><div><div><h3 class="title">
 <a name="virtual-private-methods"></a>Virtual private Methods</h3></div></div></div>
 <p>
         These are very similar to <a class="link" href="howto-gobject-methods.html#virtual-public-methods" title="Virtual public methods">virtual
         public methods</a>. They just don't
         have a public function to call directly. The header
         file contains only a declaration of the virtual function:
 </p>
 <div class="informalexample">
   <table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
     <tbody>
       <tr>
         <td class="listing_lines" align="right"><pre>1
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         <td class="listing_code"><pre class="programlisting"><span class="comment">/* declaration in viewer-file.h. */</span>
 <span class="keyword">struct</span><span class="normal"> </span><span class="classname">_ViewerFileClass</span>
 <span class="cbracket">{</span>
 <span class="normal">  </span><span class="usertype">GObjectClass</span><span class="normal"> parent</span><span class="symbol">;</span>

 <span class="normal">  </span><span class="comment">/* Public virtual method as before. */</span>
 <span class="normal">  </span><span class="type">void</span><span class="normal">     </span><span class="symbol">(*</span><span class="normal">open</span><span class="symbol">)</span><span class="normal">           </span><span class="symbol">(</span><span class="usertype">ViewerFile</span><span class="normal">  </span><span class="symbol">*</span><span class="normal">self</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="normal">  </span><span class="comment">/* Private helper function to work out whether the file can be loaded via</span>
 <span class="comment">   * memory mapped I/O, or whether it has to be read as a stream. */</span>
 <span class="normal">  </span><span class="function"><a href="../glib-Basic-Types.html#gboolean">gboolean</a></span><span class="normal"> </span><span class="symbol">(*</span><span class="normal">can_memory_map</span><span class="symbol">)</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerFile</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">self</span><span class="symbol">);</span>

 <span class="normal">  </span><span class="comment">/* Padding to allow adding up to 12 new virtual functions without</span>
 <span class="comment">   * breaking ABI. */</span>
 <span class="normal">  </span><span class="usertype">gpointer</span><span class="normal"> padding</span><span class="symbol">[</span><span class="number">12</span><span class="symbol">];</span>
 <span class="cbracket">}</span><span class="symbol">;</span>

 <span class="type">void</span><span class="normal"> </span><span class="function">viewer_file_open</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerFile</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">self</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></pre></td>
       </tr>
     </tbody>
   </table>
 </div>

 <p>
         These virtual functions are often used to delegate part of the job
         to child classes:
 </p>
 <div class="informalexample">
   <table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
     <tbody>
       <tr>
         <td class="listing_lines" align="right"><pre>1
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 27</pre></td>
         <td class="listing_code"><pre class="programlisting"><span class="comment">/* this accessor function is static: it is not exported outside of this file. */</span>
 <span class="keyword">static</span><span class="normal"> <a href="../glib-Basic-Types.html#gboolean">gboolean</a> </span>
 <span class="function">viewer_file_can_memory_map</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerFile</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="keyword">return</span><span class="normal"> </span><span class="function">VIEWER_FILE_GET_CLASS</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">self</span><span class="symbol">)-&gt;</span><span class="function">can_memory_map</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">self</span><span class="symbol">);</span>
 <span class="cbracket">}</span>

 <span class="type">void</span>
 <span class="function">viewer_file_open</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerFile</span><span class="normal">  </span><span class="symbol">*</span><span class="normal">self</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="function"><a href="../glib-Warnings-and-Assertions.html#g-return-if-fail">g_return_if_fail</a></span><span class="normal"> </span><span class="symbol">(</span><span class="function">VIEWER_IS_FILE</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"><a href="../glib-Warnings-and-Assertions.html#g-return-if-fail">g_return_if_fail</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">error </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="normal"> </span><span class="symbol">*</span><span class="normal">error </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="normal">  </span><span class="comment">/*</span>
 <span class="comment">   * Try to load the file using memory mapped I/O, if the implementation of the</span>
 <span class="comment">   * class determines that is possible using its private virtual method.</span>
 <span class="comment">   */</span>
 <span class="normal">  </span><span class="keyword">if</span><span class="normal"> </span><span class="symbol">(</span><span class="function">viewer_file_can_memory_map</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">self</span><span class="symbol">))</span>
 <span class="normal">    </span><span class="cbracket">{</span>
 <span class="normal">      </span><span class="comment">/* Load the file using memory mapped I/O. */</span>
 <span class="normal">    </span><span class="cbracket">}</span>
 <span class="normal">  </span><span class="keyword">else</span>
 <span class="normal">    </span><span class="cbracket">{</span>
 <span class="normal">      </span><span class="comment">/* Fall back to trying to load the file using streaming I/O… */</span>
 <span class="normal">    </span><span class="cbracket">}</span>
 <span class="cbracket">}</span></pre></td>
       </tr>
     </tbody>
   </table>
 </div>

 <p>
       </p>
 <p>
         Again, it is possible to provide a default implementation for this
         private virtual function:
 </p>
 <div class="informalexample">
   <table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
     <tbody>
       <tr>
         <td class="listing_lines" align="right"><pre>1
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         <td class="listing_code"><pre class="programlisting"><span class="keyword">static</span><span class="normal"> <a href="../glib-Basic-Types.html#gboolean">gboolean</a></span>
 <span class="function">viewer_file_real_can_memory_map</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerFile</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">/* As an example, always return false. Or, potentially return true if the</span>
 <span class="comment">   * file is local. */</span>
 <span class="normal">  </span><span class="keyword">return</span><span class="normal"> <a href="../glib-Standard-Macros.html#FALSE:CAPS">FALSE</a></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_file_class_init</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">ViewerFileClass</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">/* non-pure virtual method; does not have to be implemented in children. */</span>
 <span class="normal">  klass</span><span class="symbol">-&gt;</span><span class="normal">can_memory_map </span><span class="symbol">=</span><span class="normal"> viewer_file_real_can_memory_map</span><span class="symbol">;</span>
 <span class="cbracket">}</span></pre></td>
       </tr>
     </tbody>
   </table>
 </div>

 <p>
       </p>
 <p>
         Derived classes can then override the method with code such as:
 </p>
 <div class="informalexample">
   <table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
     <tbody>
       <tr>
         <td class="listing_lines" align="right"><pre>1
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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_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="usertype">ViewerFileClass</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">file_class </span><span class="symbol">=</span><span class="normal"> </span><span class="function">VIEWER_FILE_CLASS</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">klass</span><span class="symbol">);</span>

 <span class="normal">  </span><span class="comment">/* implement pure virtual function. */</span>
 <span class="normal">  file_class</span><span class="symbol">-&gt;</span><span class="normal">can_memory_map </span><span class="symbol">=</span><span class="normal"> viewer_audio_file_can_memory_map</span><span class="symbol">;</span>
 <span class="cbracket">}</span></pre></td>
       </tr>
     </tbody>
   </table>
 </div>

 <p>
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