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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="howto-interface-implement"></a>Implementing interfaces</h2></div></div></div>
<p>
      Once the interface is defined, implementing it is rather trivial.
    </p>
<p>
      The first step is to define a normal final GObject class exactly as in
      <a class="xref" href="howto-gobject.html#howto-gobject-header" title="Boilerplate header code">the section called “Boilerplate header code”</a>.
    </p>
<p>
      The second step is to implement <span class="type">ViewerFile</span> by defining
      it using
      <code class="function"><a class="link" href="gobject-Type-Information.html#G-DEFINE-TYPE-WITH-CODE:CAPS" title="G_DEFINE_TYPE_WITH_CODE()">G_DEFINE_TYPE_WITH_CODE</a></code>
      and
      <code class="function"><a class="link" href="gobject-Type-Information.html#G-IMPLEMENT-INTERFACE:CAPS" title="G_IMPLEMENT_INTERFACE()">G_IMPLEMENT_INTERFACE</a></code>
      instead of
      <code class="function"><a class="link" href="gobject-Type-Information.html#G-DEFINE-TYPE:CAPS" title="G_DEFINE_TYPE()">G_DEFINE_TYPE</a></code>:
</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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        <td class="listing_code"><pre class="programlisting"><span class="keyword">static</span><span class="normal"> </span><span class="type">void</span><span class="normal"> </span><span class="function">viewer_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="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">ViewerFile</span><span class="symbol">,</span><span class="normal"> viewer_file</span><span class="symbol">,</span><span class="normal"> <a href="gobject-Type-Information.html#G-TYPE-OBJECT:CAPS">G_TYPE_OBJECT</a></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_file_editable_interface_init</span><span class="symbol">))</span></pre></td>
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<p>
      This definition is very much like all the similar functions seen
      previously. The only interface-specific code present here is the use of
      <code class="function"><a class="link" href="gobject-Type-Information.html#G-IMPLEMENT-INTERFACE:CAPS" title="G_IMPLEMENT_INTERFACE()">G_IMPLEMENT_INTERFACE</a></code>.
    </p>
<div class="note"><p>Classes can implement multiple interfaces by using multiple calls to
    <code class="function"><a class="link" href="gobject-Type-Information.html#G-IMPLEMENT-INTERFACE:CAPS" title="G_IMPLEMENT_INTERFACE()">G_IMPLEMENT_INTERFACE</a></code>
    inside the call to
    <code class="function"><a class="link" href="gobject-Type-Information.html#G-DEFINE-TYPE-WITH-CODE:CAPS" title="G_DEFINE_TYPE_WITH_CODE()">G_DEFINE_TYPE_WITH_CODE</a></code>
    </p></div>
<p>
      <code class="function">viewer_file_editable_interface_init</code>, the interface
      initialization function: inside it every virtual method of the interface
      must be assigned to its implementation:
</p>
<div class="informalexample">
  <table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
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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_editable_save</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-print">g_print</a></span><span class="normal"> </span><span class="symbol">(</span><span class="string">"File implementation of editable interface save method: %s.</span><span class="specialchar">\n</span><span class="string">"</span><span class="symbol">,</span>
<span class="normal">           self</span><span class="symbol">-&gt;</span><span class="normal">filename</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_editable_undo</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">guint</span><span class="normal">       n_steps</span><span class="symbol">)</span>
<span class="cbracket">{</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">"File implementation of editable interface undo method: %s.</span><span class="specialchar">\n</span><span class="string">"</span><span class="symbol">,</span>
<span class="normal">           self</span><span class="symbol">-&gt;</span><span class="normal">filename</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_editable_redo</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">guint</span><span class="normal">       n_steps</span><span class="symbol">)</span>
<span class="cbracket">{</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">"File implementation of editable interface redo method: %s.</span><span class="specialchar">\n</span><span class="string">"</span><span class="symbol">,</span>
<span class="normal">           self</span><span class="symbol">-&gt;</span><span class="normal">filename</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_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">  iface</span><span class="symbol">-&gt;</span><span class="normal">save </span><span class="symbol">=</span><span class="normal"> viewer_file_editable_save</span><span class="symbol">;</span>
<span class="normal">  iface</span><span class="symbol">-&gt;</span><span class="normal">undo </span><span class="symbol">=</span><span class="normal"> viewer_file_editable_undo</span><span class="symbol">;</span>
<span class="normal">  iface</span><span class="symbol">-&gt;</span><span class="normal">redo </span><span class="symbol">=</span><span class="normal"> viewer_file_editable_redo</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_init</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">/* Instance variable initialisation code. */</span>
<span class="cbracket">}</span></pre></td>
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