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<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="howto-gobject-construction"></a>Object construction</h2></div></div></div>
<p>
      People often get confused when trying to construct their GObjects because of the
      sheer number of different ways to hook into the objects's construction process: it is
      difficult to figure which is the <span class="emphasis"><em>correct</em></span>, recommended way.
    </p>
<p>
      <a class="xref" href="chapter-gobject.html#gobject-construction-table" title="Table 4. g_object_new">Table 4, “g_object_new”</a> shows what user-provided functions
      are invoked during object instantiation and in which order they are invoked.
      A user looking for the equivalent of the simple C++ constructor function should use
      the <code class="function">instance_init</code> method. It will be invoked after
      all the parents’ <code class="function">instance_init</code>
      functions have been invoked. It cannot take arbitrary construction parameters 
      (as in C++) but if your object needs arbitrary parameters to complete initialization,
      you can use construction properties.
    </p>
<p>
      Construction properties will be set only after all
      <code class="function">instance_init</code> functions have run.
      No object reference will be returned to the client of <code class="function"><a class="link" href="gobject-The-Base-Object-Type.html#g-object-new" title="g_object_new ()">g_object_new</a></code>
      until all the construction properties have been set.
    </p>
<p>
      It is important to note that object construction cannot <span class="emphasis"><em>ever</em></span>
      fail. If you require a fallible GObject construction, you can use the
      <a href="https://developer.gnome.org/gio/unstable/GInitable.html#GInitable-struct"><span class="type">GInitable</span></a> and
      <a href="https://developer.gnome.org/gio/unstable/GAsyncInitable.html#GAsyncInitable-struct"><span class="type">GAsyncInitable</span></a>
      interfaces provided by the GIO library.
    </p>
<p>
      You should write the following code first:
</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="function"><a href="gobject-Type-Information.html#G-DEFINE-TYPE-WITH-PRIVATE:CAPS">G_DEFINE_TYPE_WITH_PRIVATE</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="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="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="usertype">ViewerFilePrivate</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">priv </span><span class="symbol">=</span><span class="normal"> </span><span class="function">viewer_file_get_instance_private</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">/* initialize all public and private members to reasonable default values.</span>
<span class="comment">   * They are all automatically initialized to 0 to begin with. */</span>
<span class="cbracket">}</span></pre></td>
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</div>

<p>
    </p>
<p>
      If you need special construction properties (with
      <a class="link" href="gobject-GParamSpec.html#G-PARAM-CONSTRUCT-ONLY:CAPS"><code class="function">G_PARAM_CONSTRUCT_ONLY</code></a>
      set), install the properties in
      the <code class="function">class_init()</code> function, override the <code class="function">set_property()</code>
      and <code class="function">get_property()</code> methods of the GObject class,
      and implement them as described by <a class="xref" href="gobject-properties.html" title="Object properties">the section called “Object properties”</a>.
    </p>
<p>
      Property IDs must start from 1, as 0 is reserved for internal use by
      GObject.
</p>
<div class="informalexample">
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        <td class="listing_code"><pre class="programlisting"><span class="keyword">enum</span>
<span class="cbracket">{</span>
<span class="normal">  PROP_FILENAME </span><span class="symbol">=</span><span class="normal"> </span><span class="number">1</span><span class="symbol">,</span>
<span class="normal">  PROP_ZOOM_LEVEL</span><span class="symbol">,</span>
<span class="normal">  N_PROPERTIES</span>
<span class="cbracket">}</span><span class="symbol">;</span>

<span class="keyword">static</span><span class="normal"> </span><span class="usertype">GParamSpec</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">obj_properties</span><span class="symbol">[</span><span class="normal">N_PROPERTIES</span><span class="symbol">]</span><span class="normal"> </span><span class="symbol">=</span><span class="normal"> </span><span class="cbracket">{</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="cbracket">}</span><span class="symbol">;</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="usertype">GObjectClass</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">object_class </span><span class="symbol">=</span><span class="normal"> </span><span class="function"><a href="gobject-The-Base-Object-Type.html#G-OBJECT-CLASS:CAPS">G_OBJECT_CLASS</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">klass</span><span class="symbol">);</span>

<span class="normal">  object_class</span><span class="symbol">-&gt;</span><span class="normal">set_property </span><span class="symbol">=</span><span class="normal"> viewer_file_set_property</span><span class="symbol">;</span>
<span class="normal">  object_class</span><span class="symbol">-&gt;</span><span class="normal">get_property </span><span class="symbol">=</span><span class="normal"> viewer_file_get_property</span><span class="symbol">;</span>

<span class="normal">  obj_properties</span><span class="symbol">[</span><span class="normal">PROP_FILENAME</span><span class="symbol">]</span><span class="normal"> </span><span class="symbol">=</span>
<span class="normal">    </span><span class="function"><a href="gobject-Standard-Parameter-and-Value-Types.html#g-param-spec-string">g_param_spec_string</a></span><span class="normal"> </span><span class="symbol">(</span><span class="string">"filename"</span><span class="symbol">,</span>
<span class="normal">                         </span><span class="string">"Filename"</span><span class="symbol">,</span>
<span class="normal">                         </span><span class="string">"Name of the file to load and display from."</span><span class="symbol">,</span>
<span class="normal">                         <a href="../glib-Standard-Macros.html#NULL:CAPS">NULL</a>  </span><span class="comment">/* default value */</span><span class="symbol">,</span>
<span class="normal">                         <a href="gobject-GParamSpec.html#G-PARAM-CONSTRUCT-ONLY:CAPS">G_PARAM_CONSTRUCT_ONLY</a> </span><span class="symbol">|</span><span class="normal"> <a href="gobject-GParamSpec.html#G-PARAM-READWRITE:CAPS">G_PARAM_READWRITE</a></span><span class="symbol">));</span>

<span class="normal">  obj_properties</span><span class="symbol">[</span><span class="normal">PROP_ZOOM_LEVEL</span><span class="symbol">]</span><span class="normal"> </span><span class="symbol">=</span>
<span class="normal">    </span><span class="function"><a href="gobject-Standard-Parameter-and-Value-Types.html#g-param-spec-uint">g_param_spec_uint</a></span><span class="normal"> </span><span class="symbol">(</span><span class="string">"zoom-level"</span><span class="symbol">,</span>
<span class="normal">                       </span><span class="string">"Zoom level"</span><span class="symbol">,</span>
<span class="normal">                       </span><span class="string">"Zoom level to view the file at."</span><span class="symbol">,</span>
<span class="normal">                       </span><span class="number">0</span><span class="normal">  </span><span class="comment">/* minimum value */</span><span class="symbol">,</span>
<span class="normal">                       </span><span class="number">10</span><span class="normal"> </span><span class="comment">/* maximum value */</span><span class="symbol">,</span>
<span class="normal">                       </span><span class="number">2</span><span class="normal">  </span><span class="comment">/* default value */</span><span class="symbol">,</span>
<span class="normal">                       <a href="gobject-GParamSpec.html#G-PARAM-READWRITE:CAPS">G_PARAM_READWRITE</a></span><span class="symbol">));</span>

<span class="normal">  </span><span class="function"><a href="gobject-The-Base-Object-Type.html#g-object-class-install-properties">g_object_class_install_properties</a></span><span class="normal"> </span><span class="symbol">(</span><span class="normal">object_class</span><span class="symbol">,</span>
<span class="normal">                                     N_PROPERTIES</span><span class="symbol">,</span>
<span class="normal">                                     obj_properties</span><span class="symbol">);</span>
<span class="cbracket">}</span></pre></td>
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</div>

<p>
      If you need this, make sure you can build and run code similar to the
      code shown above. Also, make sure your construct properties can be set
      without side effects during construction.
    </p>
<p>
      Some people sometimes need to complete the initialization of a instance
      of a type only after the properties passed to the constructors have been
      set. This is possible through the use of the <code class="function">constructor()</code>
      class method as described in <a class="xref" href="chapter-gobject.html#gobject-instantiation" title="Object instantiation">the section called “Object instantiation”</a> or,
      more simply, using the <code class="function">constructed()</code> class method.
      Note that the <code class="function">constructed()</code>
      virtual function will only be invoked after the properties marked as
      <code class="function">G_PARAM_CONSTRUCT_ONLY</code> or
      <code class="function">G_PARAM_CONSTRUCT</code> have been consumed, but
      before the regular properties passed to <code class="function">g_object_new()</code>
      have been set.
    </p>
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