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| <div class="sect1"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="howto-gobject-chainup"></a>Chaining up</h2></div></div></div> |
| <p>Chaining up is often loosely defined by the following set of |
| conditions: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"><p>Parent class A defines a public virtual method named <code class="function">foo</code> and |
| provides a default implementation.</p></li> |
| <li class="listitem"><p>Child class B re-implements method <code class="function">foo</code>.</p></li> |
| <li class="listitem"><p>B’s implementation of <code class="function">foo</code> calls (‘chains up to’) its parent class A’s implementation of <code class="function">foo</code>.</p></li> |
| </ul></div> |
| <p> |
| There are various uses of this idiom: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"><p>You need to extend the behaviour of a class without modifying its code. You create |
| a subclass to inherit its implementation, re-implement a public virtual method to modify the behaviour |
| and chain up to ensure that the previous behaviour is not really modified, just extended. |
| </p></li> |
| <li class="listitem"><p>You need to implement the |
| <a class="ulink" href="http://en.wikipedia.org/wiki/Chain-of-responsibility_pattern" target="_top">Chain |
| Of Responsibility pattern</a>: each object of the inheritance |
| tree chains up to its parent (typically, at the beginning or the end of the method) to ensure that |
| each handler is run in turn.</p></li> |
| </ul></div> |
| <p> |
| </p> |
| <p> |
| To explicitly chain up to the implementation of the virtual method in the parent class, |
| you first need a handle to the original parent class structure. This pointer can then be used to |
| access the original virtual function pointer and invoke it directly. |
| <a href="#ftn.id-1.6.3.9.3.1" class="footnote" name="id-1.6.3.9.3.1"><sup class="footnote">[7]</sup></a> |
| </p> |
| <p> |
| Use the <code class="function">parent_class</code> pointer created and initialized |
| by the |
| <a class="link" href="gobject-Type-Information.html#G-DEFINE-TYPE:CAPS" title="G_DEFINE_TYPE()"><code class="function">G_DEFINE_TYPE</code></a> |
| family of macros, for instance: |
| </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</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">b_method_to_call</span><span class="normal"> </span><span class="symbol">(</span><span class="usertype">B</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">obj</span><span class="symbol">,</span><span class="normal"> </span><span class="usertype">gint</span><span class="normal"> some_param</span><span class="symbol">)</span> |
| <span class="cbracket">{</span> |
| <span class="normal"> </span><span class="comment">/* do stuff before chain up */</span> |
| |
| <span class="normal"> </span><span class="comment">/* call the method_to_call() virtual function on the</span> |
| <span class="comment"> * parent of BClass, AClass.</span> |
| <span class="comment"> *</span> |
| <span class="comment"> * remember the explicit cast to AClass*</span> |
| <span class="comment"> */</span> |
| <span class="normal"> </span><span class="function">A_CLASS</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">b_parent_class</span><span class="symbol">)-></span><span class="function">method_to_call</span><span class="normal"> </span><span class="symbol">(</span><span class="normal">obj</span><span class="symbol">,</span><span class="normal"> some_param</span><span class="symbol">);</span> |
| |
| <span class="normal"> </span><span class="comment">/* do stuff after chain up */</span> |
| <span class="cbracket">}</span></pre></td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| |
| <p> |
| </p> |
| <div class="footnotes"> |
| <br><hr style="width:100; text-align:left;margin-left: 0"> |
| <div id="ftn.id-1.6.3.9.3.1" class="footnote"><p><a href="#id-1.6.3.9.3.1" class="para"><sup class="para">[7] </sup></a> |
| The <span class="emphasis"><em>original</em></span> adjective used in this sentence is not innocuous. To fully |
| understand its meaning, recall how class structures are initialized: for each object type, |
| the class structure associated with this object is created by first copying the class structure of its |
| parent type (a simple <code class="function">memcpy</code>) and then by invoking the <code class="function">class_init</code> callback on |
| the resulting class structure. Since the <code class="function">class_init</code> callback is responsible for overwriting the class structure |
| with the user re-implementations of the class methods, the modified copy of the parent class |
| structure stored in the derived instance cannot be used. A copy of the class structure of an instance of the parent |
| class is needed. |
| </p></div> |
| </div> |
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