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howto-gobject-chainup.html
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <title>Chaining up: GObject Reference Manual</title> <meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> <link rel="home" href="index.html" title="GObject Reference Manual"> <link rel="up" href="howto-gobject.html" title="How to define and implement a new GObject"> <link rel="prev" href="howto-gobject-methods.html" title="Object methods"> <link rel="next" href="howto-interface.html" title="How to define and implement interfaces"> <meta name="generator" content="GTK-Doc V1.32 (XML mode)"> <link rel="stylesheet" href="style.css" type="text/css"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table class="navigation" id="top" width="100%" summary="Navigation header" cellpadding="2" cellspacing="5"><tr valign="middle"> <td width="100%" align="left" class="shortcuts"></td> <td><a accesskey="h" href="index.html"><img src="home.png" width="16" height="16" border="0" alt="Home"></a></td> <td><a accesskey="u" href="howto-gobject.html"><img src="up.png" width="16" height="16" border="0" alt="Up"></a></td> <td><a accesskey="p" href="howto-gobject-methods.html"><img src="left.png" width="16" height="16" border="0" alt="Prev"></a></td> <td><a accesskey="n" href="howto-interface.html"><img src="right.png" width="16" height="16" border="0" alt="Next"></a></td> </tr></table> <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="k">static</span> <span class="kt">void</span> <span class="nf">b_method_to_call</span> <span class="p">(</span><span class="n">B</span> <span class="o">*</span><span class="n">obj</span><span class="p">,</span> <span class="n">gint</span> <span class="n">some_param</span><span class="p">)</span> <span class="p">{</span> <span class="cm">/* do stuff before chain up */</span> <span class="cm">/* call the method_to_call() virtual function on the</span> <span class="cm"> * parent of BClass, AClass.</span> <span class="cm"> *</span> <span class="cm"> * remember the explicit cast to AClass*</span> <span class="cm"> */</span> <span class="n">A_CLASS</span> <span class="p">(</span><span class="n">b_parent_class</span><span class="p">)</span><span class="o">-></span><span class="n">method_to_call</span> <span class="p">(</span><span class="n">obj</span><span class="p">,</span> <span class="n">some_param</span><span class="p">);</span> <span class="cm">/* do stuff after chain up */</span> <span class="p">}</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> </div> <div class="footer"> <hr>Generated by GTK-Doc V1.32</div> </body> </html>