blob: a4b65d81361fa5b50ab87ff67a2e499925e8c55d [file] [log] [blame]
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<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 V1.25.1 (XML mode)">
<link rel="stylesheet" href="style.css" type="text/css">
<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>
<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
<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>
There are various uses of this idiom:
<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.
<li class="listitem"><p>You need to implement the
<a class="ulink" href="" 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>
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="" class="footnote" name="id-"><sup class="footnote">[7]</sup></a>
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:
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<td class="listing_lines" align="right"><pre>1
<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">)-&gt;</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>
<div class="footnotes">
<br><hr style="width:100; text-align:left;margin-left: 0">
<div id="" class="footnote"><p><a href="#id-" 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.
<div class="footer">
<hr>Generated by GTK-Doc V1.25.1</div>