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| <div class="sect1" title="Chaining up"> |
| <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" 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>In the method B::foo, the child class B calls its parent class method A::foo.</p></li> |
| </ul></div> |
| <p> |
| There are many uses to this idiom: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"><p>You need to change 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 |
| slightly and chain up to ensure that the previous behaviour is not really modified, just extended. |
| </p></li> |
| <li class="listitem"><p>You are lazy, you have access to the source code of the parent class but you don't want |
| to modify it to add method calls to new specialized method calls: it is faster to hack the child class |
| to chain up than to modify the parent to call down.</p></li> |
| <li class="listitem"><p>You need to implement the Chain Of Responsibility pattern: each object of the inheritance |
| tree chains up to its parent (typically, at the beginning or the end of the method) to ensure that |
| they each handler is run in turn.</p></li> |
| </ul></div> |
| <p> |
| I am personally not really convinced any of the last two uses are really a good idea but since this |
| programming idiom is often used, this section attempts to explain how to implement it. |
| </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 class function pointer and invoke it directly. |
| <sup>[<a name="id637346" href="#ftn.id637346" class="footnote">11</a>]</sup> |
| </p> |
| <p>The function <code class="function"><a class="link" href="gobject-Type-Information.html#g-type-class-peek-parent" title="g_type_class_peek_parent ()">g_type_class_peek_parent</a></code> is used to access the original parent |
| class structure. Its input is a pointer to the class of the derived object and it returns a pointer |
| to the original parent class structure. The code below shows how you could use it: |
| </p> |
| <pre class="programlisting"> |
| static void |
| b_method_to_call (B *obj, int a) |
| { |
| BClass *klass; |
| AClass *parent_class; |
| |
| klass = B_GET_CLASS (obj); |
| parent_class = g_type_class_peek_parent (klass); |
| |
| /* do stuff before chain up */ |
| |
| parent_class->method_to_call (obj, a); |
| |
| /* do stuff after chain up */ |
| } |
| </pre> |
| <p> |
| </p> |
| <div class="footnotes"> |
| <br><hr width="100" align="left"> |
| <div class="footnote"><p><sup>[<a name="ftn.id637346" href="#id637346" class="para">11</a>] </sup> |
| The <span class="emphasis"><em>original</em></span> adjective used in this sentence is not innocuous. To fully |
| understand its meaning, you need to recall how class structures are initialized: for each object type, |
| the class structure associated to 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 class_init callback on |
| the resulting class structure. Since the class_init callback is responsible for overwriting the class structure |
| with the user re-implementations of the class methods, we cannot merely use the modified copy of the parent class |
| structure stored in our derived instance. We want to get a copy of the class structure of an instance of the parent |
| class. |
| </p></div> |
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