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<li class="listitem"><p>In the method B::foo, the child class B calls its parent class method A::foo.</p></li>
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There are many uses to this idiom:
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There are various uses to this idiom:
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem"><p>You need to change the behaviour of a class without modifying its code. You create
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<li class="listitem"><p>You need to extend the behaviour of a class without modifying its code. You create
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a subclass to inherit its implementation, re-implement a public virtual method to modify the behaviour
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slightly and chain up to ensure that the previous behaviour is not really modified, just extended.
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and chain up to ensure that the previous behaviour is not really modified, just extended.
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<li class="listitem"><p>You are lazy, you have access to the source code of the parent class but you don't want
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to modify it to add method calls to new specialized method calls: it is faster to hack the child class
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to chain up than to modify the parent to call down.</p></li>
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<li class="listitem"><p>You need to implement the Chain Of Responsibility pattern: each object of the inheritance
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tree chains up to its parent (typically, at the beginning or the end of the method) to ensure that
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they each handler is run in turn.</p></li>
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I am personally not really convinced any of the last two uses are really a good idea but since this
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programming idiom is often used, this section attempts to explain how to implement it.
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To explicitly chain up to the implementation of the virtual method in the parent class,
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access the original class function pointer and invoke it directly.
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<a href="#ftn.id-1.6.3.8.3.1" class="footnote" name="id-1.6.3.8.3.1"><sup class="footnote">[11]</sup></a>
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<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
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class structure. Its input is a pointer to the class of the derived object and it returns a pointer
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to the original parent class structure. The code below shows how you could use it:
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<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>
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is used to access the original parent class structure. Its input is a
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pointer to the class of the derived object and it returns a pointer to
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the original parent class structure. Instead of using this function
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directly, though, you should use the <code class="function">parent_class</code>
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pointer created and initialized for us by the G_DEFINE_TYPE_* family of
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<pre class="programlisting">
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b_method_to_call (B *obj, int a)
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klass = B_GET_CLASS (obj);
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parent_class = g_type_class_peek_parent (klass);
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/* do stuff before chain up */
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parent_class->method_to_call (obj, a);
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/* call the method_to_call() virtual function on the
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* parent of BClass, AClass.
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* remember the explicit cast to AClass*
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A_CLASS (b_parent_class)->method_to_call (obj, a);
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/* do stuff after chain up */
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docs/reference/gobject/html/howto-gobject-chainup.html
