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<title>Chaining up: GObject Reference Manual</title>
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<link rel="up" href="howto-gobject.html" title="How to define and implement a new GObject">
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<link rel="prev" href="howto-gobject-methods.html" title="Object methods">
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Chaining up
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Chaining up is often loosely defined by the following set of
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conditions:
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Parent class A defines a public virtual method named foo and
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provides a default implementation.
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Child class B re-implements method foo.
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B’s implementation of foo calls (‘chains up to’) its parent class A’s implementation of foo.
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There are various uses of this idiom:
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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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and chain up to ensure that the previous behaviour is not really modified, just extended.
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You need to implement the
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Chain
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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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each handler is run in turn.
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To explicitly chain up to the implementation of the virtual method in the parent class,
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you first need a handle to the original parent class structure. This pointer can then be used to
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access the original virtual function pointer and invoke it directly.
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<sup class="footnote">[7]</sup>
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Use the parent_class pointer created and initialized
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by the
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G_DEFINE_TYPE
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family of macros, for instance:
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1
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2
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3
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4
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5
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6
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12
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13
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14
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static void
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b_method_to_call (B *obj, gint some_param)
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{
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A_CLASS (b_parent_class)->method_to_call (obj, some_param);
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}
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The original adjective used in this sentence is not innocuous. To fully
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understand its meaning, recall how class structures are initialized: for each object type,
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the class structure associated with this object is created by first copying the class structure of its
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parent type (a simple memcpy) and then by invoking the class_init callback on
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the resulting class structure. Since the class_init callback is responsible for overwriting the class structure
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with the user re-implementations of the class methods, the modified copy of the parent class
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structure stored in the derived instance cannot be used. A copy of the class structure of an instance of the parent
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class is needed.
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