## Chaining up

Chaining up is often loosely defined by the following set of conditions:

- Parent class A defines a public virtual method named `foo` and provides a default implementation.

- Child class B re-implements method `foo`.

- In the method B::foo, the child class B calls its parent class method A::foo.

There are many uses to this idiom:

- 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.

- 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.

- 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.

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.

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. [13]

The function `g_type_class_peek_parent` 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:

    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 */
    }

A lot of people who use this idiom in GTK+ store the parent class structure pointer in a global static variable to avoid the costly call to `g_type_class_peek_parent` for each function call. Typically, the class_init callback initializes the global static variable. `gtk/gtkhscale.c` does this.

[13] The original 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 `memcpy`) 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.