subclassing lists?

[Dirk Luetjens <dirk post lfm rwth-aachen de>]

Hello,

here comes the last question in the field of programming techniques. This
is more a GTK+ question, but Iīm not subscribed to the list. And I know
that this group has the world best programmers :-)

I wanted to model a line of points in a line class wich is a subclass of a
list. In the sence of C++

class Line : GList
{
  int Length;
}

I tried to follow the object oriented approach int C using the syntax from
GTK+ and GNOME. The problem was, that I canīt subclass the GList struct,
since it does not have a control element. GLists are used in the form:

Glist *list = NULL;
list = g_list_append (list, pointer);

This results in the following :

list -> {GList}
           |
	{GList}
           |
	{GList}
	   |
          NULL

To be able to subclass, I must have an element of GList in the beginning
of the struct, and not only the pointer. But that would mean, that I
always have an unused element in the list:

struct _Line {
	GList list;
	int length;
}

A better solution would be a control element:

struct _GListNode {
	gpointer data;
	GListNode next;
	GListNode prev;
}
struct _GList {
	GListNode first;
	GListNode last;
}

graphically:

list -> {GList} -> {GListNode}
           |            |
           |       {GListNode}
           |            |
           \-----> {GListNode}
                        |
                        -

So my question: What is the reason for the other implementation and why
does the object oriented idea from GTK+ is not implemented in the glib.
Or did I miss some significant point?

Dirk


    _/         _/_/   _/      _/          Dirk Luetjens
   _/        _/      _/_/  _/_/           Lehrstuhl fuer Messtechnik
  _/        _/_/    _/  _/  _/            RWTH Aachen
 _/        _/      _/      _/             D-52056 Aachen
_/_/_/_/  _/      _/      _/              Germany

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