dev-cpp-users

[Matthews <dem...@uw...>]

I'm trying to implement a GUI management system in a project I'm working 
on.  The setup is like this:

base_gui_object:
   - all gui elements are derived from this class, which includes a virtual 
draw() function to override

window:
   - derived from base_gui_object, overrides the draw() function.

gui_controller:
   - contains linked list (subscriber_list) of all gui elements in use

subscriber_list:
   - contains linked list of all elements (subscriber_info)

subscriber_info:
   - contains a pointer to a single gui element (base_gui_object)

Now, here's the important code:

/////////////////////////////////////////////////////

class base_gui_object
{
   public:
     virtual void draw() = 0;
};

class subscriber_info
{
   public:
     int priority;
     base_gui_object *gui_item;
   public:
     subscriber_info() {}
     subscriber_info(base_gui_object *item,int priority);
     subscriber_info *next;
     int get_priority() { return priority; }
     void *get_gui_subscriber() { return gui_item; }
};

class subscriber_list
{
   private:
     int num_subscribers;
     subscriber_info *entries;
     int index;
   public:
     subscriber_list() { index = 0; num_subscribers = 0; }
     ~subscriber_list();
     void add(base_gui_object *item,int priority);
     void remove(base_gui_object *item);
     int get_num_subscribers() { return num_subscribers; }
     subscriber_info *traverse();
     subscriber_info *first() { return entries; }
     subscriber_info *next();
     subscriber_info *prev();
};

class gui_control
{
   private:
     subscriber_list subscribers;
   public:
     gui_control() {}
     void update();
     void subscribe(base_gui_object *item,int priority);
     void unsubscribe(base_gui_object *item);
     //void translate_message();
};

class window : public base_gui_object
{
   private:
     char *title;
     int x,y,w,h;
     vid_info *vid;
     game_info *game;
     char *t;

   public:
     window(char *name,char *text,int x,int y,int w,int h,vid_info 
*vid,game_info *game);
     void draw();
     void draw_text();
};

//////////////////////////////////////////////////////////////////////////////////////////////
// class Subscriber_Info - linked list items

subscriber_info::subscriber_info(base_gui_object *item,int priority)
{
   gui_item = item;
   next = NULL;
}

subscriber_info *subscriber_list::traverse()
{
   subscriber_info *p = entries;
   while(p->next) p = p->next;
   return p;
}

//////////////////////////////////////////////////////////////////////////////////////////////
// class Subscriber_List - linked list

void subscriber_list::add(base_gui_object *item,int priority)
{
   if(!entries)
   {
     entries = new subscriber_info(item,priority);
     entries->next = NULL;
   }
   else
   {
     subscriber_info *p = traverse();
     p->next = new subscriber_info(item,priority);
   }
   num_subscribers++;
}

//////////////////////////////////////////////////////////////////////////////////////////////
// class Gui_Control

void gui_control::subscribe(base_gui_object *item,int priority)
{
    subscribers.add(item,priority);
}

void gui_control::unsubscribe(base_gui_object *item)
{
    subscribers.remove(item);
}

void gui_control::update()
{
   subscriber_info *p = subscribers.first();
   while((p=subscribers.next())) p->gui_item->draw();
}

//////////////////////////////////////////////////////////////////////////////////////////////
// class Window

window::window(char *name,char *text,int x2,int y2,int w2,int h2,vid_info 
*video,game_info *g)
{
   title = name;
   x = x2; y = y2;
   w = w2; h = h2;
   vid = video;
   game = g;
   t = text;

   //draw();

   //do some gui attaching stuff
}

/////////////////////////////////////////////////////////////////////////////////////////

In main(), this is the testing code.

char *text = {
     "This is a test\n"
     "of multiple line stuff"
};
window *win = new window("Char Editor",text,5,3,20,5,vid,game);

gui_control *gui = new gui_control();
gui->subscribe(win,PRIORITY_LOW);

/////////////////////////////////////////////////////////////////////////////////////////

Now, the problem code is the single line "while((p=subscribers.next())) 
p->gui_item->draw();" in gui_controller::update().  Whenever I try to call 
the draw() function directly, I get an access violation error.  This is my 
first time trying limited object-oriented programming like this, so I'd 
appreciate learning how I'm supposed to access virtual member functions 
like this.

Thanks.