difference between class and instance structure

Question

I'm currently trying to learn how to use GObject and there's a point I absolutely don't understand: What's the difference between the class and the instance structure (like "MamanBarClass" and "MamanBar") resp. how do I use them?

The class structure is only created once, and is not instance-specific. It's where you put things which are not instance-specific, such as pointers for virtual methods (which is the most common use for the class struct).

At the moment I'd put all my object attributes into a private structure (like "MamanBarPrivate"), register it with "g_type_class_add_private" and define properties/getters/setters to access them.

Good. That's the right thing to do.

But when I leave the class structure empty I get the following error at "g_type_register_static_simple":

You should never leave the class structure empty. It should always contain the class structure for the type you're inheriting from. For example, if you're trying to create a GObject, the class structure should look like this (at a minimum):

struct _MamanBarClass {
  GObjectClass parent_class;
};

Even if you're not inheriting from GObject, you still need the base class for GType:

struct _FooClass {
  GTypeClass parent_class;
};

This is how simple inheritance is done in C.

And why are all object methods defined in the class structure (like "GtKWidgetClass")? Probably I'm just screwing up everything, but I only worked with Delphi for OOP yet (I know, nothing to be proud about :D)

Those are virtual public methods. As for why they're defined in the class structure instead of the instance structure, it's because the implementations are the same for every instance.