Re: Virtual functions and chaining up

[David Nečas (Yeti) <yeti physics muni cz>]

On Thu, Jul 27, 2006 at 02:24:23AM -0400, Nikhil Dinesh wrote:
> I'm trying to create my own object hierarchy, and I don't
> understand the part in the API reference about virtual
> functions and chaining up. To create a virtual public
> method,  the docs say we need to do something like:
>
> /* declaration in maman-bar.h. */
> struct _MamanBarClass {
>   GObjectClass parent;
>
>   /* stuff */
>   void (*do_action) (MamanBar *self, /* parameters */);
> };
> void maman_bar_do_action (MamanBar *self, /* parameters */);
> /* implementation in maman-bar.c */
> void maman_bar_do_action (MamanBar *self, /* parameters */)
> {
>   MAMAN_BAR_GET_CLASS (self)->do_action (self, /* parameters */);
> }

Correct.

> I assume that the subclass definition looks like:
>
> struct _MamanBarSubClass {
>    MamanBarClass parent;
>
>    ....
>
>    void (*do_action) (MamanBarSub* self, /* params */);
> }

No, there's already one do_action() in parent, so this is
a second one.  Moreover the parent class has no access to
this second do_action() so maman_bar_do_action() can never
invoke it.

Each class has its own class struct which contains all
ancestors's class structs (like a matryoshka).  When its
class_init() method is called, the fields defined in
ancestors classes are already filled with parent's values.
So you only need to override them.

> If the maman_bar_do_action actually needs to do something, and
> the derived class needs to chain up,

maman_bar_do_action() is *not* the parent class method, it's
the virtual method for users to call.  The child method
calls its parent's method:
MAMAN_BAR_CLASS(maman_bar_sub_parent_class)->do_action().

See Gtk+ source code for examples...

Yeti


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