Re: custom vfunc problem

[Kjell Ahlstedt <kjell ahlstedt bredband net>]

Sorry, my previous reply was wrong. What will happen with 
get_paint_volume is this (and I suppose that's what you have found):

If the user of the C++ code does not override get_paint_volume_vfunc(), 
clutter_actor_real_get_paint_volume() will be called, but 
klass->get_paint_volume will not be equal to 
clutter_actor_real_get_paint_volume.

A simple "solution" would be to not wrap get_paint_volume in C++ code. 
Then the C++ class, gtkmm__ClutterActor, would inherit 
klass->get_paint_volume from from the C class, ClutterActor. It would be 
impossible for a user of your C++ class to override 
get_paint_volume_vfunc(), because there would be no such vfunc. I 
suppose that's unacceptable or at least a serious drawback.

I don't know how to solve this problem. The long-term solution is to 
change the C code. Tests like
klass->get_paint_volume == clutter_actor_real_get_paint_volume
are bad. Comments in the C code show that the clutter developers are 
aware of that.

Kjell

Den 2016-03-21 kl. 21:38, skrev Ian Martin:
> Hi,
> Thanks.
>
> Sorry Kjell, I clipped the init function.  All of the non-deprecated 
> methods and signals are pointed to the _real_ version of the vfunc or 
> signal in clutter_actor_class_init(), and I'm wondering if using that 
> to ensure the object is not a derived class is preventing cluttermm 
> from calling the appropriate vfuncs.  Stepping through the code the 
> _real_functions are being skipped from C++ code, but being called when 
> the identical C code is run.
>
> Running a program with a debug flag( --clutter-debug=layout) I'm 
> seeing lines like the following:
> > Clutter-Message: [ +58820]:[LAYOUT]:clutter-actor.c:2342: Querying 
> > the layout manager 'gtkmm__ClutterBoxLayout'[0x2922670] for the 
> > preferred height
> where the C code gives
> > Clutter-Message: [ +85160]:[LAYOUT]:clutter-actor.c:2342: Querying 
> > the layout manager 'ClutterBoxLayout'[0xb1ea10] for the preferred height
>
> which looks to me like the C++ object used is a derived class given 
> the gtkmm_  prefix; that would  explain why it wouldn't call the base 
> methods if a check for a _real_ vfunc is present, and that would 
> explain what I'm seeing happen.  I can't see anywhere in the gtkmm 
> code where the same problem occurs; lots of gtk+ classes - GtkButton 
> for one - have a series of _real_ signals, but I don't see anywhere 
> the same use of the _real_ function as a way of blocking derived 
> classes from using C code. clutter_actor_allocate_vfunc I got around 
> by making sure a flag was set; now I'm stuck trying to get around 
> clutter_get_paint_volume, and there's no flags to use to get around 
> it- this piece of code in clutter_actor_real_get_paint_volume ensures 
> any derived class returns false, and therefore any update won't happen.
> >   if (klass->paint == clutter_actor_real_paint &&
> >       klass->get_paint_volume == clutter_actor_real_get_paint_volume)
> >     {
> >       res = TRUE;
> >     }
> > else
> >     {
> >       /* this is the default return value: we cannot know if a class
> >        * is going to paint outside its allocation, so we take the
> >        * conservative approach.
> >        */
> >       res = FALSE;
> >     }
>
>
> I can't point the C++ vfuncs to the _real_ vfuncs (as is done in 
> clutter_actor_class_init) because they're declared in the .c files, 
> not the public header.
>
> Ian