Re: Guile GC in C++

[Jonas Hahnfeld]

Am Dienstag, dem 06.07.2021 um 17:03 +0200 schrieb David Kastrup:
> Jean Abou Samra <jean@abou-samra.fr> writes:
> 
> >  For example, if I were the author of the below code, how 
> >  would I understand that the mmrest_event_ should be 
> >  unprotected? 
> > 
> >  void 
> >  Part_combine_iterator::kill_mmrest (Context *c) 
> >  { 
> >  if (!mmrest_event_) 
> >  { 
> >  mmrest_event_ = new Stream_event 
> >  (Lily::ly_make_event_class (ly_symbol2scm ("multi-measure-rest-event"))); 
> >  set_property (mmrest_event_, "duration", SCM_EOL); 
> >  mmrest_event_->unprotect (); 
> >  } 
> > 
> >  c->event_source ()->broadcast (mmrest_event_); 
> >  } 
> 
> Uh, after the call to mmrest_event_->unprotect () there is nothing that
> would protect the newly created mmrest_event_ .

Sure there is, the snippet is just too small to show the marking:

void
Part_combine_iterator::derived_mark () const
{
  Simultaneous_music_iterator::derived_mark ();

  if (mmrest_event_)
    scm_gc_mark (mmrest_event_->self_scm ());
}

The code makes perfect sense: If mmrest_event_ isn't set yet, the block
will allocate and unprotect a new object so that before the broadcast,
there is an event under the control of the iterator object for all
possible flows through the function.

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