Re: goocanvasmm: Item::get_items_at API

["Jonathon Jongsma" <jonathon quotidian org>]

On 3/11/08, Armin Burgmeier <armin arbur net> wrote:
> I believe the current implementation of Item::get_items_at is wrong:
>
>  #m4 _CONVERSION(`GList*',`Glib::ListHandle< Glib::RefPtr<Item> >',`
>  $2($3, Glib::OWNERSHIP_NONE)')
>
>  _WRAP_METHOD(Glib::ListHandle< Glib::RefPtr<Item> > get_items_at(double
>  x, double y, const Cairo::RefPtr<Cairo::Context>& context, bool
>  is_pointer_event, bool parent_is_visible, Glib::ListHandle<
>  Glib::RefPtr<Item> >& found_items), goo_canvas_item_get_items_at)
>
>  Since the ownership of the returned ListHandle is set to NONE, the
>  actual GList* will be leaked. However, if we set shallow ownership, then
>  we get memory corruption because in the C API, the returned GList* is
>  meant to be the same as the one passed in, with perhaps some items added
>  to the front and thus returning a new list head.
>
>  If I get it right, then the ownership of the found_items ListHandle
>  needs to be set to none, and a new ListHandle with shallow ownership
>  needs to be returned. We can't do this though, probably, because the
>  ownership of the ListHandle is private.
>
>  I think the C++ way to handle this is to use an insert iterator such as
>
>  template<typename InsertIter>
>  InsertIter get_items_at(double x, double y, const
>  Cairo::RefPtr<Cairo::Context>& context, bool is_pointer_event, bool
>  parent_is_visible, InsertIter iter);
>
>  to be used like this:
>
>  std::vector<Glib::RefPtr<Item> > items;
>  some_item->get_items_at(..., std::inserter(items, items.end()));
>
>  This is also more powerful because items can be inserted anywhere
>  instead of just at the beginning. However, I don't think other *mm
>  projects do anything similar, which is why I am asking for opinions
>  first.
>
>  The corresponding vfunc still needs special consideration then, since
>  virtual functions cannot be templatized. Probably it is enough to just
>  pass a specific container, such as std::vector, since the only code
>  calling it is goocanvasmm itself anyway.
>
>  Any opinions? Do you think it is OK to change it this way, or do you
>  have other ideas to tackle this?
>
>  Armin

I haven't had time to read this proposal thoroughly yet, but I just
wanted to mention that when I was wrapping this function originally, I
thought it felt like a pretty poor API at the C level. So maybe it
would be best to try to improve the API at the C level before
attempting workarounds in the wrapper.  That said, I'll try to read
your proposal more thoroughly when I get a little more time and offer
comments as well.

-- 
jonner