Re: goocanvasmm: Item::get_items_at API

[Armin Burgmeier <armin arbur net>]

On Tue, 2008-03-11 at 18:50 +0100, Murray Cumming wrote:
> On Tue, 2008-03-11 at 17:19 +0100, Armin Burgmeier 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.
> 
> Can we copy the GList* to avoid that problem?

Hm, yes, I think so. It's obviously not as efficient, but it's probably
better to stay consistent.

> > 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
> > 
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