Re: Wrapping C constructors.

[Murray Cumming <murrayc murrayc com>]

On Mon, 2007-05-21 at 03:11 -0500, Paul Davis wrote:
> Hey,
> 
> I went through and changed how I was doing all of the create methods
> for Goocanvas. I was having issues with a constructor and I seemed to
> remember Murray saying that I should hand-code constructors as opposed
> to hand-coding static create methods. Which makes quite a bit of sense
> if we want the ability to subclass the goocanvas objects.
> 
> The issue at hand is this: after calling any of the goocanvas new
> methods (ie, goo_canvas_rect_new(...)) we get an object that has a
> ref_count of 1.

You should almost never do this. You should use the _CONSTRUCT macro, so
that g_object_new() is used in the generated source code.

>  This result will almost always be put into a
> Glib::RefPtr. After the object is in the RefPtr, I would expect it to
> have a ref_count of two. Instead, they have a ref_count of 1.

A starting refcount of 2 would not be very useful. That would just leak
a reference when the RefPtr was destroyed.

> Some looking through the api has lead me to find that goocanvas always
> does an unref in the new function. I'm under the impression that this
> is to save C coders from calling unref on every object they create.

That doesn't make any sense to me. A new() function would not destroy
the object before returning it.

> In C++ land this presents a predicament. We have the one new object
> being referred to in two places (as a child of the parent object and
> from the Glib::RefPtr) and when the RefPtr goes out of scope the
> object gets destroyed. This leads to segfaults and other nasty errors.

If you can please mention some specific part of the API, I could take a
look.

> Looking through gtkmm source, I can't find an example like this. So
> almost every constructor has to be hand coded to be something along
> the lines of:
> 
> Group::Group( const Glib::RefPtr<Goocanvas::Item>& parent )
>   :
>     ItemSimple( GOO_CANVAS_ITEM_SIMPLE(goo_canvas_group_new(
> parent->gobj(), NULL ) ) )
> {
>   reference() ; //This is the un-good bit.
> }
> 
> That seems un-good. Is there a 'gmmproc' way to autogenerate this
> extra reference, or is this just a side effect of the way goocanvas is
> written?
> 
> ----
> 
> In other news, I've gotten the simple demo, a custom demo, and most of
> the first notebook page of the 'main' demo written and working. Things
> are coming along.
> 
> Paul Davis

-- 
Murray Cumming
murrayc murrayc com
www.murrayc.com
www.openismus.com