Re: parent while in INIT_INSTANCE

[Kevin Ryde <user42 zip com au>]

Torsten Schoenfeld <kaffeetisch gmx de> writes:
> If you do, however, call $label->parent, then the marshalling code sees
> that the underlying GObject has a floating reference and takes over
> ownership of it (without adding a new reference).  The GObject now has
> two references to it, one from $self and one from $parent.  With the
> change under discussion, both of these get removed when INIT_INSTANCE
> exits, leading to the destruction of the GObject.

Hmm, yes, bad.

> So I have to ask: why are you calling $label->parent in INIT_INSTANCE
> instead of simply using $self?

I had two cases.  The first was merely sharing some code by calling an
"update_some_bits()" which would normally come out of a child signal
handler, but I called it from the parent INIT_INSTANCE because it did
what I wanted.

The second I struck later was under SET_PROPERTY of the initial property
settings as from ->new(foo=>123).  I propagate a parent property to the
child, as a form of "delegate", and in the child I had a "notify" signal
handler coming back and updating some display bits in the parent, and to
get the parent it used $self->parent.  This isn't a circular update,
since the parent bits were like "output only" reflecting the child state
when it changed either by program code or user interaction.

I rearranged one of my INIT_INSTANCE to avoid that part of the problem,
but I couldn't think any clean way to mangle the initial property
settings mechanism and still have later child changes update the parent
the way I wanted.

I wondered that if almost anything returning the widget currently under
construction would suffer this reference sinking trouble, not just
$child->parent().

Dunno if there's much in Glib alone to construct a failing case by
returning the object under construction, but with gtk there's lots of
ways to return a widget.  So if it's almost any return then that could
make the problem rather subtle and rather hard to avoid for a naive
widget writer.




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