Re: generic refcounted object perl binding

[Torsten Schoenfeld <kaffeetisch gmx de>]

On 26.10.2011 07:49, YangXi wrote:
> Another question about wrapper design, in function gperl_new_object()
> where an new wrapper is newly created:
>
> ......
> obj = (SV *)g_object_get_qdata (object, wrapper_quark);
>
> if (!obj) {
> ......
> // created a hash
> obj = (SV *)newHV () ;
> _gperl_attach_mg (obj, object);
> g_object_ref (object);
>
> // created a SV wrapper for the hash
> sv = newRV_noinc (obj);
> sv_bless (sv, stash);
> ......
> }
> ......
>
> Why don't bless the HV directly, but instead creating another SV
> wrapping the HV, and bless that SV instead?

That is how references work at the perl C API level, but also kind of 
how they work in Perl directly.  A reference to something is a separate 
SV (with its own refcount, etc.), just like in Perl, where you can take 
multiple references to the same variable.  And if you to put something 
into some namespace, you have to bless the reference, not the referent.

> Why use noinc to create the wrapper? Shouldn't the SV wrapper holding a
> reference for HV?

This is a common pattern when constructing references: we use 
newRV_noinc as opposed to newRV because 'obj' from above is a new HV and 
we want 'sv' to take ownership of the initial refcount.  Thus, when 'sv' 
goes out of scope it would normally take 'obj' with it.  In Glib::Object 
this is a bit more complicated because we want 'obj' to survive so that 
we can find it later and hand it out again.  This is done by 
incrementing its refcount in Glib::Object::DESTROY (which is called when 
'sv' goes out of scope).

I wanted to link to some XS tutorial covering all this, but the example 
at <http://perldoc.perl.org/perlxstut.html#EXAMPLE-6> actually uses a 
different pattern to achieve the same thing: it mortalizes both the 
referent and the reference, so when the xsub returns both end up having 
refcount=1 as well.  (The sv_2mortal call for the reference is hidden in 
the 'SV*' typemap.)