Re: Clarification for the application of GVariant/Glib::Variant maybe types

[José Alburquerque <jaalburquerque gmail com>]

On Fri, 2011-10-07 at 15:05 -0400, José Alburquerque wrote:
> On Fri, 2011-10-07 at 14:54 -0400, José Alburquerque wrote:
> > On Fri, 2011-10-07 at 12:54 +0200, Murray Cumming wrote:
> > > On Fri, 2011-10-07 at 09:29 +0200, Markus Elfring wrote:
> > > > By which (member) function can such a variable be marked to contain an
> > > > unknown
> > > > value?
> > > > (Do I overlook a programming interface once more?)
> > > > http://developer.gnome.org/glibmm/2.30/classGlib_1_1VariantBase.html
> > > 
> > > It looks like we don't have any API yet to create a "maybe"
> > > Glib::Variant. The C API is g_variant_new_maybe() but we don't seem to
> > > use that yet. I don't know how we would support this in a template
> > > specialization.
> > 
> > One possible way might be to add a create_maybe() method to
> > Glib::VariantContainerBase that could use the
> > Glib::VariantBase::get_type() method of its children to create the maybe
> > type, but I'm not sure it would work.  I guess I can try and add it for
> > glibmm-2.32 and see if it works.
> 
> Though as you point out, Murray, it isn't as easy as it sounds because a
> create method should ideally return the type of the derived class.  I'll
> try to give it a little thought and see if something is possible for
> 2.32.

Last clarification:  A create_maybe() as described above in
Glib::VariantContainerBase should suffice.  The method declaration might
look something like:

VariantContainerBase create_maybe(const Glib::VariantBase& child);

It could use the Glib::VariantBase::get_type() method of the child to
call g_variant_new_maybe() like so:

GVariant* g_variant = g_variant_new_maybe(child.get_type().gobj(),
const_cast<GVariant*>(child.gobj()));

And then return a VariantContainerBase from the g_variant.

I don't know why I was thinking the new method return type should be the
type of one of the derived classes of VariantContainerBase.

> 
> > 
> > > 
> > > Note that GVariant generally doesn't have set functions. It's meant to
> > > be an invariant type. That probably makes it unsuitable for general use
> > > anyway.
> > > 
> > 
> 

-- 
José