[osg-users] dynamic_cast equivalent in osgDotNet

Michael Wittman wittman at acm.org
Wed Oct 3 21:16:25 PDT 2007

Hi Robert,

On Wed, Oct 03, 2007 at 09:11:21AM +0100, Robert Osfield wrote:
> On 10/3/07, Michael Wittman <wittman at acm.org> wrote:
> > It's possible to add support to osgDotNet for upcasts along non-leftmost
> > inheritance branches.  But downcasting in the same situation is a much
> > harder problem, and I don't know of a good solution for it at the moment.
> Perhaps we could add a GUIActionAdapter:asView() virtual function that
> returns a osg::View pointer.
> Another possibility would be to have a asViewer() and
> asCompositeViewer() methods too, but we'd have to forward declare
> osgViewer::Viewer and osgViewer::CompositeViewer to achieve this.  I'm
> not sure what the consequences for  the wrappers would be with this
> though.

Unfortunately I don't think those approaches would help, as there's no
analogue to C++'s incomplete types in the CLR.  And having knowledge of
full osgViewer wrapper types in the osgGA wrappers would necessitate a
circular dependency between the two assemblies, which is not permitted
(nor wise, in any case).

I'm not entirely sure it would work, but best approach I can think of
would be implemented completely within the wrappers.  It would involve
a registry of downcast conversion functions for every inheriting type
from all the types in its inheritance tree, indexed off the CLR's
type IDs for the inheriting and inherited types.  A separate function
would then be provided to index into the registry and apply the correct
conversion function based on the CLR type IDs.  The invoking code would
look something like this:

  OsgGA.GUIActionAdapter aa;
  OsgViewer.Viewer v = OsgWrapper.Convert.Downcast(aa.GetType(), typeof(OsgViewer.Viewer)) as OsgViewer.Viewer;

One thing that's not clear to me currently is how to reliably construct
a unified conversion function registry across multiple wrapper assemblies.


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