[opengoop] "Variant To Data" downcasts Class Type Inheritance Refnums

[Jim K. <ji...@ji...>]

Based on Jean-Pierre's comments about using "Variant to Data" for downcasting 
an inheritable data structure, I realized that "Variant to Data" can be used 
for downcasting Class Type Inheritance Refnums (analagous to "To More 
Specific Class" fuction).  It will even output an error at run-time (Error 
91, "The data type of the variant is not compatible with the data type wired 
to the type input."), if the class information in the variant does not match 
the more specific type.  For example, this prevents one from trying to 
Downcast to a class from the classes uncle (Car ==> DeLorian is OK, but Truck 
==> Delorian produces an ERROR).  Adopting this technique means that a class 
does not have to provide a polymorphic VI for downcasting to each descendant 
type.  The only downside, is that this method does not rely on the object 
reference to check if the object is really of the more specific type.  
However, this check will have to be done in any downstream VI that operates 
on the more specific type.

-Jim