luaL_checkudata vs. polymorphism

[Uli Kusterer <witness.of.teachtext@...>]

Hi,

 I still don't quite believe how well this is going. I've made great  
inroads integrating Lua and my C++ object model. However, I have one  
small point where, while the code works, I'm not quite happy with my  
solution: I have a C++ class UKFrame that displays an image, and I  
have a specialisation of this C++ class, UKTextFrame, whose image is  
a rendered text string. Currently, they're exported to Lua as two  
entirely separate classes. This has the downside that one other  
method's addFrame() method has to explicitly use luaL_checkudata()  
twice to find out whether one of its parameters is a UKTextFrame or a  
UKFrame, so it can treat them the same. (This works for now, but in  
the future there may be many more frame types, and I'd like to avoid  
having to manually "polymorph" all of them)

 I'm not quite comfortable with the whole __index and metatable  
stuff, and how they interact with luaL_checkudata(). Looking at the  
sources, I get the impression that luaL_checkudata() returns NULL for  
subclasses of the requested type. Is there a facility to do the  
equivalent of luaL_checkudata() but also look at metatables higher  
up? Am I even making sense? Am I mixing up __index and the metatable  
again?

 Do any of you have suggestions on how I should model this sort of  
polymorphism in Lua?

Cheers,
-- M. Uli Kusterer
http://www.zathras.de