C++/Lua oop gotcha...

[Ando Sonenblick <ando@...>]

Gang,

Just wanted to share my own 2 cents on the less-than-ideal way I 
happened to choose to implement my C++/Lua Object Oriented programming 
integration...

I have my C++ objects.
I use userdata to have these c++ object cross-over into lua-land.
These userdatas have metatables so that methods of my C++ object can be 
called from lua so scripts.

All was good up until I decided to allow scripts to create new fields 
in the userdata objects.

Let me explain:  Say I have a sprite C++ object with two intrinsic data 
fields: location and visibility.  From lua via the metatable on the 
userdata a script can do things like sprite:SetVisibility(..) or 
sprite:OffsetLocation(...) and so on.

But I then needed the flexibility for scripts to add their own fields 
for the script's own use:   sprite.isTankSprite = true

To do this, I have had to go through some unfortunate hoops.  I now 
have a global table that can contain a custom table for each of my 
userdata's.
I intercept the newIndex call to create a secondary table and replicate 
the variable being created in the table.  Then have to in the index 
call of the metatable also search the global table for a custom table 
that corresponds to the userdata and if its there, search it.

I of course also have to worry about watching for the deletion of the 
userdata and dispose of its associated custom table...

Definitely a pretty nasty corner I painted myself into.   It's too late 
to change now (and it all works well so far), but had I this insight I 
probably would have just had my C++ objects have a table representing 
them with what's currently in the userdata memory directly in the table 
(it's only a ptr to the actual C++ object)...

Thought I'd share my situation with everyone just in case it helps 
anyone else thinking of heading down similar paths...

ando "wish I had Programming in Lua before I dove in" sonenblick
-----------------------
Ando Sonenblick
SpriTec Software
www.spritec.com