Re: Hashing userdata

[Matt Campbell <mattcampbell@...>]

OK, are both "key" and "v.key" references to the same Lua userdata? 
That is, do they both come from the same invocation of lua_newuserdata?

I'm guessing the answer is no.  What you need is a table, probably only 
referenced in the Lua registry, which maps C++ pointers (as Lua light 
userdata) to the corresponding full userdata.  This table should have 
weak values; that is, its metatable's "__mode" field should be set to 
"v".  Then, whenever your C++ code needs to send an object to Lua, it 
should first look up the object's pointer (again, as light userdata) in 
this cache table.  If a corresponding full userdata object is found, the 
C++ code should send that to Lua.  Otherwise, it should create a new 
full userdata object and map that to the C++ pointer in the cache table 
before sending it to Lua.  Then there will be only one Lua userdata 
object for each C++ object.

Matt