Fast determination of userdata type

[Shaun <delius@...>]

Hi Guys,

Just wondering if there is any collected wisdom about the fastest/nicest 
way of determining the type of userdata passed into a C function (from 
LUA).

For example, assuming I have a function that can accept three different 
types of userdata (which are actually C++ objects), how do you generally 
go about determining which of the three types it is efficiently?

The most obvious idea seems to be:

   if (!lua_getmetatable(L, object))
      return(NULL);

   luaL_getmetatable(L, "Class");

   int ok = lua_rawequal(L, -1, -2);

   lua_pop(L, 2);

   if (!ok)
      return(NULL);

Not exactly fast. I'm thinking some sort of shim that adds a class 
identifier to the start of the userdata would be much faster but will make 
my implementation a pain. Any other ideas?

Cheers,
Shaun