reference to a userdata

(light_)userdata reference
class userdata : public table;

class light_userdata : public table;

These types are meant to hold a reference to a (light) userdata from Lua and make it easy to push an existing userdata onto the stack. It is essentially identical to table in every way, just with a definitive C++ type that ensures the type is some form of userdata (helpful for trapping type errors with safety features turned on). You can also use its .is<T>() and .as<T>() methods to check if its of a specific type and retrieve that type, respectively.