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Here's my check_udata function:

template <class T>
void * Udata<T> :: check_udata(lua_State *L, int ud)
	void *p = lua_touserdata(L, ud);
	if (p != NULL && lua_getmetatable(L, ud)) {
		lua_pushlightuserdata(L, (void *)T::name);
		lua_rawget(L, -2);
		if(!lua_isnil(L, -1)) {
			lua_pop(L, 2);
			return p;
		lua_pop(L, 2);

	return NULL;

T::name is a static const char * for the udata class.  I use it as a
unique ID for the userdata class.


On Fri, Feb 29, 2008 at 12:49 AM, Diego Nehab <> wrote:
> Hi,
>  As the Mike's example shows, if you are willing to be a
>  little less generic, you can store the metatable as an
>  upvalue or as the enviromnent for each function that must do
>  type checking for that class. That way, you save one rawget
>  during the checkudata call, which is by far the most common
>  case. I might do that, and offer two versions of checkudata.
>  The generic one taking the lightuserdata, and obtains the
>  metatable from it. The optimized one takes no extra
>  argument, and assumes instead that the expected metatable is
>  the environment.
>  Kind regards,
>  Diego