TYPE luaQ_optqvariant<TYPE>(lua_State *L, int index, TYPE d = TYPE())
Returns the optional Qt variant of actual type TYPE
found at position index in the stack.  If the position
corresponds to an unspecified function argument, 
this function returns d.  If the Lua value at this position
cannot be converted to the requested type, this function
causes an error.
See also luaQ_checkqvariant