Re: Lua 5 and tags

[Alex Sandro Queiroz e Silva <ventonegro@...>]

Hallo,

Vincent Penne wrote:
 > But with lua 5, there aren't any more tags, so how do I check easily
> what is the type of my userdata ?  I know I could look into the 
> methatable for some custom entry that I would call "type" for example, 
> but it looks overly complicated and inefficient. Furthermore, there 
> aren't any unified way to generate new type values (like the lua_newtag 
> function) so two libraries may accidentally generate two different kind 
> of userdata with the same "type" value in their metatable, so it would 
> not be safe either. Probably I have missed something ...

	You can say two objects (userdata or tables) are of the same type if 
they share the same metatable. If want to refer to your types by name, 
you can create entries in the global namespace using the type name as a 
key and the metatable as a value. I guess luaL_newmetatable and 
luaL_getmetatable do just that.

> Another question : I see that there are light userdata, which are much 
> like the userdata in lua 4, except that they don't have tags. The 
> documentation doesn't seem clear about these light userdata, can I 
> associate a metatable to them or not ? If not, how can we use them since 
> there are absolutely no way we could test the type of these light 
> userdata without metatable nor tag.

	You can't associate metatables with light userdata, but they are useful 
in other contexts. The I/O standard library stores FILE* pointers in 
light user data.

	Now that Programming in Lua is online (http://www.lua.org/pil/), it's 
definitely a excellent read for anyone studying Lua.

-alex