Re: How to approach this?

[Philipp Janda <siffiejoe@...>]

Am 16.09.2014 um 15:58 schröbte Andrew Starks:
> On Tue, Sep 16, 2014 at 7:42 AM, Dirk Schippers <lua@frixx-it.com> wrote:
>
> > I don't like the Lua registry (especially because of the keys that should
> > be carefully chosen to not mess things up - I use a lightuserdata adressed
> > to some static C variable as key - ), but I think I needed it for this.

That's a reasonable way to do it (and a perfect example of where to use 
lightuserdata).

> [...] 5.1 doesn't have `uservalue`s.

They are called "environments" in Lua 5.1. And that's what I would 
recommend (instead of a per-instance metatable which doesn't work with 
lightuserdata anyway). There is a storage hierarchy in Lua where you can 
store stuff:

1) Lua state local: registry (private) or globals table (public)
2) module local: function upvalues (private)
3) (userdata-)type local: metatable (public or private)
4) (userdata-)instance local: environment/uservalue (private)

Nothing prevents you from using a per-instance metatable, but there are 
some things in Lua that work better with a per-type metatable (e.g. some 
relational operators, and `luaL_checkudata` etc.). And of course you can 
always go one level higher by using a table with suitable keys ...

Now about those lightuserdata: I know "lightuserdata" sounds much nicer 
and "lighter" than full userdata, but in 90% you want full userdata. 
Lightuserdata are only useful/safe under very special circumstances. If 
used outside of those cases, they have the following problems:

1) You have to implement your own type checking (i.e. `luaL_checkudata` 
won't work for you). Lua can't distinguish between lightuserdata except 
by pointer value, so your functions must be able to determine that a 
lightuserdata really belongs to you by looking at the pointer value alone.
2) Lua doesn't manage the lifetime of the data pointed to by 
lightuserdata. That means that you can have lightuserdata to memory 
that's long gone, or memory that's already inaccessible by Lua (and you 
have no way of knowing about that).
3) Lightuserdata don't have environments/uservalues.
4) All lightuserdata share a single metatable. If you want to use more 
than one module which sets a metatable on lightuserdata, you are 
screwed. (Btw., that also means that all your windows will share the 
same `OnMouseMove` function!)


HTH,
Philipp