Re: How reliable is lua_topointer() ?

[Patrick Donnelly <batrick@...>]

Hi Lawrie,

On Tue, Aug 11, 2009 at 2:56 AM, Lawrie Nichols<lawrien@gmail.com> wrote:
> I'm looking at using the pointer returned from lua_topointer() as an index
> into a weak-keyed table, but it's not obvious whether Lua might ever change
> these pointers during the execution of my scripts (ie whether a gc cycle
> might moving things around internally).
>
> I'm guessing that for userdata, ghe pointer cannot be changed (since Lua has
> no way of knowing the relevance of the data pointed to in the userdata), but
> I'm not so sure whether this would hold true for functions/tables.
>
> I've had a look at the PiL on lua_topointer(), and all that says is that the
> pointer cannot be used to to retrieve what was being pointed to.
>
> To be honest, from trying it out, there doesn't seem to be any issues, but
> I'd like to be sure.

I have not encountered an instance where lua_topointer is necessary.
You can instead use luaL_ref and luaL_unref for an integer reference
into the lua registry. Instead of pushing your light userdata with the
void pointer you stored, you can use this:

lua_rawgeti(L, LUA_REGISTRYINDEX, index);

-- 
-Patrick Donnelly

"Let all men know thee, but no man know thee thoroughly: Men freely
ford that see the shallows."

- Benjamin Franklin