Re: C API question

[Hakki Dogusan <dogusanh@...>]

Hi,

07/01/2011 17:59, Roberto Ierusalimschy wrote:
> > I would be happy to see refs go away because they seem like a sharp,
> > tempting feature that actually doesn't add value.
> >
> > If your userdata object needs to reference other Lua data, you
> > really, really, really want to use the environment mechanism because
> > this works with the GC whereas refs are very prone to creating
> > uncollectable cycles.
> >
> > If your C code needs to reference a Lua value, I tend to recommend
> > using light userdata keys in the registry bearing in mind that this
> > creates anchors for the GC.
>
> Exactly. The archetypal use of references were a full userdata keeping
> references to other Lua values by keeping in its C structure the
> integers returned by luaL_ref. The finalizer for this userdata should be
> responsible to release its references when the userdata was collected.
> This kind of use became obsolete with environments.
>
> Does anyone still use references in real applications? For what?

( You may say to do my homework, I won't angry :) )

I'm using luna(r) -with Ignacio Burgueño's additions- for some of my 
bindings:

I found these usages in my code:

// create and push singleton
    luaopen_pxproxy(L);
    pimpl_ = new pxProxyLuaImpl(L);
    /*mtix =*/ Luna<pxProxyLuaImpl>::push(L, pimpl_);
    refimpl_ = luaL_ref(L, LUA_REGISTRYINDEX);
    lua_rawgeti(L, LUA_REGISTRYINDEX, refimpl_);
    lua_setglobal(L, "proxy");
    ....
    ....
    luaL_unref(L, LUA_REGISTRYINDEX, refimpl_);
    lua_gc(L, LUA_GCCOLLECT, 0);
    delete pimpl_;

// this is for an event handler imlemented in Lua:
// proxy.on_paint = function(evt) pxManager:on_paint(evt) end

LCB_IMPL_SET(pxProxyLuaImpl, on_paint)
{
    unref(ref_on_paint_);
    luaL_checktype(L, 1, LUA_TFUNCTION);
    lua_pushvalue(L, 1); //dup
    ref_on_paint_ = luaL_ref(L, LUA_REGISTRYINDEX);
    return 0;
}

// calling handler from host:
    lua_rawgeti(L, LUA_REGISTRYINDEX, ref_on_paint_);
    lua_rawgeti(L, LUA_REGISTRYINDEX, ref_event_);
    int rc = lua_pcall(L, 1, 0, 0); // 1-in,0-out


I should note that I'm doing -mostly- copy/paste programming here, ie. 
above usage is not my invention :)

I'm open to suggestions...

> -- Roberto

--
Regards,
Hakki Dogusan
http://www.dynaset.org/dogusanh/