Self-referring userdata

[Daan Nusman <d.nusman@...>]

Hi folks,

I'm wondering if there's a better way of doing this:

in a C userdata object, I want to call some Lua funcion with the same C 
userdata as parameter. AFAIK, for this to work I need a reference to the 
userdata, which I create when the userdata is created and then keep in 
the userdata itself. Because this is a cyclic reference, I must create 
an extra weak-valued registry to keep all those references in.

Is there a better way? In other words, can I push the userdata on the 
stack given only the pointer to that userdata? That would be nnnnnice.

Extended example (broken code, this is meant as an illustration):

-------- Definition -----------
struct NuggetHandle
{
    int m_SelfRef;            // Reference to self userdata. This is 
stored in the weak self-ref registry in m_Lib, NOT in LUA_REGISTRYINDEX! 
This makes sure the ref is removed
// moar data
};

-------- Creation of the object --------
    // Create new Lua Nugget object.
    NuggetHandle* newNugget = (NuggetHandle*)lua_newuserdata(L, someSize);
    luaL_getmetatable(L, NUGGET_LUA_TYPENAME);
    lua_setmetatable(L, -2);

    // Set self ref into weak registry
    lua_rawgeti(L, LUA_REGISTRYINDEX, 
nclass->m_Lib->m_SelfRefRegistry);     // get self ref reg
    lua_pushvalue(L, -2);
    newNugget->m_SelfRef = luaL_ref(L, -2);          // store into self 
ref registry
    lua_pop(L, 1);  // pop self ref registry


-------- Calling a callback ---------------
void NuggetHandle::passOnCallback(lua_State* L)
{
// note that we only have the userdata pointer here, for example we got
// called from a system-defined UI callback
    lua_pushSomeFunction(L);     // push some Lua callback function on 
the stack we want to call

    // Push self onto Lua stack:      ouch! get the weak registry, get 
the reference, pop weak registry
    lua_rawgeti(L, LUA_REGISTRYINDEX, m_Class->m_Lib->m_SelfRefRegistry);
    lua_rawgeti(L, -1, m_SelfRef);
    lua_remove(L, -2);

    lua_pushstring(L, "yay more params");
    lua_call(L, 2, 0);            // first param: reference to self 
userdata
}

Thanks,
Daan