Re: caching userdata pointers, lua_pushuserdata?

[Chris Marrin <chris@...>]

Kriss wrote:
> Mark Hamburg wrote:
>
> > The safe way to do this is to maintain a weak table mapping the userdata
> > pointers as light userdata to the full userdata objects. Pushing then
> > becomes something like the following:
> >
> >    lua_pushlightuserdata( L, &myUDMapKey );
> >    lua_gettable( L, LUA_REGISTRYINDEX );
> >    lua_pushlightuserdata( L, theUserData );
> >    lua_gettable( L, -2 );
> >    lua_remove( L, -2 );
> >
> > (I used a light userdata rather than an integer to key the registry 
> > because
> > it's easier to guarantee unique.)
> >  
> Thanx, that gave me a better idea for how to handle things in my case.
>
> I can key my table with light userdata of the same address as the real 
> user data. Easy to lookup from the c side, guaranteed unique keys. A 
> simple function to return a light userdata with the same address of any 
> given userdata will keep it accessible from the lua side when all I have 
> is a userdata to hand.
>
> Much simpler.

I did it this way originally and found that the overhead was too high 
for my app. I am using Lua to interact with a C++ object system, and the 
Lua userdata is my Lua aspect of a C++ object. That means it gets pushed 
a lot, so I wanted it to be as efficient as possible. I am using 
essentially the code above. This is safe for me because the userdata has 
a metatable with a __gc metamethod. So I know when it is getting 
collected and can clean up any outstanding references to it.

--
chris marrin
chris@marrin.com