Re: Light User Data

["Evan Wies" <evan@...>]

Thanks for the comprehensive answer!

I'm still figuring whether I should use light userdata or ordinary
userdata.  Since I'm most interested in wrapping my C++ objects for use
in Lua, it seems like the ordinary userdata is what I need.

I'd appreciate feedback on this idea:
If I have C++ object system with RTTI (my own homegrown or C++'s) _AND_
I ensure that all the C++ objects I expose in Lua support RTTI, then I
get the typesafety and I can play around.

But, I guess the thing I'm still missing is the ability to call Lua
functions that are tied to an object's type (i.e., invoke methods on the
light userdata's pointer), since there is no metatable.

-Evan


"Edgar Toernig" <froese@gmx.de> wrote in message
news:<3D76B77F.2AD6EA28@gmx.de>...
> Evan Wies wrote:
> > 
> > I am trying to understand the right way to use the light userdata 
> > (in Lua 5 of course).
> > 
> > As I understand it, light userdata is just a pointer value and has 
> > no metatable (since it's light) and tag (since it's Lua 5).
> 
> Right.  Like numbers.  In fact, formerly, numbers were used to store 
> C-pointers but it's not save to store pointers in floats.  So the 
> light userdata was added as a way to store C-pointers within Lua.
> 
> You need them any time you want to:
> 
>  - map C-pointers to a Lua object.  I.e. you use the registry
>    as the mapping table: KEY is the light userdata with your
>    C-pointer and VALUE is the corresponding Lua object:
> 
>      create mapping:
> 	lua_pushlightuserdata(L, <c-pointer>);
>         lua_pushvalue(L, <index-of-lua-object>);
>         lua_rawset(L, LUA_REGISTRYINDEX);
> 
>      fetch object:
>         lua_pushlightuserdata(L, <c-pointer>);
>         lua_rawget(L, LUA_REGISTRYINDEX);
>         <object on stack>
> 
>  - pass C-pointers to functions called from Lua.  You push your
>    pointer as a light userdata and attach that as an upvalue to
>    your function.
> 
>       create function:
>         lua_pushlightuserdata(L, <c-pointer>);
>         lua_pushcclosure(L, <function>, 1);
>         <function on stack>
> 
>       fetch within function:
>         void *handle = lua_touserdata(L, lua_upvalueindex(1));
> 
> 
> > But, since it lacks any tags, how are you supposed to do anything 
> > typesafe with this value?
> 
> Make sure that you cannot get the wrong type ;-)
> 
> <b>As a rule: never export light userdata to Lua scripts!</b>
> 
> Use them within your C code and never make them visible outside of 
> your module.  If you do, a Lua script may bomb your system.
> 
> Ciao, ET.
> 
> 
> Obbug: int and longs are not save to store in floats, too...
>