Re: Adding lightuserdatas that are invisible to scripts

["Peter Cawley" <lua@...>]

The registry (LUA_REGISTRYINDEX) is a table just like the globals
table. Unlike the globals table, it cannot be accessed from scripts
without using debug.getregistry().

2008/5/21 Peter Pimley <Peter.Pimley@zoemode.com>:
>
>
> Hello Lua enthusiasts :)
>
> I've been making quite light use of lua (from C/C++) on and off for a
> couple of years now and I love it to bits :)
>
> There are two things that I always wanted, but have never quite figured
> out how to do.  I'm sure they're probably quite easy.  Here's the first;
> I'll put the other in a separate mail.
>
> Quite often I'll create a lua_State, and at the same time I'll create
> some C structures or C++ objects.  Something along the lines of:
>
> lua_State * lua = luaL_newstate ();
> MyThing * thing = new MyThing ();
>
> Semantically, the C++ objects 'belong' to the lua_State.  I have more
> than one lua_State in my program, and I'd like each one to carry its
> associated C++ structures around with it.  However (and this is the
> important bit) I don't want any scripts that the lua_State executes to
> be able to access or modify the objects.  The C++ objects should be
> invisible to any lua scripts.
>
> What I've always done up to now is to make them global light user data
> variables, and give them names that are unlikely to clash with anything
> else.
>
> lua_pushlightuserdata (lua, thing);
> lua_setglobal (lua, "incredibly_unusual_variable_name");
>
> Later on a C function might called from lua, and by using lua_getglobal
> it can get at the C++ object.
>
> The obvious problem with this is that "unlikely to clash" is not good
> enough.  The following lua script might get executed:
>
> incredibly_unusual_variable_name = 42
>
> ... and then I'd be in trouble.
>
> So my question is:  Is there a more robust way of doing this?  I'd
> really like the values to be completely hidden from scripts.
>
>
> Thanks in advance,
> Peter Pimley
>
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