Adding lightuserdatas that are invisible to scripts

["Peter Pimley" <Peter.Pimley@...>]

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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