light userdata (was: Lua 5 and tags)

[Wim Couwenberg <debosberg@...>]

> If not, how can we use them since there are 
> absolutely no way we could test the type of
> these light userdata without metatable nor tag.

I tend to use light userdata in the following
situations:

1) As key for the registry.  This is a very convenient
way to make sure your registry keys are private.  An
example:

    instead of:

    /* retrieve private data */
    lua_pushstring(L, "mykey");
    lua_gettable(L, LUA_REGISTRYINDEX);

    I do it like this:

    /* in file scope: */
    static char mykey[] = "mykey";

    /* retrieve private data */
    lua_pushlightuserdata(L, mykey);
    lua_gettable(L, LUA_REGISTRYINDEX);


2) If garbage collection is not an issue you can
implement types purely in scripts and add the actual
C/C++ object as light userdata, e.g. by assigning it
to a "__data" key or such.  This can save some hassle
to create, administrate and access user data.

--
Wim



		
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