Re: light userdata (was Re: Indices in Lua)

["Juris Kalnins" <juris@...>]

> What would be the justification for doing so?

For example, storing additional tag information for implementing
lightweight built-in objects. Useful on embedded applications
that do not require C level library compatibility, and
that have sizeof(lua_Number) > sizeof(void *).

Each Value cell has sizeof(lua_Number) storage, which is
8 bytes on most 32 bit Lua configurations, but light userdata
uses only 4 bytes.

> lua_touserdata relies on
> the fact that full UD and light UD can both be a void*, so if light
> UDs are typedeffed to something else, then the function is no longer
> valid for light UD.

you can have user redefinable macro:
#define luai_lu2p(x) ((void *) (x))

but yes, additional lua_tolightuserdata would be needed...