Re: [proposal] Fast way to access Lua value in C. (op: new "empty" value in Lua?)

[Coda Highland <chighland@...>]

On Mon, Jul 1, 2013 at 3:34 PM, Xavier Wang <weasley.wx@gmail.com> wrote:
> Hi,
>
> as Tim Him proposed, A "empty" value in lua table, makes Lua have a "array"
> can contain any values.
>
> But I thought more. there are something affects me a lot:
>   - we can not "store" lua value in C. we can get pointer of any Lua value,
> but we can not store it in C and get the lua value it point, currently we
> can do it with a table to store the map with pointer and value.
>   - we can not get a cheap data structure with a/some lua values. sometimes
> we use table, it's 24 byte without any values. we just want a non-type
> container, just can hold all values we want, just like a special userdata.
>
> there is a proposal can solve all three (or more) issue: add a type only
> seen in C or other thing, it's just used to hold Lua values. just like the
> current Lua stack (internals, Roberto can implement Lua stack with this
> value).
>
> we can have several APIs to maintain this data-structure, just like this:
> lua_Values lua_newvalues(lua_State *L, size_t sz);
> void lua_reallocvalues(lua_State *L, lua_Values *v, size_t sz); /* sz == 0
> to free it */
> void lua_pushvalues(lua_State *L, lua_Values *v);
> void lua_valuesadd(lua_State *L, int idx, lua_Values *v);
> void lua_valuesset(lua_State *L, lua_Values *v, int idx);
>
> lua_Values has a defined length. the big difference between lua_Values and
> other thing is, it's just a container of some Lua value, so it can be
> changed: it's just a piece of memory that holds lua values, so we can grow
> it or shrink it.
>
> with lua_Values, we can get a fast way to hold lua value in C program, we
> can implement cheap data structure that can hold all type of Lua value, and
> we can implement a true array userdata in C, and makes it a standard library
> of Lua.
>
> local a = array.new() -- alloc a 0 size of array
> local a10 = array.new(10) -- alloc a 10 values of array
> a[100] = nil
> print(#a) --> 100
>
> it's not a type of lua, it just something only seen in C program, just a
> structure like this:
>
> struct lua_Values {
>     size_t n;
>     TValue values[1];
> };
>
> to make a array object, you need a userdata value, and hold the pointer to
> lua_Values.
>
> some things we need to solve, how to collect garbage in lua_Values? is there
> other thing we need thought?
>
> hope for comments about this idea.
>
> --
> regards,
> Xavier Wang.

There's a reason Lua doesn't expose this right now: refcounting.

Python offers this functionality and you have to be extremely cautious
in incrementing and decrementing the refcount on the object. It's a
pain in the butt. One wrong increment/decrement and you're crashing or
leaking.

The usual idiom here is to use luaL_ref() to put a reference to the
object into the registry, and then you have a nice little handle that
manages a refcount.

/s/ Adam