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

[Tim Hill <drtimhill@...>]

That's really what the Lua Registry is for, I think. And the reason you use the Registry as a way to do this is so that the GC can "see" the values so that it can correctly handle reachability. If you wanted to have C truly act as a "carrier" for Lua values you would need to restrict that to non-reference types or persist the reference types in some way (which is messy).

--Tim

On 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.