Re: Suggestion: lua_popinteger and others

[Rena <hyperhacker@...>]

On Tue, Apr 24, 2012 at 03:14, Drake Wilson <drake@dasyatidae.net> wrote:
> Quoth Rena <hyperhacker@gmail.com>, on 2012-04-24 01:00:22 -0600:
>> Right now if you want to retrieve a field using the C API, it takes
>> three lines per field:
>> lua_getfield(L, index, "field name");
>> lua_Integer val = lua_tointeger(L, -1);
>> lua_pop(L, 1);
>
> FWLIW, this isn't generally what I do if there are multiple fields
> around (and indeed I tend not to use lua_pop at all, because it's easy
> for it to get desynchronized with the surrounding code).
>
> I would tend to do something like:
>
>  int const orig_top = lua_gettop(L);
>  double const foo = (lua_getfield(L, idx, "foo"), lua_tonumber(L, -1));
>  char const *const bar = (lua_getfield(L, idx, "bar"), lua_tostring(L, -1));
>  bool const baz = (lua_getfield(L, idx, "baz"), lua_toboolean(L, -1));
>  [processing...]
>  lua_settop(L, orig_top);
>
> and those preamble/postamble lines only appear if necessary; if I'm
> about to return from the C function anyway and don't have any other
> reason to reclaim the space, I just let the stackframe teardown clear
> away all the extra values.  If there's a lot of values you might need
> a luaL_checkstack in front; if there's a number of values that is not
> bounded by a small enough constant at compile time, or you're doing a
> lot of processing afterwards, then intermediary cleanup stages become
> useful.
>
> Note that the lifetime of a string pointer isn't guaranteed to extend
> beyond its being pinned by the C-function Lua stack anyway, so
> grabbing a string data pointer and then immediately popping the value
> off is theoretically dangerous (in 5.1; the canonical implementation
> doesn't use a moving GC last I checked, but the manual doesn't forbid
> it).
>
>   ---> Drake Wilson
>

True, I guess lua_popstring() wouldn't be able to exist, or would have
to assure that another reference to the string exists somewhere. (Or
leave it to you to worry about that. :-))

-- 
Sent from my toaster.