Re: Faster checking of userdata

[Edgar Toernig <froese@...>]

Fabio Mascarenhas wrote:
>
> A safe alternative, that does not use strcmp, [...]

I think one of the fastest methods is to store the metatable
as an upvalue in all Cclosures that make use of the userdata
and then use lua_rawequal for checking:

void *
my_checkudata(lua_State *L, int idx, int metaidx /*not rel*/)
{
    if (lua_getmetatable(L, idx))
    {
        int ok = lua_rawequal(L, -1, metaidx);

        lua_pop(L, 1);

        if (ok)
            return lua_touserdata(L, idx);
    }
    return NULL;
}

#define META_FOO lua_upvalueindex(1)

    struct foo *p = my_checkudata(L, META_FOO);
    ...

    struct foo *p = lua_newuserdata(L, sizeof foo);
    lua_pushvalue(L, META_FOO);
    lua_setmetatable(L, -2);
    ...

All operations are cheap - no table lookups or string handling.

Of course this method has a downside: managing which closures
need which upvalues may become a nuisance...

Ciao, ET.