Re: The __newindex metamethod

[Axel Kittenberger <axkibe@...>]

It's all about speed.

Am 10.01.2011 20:28 schrieb "Dirk Laurie" <dpl@sun.ac.za>:
>
> The reference manual states:
> --
> "newindex": The indexing assignment table[key] = value.
> --
> The operation can be overridden by redefining __newindex
> in the metatable.  However, the overriding function is
> only invoked when table[key] does not yet exist.  Otherwise,
>    table[key]=value <==> rawset(table,key,value).
>
> This behaviour is documented in the reference manual by the
> Lua code for settable_event,
>         local v = rawget(table, key)
>         if v ~= nil then rawset(table, key, value); return end
>         h = metatable(table).__newindex
>         if h == nil then rawset(table, key, value); return end
> which corresponds closely to the C code for luaV_settable in lvm.c.
>
> I can't guess why the overriding function is not always invoked
> when available, i.e. simply
>         h = metatable(table).__newindex
>         if h == nil then rawset(table, key, value); return end
> unless it is because the user is not trusted to call rawset in
> the function supplied, which could cause dangling references.
>
> In the meantime, if I want this simpler behaviour, it seems that
> I will have to change the code in lvm.c and recompile Lua.
>
> Dirk
>