Re: The __newindex metamethod

[HyperHacker <hyperhacker@...>]

On Mon, Jan 10, 2011 at 14:59, Axel Kittenberger <axkibe@gmail.com> wrote:
> 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
>>
>

There is a patch to do something like this (__usedindex), but I
imagine the performance hit is rather noticeable.

-- 
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