Re: Table constructor question

[Marc Balmer <marc@...>]

Am 18.12.10 11:09, schrieb Drake Wilson:
> Quoth Marc Balmer <marc@msys.ch>, on 2010-12-18 10:51:08 +0100:
>> Indeed, when a new table is created on the hdf object, it's metatable is
>> set so that I can catch index/newindex.  What I am trying to accomplish
>> is to access a C datastructure (the HDF containers) using the Lua table
>> syntax (or at least as close as possible) without having to do deep
>> copies of the contents of (real) Lua tables.
>>
>> And it seems that the assignment in the table constructors don't call
>> newindex, so I end up with a (real) Lua table, but can not access the
>> values later using my methods.
> 
> No, of course they don't.  A Lua table constructor is a Lua table
> constructor.  You don't get control over the computation of the entire
> expression that just happens to be on the RHS of a table write that
> you've overridden with a newindex metamethod; you get the resulting
> value.  The code:
> 
>   t[x] = { c = d, e = f, g = h }
> 
> first creates the table on the RHS, then writes it into t[x].  Period.
> 
> So as I said, if you want to support that style, you have to iterate
> incoming tables to get any existing values out and treat them as
> additional writes.  Yes, you wind up creating an intermediary table
> that way, but the example you gave showed a three-element table, which
> is not a huge cost unless that's a sensitive inner loop.  (In the
> other case, you should add locals to cache the values of hdf.bset and
> hdf.bset[n], which would be slightly clearer anyway.)
> 
> If we're talking about 100x100 arrays or something, then it's unlikely
> that they're going to be specified with a table literal.  You can then
> provide an interface to create them (in whatever efficient form you
> like) unattached and then just unbox a userdata and transfer a
> reference in C when placing them in those slots.
> 
> Am I missing something here?

No precise to the point and I just found out that newindex gets the
table as third argument.  So I will now just do as you suggest, parse
the incoming object and handle it according to the object types.  That
should be fairly easy.

Thanks for your help!