Re: "Hidden" data in Lua objects

[Henk Boom <henk@...>]

On 21 December 2010 10:59, Marc Balmer <marc@msys.ch> wrote:
> In Lua, I create a hierarchy of "objects" (Lua tables) that finally
> represent a GUI widget hierarchy.
>
> Now I want to somehow store C data in these objects, or link them with
> such information.
>
> E.g. I have to store the Widget pointer (a C lightuserdata value) for
> each widget represented in the hierarchy.
>
> What is a good way to do this?  My adventourous approach at the moment
> is to create an additional table when an object is created, store my
> values in this additional table and add it as metatable to the created
> object...

I'd start by saying that the easiest way is to simply store the values
a a field on the table. Something hidden-looking (like ._c_data) might
be good enough for your use.

A more insulated way is to use a weak-keyed table, similar to what you
were suggesting. Something like this:

local data_table = setmetatable({}, {__mode='k'})
function attach_value(table, data)
  data_table[table] = data
end

function get_value(table)
  return data_table[table]
end

If the table is garbage collected, its entry is automatically  removed
from the table. The downside to this approach is that if 'data' is an
object which refers directly or indirectly to 'table', they will never
be collected. If it's a lightuserdata though there shouldn't be a
problem. In Lua 5.2 this issue is supposed to be completely resolved.

    henk