Re: Proxying in Lua 5.1 vs Lua 5.2 (Was: Re: Possible bug with the length operator)

[Henk Boom <henk@...>]

On 1 April 2011 14:10, Jan Behrens <public@magnetkern.de> wrote:
>> On Fri, Apr 1, 2011 at 08:58, Julien Duminil <julien.duminil@ubisoft.com> wrote:
>> > But I was wondering if __len and __newindex are enough to easily emulate
>> > all four of these options, as you can't detect adding some "holes" in
>> > your table. So here is my question: would it be interesting to add a
>> > __delindex or __remindex metamethod called when an existing table
>> > index/key is setted to nil?
>
> On Friday 01 April 2011 17:28:57 HyperHacker wrote:
>> __newindex is called even if the value being set is nil. The problem
>> is that __newindex isn't called if the key already exists (presumably
>> for optimization). So you need to use __index and __newindex to proxy
>> all accesses to a hidden table somewhere so that you can catch those
>> nil assignments. Then you're free to handle "holes" however you like.
>
>
> One way to do it is this one:
>
> do
>  local data = setmetatable({}, {__mode = "k"})  -- ephemeron table since Lua 5.2
>  local mt = {
>    __index = function(t, k)
>      return data[t][k]
>    end,
>    __newindex = function(t, k, v)
>      if v == nil then
>        print("NOTICE: Deleted key " .. tostring(k) .. " in (proxy) table " .. tostring(t))
>      end
>      data[t][k] = v
>    end,
>    -- TODO: add __len, __pairs, __ipairs as needed (needs Lua 5.2)
>  }
>  function makeproxy(t)  -- NOTE: don't mix with undocumented function "newproxy"
>    local proxy = setmetatable({}, mt)
>    data[proxy] = {}
>    for k, v in pairs(t) do
>      data[proxy][k] = v
>    end
>    return proxy
>  end
> end
>
>
> The results are as follows:
>
> Lua 5.1.4  Copyright (C) 1994-2008 Lua.org, PUC-Rio
>> t = makeproxy{"Hello", "World"}
>> t[1] = nil
> NOTICE: Deleted key 1 in (proxy) table table: 0x800e12a00
>
>
> If I understand correctly, there are two problems with this
> approach in Lua 5.1, which will BOTH be solved in Lua 5.2:
>
> a) Lua 5.1 supports no ephemeron tables, which means that
>   t[1] = t; t = nil would cause a memory leak, as there are
>   remaining references to t in the proxied table.
> b) The metamethod __len does not work for tables and the
>   metamethods __pairs and __ipairs do not exist in Lua 5.1.

What about next(t)?

    henk