Re: advice needed: OO style

[Emmanuel Oga <emmanueloga@...>]

On Fri, Apr 29, 2011 at 1:13 AM, Lorenzo Donati
<lorenzodonatibz@interfree.it> wrote:
> On 28/04/2011 16.12, Mark Hamburg wrote:
>>
>> If your goal is keeping clients from accidentally messing with state, then
>> you can often deal with this with simple naming conventions -- e.g., private
>> fields start with _ or priv_ or whatever.
>>
>> If you are a little more paranoid and can live with somewhat more typing
>> and a loss in debugger support (or a need for a more sophisticated
>> debugger), then define private keys using tables:
>>
>>        local keyMyPrivateField = {}
>>
>>        self[ keyMyPrivateField ] = 'can't see this'
>>
>> If you never give out the keys, then clients can't mess with the field
>> contents.
>>
>> Except, of course, they can if they use pairs or next to iterate the table
>> and recover them. So, that's where you have to decide whether you are
>> concerned with sloppy coding or mischievous coding.
>
> Ouch (*big slap on forehead*)! I didn't consider that!
> I knew I could iterate the proxy, but somehow my brain ruled that out
> automatically! Here is a big gotcha of my approach! I'm giving out the
> metatable to anyone who might iterate my proxies (so the whole point of
> keeping it private is moot)!
>
> Well, as I said I don't need strict access control, only - perhaps - robust
> data hiding, so I don't think of banning next, pairs, etc, from the client
> code. Therefore probably the simpler and best approach is to stick to a
> simple naming convention for the private data, as you suggested:
>
> local proxy = {}
> proxy._obj = {} -- private data
>
> I wonder if a better crafted newproxy function could help here. I read in
> the archive that Lua team considered it not robust enough to give it
> support, and with upcoming 5.2 it is going to be removed.
> (I never used it actually - I don't like to depend on undocumented
> features).
>

For data hiding you could consider too saving the private data on a closure:

function Object()
  local new = {key="PUBLIC"}
  local private = {key="PRIVATE"}

  new.method = function()
    print(private.key, new.key)
  end

  return new
end

new = Object()
new.method()


--------------------------------------------------------------
EmmanuelOga.com - Software Developer


> Thank you very much Mark
>
> --
> Lorenzo
>
>
>