Re: Getting the "ID" of an object.

[Lorenzo Donati <lorenzodonatibz@...>]

On 02/05/2011 7.16, Hisham wrote:
> On Mon, May 2, 2011 at 1:56 AM, Lorenzo Donati
> <lorenzodonatibz@interfree.it>  wrote:

[snip]

> > I need the "NNN" above to include that ID into the string representation for
> > my OOP objects when defining __tostring.
> >
> > I know that I could either:
> > 1) generate an unique ID with a custom algorithm
> > 2) upon creation, before attaching the metatable to the object being
> > created, call tostring on the object and store the ID in a (weak?) table.
> > 3) use a proxy and return the ID of the "private" object instead (preserves
> > uniqueness because of the 1-to-1 relationship between proxies and objects)
>
> Besides the options you gave, if what you want is to use the original
> tostring() value within your custom __tostring, an option (hack?) is
> to temporarily disable your custom __tostring, like this:
>
> -------------
> local tbl = {}
> local meta = {}
>
> local function meta_tostring(t)
>     meta.__tostring = nil
>     local s = tostring(t)
>     meta.__tostring = meta_tostring
>     return "Original tostring is "..s
> end
> meta.__tostring = meta_tostring
>
> setmetatable(tbl, meta)
>
> print(tbl)
> -------------

Right! Nice trick! Thanks!


I really didn't think of such a solution. My mindset is still too 
accustomed to classical OOP behaviour of languages like Java & C++ 
(changing the methods of an object at runtime - really weird from such 
languages' standpoint!).

I really wouldn't call it a hack: it just uses the dynamic nature of 
Lua. Now that I see it I find it rather clean and elegant (unless I miss 
something, such as hidden side effects or loopholes).

> In case you're using it for something else, I'd use one of the methods
> you mentioned, like the weak table. But make sure you really need the
> id -- for most situations where one would use an object id in other
> languages, one can simply use the table itself as a key when coding in
> Lua.

Yes, I know that I can use tables as keys, thanks.
I need the ID only for debugging/logging purposes, just to build a more 
detailed string representation of an object and tell apart two objects 
which may happen to have the same "content", but different identity 
(say, for example, two complex numbers having the same cartesian 
components).


Thank you very much for the suggestion.


--
Lorenzo