Re: Getting the "ID" of an object.

["Dimiter \"malkia\" Stanev" <malkia@...>]

Not sure I understand exactly, but here is my poor attempt at solving it 
(poor because it has local side effects - I have to turn off the 
metatable, then turn it back on if tostring is the lua tostring)

local test = {
}

local function print_old_new(obj, old_tostring, args)
   return "OLD: " .. old_tostring(obj) .. " NEW: " .. 
table.concat(args, ' ')
end

local function wrap_tostring(obj, new_tostring, ...)
   local args = { ... }
   local mt = getmetatable(obj)
   local old_tostring = mt and mt.__tostring or tostring
   return function(obj)
	     local mt = getmetatable(obj)
	     setmetatable(obj, {})
	     local r = new_tostring(obj, old_tostring, args)
	     setmetatable(obj, mt)
	     return r
	  end
end

setmetatable(
   test,
   { __tostring = function(obj) return "PRIMARY?" end }
)

setmetatable(
   test,
   { __tostring = wrap_tostring(test, print_old_new, "SECONDARY") }
)

setmetatable(
   test,
   { __tostring = wrap_tostring(test, print_old_new, "ADVANCED!") }
)

print(test)



On 5/1/11 9:56 PM, Lorenzo Donati wrote:
> Hi all!
>
> I recently stumbled on an issue I cannot find a workaround for.
>
> When defining the __tostring metamethod for a a table, the old behaviour
> is completely lost, i.e. there is no "rawtostring" and no apparent way
> to concoct a substitute (without resorting to C).
>
> What I'd like is a way to get the unique IDs (I suppose they are the
> memory address) that Lua generates for the default string representation
> of tables, i.e. the NNN in
>
> tostring(t) --> "table: NNN"
>
> It doesn't seem that the debug library helps here either.
>
> 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)
>
> But both these approaches are lots of boilerplate code and/or useless
> overhead (the logic for that ID is already somewhere in the bowel of Lua
> - it just need a way to be exposed cleanly).
>
> I wonder if there is a nifty trick to recover that NNN only when needed
> without too much overhead or code.
>
>
> If not, why not adding a new method to the debug library, such as
> "debug.getid". Maybe it could be useful in other contexts and it
> shouldn't be to "heavy" to implement (just expose what's already in
> Lua). I see that upcoming Lua 5.2 has a debug.upvalueid. Maybe rename it
> as debug.getid and make it applicable to any Lua object? Wouldn't it be
> more in Lua spirit (less functions, with more general application)?
>
> Or even enhance debug.getinfo to give info also on tables (and other
> types too?).
>
> Or, still another option, add a second return value to "type" (I admit
> that this solution is maybe not very clean semantically).
>
> thanks!