Lua table->address and address->table

Question

Imagine the following code:

Mytable={}
print(Mytable)

Prints something like Table: 12345. How do I get the "address" part from Lua without messing with tostring's return value, and more importantly, how do I get the table back?

In code:

addr=table2address(Mytable)
-- type(addr) is number, addr is 12345
Othertable=address2table(addr)
-- type(Othertable) is table, Othertable==Mytable is true (same reference)

Is there any way to implement these 2 functions in Lua? If not, (how) do I do this in C?

Edit: the table2address could be done by cutting the Table: off from tostring(Mytable), but only if metamethod __tostring isn't defined, so I want to avoid this.

Answer 1

A trivial implementation meets all your criteria but one:

function table2address(Mytable) return Mytable end
function address2table(addr) return addr end

demo:

> Mytable={}
> print(Mytable)
table: 0x7fe511c0a190
> addr = table2address(Mytable)
> Othertable=address2table(addr)
> =type(Othertable)
table
> print(Othertable==Mytable)
true

A slightly more complicated implementation meets all your criteria:

t2at = {}

function table2address(Mytable) 
  local addr = t2at[Mytable]
  if addr == nil then
    addr = #t2at + 1
    t2at[Mytable] = addr
    t2at[addr] = Mytable
  end
  return addr
end

function address2table(addr)
  return t2at[addr]
end

demo:

> Mytable={}
> addr = table2address(Mytable)
> Othertable=address2table(addr)
> =type(Othertable)
table
> print(Othertable==Mytable)
true
> =type(addr)
number

So, why is the address important to you?

In a garbage collected language such as Lua, it is only possible to hold a reference to an object, not an address. [The present implementation may or may not move objects during GC, but other than userdata and Lua states, Lua has license to move anything.]

ADDENDUM

Re: "the addresses never get randomized (try print({}) in 2 new interactive lua instances)"

e$ lua
Lua 5.2.2  Copyright (C) 1994-2013 Lua.org, PUC-Rio
> print({})
table: 0x7fdaca4098c0
> ^D
e$ lua
Lua 5.2.2  Copyright (C) 1994-2013 Lua.org, PUC-Rio
> print({})
table: 0x7fb02a4098c0
> ^D
e$ 

Re: the physical address is really needed

Look at the function luaL_tolstring that implements the guts of print; it has (in Lua 5.2.2):

  default:
    lua_pushfstring(L, "%s: %p", luaL_typename(L, idx),
                                        lua_topointer(L, idx));
    break;

So, lua_topointer(L, idx) is the function you need to get a table's address.