Slicing / Object identity problem

Hi everyone,
I'm trying to use some classe instances from Lua to C++ then back to 
Lua, and I'm having trouble to preserve my Lua content.
Here is my test:

********* C++ *********
     struct A
     {
         bool operator == (const A& ref) const {return this == &ref;}
     };
     struct A_wrap : A, luabind::wrap_base
     {
     };

     A* Identity(A* ptr) { return ptr; }
...
     module(pLua)
     [
         def("Identity", &Identity),
         class_<A, A_wrap>("A")
         .def(constructor<>())
         .def(const_self == const_self)
     ];

********* Lua *********
class 'B' (A)
function B:__init()
     A.__init(self)
end
function B:Test()
     print("OK")
end

x = B()
x.foo = 0
y = Identity(x)
assert(x == y)    -- no problem but doesn't help for next tests
print(x.foo)    -- print "0"
print(y.foo)    -- print "nil"
x:Test()        -- print "OK"
y:Test()        -- error "attempt to call method 'Test' (a nil value)"

The problem is that I would like to achieve "y" still having the "foo" 
and "Test()" members.
Is there any way to prevent this slicing loss and having "y" still being 
a real lua identity of "x" ?
The example is quite simple but I'm looking for any possible solution : 
some wrapper on my pointers, instance manager in C++, etc.

Thanks

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