decompiling back to C++?

It is worth seeing the home page of The International Obfuscated C Code Contest. (hxxp://www.ioccc.org)
I would be surprised if there would ever be such an AI that could retrieve those sources.
Just an example to taste it:

#include <stdio.h>
int l;int main(int o,char **O,
int I){char c,*D=O[1];if(o>0){
for(l=0;D[l              ];D[l
++]-=10){D   [l++]-=120;D[l]-=
110;while   (!main(0,O,l))D[l]
+=   20;   putchar((D[l]+1032)
/20   )   ;}putchar(10);}else{
c=o+     (D[I]+82)%10-(I>l/2)*
(D[I-l+I]+72)/10-9;D[I]+=I<0?0
:!(o=main(c/10,O,I-1))*((c+999
)%10-(D[I]+92)%10);}return o;}

This is a square root calculator, note the form of the whitespaces

Ok, this (and the others on the IOCCC page) are not real-life examples, but as LoveExeZ pointed there are substantial information in the source code that is simply impossible to get back.

On the other hand if we just get back only a small subset of this extra info, it can help a lot. If one gets back a part of the inheritance hierarchy, then it can be very useful.
Polymorph classes and virtual function calls can be recognized because they use the vptr (exact implementation details differ from compiler to compiler). The hierarchy can be reproduced from the constructors and the destructors as they again have a certain structure (calling the ctor of base's base, the ctor of base etc.)
Finding constructors and destructors is easy from the virtual table, and having these functions identified, lots of info can be given.
Just imagine the following:

Originally:

function1()
{
   int i1, i2, i3, i4, i5;
   function2( &i1 );
   function3( &i4 );
   function4( &i1 );
   function5( &i4 );
   function6( &i4 );
   function7( &i1 );
}

Having ctor/dtor pairs identified:

function1()
{
   Class1 Object1;
   Class2 Object2;
   Object1.Member1();
   Object2.Member2();
}