Potential Problem in "Swapping values of two variables without using a third variable"

Question

I recently came along this method for swapping the values of two variables without using a third variable.

a^=b^=a^=b

But when I tried the above code on different compilers, I got different results, some gave correct results, some didn't.

Is anything terribly wrong with the code?

Answer 1

If you're using C++, why not use the swap algorithm in STL? It is ideal for this purpose and it's very clear what it does:

#include <algorithm>
using namespace std;

// ...

int x=5, y=10;    // x:5 y:10
swap(x,y);        // x:10 y:5

Answer 2

Is anything terribly wrong with the code?

Yes!

a^=b^=a^=b in fact invokes Undefined Behaviour in C and in C++ because you are trying to change the value of a more than once between two sequence points.

---

Try writing (although not foolproof )

a ^= b;
b ^= a;
a ^= b;

instead of a^=b^=a^=b.

P.S : Never try to swap the values of two variables without using a third one. Always use a third variable.

EDIT :

As @caf noticed b^=a^=b is fine even though the order of evaluation of arguments of ^= operator is unspecified, since all the accesses of b within the expression are being used to compute the final value that is being stored in b, the behaviour is well defined.