c++ - 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

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.

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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.