The standard hoary old assembly language programmer's trick is:

a ^= b;

b ^= a;

a ^= b;

But this sort of code has little place in modern, HLL programming. Temporary variables are essentially free, and the idiomatic code using three assignments, namely

int t = a;

a = b;

b = t;

is not only clearer to the human reader, it is more likely to be recognized by the compiler and turned into the most-efficient code (e.g. perhaps even using an EXCH instruction). The latter code is obviously also amenable to use with pointers and floating-point values, unlike the XOR trick.