Someone says this is an old lame trick. I think it’s simple and clever use of XOR.

How/Why does it work?

It’s built around the properties of the XOR ^ operator, who has the following properties:

  • A ^ B = B ^ A (commutative)

  • A ^ 0 = A

  • A ^ 1 = ~A

  • A ^ A = 0

So, you can see how it get’s applied here:

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#include <stdio .h>
int main(void)
{
   unsigned int a, b;
   // ... populate somehow "a" and "b"...
   
   printf("a = %d - b = %d\n", a, b);

   a ^= b; // store in "a" the value of "a XOR b"
   b ^= a; // store in "b" the value of "a XOR b XOR b" = "a XOR 0" = "a"
   a ^= b; // store in "a" the velue of "a XOR b XOR a" = "b XOR 0" = "b"

   printf("a = %d - b = %d\n", a, b);
}

Neat.