There are several ways I’ve seen over the years of interchanging 2 variables. I’ll show you here how you can do it with bitwise operations. This method is most efficient when used in combination with assembler. In C++ it is looked at more like a curiosity than a real method by most. However if you would want to make your C++ code harder to read, then this can be helpful.

    A byte of theory

     Before seeing how we can interchange two variables we will have to learn the use of the XOR logical operator. It is sometimes called exclusive OR, or exclusive disjunction. It is true when one of the operands (the numbers between which we are using XOR) is true, but not both, while its close brother the OR operator is true even when both are true.

a	b	a XOR b
0	0	0
0	1	1
1	0	1
1	1	0

    You can remember it more easily if you think of it like adding binary numbers without carry. So: 0+0=0 ; 0+1=1 ; 1+0=1 ; 1+1=0 carry 1. So by ignoring the carry we obtain the XOR logical function.

    So how can we use it?

Let’s say that we have two variables say a=5 and b=3. The steps we would have to take to switch the two variables (registers) are shown in the table below:

a	0101
b	0011
a = a XOR b	0110
b = a XOR b	0101
a = a XOR b	0011

    i)    In assembler ( asm )

Let’s suppose that a and b are in two of the CPU registers: AL and respectively BL.

The coding sequence looks like this:


Note: As you can see the XOR operator in ASM delivers the result of the operation in the first register you mention.


    ii)    In C/C++

In C++ the XOR operator is represented by the operator "^" (without the quotes). So the coding sequence for C++ is:


By using the operator ^= we could make our code harder to read :


    In the end I would like to mention another interesting use of the XOR function especially for ASM. If we want to empty a register (let’s say AL) we would use "XOR AL, AL". So by XORing a number with itself we obtain 0 because 0 xor 0 = 0 and 1 xor 1 = 0.