swapping in one line

Using * especially has a risk of causing overflow, which is undefined behavior. (Unsigned overflow is not defined, but (x * y) / y is not always x in unsigned arithmetic). The + option is better, but can still overflow. A better option would be ^ (xor), since there's no overflow possible.

However, while this solution is cute, it's still undefined behavior. You're not allowed to both read and write a variable within one expression, because there's no specified sequence determining which happens first. It might happen to work in this example on the compiler you're using, but with optimizations enabled any number of things could break it. (Both gcc and clang rightfully emit a warning when you try this code).

In C++, you should use std::swap(a, b).

You can define your own swap macro in C if you really need it, for example from this StackOverflow question:

#define swap(T, x, y) {T _swap_tmp = x; x = y; y = _swap_tmp; }