Assign a negative number to an unsigned int

Question

This code gives the meaningful output

#include <iostream>

int main() {
    unsigned int ui = 100;
    unsigned int negative_ui = -22u;

    std::cout << ui + negative_ui << std::endl;

}

Output:

78

The variable negative_ui stores -22, but is an unsigned int. My question is why does unsigned int negative_ui = -22u; work. How can an unsigned int store a negative number? Is it save to be used or does this yield undefined behaviour?

I use the intel compiler 18.0.3. With the option -Wall no warnings occurred.

Ps. I have read What happens if I assign a negative value to an unsigned variable? and Why unsigned int contained negative number

Answer 1

How can an unsigned int store a negative number?

It doesn't. Instead, it stores a representable number that is congruent with that negative number modulo the number of all representable values. The same is also true with results that are larger than the largest representable value.

Is it save to be used or does this yield undefined behaviour?

There is no UB. Unsigned arithmetic overflow is well defined.

It is safe to rely on the result. However, it can be brittle. For example, if you add -22u and 100ull, then you get UINT_MAX + 79 (i.e. a large value assuming unsigned long long is a larger type than unsigned) which is congruent with 78 modulo UINT_MAX + 1 that is representable in unsigned long long but not representable in unsigned.

Note that signed arithmetic overflow is undefined.