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c++bitwise-not

Is ~i really equivalent to i != -1?

How does ~i work in C++?

I just noticed it is equivalent to i != -1, but I'm not certain about that. 

int arr[3] {1, 2, 3};
int n = 3;
for (int i = n - 1; ~i; i--) {
    cout << arr[i] << ' ';
}

It printed the array in reverse.

Solution

  • ~ is the bitwise NOT operator. ~i is 0 if and only if i has 1 in all its bits. Whether -1 has all bits 1 depends on how signed numbers are represented on the system. In two's complement representation, -1 is represented with all bits 1, so on such systems ~(-1) == 0. Neither in one's complement, nor in sign-and-magnitude does that hold true.

    Therefore, the answer is no; not on all systems. That said, two's complement is fairly ubiquitous in modern machines (everything made since the 90's), and on such systems, the answer is yes. Regardless of the sign representation however, i != -1 is much more readable.