Does float have a negative zero? (-0f)

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According to the standard, negative zero exists but it is equal to positive zero. For almost all purposes, the two behave the same way and many consider the existence of a negative to be an implementation detail. There are, however, some functions that behave quite differently, namely division and atan2:

#include <math.h>
#include <stdio.h>

int main() {
    double x = 0.0;
    double y = -0.0;
    printf("%.08f == %.08f: %d\n", x, y, x == y);
    printf("%.08f == %.08f: %d\n", 1 / x, 1 / y, 1 / x == 1 / y);
    printf("%.08f == %.08f: %d\n", atan2(x, y), atan2(y, y), atan2(x, y) == atan2(y, y));
}

The result from this code is:

0.00000000 == -0.00000000: 1
1.#INF0000 == -1.#INF0000: 0
3.14159265 == -3.14159265: 0

This would mean that code would correctly handle certain limits without a need for explicit handling. It’s not certain that relying on this feature for values close to the limits is a good idea, since a simple calculation error can change the sign and make the value far from correct, but you can still take advantage of it if you avoid calculations that would change the sign.

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