Detecting a negative 0 stored as a double in C++

Question

I am doing some mathematical calculations (trying to convert Matlab code into C++, using VS2010) and I need to be able to tell if at some point I get a negative 0.

According to the IEEE standard -0/+0 differ only in the sign bit (the rest are 0).

I have used the following piece of code (post) to interpret my double as a unsigned char

double f = -5;
    unsigned char *c = reinterpret_cast<unsigned char *>(&f);
    for(int i=(sizeof(double)-1); i>=0; i--){
        printf("%02X", c[i]);
   }

Trying it with 5/-5 I get the expected result:

C014000000000000 (-5)
4014000000000000 (5)

But when I try it with 0/-0 I get only zeros in both case. VS2010 states that they are IEEE compliant (msdn) so I'm not sure which part of it I'm not getting.

If 0/-0 are indeed stored in the memory in the exact same way, there is no way I can tell them apart if I need to so I should stop wasting my time :) Right?

Answer 1

If you write

double d = -0; 

the following will happen:

First, -0 will be evaluated, which is of type int, because 0 is of type int. The result will be 0. Then 0 will be converted to double and assigned thus being +0.0, and not -0.0

double d = -0.0; // is your solution.

Answer 2

In addition to Armen's good answer, you should use signbit to detect this. Doing so will protect you from endian issues:

#include <iostream>
#include <cmath>

int main()
{
    std::cout << std::signbit(0.0) << '\n';
    std::cout << std::signbit(-0.0) << '\n';
}

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