Detecting and adjusting for negative zero

Question

I have some code that has been port from Java to C++

// since this point is a vector from (0,0,0), we can just take the
// dot product and compare
double r = point.dot(normal);
return (r>=0.0);

But in C++ r can be either +0.0 or -0.0, when r equals -0.0 it fails the check.

I've tried to adjust for negative zero in the code below but it never hits the DEBUG("Negative zero") line. But r2 does print out equal to +0.0.

// since this point is a vector from (0,0,0), we can just take the
// dot product and compare
double r = point.dot(normal);
if (std::signbit(r)){
    double r2 = r*-1;
    DEBUG("r=%f r=%f", r,r2);
    if (r2==0.0) {
        DEBUG("Negative zero");
        r = 0.0; //Handle negative zero
    }
}
return (r>=0.0);

Any suggestion?

TESTING CODE:

DEBUG("point=%s", point.toString().c_str());
DEBUG("normal=%s", normal->toString().c_str());
double r = point.dot(normal);
DEBUG("r=%f", r);
bool b = (r>=0.0);
DEBUG("b=%u", b);

TESTING RESULTS:

DEBUG - point=Vector3D[ x=1,y=0,z=0 ]
DEBUG - normal=Vector3D[ x=0,y=-0.0348995,z=0.0348782 ]
DEBUG - r=0.000000
DEBUG - b=1
DEBUG - point=Vector3D[ x=1,y=0,z=0 ]
DEBUG - normal=Vector3D[ x=-2.78269e-07,y=0.0174577,z=-0.0174391 ]
DEBUG - r=-0.000000
DEBUG - b=0

GCC:

Target: x86_64-linux-gnu
--enable-languages=c,c++,fortran,objc,obj-c++ 
--prefix=/usr 
--program-suffix=-4.6 
--enable-shared 
--enable-linker-build-id 
--with-system-zlib 
--libexecdir=/usr/lib 
--without-included-gettext 
--enable-threads=posix 
--with-gxx-include-dir=/usr/include/c++/4.6 
--libdir=/usr/lib 
--enable-nls 
--with-sysroot=/ 
--enable-clocale=gnu 
--enable-libstdcxx-debug 
--enable-libstdcxx-time=yes 
--enable-gnu-unique-object 
--enable-plugin 
--enable-objc-gc 
--disable-werror 
--with-arch-32=i686 
--with-tune=generic 
--enable-checking=release 
--build=x86_64-linux-gnu 
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.6.3 (Ubuntu/Linaro 4.6.3-1ubuntu5) 

FLAGS:

CXXFLAGS += -g -Wall -fPIC

ANSWER:

I've used @amit's answer to do the following.

return (r>=(0.0-std::numeric_limits<double>::epsilon()));

Which seems to work.

Answer 1

Well, a generic suggestion when using doubles is remembering they are not exact. Thus, if equality is important - using some tolerance factor is usually advised.

In your case:

if (|r - 0.0| >= EPSILON)

where EPSILON is your tolerance factor, will yield true if r is not 0.0, with at least EPSILON interval.