Consider the following code:
#include <boost/units/io.hpp>
#include <boost/units/systems/si/plane_angle.hpp>
#include <boost/units/systems/angle/degrees.hpp>
#include <iostream>
#include <cmath>
#include <limits>
int main()
{
using namespace boost::units;
std::cout.precision(std::numeric_limits<double>::digits10);
std::cout << "Everyone knows that 180 deg = " << std::acos(-1.) << " rad\n";
std::cout << "Boost thinks that 180 deg = "
<< quantity<si::plane_angle,double>(180.*degree::degree) << "\n";
}
I get the following output:
Everyone knows that 180 deg = 3.14159265358979 rad
Boost thinks that 180 deg = 3.14159265359 rad
Apparently, Boost.Units gets very low precision M_PI defined somewhere manually, because it's just truncated after 12th decimal place. But as I grep
ped my /usr/include/
, I only found this imprecise definition in /usr/include/python2.7/Imaging.h
, which looks quite unrelated. All others have more decimal figures specified.
So, my question is: how does Boost come up with this result?
From
boost/units/base_units/angle/degree.hpp
:Dividing 6.28318530718 by 2 returns the value you are seeing. This certainly doesn't seem quite right, but alas, this seems to be the case.
Update: I found a bug report for this problem: https://svn.boost.org/trac/boost/ticket/6893