c++ - Best platform independent pi constant?

Question

I know that you can use:

#define _USE_MATH_DEFINES

and then:

M_PI

to get the constant pi. However, if I remember correctly (comments welcome) this is compiler/platform dependent. So, what would be the most reliable way to use a pi constant that won't cause any problems when I port it from Linux to other systems?

I know that I could just define a float/double and then set it to a rounded pi value myself, but I'd really like to know if there is a designated mechanism.

Answer 1

Meeting C++ has an article on the different options for generating pi: C++ & π they discuss some of the options, from cmath, which is not platform independent:

double pi = M_PI;
std::cout << pi << std::endl;

and from boost:

std::cout << boost::math::constants::pi<double>() << std::endl

and using atan, with constexpr removed since as SchighSchagh points out that is not platform independent:

 double const_pi() { return std::atan(1)*4; }

I gathered all the methods into a live example:

#include <iostream>
#include <cmath>
#include <boost/math/constants/constants.hpp>

double piFunc() { return std::atan(1)*4; }

int main()
{
    double pi = M_PI;
    std::cout << pi << std::endl;
    std::cout << boost::math::constants::pi<double>() << std::endl ;
    std::cout << piFunc() << std::endl;
}

C++2a pi_v

In C++2a we should get pi_v:

#include <numbers>
#include <iostream>

int main() {
     std::cout<< std::numbers::pi_v<double> <<"
";
}