How to get mathematical constants?

  #include <boost/math/constants/constants.hpp> // header file
  #include <iostream> // for output

  int main(void)
  {
      double PI = boost::math::constants::pi<double>();  // Define pi as the variable "PI"
      double E = boost::math::constants::e<double>();  // Define e as the variable "E"
      std::cout.precision(16);   // output with 16 digits
      std::cout << "pi=" << PI << std::endl;
      std::cout << "e=" << E << std::endl;
      return 0;
  }