The cmath library actually provides many mathematical constants that you can make use of.

To include the mathematical constants, you need to use a #define macro called _USE_MATH_DEFINES and add it before importing the cmath library:

#define _USE_MATH_DEFINES
 
#include <cmath>
#include <iostream>
 
int main() { 
  std::cout << M_PI << " " << M_E << " " << M_SQRT2 << endl;
  return 0;
}

There are quite a few constants on offer.