Takes advantage of the C++20 mathematical constants in the new <numbers> header

pi.cpp

#include <iomanip>
#include <iostream>
#include <numbers>

// Uses C++20 features.
void PrintPi() {
  std::cout << std::setprecision(16) << std::numbers::pi_v<long double> << '\n';
}

int main() {
  PrintPi();
  return 0;
}