// Code derived from Stroustrup's PPP2 book
// § 4.3.1 Constant expressions
// -beginning on p 96

#include <iostream>

using std::cout;

int main()
{
  // constexpr double pi = 3.14159;
  constexpr double pi = 3.14159265359;  // using more-accurate def'n instead
  // pi = 7;  // error: assignment to constant

  constexpr double r = 2.0;
  // r = 2.1;  // error: assignment to constant

  double c = 2 * pi * r;  // OK: we just read pi; we don’t try to change it here

  cout << pi << '\n'  // output above variable values to console...
       << r << '\n'   //
       << c << '\n';
}