C++ Constant for PI

Where's the definition for pi? Doesn't C++ provide a constant? What about C?

The C++ standard doesn't provide a value for pi but it's simple to define yourself:

#include <iostream>
#include <cmath> // M_PI is not standard

using namespace std;

class MathConst {
  public:
    static const long double PI;
};

const long double MathConst::PI = acos((long double) -1);

int main() {
  cout.precision(100);
  cout << "PI ~ " << MathConst::PI << endl;
}

Output:

$ g++ MathConst.cpp -o pi
$ ./pi 
PI ~ 3.14159265358979323851280895940618620443274267017841339111328125

That is accurate to 18 decimal places.

Here's a definition in C:

#include <math.h>
#include <stdio.h>

double pi() {
  const double pi = acos((double) - 1);
  return pi;
}

int main() {
  printf("%.20f\n", pi());
  return 0;
}