Calculate e http://en.wikipedia.org/wiki/E_(mathematical_constant)

e.cc

#include <algorithm>
#include <chrono>
#include <cmath>
#include <iostream>

#define STR_(X)  (#X)
#define STR(X)  (STR_(X))
#define ACTUAL  2.71828182845904523536028747135266249776

typedef long long int Int;
typedef long double Real;
typedef std::chrono::high_resolution_clock Clock;

int main() {
  Real actual = ACTUAL;
  Real e = 1;//0
  Int fact = 1;
  int i = 1;//0

  auto t1 = Clock::now();
  while (fact > 0) {
    fact *= i;//std::max(1, i);
    e += 1.0 / fact;
    ++i;
  }
  auto t2 = Clock::now();

  std::cout.precision(50);
  std::cout
    << i << " iterations in "
    << (t2-t1).count() << " ns\n"
    << e << " computed\n"
    << STR(ACTUAL) << " actual_str\n"
    << actual << " actual\n"
    << std::abs(actual - e) << " error\n";

  return 0;
}