Example of boost::optional to write a safe sqrt function.

safe_sqrt.cc

// clang++ -std=c++11 safe_sqrt.cc

#include <iostream>
#include <cmath>
#include <boost/optional.hpp>

auto safe_sqrt(double x) -> boost::optional<double> {
  if (x >= 0.0)
    return std::sqrt(x);
  return boost::none;
}

auto main() -> int {
  const auto xs = std::vector<double> { -0.001, 0.5, 23333.9182, -78.0183 };
  for (const auto x : xs) {
    const auto s = safe_sqrt(x);
    if (s)
      std::cout << "sqrt(" << x << ") = " << *s << '\n';
    else
      std::cout << "No square roots for " << x << " on the real line.\n";
  }
  return 0;
}