Optional.h

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#pragma once

#include <ostream>

#if __has_include(<optional>)
#  include <optional>
#elif __has_include(<experimental/optional>)
#  include <experimental/optional>

namespace std {
using std::experimental::nullopt;
using std::experimental::nullopt_t;
using std::experimental::optional;

using std::experimental::make_optional;

template <class T>
std::ostream &operator<<(std::ostream &os, optional<T> const &opt) {
  return opt ? (os << *opt) : (os << "nullopt");
}

} // namespace std

#else
#  error C++ Standard libary has no optional type
#endif