boost/libs/optional/doc/14_io.qbk


[section IO operators]

It is possible to use `optional<T>` with IO streams, provided that `T` can be used with streams. IOStream operators are defined in a separate header.

``
#include <iostream>
#include <boost/optional/optional_io.hpp>

int main()
{
  boost::optional<int> o1 = 1, oN = boost::none;
  std::cout << o1;
  std::cin >> oN;
}
``

The current implementation does not guarantee any particular output. What it guarantees is that if streaming out and then back in `T` gives the same value, then streaming out and then back in `optional<T>` will also give back the same result:

``
#include <cassert>
#include <sstream>
#include <boost/optional/optional_io.hpp>

int main()
{
  boost::optional<int> o1 = 1, oN = boost::none;
  boost::optional<int> x1, x2;
  std::stringstream s;
  s << o1 << oN;
  s >> x1 >> x2;
  assert (o1 == x1);
  assert (oN == x2);
}
``

[endsect]