Hello,

 

I've just noticed that the following code might be missleading:

 

boost::optional<int> opt;

opt = 0;

std::cout << opt;

 

If boost/optional/optional_io.hpp is not included, the above code outputs "1" - because boost::optional is implicitly convertible to bool.

Maybe it's worth either including boost/optional/optional_io.hpp in boost/optional.hpp or making some trick to generate compiler error if operator <<() used and optional_io.hpp is not included?