...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
boost::log::optional_manipulator
// In header: <boost/log/utility/manipulators/optional.hpp> template<typename OptionalT, typename NoneT> class optional_manipulator { public: // construct/copy/destruct optional_manipulator(stored_optional_type, stored_none_type) noexcept; // public member functions template<typename StreamT> void output(StreamT &) const; };