constexpr I think is the big difference. Also the name of the empty class to reset the value to none, boost is boost::none whereas the std has std::nullopt_t.

constexpr I think is the big difference. Also the name of the empty class to reset the value to none, boost is boost::none whereas the std has std::nullopt_t. I prefer the boost name of none as nullopt sounds and looks too much like null.