The std::optional proposed for C++14 uses std::nullopt and it was purposely kept separate from any other nulls because of implicit conversion. If you put a tribool into a optional then set it to some_common_null, are you setting the optional to null/disengaged or setting the tribool to indeterminate (and the optional is thus engaged). ie: optional<tribool> op = some_null; Sure an optional tribool is odd (in effect a quadbool) but probably will happen somewhere, and not obvious if it happens in templates... Tony Sent from my portable Analytical Engine ------------------------------ *From:* "Bjorn Reese" <breese@mail1.stofanet.dk> *To:* "boost@lists.boost.org" <boost@lists.boost.org> *Sent:* 13 July, 2013 8:58 AM *Subject:* [boost] Nullable types It seems like different Boost libraries use different nullable types. * Optional uses boost::none * Tribool uses boost::logic::indeterminate_keyword_t * Tuple uses boost::tuples::null_type * Variant uses boost::blank And there are possibly more. What are your opinions about unifying these? _______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost