From: Jonathan Wakely [mailto:jwakely.gcc@xxxxxxxxx] > N.B. I was forgetting the has_trivial_xxx traits were renamed/replaced by http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2010/n3142.html > but GCC doesn't implement that yet. Ok, so they're changing the names, but did they change the meaning? Do they say wether a compiler's impl of is_trivially* is allowed generate false negatives? > I'm not sure, but I think std::is_trivially_default_constructible > should be true for your optional<std::string> union, because although the default constructor is not trivial (in terms of the core language) it doesn't actually call std::string's constructor or any other non-trivial functions. I think so to. The frustrating thing is I have to define the ctor/dtor even if they do nothing, but then they appear not to be trivial. > Again, do-nothing and trivial are not the same thing. And a do-nothing constructor for a struct/class (which default-initiailizes bases and > members) is not the same as a do-nothing constructor for a union (which does not initialize its members.) I know. Theoertically the compiler could go through all base classes and members. By "do-nothing" constructor I really meant on that generates no code, directly or indirrectly. > > Is there some proof that doing these things are impossible? > Not that I know of, but I'd be surprised if is_trivially_xxx or the old has_trivial_xxx can be implemented without compiler magic. Wow, I'm surprised too! Can you send me a link. Anything that detects trivial deconstruction seems like it would need magic. It talks about calling " any operation that is not trivial. " http://en.cppreference.com/w/cpp/types/is_destructible What I don't get 100% is: 1) wasn't this stuff supposed to be finished in the standard? 2) are the is_trvial* different from has_trivial* ? 3) does gcc need magic? Will it do the magic any time soon? Thanks Jonathan. Chris
