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. 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. On 22 February 2012 18:24, Hite, Christopher wrote: > > The whole point of N2544 was to allow you to do stuff like this. I can declare a deconstructor/constructor for the union which does nothing. Unfortunately has_trivial_destructor doesn't get it. Because it's not trivial. The is_trivial_xxx traits tested very specific properties with well-defined meanings according to the standard, they didn't just test for empty constructors/destructors, because that's not the same thing. struct X { std::string s; }; Even though it's not user-declared, X does not have a trivial default constructor, because it calls std::string's default constructor which is non-trivial. But as N3142 says, that's not very useful for library writers. > That's just my use case though. A more general question: can you detect do nothing/trivial constructors/destructors/assignment on any struct/union/class? 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.) The unimplemented is_trivially_xxx traits should work for both cases if the constructor really is do-nothing, unlike the X example above, or this example which allocates memory in its default constructor: struct Y { std::string s = "foo"; Y() { } }; > For example if std::pair<bool,char> defines a copy constructor or assignment does it have to lose the "trivial" property? It would, yes, but that's why std::pair has defaulted copy/move constructors, so those members are trivial if the template arguments are trivially constructible. > 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.
