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Re: has_trivial_destructor improvable?
On 27 February 2012 10:14, Hite, Christopher wrote:
> From: Jonathan Wakely [mailto:jwakely.gcc@gmail.com]
>
>> 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?
Yes.
> ?Do they say wether a compiler's impl of is_trivially* is allowed generate false negatives?
It is not allowed.
>> 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.
That's what is_trivially_xxx_constructible should detect.
>> > 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
Huh? A link to what?
> What I don't get 100% is:
> 1) wasn't this stuff supposed to be finished in the standard?
It is finished.
> 2) are the is_trvial* different from has_trivial* ?
Yes, did you read the link I sent?
> 3) does gcc need magic? ?Will it do the magic any time soon?
Yes! They can't be implemented without compiler support. I don't
know when that will be available.