Re: [boost] [type_traits] is_base_of<B, D> should work when B is incomplete

Subject: Re: [boost] [type_traits] is_base_of<B, D> should work when B is incomplete
From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2016-03-04 03:36:30

2016-03-04 9:24 GMT+01:00 Andrzej Krzemienski <akrzemi1_at_[hidden]>:

>
>
> 2016-03-03 23:33 GMT+01:00 Matt Calabrese <rivorus_at_[hidden]>:
>
>> On Thu, Mar 3, 2016 at 5:18 PM, Daniel Frey <d.frey_at_[hidden]> wrote:
>>
>> > > On 03.03.2016, at 22:57, Andrzej Krzemienski <akrzemi1_at_[hidden]>
>> wrote:
>> > >
>> > > The Standard ([meta.rel]) requires that in std::is_base_of<B, D> D
>> must
>> > be
>> > > a complete type, but it does not require the same of B. This makes
>> sense,
>> > > because if we compare a complete type D and an incomplete type B, the
>> > > former is surely not derived from the latter.
>> > > In contrast, boost::is_base_of imposes an additional constraint that B
>> > must
>> > > also be complete. Could this restriction be lifted?
>> >
>> > That would probably be possible for all modern compilers that have a
>> > proper std::is_base_of because it requires a compiler intrinsic (unless
>> you
>> > can implement it without such an intrinsic and in C++98).
>> Boost.TypeTraits
>> > have quite a history and by lifting the restriction, you would drop
>> support
>> > for old compilers (think pre-C++11).
>> >
>>
>> Excluding C++98 support, why do you need a compiler intrinsic for this
>> change? If the reason is the incompleteness check, I've implemented
>> completeness checking metafunctions before and it's fully possible without
>> intrinsics in C++11. I described a completeness metafunction that works in
>> practice a few years ago in a discussion on the mailing list here:
>>
>> https://groups.google.com/forum/#!topic/boost-developers-archive/zp7u3XHv6zc
>> which links to an implementation here: http://codepaste.net/xfgcu8 (there
>> are subtleties to usage, of course). The code that is linked is old and
>> not
>> fully correct, although in the time since that thread, I have produced
>> what
>> I believe to be a fully correct and well tested implementation.
>>
>
> This is quite interesting. However, for my particular purpose, I may not
> need to employ a C++11 solution.
>
> I have an incomplete type boost::in_place_factory. I want to check if a
> given type T is either derived from boost::in_place_factory. It doesn't
> have to be very strict: it can get false negative on multiple inheritance;
> it can give a false positive when T is implicitly convertible to
> boost::in_place_factory by other means, or simply return false_type when
> boost::in_place_factory is incomplete.
>
> Perhaps there is a C++03-compatible solution to this particular problem?
>

Actually, I do not even need a true-false answer; I just need to disable an
overload (in SFINAE sense) if boost::in_place_factory is either incomplete
or not a base class of T.