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 03:24:11

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?

Regards,
&rzej